Anyone know of a good (and economical) way to make a motor controlled arm become a passive link on trigger? That is, the joint can be active (motor controlled) or passive (rotate freely). The tip of the arm weights 3kg and the length is 15cm. There's a 5kg load as well. I suppose the motor and gearbox will need to provide around at least 25Nm torque. A gearbox with that much torque would be very difficult to back drive.

I guess one way could be to use an electric clutch, but electric clutches are expensive and I don't think I have enough battery capacity to power the electromagnets. Another way I've thought of is to use planetary gears by braking the ring gear with a servo-controlled ratchet to make the link passive. But planetary gears are hard and expensive to make as well. Anyone know of any alternative?