Tech Tips: Considering Payload and Inertia in 6-Axis Robot EoAT Design

When considering gripping force and the method for the end-of-arm tooling (EoAT) design, it's crucial to carefully assess the application's specific requirements. This consideration involves understanding the properties of both the object and the gripper itself, such as shape, size, weight, material, surface characteristics, and the environment.

A separate topic is the discussion of gripper power sources and sensing mechanisms, which are very important factors in their own rights, but are usually considered far before the final engineering design.

 

Grip Every Component

Customization and adaptability capabilities within the tooling are essential for an end-of-arm tooling design, especially when handling complex objects or varying payloads. In some scenarios, multiple gripping methods may be required to handle all objects within an application. Therefore, engineers often combine multiple gripping methods to enhance versatility without constant changeovers.

One approach to achieving this is through modular gripper designs. By incorporating interchangeable components or modules, such as jaws, fingers, or suction cups, engineers can easily configure the gripper to suit different objects or tasks. For example, a gripper system may include suction cups for handling smooth, flat objects and mechanical jaws for gripping irregularly shaped items.

 

Adaptability

Engineers may implement adjustable features within the gripper design to accommodate object size, shape, or weight variations. This implementation could incorporate mechanisms for adjusting the width or opening of the gripper jaws, allowing for a snug fit around objects of different dimensions. Similarly, pneumatic or hydraulic systems may be integrated to enable variable gripping force based on the specific requirements of each task.

 

Control Technology

Besides mechanical adjustments, EoAT designs may leverage advanced sensing and control technologies to enhance adaptability. For instance, force sensors or feedback systems can be integrated into the gripper to provide real-time information about the object's properties and grip strength, allowing for dynamic adjustments during handling programs.

 

Payload Considerations

Payload and weight distribution are pivotal when designing end-of-arm tooling for a 6-axis robot. Knowing the precise weight capacity the robot arm can handle is extremely important.

Payload is one of the key specifications to acknowledge when designing end-of-arm tooling for your 6-axis robot. It includes not only the item to be gripped but also the tooling itself.

Several considerations should be taken into account when accommodating payload limitations.

• Weight: Determine the maximum weight the EoAT will be required to handle. Knowing the weight of the heaviest object gives designers the weight range the EoAT should stay within.
• Material Properties: When developing your end-of-arm tooling, it's crucial to consider material properties to ensure it remains within weight limitations. For instance, aluminum, being approximately one-third lighter than steel, can be a strategic choice to minimize overall weight and is commonly used. It’s also resistant to corrosion and is non-ferrous for inductive environments. Plastic is also very common, but it’s not as durable.
• Center of Gravity: Knowing the CoG allows engineers to optimize the design of the EoAT for efficient operation. By aligning the payload's center of gravity with the robot’s axes of rotation, engineers can reduce the torque required to move the payload, thereby improving the robot's overall energy efficiency and performance. An aligned CoG is essential in applications where the robot needs to handle heavy or bulky objects, as minimizing torque requirements can extend the lifespan of the robot's motors and actuators.
• Factors of Safety: Incorporating safety margins into the design of the tooling is crucial for ensuring the reliable operation of the robot arm. This is especially true when dealing with variations in payload weight or distribution. Safety margins act as a cushion, providing extra protection against overloading the robot’s payload and exceeding its intended capacity during acceleration. For example, If my part weighs 5 lb and the max capacity of the robot is 15 lb, I cannot simply allow my EoAT to weigh 10 lb; I need to include a safety margin and lower the weight of my EoAT.

 

Figure 1. Visual representation of equation factors used to calculate your center of gravity. Image courtesy of FANUC

 

Center of Gravity and Moments of Inertia

Although it involves a lot of calculation and design adjustment, always pay close attention to the center of gravity, inertia, and moment load capabilities in your project planning. Neglecting these factors could lead to premature failure of servos within the robot’s wrist. It is imperative to review the robot's manual and specifications sheet thoroughly. While the static payload limitations are crucial, it is equally important to closely examine the allowable load moment and inertia at the wrist.

Most robots are accompanied by a wrist load diagram, offering insights into the permissible distances for tool manipulation in the X, Y, and Z axes before experiencing a decline in payload capabilities.

 

Figure 2. The further the robot reaches across all 3 linear axes, the more the payload capabilities decrease. Wrist load diagram used courtesy of FANUC

 

For example, despite a robot's nominal payload capacity, exceeding specified distances in the z-axis can significantly reduce payload capacity. Therefore, it is vital to consider the robot's reach within the workspace and refer to the wrist load diagram before designing your end-of-arm tooling. Without such guidance in the manual, reaching out to the robot supplier for clarification is advisable.

 

Figure 3. Although it's easier to calculate your inertia in Solidworks or AutoCAD (or alternative), you can manually calculate it. Image used courtesy of FANUC

 

Correcting an EoAT Moment of Inertia Error

If you find yourself in a situation where your end-of-arm tooling design somehow overlooked the inertia and moment allowable for the load specifications, and you encounter operational challenges, consider adjusting your robot's movements. Try redistributing the load so that joints J4 and J5 bear more burden than J6. Typically, J4 and J5 axes can withstand approximately three times the inertia/moment load as J6 due to the larger size of the servos.

If you continue encountering difficulties, perform calculations to determine the mass moments of inertia and identify which factor influences your robot's motion. Compare the calculations of the mass moments of inertia of the tooling to the allowable load for your robot's joints according to how you move your robot. This assessment will provide insight into the extent of deviation and enable you to adjust the robot's movements accordingly or consider redesigning a new end-of-arm tooling solution.