Pneumatic Control and Automation Fluid power is commonly used in industry to move linear and rotary actuators, and also to transmit control signals that allow a series of events to take place in a pre-determined order. Hydraulic power using oil is preferred when large forces or loads are involved, or if precise positioning is required, as oil is considered incompressible. Pneumatic power is faster, as air is lighter to move and produces less friction, but as we are limited to a typical air supply pressure of 120 psi (8 bar), it is normally used to move lighter loads, unless very large actuators are used (Force = pressure x area). As air is compressible, precise positioning is not possible. However, it has another distinct advantage of being safe to use in hazardous situations where explosions or fires may occur. The following designs will illustrate how pneumatics can be used to provide linear motion, starting with a very cheap and simple circuit, and then gradually increasing in complexity and sophistication in order to demonstrate the use of safety interlocks and automation in mechanical- and electro-pneumatic systems. A typical design scenario is presented with each design, which normally starts in the bottom left-hand corner of the page with the service units (compressed air source, filter, regulator, lubricator and isolation switch). The design then rises diagonally to the top right-hand corner with input, processing and final control functions in horizontal layers for easy recognition, and culminating in output or actuation. A numerical system using 0 for service equipment, 1 for devices operating the first actuator, and 2 for devices operating the second actuator, followed by a second number to differentiate between components, is used to make schematics easy and logical to follow. Sometimes a letter (S for input switch, V for valve and A for actuator is used between the numbers. An example would 1.2 or 1.V.2 which would indicate the second valve in the schematic associated with the operation of actuator #1.

Circuit 1 In the first scenario, a component comes down a line (conveyor) and stops at a workstation. A worker attaches a part to the component and presses a push-button to eject it off the line. Assemble the circuit using the following equipment list: 3/2-way push-button valve, normally closed, spring return Single-acting cylinder, spring return With the valve, 3 refers to the number of connections, and 2 to the number of positions it can take. Normally closed refers to its de-energized status (the box next to the spring), and is opposite to electrical terminology in that normally closed means that no air can pass through it. WARNING: Make sure that the isolation switch is turned off before connecting or disconnecting tubing, otherwise tubing will whip and may cause injury!

LAB 1

Once you have assembled the circuit, turn the isolation switch on and test the circuit. If it works, proceed to lab 2. If it doesn’t, then you have an ideal opportunity to test your trouble-shooting skills!

Circuit 2 In lab 2, a pneumatic actuator is use at a sorting station to open and close a gate to divert defective components onto a branch conveyor. The gate is too heavy to use a single-acting cylinder, and its position needs to be maintained after the switch is released. Equipment list: 5/2-way valve with selector switch Double-acting cylinder

LAB 2

Notice in labs 1 and 2 that the same air is used for both the operator’s input switch and the actuator. If the actuator were large, and required a large flow of air to operate it quickly, the input switch would also have to be large enough to accommodate the volume. If an air conductor ruptured in the vicinity of the operator, there would be the potential for personal injury. Lastly, if the operator were positioned some distance away from the sorting station, the added length of large-diameter tubing would be expensive.

Circuit 3 In lab 3, we are going to simulate the operation of a large double-acting cylinder that must be actuated from a push-button situated 100 feet away. Upon release of the push-button, the cylinder must retract. We are going to isolate the input signal from the final control by use of a piloted signal. This will allow smaller diameter tubing to be used to the remote operator’s station, and if necessary, would also allow the use of a regulated lower air pressure signal line for operator safety (similar to using a 120v switch and relay to start a 600v electric motor). Equipment list: 3/2-way push-button valve, normally closed, spring return 5/2-way single pilot valve, spring return Double-acting cylinder

LAB 3

Circuit 4 In lab 4, we will use a valve with a cam follower (roller lever) that is actuated by a guard being closed to provide operator safety. A double-acting cylinder operating a shear must extend only when a guard is in place, and a push-button is activated. If the button is released, or the guard is lifted, then the cylinder must retract. Equipment list: 3/2-way push-button valve, normally closed, spring return 3/2-way roller lever valve, normally closed, spring return Dual-pressure valve 5/2-way single pilot valve, spring return Double-acting cylinder

LAB 4

Notice that the dual-pressure valve requires two input signals in order to provide one output signal (AND logic).

Circuit 5 Lab 5 is similar to the previous one, except that we wish to be able to operate this shear from either of two controls, as well as ensuring that the safety interlock on the guard is always operational. In order to accomplish this, we will also use a shuttle valve, which will transmit one of two input signals (OR logic). Equipment list: 2 - 3/2-way push-button valves, normally closed, spring return 3/2-way roller lever valve, normally closed, spring return Shuttle valve Dual-pressure valve 5/2-way single pilot valve, spring return Double-acting cylinder

LAB 5

Circuit 6 The previous circuits all required a continuous signal to operate them, either by holding the push-button or using a détente. This is useful in many situations, for example where the operator uses judgement in determining the operation or as a safety measure in that the circuit deactivates immediately upon release of the button. However, it may be desirable to have a circuit perform a series of functions with only one short impulse (i.e. push the button and release). The circuit may need to operate until some other signal is received, then reset. This is accomplished by means of a memory valve, also known as a bistable, double-piloted, or impulse valve. Lab 6 is to be built using a memory valve to operate the cylinder. One push-button will be required to extend the cylinder, and another push-button to retract the cylinder. Equipment list: 2 - 3/2-way push-button valves, normally closed, spring return 5/2-way double pilot valve Double-acting cylinder

LAB 6

Circuit 7 In circuit 7, a cylinder presses a bearing into a housing. It must actuate only when a push-button is depressed and a safety gate is closed. The circuit must also incorporate an alternative activation method, automatic return and speed control on both extension and retraction strokes. The speed control must be effective no matter which way the load acts on the cylinder. The automatic return is provided by 1S4, which is tripped by the cam on the actuator piston at the end of its extension stroke, and thus changes the position of the memory valve. Speed control is effected by the use of flow control valves which meter the amount of air passing through them. Notice that they meter the amount of air coming out of the cylinder, thus providing a back pressure which will prevent a load from overrunning. Air travelling in the opposite direction will bypass the flow control valve via the built-in check valve. This is referred to as meter-out flow control. Equipment list: 2 - 3/2-way push-button valves, normally closed, spring return 2 - 3/2-way roller lever valves, normally closed, spring return Shuttle valve Dual-pressure valve 5/2-way double pilot valve Double-acting cylinder

LAB 7

Circuit 8 Lab 8 features the use of a time-delay valve. This is a combination valve that incorporates more than one individual component – in this case, a flow control valve, direction control valve and an accumulator. The accumulator must fill up before pressure increases sufficiently to overcome the spring and move the direction control valve. The flow control valve is adjusted to vary the flow of air into the accumulator, and hence the amount of time before the valve will act. In lab 8, a vertically-mounted pneumatic cylinder provides a clamping force for an operation that glues two components together. The cylinder must extend at a controlled speed until it reaches full extension, remain at full extension for 5 seconds while the glue sets, and then return as quickly as possible. The procedure is started by depressing a push-button, and will not initiate unless a safety guard is closed, thus activating a roller lever. The cylinder is too large for direct control. Equipment list: 3/2-way push-button valve, normally closed, spring return 2 - 3/2-way roller lever valves, normally closed, spring return Dual-pressure valve 5/2-way double pilot valve Time delay valve One-way flow control valve Double-acting cylinder

LAB 8

Circuit 9 In lab 9, another combination valve called a pressure sequence valve is used. It consists of a pressure control valve and a direction control valve. The pressure control valve will only operate the direction control valve when it receives a pre-set signal. A pneumatic cylinder is used to press a bearing into a seat. The cylinder is activated when a roller switch on a safety gate is actuated, and a push-button is pressed. The cylinder will continue to press until a pressure of 2 bar is reached, regardless of cylinder position. When 2 bar pressure is reached in the cylinder, the cylinder retracts. The cylinder extension speed must be controlled. Equipment list: 3/2-way push-button valve, normally closed, spring return 3/2-way roller lever valve, normally closed, spring return Dual-pressure valve 5/2-way double pilot valve Pressure sequence valve One-way flow control valve Double-acting cylinder Pressure gauge

LAB 9

Circuit 10 In lab 10, a package must be raised from one conveyor to the level of another conveyor, and pushed onto the second conveyor. When the package arrives at the elevator table, it will trigger a push-button, which starts the sequence. Cylinder 1 extends at a controlled rate, raising the package. Upon complete extension of cylinder 1, cylinder 2 extends, also at a controlled rate, and pushes the package onto the upper conveyor. After complete extension of cylinder 2, both cylinders are to retract simultaneously at full speed. Confirmation that both cylinders have retracted is required before the system will start again. The valves that provide the confirmation signals need to be connected in series with each other.

DISPLACEMENT STEP DIAGRAM

When we incorporate more than one cylinder, it is helpful to draw a displacement step diagram to show the sequence in which the cylinders extend and retract. Notice the use of 2S1 – an idle return roller lever valve. This valve should be positioned so that when cylinder 1 extends, it passes over the cam follower, activating it, but also allowing the signal to be immediately cancelled. The idle return feature allows cylinder 1 to retract without re-activating 2S1. If we position the valve right at the end of the stroke, there is a possibility that the signal will keep 2V1 permanently energized in the left-hand envelope, hence a signal to the other side of 2V1 would have no effect. This condition is known as signal overlap. If we use an ordinary roller lever valve, we would get a second signal sent to 2V1 as cylinder 1 retracted.

Equipment list: 3/2-way push-button valve, normally closed, spring return 3 - 3/2-way roller lever valves, normally closed, spring return 3/2-way idle return roller lever valve, normally closed, spring return Dual-pressure valve 2 - 5/2-way double pilot valves 2 - one-way flow control valves 2 - double-acting cylinders

LAB 10

Circuit 11

TRANSFER STATION

Lab 11 further demonstrates signal overlap by deliberately incorporating it into the following circuit. Two cylinders are used to transfer parts from a magazine onto a chute. When a push-button is pressed, cylinder 1 extends and pushes the part from the magazine onto a transfer table. Cylinder 2 then pushes the part onto the out-feed chute. Cylinder 2 then retracts, followed by cylinder 1. Notice that the sequence can not be re-activated until confirmation is received that the two cylinders are fully retracted (1S2 and 1S3).

DISPLACEMENT STEP DIAGRAM

Equipment list: 3/2-way push-button valve, normally closed, spring return 4 - 3/2-way roller lever valves, normally closed, spring return 2 - 5/2-way double pilot valves 2 - one-way flow control valves 2 - double-acting cylinders

LAB 11

Notice that there are two signal overlap problems in this circuit. In the first step (see the displacement step diagram), the double pilot valve 1V1 can receive signals from valves 1S1 and 1S2 at the same time, and hence will not change positions. The first of these signals (1S2) will need to be cut short so that it is not active past its requirement in step 4. The second overlap problem is with double pilot valve 2V2. In step 3, it receives a signal from limit valve 2S1 when cylinder 1 (1A) has extended. This signal, which allows cylinder 2 (2A) to extend, is never cancelled. After 2A has extended, it trips limit valve 2S2, which in turn sends a signal to 2V2 to reverse itself, and of course it cannot. Once again, the first signal (2S1) needs to be cut short so that it is not active past its requirement in step 3.

Circuit 12 This circuit is the same as lab 11, except that the signal overlap is eliminated by replacing the two roller lever valves 1S2 and 2S1 with idle return roller lever valves. These are not the most accurate way of eliminating signal overlap, as they have to be positioned fairly accurately to work properly, and may move with use. Equipment list: 3/2-way push-button valve, normally closed, spring return 2 - 3/2-way roller lever valves, normally closed, spring return 2 - 3/2-way idle return roller lever valves, normally closed, spring return 2 - 5/2-way double pilot valves 2 - one-way flow control valves 2 - double-acting cylinders

LAB 12

Circuit 13 Lab 13 uses the same scenario as the previous two circuits, but adopts a more sophisticated (and reliable) approach by using a single reversing valve. This valve (a double pilot valve 0V) ensures that a pressurized air signal is present at one side only of the memory valves 1V1 and 2V1 at any given time, and that the other side of the valves is always exhausted to atmosphere. Equipment list: 3/2-way push-button valve, normally closed, spring return 4 - 3/2-way roller lever valves, normally closed, spring return 3 - 5/2-way double pilot valves 2 - one-way flow control valves 2 - double-acting cylinders

LAB 13

Circuit 14 Once again we use the same scenario. Notice in the displacement step diagram that we have reduced the number of steps by having the first cylinder retract as the second cylinder extends. One very reliable way of achieving this is to use two reversing valves.

DISPLACEMENT STEP DIAGRAM

Once again there is the potential for two signal overlap conditions, one at each of the memory valves 1V1 and 2V1. Both of the cylinders have to extend and immediately retract, so the first signals to both 1V1 and 2V1 must be short in duration. This is accomplished by using the two limit valves 1S3 and 2S2. These are activated by the cylinders after they have extended, and send signals to the two reversing valves, which it turn cancel the first signal to the memory valves 1V1 and 2V1 and send a signal to the opposite side. Equipment list: 3/2-way push-button valve, normally closed, spring return 4 - 3/2-way roller lever valves, normally closed, spring return 4 - 5/2-way double pilot valves 2 - one-way flow control valves 2 - double-acting cylinders

LAB 14

Circuit 15 Finally, lab 15 is an example of a fully automated electro-pneumatic circuit. A component arrives at a punching station and activates a photo sensor, which in turn initiates the extension of a clamping cylinder. A second cylinder then extends and punches a hole in the component and then retracts, followed by retraction of the first cylinder. The extension speed of both cylinders must be controlled, but retraction must be as fast as possible. Confirmation of retraction is required before the circuit can be re-activated. In order to provide automatic control, magnetically actuated proximity switches are used for the first cylinder and roller lever switches for the second cylinder. Relays, normally-open and normally-closed contacts are used to transmit signals in the correct sequence and also to eliminate signal overlap.

DISPLACEMENT STEP DIAGRAM

Equipment list: 4 - 5/2-way double solenoid pilot valves 2 - one-way flow control valves 2 - double-acting cylinders with proximity switches 2 - roller lever switches 3 - relays

LAB 15 – ANSI

1A+

-

2B+

-

2B

1V 2V

LS1 LS2 LS3 LS41A

1A+ 1A- 2B+ 2B-

+24vdc 0vdc

1

2

3

4

5

6

7

8

Magnetic ProximityCylinder 1ARetracted

Magnetic ProximityCylinder 1AExtended

Start Cycle

LS1CR1

LS2 CR2

PB2CR1 LS4 CR3

CR3

CR3

CR2

1A+

2B+

Extend Cylinder 1A

Extend Cylinder 2B

CR3

LS3

2B-

1A-

Retract Cylinder 2B

Retract Cylinder 1A

LAB 15 – ISO

For convenience, the electrical schematic is shown in both ISO and ANSI configurations. A logical progression from here would be to write a PLC program to operate the same circuit and eliminate the relay logic.