It seems obvious that a robotic arm can’t perform a specific task until an end-of-arm tool (EOAT), sometimes called an end effector, is added. It might seem as simple as buying a tool for the task, but it’s actually a complex decision with several interconnected factors to consider. These are the top five:
1. Payload Limits
A robotic arm’s payload includes the weight of the part being manipulated as well as the weight of the tool itself. It seems like basic addition, but it’s not as simple as knowing the maximum amount of weight you can attach to the arm. Higher weight also means more force is required to move the part, which means more vibration and force absorbed by the machine, which results in more wear and tear over time. Because of this, inertia actually becomes the limiting factor to performance.
Fortunately, within today’s broad range of gripper and tool options there is room for tweaks. One possibility is lightweight end-of-arm tools made with aircraft aluminum, which is generally lightweight and strong enough to be used with hollow tool designs.
Another emerging option is tools made by additive manufacturing. As this article in Assembly Magazine notes, by building up a tool in thin layers, it’s possible to design and create shapes that are difficult, expensive, or impossible to achieve with metals. The technology includes “software that optimizes the design of structures organically, following the lines of the geometry and adjusting the thickness as needed for strength or flexibility.” The end result is a custom tool engineered with as little material (i.e. weight) as necessary for the task.
2. Actuator Types
There are four types of actuators, often suited to different types of tasks and motions:
- Pneumatic – These are generally capable of fast movement and are comparably inexpensive. In most cases, pneumatic tools move between one or two positions, such as open and close for a plier-style gripper.
- Electric – More expensive than others, electric actuators provide far greater flexibility for moving or positioning parts, finishing applications, and welding. Traditionally electric actuators and tools required programming skill, but new software and sensor technology is making it possible to “train” robots to perform a set of steps at the point of need. Other advantages include software that gives greater control over movements and the ability to move through many different strokes and positions, making them re-deployable in high-mix environments.
- Vacuum – Vacuum technology generally uses two air lines, one for suction and one for pushing/blowing away. The end effector is usually either a curved suction cup or a cup with accordion-fold bellows. The strength of the suction can be adjusted. This technology works well with flat surfaces that allow good suction (as opposed to porous or highly perforated); however, the cups could leave a mark or damage certain materials if suction is improperly adjusted. Dust or particles may also clog air lines, requiring additional maintenance.
- Hydraulic – Hydraulic actuators are powered by liquids, often oil, which means they can generate great force and move heavy objects. Maintenance of the oil and supply lines can add time and expense, and spills and leaks are messy, which may contaminate parts and pose health and safety risk for workers.
3. Tool Types
In general, the application will determine the tool type. Think of tools in these broad categories:
- Material removal – These are used for tasks such as grinding, buffing, deburring, cutting, trimming, or polishing, which are generally accomplished with a specialized tool for the job.
- Welding and soldering – These can include TIG, MIG, or laser welding tools as well as soldering irons. As with automated material removal and finishing processes, robotic welding has the advantage of being faster and more consistent than its manual counterpart.
- Grippers – Because there are many types of gripper tools, they are both flexible and popular. In fact, global demand for grippers is estimated to exceed $1.14 billion in 2019. Grippers are often the end effector of choice for pick and place, moving parts in or out of a rack or pallet, and positioning items. Common configurations include jaws, pliers, claws, and even opposable “fingers” that look surprisingly hand-like. The shape of the part and where to pick it up determine the best grasping style, but things like surface material and hardness, friction, and weight are also taken into consideration.
4. Accessories and Features
Newer robotic arms can be equipped with accessories to enhance efficiency and safety, including:
- Vision cameras – Both 2D and 3D cameras positioned at various angles on the arm send critical data to computer software to identify product presence, orientation, and features. These vision systems can also be used to recognize potential collisions that trigger the robot to react in time.
- Automatic tool changers (ATC) – These enable faster tool changes and shorter changeover times, often with tools that latch or lock into place instead of time-consuming installation.
- Force torque (FT) sensors – Gripper fingers enhanced with FT sensors can “feel” forces in all axes to synthesize the sense of touch and rudimentary hand-eye coordination. Signals picked up by the sensors can determine if a part was picked up, how much pressure is being exerted, and if an item is slipping or moving. Some can even use this data to pause and readjust grip before proceeding. FT sensors are also one of the features that allow some collaborative robots to be “trained” for new tasks by being guided through the necessary movements.
5. Available Resources
The most advanced, flexible EOAT is only as useful as your ability to take full advantage of it. Equipment and installation costs are common concerns, but there are other resources you’ll need to bring to the table. These include any required programming skills for initial configuration, troubleshooting, and future adjustments, training requirements, supplies like compressed air and cleaning agents (especially when contamination is a concern), maintenance and replacement parts. Always factor in time and money needed before beginning an automation project.
Keep the Big Picture in Mind
Choosing the right EOAT can be a balancing act since the above considerations don’t exist in isolation. For example, taking advantage of the arm’s maximum payload will mean that the robot will have to move at a slower speed. Heavier parts also require the right gripper selection to reduce the risk of accidental dropping.
When cycle time is a concern, the speed of pneumatic actuators can be helpful, but if the part is heavy, the acceleration and force required to pick it up coupled with the fast-moving actuator can increase wear on the tool. Likewise, flat parts can be grasped and rotated into position with great accuracy with an electric gripper, but if your application requires cleanliness specifications for the surface, you’ll need to consider how to keep the gripper fingers free of dirt, oil, and other contaminants, which might be easier to accomplish with a vacuum cup.
And of course, while efficiency is a major goal of automation, worker and facility safety is even more important. This includes the safety of the tooling itself and of the robotic machine (for example, using proper screening with large robots and choosing the right power and force limiting collaborative robot for your application). “Not all processes are applicable to collaborative robots, and not all end-of-arm tooling is safe to work in a collaborative manner,” notes Manufacturing Automation. Always perform a risk assessment including, tool pinch points, sharp edges, potential for drops, spills, and exposure to hazardous materials.
Remember that while there are standard mechanics and common components and applications for EOATs, each robotic system is usually a piece of custom automation equipment that should be tailored to your needs. These considerations, along with the specifics of parts and tasks mean there’s a range of possibilities for selecting the right tool. If you’re trying to decide what end of arm tool design is right for your situation, an experienced automation vendor can help you match the equipment to your process. Tell us about your application, we can help.