Balancing Ball Final Report V3

7/30/2019 Balancing Ball Final Report V3

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MECHATRONICS FINAL YEAR PROJECTFONTYS UNIVERSITY OF APPLIED SCIENCES

Ball Balancing Project

Group Members:

-Rabindra Rijal

-Huynh Phu Khanh

-Frank Dooms

-Bruce Appleton

Author: Bruce Appleton


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Ball Balancing Project Page 1

Summary

From the early stages of the project it became clear that a good construction was one of the key

elements needed for this project to work. A couple of concepts were sketched and finally a

construction concept with two independent axes was used. Some of the parts were to be designedfrom scratch in Autodesk Inventor (CAD) and later fabricated using the machinery in the mechanical

engineering labs.

The project was to balance a ball on a flat surface and to achieve this some form of feedback for the

balls exact position on the flat surface was needed. For this it was decided to use a ten inch resistive

touch screen. The touch screen can then provide the necessary feedback for position. A dedicated

touch screen controller was added to read the coordinates and relay them to the PC via RS232.

The touch screen controller relayed the touch screen coordinates via RS232 into labview. Although

the touch screen controller supports USB, it was decided to use the RS232 interface due to protocol

simplicity. The means to read the coordinates via RS232 had to be implemented in labview.

To control the actuators a control system would have to be implemented using the touchscreen for

feedback. It was decided to implement the controller in labview so we could change its parameters on

the fly. To control the servo-motors a PWM signal would need to be generated within labview and

supplied to each motor.

The Ball Balancing project was a challenge, but we worked well together to achieve a lot.


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Glossary

SISOSingle Input Single Output

MIMOMultiple Input Multiple Output

PWMPulse Width Modulation

CADComputer Aided Design

VIVirtual Instrument


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List of Figures, Graphs and Tables

Figure 1 5-wire resistive touch screen concept ....................................................................................... 8

Figure 2 touch screen controller.............................................................................................................. 8

Figure 3 construction concept sketch...................................................................................................... 9

Figure 4 aluminium brace (CAD view) ................................................................................................ 10Figure 5 aluminium brace dimensions.................................................................................................. 11

Figure 6 Translation arm (CAD view) .................................................................................................. 11

Figure 7 translation arm dimensions..................................................................................................... 12

Figure 8 universal joint (CAD) ............................................................................................................. 12

Figure 9 HUCO universal joint dimensions.......................................................................................... 13

Figure 10 Rod-end bearing (CAD) ....................................................................................................... 13

Figure 11 IGUS rod-end bearing (EBRM-05) dimensions ................................................................... 14

Figure 12 Construction CAD model ..................................................................................................... 14

Figure 13 Final Construction (real image) ............................................................................................ 15

Figure 14 Ball on plate model concept ................................................................................................. 16Figure 15 Motor time constant measurement circuit ............................................................................ 18

Figure 16 System Connections .............................................................................................................. 23
http://e/DocD/Mechatronics%20Project/Documents/Balancing_Ball_Final_Report_V2.docx%23_Toc284185048http://e/DocD/Mechatronics%20Project/Documents/Balancing_Ball_Final_Report_V2.docx%23_Toc284185048


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Introduction

For our final year of study we were to undertake a project in mechatronics lasting the semester. The

project chosen was the Ball Balancing project. We were to design a system whereby we could

balance a ball on a flat surface using actuators. This project was a great opportunity to gainknowledge in the field of mechatronics.

The goal of the project is to build and implement a device that can balance a ball at any set point by

tilting the flat surface allowing the ball to roll. The surface is tilted by actuators located below it. The

mechanics by which the actuators transfer torque to the flat surface will have to be designed and built.

The control system will have to drive the motors to make the ball move to the allocated position. In

addition a feedback system will need to be implemented so the balls current position can be known at

all times.


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Project overview

Block schematic of the system

Below is a block schematic of the entire system.

Description of Individual Blocks

PC:

The controllers are implemented on a Personal Computer using National Instruments LabVIEW. The

system must be running Windows XP or higher with a RS 232 COM port and PCI cards installed.

- The COMport is required to input current position of the ball via Touch Screen Interface.A USB to RS232 converter maybe used where no COM port is available.

- A PCI card is required to connect the National Instruments Adapter BNC 2110.Motors:

The existing servo motors will be used for movementon each axis. The servo motors used are BMS-620MG from Blue Bird Technology.

They have following specifications:

Weight: 50 grams

Dimensions: 40.5 X 20 X 42 mm

Torque at 4.8V: 9.1 kg-cm

Torque at 6.0V: 10.6 kg-cm

Speed at 4.8V: 0.15 sec/ 60 deg at no loadSpeed at 6.0V: 0.13 sec/60 deg at no load

Touchscreen:

The Touch Screen used for the project is a Five-Wire Analog Touch Panel from Higgstec Company.

Major features and specifications of this device are noted below:

Input Mode: Stylus or Finger

Cable: FFC (Flat Flexible Cable)

Frame Size: 248.920.30 X 186.690.30 mm

View Area: 219.900.20 X 167.830.20 mm

Active Area: 215.900.20 X 163.830.20 mm`

Total Thickness: 2.200.20 mmTail Length: 2056.00 mm

PC

(LabVIEW)

Motor Construction

+ Touch Screen

Touch ScreenInterface

X, Y


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Operational Temperature: -100C to +700C

Loop Resistance: X: 20 to 500 Y: 20 to 500

Figure 1 5-wire resistive touch screen concept

Touch Screen Contr oller :

As we intended to build a control system for our project, we need feedback from the touchscreen to

know the current exact location of the ball. However the touch screen described above gives us only

analogue values from different locations and our controller will be implemented in LabVIEW inside a

PC (digital form). Additionally, these analogue signals from the screen need further calculation to be

read as X & Y positions of the ball. For these reasons, and also to save time, we intend to use a

dedicated touchscreen controller with USB interface to PC (USB-R5W-HT580-R) from Higgstec

Company (same as Touch screen manufacturer).

Figure 2 touch screen controller

Power required: +5VDC (Max. 150mA, 50mV peak to peak max ripple)

Protocol: COM: 9600 BPS baudrate 8 Data Bits

USB: USB1.1 full speedResolution: Sample 1024 X 1024 (12 Bits)

Report Rate: COM: Min. 63 Points Per Second (PPS)

USB: Min. 86 PPS

Response Time: Max 20 ms

Requirements:Resistance 30 to 500


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Construction

Concept

The figure below shows the construction concept for the balancing ball project.

Figure 3 construction concept sketch

The main parts:The main parts described below can be found in the concept sketch above.

LINKAGE JOINTSLocated either side of the linkage bars, providing connections between the

motor and the backing plate.

U-JOINT- Allows the screen plane to move in X and Y directions only.ALUMINIUM BRACEProvides a means for the linkage bars to be connected to the backing plate.TOUCHPAD SENSORResistive touch screen used for position information.BACKING PLATE - Provides added strength and a supporting base for the glass touch screen.GAURDRAILSKeeps the ball on the touch screen.GROUND PLATEProvides a stable base for the whole construction.

TRANSLATION BARConnects motor shaft to the linkage bar.


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After some thought we decided this would be the best construction for our project. Reasons forchoosing this construction:

Axes are independent. This gives us the advantage of being able to develop and test each axisseparately; because a single motor works on each axis.

Firm and stable if built correctly.

Project implementation time is reduced using a simpler axes concept, whereby the motors arenot coupled.

Modelling and controller development are less complex with independent axes.

Construction components

However some of the parts such as the aluminium brace and translation arms would have to be

fabricated out of aluminium from scratch. The linkage bars would need to be threaded both ends to fit

into the rod end bearings as well as building the whole supporting structure. All in all there was quite

a lot of work to be done in the mechanical engineering labs. The parts are described below in more

detail.

Aluminium Brace

The aluminium brace is fabricated from a single piece of aluminium and is located underneath the

perspex plane. Two rod-end bearings are located either side, fixed in place with pins. At the centre of

the brace there is an extrusion for the universal joint to fit in. The aluminium brace provides a means

to transfer torque to the plane.

Figure 4 aluminium brace (CAD view)


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Figure 5 aluminium brace dimensions

Aluminium Tr anslation Arms

The translation arms are connected to each motor.

Figure 6 Translation arm (CAD view)


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Figure 9 HUCO universal joint dimensions

Rod-end Beari ng

Type 1: IGUS rod-end bearings (EBRM-05)

Figure 10 Rod-end bearing (CAD)


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Figure 11 IGUS rod-end bearing (EBRM-05) dimensions

The Final Construction

CAD Model

The final CAD model has motion just like the real construction. Each axis can be moved

independently and an animation for the working principal can hence be created.

Figure 12 Construction CAD model


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Construction implemented in reality

The images below are taken of the real construction.

Figure 13 Final Construction (real image)


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Modelling Section

Our system model is divided into 2 sub models:

- The motors + the construction: input is the duty cycle of the PWM signal controlling themotor and output is the angle of the plate: H(s)

- The ball on a plate with an angle: input is the angle of the plate and output is the position ofthe ball: G(s)

Ball and plate

Figure 14 Ball on plate model concept

The angle of the plate will create force acting on the ball which then again result in a change of theball position.

The inclination is considered the x-coordinate.

Let acceleration of the ball be denoted as ''2

2

xdt

xd

The force due to translational motion is then

Ftx= mx

Rotational kinetic energy of the ball is:

Krot =

Prot = Tr* =

=

''/)/( 22

xR

J

dt

RxdJ

dt

RvdJ

dt

dJT br

H(s) G(s)PWM dut Angle Position


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Motors and construction

As a servo has internal feedback and control, it would take a lot of time and effort to dismantle the

motor and model it correctly. To simplify our motor model through observation of the motor

behaviour, we will just assume the model as a 1st order transfer function. The only value we will have

to determine in this case is the time constant tau.

In order to obtain this, we will put a load on the motor that is similar to our construction. Then a step

input of the PWM duty cycle (D) will be apply to the motor and we will measure the output angle

over time to find the time constant. After measuring the time required to get from a position zero to a

45 degree position, one can determine the motor constant m which can then be added to the Matlab

system model.

The motor will be attached to a potentiometer, and with a one shot logger, we can measure the voltage

over the potentiometer (which is then proportional to the motor angle) and then determine the time

constant of the motor + construction.

Figure 15 Motor time constant measurement circuit

Motor torques:

On the motor with long shaft:

BARHorriBARVertBARHorriBARHorriBARHorriHingeBARHorriScreenLong LMLMLMLMT 2

1

4

1

4

3

On the motor with short shaft:

BARHorriBARVertBARHorriBARHorriBARHorriHingeBARHorr iScreenShortLMLMLMLMT

2

1

4

1

2

1


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mscreen = 564g

mhinge = 96 + 2*24 = 144g

mhoribar= 53.5g

mverbar = 4.5g

Base on the formula and data above, we can calculate the load weight use to replace the construction

for the motor with short shaft is 349.25g and one with long shaft is 490.25g.

With the time constant measured as 16ms, we can determine the transfer function of the motor and the

construction:

As the time constant is 16ms, the angle reaches the wanted position after 5 times of = 80ms.


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Simulation results

A simulation with the 2 models combined show the following result:

With a 0.005 duty cycle step input:

Angle Distance

Potential energy


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Translation kinetic energy Rotational kinetic energy

As we can see from the simulation result, the change of the potential energy is always equal to the

sum of changes of 2 types of kinetic energy (law of conservation of energy).

The system is then connected to a PID controller which gives the following closed loop response:

Distance


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Angle

With the help of the PID controller, the ball reached the desired position after approximately 10

seconds. After the ball reached the desired position, the angle of the plate is fluctuating around. This

is an expected behaviour as the plate should change angle to compensate any change in position of the

ball away from the desired position.


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Implementation in LabVIEW

Connection Diagram

As shown in the above diagram, the controlling of the ball position with LabVIEW is mostly defined

by the interfacing between:

(a)The Touch Screen Controller [via Serial COM Port in PC> 5 wire resistive Touch

Screen]

(b) The NI PCI 6024E - Card [via BNC-2110> Motors [counter 1 and counter 0]] (see

below)

USB/COM Port

[FTDI drivers]

PCI 6024E card

[NI DAQ drivers]

USB Serial

ConverterTouch Screen

Touch Screen

Controller

[COM Mode]

PC

with

LabVIEW

BNC 2110

Connector

X Motor

Y- Motor

Counter -1

Counter -0

5 Wire

Figure 16 System Connections


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The program layout is such that we read the current position of the ball on the screen and send PWM

signals to the respective motors to bring the ball towards center position.

In short, we will read from the TS controller in Serial mode [COM3 in this case]; condition the signals

via PID Controller and generate PWM signals [Dev1/Ctr0 and Dev2/Ctr1]. These three input and

output processes must be done at the fastest possible rate; for which we will use 3 parallel while

loops:

One for reading from the touch screen One for generating PWM signals based on PID controlled variable for X Motor One for generating PWM signals based on PID controlled variable for Y Motor

For this purpose, following 7 VIs have been generated:

Reading from Touch Screen

Reading from the Touch screen involves following processes:

- Configuring and opening port- Read 5 bytes at a time- The bytes come in following order

o Byte 1: A0this value is prefixed every time with the coordinates, so it is easy tointerpret, validate and convert.

o Byte 2 and 3X Coordinateso Byte 4 and 5Y Coordinates

- Initially, the Read Buffer is filled with :o Byte 1 : A0o Byte 2 : 08o Byte 3: 00o Byte 4: 08o Byte 5: 00

- The conversion of 08 00 gives 512 (described later). Since the touch screen co-ordinatesare (1024, 1024) we always start with (512,512) the center of screen.


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VISA Open

These are the settings to open the COM Port where the Touch Screen Controller is connected.

By installing the Virtual COM Port drivers from FTDI, the USB ports are now accessibleserially. The Touch Screen Controllers datasheet specifies:

o Baud Rate: 9600 Bpso RTS [Request To Send] set to HIGH

The rest are general settings for a serial transmission.

Validate Data

The coordinates are prefixed with A0, so we search the array for value A0 (=160 decimal) and

take 4 consecutive bytes out of that and convert them to X and Y positions (integers).

Byte to (x, y)


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For each axis, the controller gives 2 bytes. During the conversion process, each nibble in

hexadecimal system is converted as follows:

If received bytes = 08 and 00 then x-position = 0X163

+ 8X162

+ 0X161

+ 0X160

= 512

So there is a byte to nibble conversion required because computers can only represent 8-bit

data. We ignored the MSB value because it is always 0.

Byte to nibble

This VI splits the byte to 8bit array and takes only last 4 bits (nibble) and converts it to a

number again.

Then the X and Y Values are displayed by configuring the Display Graph accordingly.


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Controlling the Motor

The loop controlling the motor is as shown below:

This loop interacts with the motor via PCI 6024E card installed inside the CPU and connected via a

BNC2011 connection toolbox. By using the National Instruments Measurement & Automation

Explorer program, one can see that this device is installed as Device 1 and it has 2 counters Ctr0 andCtr1. We will generate PWM signals via these counters and drive the motors on each axis.

From the left side the blocks are described as below:

- DAQmx Create Virtual Channel: Create and initialize a output channel [Dev1/ctr0].The most important parameters are:

o Frequency: we use 50Hz by default, so the duration is 20ms standard.o Starting Duty cycle: 0.06

- DAQmx Timing:The second block is about timing. The Implicit timing is used when nosampling time is needed and continuous samples are taken.

- DAQmx Start Task: The DAQMx VI block initiates the pulse generation.- Then firstly, for each variable PWM controlled variable is determined. We take the input

(a local variablefrom Touch Screen Readers X value).

- DAQmx Write: This sends the actual pulse trends to the hardware. Since the duty cyclechanges with the position of the ball, the duty cycle has to change dynamically. So, the

local variable has been introduced and new values are processed with a CASE

STRUCTURE .

- A delay [50ms] higher than the pulse width of PWM [20ms] is required for efficientoperation


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PWM_X

The duty cycle required for the motor is a controlled variable with PID Controller. For the PID

Controller:

-- The set-point is 512, the center position.- The process variable is the current ball position.- The gains are and output range are real-time adjustable from front panel.- Time constant = 0.2msBut in this case we brought down the values by decade because the contact surface of the ball never

reached 512 and it kept oscillating. The final control was not fully accurate but the results were

improved. So the value would be floating around 510 ~ 519.

Here are the PID parameters we used:

P = 2.5

I = 0.2

D = 0

Output range: High = 0.0015, Low = -0.0035

PWM_Y


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Similar to X, but since the motors are aligned in opposite directions, the duty cycle has to be negated.

The dtfor X is 0.2ms and 0.5ms for Y that is because X is longer than Y, so the ball needs a larger

push in X direction.

The parameters used are:

P = 2.5

I = 0.2

D = 0

Output Range: HIGH = 0.0013 LOW = -0.0025

Operators Control Panel

This is the operators control panel with indicators and controls. The PID gains for X and Y shown

and output range for X and Y are default values which we choose. The red dot in the graph shows the

current ball position in touch screen.


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Problems and Recommendations

The system is mechanically stable and well controllable. In LabVIEW, by using a better

implementation of PID Controller, a stable end result could be obtained.

There is a problem with PWM generation. The

problem says that it cannot update the pulse

trend fast enough. The solution is to keep a

delay greater than the width of the PWM. In

our case the PWM was 50Hz signal (= 20ms)

and the while loop had a 50ms, but it still

didnt fix the problem. More detailed study is

required here.

The two counters must be synchronized for

stable operation which we could not

implement due to time shortage.

This will be the best educational project for

both for mechanically building and controlling the

system.


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Conclusion

Although this project was a challenge our group succeeded in creating a working system. The ball

does not settle at the desired position but instead moves near or around it with close proximity.

Further fine tuning of controllers as well as construction improvements can improve this. Themodelling of the system helped group members to understand but did not help in controlling andtesting the system as we model the process in continuous time, but the actual system works withdigital (discrete) signal.

Recommendations

Improvements to the construction to reduce play between joints.

Servo motors with higher position accuracy (improved construction may also contribute to this).

Adding springs to the construction to dampen certain oscillations.

A MIMO and/or digital control system would work better.

The current system could be enhanced by introducing a profile that the ball could follow as opposed

to a single point.

Balancing Ball Final Report V3

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