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 noticeable 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)
• The first step to this problem is to dismantle pivot #4 and assess the mechanical condition of the pivot, steering and sensor. If it is not sound mechanically, no further analysis of the programming issues can be done. We have spares of all parts.h

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 slightly more off center than the other. This was supposedly corrected during mounting. Maybe it is not. I do not believe this would give the error that we see. Off center acceptable alignment 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 ?????

Gary> There are several things about the program that concern me. The Cherry sensors should have very good repeatability. In other words the pivot drive motors and sensors once assembled should always have the same calibration coefficient's. Since day one after calibration this value for each pivot should stay the same. Only if we take apart the chain on the steering motors would this change. If it varies some thing is very wrong.

The potentiometers should only be used for moving the steering motors to obtain the calibration. Why do we have to keep calibrating? What bothers me is that the programmers talk about the calibration routine at auto start up. Are they using the pots at all then? They shouldn't.

The calibration values should be able to be hard coded into the least distance routine as a constant. The PID drive VI should be able to be given a position to drive to at the start of Auto (straight forward) and the steering motors should reach that position in less than 1 second. Why does it take so long to get the wheels pointing forward?

Could part of the lag be attributed to the time to read the offset values from flash? Are the values from flash assigned to a variable by value? Could Labview be using a pointer to a flash memory location? This would be very bad. Flash reads are much slower than ram. A whole sector has to be read to get to one byte. Question? If the offset values are written to flash and stored after the compiled code, could they be over written if the code grew considerably and they are not re-written?

• The original (Foster) idea was to read the positions and store them into Flash. The team opted to create a board with 4-10K pots on it. The pots are adjusted to align the wheels. During startup, the pot values are read into memory (ADC conversion of ~10ms) and kept for that run of the program. The pot values are only read during alignment.
• On of the changes made on Sunday before Drexel was to "force" the wheels to go to the zero position. This had the effect of moving all the pivots to the same orientation. When doing this we saw "pivot palsy" where the pivot moved back and forth from 355-0-355 degrees due to the PID trying hard to get exactly to zero.

Could there be something about auto mode that is different than telop that affects the loop rate on the PID of auto? Does something in the code affect the loop rate?

• Carly noted that the robot drifts to one side during auto mode. Could this be a code issue or mechanical drag?

Gary> A comment by Paul bothers me. He said that at Drexel he had to change the PID values back to what they were and the lag went away. This does not make sense to me. Sunday before Drexel the test bed set up showed very sluggish response. When the PID coefficient where adjusted the response was very good. This should have nothing to do with lag. Remember the motors had no load on them then. When the robot is on the floor there is more friction. If anything they may need to be higher under load. Additional friction from a damaged pivot #4 would make the PID lack of responsiveness much more apparent. The 2 together could be the problem.

Foster>What are the PID values from Drexel?

Tests and Results

• Talk about how the calibration routines work with the entire team so we understand how it works.
• Why does it take so long to get the wheels pointing forward?
• Examine pivot #4 for additional friction issues

Thesis: Bent Axle

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

Information

Gary Deaver and Clem McKown inspected the drive-train and especially Pivot 4 on 10-April.