CIM Motor Reversing

[gpetilli]

Originally Posted by MrForbes

...or mount the switch so that it is activated as the arm moves past it and hits the hard stop. There are many solutions.

Thats what i meant, but also tried to suggest that it to stays activated while resting on the hard stop. I have seen fast moving arms fly past the limit switch, the motor pauses, and then restarts when it clears the switch. It then powers into the hard stop as if the switch wasn't there.

I think Ether, MrForbes and myself are saying the same thing. Limit switches should be mounted next to the moving arm such that the arm sweeps past the switch; NOT directly in the arm's path such that the arm smacks the switch and uses it as the hard stop.

[swaxman12345]

Originally Posted by Brian C

Electric motors are designed to operate with more effective power in one direction than another. This is done through the timing of where the armature and commutator in relation to the field magnets.

It sounds as if your initial button push is spinning the motor in "reverse" and your second movement is the motors "forward" rotation. If you're trying to stop an arm at a specific point then limit switches or a potentiometer would really be the best option.

Yes, or an encoder and PID if you feel like doing extra work.

[GeeTwo]

Originally Posted by maxnz

That happened enough times that we had to fashion a new 'detector' (the part that is hit and turns on the switch, not sure what it's called) out of a zip tie. It actually worked better than the metal did by not bending out of shape and staying there for the rest of the offseason event.

I've seen this referred to as a whisker or cat's whisker.

[Knufire]

Originally Posted by gpetilli

One way to make the "switch" more robust from damage with fast moving arms is to use a hall effect sensor like http://www.andymark.com/Electrical-p/am-3313.htm an put a small magnet on the arm.

I encourage this as well. We've found hall effect sensors to be much easier to work with than limit switches.

[Ether]

Originally Posted by Knufire

We've found hall effect sensors to be much easier to work with than limit switches.

In what ways are they easier to work with?

[Richard Wallace]

Originally Posted by Ether

In what ways are they easier to work with?

They are non-contacting sensors, so less susceptible to mechanical damage. They can be easier to coordinate with hard stop protection also. However, they do require repeatable clearance from sensor to target at the desired switch point, so their function will change if the arm or frame is distorted.

Generally, a stronger target magnet makes the Hall sensor less susceptible to switch point shifting when clearances change slightly. Sensor manufacturers usually provide details (sensitivity curves, operate/release magnetic field levels, etc.) in their data sheets and application notes.

[philso]

Originally Posted by gpetilli

Thats what i meant, but also tried to suggest that it to stays activated while resting on the hard stop. I have seen fast moving arms fly past the limit switch, the motor pauses, and then restarts when it clears the switch. It then powers into the hard stop as if the switch wasn't there.

I think Ether, MrForbes and myself are saying the same thing. Limit switches should be mounted next to the moving arm such that the arm sweeps past the switch; NOT directly in the arm's path such that the arm smacks the switch and uses it as the hard stop.

Or the limit switch contact closes. Milliseconds later, the arm crushes the limit switch. Another few milliseconds later, the switch contacts open again, never to close again, and the mechanism with over 400 ft.-lb. of torque procedes to destroy itself...