End effectors and grippers

The end effector is where a manipulator meets the world, and choosing it is often the single most important decision in a manipulation system. A clever grasp planner cannot rescue a gripper that cannot physically hold the object, and a simple gripper matched well to the task can make the rest of the system easy. This lesson surveys the main families of end effectors, how each works, what it is good and bad at, and how engineers choose between them.

What an end effector must do

Every end effector has to do some combination of the following: make contact with the object at suitable locations, apply enough force or suction to resist gravity and the accelerations of transport, conform to the object's shape well enough to hold it securely, release cleanly, and survive the environment (dust, washdown, temperature, impacts). Many also need to sense: report whether an object is present, measure grip force, or detect slip.

Requirements come from the objects and the task. Object properties include mass, size range, shape, surface (porous, smooth, oily, textured), stiffness and fragility, and whether the object may be marked or deformed. Task properties include cycle time, required placement accuracy, whether the object must be reoriented, how cluttered the pick location is, and hygiene or safety rules. Robot properties also constrain the choice: payload capacity must cover the gripper plus the object, the tool flange has a standard bolt pattern (commonly ISO 9409-1), and the robot's controller determines what electrical or pneumatic signals are available.

Parallel-jaw grippers

The parallel-jaw gripper is the workhorse of industry and research. Two fingers move toward each other along a straight line, driven by a pneumatic cylinder, an electric motor with a lead screw or rack, or a servo. The object is held by friction between the fingertips and its surfaces, or by the finger shape enclosing it.

Pneumatic grippers are cheap, fast and strong, with grip forces from tens to thousands of newtons. Their limitation is control: they are usually either open or closed, with force set by supply pressure. Electric grippers allow programmable position, speed and force, can report finger width (useful for grasp verification and rough size measurement), and need only electrical connections. Many collaborative-robot grippers are electric for this reason.

Finger design matters as much as the actuator. Flat fingertips work for boxes. V-groove fingertips center cylinders and give two contact lines per finger. Custom fingers shaped to a specific part, often 3-D printed, can make an awkward part trivial to hold. Fingertip material controls friction: polyurethane or silicone pads give friction coefficients well above bare aluminum, while hard fingers are better for precise location.

Multi-finger and adaptive grippers

Three-finger grippers can hold cylinders and spheres more securely than two fingers, because three contacts surround the object. Underactuated or adaptive grippers have fewer motors than joints: a single motor closes several finger links, and springs or linkages let the fingers wrap around whatever they touch. The result is a hand that conforms to varied shapes without complex control. The well-known Robotiq adaptive grippers and several research hands use this principle.

Fully actuated anthropomorphic hands with twenty or more joints exist, but they are expensive, delicate and hard to control. Outside research and a few humanoid platforms, industry favors simpler grippers with good finger design. Lesson 9 returns to dexterous hands.

Grasp types for fingered grippers are often described as power grasps, in which the palm and fingers wrap the object for maximum security, and precision grasps, in which only fingertips touch so the object can be positioned finely. Power grasps resist large loads; precision grasps allow small adjustments.

Suction and vacuum grippers

Vacuum grippers hold objects by lowering the air pressure inside a cup sealed against the surface. Atmospheric pressure on the outside then pushes the object against the cup. The holding force is roughly the pressure difference times the effective cup area, reduced by a safety factor; lesson 7 does the arithmetic. Vacuum is generated by an ejector, which uses compressed air through a venturi, or by an electric vacuum pump or blower.

Suction dominates e-commerce picking because it needs access to only one face of an object, works well on boxes, bags and flat items, and tolerates position error, since the cup just needs to land somewhere on a suitable surface. Cup shape and material matter. Flat cups give stiff, precise holding on smooth surfaces. Bellows cups, with one or more folds, conform to uneven or tilted surfaces and add a short vertical compliance that cushions contact. Soft silicone lips seal on textured surfaces. Foam pads with many small holes can grip irregular or porous items with a high-flow blower.

Suction struggles with porous items (fabric, open-weave bags, some cardboard), very small or curved surfaces where a seal cannot form, heavy items that would need large cups, and items with labels or loose plastic that peel away. Leaks reduce holding force, so suction systems monitor vacuum level to verify grasps and detect slip.

Magnetic, electroadhesive and needle grippers

Magnetic grippers hold ferromagnetic parts such as steel sheet and machined parts. Electromagnets switch on and off with current; permanent magnets with a mechanical or electrical switching mechanism hold without power, which is safer if power fails. Magnetic grippers cannot handle non-ferrous metals or most other materials, and residual magnetism can make thin sheets stick together.

Electroadhesive grippers use high voltage across interleaved electrodes to induce electrostatic attraction, and can hold fabrics, paper and flat items with low force. Needle grippers drive small needles into textiles or composites. Freeze grippers and Bernoulli grippers (which hold wafers without contact using airflow) serve specialist niches.

Soft grippers

Soft grippers are made from elastomers and other compliant materials. A common design has fingers with internal air chambers that bend when pressurized, wrapping gently around objects of varied shape. Granular jamming grippers use a bag of granular material that flows around an object and then stiffens when air is evacuated. Fin-ray fingers, inspired by fish fins, bend toward the object when pushed. Soft grippers excel at fragile and irregular items such as fruit, baked goods and some medical supplies, and are cheap to make. Their weaknesses are lower precision, lower load capacity and wear.

Tool changers and multi-tool heads

A single robot often needs more than one end effector. Automatic tool changers have a robot-side plate and tool-side plates that lock together mechanically, pneumatically or electrically, passing air, power and signals through. The robot sets one tool in a stand and picks up another in a few seconds. Alternatively, a combined head may carry a suction cup and fingers together, so the system can choose per item without changing tools. Many warehouse picking systems use exactly this hybrid.

Choosing an end effector

Engineers often use a weighted selection matrix: list the criteria that matter for the application, weight them, score each candidate, and compare totals. The matrix forces explicit discussion of what matters and exposes where a candidate fails a hard requirement. Hard requirements, such as food-safe materials or a payload limit, eliminate candidates before scoring.

# Weighted selection matrix for a mixed-item picking cell
import numpy as np
criteria = ["item coverage", "speed", "reliability", "cost", "maintenance"]
weights = np.array([0.35, 0.20, 0.25, 0.10, 0.10])
scores = {                       # 1 (poor) to 5 (excellent)
    "parallel jaw (electric)": [3, 3, 4, 3, 4],
    "bellows suction":         [4, 5, 4, 4, 3],
    "suction + fingers combo": [5, 4, 4, 2, 2],
    "soft pneumatic fingers":  [3, 3, 3, 4, 3],
}
for name, s in scores.items():
    print(f"{name:26s} total = {weights @ np.array(s):.2f}")
best = max(scores, key=lambda k: weights @ np.array(scores[k]))
print("highest score:", best)

parallel jaw (electric)    total = 3.35
bellows suction            total = 4.10
suction + fingers combo    total = 3.95
soft pneumatic fingers     total = 3.10
highest score: bellows suction

Sensing in the end effector

Simple presence sensing, such as a proximity sensor in the palm or a vacuum switch, confirms that something is held. Electric grippers report finger position and motor current, from which grip force can be estimated. Wrist force-torque sensors measure the full six-axis wrench between flange and gripper, which reveals the object's weight, contact forces during insertion, and collisions. Tactile sensors in the fingertips, covered in ROB-220, measure the pressure distribution and can detect incipient slip. Cameras mounted on the wrist give close-up views for final alignment.

Sensing adds cost and cabling but often pays for itself through better verification and recovery. A rule of thumb in industrial cells is that every grasp should be verified by at least one independent signal before the arm moves away at speed.

Practical details that decide success

Several practical details repeatedly decide whether an end effector works in production. Compliance in the right place, such as a bellows or a spring-loaded finger, absorbs small position errors. Fingertip pads wear and must be inspected and replaced. Hoses and cables must be routed so they do not snag during wrist rotation. Pneumatic systems need clean, dry air and correctly sized lines, or response slows. And the end effector must be safe if power or air fails: a gripper that opens on air loss will drop its object, while a spring-closed or self-locking gripper will hold it.