Authors: Ebbe Overgaard Fuglsang and Martin Rytter
This guide presents knowledge and considerations for selecting, customizing, designing, connecting and controlling end-of-arm tools.
End-of-arm tools are designed to solve specific tasks. While a cobot arm can solve a wide variety of tasks, an end-of-arm tool does only one thing well. There are tools for handling cardboard boxes, tools for TIG welding, tools for high torque screwdriving, and tools for many more very specific tasks.
There is a trade-off between selecting a highly specialized tool that solves one task very well, and selecting a more general-purpose tool that solves similar tasks reasonably. There are end-of-arm tools that can handle a wide variety of cardboard boxes reasonably well, but if you know that all boxes are 1 kg, made of the same material and have fixed dimensions, then it is possible to find a tool that is more optimal for that specific task. Even though there is a trade-off between specialized and general-purpose tools, any qualified tool selection can only be made with specific tasks in mind.
In many cases it is possible to customize a general-purpose tool to solve a more specific task better. An example is customization of jaws, fingertips or suction cups. Customization is also only possible with specific tasks in mind.
There is no single right tool. There is only the right tool for the task.
While each tool is for a specific task, all end-of-arm tools have common properties that are almost always important when used with a cobot.
Tool center point (TCP): When an end-of-arm tool is used with the cobot, you want to move the tool to certain positions, you want to move the tool along a linear trajectory, and so on. The “point of interest” for your task is a point on the tool – not the cobot tool flange. Therefore, the cobot needs to be configured with a tool center point (TCP). The TCP describes where the point of interest is with respect to the cobot’s tool flange. The TCP is defined by both a translation (displacement in X, Y and Z) and a rotation (RX, RY and RZ). If you cannot move the tool along a linear trajectory (MoveL), then it’s likely because your TCP is not set correctly.
Payload: When an end-of-arm tool is used on the cobot, it adds payload to the end of the arm. The tool has its own payload (or mass). When the tool is a gripper, workpieces add additional payload when being handled. Tool and workpiece payloads are important to the cobot and needs to be configured. If the cobot does not behave correctly in free drive mode, or if the cobot makes unexpected protective stops, then it is likely because the payload is set incorrectly. As a rule of thumb, higher payloads mean more challenges. Consider our technical article dedicated to designing applications with high payloads.
Center of gravity (CoG): The distribution of payload is also important to the cobot. You specify this by configuring the tool’s center of gravity (CoG). When the cobot needs to make a rotation around the CoG, the mass will not impact the robot effort, but mass distribution will. That measure is the moment of inertia. In the cartesian space this is a 3 by 3 inertia matrix that due to symmetry can be given as 6 values (Ixx, Iyy, Izz, Ixy, Ixz, Iyz). As with the mass, the mass distribution is also the result of both the tool and the potential workpiece attached to the robot.
TCP, payload and CoG is configured using the teach pendant. Step-by-step guidance on tool configuration is also covered in our E-learning Core Track. The tool configuration values are often available when you buy a standard tool. If they are not available, then you need to resort to estimating, measuring or calculating them.
Many applications require an end-of-arm tool to interact with the environment around the cobot in ways that result in external forces. This is the case for part handling, screwdriving and sanding. It is usually not the case for welding, quality inspection with vision and painting, because tools in these applications don’t touch anything in the environment.
For most purposes it is sufficient to talk about external forces in the broad sense. However, external forces can really be several forces including physical force (measured in Newton) and torque (rotational force).
When using an end-of-arm tool, it is useful to consider these forces:
Collisions forces: Collisions with the environment produce forces. If the forces are large enough, they will cause protective stops that threaten the reliability of your application. Mechanical damage to the end-of-arm tool, or even the cobot arm, is another problem with collisions.
Part-handling forces: Parts being handled by the end-of-arm tool is another source of external forces. The mass of parts obviously results in a force on the tool. External force changes can also come from “picking up a part”, “placing a part”, “dropping a part”, “engaging a chuck on a CNC machine” or similar events. It is often difficult to know exactly when these changes in force take place. This makes them particularly challenging to deal with. Especially when the forces are so significant that they result in protective stops or serious mechanical damage. Examples of tools that are subject to this challenge include grippers for high payload palletizing and tools that hold parts while they are being fixated with a chuck in a CNC machine or similar.
High torque processes: Some tools exist to produce high forces. This is the case for high torque screwdriving tools. For this type of tool, the dimensions, mounting and resulting TCP are extremely important. A long screwdriver with a carefully selected mounting angle may result in a high force going into a bolt being tightened while minimizing the force going into the robot arm. This is basically the same principle that goes into tightening a bolt with a wrench or a spanner – the longer the tool, the more force can be applied. Read also our technical article on high torque screwdriving.
Vibrating processes: Some tools produce forces in the form of vibrations. This is the case for sanders and grinders. For these tools we recommend incorporating dampening into the tool or into the tool mounting to reduce forces going into the cobot arm.
Our general recommendation is to minimize external forces as much as possible. If use of your tool results in many protective stops, then consider what external forces are present, and think about how they can be minimized. Are your waypoints set correctly? Do you set payload changes at the right time? When high forces are necessary at the end of your tool, then think about how your tool’s TCP or dampening can be used to minimize repelling forces going into the cobot arm.
Are you going to design, customize or buy a tool? The answer depends on your task.
A specialised task will require either a design or customization effort. But even with highly specialised tools, prefabricated components like vacuum ejectors, pumps, pistons, or motors can speed up the process and make the result more reliable than inventing everything yourself.
Customize fingers for workpiece: A common customization involves creating custom fingers for a standard gripper. This allows your end-of-arm tool to be optimized for exactly your workpieces. For high mix applications you can consider having multiple sets of fingers for the same tool.
Create mounting for existing tool: Another way to create an end-of-arm tool is to start with an existing tool designed for solving the task manually. In this case you reuse the existing tool and customize the mounting (the part that connects to tool to the cobot) and any IO integration (if the tool is actuated).
Seek inspiration: No matter whether you design, customize or buy your end-of-arm tooling, we recommend that you seek inspiration. Try to talk to someone that already solved a similar task with an end-of-arm tool. Seek also inspiration from UR+ products (https://www.universal-robots.com/plus/).
Active tools need to be supplied with both signal/communication and power. In case you buy an integrated UR+ product, the choice is often made by the tool vendor. The options that are available on the cobot are:
Tool connector: The cobot has an M8 tool connector dedicated to end-of-arm tools. With this connector you 1) avoid cables along the arm, 2) can supply power up to 48 watt peak, 3) have access to 2 digital inputs, 2 digital outputs and 2 analog inputs at the end of the arm, and 4) can optionally use the analog inputs for RS485 communication. Many UR+ products use this option.
Control box IOs, ethernet and USB: You can connect your tool using all the connectivity options available in the control box. The main limitation of this approach is that it implies external cabling along the cobot arm.
External power sources: It is also possible to use an external power source such as external compressed air or an external electrical power supply. These solutions add additional hardware to your end-of-arm tool integration. This may contribute additional cost and complexity of your tooling, but in some cases, it can be the simplest and cheapest solution. Obviously, this solution also implies external cabling.
There are two main ways to control tools. With and without a URCap. Both approaches are integrated into the user experience on the teach pendant.
Control with manual setup: Most connectivity options in the control box (IOs and fieldbusses) can be configured in the installation settings and controlled using both program nodes and URScript.
Control with URCap: End-of-arm tools that are supplied from a UR+ partner often comes with a URCap that is designed to make control of the tool easy. It is normal that a URCap comes with one or more program nodes for control during program execution, an installation/application node for configuration, and a toolbar for controlling the tool when no program is running. If you would like to use a UR+ product without a URCap (for whatever reason), then there is usually a way to do that, but you might have to talk to the vendor.
USB in the control box and RS485 via the tool connector: These interfaces require a URCap with a daemon process that controls the tool. Ask your tool vendor. Are you developing your own tool, then learn how to develop your own URCap, or find someone that can help you develop one.
In summary, there are many ways to use end-of-arm tooling on a cobot, and the best approach very much depends on the task you are solving. Our recommendation is to always seek inspiration and try to talk to someone that solved a similar task. A great place to start is our forum.