Tiny high torque actuator/sensor design

Question

I need to assemble a small (about 8cm x 5cm x 5cm maximum), actuator with as much torque as I can get at that size. It will be driving a small reel and pulley (reel is ~1.25cm^3, 5mm center diameter), and needs to respond to load (eg. stop if the load increases beyond a certain threshold). Power to the actuator will be provided via a common bus line, so the space limit isn't further limited by the size of the battery.

My thought is to use a worm drive for this (for torque) and watch for change in current/voltage (for load), but I'm not sure if that is mechanically sound. It seems like the mechanical advantage provided by the worm would make it hard to detect a change in load.

Plan B

I could add another sensor that would gauge the force being exerted. I'd prefer to avoid adding too many points of failure to the system, but if I did what sort of sensor would I use?

How should I approach this?

Answer 1

With a series of planetary gear sets aligned axially, one can gear down by high ratios. For example, the picture below (a wikipedia commons image used in the planetary gear article) shows a 2.5-cm gearset with ratio -5/352, about 1:70. Stacking three of these would give a ratio of about 1:343000. Some torque and power would be lost to friction, and plastic gears would not be strong enough to deliver all of the torque possible at the output, but the concept should be clear. If multiplying motor torque by 1/3 of a million isn't enough, add a fourth gearset, giving a torque multiplication from motor to output of about 24 million. Note that rotation speed would be correspondingly divided (by about 24 million) and only about 1/2 to 3/4 of the motor power would arrive at the output, due to friction losses.

To stop turning the pulley if load gets too high, you could use a torque-limiter (or, safety clutch or slip clutch) at the output.