DEWBOT VI Drive Train Tread Wear

One of the issues we had at Drexel is that one wheel had extreme tread wear. This page is a discussion on what the problem is, what possible reasons are and how to test for the reason before we do a repair.

Problem

The wheel on pivot #4 exhibits extreme tread wear when compared to the other wheels.

Wheels all were retreaded on xxx. Wear was noticed at the 7th match at Drexel and replaced. Wheel had only yyy minutes of wear.

Wear was even across the entire tread.

Other Observed Inconsistencies

• Wheel #4 does not calibrate correctly zz% of the time in the aa version of the calibration code. It is the only wheel with this problem.
  • It can however, be aligned manually in the pit pre-match. When this is done and the calibration code is not run in autonomous, there the wheel doesn't experience a calibration problem (true?)
• Simultaneously with noticing the tread wear, the pit crew discovered the wheel's locking collar was completely unsecured. It appeared to have loosened rather than sheared (i.e. it could be repositioned and tighten again).
• This wheel appears to have a binding spot in the drive mechanism as well (is this confirmed?)
• Gary> When pivot #4 was serviced before ship. there was not noticible wear on the tread worse than the others. The chain was much looser than the others from the time it was installed. This is most likely from improperly drilled steering motor mounting holes and play in the 1' pivot tube and bearings. This can be addressed by removing a link and adding a 1/2 link and/or oblonging the mounting holes. The chain tensioner may be adding friction that can cause problems with PID. (see PID comments later)
• Gary>One side effect from the method of 360 wheel drive is that there is a torque opposite the rotation of the wheel drive gear rotation. This load is felt by the steering motor and the motor must over come this force to move against it. Turning the other way aides the wheel steering. If from wear pivot #4 can not turn the wheel as freely as the others the steering motor will have to over come this higher torque. Is the pivot drive train damaged? ( PID constants affect the motors ability to deal with this added load)

Backwards Induction

Even wear across an entire Plaction Wheel tread that is inconsistent with the other wheels can be caused by

• that wheel spinning (consistently) at a different rpm than the others
• that wheel being driven while the others are stationary and vice versa (causing it to slip)
• a tread defect (this seems unlikely given the other issues, but is easily checked by continued use of the new tread)
• ????

Thesis: Brake Jumper on Jaguar

The brake jumper is installed on the drive cRio. This means every time the robot changes direction the wheel will stop, dragging the tread. The other wheels would continue to spin along the carpet.

Tests and Results

Look at the drive Jaguar for that pivot.

Thesis: Pivot Plate Dimension

The holes that the steering chain run between on the #4 pivot plate are slightly off, making the chain loose. Though it was subsequently tensioned, the drag of the tensioner or the looseness of the chain delays the wheel's steering. This makes it point in an incorrect direction (at least initially), causing it to slip.

Tests and Results

Attempt to tighten chain (sans tensioner). Switch pivot plate? (seems drastic)

Thesis: Loose Axle Bearing

The axle bearing collar was loose, this would allow the wheel to slide back and forth across the axle. This would lead to wear because of ???????

Thesis: Position Sensor Mounted Differently

The magnet for the Cherry Position Sensor is mounted differently than the other three. This would cause wear because ?????

• Gary> The Cherry sensor mounting plate on #4 was slighty more off center than the other. This was suposedly corrected during mounting. Maybe it is not. I do not believe this would give the error that we see. Off center aceptable alighnment is up to 2 MM. I'm almost certain it is within that amount. Offset error Would show up as a +- sinusoidal offset error around 4 points on the linear output line and be very repeatable. The magnet on pivot #4 was too close to the Cherry sensor on pivot # 4. This was corrected at install time by adding 2 washers instead of 1 washer on top off the 1" standoffs. This should give a 2 - 2.5 MM magnet to sensor distance. Is it?

Tests and Results

Look at the sensor and see how many washers are on it. Should be 2 - 2.5 MM magnet to sensor distance.

Thesis: Different Drive Code

There is different drive code for that pivot. This would cause wear because ?????

Thesis: Bent Axle

There is a bent axle at xxx location in the pivot mechanism, this would lead to wear because of ?????