How to calculate the force created by the servo motor using the rosserial_arduino library

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There are two ways I can think of to measure the force or torque of a motor.

1) Measure the current with a Hall effect current sensor. Torque is proportional to current. Or go "openlop" just record the PWM duty cycle and assume current and force are proportional to PWM.

2) Use a resilient mount for the motor so that the motor moves against some kind of spring or rubber as it applies torque then you measure the motor's displacement. ANy kind of spring in the link can be used. One system I say used a flexible shaft coupler and measure the amount of "twist" Also, the reaction movement need not be large. You can measure torque with a stain gauge or load cell