Neverest 40 Brake vs Float

Hi everyone,

we have a arm that tilt up and down and we would like the arm to stay in the position that the controller stop.

It is a neverest 40 with 40 tooth gear on motor shaft and 120 gear on the arm lifting device. The motor can handle the motion but when we reach the desired position, it fall back due to the weight of the arm.

Is there a way to program the motor so it stay in position?

We are using MIT APPInventor.

Thanks

Sebastien

The best way to make your arm stay where you want it is to use PID control on the arm to hold it at a setpoint. There’s plenty of documentation on how to do PID loops both on CD and on the web.

I would find a way to hold the position mechanically. You could design using a non-backdrivable gearbox (ex. worm), or you could come up with a brake system to lock the arm in place. Depending on your application you could “offset” some weight on the arm using springs, gas struts, or a counterweight.

I would also look for a mechanical solution. Holding an arm in place with a motor means stalling the motor, and a lot of motors don’t like that - they could burn out on you. That problem increases as the weight the motor has to hold (arm + game piece typically) increases. Decreasing that weight makes it much easier for the motor to hold- if the weight was low enough, I would be comfortable just stalling the motor. For example, in 2013 we had an arm that would move a set of hooks from one level of the pyramid to the next. The arm weighed very little, it wasn’t lifting game pieces, and only had to hold its position briefly before the winch would take over and pull on the hooks. So, we let the motors handle it that year.

I would start by looking for a way to coubterbalance the arm - weights or springs can work wonders, and give you almost a neutral weight for the arm! After that, mechanical brakes are certainly a good option. I’ve seen teams use disc brakes from bikes (light weight, able to hold a large force, and easy to work with a pneumatic piston). Using something similar to the ratchet-modified gearbox a lot of teams used in 2015 is also an option, but is a little less flexible and needs a little more thought to make work properly.

Maybe one of my favorite arms from a few years ago (I can’t remember the team it is as on) used a 500-series motor driving a long worm drive that turned the gear on the arm pivot. Simple, worked well with their design, got the weight of the motor and gearbox in the bottom of the robot, and prevented the arm from back driving.

That would work fine as long as the arm and gearing doesn’t have enough torque to strip the worm gear.

I’m not sure the status of FTC pneumatics, but it could also be possible to use a servo actuated brake. (I think 68 used one many years ago, and I know 25 used a pin through their gear to act as a break too) Just jam a piece of metal across a gap such that the gear teeth hit that before rotating. Dirty but if you can’t get a friction brake working it might be passable. The trick is making sure whatever material you are jamming your gear/sprocket with is taking the load and not your servo.

Use a constant force spring(aka gas shocks) to balance some of the arm weight, put a potentiometer or encoder of some kind on the arm shaft, then tune it to move to a spot and hold position. I am a very big fan of gas springs/shocks, there are a fantastic way of supplying a persistent amount of force to a mechanism. In this case you would be using them to make your arm almost weightless and has the added bonus of acting like a motion damper to slow down any rapid motions.