sensorless motor stall detector?

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rob_fed

Member - Posts: 15

Joined: June 10, 2004

Location: United States

Posted: September 09, 2008 at 6:13 PM / IP Logged

I have been experimenting with a few small gear head motors, I have managed to motorize a hinge on small lid that covers a compartment centered on my dash. I used two micro switches to detect whether the lid is full open or full closed. I was wondering if someone had a circuit that would interrupt the power to a motor when it stalls out due to a physical load. From memory, a power window module that hooks up to an alarm seems to perform the same function. But if there is a quick and simple way to detect when a motor is stopped when power is still applied, I could then build off of this concept.

Thanks - rob

KPierson

Platinum - Posts: 3,527

This member consistently provides reliable information

Joined: April 14, 2005

Location: Ohio, United States

Posted: September 09, 2008 at 8:05 PM / IP Logged

One way to go about it is with a very low resistance shunt resistor between ground and the motor. Something like a 0.005 - 0.025 ohm resistor would work. You can then monitor the voltage at the junction between the resistor and the motor. Due to Ohm's law, if the current of the circuit goes up while the resistance stays the same, voltage must change. You can set a voltage threshold with a comparator circuit (or an A/D converter) to kill power after the motor has reached the preset current.

You may run in to issues at start up, so you may need to set two different thresholds - a start up and a running value. That way when the motor first starts and the current spikes the circuit won't shut the output off (but if the motor is already at the end of travel then it won't keep pushing because it will be monitoring the higher preset).

I've never actually built a circuit like this but I've always wanted to. You can actually find formulas on the net to create the shunt resistor with a copper trace on a circuit board.

Kevin Pierson

noisy crow

Member - Posts: 13

Joined: April 06, 2008

Location: Ontario, Canada

Posted: September 10, 2008 at 5:35 PM / IP Logged

Another way.... stick a timer on it. In fact, you can probably get rid of the limit switches if you use a timer. Just run the motor for 5 seconds (or whatever works) and then have turn it off. With a two or three relays

If you search the site for "relay timer" there a bunch of good ideas... like this one: https://www.the12volt.com/installbay/forum_posts.asp~TID~105514~PN~1

rob_fed

Member - Posts: 15

Joined: June 10, 2004

Location: United States

Posted: September 15, 2008 at 1:01 AM / IP Logged

Thanks for the replies. Both the comparator and timer ideas are great. But I was still looking for something that was slight less complicated. I think I have a circuit that is working, but not fully tested. It consists of a latching relay in series with motor. The motor stalling causes the latching relay to latch which opens up the power to the motor. I will need to test further and will post a diagram appropriately.

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sensorless motor stall detector?

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