Using IMEs on the ClawBot to eliminate "sagging" of the arm

Bpalms · November 22, 2015, 4:07pm

AppleDavidJeans:

I have built a ClawBot like this one for my class’s Nothing But Net challenge. It’s going to get an overhaul, but I have a question that I’d like to float up. (the overhaul’s irrelevant to this question.)

Let’s take the arm in this picture:
http://www.vexrobotics.com/media/catalog/product/cache/11/image/296x/5e06319eda06f020e43594a9c230972d/c/l/clawbot-3ql.jpg

Ideally, the arm should be moving up and down flawlessly every time, right? But in reality, with my ClawBot, the arm continues to move downward for a bit after I’ve moved it in the downward direction, after the robot is no longer commanded to move its arm, as if gravity were pulling it down. It does the upward direction fine.

Here’s where my question comes in: Would installing an IME onto the arm motor fix the problem? My theory is that the error-based correcting algorithm of the IME should catch this drift and immediately apply counter-torque to keep the arm from dropping, or “sagging” as I call it.

Would this work?

An encoder would simply allow you to determine the position of the arm (and therefore speed). If you program an algorithm like PID (or even just a proportional controller) to use this information to send the correct power levels to the motors in order to eliminate the “sag” and improve performance of your arm.

If you have RobotC version 4, PID is already built into it. More information on that in this thread:

There are also many other threads on this forum about PID that you could check out, or ask me if you have any more questions.