Robotic Gripper Control

What is Robotic Gripper Control?

Robotic Gripper Control specifically means control methods associated with getting a robotic 'hand' to grip an object safely - applying neither too much or too little force. Too much and the object may be crushed or damaged, and too little and the object may be dropped during robot arm motion. Looking at this problem more generally though this combined position and torque control problem is relevant to a broad category of applications including screw cap applicators, cobots, press fit equipment, surgical robotics, and more.

Design Considerations for Robotic Gripper Control

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).

Gripper Control Application Image 7-01


Vital Robotic Gripper Control Techniques

  • Combined position torque control
  • High precision current measurement
  • Cascaded position/torque control loop
  • Strain gauge signal input
  • Torque limiting
  • Encoder input
  • Outer loop controller

The second approach has the advantage that once engagement occurs the gripper location can continuously change so as to continuously apply the threshold force. This may be important if the gripped object's shape or engagement point with the gripper changes during motion.

In any case, what makes either of these techniques work is two things - first the use of a motor/actuator mechanism with relatively little friction or gearing. This allows the motion servo to 'feel' the resistive force at the gripper by measuring the motor torque command output. The second is a cascaded position/torque loop, shown diagrammatically in the architecture diagram below. This type of control loop  generates move profiles but also precisely measures motor coil current using Hall sensors or current sensing resistors.

It's worth nothing that mechanical actuator systems which do have high friction or large gear ratios can still control force by adding a load cell sensor (also called a strain gauge) sensorto explicitly measures force. This requires a somewhat different control loop known as an outer loop controller but the principle is the same. A move is commanded and the resultant force is measured and limited via the signal from the load cell.

Application Architecture & Approach

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.

Gripper Control Diagram V1-01


Robotic Gripper Control Solutions from PMD

Since 1994 Performance Motion Devices products have been used in a range of force control applications including robotic grippers, automated fastening systems, surgical robotics, and virtual motion systems. PMD’s ultra-compact and powerful N-Series ION Drives is especially well suited for robotic gripper control, as is the MC58113 Motion Control IC family. For applications that require force control but not positioning control PMD's Juno IC family is another cost effective solution well suited to the task.

Machines That Frequently Use Robotic Gripper Control

  • Screw Cap Applicators
  • Automated Fastener Systems
  • Door Control
  • Automated Turnstile Control
  • Cobots
  • Press Fit Machinery
  • Teleoperation
  • Surgical Robotics
  • Virtual Motion Systems
  • Entertainment Robotics
  • Animatronics
  • Exoskeletons
  • Patient Assist Robotics

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