IMEs in Autonomous

So I am considering using 4 IME motors on my drive. During autonomous, I’ll probably use some sort of autocorrecting PID loop, but is this a good idea? I’ve heard that the fields at worlds can reset IMEs with static; is there any way to prevent that? Also, I’m curious, what do some top teams use in their autonomous, like gyro, encoders, or IMEs?

Don’t use IME’s, they’re famous for resetting at very random times and also not being as accurate as you’d think. You’d be better off with one of the red optical shaft encoders on each side of the drive. Most “top” teams, to my knowledge, use these on the drive.

  1. Why would you need 4 IMEs?
  2. Use quadrature encoders.

Please never use IMEs. They have burnt out my I2C ports and literally started smoking and blew up my motor. Also, at worlds they use an egregious amount of anti-static spray, to the point that it makes extra friction. If you need an IME functionality but even better please use quadrature encoders. In regards to your curiosity: I use 2 quad encoders on my drive, 1 quad encoder on my mobile goal lift, 1 quad encoder on my dr4b, a potentiometer auton selector, and a gyro. (and I am hoping to be able to add a quad encoder onto my secondary 4 bar). PID however, is the best thing ever and you should definitely learn it to increase autonomous speed, accuracy, and consistency.

Is your drive chained, and if not which motors do you have your encoders on?

EDIT: The reason I thought I would need 4 is so that each motor would be going at the correct speed, virtually a chained drive but with no chain.

Don’t waste your times with IMEs. They reset randomly and are not accurate. Use quadrature encoders, you’ll be much better off.

@Baguette123 My drive is not chained due to the inherent issues with using chain on your drive. VEX chain has a slack and slop problem which causes extra friction in the system and more strain on the motors causing the PTC’s to trip more often. I use the quadrature encoders on the back wheels since they never have to go over the 10 point bar(at least on my robot they don’t). I just send the front wheels the same value as the back wheels, I have not noticed any issues with this.

Why do you need an encoder on each side of the drive?

@Naman I use an encoder on each side of the drive as a failsafe. If the encoder isn’t changing when it should then it switches to the other encoder. I also use independent PID on the sides of the drive to ensure that each side is traveling at the same speed (given that both are working, if they aren’t then it just goes off of the working encoder). I could use a feedback loop with the gyro to ensure it is straight (which I do) but I prefer to have feedforward programming to make sure that it works correctly in the first place.

More accurate readings, and you could theoretically use them to make sure your autonomous routines drive perfectly straight (although a gyro is arguably better for that purpose).

That and if you have an encoder on only one side, there will be a tiny marginal amount more friction on that side, which over long distances add up. I don’t think this matters too too much, but it’s worth noting.

DO NOT, I REPEAT, DO NOT USE IME’S ON YOUR DRIVE FOR COMPETITION. At CREATE US OPEN, all of our autons failed to work reliably because of the static to the IME’s. Even with anti-static, our IME’s have been given false readings countlessly on the field, making us lose a great amount of autons. If you are going to the world championship, DO NOT USE THEM, AND DO NOT USE THEM ESPECIALLY BECAUSE OF THE BO1 UPDATE.

OK thanks I figured out how to mount encoders on my back wheels.

In HS I used IMEs on our worlds bot in skyrise. Every worlds robot since then has used two shaft encoders. They work a lot better and are easier to wire and the code is easier to transfer over year to year.

Not on topic of IMEs, just commenting on the use of chain. Some people are going to learn to love chain in the next few months. Given the motor limits for V5, I imagine we’ll see a lot more chain drives next year. There are, of course, other ways to do it: front or rear wheel drive, mid drive, drop center drive. But I imagine we won’t see many 4 motor V5 drives.

@kypyro first off I am a senior so it won’t really matter. Nevertheless I think we will see a lot of 4 motor V5 drives since a lot of teams use 6 motor drives currently. Either way, I did a test with having chain on my drive vs gears vs direct and the difference between gears and direct wasn’t huge but using chain (I tried a lot of different tensions) either drastically decreased power, speed, or slop(slop was with very loose chain that was falling off the sprocket). I would prefer to use front/back wheel drive instead of chain.

Edit: This is only for drives. Chain is very useful in other places in my opinion. I would just steer clear of it on drives.

You could very well be right about their being many 4 motor drives; I guess it depends on how complex the manipulation of game objects is going to be. I was mostly thinking you just won’t need the extra torque on the drive, and you might want lots of motors for other things. (lifts, intakes, launchers, etc.) But if it turns out that not much manipulation is required, then 4 motor drives could be quite common.

I haven’t been a big fan of chained drives either, with it either being sloppy loose or too tight and adding friction; neither seems good. But some people seem to use it effectively.

I learned very quickly from Sack Attack/Toss Up - IMEs are the worst. The literal worst. Never use them unless it’s a last resort.

That said, I agree that chain drives will be more prevalent with V5. All the teams I have currently use chain drives. Most teams in I see here use Chain Drives.

We had terrible trouble with IME’s as position sensors on lifts and as location sensors on wheels. However, they worked great as compact velocity detectors in NBN for flywheel control. The reason being, the team was taking the difference between the latest and the last IME readings every 20ms, instead of absolute position, so if the IME glitched then that reading would be wiped out of the history in just a few samples.

We had more than 5 IME’s on our previous robot. Huge mistake. 8/6 do not recommend.