PID control for a robotic gripper

Hi everyone, Im working on a control system for a robotic gripper to control the gripping force of the gripper by sensing the amount of current the motor is drawing. Im utilizing a dc motor as the actuator. The aim of the project is to control the grasping force of the gripper in order to not damage the object its gripping. Need some advice on where to start on this project. :)

dnth: Hi everyone, Im working on a control system for a robotic gripper to control the gripping force of the gripper by sensing the amount of current the motor is drawing. Im utilizing a dc motor as the actuator. The aim of the project is to control the grasping force of the gripper in order to not damage the object its gripping. Need some advice on where to start on this project. :)

Current would be proportional to torque on average during the motor's revolution, but the instantaneous relationship might not be at all straight forward and I suspect for typical DC motors the relationship would vary hugely as different poles came in and out of alignment. You would also need to know how the mechanical leverage of the motor varied with position - in some systems the leverage might be constant but in others it could vary a lot.

If you manage to overcome all those issues, it sounds as if what you're trying to produce would be a constant current supply. And you can create that electronically without any clever programming.

The ZXCT1009 is used in the open servo project for current monitoring.

http://www.diodes.com/datasheets/ZXCT1009.pdf

Im varying the torque of the motor using PWM after getting feedback from the current sense resistor (using L298 IC). May i ask what is the purpose of the diode posted above in layman's term? thanks. Any reference on the net u guys came across that can possibly assist me in this project?

dnth: Im varying the torque of the motor using PWM after getting feedback from the current sense resistor (using L298 IC). May i ask what is the purpose of the diode posted above in layman's term? thanks. Any reference on the net u guys came across that can possibly assist me in this project?

Have you measured the relationship between torque and current with the motor in different positions to see if the relationship is linear? All the DC motors I can think of have been very 'lump' in terms of torque characteristics when stationary. Unless the torque is constant at all shaft positions I think you will be flogging a dead horse.