End Effector Design Factors

End Effector Design Factors

By on February 13th, 2019 in Robotics

The process of selecting/designing a robotic end effector can be overwhelming due to the various design factors. There are so many options out there and how do you know when to customize an existing end effector vs. a full-custom build?! I use the following table to help organize my design and ensure that all the applicable factors have been addressed. This process also allows me to tackle one item at a time instead of rushing into it and getting stuck half-way through.

For your convenience, I have high-lighted my top 15 considerations.

 

CategoryDesign FactorClarification / Example
Part CharacteristicsWeightMass of the object, which could change due to a machining process.
SizeAny internal and external dimensions that may be in contact with the gripper or possibly interfere with the robot arm movement.
Surface ProtectionSoft materials and high polish surfaces may be susceptible to scratches or scuffing.
ShapeThe part is asymmetrical, spherical, angular, or has drilled holes or channels.
Tolerance VariabilityParts within a batch could vary in size or weight.
Part StabilitySoft and thin parts may deform easily when gripped or moved. Top-heavy parts may need added support to avoid tipping.
Surface CleanlinessPart or finger surface may have oil, dirt, dust, chemicals, or residue, which could affect the gripping ability of the end effector.
Gripping MethodMechanical PinchPinching action caused by actuating fingers, which can contact internal or external features of the part.
ScoopScooping action that cradles the part like a forklift.
Vacuum CupVacuum Cup or plate that uses vacuum pressure to lift an object.
MagnetElectro-magnetic force that can pick up metallic objects.
BalloonRubber Balloon that inflates around or inside the object. Very useful for odd-shapes and variable sizes.
Process – OtherA grinding wheel, paint sprayer, or drill that is installed on the robot arm and performs a task without actually gripping the part.
Power TransmissionPneumaticUses air or gas as the working fluid.
HydraulicUses oil or similar liquid as the working fluid.
ElectricalA motor or magnetic coil that uses electro-magnetic force to rotate a pivot or slide.
MechanicalSprings, rubber bands, or flexed materials that uses deflection force.
Gripping ForceWeight of the ObjectThe end effector can withstand the static load of the part, as well as the process momentum of the part.
Method of HoldingRefer to “Gripping Method”.
Finger FrictionFriction needed to keep the part from slipping out the fingers. Cleanliness may be a factor as well, see “Part Characteristics – Surface Cleanliness”.
Process Speed and AccelerationHigh speed and acceleration will result in high momentum forces on the gripper, possible causing the part to fly out.
Part PositioningLength of Fingers or CupsLonger fingers will allow for deeper access, but also requires clearance above the part for retraction.
Robot AccuracyLow accuracy robots may need external guides or fixtures to achieve repeatable actions.
Upstream VariabilityPart location of the part may vary. Possible solutions would be a vision system or bowl feeder.
Downstream ToleranceHigh-precision processes after the robot may require very high-precision placement.
Access RestrictionsConfined areas may not allow for large end effectors, such as a CNC machine or assembly line.
Gripper ServiceLifetime of ComponentsRubber fingers may wear out. Plastic may become brittle due to UV radiation or chemicals.
Removal of ComponentsSuction cups or rubber fingers are easily removable. End Effector design has bolts or quick-turn knobs instead of glue or welds.
Availability of PartsSuction cup sizes are standard and materials are easily obtainable or widely sold.
Operating EnvironmentTemperatureLow and high temperatures may affect the end effector’s strength, causing premature failure.
Humidity, Water sprayEspecially prevalent with electronics, you may need to source IP rated components Water spray protection is the second number of an IP rating (IP45).
Dirt, DustIngress of dirt and dust may cause electrical shorts or excessive heat, so you may need to source IP rated components. Dust protection is the first number of an IP rating (IP45).
SafetyThe robot area may be situated in a high-traffic area, which would require certain safety protocols. See “Safety” for more details.
Temperature ProtectionHeat ShieldsIncorporate foil or temperature blankets that reflect heat away from critical components.
Long FingersBy extending the heat source away from the end effector, less heat will reach the gripper body.
Forced CoolingAir or liquid can be used to accelerate the cooling process.
Heat-resistant MaterialsThe gripping material can withstand direct contact with high heat, such as silicon.
Gripper MaterialsStrength, RigidityLonger fingers may require added structure to prevent flexing.
Fatigue ProtectionHighly repetitive actions can cause material failure even though the payload is low.
Cost and Availability of ComponentsTitanium is a lightweight and strong material, but extremely expensive compared to aluminum.
Gripping FrictionRubber materials offer great friction force, but only for certain materials and environments.
Chemical CompatibilityParts that are contaminated with acids or chemicals may require special handling. Even water and oil can degrade certain materials.
SafetySharp EdgesEnd effector has sharp edges or corners that could cut wires or people, especially for high-interaction automation such as collaborative robots.
Gripping ForceFinger actuation could crush a human hand and may require a lower gripping force.
Pinch PointsPivoting joints can pinch wires or tubing, which can be guarded with plastic or fabric.
Collaborative DesignIn general, all corners and pinch points are guarded or padded and gripping force is limited to the minimum value to grip the part.
SensorsArea scanners around the robot can disable the end effector. Force sensors in the fingers could detect an unidentified object, limiting the gripper power.
Mounting RequirementsRobot Bolt PatternThe bolt pattern on the robot wrist may require a custom mount, especially for custom end effectors.
Robot StrengthThe wrist bolt pattern and arm can physically handle the payload and forces of the end effector.
End Effector Control and PowerThe end effector may require air, electric power, or control connections. Some Robot arms already have I/O connections at the wrist for easy installation of end effectors.
End Effector ProfileThe overall profile of the end effector won’t interfere with the arm joints. A 2ft long end effector or part may hit the robot arm, causing major damage.
OtherQuick-disconnect Tool ChangeNon-tool disconnection such as pull pins, quarter turn knobs, or air-brake couplers can limit the amount of time needed for tool-changes. In some cases, the robot can self-change the gripper and continue the process.
Interchangeable FingersThe fingers can be swapped out for various part profiles. You will want to utilize easy-to-remove fingers, as noted in “Gripper Service – Removal of Components”.
ColorIt may seem trivial, but having a similar color to the robot will make your design look professional and seamless.
Design for BreakageThe motor cam or fingers would break before damaging the part, especially for highly delicate or expensive applications. You could also integrate compliant fingers that would flex in the event of a collision.
Multi-functionDual-grip end effectors can offer higher cycle rates compared to a single grip, due to the reduced arm movement. Dual-grip designs can also offer multi-size picking, which would otherwise require a tool-change.
Fully Custom or CustomizeIn some cases, it’s cheaper and easier to customize an existing end effector platform instead of designing a fully custom gripper. For example, you can 3D print some custom fingers and install them on an off-the-shelf gripper.

It’s easy as an engineer to overcomplicate the design, so my best recommendation is to list the critical functions and work out from there. By listing the design requirements, it’s easier to prioritize one feature over another. This also helps motivate yourself to finish one problem before moving on to the next.

If you have any questions about the information above, need assistance with a robotic application, or have suggestions to add to the list, I would be glad to discuss it with you!