FINAL PROJECT REPORT

Mechanical Engineering 224

Professor Espinosa

June 9, 2005

Kendra Armstrong

Nick Eccles

Cary Maguire

Alex Taam

Paul Williams

2

Table of Contents

I. Introduction

II. Boe-Bot Assembly

III. Servo Calibration

IV. Gyroscope Calibration

V. Analog-to-Digital Converter

VI. Programming and Debugging

VII. Results and Conclusions

VIII. Improvements and Future Considerations

IX. Appendix A: Servo Calibration Program

X. Appendix B: LabView Program for Gyroscope Calibration

XI. Appendix C: Turn Test Program

XII. Appendix D: Final Path Program

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I. Introduction

The objective of this project is to use our knowledge acquired in Mechanical Engineering

224 to control a Boe-Bot along a specified path,

Figure 1 – Robot Path

using gyroscope sensing in a closed-loop feedback system (see figure 1). The Boe-Bot robot is

mainly composed of two servo motors to operate the wheels and a Board of Education carrier

board, which is controlled by a program called BASIC Stamp. Specifically, we will create a

BASIC Stamp program that will use a specified voltage, which corresponds to a certain angular

velocity, to control the direction of travel of the Boe-Bot. Then, using closed-loop feedback from

a gyroscope and ADC (Analog-to-Digital Converter), the Boe-Bot will correct itself to travel in a

completely straight line.

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II. Boe-Bot Assembly

Figure 2—Boe Bot

Our Boe-Bot was brand new, therefore unassembled. First, we attached the four

standoffs to the four corners of the chassis. The standoffs support the Board of Education from

which the Boe-Bot runs. After centering the Parallax Continuous Rotation servos, we attached

the servos to the chassis using Philips screws and nuts. We then attached our power source, the

battery pack, to the underside of the chassis. After that, we attached the tail wheel ball and high

quality rubber band tires. Lastly, we connected the Board of Education onto the four standoffs,

with the breadboard closest to the drive wheels. And the Boe-Bot was born! (See figure 2 for a

photo of an assembled Boe-Bot).

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III. Servo Calibration

Figure 3 – Boe Bot Servo

Before assembling the Boe-Bot, we had to calibrate the servo motors (see figure 3). We

used a program (see Appendix A) that sends the servos a signal, telling them to stay still.

Because the servos are not pre-adjusted at the manufacturing facility from which they came, they

will actually start spinning. We then had to use a screwdriver and adjust the servos until they

were still. This calibration is called centering the servos. When the program input is PULSOUT

12, 750, it is centering the right (designated by 12) servo to stay still (750 designates no

movement in either direction). When the program input is PULSOUT 13, 750, it is centering the

left (designated by 13) servo to stay still. When the input 750 is increased, the servo will travel in

one direction, and when it is decreased, the servo will travel in the opposite direction.

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IV. Gyroscope Calibration

The main programming softwares we used in this project were LabView and BASIC

Stamp. While we were capable of programming the Boe-Bot to travel in straight lines and make

various turns with BASIC Stamp, we did not know the actual angles the Boe-Bot turned during

its test trials. In order to be more accurate with our Boe-Bot following its respective path, we

integrated the response of a gyroscope in a closed feedback loop, which will allow us to program

the robot to make turns at specified angles and will adjust the Boe-Bot so it does not deviate from

its straight path.

In this project, we used an ADXRSS150EB gyroscope from Analog Devices. It operates

on a 5 Volt power supply and is capable of sensing up to 150 degrees in angular motion. This

gyroscope contains two polysilicon sensing structures which have capacitive pickoff structures

that are capable of detecting motion caused by a Coriolis force. This Coriolis force is produced

when the Boe-Bot rotates. After the Boe-Bot rotates, the Coriolis force causes the two

polysilicon sensing structures to be displaced orthogonal to the vibrating motion of the Boe-Bot.

The capacitive pickoff structures on the polysilicon sensors then pick up the Coriolis motion and

a rate signal output is produced. This rate signal is the feedback we need in order to ensure that

our Boe-Bot turns at specified angles and follows a straight path.

Before we wrote our final program, we needed to calibrate the gyroscope and determine

the relationship between its angular velocities and their respective output signals. We first

created a LabView program (see Appendix B) that plotted our gyroscope output signals versus

time:

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Figure 4 – Gyroscope Response

This allowed us to see how the gyroscope output signals varied as the Boe-Bot spun in a

clockwise (from t = 5 s to t = 17 s) and then counterclockwise (t = 17 s to t = 26 s) motion (see

figure 4).

The next step was to determine the relationship between the gyroscope’s angular

velocities and their respective output signals. In order to find this relationship we created a

BASIC Stamp program (see Appendix C) that spun the Boe-Bot at various angular velocities.

After each run, we recorded the number of rotations, and the respective times for each run, the

Boe-Bot completed. This enabled find a relationship between the gyroscopes output signals and

their respective angular velocities. (See figure 5).

Voltage vs Time

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 5 10 15 20 25 30

Time (s)

Vo

ltag

e (

V)

8

Figure 5 --Gyroscope Calibration Curve

Angular Velocity vs Voltage

y = 1.3361x - 3.0312

R2 = 0.9999

-4

-3

-2

-1

0

1

2

3

4

0 1 2 3 4 5

Voltage (V)

An

gu

lar

Velo

cit

y (

rad

/s)

Test Data

Linear (Test Data)

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V. Analog-to-Digital Converter

After mounting and calibrating the gyroscope, we encountered a major problem. The

gyroscope was outputting a voltage in the range of about 0.2V to 4.6V. We needed to incorporate

the full scope of these values in our control of the Boe-Bot. However, the pins on the Board of

Education of the Boe-Bot can only read high or low. To rectify this problem, we decided to look

into ADC’s, which could then provide us with a range of values instead of just 1 or 0. After

researching various options online, we decided to use the TLC0820AIN, produced by Texas

Instruments. (Specification sheet can be found at http://www-s.ti.com/sc/ds/tlc0820a.pdf).

The converter is an 8-bit analog-to-digital converter that uses the output of the gyroscope

(RATEOUT) as its input and then writes the values to pins zero through seven on the Boe-Bot.

Thus, we could theoretically acquire a range of values from 0 to 255, instead of the original 0 or

1. The converter uses the 5V power of the Boe-Bot as its VCC, which is the desired value on the

specification sheet, thereby eliminating the need for an op-amp circuit to power the converter.

When wiring the converter onto the Board of Education, we encountered another problem,

namely a shortage of possible connections. The Board only has 17 rows of pins, but the converter

and the gyroscope each need 10 rows. To solve this problem, we purchased another small

breadboard from RadioShack and mounted it to the cart of the Boe-Bot, beneath the original

circuitry. (See figures 6 and 7).

Before starting to write a comprehensive BASIC Stamp program for the robot’s route, we

needed to determine if the converter was working as expected and was compatible with the Board

of Education. To do this, we used a variable voltage generator as the input, and BASIC Stamp to

read in the values from the pins of the Boe-Bot. Since the maximum voltage the converter could

handle was 5V, and the maximum output of the gyroscope was around 4.6V, we found that it

worked nicely. We varied the voltage generator from zero to 4.9V, and found that the converter

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was wired correctly and working as expected. We were now ready to incorporate the gyroscope

as the input and control the Boe-Bot with BASIC Stamp.

Figure 6 –Robot with both breadboards Figure 7 – ADC, Installed

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VI. Programming and Debugging

Because we chose to use only the BASIC Stamp programming language to control our

robot, we developed a single program encompassing both the feedback and control of the robot.

Development of this program required the integration of the information obtained in the

gyroscope and ADC calibration phases with the programs we had created to perform basic robot

maneuvers. The calibration we achieved of the gyroscope produced a correlation between

angular velocity of the robot and the voltage generated by the gyroscope. The output of the

gyroscope is the input to the ADC, which produces an 8-bit binary number that the Board of

Education can read. Testing of the ADC gave us a correlation between the digital numbers

produced by the ADC and the analog voltages that create them.

Figure 8 – The team at work

The program reads the output of the ADC and converts the binary signal to an integer

value, and then converts this integer to the corresponding voltage of the gyroscope output. Using

the gyroscope calibration equation, this voltage is then converted to an angular velocity.

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Knowing the angular velocity of the robot and the sampling time, a simple integration can be

performed by summing the product of the angular velocity and the time step to produce the total

angular displacement of the robot. The program written performs these tasks and instructs the

robot to turn until the proper displacement has been reached, regardless of surface conditions or

the time required to do so. The initial runs were far from ideal, but with iteration we were able to

produce a program that causes the robot to describe a parallelogram path, with 45° and 135°

angles and equal length sides.

In order to debug the program we tested the small programs that performed portions of

the route (traveling straight, turning 45°, etc.) individually before integrating them into the final

program as subroutines. The proper turn durations were achieved by iteratively modifying the

calibration in the calculation routine. Straight line performance was established by first creating a

baseline servo setting that produced the straightest possible path without feedback, then running a

feedback loop to correct the path if the robot deviated from straight ahead (if the gyroscope

detected an angular velocity). (Figure 8 shows the team hard at work).

The robot’s performance is not as consistent as it could be, nor are the measurements as

precise as we would like, due to BASIC Stamp’s inability to handle decimal numbers. (See

figure 9 for a sample of the testing procedure). This introduced error as any decimal value is

truncated to leave only the integer. This is particularly important during division operations,

when any remainder is discarded. Because BASIC Stamp can only handle numbers up to 65535,

the scale of numbers becomes an issue—it is difficult to maintain accuracy when dealing with

numbers of varying magnitudes. An additional problem is that there is a finite, non-zero time

step for the summing operation. Ideally, as the time step goes to zero, the sum becomes an

integral, but the Board of Education and the BASIC Stamp software require a finite amount of

time to run the program each time it loops. These errors multiply over the integration to create

less accurate results. The sensitivity of the instruments was another concern. After processing

the gyroscope’s output (with the aforementioned BASIC Stamp mathematical complications), the

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robot’s closed loop straight line feedback is not robust enough to detect small path deviations,

only large scale errors. This could be corrected with more sensitive components, faster

processing, more precise computations, or a combination thereof.

Figure 9 – The robot during testing for path accuracy

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VII. Results and Conclusions

Our project was successful. The Boe-Bot follows the specified route successfully most of

the time. It races smoothly along as it takes into account the feedback it receives from the

gyroscope. Although this fulfills the basic requirements of the project, there are many problems

with our current system that detract from the accuracy and consistency of our Boe-Bot operation.

First, there are multiple problems with the BASIC Stamp program. The largest problem is that

BASIC Stamp does not allow the use of decimal points. This means that the numbers we use

must be scaled up, and then subsequently scaled back down as we make our calculations that

determine the Boe-Bot’s route. This scaling results in a large loss of precision. Also, variables

can only be declared to about 60,000, which makes it difficult to scale numbers to a large enough

number to prevent important values from being truncated. Secondly, there are problems with our

ADC. While it converts the analog input from the gyro into a digital signal, it has a number of

sensitivity issues. In its current configuration, the ADC can’t detect small changes in angle. This

allows BASIC Stamp to control the Boe-Bot to some degree, but it often produces inaccurate and

inconsistent results. Ultimately, the Boe-Bot works, however, there are many improvements that

could be implemented.

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VIII. Improvements and Future Considerations

As mentioned above, there are numerous improvements that could be employed to

improve the Boe-Bot’s operation. . (Figure 10 shows the robot during testing). The most

important improvement involves making the ADC more sensitive. This can be done in various

ways. The most logical way is amplifying the signal with an operational amplifier. There is a

detriment to this method, however. Amplifying the signal would improve the sensitivity, but it

would also reduce the range of values that would be recognized. The Boe-Bot would

subsequently have to be operated at lower voltages to ensure that it didn’t go out of the narrowed

range. In addition, the method of scaling number in BASIC Stamp to account for the lack of

decimal points could be refined. There are other methods that produce higher precision, but the

process is more tedious. For our calculations, the more simple method was used. Finally, all of

the equations and calculations would benefit from being recalibrated. Many hours were spent in

the lab tweaking numbers, and all of the tweaking resulted in better control of the Boe-Bot. With

more time, there could be a larger amount of time dedicated to calibrating the Boe-Bot and

refining numbers. We had a limited amount of time for the project, and the equations were

refined to be as accurate and consistent as possible

Figure 10 – The robot connected to the computer during testing

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IX. Appendix A: Servo Calibration Program

' {$STAMP BS2}

' {$PBASIC 2.5}

counter VAR Word

FOR counter = 1 TO 100

PULSOUT 13, 750

next

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X. Appendix B: LabView Program for Gyroscope

Calibration

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XI. Appendix C: Turn Test Program

' {$STAMP BS2}

' {$PBASIC 2.5}

value VAR Word

theta VAR Word

sum VAR Word

V VAR Word

ts VAR Word

counter VAR Word

dsum VAR Word

pulsecount VAR Word

ts = 6

GOSUB Ramp

GOSUB Turn45

END

Turn1:

PULSOUT 13, 770

PULSOUT 12, 770

PAUSE 20

RETURN

Ramp:

FOR pulseCount = 1 TO 20

PULSOUT 13, 750 + pulseCount

PULSOUT 12, 750 + pulseCount

PAUSE 20

GOSUB Calc

NEXT

RETURN

Calc:

LOW 8

value = (1*IN0) + (2*IN1) + (4*IN2) + (8*IN3) + (16*IN4) + (32*IN5) + (64*IN6)

+ (128*IN7)

HIGH 8

V = (value*420/256+50)

theta = (133*V - 30590)/100

dsum = theta*ts/100

sum = sum + dsum

RETURN

Turn45:

DO WHILE (sum < 1225)

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GOSUB Turn1

GOSUB Calc

LOOP

RETURN

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XII. Final Path Program

' {$STAMP BS2}

' {$PBASIC 2.5}

value VAR Word

theta VAR Word

sum VAR Word

V VAR Word

ts VAR Word

counter VAR Word

dsum VAR Word

pulsecount VAR Word

value2 VAR Word

value3 VAR Word

counter2 VAR Word

ts = 2

GOSUB Straight

PAUSE 500

GOSUB Ramp

GOSUB Turn45

PAUSE 500

GOSUB Straight

PAUSE 500

GOSUB Zerovar

GOSUB Ramp

GOSUB Turn135

PAUSE 500

GOSUB Straight

PAUSE 500

GOSUB Zerovar

GOSUB Ramp

GOSUB Turn45

PAUSE 500

GOSUB Straight

PAUSE 500

GOSUB Zerovar

GOSUB Ramp

GOSUB Turn135

END

'%%%%%%%%%%%%%%%%%%%%%%%%%%% subroutines

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Zerovar:

value = 0

V = 0

dsum = 0

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theta = 0

sum = 0

RETURN

Turn135:

DO WHILE (sum < 270)

GOSUB Turn1

GOSUB Calc

LOOP

RETURN

Turn45:

DO WHILE (sum < 85)

GOSUB Turn1

GOSUB Calc

LOOP

RETURN

Turn1:

PULSOUT 13, 780

PULSOUT 12, 780

PAUSE 20

RETURN

Straight:

FOR counter2 = 1 TO 10

value2 = 0

value3 = 0

FOR counter = 1 TO 10

PULSOUT 13, 807

PULSOUT 12, 650

PAUSE 20

GOSUB Valread

value3 = value3 + value2

NEXT

IF value3/10 > 120 THEN

GOSUB Correctleft

ELSEIF value3/10 < 100 THEN

GOSUB Correctright

ENDIF

NEXT

RETURN

Ramp:

FOR pulseCount = 1 TO 20

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PULSOUT 13, 750 + pulseCount

PULSOUT 12, 750 + pulseCount

PAUSE 20

GOSUB Calc

NEXT

RETURN

Calc:

LOW 8

value = (1*IN0) + (2*IN1) + (4*IN2) + (8*IN3) + (16*IN4) + (32*IN5) + (64*IN6)

+ (128*IN7)

HIGH 8

V = (value*420/256+50)

theta = (133*V - 30590)/100

dsum = theta*ts/100

sum = sum + dsum

RETURN

Valread:

LOW 8

value2 = (1*IN0) + (2*IN1) + (4*IN2) + (8*IN3) + (16*IN4) + (32*IN5) + (64*IN6)

+ (128*IN7)

HIGH 8

RETURN

Correctright:

FOR counter = 1 TO 3

PULSOUT 13, 770

PULSOUT 12, 770

PAUSE 20

NEXT

RETURN

Correctleft:

FOR counter = 1 TO 3

PULSOUT 13, 730

PULSOUT 12, 730

PAUSE 20

NEXT

RETURN