To stop a robot gripper dropping wet or delicate food, do not begin by simply increasing squeeze force. Combine a wet-contact surface that can develop adhesion, compliant fingers that spread pressure, feedback that detects contact or incipient slip, and—when friction is inherently unreliable—mechanical support from underneath. Then validate the complete setup on the exact food, moisture level, orientation, motion and cleaning process you will use in production.
Why soft and wet food slips
A gripper must generate enough tangential resistance to support an item’s weight and acceleration. Wet surfaces complicate that requirement in two ways. A film of water can reduce ordinary dry friction, while a soft food deforms under the finger. If the finger pushes harder, the food may flatten, bruise, split or squeeze out of the contact patch instead of becoming more secure. Irregular shapes and thin profiles provide even less area for a conventional pinch grasp.
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That is why “soft gripper” is not a complete solution. A compliant pad can conform to a tomato or tofu block, but the pad still needs an interface that works with water, a force level the food can tolerate, and a grasp geometry that keeps the load inside the contact region. A 2025 study on robotic handling of soft fruit specifically cautions that softness alone does not establish suitability for soft produce.
Choose the load path before changing the force
Decide whether the item should be held by friction at the sides, stabilized by feedback, or physically supported from below. The right choice depends on surface wetness, shape, allowable deformation and how much integration complexity your cell can accept.
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| Approach | What carries the load | Best fit | Main trade-off |
|---|---|---|---|
| Textured, compliant contact pads | Wet adhesion and friction distributed over a conforming area | Moist, deformable items such as tofu or rounded produce | Pattern geometry, pad material and cleaning must be matched to the food |
| Force or slip feedback | A controller adjusts actuator force as contact changes | Processes with varying size, moisture or orientation | Requires sensors, signal processing, calibration and a responsive actuator |
| Bottom support plus wrap or binding | Thin plates carry weight; strings or bands stabilize the item | Thin, slippery or irregular products that defeat a side pinch | More moving parts and possible gaps, sanitation and access concerns |
| Hybrid design | Surface adhesion, controlled force and mechanical support together | High variation or costly product damage | Highest design and validation effort |
These are design patterns rather than universal settings. Prototype results depend on the particular food, pad, actuator, speed and environment.
Improve the wet-contact interface
Use texture to retain a water film
Increasing normal force is not the only way to increase holding ability. JAIST’s November 8, 2021 newsletter describes work presented at IROS 2020 in which a soft gripper pad used square micropattern cells measuring 85 μm × 85 μm. Narrow grooves between the cells supported capillary action at the wet interface. In the reported tofu experiments, the patterned pad suppressed slippage with less squeeze force than a flat pad, reducing deformation.
The result is important because it changes the design question from “How hard can the fingers squeeze?” to “Can the interface create useful adhesion at a lower force?” The JAIST account says the potential of the micropattern pad to prevent slippage between a robotic gripper and a fragile deformable object in wet conditions without a complicated control method was demonstrated. That statement applies to the reported pattern, materials and test conditions; it does not establish that any textured rubber pad will behave the same way.
Design the texture around the contamination you actually have
A groove that helps retain water may trap pulp, starch, protein or cleaning residue. Before selecting a pattern, document whether the product is merely damp, coated with oil, covered in free water, or releasing juice under pressure. Record the smallest feature that can be cleaned and inspected with your sanitation method. A pattern that performs well in a clean laboratory test may lose adhesion after repeated loading or after a food film fills its grooves.
Keep a removable test coupon or spare pad for each candidate texture. Compare it with a flat control using the same food and motion. Measure the minimum force at which the item begins to slide, the force at which visible deformation starts, and the number of successful transfers after cleaning cycles. Do not treat a one-time successful lift as proof of repeatability.
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Compliant pads spread pressure
Compliance lets a finger conform to an uneven surface instead of concentrating force at one edge. It can increase real contact area and reduce local bruising, but it also introduces deflection: the actuator position is no longer the same as the contact force. Use a defined closing speed and a force limit, and verify that the fingers do not continue closing after the food has begun to deform.
A 2026 paper by Kargar and Berselli describes a sensorized compliant gripper with nylon compliant fingers, PLA structural components, Dragon Skin 30A silicone pads and force-sensitive resistor (FSR) tactile feedback. Its experiments involved lemons, zucchini, potatoes, cherry tomatoes, carrots and onions, and the authors report grasping those items without damage in their setup. Those material choices and results are study-specific. They are useful starting points for a prototype, not a food-contact approval or a recipe that can be transferred unchanged to another robot.
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Control the minimum secure force
The control objective is a stable grasp with the lowest force that survives acceleration, vibration and contact disturbances. A practical sequence is:
- Approach at a speed that does not push a liquid film ahead of the pad.
- Close until the sensor indicates first contact rather than relying only on motor position.
- Increase force in small increments while checking for slip and deformation.
- Hold a force margin appropriate to the planned acceleration, then reduce force if the item is stable.
- Release only after the receiving surface or support has taken the load.
The numerical thresholds must be calibrated on the actual food and end-effector. The cited studies do not establish a universal slip force, pressure limit or controller gain.
Detect incipient slip instead of waiting for a drop
Slip can appear as small vibrations or changes in tactile signals before the object visibly moves. A May 2024 article in Computers and Electronics in Agriculture describes contact-force and bending sensors, short-time Fourier transform and discrete wavelet transform analysis of tactile signals, and machine-learning-assisted slip detection and grasp-force optimization in experiments with fragile fruits. This supports evaluating a tactile slip detector, but it does not supply a transferable threshold for every gripper.
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FSRs, bending sensors or other tactile devices should be evaluated as part of a loop: sensor, signal conditioning, decision rule and actuator response. A sensor that detects slip but cannot command a fast, repeatable force change will not protect the food. Conversely, a fast actuator with no contact measurement may over-compress a soft item before the controller recognizes a problem.
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- Zero each sensor with the pad unloaded and at the operating temperature.
- Check whether water or food residue changes the baseline or sensitivity.
- Repeat calibration after the cleaning and drying procedure used between batches.
- Record force and tactile signals during successful lifts, visible deformation and actual slips.
- Set separate detection rules for contact, stable hold, incipient slip and release.
Use the data to define an operating window rather than a single “correct” force: below the window, the item slips; above it, the item deforms or the pad becomes contaminated.
Use bottom support when friction is the wrong load path
Scoop first, then stabilize
For a thin or very slippery item, side friction may never provide a robust safety margin. A scooping-binding gripper described in Frontiers in Robotics and AI (2021) uses thin plates that slide beneath the food and flexible strings that wrap around it. The plates carry weight from below while the strings keep the item from rolling or sliding away.
The authors tested 20 food items. Raw shrimp succeeded in 4 of 5 trials, carangidae fish in 3 of 5, and raw oyster in 4 of 5. Failures were associated with slippery properties and minimum height profiles: an item could pass through the gap between the lowest string and the thin plate. The authors propose reducing that gap and increasing the friction coefficient of both string and plate.
Close the geometry around the thinnest product
Measure the minimum thickness, edge radius and likely deflection of every product variant. Set the plate-to-string gap below the smallest profile that must be retained, while leaving enough clearance to avoid cutting or pinching. Test tilted, rotated and partially overlapping items; a gap that is safe in a centered lift can become an escape path when the food enters at an angle.
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The same study reports an approximately four-second takt time in its handling tests and says significant food damage was not observed with the flexible rubber strings. Both figures belong to that prototype and test setup. They are not production benchmarks for another robot, speed or food set.
Plan food-contact materials and sanitation early
The scooping-binding authors identify food compatibility as a limitation. They regard stainless steel as a logical preference for product-contact structures and say that food-compatible elastomers for the flexible string would need further investigation. Treat every pad, string, adhesive, coating and fastener as part of the hygienic design. Verify cleanability, chemical resistance, water ingress protection and the applicable food-contact requirements for your jurisdiction before deploying the mechanism.
A step-by-step development workflow
1. Characterize the product and task
- Measure mass, dimensions, minimum thickness, surface roughness and temperature.
- Classify the surface as dry, damp, free-water wet, oily, slimy or juice-releasing.
- Define the pick orientation, acceleration, travel distance, handoff and release height.
- Set an acceptable limit for bruising, indentation, tearing, leakage and cosmetic damage.
2. Start with the least invasive grasp
Use the largest practical compliant contact area and the lowest closing speed. Establish a flat-pad baseline before adding texture, sensors or a support mechanism. The baseline tells you whether the dominant problem is inadequate wet friction, excessive deformation, poor geometry or motion-induced disturbance.
3. Screen pad designs
Compare a flat pad with one or more textured, compliant candidates. Keep actuator force, contact area, food temperature and motion constant. For each candidate, record successful transfers, slip onset, peak force, visible deformation and cleaning time. If a micropattern is inspired by the 85 μm × 85 μm JAIST cells, describe it as an experimental adaptation and revalidate its performance; do not assume the same dimensions are optimal for a different food.
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Install the sensor where it measures the force that actually reaches the food, not merely motor current or gearbox torque. Characterize hysteresis, repeatability and saturation. Protect the sensor and wiring from washdown, but make the protective layer thin and compliant enough not to mask contact changes.
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5. Tune a force-and-slip controller
- Collect labeled traces for no contact, stable contact, slow slip, fast slip and release.
- Choose a detection feature, such as a tactile-frequency change or force derivative, and verify it against filmed motion.
- Command a small force increase when incipient slip is detected.
- Abort or transfer to a support mode if force rises while the item continues to move.
- Log every intervention so repeated false alarms can be separated from genuine slips.
6. Add underside support for failure modes that force cannot fix
If the item still escapes at acceptable force, test a thin plate, spoon-like support or scoop. Add a wrap, string or compliant retaining feature only after checking that it cannot cut the product or create an uncleanable crevice. Keep the support path short and visible so an operator can inspect it.
7. Validate the complete operating envelope
Run repeated trials across the smallest and largest items, wettest and driest conditions, every allowed orientation, normal and maximum speed, and after representative cleaning cycles. Include deliberate disturbances such as a brief acceleration change. Report failures as well as successes; a best-case demonstration does not reveal the margin you have in production.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare candidate designs
Use one food set and one task when comparing alternatives. A useful test sheet includes the following axes:
| Measure | What to record | Why it matters |
|---|---|---|
| Wet and oily slip resistance | Slip force, slip distance and successful transfers under controlled moisture | Shows whether the interface works beyond a dry demonstration |
| Food deformation | Peak force, indentation depth, bruising, tearing or leakage | Separates a secure grasp from an unacceptable one |
| Feedback performance | Detection latency, false alarms and recovery success | Indicates whether sensing improves the process rather than adding noise |
| Thin-profile handling | Minimum retained thickness, gap escapes and tilted-entry results | Reveals failures that a centered, thick sample hides |
| Repeatability | Success rate and failure mode over repeated trials | Prevents a single successful lift from being mistaken for reliability |
| Hygiene and maintenance | Food-contact material documentation, cleanability, drying time and inspection access | Connects mechanical performance with real operation |
| Integration burden | Changes to fingers, controller, wiring, tooling and cycle time | Quantifies the cost of moving from a prototype to a cell |
The available studies do not provide a controlled head-to-head benchmark proving one approach is best for all foods. Make the comparison on your own representative samples.
Troubleshoot by symptom
The food slides while the fingers appear to hold position
- Check for free water, oil or juice on the pad and food.
- Increase contact area or trial a wet-adhesion texture before increasing force.
- Reduce acceleration and closing speed during diagnosis.
- If the food is thin or low-profile, test underside support instead of relying on side friction.
The food is secure but bruised or flattened
- Lower the force ceiling and slow the final closing motion.
- Use a more compliant pad or distribute force over a larger area.
- Check whether the controller continues closing after first contact.
- Use slip feedback to add force only when needed rather than applying a high preset.
The gripper works on one size but not another
- Measure how pad curvature and contact area change with size.
- Check whether a larger item bottoms out on the finger or a smaller one reaches only an edge.
- Revisit the plate-to-string gap and support geometry for the smallest profile.
- Use size-specific recipes only when automatic sensing cannot cover the variation safely.
Slip detection triggers constantly
- Inspect sensor mounting for vibration or cable movement.
- Re-zero after cleaning and drying; residue can shift the baseline.
- Label real slips separately from normal pad settling and actuator noise.
- Retune features and thresholds for each food class instead of assuming one tactile signature.
The prototype is hard to clean
- Remove trapped-food pockets, exposed threads and inaccessible sensor cavities.
- Prefer smooth, inspectable product-contact structures where texture is not essential.
- Document disassembly, washdown, drying and reassembly time.
- Do not claim food-safe operation until the actual materials and cleaning process have been assessed.
What the evidence supports—and what it does not
The JAIST result supports investigating micropatterned wet-adhesion pads when a flat pad requires damaging squeeze force. The 2026 compliant-gripper study supports combining compliant contact surfaces with FSR tactile feedback for produce handling. The 2024 fruit-handling study supports investigating tactile slip detection and force optimization using contact and bending signals. The 2021 scooping-binding study supports changing the load path when thin, slippery foods can escape a pinch grasp.
None of these studies establishes a universal pad material, force threshold, slip algorithm, cycle-time guarantee or food-safety certification. Their outcomes depend on their own objects, moisture, motion, dimensions, materials and control systems. Treat every cited number as a result from the named prototype, then repeat the measurement on the gripper and foods you intend to operate.
Practical decision rule
Choose a textured compliant pad first when the item has enough profile for a side grasp but wet friction is the main failure. Add force and slip feedback when product variation or acceleration makes a fixed force unreliable. Move to a plate, scoop or wrap when the item is too thin, slippery or irregular for friction to carry its weight without excessive compression. In demanding cells, combine all three: a cleanable wet-contact interface, feedback-controlled force and a support feature that provides a positive mechanical backup.
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