Robot won't go straight

[kmcclary]

Originally Posted by svanderlugt

Our robot has 2 cim motors attached to wheels via chain and sproket. We have used the default code and the robot will not go straight, it turns in one direction weather we go forward or backwards! Is anyone else having this problem? It could be the transmission between the motors and the chain, and the chains on each side are almost the same. We will continue to troubleshoot.
Thanks,

[b]Quantum Ninjas[b]

Is it SPINNING, or just making a gentle "arc" to one side?

(A) IF spinning: Reverse the polarity on one motor. Remember that one runs forward and one runs backwards on a standard outward facing differential drive, so one will have the red lead on the Victor's M+, whereas the other will have the black lead on it (and vice versa for the M- connection).

(B) IF arcing in a gentle "S" curve (arc one way forward, the other backward): You have "motor timing" issues, where the motor favors one direction over the other. Others have posted that the CIMs are not SUPPOSED to be "timed" (favor one direction), but in reality they always seem to favor one way or the other slightly.

This can be compensated for via several methods: a power factor lookup table to balance the power to each side based on command, closed loop steering control with the gyro (slave one motor's speed to the other via integrating the gyro data - see the Whitepapers or kevin.org for more information), mechanical coupling of the two sides (tough to do without designing a clutch), or a different drivetrain geometry, like skid steer 4WD or 6WD (also tough to change now, since we're in week six!)

(C) If it is favoring the SAME side both forward and backward: You either have one motor more powerful than the other (CIM output is rated +/- 10 percent per copy), or the drivetrain is BINDING more on one side than the other. Check for binding first. That is the most common reason when this occurs. Check for: binding in the CIM-Transmission coupling (VERY common source!), for misalignment in gear and/or sprocket-chain sets, binding wheel bearings, and if live shafts are used, defective/dry pillow blocks or a misaligned/bent shaft.

Also be sure that the far end of the motor/gearbox shaft is supported with a pillow block! If it's not, chain tension can bind up a gearbox.

If you suspect motor-to-motor variance, try a different combination of CIMs.

(D) It COULD be a programming problem, but that's unlikely. You can easily disprove ANY variance in the control system & programming is responsible, by simply switching which side each Victor controls (IOW, swap motor wiring to the Victors, keeping everything else identical). This is an easy test to do. If the symptom shifts sides, then you actually have a programming or electrical problem! If not, then it's a motor, transmission, drivetrain, wheel, or other mechanical problem.


I hope this helps you diagnose your problem.

- Keith McClary
Chief Engineer, Team 1502 "Technical Difficulties"