DC Motor/End of Travel

[reggie]

I have an application where a DC motor drives a leadscrew. At the end of travel, there is no limit switch. The current design uses a poly-switch to determine the end of travel has been reached. I'd like to change this to something a bit more fast acting. My first thought is to use a current sensing ckt. (cheap op amp and a few resistors). Any other ideas? Thanks.

 

[cbarn24050]

Hi, current sensing will work but you have to remember that you will get a surge on startup so you have to account for that in some way. Also there will be a lot of tourque at the end of travel which could cause mechanical problems you might need some kind of slipping clutch in the drive train. Its much better to shut off the drive before it reaches a machanical end point with some kind of sensor.

 

[Lewish]

Another option, which is used in the lift control for many treadmills, is to gear a potentiometer to the shaft, such that full travel of the lead screw equals one turn of the pot. Then you just need to sense minimum or maximum resistance across the pot. Be careful to allow some margin at the ends for changes due to temperature, etc. Most have a 10 per cent safety margin. So, maybe it trips at 1000 ohms at the minimum and 45,000 ohms for the maximum for a 50Kohm pot. Then you use an opamp comparator to sense the 2 conditions and stop your travel.

 

[nbucska]

Give me a FAX # for schematics <nbucska@pcperipherals.com>

 

[jbartos]

Suggestion: Try LVDT (Linear Voltage Differential Transformer)