A pinch gripper linkage is a mechanical system that lets a robot grasp small objects using two opposing fingers. In many designs, one servo motor rotates a horn or crank, and the linkage converts that rotation into equal inward motion of both fingers. This matters because a simple, lightweight gripper can pick up parts reliably without needing a separate motor for each finger.
The linkage geometry controls how far the fingers move and how much gripping force they can apply.
Key Facts
- Servo torque relates to output force by tau = F r, where r is the effective lever arm.
- Mechanical advantage can be estimated by MA = F_out / F_in.
- For symmetric gripping, the left and right finger linkages must have matching lengths and pivot locations.
- A crank converts servo rotation into linkage motion, and a rocker converts linkage motion into finger rotation.
- Finger closing speed is not constant because linkage angles change the velocity ratio during motion.
- Grip force usually increases when the linkage approaches a toggle position, but motion range decreases near that point.
Vocabulary
- Pinch gripper
- A robotic end effector that holds an object by squeezing it between two opposing fingers.
- Servo motor
- A motor with position control that rotates to a commanded angle using internal feedback.
- Linkage
- A set of rigid bars connected by pivots that transfers and transforms motion.
- Mechanical advantage
- The ratio of output force to input force produced by a machine or mechanism.
- Pivot
- A joint that allows one part of a mechanism to rotate relative to another part.
Common Mistakes to Avoid
- Using servo torque as the finger force directly is wrong because torque must be divided by an effective lever arm to estimate force.
- Assuming both fingers close symmetrically without matching the linkage geometry is wrong because unequal link lengths or pivot locations create unequal motion.
- Ignoring changing linkage angles is wrong because force and speed vary throughout the gripper stroke.
- Designing the gripper to clamp exactly at a locked toggle position is risky because the mechanism may jam or fail to release under load.
Practice Questions
- 1 A servo provides 0.80 N m of torque to a crank with an effective radius of 0.020 m. Estimate the input linkage force using F = tau / r.
- 2 A gripper linkage has an input force of 40 N and a mechanical advantage of 2.5 at a certain position. What output grip force is applied at one finger?
- 3 A single-servo pinch gripper closes unevenly, with the left finger moving farther than the right. Explain two linkage design features that could cause this and how you would correct them.