Calibrating 3pi to drive straight without sensor feedback

Support Robots
1

My 3pi has slightly mismatched motors and setting both motors to move forward at the same speed always causes the 3pi to veer to the right. For example, In a run of 20cm, it can veer as much as 7.5cm off heading. The tires, motor shafts, and running surface are clean and the robot has done this since I pulled it out of the box 2 weeks ago.
The degree to which it veers is not a constant difference between motors, nor is it directly proportional to the speed at which the 3pi is run. So I am not able to compensate for the difference by increasing the right motor speed by a constant offset:

set_motors(speed,speed + offset); // eg. offset = 10

I also cannot compensate by increasing right motor speed by a percentage of the current speed:

set_motors(speed, speed * offset); // eg. offset = 1.08 to increase by 8%

I will be adding PID control which will allow my 3pi to correct itself and drive perfectly straight with sensor feedback; however, I anticipate the requirement to run the 3pi in a straight line for periods when the sensors do not see anything so it appears my only option is to generate a look-up table with calibration values. The table may provide me with an alternate proportionality (exponential?); although, once the table exists, I would of course not waste cycles with unnecessary math.
I also have a design which will allow me add encoders to the 3pi, but do not have spare i/o pins on this version.
I was hoping the constant voltage to the motors would provide more consistent motion, but of course the veering is only consistent at a single speed. I may also have to create another table to enable accurate 90º and 180º turns at various speeds.
Have others experienced this same issue? Am I missing alternatives to a calibration table?
Thanks,
Z

2

Hello.

I don’t think you’re missing a simple alternative. In general, going straight is difficult, especially without encoders.

- Jan

3

I had the same problem. My motivation was a bit different, I wanted to calculate position by dead reckoning while following a line (to solve looped mazes).

There’s no alternative to calibrating the motors. Their speed response is non-linear with the applied speed setting, especially at lower speeds. Once I had them calibrated I found that the distance/speed/time relationship is consistent to within about 1-2%, which is good enough for what I wanted.

I also found the problem you did. I was able to calibrate it out, by adding a “swerve factor” for reach speed range (16 ranges of 16 speed settings each). The result isn’t perfect, but it is good enough. To get a better result you would have to continuously vary the swerve correction in a kind of pwm. At some point life is just too short.

John