Trouble-shooting Low Traction

[alecmuller]

Thank you for the ideas. That 3/4" plywood is essentially the whole chassis (all 4 drive pods are screwed to it), so it's pretty stiff relative to the weight of the robot. I don't have a whole lot of margin on the pneumatics (which deploy the traction wheels), so I don't think I'd be able to get away with reducing the pressure very much. I believe all 4 wheels are carrying pretty close to the same weight (due to symmetry), but I don't have 4 identical scales to verify that with.

Coupling the front & rear wheels tank-style sounds like a great solution for most 4 & 6 wheel robots; unfortunately it defeats (or complicates the heck out of) the mecanum part of the Octocanum in my case.

At the moment I'm still trying to debug the power-ramping code (to avoid going straight to full power and slipping the wheels). My first attempt (based on the arduino clock time) didn't move at all. I need to find an easy-to-use clock function in Arduino . . .

[JamesBrown]

Originally Posted by alecmuller

[*]each brick weighs ~20 lbs alone and requires about 26 lbs of tension to lift through the pulleys (measured with bathroom scale)

How did you measure the tension to lift through the pulleys with a bathroom scale?

[IKE]

You could have them all hooked together and at a higher weight than robot can pull (say 300 lbs), then record initial weight and weight at stall and the difference would be the pull weight.

[alecmuller]

Originally Posted by JamesBrown

How did you measure the tension to lift through the pulleys with a bathroom scale?

I stood on the scale, pulled the cord to raise and lower the weight a few times, and watched the scale move. The friction in the pulleys works against the direction of travel both ways, so the scale goes down a few pounds when you're lowering (i.e. friction offsets the weight), and goes up a few pounds when you're lifting (i.e. friction adds to the weight). The average should be: [weight of Alec] + [weight of the paver], and the max should be [weight of Alec] + [force needed to lift paver through pulleys].

Originally Posted by IKE

You could have them all hooked together and at a higher weight than robot can pull (say 300 lbs), then record initial weight and weight at stall and the difference would be the pull weight.

I like that - like some of the teams used with their hooks to lift totes & hold them a fixed distance apart this past year. It would also provide a "stair-stepped" resistance instead of suddenly hitting the full load all at once. Unfortunately I'd need to get lighter weights (because my ~20lb weights are very low resolution).


I realize I neglected to point directly to my calculations. This Google Sheet (specifically the "Architecture" tab) has my traction calculations (lines 220 to 410).

[GeeTwo]

Originally Posted by IKE

You could have them all hooked together and at a higher weight than robot can pull (say 300 lbs), then record initial weight and weight at stall and the difference would be the pull weight.

Originally Posted by alecmuller

I like that - like some of the teams used with their hooks to lift totes & hold them a fixed distance apart this past year. It would also provide a "stair-stepped" resistance instead of suddenly hitting the full load all at once. Unfortunately I'd need to get lighter weights (because my ~20lb weights are very low resolution).

Not quite what I think Ike had in mind - something more like:

• Place a single load on the back end of the cord that is greater than the maximum pull your robot can lift, but less than the top limit of your scale.
• Put the scale under the load, in the configuration ready to be lifted.
• Record the reading on the scale.
• With someone watching the scale, turn the robot on, and gradually increase the pull until a wheel slips.
• Record the lowest weight on the scale just before spin-out.
• Subtract the pre-spin-out weight from the full weight to get tension in the cord at the point of maximum traction.
• Add/multiply whatever fudge factor is needed to account for the friction in the pulleys to get the maximum traction force.

[Labman]

Those interested in this type of testing may be interested in American Weigh Scales model PK-110. This small digital hanging-style scale can be set to record peak force pulling on the hook. It operates horizontally as well as vertically so it is easy to use to measure robot pulling power, as well as weighing parts, testing robotic arm lifting power, checking crimp pull-out strength and many other force measurements. The scale reports weights below 44 pounds to 0.02 pounds, and weights above 44 pounds to 0.1 pounds. I tested two scales against highly accurate lab scales and found that one was right on while the second averaged 0.1 pound too low. The scales cost about $45 and have held up well over a year. The only downside is their max capacity is 110 pounds, so we use two in tandem to measure larger weights/forces.

[GeeTwo]

Originally Posted by Labman

hanging-style scale

In addition to our digital flat scale, we have four old-school fish-weighing scales from the sporting goods store. We have four so that we can hang a robot by the axles of the four corner wheels, and figure out where the CoG is (we did Mecanum in 2014, and H/slide in 2015). Not as elegant as the digital peak-recording variety, but they've met our needs.