Gear motor rigidity / stiffness / slippage

Other Hardware Motors, Mechanics, Power and CNC
1

I'm working on a project which is a sort of exoskeleton. I'd like to a motor to flex the knee actively, but then to let the user bring it back to extension by pushing in the opposite direction. This means that the motor needs to the rigid and forceful during flexion, but then to relax and allow itself to be passively pushed back. I've worked out a way to do this with a servo, but I'm wondering if a gear motor could also be used.

I bought the gear motor based on this site (Bipolar Transistor HBridge Motor Driver - Robot Room), which referred to a state where the motor is unpowered as "Coast/Roll/Off", and referred to a separate state where the H-bridge activates rotation equally in both directions, as "Brake/Slow Down". But my concern is that, even unpowered, the, motor is stiff enough that I would consider it a "braking" condition.

Is this stiffness just a fundamental property of gear motors, or is there something I can do to relax it and make it easily pushed?

Thanks, everyone.

2

reduction gearing introduces a lot of friction, that's how gears are. The fewer the
steps the less friction and using high quality gears will help (but for metal gears
light machine oil, not grease, is the lubricant to choose). Also reduction gears
multiply the braking effect of the brush friction for a brushed DC motor.

Sounds like you want a low speed high torque brushless motor so less reduction is needed
and there is no brush-friction to overcome. Basically a decent servo motor with configurable
drive torque, or to create one using a PID loop yourself.