The Gripper That Crushed the Plastic Parts
We used a parallel gripper (pneumatic) to pick up plastic housings. The gripper had adjustable fingers. We set the air pressure to 6 bar (standard). On the bench, the parts were fine. On the floor, about 5% of the plastic housings had indentations on the sides — the gripper squeezed too hard. The problem: the gripper’s force at 6 bar was about 120 N. The plastic housing only needs 30 N to hold (it’s light, 200 g). The extra 90 N crushed it. We reduced the air pressure to 2 bar (using a regulator dedicated to the gripper). The gripping force dropped to 40 N. The parts held without damage. The mistake was using the main line pressure (6 bar) for a delicate part. The gripper needs its own pressure regulation.
Robot gripper force control is about holding without damaging. Too much force crushes the part; too little drops it. This article covers the options.
Gripper Force Sources
Pneumatic Gripper (Fixed Force)
A pneumatic gripper has a piston. The gripping force is F = P × A (like a cylinder, article 53). The force is set by the air pressure. But it’s fixed (no closed-loop control). The gripper applies the same force every cycle.
Best for: Rigid parts (metal, glass) where a fixed force is fine.
Pneumatic Gripper with Regulator (Adjustable Force)
Add a pressure regulator on the gripper’s air supply. Turn the pressure down for delicate parts. The force is adjustable (manually). Still no closed loop, but you can tune it.
Best for: Most automation. Cheap and adjustable.
Servo/Electric Gripper (Force-Controlled)
An electric (servo) gripper has a motor that controls the force. The PLC commands a gripping force (e.g., 50 N), and the gripper closes to that force. Closed-loop. The force is programmable (different parts = different forces).
Best for: Delicate parts (electronics, soft materials), multi-part cells, where the force must vary.
| Gripper Type | Force Control | Cost | Best For |
|---|---|---|---|
| Pneumatic (fixed 6 bar) | Fixed, high force | Low | Rigid parts |
| Pneumatic + regulator | Adjustable (manual) | Low | Delicate parts, tuned |
| Servo/electric gripper | Programmable, closed-loop | High | Multi-part, delicate, force-sensitive |
Step 1: Estimate the Required Holding Force
The gripper must hold the part against gravity and acceleration. The holding force (normal force on the fingers) is:
F_hold = (m × (g + a)) / (2 × μ)
Where:
- m = part mass (kg)
- g = gravity (9.8 m/s²)
- a = robot acceleration (m/s², about 10–20 for fast pick-and-place)
- μ = friction between the finger pad and the part (about 0.3–0.5 for rubber pads)
- The 2 = two fingers (two contact points)
For m = 0.2 kg (200 g), a = 15 m/s², μ = 0.4: F_hold = 0.2 × (9.8 + 15) / (2 × 0.4) = 0.2 × 24.8 / 0.8 = 6.2 N. The gripper needs about 6 N (plus safety factor, so 15–20 N).
The pneumatic gripper at 6 bar delivered 120 N — way over. Set the regulator to deliver 20–30 N (about 1.5–2 bar). The part holds without crushing.
Step 2: Safety Factor
The calculated F_hold is the minimum. Add a safety factor (2–3×) for real-world variation (slippery parts, robot acceleration spikes, part variations).
For F_hold = 6 N, use F_grip = 15–20 N. That’s enough to hold the part, not enough to crush it.
The gripper force rule: Calculate F_hold = m(g+a)/(2μ). Add 2–3× safety factor. The gripper that crushed plastic parts was at 6 bar (120 N) against a 15 N requirement. Add a regulator on the gripper and turn it down. For multi-part cells, use a servo gripper (programmable force).
Step 3: Soft Finger Pads
Use soft finger pads (rubber, silicone, or polyurethane) on the gripper fingers. The soft pad:
- Increases friction (μ higher, so less normal force needed).
- Distributes the force over a larger area (lower pressure, less mark).
- Conforms to the part (better grip on irregular shapes).
For a plastic part, a silicone pad (Shore 40A) holds with less force and doesn’t mark. For a metal part, a harder pad (Shore 70A) or steel pad is fine.
Step 4: Force Limiting (Current Limiting)
For a servo gripper, the force is controlled by the motor current. The gripper closes until the motor current (torque) reaches the setpoint. When it touches the part, it senses the force and stops. This is “soft gripping” — the gripper holds without crushing.
For a pneumatic gripper, add a pressure regulator (and a flow control) to limit the force. The pneumatic gripper can’t sense the force, but the regulator sets the maximum.
Step 5: Part Damage Inspection
After setting the force, test on actual parts. Inspect for marks (indentations, scuffing, deformation). If there are marks, reduce the force. If parts drop, increase it. Tune on real parts, not just the calculation.
A Gripper Force Checklist
- What is the part mass? (kg)
- What is the robot acceleration? (m/s²)
- What is the finger pad friction? (μ)
- Required F_hold = m(g+a)/(2μ)?
- Safety factor? (2–3×?)
- Is the gripper force adjustable? (Regulator or servo?)
- Is there a dedicated regulator on the gripper? (Not main line?)
- Finger pad material? (Soft rubber for delicate parts?)
- Are parts inspected for marks after setting? (Tune on real parts?)
- For multi-part cells: is the force programmable? (Servo gripper?)
- Is there a part-present sensor? (Confirms grip?)
- Does the gripper sense dropped parts? (Force or vacuum?)
The Bottom Line
Robot gripper force control is holding without crushing. The gripper that crushed plastic parts was at 6 bar (120 N) against a 15 N requirement. Calculate F_hold = m(g+a)/(2μ), add 2–3× margin, and set the gripper force (with a regulator or a servo gripper). Use soft finger pads to increase friction and reduce marking. The gripper that picks every part without damage wasn’t the strongest one — it was set to the right force.
