[FTC]: Motor encoders Love'em? or leave'em?

[Ether]

Originally Posted by MattRain 15Nov2013 11:00

I will have to students try that out today. I shall post the results tonight.

Did you forget?

[aakerberg]

Well, accurate distance isn't all that important. Presumably you should have some opportunity to tune your program to account for any inaccuracy in the encoder. However, consistent results ARE important. A better experiment would be:

1. Mark a very specific start point
2. Program the robot to go any distance
3. Mark where it finishes moving.
4. Repeat several times - preferably with different loads (add/subtract weight) and/or different levels of battery charge.

How close does it get to your original finish mark each time? if the encoders are reasonably consistent, you should end up in the same spot (or very close) each time, regardless of load or battery condition.

The problem with using encoders for measuring robot position in autonomous mode is they offer no defense against the things that are most likely to trip you up - getting bumped off course by another robot, wheels slipping on the edge of the board, etc.. I would argue that provided you don't make radical changes to the robot during the course of a single competition and manage your battery pack properly, you can get equally effective results using time for judging distances driven.

If they are consistent, I can see them being more useful for measuring things like precision arm movements, or maybe feedback for a sophisticated driving control system...

We have been also been unhappy with they way they are mounted to the outside of the motor. We are experimenting this year with other ways to give us the information we need.

[Ether]

Originally Posted by aakerberg

Well, accurate distance isn't all that important.

That may well be true in many cases, but the specific point of posts 7, 11, 12, 13, 14, and 15 was to think through the statement in post 2.

[Sasha]

We do use encoders for autonomous. They work fine for straight motion (of course, not to 0.01 inch - but it is not necessary). However, we found them rather unreliable for measuring turns (when turning, both wheels slip), so we use gyro sensor for this. Works much better for us. Your mileage may vary.

[Ether]

Originally Posted by MattRain 15 Nov 2013, 11:00 AM

I will have to students try that out today. I shall post the results tonight.

Still interested in your test results.

[DavisDad]

Originally Posted by DocMartin

My fellow Mentor and I are at odds on using encoders or not.

I want to hear from you.

Does your team use them or not? and why?

Go!

We're working on a design using VEX 4" Mecanums. We built a prototype to test performance of wheels, motor controller, and gearmotor/encoder combination. We used the Matrix system for cost and availability reasons; the Matrix gearmotor has the encoder integral to the motor.

Our competition design is currently:



For the testing prototype we built:





Things we like about the prototype:

• Motor controller built in PID speed control worked well (RobotC code)
• Teleop control is very nice for omni-directional control
• Speed vs. power is good (quantitative data to follow if we can wrestle the bot away from the drivers )
• Fabrication was fairly straight forward (see cons)
• Square shaft for wheel eliminates hub

Cons:

• Haven't been able to get encoder position control to work with the built in Matrix RobotC code
• Square shaft VEX to 4mm motor shaft coupling was tricky. We had to learn how to broach a square hole and weld 2 pieces of the coupler together.

I think using the motor controller encoder speed control is excellent; you're telling the motor how fast to go rather than sending a power level that is blind to load and variations in the wheels'/gearbox response to voltage. We pushed chairs around the kitchen at the same speed with no load. Teleop control of the prototype is smoother, more responsive and predictable than any other drive we've tested in 3 seasons; 6 wheel tank, 4-wheel mecanum without encoder feedback.

[MattRain]

Sorry about the delay.

Test Results: (Average of three tests each)

FTC Regulated Foam pads: 20ft exact
Carpet: 19.76
School Tile: 19.5
Home Tile/with bumps: 19.4

Yes, as you change the flooring options, the distance did change. But for on foam tiles, it went the distance. Even though the field wouldn't allow that in a game.

[RRLedford]

DavisDad, regarding the VEX mechanum wheels, I am not sure if their square axle material is considered legal for FTC. We also used these wheels this year and made 1st seed in our 1st regional, captaining the winning alliance. We also received a Rockwell Design Innovation Award (primarily for the drive system) at our second regional. Our robot was very consistently best positioned for flag winding and still able to get back on the bridge in correct location for executing a potential double hang, when possible.

We used Tetrix motors at 1:1 with external encoders and chain direct driven wheel attached sprockets. We were doing consistent autonomous block drop and getting on bridge from the level of precise driving we could get with encoders on our practice field, but at competition, we kept hitting the wall enroute to block drop.

We opted to drill & ream out the VEX wheel square holes to .188" and then used extra long, hardened 10-32 SHCS with unthreaded portion polished for fit to terix sleeve bearings. Protruding overlong Tetrix hub screws engaged into holes in plastic of the VEX 7-spoke wheel hubs, and Shoe Goo urethane adhesive strengthened the hub-to-wheel attachment. Bolt axles free spin in nested channel bearing blocks, and threaded portion captures wheel to adjacent hub/sprocket. Direct driven chain sprocket that is close mounted to wheel is our preferred FTC drive scheme. Driving wheels through axles has proven too unreliable for us.

Double bar H-frame of 20mm Bosch-Rexroth slotted aluminum, using 80/20 10-32 slot nuts. Sliding motor plate adapter and sliding bearing block on opposite sides of T-slot frame allow variable positioning for setting wheelbase length and chain tensioning. This encoder supplemented drive was very precise & reliable in competition and had excellent traction. The VEX wheels are an excellent value.

PICs below:







-Dick Ledford

[DavisDad]

Originally Posted by RRLedford

DavisDad, regarding the VEX mechanum wheels, I am not sure if their square axle material is considered legal for FTC.

Hi RRLedford,

I've been following your work in another thread. Nice work!

To be honest, it never occurred to me that the square shaft axle might be an illegal part. I'll have to look into this. Thanks for the heads-up.

We had initially planed to bore to 3/16" and make a hub, as I thought the VEX design would be prone to strip. After working with the system, I'm comfortable that it'll hold up; hope we can keep it.

Questions from a first year FTC team:

• Have you seen teams using the Matrix 9V motors? If so, was there an appreciable difference in power?
• Are you using the Tetrix motor controller internal PID control for encoders? If so, have you been able to use Tetrix motor controllers for position control? We haven't gotten this to work with Matrix.
• How important is power in FTC for blocking and shoving? The videos I've seen from early scrimmages don't have much "combat".

We custom weld aluminum structures as it's a lot of fun, quick and strong. Anyone interested in Al welding, I'd be glad to post on the subject. We've been MIG welding steel for a while, but had thought Al was difficult; not true with a "spool gun". The welder and spool gun set-up is about $1,000 equipment costs. The up side is that the Al stock is cheaper than vendor structural components. Welding is also good for prototyping as you can tack weld a design together more quickly than bolting and it's easy to grind away the spot welds for changes. We've yet to have a weld failure.

Thanks for the post

[RRLedford]

DavisDad,

I have not seen the Matrix motors used so far. For us they seemed too weak for a 1:1 drive train.

We are using the Tetrix motor controllers with our Tetrix motor-encoder combos. We use LabView for programming.

This year the penalties are rather high and easy to get if you plan to use blocking and pushing for a defense strategy. We wanted the more powerful Tetrix motors for better acceleration to peak speed.

The AL welding makes more sense for our team at the FRC level than the FTC level, especially since we have already accumulated a lot of the Tetrix channel and related parts. It does really annoy me though when I see too many screws and nuts falling off the robot, and wiggling joints negatively impacting performance.

Good luck using the VEX square shafts to drive your VEX mechanum wheels.

-Dick Ledford