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Chapter 2

Computers and Control - Mechatronic Systems

A Summary... An overview and history of industrial computer control systems. General-purposecomputer systems adapted for process and system control. Specialised computercontrol systems (micro-controllers, Computer Numerical Controllers (CNC) and robotcontrollers). Industrial computer interfaces for process control - the ProgrammableLogic Controller. Mechatronic elements and systems - servo motors, transfer lines,FMS, etc.

Digital to

to Digital

Scaling orAmplification

External Voltage Supply

EnergyConversionIsolation

IsolationScaling orProtectionCircuits

EnergyConversion

ExternalSystem

Computer

AnalogConversion

Analog

Conversion Amplification

External Voltage Supply

Analog Energy FormsAnalog VoltagesDigital Voltages

(Actuators)

(Transducers)


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14 D.J. Toncich - Computer Architecture and Interfacing to Mechatronic Systems

2.1 A Review of Developments in Digital Computer Control

Digital computer control is a relatively recent phenomenon. It began toproliferate in the 1970s as a result of the increased computational capacity of digitalcomputers and the increased availability (and "affordability") of integrated digitalcircuits. However, early developers in digital computer controls faced a difficult task because the external systems requiring control were predominantly analog in nature.Ironically, now that we have an enormous supply of devices to simplify the process of interfacing to analog systems, we increasingly find that the "systems" themselves arebecoming more digital in nature as a result of the increasing intelligence of constituentcomponents.

Unfortunately for early system designers, the cost of control computers was

relatively high and so, as a result, only the most expensive and complex processes weregenerally considered for computerisation. The cost of applying computers to simplecontrol systems was prohibitive and so designers had to add to their professional skillsby first tackling the worst possible problems, with few experts to turn to for advice.Typical applications included:

Power station control systems Chemical plant and refinery controllers Metal smelting plant controllers Large-scale food processing control systems.

All of these types of computer applications can be classified under the umbrella of "real-time control" and all suffer from similar problems. The problems include:

(i) The need to extract information from hundreds of sensors and energytransducers

(ii) The need to process incoming information in "real-time" (ie: before thenext change of information occurs)

(iii) The need to output signals to hundreds of sensors and transducers.

Very few real-time control problems had been tackled by computermanufacturers up until the 1970s. Most manufacturers had been busy enough justdeveloping computers to handle the growing number of data processing tasks that had

arisen during the 1960s. However, the requirements of real-time control were quitedifferent and, given the limitations of the technologies available in the 1970s, designersdid an outstanding job in creating relatively reliable end systems.

Typical computer control systems in the 1970s had an architecture of the formshown in Figure 2.1. This is commonly referred to as a "hierarchical" controlarchitecture. It is composed of an intelligent control device (computer), referred to asthe host, and a number of unintelligent slave devices (sensors, transducers, actuators,amplifiers, etc.) that together make up a functioning system.


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Computers and Control - Mechatronic Systems 15

Control Computer

(Host)

Interfacing Boards

Sensor 1 Transducer 1

Sensor N Transducer N Relay 1 Amplifier 1 Solenoid 1

Relay N Amplifier N Solenoid N

Feedback Signals from System Computer Outputs to System

Input/Output

Figure 2.1 - Hierarchical Computer Control Architecture

The hierarchical control architecture came about as a result of necessity ratherthan outright design acumen. Computer processing was a relatively expensivecommodity in the 1970s and as a result it was most uncommon (and expensive) toconsider the use of more than one computer for a control problem. For this reason, the

host computer had to carry out all the functions associated with real-time control,including:

Input/output (normally abbreviated to I/O) Control algorithm execution Interaction with the system supervisors (users) Display of current status on screens and mimic panels.

This level of functionality was a radical departure from the traditional concept of

computing that had developed in the late 1950s and throughout the 1960s. Computershad to become devices that could execute programs within a given time-frame andinteract with the outside world to an extent that had previously been difficult toimagine.


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There were a number of important issues to be overcome before a transition couldbe made from the data-processing role of the computer to a real-time control scenario.Firstly, specialised input/output (I/O) boards and circuits had to be devised in order to

enable the computer to interact with the wide range of analog signals that emanatedfrom external systems and to enable the computer to drive high level analog outputs.Secondly, the most fundamental piece of software within a computer system, theoperating system, had to change its functionality. The operating system still had toremain as a resource manager and a scheduler and an interface between the computerhardware and software. The big change was that it now had to perform these functionsin such a way that programs could execute quickly enough to process incoming signalsbefore the status of those signals altered (otherwise data would be lost). This was thecritical, "real-time" issue.

In the 1950s and 1960s, operating systems were really designed so that large

computer systems (main-frames) could process office data, entered on punch-cards(known as Holorith cards), as efficiently as possible. The basic principle was that itdidn't really matter which data the computer processed first as long as, over a giventime-period (eg: one day), the total amount of information processed was maximised.A small amount of task prioritisation was allowed, but in general, operating systemstended to treat all data in terms of "files". In strict terms, a file was a quantum of datastored on either magnetic disk or ferrite-core memory. However, the changing natureof data input and output (from punch-cards and print-outs through to video terminalsand serial communications links) could not readily be accommodated in terms of theolder operating systems. The operating systems were therefore modified to considernewer forms of data as though they were files (even though they were physicallysomething else).

Although the concept sounds rather convoluted, treating all inputs and outputs asthough they were files on a disk worked satisfactorily in the office but it becameunwieldy for control purposes. If inputs and outputs related to an external systemunder control were treated as though they were files, then they were subject to the samelevel of prioritisation as files. In simple terms this meant that the operating systemsassociated with office computing could not deliver the time responses required for real-time control. In other words, it could take the older computers longer to process datafrom external systems than it may take for the data to change - information could belost.

One of the first organisations to recognise the limitations of the older stylecomputer architectures and operating systems was the Digital Equipment Corporation(DEC). Their response was to develop a range of computers that are still revered todayfor their innovative hardware and software. The computer range was given the titleprefix PDP-11 and was really the first computer series that engineers would claim forthemselves.


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Several factors made the DEC PDP-11 series revolutionary in its time (and led toits survival for some 20 years). The first was an innovative hardware design thatsimplified the control system development task for engineers through an enormous

range of powerful instructions. The second advantage of the PDP-11 was that thecompany began to produce a range of products that enabled the computer to interactwith the outside world, at a time when other manufacturers were still producingcomputers that worked in isolation. The third major advantage of the PDP-11 was itsnew operating system, specifically designed for real-time control applications. Theoperating system was given the acronym RSX-11 and also endured for some 20 years.

The DEC PDP-11 series of computers became a bench-mark for digital controlsystems and were widely used in the complex types of applications cited earlier.Indeed, even in the 1980s (when computer processing costs had diminished), systemdesigners still worked with the sort of hierarchical control architecture made popular

(and feasible) by DEC. However, by the mid-1980s, the cost of microprocessor chipsand microprocessor-based devices (including personal computers) had plummeted anda new trend in both computing and control emerged.

Mid-range computers such as the DEC PDP-11 became relatively expensive incomparison to microprocessor based computers and controllers and so, a new industryarose, focused on the task of minimising the use of mid-range computers andmaximising the use of low-range (single microprocessor) based devices. This wasgreatly assisted by the proliferation of intelligent (microprocessor controlled) devices inthe early 1980s. The net effect was to make possible a different form of controlarchitecture, which we now refer to as "distributed control". The concept of distributedcontrol is shown schematically in Figure 2.2.

Host Computer(Event Scheduler)

Local

Processor 1

Local

Processor 2

Local

Processor N

CommunicationsData

Links

Inputs / Outputs Inputs / Outputs Inputs / Outputsto local system 1 to local system 2 to local system N

Figure 2.2 - Distributed Computer Control Architecture


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The basic principle of distributed control is that a complex control system isdivided up into a number of components and each component is controlled by a localcomputer (which may be microprocessor or Digital Signal Processor (DSP) based).

The role of the host computer is then only to coordinate (schedule) the activities of each of the local (slave) processors and to interact with the system users. The hostcomputer and local processors are normally connected to one another via datacommunications links or a network, both of which unfortunately create a new range of design problems that have to be resolved. However, if the communications issue canbe resolved then the distributed control architecture has a great deal of potential.

The idea of distributed control is to make each local control unit modular andsimple so that the overall control system becomes more robust than one large andcomplex system. This means that the host system can be a much smaller computerthan would be required for a hierarchical control system. The collective cost of the

smaller host computer and the local processors can be comparable, or lower, than thecost of one large computer. Moreover, when a local processor fails, it is often muchmore cost effective to replace it entirely than it is to repair a mid-range or mainframecomputer.

A typical area of control that is commonly allocated to some form of localprocessor is the collection and processing of signals from external systems. If the localprocessor is designed to gather and process signals and it is not loaded down with othertasks, then it may result in a total system that can perform time-critical functions moreefficiently than a large computer carrying out many tasks.

There is some ambiguity amongst different text books on the subject of "hierarchical" and "distributed" control, largely because the definitions are rathersubjective. In this book, we will simply define a distributed control system as beingone in which the total control structure is divided up amongst a number of computers orprocessors. One could also argue that the structure shown in Figure 2.2 is bothhierarchical and distributed since there is a host computer (ie: higher level computer)that controls the local processors.

The distributed control architecture concept can be further extended to the pointwhere there is no longer a need for a host computer. In other words, a system iscontrolled by a collection of computers (processors) that all work together in order toachieve some particular objective. This is referred to as a "heterarchical" control

structure and is shown schematically in Figure 2.3. This is a departure from the othertwo computer control architectures in the sense that there is no coordinating device inthe control system. The structure is sometimes referred to as a "functionallydecomposed" control architecture, since all control functions have been devolved downto local devices. The heterarchical control system looks interesting and has a numberof characteristics in common with the way in which the human brain operates - that is,as a collection of equally intelligent nodes operating together for some commonpurpose.


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LocalProcessor 1

LocalProcessor 2

LocalProcessor N


Data Communications Network (Local Area Network)

Figure 2.3 - Heterarchical Control Architecture

The problem with the heterarchical control architecture is that it makes thedevelopment of control software rather difficult because no single node has acoordinating role. This really requires a new way of thinking and many modern systemdesigners are graduates of the hierarchical control school and have difficulty intranslating their existing techniques to heterarchical control. Over and above theproblems related to the development of control systems in heterarchical structures,there is the issue of networking. Networking has always been one of the irritatingbottlenecks in the development of digital computers and computer control systems.Since the need for networking emerged in the 1970s, the progress towardsstandardisation has been painfully slow. In the case of heterarchical control systems,the networks form the backbone for communications between devices and are critical

to the success or failure of the system. However, the speed of communicationsbetween devices across a network has always been a limiting factor in the use of heterarchical control.

The heterarchical control architecture is making some progress as theperformance of computer networks improves and system designers begin to changetheir ways of thinking about control problems. However, all three forms of controlarchitecture are currently in use and all have advantages and limitations that need to beconsidered. It is to be hoped that once you have completed reading this book, you willhave a much greater understanding of the issues that are involved in selecting a digitalcomputer control architecture for a mechatronic control system.

The remainder of this chapter is devoted to exploring a number of differentcomputer devices that are available in common mechatronic systems and the way inwhich they are used in those systems.


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2.2 Programmable Logic Controllers

The Programmable Logic Controller, more commonly known as a PLC, is anunusual device in many ways. The modern PLC is an industrial control computer inevery sense but its predecessors were really only designed to be low-level digitalreplacements for the electromechanical control systems found on industrial equipmentduring the 1940s, 1950s and 1960s. As a result of this rather unusual heritage, the PLChas been developed in a rather peculiar way relative to other computer systems.

The first point to note about the PLC's heritage is that it was originally developedas a "tradesman's tool" rather than the "professional's tool" that the traditional officecomputer was designed to be. For this reason, a great deal of emphasis in early PLCdesign was to create a programming language that was best understood by industrial

electricians rather than professional programmers. This language became known as"relay-ladder-logic" and can still be found today on older equipment. The second pointto note about the PLC is that it was always designed as a computerised device withextensive input and output facilities that were very rare in traditional office computers.

PLCs are now amongst the most prolific of all modern industrial control systems.They are used for a wide range of applications and are very diverse in their capabilities.A PLC is shown schematically at its simplest level in Figure 2.4.

CurrentInputs

VoltageInputs VoltageOutputs

CurrentOutputs

PowerTransistorFront-End

Microprocessor

Digital

Circuitry

PowerTransistorBack-End

Control

and

Figure 2.4 - Schematics of a Programmable Logic Controller


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PLCs use power-transistor technology, in combination with microprocessors anddigital circuitry, in order to produce a specialised computer system for high powerswitching and control. The power transistor front and back ends, are used to buffer the

low-voltage microprocessor computer circuitry from high power industrial inputs andoutputs. PLCs therefore provide the ideal combination of a small computer systemtogether with interfacing to the industrial environment.

We earlier noted that PLCs were introduced to replace the electromechanicalrelay-ladder logic systems used to implement industrial controls. A typical functioncould be as follows: "If Input A is high and Input B is greater than 50 volts, then delay 10 seconds and then set Output C to High."

The starting point in PLC design was to give the devices the ability to performsuch functions with minimum programming effort. Moreover, to enable the industrialelectricians who once created relay ladders to program the newer technology PLCs.The inherent ability of PLCs to perform such functions makes them ideal for sequentialcontrol functions where (say) a number of hydraulic and pneumatic actuators andsensors have to be governed. For example, the opening and closing of safety doors orthe switching of fluid pumps in a production system.

Modern PLCs are relatively inexpensive items, which are industrially rugged indesign and extremely modular in structure. It is commonplace to buy a CentralProcessing Unit PLC, together with any number of bus-connected Input/Output (I/O)modules. This allows both simple and complex machines to be based upon the same,

basic PLC unit. An Original Equipment Manufacturer (OEM) may choose to design amachine using a basic PLC system (with say 10 to 20 inputs and outputs) and thenpurchase expansion I/O modules as customer, design requirements change.

Programming languages for Programmable Logic Controllers are as diverse as thecontrollers themselves. Early PLCs were only programmable in " ladder-logicdiagrams " that were a pictorial representation of Boolean circuits coupled with delayand timer elements. However, as people grew to realise the enormous range of designapplications for these programmable devices, it became less and less attractive to usethe now dated, ladder-logic, diagrams.

Many modern PLCs are sold with specialised implementations of the BASICprogramming language as a built-in feature. This allows a much more sensible andstructured approach to system design to be used. It has also been a logical step, sincethe proliferation of Personal Computers meant that more and more technical peoplewere comfortable with the concept of programming in a language, rather than usingdiagrams. In recent years, PLCs have now become available with specialised "C" or"PASCAL" compilers, that allow complex program development for controlapplications.


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Despite the availability of high-level languages on PLCs, they still remain verymuch a "tradesman's tool". PLCs are generally weak in terms of their ability to carryout complex computations and control algorithms. The real strength of the PLC is in

its ability to interact with high voltage and current inputs and outputs. Moreover, sincePLCs are designed in a modular (building-block) manner, an enormous range of energyconversion transducers can be used to turn PLC outputs into useful drivers formechatronic equipment. For example, PLCs can readily be connected to a range of solenoids and pneumatic actuators to convert a voltage output into a mechanicalmovement.

The functionality of the PLC makes it ideal for controlling sequential "event-oriented" systems such as conveyors, dedicated transfer machining lines and so on.However, the ability of the modern PLC to communicate with higher level computersystems through a data communications link or network has made it into a useful

processing device for distributed control. Figure 2.5 illustrates a commonly useddistributed control system (similar to that shown in Figure 2.2) in which the PLCs areresponsible for interacting with the outside world while the host computer systemcarries out some complex control algorithm.

Host Computer(Control Algorithm)

PLC 1 PLC 2 PLC 3

Communications

Data

Links or Network


Figure 2.5 - Distributed Control Using PLCs

The distributed system of Figure 2.5 enables the PLCs to input information fromhundreds of external sources, carry out minor processing on that information and thenfeed it as control input data to the host computer. Once the host computer hascalculated the next set of required outputs (as determined by the control algorithm) itsends the information to the PLCs via the communications links or network. The PLCsare then responsible for creating the high current or voltage output.


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The problem with the sort of architecture shown in Figure 2.5 is that thecommonly used data communications links and networks are relatively slow. If, forexample, the system shown therein was used for some form of continuous process

control (such as in a power station or chemical plant) then the communications link between the PLC and the host computer could form an unacceptable bottle-neck interms of data flow. For this reason, a number of PLC manufacturers are nowdeveloping devices that have a much closer coupling to the host computer system sothat the external communications link can be avoided. This form of design involvesconnecting the PLC directly into the internal architecture of the host computer systemand is referred to as a "back-plane" connection.

The range of PLCs currently on the market is quite extensive and can includesmall devices costing less than a personal computer up to large systems costing morethan most mid-range computer systems. Simple PLCs have an appearance similar to a

programmable pocket calculator - however, the PLC is equipped with a number of high-voltage, high-current input/output terminals which are used for interaction withthe outside world. These systems are useful for simple sequential control.

Sophisticated PLCs can resemble computer workstations in both appearance andfunctionality. Most of the high-end PLCs are capable of executing multiple programssimultaneously, and interacting with the user through graphical software interfaces.These systems normally represent the "break-even" point in control systems design. Acontrol system designer would sometimes need to decide whether to use one of thesecostly PLCs to carry out a complete control system or whether it would be more costeffective to use a traditional computer and a lower cost PLC system.

The criteria typically used to select a PLC for a particular application include thefollowing:

PLC Programming Language Number of Inputs and Outputs (I/O capability) Ability to interact with the user and/or display graphical system information Expansion Capability Processor Execution Speed Modularity of Design Ruggedness of Design Capacity for Integration with other systems through:

Serial Communication Back-plane (Bus) Communication Local Area Network Communication.

Overriding the technical factors are the always considerable political factors,

which cause a company to choose a PLC vendor based upon conformance with othersystems already installed in a plant. This reduces the need for maintenance personnelto become familiar with a wide range of programming languages and implementationtechniques.


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2.3 Intelligent Indexers and Servo-Drive Systems

One of the most important issues in the development of mechatronic controlsystems for industrial applications is the ability to accurately move some mechanicalelement such as a cutting tool or end-effector from one position to another. This isessential to the development of precision devices such as robots, Computer NumericalControl (CNC) machine tools, indexing tables for laboratory equipment, etc.

There are many ways in which an element can be moved from one position toanother. Older industrial systems traditionally used hydraulics and pneumatics topropel mechanical elements. For centuries, mechanical clocks have used springs forenergy and gears to index the arms with a relatively high degree of accuracy. However,in modern industrial systems the most common approach is to use electromagnetic

techniques - that is, electric motors whose rotational positions can be accuratelycontrolled. Rotational movement is transformed into linear motion by driving simple"screw-feeds" or low-friction, low-backlash, recirculating ball-bearing, screw-feeds(ball-screw-feeds).

Those who are unfamiliar with the intricacies of electric motors will assume thatthe only function of a motor is to rotate continuously within a required velocity range.However, when we talk about the accurate positioning of a robot arm or CNC machineaxis, we are really talking about motors that are designed to rotate a fraction of arevolution and then stop. The actual motors used for these applications are similar tothe a.c. and d.c. motors that rotate continuously in other electrical machinery - thedifference is in the way they are controlled.

There are essentially two types of motor control systems that can be used foraccurate positioning of mechanical elements:

Stepper Motor or Indexer Control (Open loop control) Servo Motor Control (Closed loop control).

The stepper motor system is based upon a special type of motor that rotates

(indexes) by a fraction of a revolution each time a voltage pulse is applied to one of itswindings. Unlike traditional motors, the stepper motor does not rotate smoothly butrather, steps from one position to another and hence its name. The overall stepper drive

system is shown schematically in Figure 2.6. Stepper motors were originally used forsmall scale applications such as in printers, plotters, etc. However, they are now alsoused in industrial applications where the mechanical load on the motor is known (andstable). Although the control principles are similar, the industrial systems tend to bereferred to as "indexers" in commercial literature.


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Stepper Motor

Stepper Motor Drive

VoltagesRepresentingRequired Position

OutputsElectrical

MechanicalOutput

Electrical Energy Input

Figure 2.6 - Stepper Motor Arrangement

The nature of the actual stepper motor drive can vary considerably. For verysmall motors, the drive can be implemented on a single silicon chip. On large systems,the stepper motor drive has to include power electronics and cooling fins to dissipate

heat and hence has to be implemented on a circuit board, with discrete transistors anddigital circuits.

The fundamental limitation of stepper motors arises when they are used as shownin Figure 2.6 - that is, as "open-loop" devices. If the load on the motor shaft is largerthan the torque generated by the electrical energy input to the motor windings, then themotor will not index from one position to another in a predictable manner. Theabsolute positioning characteristic of the motor is therefore lost. As a result, open-loopstepper motors are only used in situations where the load is always well defined - forexample, in parts transfer (shuttle or indexing) systems where the maximum load on themotor can be calculated during the system design phase. A stepper motor, running in"open-loop" mode would be inappropriate for positioning a cutting tool, since the loadcaused by tool could vary substantially depending on the work-piece properties andamount of material being removed.

Some stepper motor controllers can function in a closed-loop, where the positionof the motor shaft is fed back to the controller from a resolver or position encoder.However, once these devices are converted into closed-loop systems, then they losetheir cost advantage over traditional servo drives that can provide a "smoother"rotation.


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The servo motor system is the embodiment of classical control theory andoperates in a closed loop that enables the controller to drive the motor according to itscurrent velocity or position. The traditional servo motor system is shown in Figure 2.7.

It is composed of:

An a.c. or d.c. motor An analog resolver or digital encoder device that provides a voltage signal

or signals corresponding to the orientation of the motor shaft The servo drive controller itself An electrical energy supply.

a.c. or d.c MotorEncoder

orResolver

Servo Drive

Electrical EnergyInput

ElectricalEnergyOutput

PositionFeedback

MechanicalOutput

Input voltageproportional torequired position

(Controller)

Figure 2.7 - Schematic of Traditional Servo Drive System Arrangement

The servo drive is an electronic device that is used to provide a regulated flow of electrical energy from an external power supply to the motor, based upon the differencebetween a specified voltage signal (the set-point or reference position) and the feedback

signal from the encoder or resolver on the motor shaft. This provides closed-positionloop control of the shaft.


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A number of servo motor systems have servo drives that do not provide closed-position loop control. Instead, their purpose is to provide closed-velocity-loop control.In these systems, the servo drive provides an output proportional to the difference

between the actual velocity of the motor and a specified voltage signal (the set-point orreference velocity). In servo drives such as this, the velocity feedback can either beobtained by differentiating the position feedback signal (readily achieved in bothanalog and digital servo drives) or from an additional element known as a tacho-generator. A tacho-generator is a d.c. machine that is mounted onto the same shaft asthe main motor and provides an output voltage proportional to the speed of rotation of the shaft. This closed-velocity loop form of servo drive is shown in Figure 2.8.

a.c. or d.c Motor

Servo-Drive

Electrical EnergyInput

ElectricalEnergyOutput

VelocityFeedback

MechanicalOutput

Input voltageproportional torequired velocity

(Controller)

Encoderor

Resolver

Tacho-Generator

PositionFeedback

Figure 2.8 - Closed-Velocity-Loop Servo Motor System

The "velocity" terminology in regard to servo drives will cause some annoyanceto those concerned with engineering etiquette. Strictly, of course we are referring tothe speed of rotation (not velocity). However, this speed is normally directly related tothe linear velocity of some end-effector and so the terms tend to be used

interchangeably.

The closed-position and closed-velocity loop servo drives both have roles to fulfilin industrial applications. The closed-position loop system is most useful where onlyone independent axis of movement is required in order to move some element or "end-effector" to a given position. However, where two or more motors are used to drive anend-effector to a given position, the axes are often interdependent. The path taken toreach that position also needs to be controlled through the velocities of the servomotors.


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The difference between velocity and position control is best demonstrated with asimple "XY" machine, as shown in Figure 2.9 (a), where one motor controls Xmovement and the other controls Y movement. Figure 2.9 (b) shows two paths taken

by the end-effector in order to reach the point "P". Path (i) is obtained using velocitycontrol of servo motors so that a straight line is generated between the starting pointand the final destination. This is called "linear interpolation". Path (ii) is what canresult from using two position controlled servo motors, which each attempt to reachtheir final positions independently. In path (ii) it is clear that the "X" movement isfaster than the "Y" movement and hence when the X motor reaches its final position,the Y motor still has to continue for some time. Since it is obviously not possible tohave "total velocity control" over a motor, because of acceleration and deceleration,most multiple-axis machines (robots and CNCs) use a dual feedback loop arrangementincorporating both velocity and position.

X

Y

X

Y

Drive

Drive

(a)

End-EffectorPosition

Y

X (b)

(i)

(ii)

P

Figure 2.9 - (a) A Simple XY-Machine(b) Problems with Using Position Control for Interdependent Axes


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There are many different types of servo drive control available for industrialapplications. The first distinction between drives is that some are designed for d.c.motors and others are designed for a.c. motors (induction and synchronous motors).

Direct current motors are much simpler to control than alternating current motors butcost considerably more and are less reliable. As a result of technology limitations,older servo drives were only designed to control d.c. motors and a.c. drives did notemerge until the 1980s.

The older types of servo drives (both d.c. and a.c.) are analog in nature and arecharacterised by the fact that the circuit boards are relatively bulky. Modern drivesutilise digital technology to control the power flow to the motors and as a result, less"waste" heat is generated and hence the drives can be much smaller. Traditional servodrives did not have any in-built "intelligence" and could thus only carry out simpleforms of closed-loop control. The most prolific form of control was (and still is) the

so-called Proportional-Integral-Differential or PID control which is a classical, closed-loop control methodology.

In recent years, servo drives have also begun to utilise the low-cost processingpower that has become available through microprocessors and Digital Signal Processors(DSPs). This enables manufacturers to design servo drives that can "intelligently"control the flow of energy to the motors through some complex algorithm. A numberof commercially available indexers (stepper motors) are now also equipped withmicroprocessor control. In addition to allowing a broader range of control algorithmsto be implemented (in addition to PID), the on-board processor can also allow the servodrive controller to be networked, so that it will respond to positioning or velocitycommands.

Servo motors and drives, both a.c. and d.c., are not only the basis for a greatrange of modern machinery design but are also contributors to the improved factoryenvironment that now exists in many Western countries. Servo drive systems are muchquieter than hydraulic and pneumatic systems and considerably reduce noise emissionlevels in the factory when they replace these older drive systems in low powerapplications (up to a few kilowatts). The servo motors not only provide quieteroperation but also the ability to position elements with a high degree of accuracy overan entire range of displacements. Hydraulic and pneumatic systems, on the other hand,tend to be used only for point to point positioning and are not suitable for graduatedpositioning. However, the hydraulic and pneumatic drives still have advantages in

situations where extremely high forces need to be applied to move actuators or end-effectors from one position to another.


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2.4 CNC and Robotic Controllers

The servo motor drive system is a basic building block for most modern industrialequipment. In particular, it is the basis for Computer Numerical Control (CNC)machine and robot design. These two elements are, in turn, amongst the most prolificpieces of machinery in a modern industrial complex.

The CNC machine is very much a "fish out of water" in the worlds of computingand manufacturing. CNC has never provided a full-fledged computer control system(in the way most engineers would understand it) and the machines which are controlledby CNC are still designed like machines that should be manually driven. Robotics onthe other hand, benefited enormously by arriving at a more opportune "technologicaltime" after suitable electronics and processing became available to provide sensible

controls.

In the middle of the twentieth century, the majority of lathes and mills weremanually driven by operators that moved axes by turning hand-wheels. The axes werecomposed of "screw-feeds" that were used to move some end-effector (such as acutting tool or work-piece fixture) so that the work-piece could be processed. Thesemachines were designed around a rudimentary, geometric axis system (based on eitherorthogonal or cylindrical coordinates) so that the operators could easily relate therequired geometry of a work-piece to the movement of a single hand-wheel on themachine.

The most logical first step in automating these machines in the 1950s was toreplace each of the manual hand-wheels with a servo motor drive. Initially, the servomotor drives were all connected to a controller, known as a Numerical Controller orNC, that would cause each of the drives to move, based on a punched, paper-tapeprogram. Considering the high cost of computing in the 1950s, these machinesprovided an extremely good mechanism for automated processing of work-pieces.Unfortunately however, as computer technology evolved and low cost microprocessorsproliferated in the 1970s, NC became Computer Numerical Control or CNC, with littlerevision of the fundamental concepts. CNC is conceptually little more advanced thanNC and its basic advantage is that it provides the ability for programmers to enter, editand simulate cutter paths on the controller itself.

Few people question the design of CNC machines. As with many other industrialsystems, features that were ill-designed due to lack of technology have remained asindustry standards, long after the enabling technologies have emerged. There are manyother limitations that have arisen in CNC, in terms of the design of the computercontrol itself. Many CNCs are still designed as though machines were intended to existin isolation from other computers and the outside world - that is, as "islands of automation". However, in industry we now know that it is important for all computercontrollers to either receive instructions from the outside world or send data to theoutside world in order to simplify the task of factory automation.


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Robots, unlike CNC machines, really only came into being (on a large scale) afterthe advent of microprocessors and the dawn of "low-cost" computing. Modern robotdesign has therefore suffered far less from the manacles of the by-gone manual era than

has CNC machine design. As a result, robots tend to look and perform like devicesdesigned for a specific function using modern concepts of computer control. Even afterall the advances in CNC design, robots still tend to interact with the outside world in afar more proficient manner than the CNCs. However, despite the obvious physicaldifferences between, say, an articulated welding robot and a CNC milling machine, theprinciples behind the actual control systems are essentially similar. The schematics of CNC and robot control systems are shown in Figure 2.10.

Supervisory Controller(Executing Program)

Axis Controller

Servo DriveController 1

Motor 1 EndEffector

Position Feedback

Velocity Feedback

Voltage

or Current

ReferenceVelocity

ReferencePosition

Position Transducer

MechanicalCoupling

(Axes 1 to N)

Figure 2.10 - Schematics of a CNC or Robotic Axis Control System

The supervisory controller in robotic and CNC systems is responsible for anumber of simultaneous activities including the user (system) interface and partprogram parsing and execution. As each step (block) of a part program executes, thesupervisory controller passes down positioning information to the axis controlcomputer.

The axis controller is responsible for achieving the desired position, usingappropriate acceleration and deceleration curves. It does this by sending referencevelocities to the servo-drive controller, on the basis of the actual position that has beenachieved. As a result, a double feedback loop (velocity and position) is established.


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The relationship between the servo-drive controller and the servo-motor isdependent upon the type of motor in use. In d.c. systems, the servo-controller variesthe average voltage (hence current) applied to the motor windings. In both

synchronous and induction motor, variable-speed a.c. systems, the servo controllersupplies the armature of the motor with a variable frequency supply voltage. Note also,that the servo motor drives in traditional CNC and robotic control systems are basedupon closed-velocity-loop control. The CNC or robotic axis controller is thenresponsible for closing the position loop.

In older CNC and robotic control systems, a number of processors were used toimplement the entire control. One processor acted as the supervisory controller,another as the axis controller and so on. In some cases, each axis had its own processorbecause the task of closing the position loop was so "processor-intensive". Currentprocessor technologies enable a typical four axis machine to be controlled by one

microprocessor which performs all processing (supervisory, axis control, etc.) functionsin real-time through multi-tasking.

The transducers used to provide velocity and position feedback on both robotsand CNC machines vary according to the specific applications. Commonly, shaft-mounted tacho-generators are used to provide velocity feedback and linear resolvers orpulse-code transducers (encoders) provide position feedback.

Both CNC and robot control systems share another common trait in that they tendto be very specialised in their design. They are optimised to achieve multi-axis control,with minimum user programming, and hence the languages that they use tend to besomewhat restrictive.

For historical reasons, related to early hardware limitations, CNC machines were(and still are) traditionally programmed in a "G-Code" language. This dated systemprovides a user with a number of sub-programs, commonly prefaced with either a "G","F", "S", or "T" and suffixed with a subroutine number or parameter. These facilitatethe movement of a cutting tool through a predefined path; the selection of a cutting tooland so on. However, few (if any) of these languages will allow a programmer to domore than this.

G-Code languages were never intended to provide the user with routines foraccessing various aspects of the machine controller itself and for interfacing it to the

outside world. These features, which are now both desirable and important, aredifficult or impractical to implement on CNCs. For example, with older CNCs, it isoften difficult to display user programmed screens as a part program executes. Further,it is generally not possible to access the serial communications facilities of a CNCthrough the G-Code language itself.


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CNC designers often attempt to augment the limited features of the G-Codelanguages by running additional (concurrent) tasks on the CNC. For example, manyCNCs have programs designed to handle serial communications and remote commands,

running as tasks, while a part program executes. This type of task is referred to as aDirect Numerical Control or DNC task/facility. It generally enables a host computer toremotely control a CNC machine through a serial link. There are other tasks, such asconcurrent, graphic information displays, which are also added by a number of manufacturers.

The deficiency with older forms of CNC architecture is that they ultimatelyprovide a closed (black) box to the end-user. It is often difficult for end-users to re-program CNCs in order to change graphics displays, or the way in which serialcommunication occurs. So, while traditional CNCs provided great flexibility in termsof cutting and shaping materials, they generally provided very little flexibility in terms

of tailoring the user environment. Modern CNC designs, however, have improvedsince the early 1990s and are gradually moving towards high-level, structuredlanguages and open-architecture programming capabilities.

Robot controllers have generally been better than traditional CNCs in terms of programming flexibility. Unlike CNC machines, robots seldom, if ever, work asdevices in total isolation from other systems. They are generally linked to othercomputer controlled or logic controlled mechanical systems. For example, a spray-painting robot must be linked to the production line that feeds it with work-pieces forpainting - otherwise there can be no inter-locking between line movement and robotcycles. As a result, programming languages on robots have tended to reflect thesystems oriented nature of these devices. However, it is far more difficult to categorisethe capabilities of robot controllers, because they are far more diverse in softwarearchitecture than CNC systems.

Some robot-controllers can be programmed in PASCAL or "C" (or specialstructured languages such as VAL) in the same manner as any normal computersystem. These systems provide users with a high level of access to the internalhardware of the controller itself. This makes such controllers more amenable tointerfacing with the outside world. A few, "less sophisticated", robot controllers areanalogous to older CNCs and can only be programmed in restrictive, specialised,movement languages (similar to proprietary G-Codes). These systems suffer from thesame interfacing disadvantages as CNC systems.


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CNC systems and robot controllers generally come with built-in ProgrammableLogic Controllers, usually of a specialised and complementary design, and normallyproduced by the CNC or robot manufacturer. The PLCs are integrated into the CNC or

robot control system, under the control of the main processor. They are used to controla range of sundry functions requiring high voltage or current switching. For example,on a CNC machine, the internal PLC may control the switching of coolant pumps, theopening and closing of doors, etc. On a robot, the PLC may control the opening andclosing of grippers and the switching of inter-locked equipment. The inputs andoutputs of PLCs on both robot and CNC systems are accessible from within the robotprogramming language or G-Code programming language. This scheme is shown inFigure 2.11.

Inputs

Outputs

CNC

or

Robot Control

Integrated

Purpose-Built

PLC

ServoDrive 1

ServoDrive N..

Figure 2.11 - Integrated PLC Control of High Power Peripherals

CNC and robot controls are generally both provided with a "hard-wire" interfaceto the outside world. This provides a simple means of integrating the devices intoautomated systems. In a hard-wire interface, spare inputs and outputs from theintegrated PLC are selectively connected to external devices so that they can be inter-locked. Program execution is then made dependent upon the condition of inputs.

For example, if we wished to use a robot to feed a CNC machine with work-pieces, a hard-wire inter-locking arrangement, such as the one shown in Figure 2.12may be used.


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CNC Output x

Input y

Input m

Output n

Robot

PLC

Controller

PLC

Figure 2.12 - Inter-locking a CNC Machine to a Robot

In such a system, the CNC machine program should start execution as soon as therobot has loaded a part (ie: when the robot program has been completed). The robotshould unload a part when the CNC machine program has ended. This can be achievedby the robot setting output number " n" high when it completes a program (lastexecutable line of code) and the CNC machine setting output " x" high when itcompletes a program (last executable line of code). The first lines of code on both theCNC and robot are to wait for inputs " y" and " m ", respectively to go high beforecontinuing.

This form of inter-locking is suitable for simple systems, but is unable to dealwith problems that occur during the execution of a robot program or CNC program.For example, the robot may jam a component while loading the CNC machine and stop

while its grippers are still inside the machine. The CNC has no way of knowing theactual position of the robot and so may damage the grippers. These sorts of issues canonly be resolved by having the robot and CNC intelligently communicate with oneanother through a data communications link. These links require specialisedcommunications software packages (known as protocols) to execute on each of thedevices. Robots have been equipped with communications protocols since the early1980s and CNCs became available with similar protocols shortly thereafter. The actualimplementation of a control system via a communications protocol is the subject of another book and is outside the scope of this text.


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2.5 Development Systems for Mechatronic Control

The advances in computer processing power which took place during the 1970sproduced an enormous number of low cost personal computers in the 1980s. However,by the mid-1980s it was evident that these personal computers were limited by theirinability to communicate with the outside world. As a result, their use in engineeringcontrol was limited. Engineers with a good background knowledge of electronics wereable to design interfaces between personal computers and real-world systems but thesewere "one-off" solutions that were very costly to pursue. For those without anelectronic engineering background, the concept of linking personal computers andworkstations to the outside world presented serious problems.

By the latter part of the 1980s, a number of companies had recognised the need

and potential for equipment suitable for interfacing personal computers to the outsideworld. Vendors began to launch a range of "building-block" products that could beused by engineers, with limited knowledge of computer hardware, to implementengineering control systems. Since that time, the interfacing-board market has grownto the extent where plug-in interfaces have almost become a commodity.

There are literally thousands of different types of products that are suitable formechatronic control systems in industrial and laboratory environments. In general,these are designed to plug directly into a variety of commonly available personalcomputers and workstations. At their most basic level, these interfacing boards providethe ability for computers to input and output analog voltage signals through a numberof different channels. This is shown schematically in Figure 2.13.

Analog Outputs

Analog Inputs

N

1

1

N

:

:

InterfaceBoard

Operating System

UserProgram

InterfaceBoardSoftware Library

Host Personal Computer

Figure 2.13 - Basic Interface Board Arrangement for Data-Acquisition and Control in Analog Systems


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An interface board such as the one shown in Figure 2.13 would also typicallyprovide a library of software routines that would carry out the low level hardwareaccess to the input and output channels of the board. The end user can utilise these

routines in a common high-level language program such as C or Pascal without everunderstanding the complexities of the board or the processes that transfer data betweenthe board and the computer's memory areas. As with nearly all consumer items, themore one pays, the more one gets. A more sophisticated version of the board couldprovide protection and isolation between the external signals and the computerhardware.

The basic arrangement of Figure 2.13 assumes that the end-user will wish todevelop control software entirely on the "host" personal computer. However, there aremany instances where the development of a control algorithm may be a case of "reinventing the wheel". A typical example would be the implementation of a basic

PID control, where an incoming feedback signal is processed via a standard algorithmto produce a required output signal. A number of boards are equipped with their own"on-board" microprocessors or Digital Signal Processors that are available to carry outbasic "closed-loop" control functions. The host personal computer then essentiallybecomes a development tool that provides a screen and keyboard/mouse inputarrangement. However, when the control system is fully implemented, the systemdesigner can develop software that will enable the personal computer screen andkeyboard to become the interface between the user and the control system. This isshown schematically in Figure 2.14.

Analog Outputs

Analog Inputs

N

1

1

N

:

:

InterfaceBoard with

Operating System

UserProgram

InterfaceBoardSoftware Library

Host Personal Computer

On-BoardProcessor

StandardControl LoopSoftware

Figure 2.14 - Using Advanced Interface Boards with "On-Board Processors" forStandard Closed-Loop Control Functions


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The concept shown in Figure 2.14 is often extended to provide more than just asimple general-purpose control. A number of intelligent controller boards are availablefor specific functions including:

Servo motor control PC-based digital storage oscilloscope applications Waveform synthesis Micro-stepper and stepper motor control Video acquisition (frame-grabbing and image-processing, etc.).

To some extent, a number of plug-in boards available for personal computers and

workstations really do little more than replicate the functionality available in aProgrammable Logic Controller. There is however, one major advantage in using plug-in interface boards in preference to PLCs and that is the fact that the interface board

connects directly into the bus structure of the host computer. This provides the fastestpossible link between the outside world and the computer. Most PLCs can only beconnected to a computer via a network or point-to-point communications link, both of which are relatively slow for real-time control. In some instances, PLCs have evolvedto a level where there is really no difference between the functionality of an interfaceboard and a PLC - this is particularly true of PLCs that plug directly into the busstructure of a personal computer or workstation in much the same way as thearrangement of Figure 2.14.

The major disadvantage of nearly all plug-in boards (and PLCs) is their relativecost. An interface board can often cost as much or more than the host computersystem. If the board is to be used for a "one-off" system design then this is not anissue, since the cost of developing an interface board would be a much more expensiveproposition. However, if the objective is to develop control systems for massproduction, then clearly the off-the-shelf interfaces are unacceptable.

One of the reasons for the relatively high cost of interface boards is the fact thatthey are designed to be "general-purpose". Most of the boards accept a wide range of voltage and current inputs and can provide a wide range of output voltages andcurrents. This sort of functionality is expensive. In situations where many boards needto be produced and cost is of the essence, specialised boards with strictly limitedfunctionality (in other words, purpose-built) need to be designed from first principles.The focus of this book is to help you to come to terms with the basic principles behind

the design of interfaces so that you can understand how commercial systems operate,their limitations and the appropriateness of designing from first principles.


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In some cases, the decisions in regard to control system design are already fixedby overriding physical or commercial factors. For simple controller applications, thecost of a personal computer plus interfacing boards may be too high to make such a

solution viable. In other situations, the physical size of a personal computer andinterfacing boards precludes the use of such a solution. There are several ways toresolve these control problems. The first is to design a complete microprocessor orDSP based controller from first principles. This is normally the most cost-effectivesolution where mass production is involved, but is expensive for low volumeapplications. The other alternatives are based upon the tailoring of so-called "miniaturecontrollers" or "micro-controllers".

Miniature controllers are microprocessor based computers that are usuallydesigned to fit onto a relatively small printed-circuit board. Unlike the mother-board of a personal computer or workstation, a miniature controller already has built-in control

functionality such as analog inputs and outputs and relay-drivers. The software for theminiature controllers is normally stored in special memory chips known as "ElectricallyErasable Programmable Read Only Memory" or EEPROM. On a normal computer,programs are stored on magnetic disks and transferred to memory later for execution.This allows for much greater storage but also makes the overall computer system largerin size than the miniature controller. A typical arrangement is shown in Figure 2.15.

P EEPROM

A/D D/A

Miniature Controller

Strain Gauge

TemperatureSensor

Load Cells

Linear PositionTransducer

Serial Link

PC Workstation

Operating System

SoftwareDevelopmentKit

Analog Output 1

Analog Output 2

Analog Output 3

Analog Output N

Key-Pad LCD Display

Figure 2.15 - Miniature Controller Development System


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The beauty of miniature controller development systems is that a system designercan develop software on a standard PC workstation using a special developmentcompiler (normally programmed in the "C" language). The software is then down-

loaded to the miniature controller memory via a serial communications link.Thereafter, the miniature controller can be disconnected from the PC and act as a stand-alone unit.

Miniature controllers are designed in a building-block fashion so that thedevelopment does not require a highly skilled electronics engineer nor the productionof printed circuit boards and so on. Typical accessories include liquid crystal displayscreens and operator keypads that enable the final users of such controllers to interactwith them at a very basic level. As with all other general-purpose "building-block"devices, the cost of a miniature controller can become excessive when large volumesneed to be produced. However, when one takes into account the fact that development

times are minimised and hardware reliability is much higher than for "one-off" designs,the miniature controller is a very useful device.


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2.6 Manufacturing Systems

There are very few areas of engineering where control systems proliferate to theextent that they do in modern manufacturing systems. A sophisticated manufacturingsystem can contain control elements including:

Microprocessor or DSP controlled Servo Drives CNCs Robot Controllers PLCs Cell controllers (PC workstations or dedicated computers).

The difficulty, of course, lies in getting all these different types of controllers to talk to

one another so that a cohesive manufacturing system can be produced.

In continuous processes (such as in chemical, food and petrochemical productionand power generation) the interaction between different levels of control is very tightlygoverned because the level of intelligence ascribed to each element tends to be limited.However, in discrete processes, such as in metal-cutting manufacturing systems ortextile production systems, the boundaries between the different levels of control aresomewhat blurred and cohesive control is more difficult to achieve.

A number of different, metal-cutting, manufacturing systems are used in order tosatisfy the performance criteria demanded by a wide range of industries, includingworkshops, automotive and aerospace manufacturers. The common systemconfigurations are shown in Figure 2.16, which gives an indication of how each systemfits into annual volume / variety regions in the production environment.

1 10 100 1000

1

10

100

1000

10000

100000

FMS with CNCs, AGVs & Robots

F.T.L.

DedicatedSystems

Stand-alone CNC

RobotFed CNC

Part Variety

Annual Volume

Figure 2.16 - Realms of Metal-Cutting Manufacturing Systems


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The systems mapped onto the graph in Figure 2.16 have differing performancecharacteristics and also place different demands upon the data communications used bytheir control systems. We shall now examine the structure of each of the integrated

systems and the way in which communication between devices occurs.

The dedicated, in-line transfer machine is shown in Figure 2.17 and is a high-volume, low part variety system. It is composed of a number of machining stations anda transfer conveyor. Each of the machining stations is designed and tooled for aspecific application. For each station, tools are loaded into an induction motor driven,multi-spindle cutting head, which has an advance and retract motion. When a work-piece comes into position within a machine, the head advances for a fixed period, thenretracts, to allow the work-piece to move down-line to the next destination. Eachmachining module is generally controlled by a PLC, which is hard-wired to its sensors,coolant pumps, etc. These machining modules are generally not user-programmable

devices. They are pre-programmed to perform only a fixed task.

PLC PLC PLC

PLC PLC PLC

PLC

Transfer Mechanism(Conveyor)

Supervisory PLC

DedicatedMachining Modules

Figure 2.17 - Schematic of Dedicated, In-Line Transfer Machine

The transport mechanism for in-line transfer machines is also a PLC controlleddevice. In simple systems, the transport mechanism controller is also the systemcontroller, and is hard-wire inter-locked to the dedicated machining modules. In morecomplex systems, a separate, high powered PLC is used to coordinate the running of the system and drive mimic-panels and graphic information displays.


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In transfer machines, where individual modules are controlled by a range of PLCs, produced by different vendors, it is common practice to simply hard-wire fromthe supervisory PLC to other PLCs in the system. However, in a single-PLC-vendor

environment, a number of proprietary solutions are generally feasible. Some of thesesolutions allow PLC data buses to be inter-connected through a back-plane system forinformation exchange. Other solutions allow for interconnection of PLCs through highspeed Local Area Networks. Regardless of which system is chosen, the objective is forthe supervisory PLC to implement sequential control over the system throughinput/output inter-locking with individual module controllers.

Rotary transfer machines are analogous to in-line transfer machines except thatparts transfer, from machine to machine, occurs through via an indexing mechanism ina circular path. The control principles however are almost identical.

The hard-wire, inter-locking, communications techniques, shown in Figure 2.17,for dedicated systems are generally adequate because:

Individual machining modules are relatively simple devices, executingsimple, fixed, programs

The amount of information which any, one machine can feed back to a

supervisor is comprised of little more than off/on limit-switch status The supervisory controller does not need to change programs on individual

modules in the system. Dedicated manufacturing systems, of the type shown in Figure 2.17, fulfil a vital

role in the high-volume production of a small variety of parts. However, in order tovary the type of part that passes through such a system, it is necessary to manually re-tool each of the machining stations. If the type of part to be produced is radicallyaltered, then such systems require major re-engineering, or as is often the case,complete replacement. Since these systems are designed for the production of aspecific item, their cost and production life are calculated on the basis of anticipatedproduct life.

Increased competition in manufacturing, coupled with increasing consumerdemands for new products, mean that product life-spans are decreasing. The cost

effectiveness of dedicated production systems is therefore diminished accordingly. Inaddition, companies driving towards export competitiveness with products now findthat they need to produce a "family" of products, tailored to specific global markets.These requirements engender a need for flexibility in production systems.


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Flexibility in production systems is achieved through the ability of individualmodules in those systems to respond to changes in part variety. This is in turn,achieved through the use of fully programmable machining modules and flexible parts

transport techniques.

It would be sensible to suggest that flexibility in manufacturing could be achievedby taking a dedicated system, such as that shown in Figure 2.17, and replacing some orall the fixed machining modules with CNC machines. This is common practice, andthe result is referred to as a "Flexible Transfer Line" or FTL. However, it should benoted that the cost ratio of a CNC machining module to a dedicated module can be ashigh as 10 to 1. It is therefore not economically feasible to replace, say 50 dedicatedmachining modules in a fixed system with 50 CNC machines. Generally, each CNCmachine uses automatic, programmable tool changing in order to perform the functionsof a number of dedicated modules. Thus, production flexibility is increased but

throughput is decreased in what becomes a normal trade-off situation. While it may be common in a dedicated production line to have 50 to 100

machining stations, a flexible production system may have only 5 to 10 CNCmachining stations, performing the same net function at a lower production rate.However, the benefits in flexible production become self-evident when productionneeds change, because flexible systems can respond very quickly to new demands, withminimal human intervention.

The transfer line arrangement of Figure 2.17, whilst very fast, does not providethe optimal transport mechanism for maximum production flexibility. Robots, GantryRobots and Automated Guided Vehicles (AGVs), on the other hand, provide a highdegree of transportation flexibility at the cost of production throughput. All threedevices use relatively sophisticated control systems. AGVs in particular, commonlyuse a powerful PLC as a Constant System Monitor (CSM), which governs the positionsto which vehicles move.

The Flexible Manufacturing System (FMS), designed for a very wide variety of parts, is more likely to resemble the schematic shown in Figure 2.18, rather than that of 2.17. The intelligence level of each module (machine) within the system is muchgreater than that within the dedicated production line. CNC machines in sophisticatedFMS environments may even be augmented with specialised robots to transfer toolsfrom AGVs to machine tool carousels and vice-versa. The practicality of such systems

has long been questioned by industrialists and their reliability has been unsatisfactory.The primary reason for this is the difficulty of creating a cohesive and robust controlsystem that can recover from the numerous system faults that arise when handling awide range of parts.


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CNC

CNC CNC

CNC

AGV

AGV

AGV

FMS Controller AGV Controller

ProgrammableMachining Modules

Figure 2.18 - Schematic of a Flexible Manufacturing System

In a complex FMS environment, where a number of different part-types may bewithin the system simultaneously, the controller is required to:

Coordinate the flow of work-pieces of differing types, from one machine toanother, based upon a rolling schedule

Activate different part programs on CNC machines, as required by the part-

types present in the system Down-load part programs to CNC machines as required by the machines Coordinate (inter-lock) the role of the work-piece transport system with the

operation of CNC machines.


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Some of these functions can be (and sometimes are) implemented through thehard-wire inter-locking of devices to the FMS controller, which can be a powerful PC,PLC, workstation or mini-computer. However, FMS control is more appropriately

achieved through data communications between the controller and the othercomputerised (intelligent) modules within the system. In a complex, FMSenvironment, the system controller must have the capacity to interrogate otherequipment whilst programs are running. This gives the controller access to a widevariety of information regarding the status and error-conditions of machines, therebyallowing for intelligent decision making in the control algorithm.

Simple, hard-wiring techniques only allow devices to exchange one piece of dataper wire (say an on/off state or transducer voltage). They do not allow one computer totransfer data files to another computer. This of course means that down-loading of CNC machine programs from a supervisory computer cannot be achieved with the

hard-wired system alone. In simple hard-wired, FMS systems, machine programs arenormally resident in the local memories of each machine during a production run.Programs are generally down-loaded (or file-dumped) to machines, via datacommunications links, prior to the start of automatic FMS control.

One of the major benchmarks of FMS is the ability to tolerate and reconcile faultconditions. Each module in the system performs a complex task and is thereforesubject to a large number of possible faults or errors. It is costly for an FMS controllerto shut down an entire system, simply because one machine has developed a fault. Theobjective is for the controller to attempt to maintain orderly and safe system operationeven under certain fault conditions. However, as previously stated, this is far morereadily achieved in a rarefied academic environment than it ever has been in industry.

One of the major problems in FMS is the difficulty involved in integrating arange of different and proprietary computer controllers (PLCs, CNCs, robot controllers,etc.) via computer networks. There are considerable problems with lack of standardisation and an overwhelming sensation that many of the "intelligent"controlling devices to be linked in an FMS were designed to operate as "islands" of control without the necessary functionality for interaction. These sorts of integrationproblems are the subject of another book, but as we progress through this text we shallsee why it is that integration problems arise with computer based devices.

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