CIM Motor Reversing

[GeeTwo]

Originally Posted by cbale2000

To add to this, in a pinch, you can add a dead stop to the mechanism and then program it to slightly overshoot where you want it to go and always hit the dead stop, thus returning it to a consistent starting point. You do briefly stall the motor by doing this ..

If you go this route, also monitor the current on that motor. A spike in current (other than during startup) will alert you to hitting the stop. Not elegant, but better than nothing.

[aeastet]

Encoders are easy to use and take no time to install. Look into that before doing something like dead stopping your arm. That could cause many other issues with voltage spikes and brownouts.

[gpetilli]

Originally Posted by aeastet

Encoders are easy to use and take no time to install. Look into that before doing something like dead stopping your arm. That could cause many other issues with voltage spikes and brownouts.

Encoders are not hard, but limit switches are even easier - especially with the new SPARK speed controllers which have limit switch inputs. No software!!! You dont even need to stop driving up or down based on time, just leave the motor drive in the direction you want the arm. The limit switch cuts the power at precisely the right time. I strongly recommend limit switches tied directly to a speed controller (Spark, Jaguar or SRX) for this particular problem.

Also, a previous post talked about loads. Not knowing your system, I am going to make a wild guess that the first direction arm moves is "up" (against gravity) and the second, faster direction is "down" (with gravity). Gravity always wins - common design challenge with rotating arms.

[maxnz]

Potentiometers work too, but they can be harder to program.

[Ether]

If you decide to go with limit switches, make sure to mount them in such a way that they don't also serve as a hard stop.

Common mistake.

[Zebra_Fact_Man]

Originally Posted by Ether

If you decide to go with limit switches, make sure to mount them in such a way that they don't also serve as a hard stop.

Common mistake.

Seconded. You'll destroy your limit switch if you do.
-Knowledge from experience.

It boggles my mind that there are still people that think motors/gearboxes/axles are going to perfectly turn in opposite directions at the same speed/distance/force.

[maxnz]

Originally Posted by Zebra_Fact_Man

Seconded. You'll destroy your limit switch if you do.
-Knowledge from experience.

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.

[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.

[gpetilli]

Originally Posted by Ether

If you decide to go with limit switches, make sure to mount them in such a way that they don't also serve as a hard stop.

Common mistake.

Agreed. I have seen many smashed/mangled limit switches. This can be a huge issue, especially if the arm is moving fast. Locate the switch trip point about a half inch before the hard stop, such that when at the hard stop the switch is still tripped.

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.

[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.

[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.

[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...

[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.