Cost-Effective Pneumatic Grippers for Electric Robots
Combining pneumatic power source for grippers on the end of electric robots provides the best of both technologies. In many cases, the use of a pneumatic end effector is the most cost-effective solution for grasping the parts being handled by the robot.
Gripper Defined
A gripper is a device which grasps a part or object being moved by a robot or other automation device. The gripper acts like a human hand, but in most cases is not as articulate and sophisticated. Like your hand, a gripper provides the ability to grasp an object, holds the object while it is being moved or manipulated, and then releases the object once the movement or manipulation is completed. The gripper is just one type of end effector on a robot or automation device. Grippers come in a wide range of styles and sizes to handle an almost infinite range of objects. Grippers can be powered pneumatically or electrically and can include vacuum cups or bellows that expand to grasp an object. This article will focus on the use of pneumatic grippers on robots as a cost-effective method of grasping and manipulating the objects being handled by the robot.
Pneumatic Gripper Basics
A pneumatic gripper operates by providing compressed air to the cylinder contained in the body of the gripper. The compressed air is controlled by a directional control valve forcing the cylinder’s piston up and down. This drives various types of mechanical linkage forcing the gripper’s jaws open and close. The types of linkage and the jaw configuration allows for different types of jaw movement. These are typically either angular with pivoting jaws or parallel with linear jaw movement.
Standard pneumatic grippers come in a very large array of sizes and styles. These grippers can be very small and precise for grasping miniature parts such as electronic components and intricate medical devices. Pneumatic grippers are also available in very large configurations for grasping objects such as engine blocks and automotive wheels.
Jaw configurations vary widely allowing grippers to be easily adapted to the objects being grasped. Two jaw and three jaw units are common, and grippers can be designed with multiple jaws for specific appli
cations.
Various types of sensors on pneumatic grippers provide inputs to the robot’s controller indicating the position of the jaws. Some sensors are digital and provide an input at a specific point in the jaw’s travel. These are used to indicate when the jaw is open or closed and can be adjusted to indicate the jaw position when a part is grasped. Analog sensors are also available on some grippers providing constant jaw position feedback to the controller.
Customization
The mechanical characteristics of a pneumatic gripper allow for easy customization of the units for specific applications and environments. For example, units can be designed with multiple jaws for installing o-rings. Grippers can also be modified to function in harsh environments such as high temperatures or areas where a high degree of dirt or contamination exists. The materials pneumatic grippers are made from can also be customized. As an example, grippers can be constructed from plastic composites or food grade materials for specific industry applications.
Pneumatic Gripper Advantages
There are a number of unique advantages of using a pneumatic gripper as an end effector on an electric robot. Pneumatic actuators are known for having high force to weight ratios. This factor especially applies to a pneumatic gripper. Because of this efficiency, the gripper size and weight can be minimized allowing for higher overall payloads carried by the robot. Pneumatic grippers also have a very quick response time for opening and closing the jaws. This reduces dwell times for pick up and release providing for fast cycle times. The cost of a pneumatic gripper is typically much less than that of an electric gripper.
There is also a very wide variety of standard pneumatic grippers to chose from in the marketplace. This makes it easy to select a gripper that best fits an application saving design time and providing quicker lead times. One particular feature on some pneumatic grippers is the ability to make the gripper ‘single acting.’ This is typically done with internal springs that provide retention of the part even if power is lost. This can be used as a safety feature and for highly valuable parts that you cannot afford to drop.
Pneumatic Gripper Disadvantages
There are a few drawbacks to pneumatic grippers as compared to an electric gripper. The pneumatic version requires a pneumatic power source and a control valve for opening and closing the gripper. However, most automation systems are already using other pneumatic actuators in which case the power source would be readily available. The jaw travels and jaw forces of a pneumatic gripper cannot easily be changed once the gripper is installed. Jaw positions are typically open or closed with no positions in between, except when an analog sensor-compatible gripper is available.
Ideal Applications
Despite the few disadvantages, there are many applications that are ideal for pneumatic grippers as end effectors on electric robots. Pneumatic grippers are ideal for applications where the robot is handling parts that are a consistent size. When more flexibility is required, two or more pneumatic grippers may be employed on a single robot. This is made possible due to the lighter weight characteristics of these grippers. This lighter weight is also an advantage when the robot’s payload is critical. The lighter gripper may allow for higher total payloads without increasing the size of the robot. Pneumatic grippers are also ideal for applications requiring high grip forces for handling larger payloads due to their high force to weight ratio.
Application Considerations
There are a number of application considerations for using a pneumatic gripper on an electric robot. As stated earlier, a compressed air supply is required to operate the pneumatic gripper. The available consistent air pressure is an important application consideration. Typically, a minimum of 60 psi should be used. It is important to ensure that the minimum air pressure used to select the gripper for the application is always able to be maintained. The grip force created by the gripper is in direct relationship to the input air pressure to the unit. Using the correct directional control valve is part of the air supply equation. The valve must have enough flow capacity to properly open and close the pneumatic gripper. The valve should also be capable of maintaining air pressure to the gripper throughout the grip cycle.
Proper sizing of the pneumatic gripper includes a number of factors in addition to the input air pressure. Jaw motion (angular or parallel), length of the tooling attached to the jaw, and the moment loads that the jaws will encounter are all factors to consider. Calculations can be made using the published technical date provided by the gripper manufacturer. Software is often available to help in the gripper selection process.
The design of the finger tooling that contacts the part is also an important consideration. The tooling needs to be as light as possible, yet able to handle the grip force and moment loads. The finger tooling should be designed to conform to the part shape and encapsulate the part being gripped whenever possible. It is always best to keep the part being gripped as close as possible to the actual gripper. This helps with the grip force provided and minimizes the moment loads.
A pneumatic gripper with internal springs can be used to maintain grip force even of air pressure is lost. These grippers can be used as a single acting unit with the internal springs providing the grip force or a double acting gripper using both the springs and the air pressure to grip the part. In either case the gripper should be sized such that the internal springs alone can maintain a hold on the part.
Pneumatic Grippers on ‘Cobots’
In recent years there has been a trend toward the increased use of ‘collaborative robots’ also known as ‘Cobots.’ Cobots are typically power and force limiting. The robot can feel abnormal forces in its path and may be programmed to stop when it reads an overload in terms of force. The cobots are designed to dissipate forces in case of impact on a wide surface. As a result, the cobot may be used for direct collaboration with the worker for various tasks. The cobots can work alongside humans without any additional safety devices.
Cobots are typically highly adaptable and virtually plug-and-play. This means a user does not need a great deal of experience to program and apply the device in an effective manner. Some companies are using them because they can be placed alongside humans. Many others are applying the cobots due to their extreme ease of programming, quick uptime, and great flexibility.
Pneumatic grippers with tooling that encapsulates a part can be operated at lower air pressures which is ideal if the cobot is being used in a human collaborative environment. The lower air pressures mean less force and less chance of injury. As stated earlier, many of the collaborative robots are applied because of their ease of use versus being used in a human collaborative environment. In these cases, the pneumatic gripper can run at high pressures producing higher grip force and potentially allowing a smaller, lighter gripper to be used. This has a positive effect on the total payload of the cobot.
Adapters are available that make it easy to attach pneumatic grippers to cobots manufactured by Universal Robots. These adaptors have embedded directional control valves for control of an end-effector through the robot’s tool port. The system easily attaches to the robot arm and is intuitively operated from the Universal Robot interface.
Summary
Pneumatic grippers on electric robots are a good combination of varying technologies. The electric robots provide a great deal of flexibility and accuracy. Pneumatic grippers offer a cost effective, high force to weight ratio end effector on these robots. The wide variety of standard grippers along with the ability to easily customize the units makes them adaptable to a wide range of applications. The pneumatic gripper’s use on the very popular cobots provide even more application capabilities for these grippers.