My goal is to build a balancing robot that can roam and avoid obstacles. As a first step I built a simpler robot that can balance in place or move slowly forward or backward over a level surface.

I posted a video at http://www.youtube.com/watch?v=L6SGsV9uHfs

I used mostly components from my last Boe-Bot project: Parallax continuous rotation servos, a BOE development board with a BS2sx chip and Ping to measure tilt angle. The balance point can be moved slightly fore or aft with on-board buttons to choose stationary or slowly moving behavior and to fine tune for different environments. The chassis is made from Radio Shack perf board. For power I’m using five Ni-CDs, whose low internal resistance provides quicker surges as the servos constantly start, stop, and change direction. The only non-Parallax working parts are 3” model airplane wheels.

I chose the Ping as tilt sensor after tests showed it to be the most accurate of the sensors I had on hand from previous projects. I collected data by running the robot with the serial cable attached, holding the cable to minimize it's affect on balance. Not elegant, but it worked.

The closed feedback loop is based on Andy Lindsay’s wonderfully clear explanation of how to implement a PID loop on a Basic Stamp. For this application I found that for the integral term, rather than maintaining a running total,  I got better results with a leaky accumulator which remembers only the previous two errors.

I’m eager to start a more sophisticated balancing robot and I’d appreciate any ideas or advice:

I’d like to go with a gyro/accelerometer combination. I already have a Memsic 2125. What’s the best choice for gyro? Perhaps the ADXRS300 on a breakout board from SparkFun.com? Does anyone have ideas or leads on information about algorithms to utilize these sensors on a Basic Stamp or SX chip (preferably in SX/B)?

I’d like to use DC motors. Any suggestions on motors that have a little more torque, a little more speed, and a little less latency than hobby servos? 

Thanks!

Below is the latest version of the pBasic code.

/John

' {$STAMP BS2sx}

' {$PBASIC 2.5}

'

' Balancer.BS2X. A simple two wheel balancing robot using a

' Ping))) sensor for tilt angle.

'

' John Fisher, January 2007

'

' -----[ I/O DEFINITIONS ]---------------------------------------

  buttonBottom       PIN     10

  buttonTop          PIN     11

  rwheel             PIN     12

  lwheel             PIN     13

  ping               PIN     14

  speaker            PIN     15

' -----[ CONSTANTS/PARAMETERS ]----------------------------------

' PID contstnts:

  Kp                 CON       2    

  Ki                 CON       2    

  Kd                 CON       4    

'Meaningful names for error array index:

  Current            CON       0

  Sum                CON       1

  Last               CON       2

  SecondtoLast       CON       3

  Delta              CON       4

' -----[ VARIABLES ]--------------------------------------------

  pTime              VAR     Word    ' raw value from Ping)))

  drive              VAR     Word    ' value sent to Motors

  error              VAR     Word(5)

  p                  VAR     Word    ' proportional term

  i                  VAR     Word    ' integral term

  d                  VAR     Word    ' derivative term

  SetPoint           VAR     Word   

  counter            VAR     Nib    

' -----[ INITIALIZATION ]----------------------------------------

Reset:

GOSUB ProgramStartSound

SetPoint = 540

' -----[PROGRAM CODE ]-------------------------------------------

Main:

IF buttonTop = 1 THEN

  SetPoint = SetPoint + 1

  FREQOUT speaker, 12, 1350    ' confirm button push

  PAUSE 175

ENDIF

IF buttonBottom = 1 THEN

  SetPoint = SetPoint - 1     

  FREQOUT speaker, 12, 1250    ' confirm button push

  PAUSE 175

ENDIF

DO

 PULSOUT ping, 12

 PULSIN ping, 1, pTime

 IF (pTime<425)OR(pTime>625)THEN GOTO Main 'Shut down motors

                                           'if beyond recoverable

                                           'tilt angle.

 error(current) = setPoint - pTime

 p = error * Kp

 error(Sum)=error(Current)+error(Last)+error(SecondtoLast)

 i = Ki * error(Sum)

 error(Delta) = error(Current) - error(Last)

 d = Kd * error(Delta)

 drive =  p + d + i

 GOSUB Motors

 error(SecondtoLast) = error(Last)

 error(Last) = error(Current)

' Code to send runtime data to debug goes here. The

' Parallax oscilloscope was used to check the actual

' time between motor pulses so the pause in the motor routine

' could be adjusted to stay close to 20 ms. The main value

' of using the BS2sx was that the BS2 was not fast enough

' to read the sensor, calculate the motor input, send

' data to debug, and get back to the motors within 20 ms.

LOOP

END

' -----[ SUBROUTINES ]-------------------------------------------

ProgramStartSound:

  FOR counter = 4 TO 1

    FREQOUT speaker, 30, (1320) - (counter * 36)

    PAUSE 100

  NEXT

RETURN

Motors:

  PULSOUT lwheel, (1871 + drive) MIN 1625 MAX 2111

  PULSOUT rWheel, (1875 - drive) MIN 1625 MAX 2125

  PAUSE 3

RETURN

Image Attachment : completed balancing robot dec 31 06.JPG   706KB (image/pjpeg) This image has been viewed 1602 time(s).

Thanks. I was surprised as well. I tested a variety of IR arrangements and the Ping was better, at least within a very narrow range of tilt angles. But the narrow range is all that's useful anyway.

The Ping reading only takes a few milliseconds. If you run them back to back strange things happen - probably the echoes are still bouncing around the walls. But there's no reason to update the readings faster than the 20 msecond servo pause, and at this rate everything works fine.

/John

Good work.

I have read over both yours and Tronics, you both have done good work.

Hey Everyone. Was inspired to try and make one of my own. The only extras I used were bigger wheels I had lying around and adding an extra battery pack to handle the current needs of the servos. Defintely found a lot of odd things happening to the bs2 and ping when the current is being pulled so hard. But with alot of tuning I finally got the thing to balance for over a minute. I believe that if I raise the batters up 3 or so inches the stability will greatly improve and will post again once I try this.

I included my dirty dirty code if anyones interested. Some math in the code does not really make sense. I think im getting errors due to current fluctuations but not sure. I need to go thru it when im not tired and really debug it.

And here is the video on you tube.

Finally got my bs2 controlled boe to be stable and balance on two wheels.

http://www.youtube.com/watch?v=vhJIWGOb8hg

D. Faust,

Thanks! I made the frame from Radio Shack perf board, glued up with epoxy. I live in an apartment and my workshop is also the dining room table, and everything gets folded up and stowed in a closet after every work session, so I have no choice but to keep everything very simple. Maybe that's a blessing in disguise.

In the middle of experimenting with various combinations of accelerometers and gyros, I got even more interested in this guitar tuner project: http://forums.parallax.com/forums/default.aspx?f=21&m=181996

At the moment I'm getting ready for a trip to Germany and I hope to get back to both the tuner and the balancing robot project in July.

Looking forward to seeing progress by others with balancing robots, particularly using combinations of prop/accelerometer/gyros.

/John

Image Attachment : balancer_chassis.jpg   118KB (image/pjpeg) This image has been viewed 417 time(s).

lboucher,

I just saw the video

This is a nice design, apparently based on standard parts

Would you let us have more info on your baby, and post some more photos

If you allready did, please direct me to the right direction

Yves