Taking the hand (palm+fingers) to act as the load, I want to bring about wrist flexion and extension using a pulley mechanism mounted on the forearm for the purpose. Considering the load= 0.7kg and torque=0.49Nm and the rate of flexion/ extension to be 5 degrees/s , small size and light weight motor, which type of motor should I go for?
DC shunt motor, self speed regulation through wounded shunt motor ..
Speed is directly proportional to applied voltage, hence can be adjusted as needed, up to rated speed at rated voltage, with constant torque you needed. Permanent magnet D.C motor, with push button control, would be cheaper compare to servo motor, for your requirement in process, as not very accurate control is needed.
A servo motor will provide the torque and speed that you need and weigh in at less than 50g and cost less than $20. Plus it's super easy to control with an Arduino or other cheap microcontroller.
If the subject is a 'dead weight', all suggestions here might apply, but:
- speed is only proportional to voltage for a DC motor with a constant load. A living person is almost never a constant load. Also check whether you've taken this into account for your torque / weight requirements.
- A servo is nice, but you don't know what the inner control loop is, and most servo's can not handle continuous overload nicely. This might be a good choice if the servo is powerful enough (most RC servo's aren't for a hand).
- A stepper motor is very heavy , but does produce quite a bit of torque without a gearbox. However: stepper motor drivers are open loop, so you never now whether you've missed a step. Maybe your subject could overdrive the stepper motor...
Also take safety into account, so think about how you can limit / disconnect the motor when your subject is not cooperating or when your control goes wrong.
The R/C servo type motor with high torque would be the ideal choice as it is even easier to control its position accurately with a simple PWM signal drive.
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