Key concepts to follow when designing custom gripping components for automation systems.
One key component of the configurability of the automation system is the robot gripper module. The gripper module is an actuator with jaws that grasps an object and allows the workpiece to be picked up, transferred and placed by the robot. They are typically mounted at the end of a robot arm and at the point of contact with the workpiece. Automation components such as robot arms and software are flexible enough to be programmed for a wide variety of tasks and therefore, can be reused to produce multiple variants of products. However, the gripper module can be very specific to one task and is often specifically chosen and outfitted with custom gripper finger tooling to help adapt.
Designing functional robot gripper fingers can be complex and sensitive. Moderate to advance engineering techniques and an iterative process is typically used to validate designs and arrive at an optimal solution. Some of the key concepts are outlined below to give a basic overview of considerations when designing gripping components.
Automation is geared towards increasing throughput and the impact of having gripper finger tooling that is designed to work efficiently is often underestimated.
Minimizing Interference between the gripper finger and other objects around the workpiece intended to be grasped can dramatically increase the effectiveness of the gripper system. This may be standard for parts that are presented in traditional methods, but with the advancement of vision systems, parts are now being presented in flexible feeding applications such as on a conveyor. Even if the gripper can grasp the workpiece properly, interference by having too large of a gripper finger can cause the robot pick attempt to fail potentially resulting in a reduction of throughput the system can handle. The best way to prevent interference is to design the gripper finger with the smallest profile possible while maintaining good structural strength for the application.
Chamfering Exterior Surfaces including the leading edge of the gripper can help when workpieces are being presented close together. This is especially useful for cells that have vision capabilities. By having the edges angled, it allows the gripper finger to push and move other parts out of the way giving the clearance needed to grasp properly.
Minimized Weight in the gripper finger design is critical as robot arms have a fixed payload which could be further reduced by other components attached to the arm like actuators, tool changers, vision systems, etc. Weight can affect the speed of the robot arm in two ways. The first is due to the finite amount of torque that can be applied to each joint in the robot arm. By increasing the mass at the end of the arm, the robot will be forced to move at slower speeds when trying to move the load with the same amount of torque being applied. Momentum can also be affected by increased mass. When the load is set into motion, the increased inertia could cause unwanted movement in the arm or even overshooting to occur. This could lead to damaging the gripper finger or the object it is coming in contact with. To help decrease weight, gripper fingers could be made of a lighter weight material such as aluminum, composites or even 3D printed polymers. If the application requires the stiffness or wear properties of steel, care should be taken to minimize the size of the finger and to remove material in any non-structural areas.
Ensuring a Secure Grasp is important to offset the speeds at which robots can move by designing the correct contact points into the gripper. An encompassing grip, where the gripper finger matches the contours of the workpiece and overlaps a portion of the geometry, should be used whenever possible. It allows a positive, mechanical grip without relying on friction of the grip alone to secure the part. This type of grip also spreads the gripping force over a wider section of the parts surface instead of concentrating it on a single set of points where the gripper comes into contact with the workpiece. The spreading of the load will prevent deformation of the part due to the gripping forces being exerted on the part. When an encompassing grip is used, care should be taken to add reliefs or chamfers to areas that may cause stiction and not allow the part to be dislodged from the gripper finger. If a flat grip must be used there are ways to increase the coefficient of friction by designing rubber pads into the gripping area. The rubber will provide a compliant surface that can prevent damage to parts and help increase the holding force of the gripper. Having a secure grip ensures the efficiency of the robot and increases safety.
Avoiding Tool Changes can reduce cycle times and free up valuable real-estate within the robots working environment. Although there are times where a tool change must be made, there are some ways to avoid unnecessary slowdowns.
Grasp Multiple Parts with a single gripper can significantly increase the efficiency of the gripping system. This can be accomplished by creating multiple gripping surfaces in the same gripper finger set. By doing this, the gripper can be used to pick up a variety of parts without the need to switch out the gripper tooling.
Multiple Grippers can be added to the end of arm tooling allowing the robot to handle multiple parts at the same time. Machine tending applications are a great place to use multiple grippers as the finished part can be removed and a blank inserted in a single operation from the robot.
Auxiliary Functions can be performed by the robot gripper finger by designing features that go beyond grasping the workpiece. By eliminating the need for another operation, the system efficiency and throughput is increase while reducing costs associated with having another machine to do the work. Hooks to aid in opening doors on CNC machines before loading the parts, air nozzles to blow chips away before grasping a workpiece, rotary devices to drive screws after placement of assembly components, and locating geometry to align parts are just a few ways to increase the functionality of the gripper.
Mechanical grippers can be grouped into three main categories when describing how the gripper actuates and can aid when selecting which gripper module will best suit your application. The most common actuation patterns are parallel, angular and translational.
Stabilizing the Workpiece during the movement of the robot is very important and can ensure proper placement in the final position. As suggested in the sections above, an encompassing grip should be used. The matching geometry and increased surface area will help keep the part from shifting during high speeds of transportation.
Minimize Finger Lengths whenever possible to reduce the amount of bending momentum (flex) which causes the gripping force transferred to be reduced. The fingers should be just long enough to grasp the workpiece without causing interference with the gripper jaws or body of the gripper module. By staying within the recommended finger lengths of the manufacturer, it ensures the designed gripping force will be transferred to the part and not lost due to displacement of the finger caused by unnecessary deflection. There are certain situations that would call for longer fingers to maneuver around geometries when picking up a part or for inserting parts deep into cavities, but extra design consideration should be taken to make sure the gripping force is calculated correctlyand the gripper will not be overstressed.
Approach Clearances keep the gripper fingers from interfering with the workpiece it is trying to grasp and can be vital to the system performing reliably. It is important to make sure the percent of stroke at contact allows the fingers enough stroke to open wide enough to approach the part without interference. Features like chamfers along contact surfaces are designed into the gripper finger for situations where the part being picked needs a high error tolerance or may be off-center and the geometry of the gripper design can help shift the workpiece into position when the gripper is approaching ensuring that when the gripper closes, the workpiece is in position to be securely grasped.
Gripping Surface is the area of the gripper finger that comes into contact with the workpiece and can be critical in getting a secure grasp. The materials of the workpiece and the gripper should be considered when determining if the grasp will be sufficient. With an encompassing grip matching the profile of the workpiece, the coefficient of friction between the workpiece and the gripper material should be low to allow the part to center itself into the contoured grip. When an encompassing grip is not possible and a flat contact surface must be used, the coefficient of friction needs to be higher because the grip is relying on the friction to keep the part secure. Rubber material can be added to the contact surface to give the grip a small amount of compliance and create the friction needed to maintain a secure grasp when transporting the workpiece.
Gripper Mounting should be designed in a way that provides secure, aligned positioning of the gripper finger. The mounting hardware itself is often times not enough to keep the finger in position and can cause issues when trying to grasp an object. The finger geometry should encompass the jaws of the actuator when possible to secure the finger in all three axis and to help located. If this is not possible, the dowel or centering pins should be used to maintain an accurate position increasing the reliability of the gripper.
Determining Where to Grip a part is a function of where the part will be going. The gripper should be designed to grasp the part on an area that will not interfere with the placement of the workpiece at the next step. It is also important in assembly operations to grasp the workpiece on a feature where parts will be added instead of relying on areas of the workpiece where parts have already been placed into the assembly. If the previous part was broken or missing and the gripper is relying on this geometry to grasp the workpiece securely, it could cause issues with picking the object up or even damage the grippers. If this is the only option, the assembled components should be verified at a previous step to ensure the geometry is there for the gripper to grab on to.
Assembly Specific Grippers can need special geometry to help align parts into subassemblies. An example of this could be large chamfers or other mating features that extend past the part being grasped to match up with existing parts on the subassembly prior to being inserted. This acts as a mechanical guide and helps locate the part correctly during placement. Not all applications will allow for this due to limitations or restrictions in how the part is being presented for grasping but should be included when it does not cause interface issues.
Adding Functionality in grippers can increase throughput by reducing the risk of error associated with handling the parts multiple times. Designing the gripper system to perform more than one task while maintaining contact with the workpiece simply reduces the number of times the robot must pick an object up, therefore reduces the possibilities of a grasping error.
The gripping component costs should be managed in a way that adds value to the overall cell design. By doing this, it will ensure that the gripper finger was not implemented in such a way that will cause more expense in operator costs due to excessive dropping of the workpiece or in downstream operations through time or additional hardware needed in the cell.
Actuator Selection is a simple way to reduce costs by designing around actuators that have simple linear or rotary motions. These types of actuators are much less expensive than others that use more complex ranges of motion. Being more creative with the finger design can often result in the same effect without the need of the more expensive unit.
Standard Components save time and money and readily available actuators should be used whenever possible. Gripper fingers can then be designed around the actuator to further customize the gripper for the specific application. One benefit to using standard actuators is the repair and maintenance process is much easier than with custom components.
Multipurpose Grippers are a great way to reduce overall hardware costs. In most cases, a system that uses two grippers or a rotary device can be eliminated by designing the gripper fingers to handle multiple parts. This works well unless they need to be transported at the same time like in a machine tending operation or the component geometry is so different they require an alternate style of gripper such as a vacuum cup.
Fairlane was established in 1957 in Fraser, MI and designs, manufactures and stocks a wide range of standard components used in positioning and workholding applications. Fixtureworks, a related business, supplies an extended range of standard machine components and complete workholding solutions from work leading manufacturers. Recently Fairlane has developed a web-based design application aimed at the end of arm tooling industry. It uses intuitive CAD to help users design and purchase custom gripping components in minutes.
To learn more visit www.FairlaneProducts.com
Ref Material: GREGORY CARYLEE CAUSEY, ELEMENTS OF AGILITY IN MANUFACTURING, Thesis, Case Western Reserve University, January, 1999.
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