DC motor control, de-accelarating to stop

[triumph406]

Hi,
I would like to control a DC Gearmotor.

What I have done in the past is to have 2 limit switches that limit the travel of a part attached to a leadscrew. The limit switches kill the power to the motor to stop the mecahanism.

However the precise point at which the mechanism stops is important. Motor Inertia complicates this, and setting the limit switches to account for run-on is not precise enough.

I have tried a third limit switch in the circuit (about .75" before the end of the travel), wired NC, with resistors across the NO terminals. When the device hits the limit switch, the switch opens, and the resistors are now in the circuit, this slows the motor down (depending on resistance), and it’s arrival at the last limit switch is slower and more predictable.

An option I haven’t tried is too add a relay wired to the last limit switch, so that when the device hits the last limit switch, the relay is energized which will short out the motor and bring it too a halt in a few turns of the armature.

I have found that the addition of a third limit switch not particulary reliable. I’m also not convinced that having a relay shorting the motor at the end of the travel will be very reliable either.

I guess my question is:
1) is there a better way to do this with a 12VDC motor?
2) is there a COTS controller available (cheaply) that could do all this? i.e work with limit switches and have deacceletaion points to de-accelerate to a limit switch.
3) what have other people on the forum done to solve this dilemma?

Thank you for any help

 

[nbucska]

There are several possible solution depending specs.

Plesae read FAQ240-1032
WEB: <

http://geocities.com/nbucska/>

 

[BobM3]

Dynamic braking will stop the motor quickly.

 

[MikeHalloran]

Shorting the motor for dynamic braking is pretty brutal. Better to put a resistor across it.

Still better, given that you need position control, is to add feedback and closed loop control.

I know, you want it 'cheap', but industrial strength switches and relays are not coming down in price, while motion control is. Check your assumptions.





Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

You can put a free wheeling diode across the motor. The current will be limited by the motor's internal resistance and will be equal to a short circuit minus the forward drop of the diode. If that is too brutal you can add a resistor in series with the diode. If you reverse the field connections to reverse the motor you can leave the diode in the circuit. If you have to reverse the polarity to the motor, connecting the diode to the motor before disconnecting the power is a little more difficult but still possible.
What Mike said about checking motion control may be starting to look like a more attractive avenue.
yours

 

[blacksea]

Further two last postings, a motion controller with build-in amp* have two special Limit switch inputs (for both directions). The decceleration with activating of ones is significant higher vs normal operation.

*See Galil (

http://www.galilmc.com)

or Elmo (

http://www.elmomc.com)

for example.

 

[triumph406]

Thank you all for your help, looks like motion control may be the way to go. I'm trying to keep it simple and reliable, I'll post on the forum, what I ended up using, and the end result. Thanks again