Electronics > Mechanical & Automation Engineering

Is there a motor/drive system with intentional slip?

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I want to replace a clock spring cable retractor with a motorized spool. The retractor has to reel up 50 feet and any mechanical retractor I find is too uneven in force over that distance and also occasionally jams, which in unacceptable since it will be unattended. It seems like this could be done with a motor, and might even be cheaper than coil spring mechanisms which are about $200 for industrial grade versions. The idea is to create 1 pound or so of "drag" force on a spool as the wire is pulled out and let back in. The motor driving the spool would have to allow slip while still trying to reel in the cord. Any ideas what sort of motor/driver can do this?

Shaded pole AC motors can be designed to slip indefinitely; just producing heat.  They are often used in HVAC duct fire dampers where they are used to wind up a spring and stay powered to keep the damper open.  When power is removed, the spring closes the damper. 

Seems like an obvious job for a DC motor with brake resistor.
When pulling out the cable, the brake resistor will provide mechanical drag/resistance.
When pulling in, current limiting will limit torque if the cable jams.

Sounds like any motor type able to cope with long term continuous stalls would be suitable, using a gearbox if necessary to increase torque up to the "pound" (torque = pound * what radius) you want. I am guessing you'll run the motor in the retracting direction at stall "all"* the time, if the force pulling the cable out reduces below that which the stalled motor provides then the cable starts retracting. You could also perhaps supply a higher voltage to the motor at times when you want a higher stall torque, to actively pull the cable in, and a lower voltage when you only want the stall to serve to apply force (for plenty of motor types the stall torque, and stall current when stalls occur, rise with the voltage supplied). Some types of BLDC motor could be suitable, especially with the use of field oriented control electronics to drive them, this site has some exmaples of FOC with BLDCs https://odriverobotics.com/ you might not need something so high performance though. Unless I've misinterpreted your requirement slipping isn't really involved anywhere, just stalling where the motor applies enough force to keep the cable tensioned but not enough to resistwhatever force on the cable end is causing it to extend.

*you might have some logic which tells it not to bother at times when the cable is already fully retracted as determiend by a sensor somewhere

What you need is a torque motor.


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