Free Guide: How to select the right robot gripper

In the last two years there has been an explosion of interest in compact collaborative robots (cobots) that cost much less than traditional industrial robots, are designed for installation without the need for physical guarding, and are so easy to relocate and program that they can be used for different tasks on a weekly or even a daily basis. However, to get the most out of such robots it is essential that the optimum gripper is selected. The following guide will help users choose the type of gripper that is right for their applications.

What are you gripping?

Consider what you will be using the robot gripper for, both immediately and in the future. Some points to think about include:

• Size
• Weight
• Geometry (eg cylindrical, tapered, polygonal, irregular)
• Is the material solid or deformable?
• Is the surface easily marked?
• Will the object change during the task?

If, for example, the object is a machined steel component, an appropriately sized two-finger pneumatic or electric gripper may suffice - or a three-finger self-centring gripper may be better for a cylindrical component. However, a soft or irregularly shaped item may require something different. Suction cups can be useful on flat or gently curved surfaces, but some products might be marked by suction cups. A viable option for these might be the Festo OGGB gripper that uses the Bernoulli principle to lift and transport components with smooth, flat surfaces - such as silicon wafers, solar cells and food packaging. The Bernoulli principle uses high-speed airflow to create a low-pressure zone that generates a lifting force for transporting workpieces quietly, reliably and quickly. The unusual OGGB gripper can also handle porous, perforated or pliable materials such as fabric, metallic mesh and bubble wrap.

For objects that have a geometry that would be very challenging for conventional grippers, Empire Robotics of the USA manufactures the VERSABALL Gripper. This device uses a soft spherical bag filled with loose particles. After the gripper has been lowered onto the object to be lifted, and the bag of particles has conformed to the surface, a suction is applied to draw the air from the voids between the particles, so the soft bag of particles hardens and securely yet gently grips the object so it can be moved. After moving, the suction is removed, the particles become free to move, and the object is released. Empire Robotics has used the VERSABALL on a wide variety of objects including compact fluorescent lightbulbs, engineering components and bricks.

Objects that have a complex geometry can be difficult to grip unless they are in a known position and orientation (or their position can be identified accurately using a machine vision system). If that is the case, gripper jaws can be manufactured to match the surface geometry for an accurate, repeatable and secure grip. Options here are for a designer to create the jaw design by working from CAD files, either manually or using appropriate computer-aided tools, or an interesting recent development from Schunk is the eGrip web tool for creating 3D printed gripper jaws. 3D CAD files for the workpiece or components are uploaded in STEP or STL format using the intelligent web-based software, then the user enters information such as the object's weight, the installation position of the gripper and the finger length. In a few seconds, the software generates a detailed offer containing the 3D contour, the delivery time and the price for 3D printing the gripper jaws in polyamide 12, with an option of an FDA-approved grade for pharmaceutical and medical applications.

3D printing certainly offers advantages for producing complex gripper jaw designs, whether or not the Schunk eGrip process is used. For handling components that are delicate or require a small amount of compliance in the gripper, the Freeformer additive manufacturing process from Arburg can manufacture, in one piece, components that have both hard and soft regions. For example, gripper jaws could be manufactured predominantly from a relatively hard polymer, but with elastomer regions for the features that come into contact with the object being gripped.

In the bullet-point list above, the last item asks 'Will the object change during the task?' In a machine tending application, the robot loads a blank or semi-finished component into the machine tool, then unloads the part after the machining operation has finished. In some cases the shape might change substantially, so a gripper with a relatively long stroke might be required (the blank would be gripped with the gripper jaws near their fully open position, and the machined component would be gripped with the jaws nearer to their fully closed position).

What is the operating environment like?

Industrial robots are capable of operating in a variety of different environments, but not all types of gripper are suitable for all environments. For example, in food processing and packaging, pneumatics are often avoided because of the risk of 'dirty' air being emitted from valve exhaust ports or leaks. Clearly this steers the selection towards electric grippers. If, however, a suction cup is desirable but compressed air is not, an innovative gripper from DE-STA-CO is the EcoCup electric vacuum cup that can be used singly or in multiples attached to a mounting plate. Quiet, fast, long-lasting and requiring just a 0.5A 24V DC supply, each EcoCup can lift 0.45kg (or up to 1.8kg if acceleration and deceleration rates are reduced).

For environments that are in themselves dirty or dusty, such as machine tools where there may be cutting fluids and/or swarf present, suction cups can be problematic due to the ingestion of fluids and particles that can clog filters and airlines. Also, while magnetic grippers (normally of the permanent-electro-magnetic type) might appear to be a good choice for handling iron and steel components, swarf can cause problems if accurate positioning is necessary.

How demanding are the performance requirements?

Every robot application is different, so the performance requirements for grippers will vary accordingly. For instance, a high-speed delta robot picking and placing small components will need a gripper that is lightweight yet capable of withstanding the loads imposed by high accelerations; the gripper also needs to open and close very quickly so that it does not contribute significantly to the cycle time. At the other end of the scale, a six-axis collaborative robot may be performing an operation that is not a process bottleneck, so the weight of the gripper will not be as important (unless the workpiece is already close to the robot's maximum payload), fast opening and closing is desirable rather than essential and, depending on other factors such as workpiece weight, acceleration/deceleration and the type of grip, the gripping force can be lower.

What type of grip is required?

Let us consider the type of grip in a little more detail. Sometimes a pair of flat jaws acting on parallel flat surfaces on the workpiece is adequate. However, the strength of the grip is then a function of the gripping force and the friction between the jaws and the workpiece (which may not be clean and dry, of course). It should also be borne in mind that the grip strength required is not just down to the weight of the workpiece because robots can generate high accelerations that increase the load exerted on the gripper by the workpiece. If a more secure grip is necessary, either a more powerful gripper can be selected or a set of gripper jaws can be specified that locate positively on geometric features on the workpiece.

If the gripper is required to act on a cylindrical surface, the options are a pair of flat jaws with line contact on opposite sides of the cylinder, or the jaws may be fitted with inserts having either V-grooves or radiused surfaces to correspond with the cylinder's surface. If access to the cylinder can be gained from one end, then it is possible to use a three-jaw gripper. Another option that is attractive, particularly if a versatile gripper is required, is the 2-Finger Adaptive Robot Grippers from Robotiq. When approaching from the side of a cylindrical feature, the fingers start moving in a parallel fashion but, once they have made contact with the cylinder, they continue to move so that they partially encompass the cylinder, which provides a far more secure grip. Of course, this encompassing grip can be applied to other geometric forms as well as cylinders.

How often will the application change?

Robots are inherently programmable, so they will usually be required to perform more than one role in their lifetime. In some cases the robot may be called upon to perform new tasks on a weekly basis or even daily, whereas others may be reprogrammed far less frequently. If a new gripper is necessary for each task, the costs can soon become significant. While one option might be to specify a gripper than has a long stroke and can be fitted with different jaws, an alternative is the adaptive grippers manufactured by Robotiq - and available in the UK through R.A. Rodriguez (UK) Ltd. Two sizes of two-finger gripper are offered, with key characteristics shown in the table below (though the base unit is common and it is possible to swap fingers to convert a 2-F85 to a 2F-140 and vice versa):

tstart{c,80%}

thead{Characteristic|2-Finger 85|2-Finger 140}

tdata{Stroke|0-85mm|0-140mm}

tdata{Grip force|5-220N|10-110N}

tdata{Recommended max payload|5kg|2.5kg}

tdata{Closing speed|20-150mm/s|30-250mm/s}

tdata{Gripper mass|850g|1000g}

tend{}

Unlike pneumatic grippers, these servo-controlled, all-electric adaptive grippers are programmable for stroke, grip force and speed. Furthermore, the gripper can sense when the workpiece has been gripped and feed that status information back to the robot controller. Thanks to the multi-link finger design, the two-finger grippers can grasp in three modes: parallel external, parallel internal and encompassing. These grippers are therefore considerably more versatile than conventional robot grippers and are therefore an excellent choice for collaborative robots that are frequently reprogrammed to perform new tasks. Indeed, the Robotiq grippers are particularly easy to integrate with cobots from Universal Robots because of the physical interface with the robot wrist joint and the communications link with the controller.

How many fingers on your gripper?

Robotiq has also developed a 3-Finger Adaptive Robot Gripper for use with the three sizes of robot from Universal Robots. Why would you need three fingers for a robot gripper? First, three points of contact gives a more stable grip, particularly on spherical objects, and also enables the contact force to be reduced when handling soft or delicate objects. Unlike most three-finger grippers that have a set of stiff, self-centring fingers, the Robotiq 3-finger gripper has jointed fingers that can close around an object to grasp it securely (rather like the encompassing action of the Robotiq two-finger grippers). This gripper is also programmable and can operate in three modes: normal, scissor and wide. To help choose between a two-finger and three-finger gripper, Robotiq suggests attempting the task manually to see if it can be done with two fingers or if three are better!

Do you need to know when an object is grasped?

Depending on the application it may be acceptable to program a pick-and-place robot so that it moves to the 'pick' point, actuates the gripper, waits a set time interval for the gripper to close, then moves to the 'place' point - with the assumptions being that the correct object was in the right position ready to be picked, and that the gripper closed as required and did, in fact, grasp the object. However, most applications require a feedback signal to indicate 'object grasped' to the robot controller - perhaps if the cycle time is critical and the robot needs to start moving as soon as possible rather than waiting for an elapsed 'grasping time' interval, or if the process needs to be paused and/or an alarm raised if there is no object ready for picking or if the object is not picked correctly. Feedback can take many forms, such as a pressure switch on a suction cup or a sensor detecting the position of a magnetic piston in a pneumatic gripper. There are also applications in which a dedicated sensor is mounted on the gripper to detect the actual workpiece rather than relying on an inference that a part has been grasped from a sensor acting on the gripper.

Robotiq's two- and three-finger grippers, however, are servo-controlled devices that can be programmed to detect when an object is grasped. In one mode, the gripper can be programmed to close to a certain position (just beyond the dimensions of the object); when the object is encountered, the gripper will continue to attempt to move to the set position but, if it remains at an alternative position for a preset (but short) period of time, the gripper controller assumes the object has been grasped. Another mode, known as Secure Grip Mode, is useful for objects that are not rigid; in this mode the gripper continuously monitors the grip force and closes the gripper further if a reduction in force is detected. One more mode that can be used with the Robotiq adaptive grippers is Low Force Mode, in which the closing force is controlled in order that soft objects are not unduly deformed - yet force and position are monitored in order that the robot controller receives feedback that the object has been grasped.

Conclusion

Robots can be used for an almost unlimited range of manufacturing and assembly tasks, but it is critical to select the correct gripper if the full benefits of automation are to be enjoyed, otherwise the task might be unreliable, overly expensive to implement, or the robot performance might be compromised due to a heavy gripper or one that cannot maintain a secure grasp on the workpiece. While simple suction cups, pneumatic grippers or flat permanent-electro-magnetic grippers might be appropriate for some applications, robots that will be used for many different tasks are more likely to benefit from highly versatile grippers such as the Robotiq adaptive grippers. R.A. Rodriguez (UK) Ltd has found these grippers to be ideal for demonstrating the collaborative robots from Universal Robots, as the same gripper can simply be programmed to handle most components that customers are interested in manipulating - plus there is no need for a compressed air supply. Although a Robotiq gripper will cost more than a simple suction cup or pneumatic gripper, its versatility means that one gripper can be used for most if not all tasks, so there is no need to continually procure new suction cups or grippers - which will be both costly and inconvenient if the robot is frequently being programmed for new tasks.

Follow the link for more information about Robotiq adaptive grippers from R.A. Rodriguez (UK) Ltd.