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The key difference between motion control of motors used to transport objects from point A to point B and motion control of motors used as grippers used to grasp and perhaps manipulate objects is the additional requirement for force control. In the robotic gripping control problem we care just as much (and often more) about the force applied by the actuator as the position of the actuator.
When operating a robotic gripper the object being carried may vary in size or orientation. This results in a variety of mechanical engagement points and so we are likely to need to move the gripper just to reach an initial engagement point with the object. Once contact is made the gripper generally seeks to apply a consistent force on the object – large enough to hold it securely but not so large as to damage it.
How can this kind of motion control be achieved? There are several techniques but two popular approaches could be described as 'Stop Trajectory When Force Threshold Reached', and 'Continue Trajectory But Limit Force Output'. What these techniques share in common is that they provide positioning control in combination with force control.
In the first approach the gripper is controlled by a servo PID loop. At the start, the gripper is not in contact with the object and begins a trajectory move to approach the object to be gripped. As the move proceeds, at some point the gripper begins to make contact and this is sensed in the servo loop by an increase of the motor command torque needed to follow the commanded trajectory (minimize the servo position error). Once the desired grip force (as determined by the torque output of the motor) is achieved the motion profile and therefore gripper motion is halted.
In the second approach the gripper similarly approaches the object using a commanded trajectory move but the servo loop does not monitor the force output of the servo loop. Instead, a current output limit is installed in the servo loop so that even if the position PID tries to follow the trajectory, the force it can apply is never greater than the desired force threshold (the target gripping strength).
The diagram below shows a cascaded position/torque control loop often used in force control applications. When the gripper or actuator has not yet made contact with the target object the Torque Command is generally very small because nothing (yet) resists the motion of the actuator. Once contact is made the Torque Command increases rapidly as compression of the gripped/contacted object creates a countervailing force to the position control loop. By limiting or monitoring this torque output precise torque control can be achieved, whether for a robot gripper, a screw cap applicator, or any application where a specific torque is needed.
