ROBOT GRIPPERS & END EFFECTORS

One of the most important areas in the design of robot systems is the design of end effectors[4]. Most of the problems that occur in production are caused by badly designed tooling and not by faults in the robots. There are many different types of gripper available along with the vast number of specialist tools for nut running, arc welding, paint spraying etc. These grippers are not used solely with robots however. They can be used for fixturing components anywhere in an automated or semi-automated line.

Gripper Types

The most commonly used grippers are finger grippers. These will generally have two opposing fingers or three fingers like a lathe chuck. The fingers are driven together such that once gripped any part is centred in the gripper. This gives some flexibility to the location of components at the pick-up point. Two finger grippers can be further split into parallel motion or angular motion fingers.

For some tasks however where flexible or fragile objects are being handled the use of either vacuum or magnetic grippers is preferable. With these the surface of the gripper is placed in contact with the object and either a magnetic field or a vacuum is applied to hold them in contact. Any errors in placement of the object at the pick-up point will be reflected in a similar error at the destination so these grippers are not usually used for high accuracy applications.

Accessories

Along with a wide range of standard grippers being available there are also a number of standard accessories for mounting them. If motion of the gripper is required which cannot easily be provided by the robot then this can be provided in the end effector. Linear and rotary units are available in many different configurations. The linear units tend to be used where reaching into confined spaces is required while the rotary units are often used where more than one gripper is mounted to switch from one to the other.

Another mechanism that is frequently used is a collision detection system. A collision detection unit works in the same manner as a probe on a co-ordinate measuring machine. It is attached between the robot flange and the end effector. The device consists of two plates held together by a spring. The two plates contact each other at three electrical contacts arranged in a triangle. These contacts are wired in series such that any motion of one plate relative to the other will cause the circuit to break. This circuit can then be wired either to the emergency stop circuit of the robot or an interrupt in the program.

Where more than one job is being performed by a robot and all of the necessary tools cannot be assembled together, it may be necessary to use a tool changer. Devices exist such that all of the services to the tools (electricity, air, etc.) can be fed through the changer so that a tangle of different wires, pipes etc does not have to exist for each different tool. Where possible however it is preferable not to use tool changers as they add weight to the endeffectors and cause an increase in cycle time when changes are required.

Gripper Drives

The same options are available for grippers as for robots. The most popular one here is pneumatic however because of the ease with which it can be supplied to the end of a robot arm and controlled. The only disadvantages of pneumatics are that it has a slightly lower power to weight ratio than hydraulics and it is not as controllable or easy to feed as electricity. In most situations however these are not important.

Gripper Design Considerations

When selecting a gripper there are a number of factors that may need consideration.

Gripping force

When considering the gripping force required a number of factors must be considered. Not only must the mass of the object to be gripped be taken into account but also the accelerations imposed on it by the robot. Where accelerations are not known a figure of 2.5g is generally assumed. The coefficient of friction between the gripper and the object may also be an important factor. This can often be increased by using one of the special rubber based materials that have been developed. The use of these materials can create maintenance issues however as they have a finite life.

One other way of reducing the gripping force required is to use fingers designed for the form of the component. This reduces the flexibility of the gripper but dramatically increases the weight carrying capacity.

Weight

Industrial robots have fixed lifting capabilities and the combined weight of the gripper and gripped component may be important. Even where this weight is within the capability of the robot it may cause an unacceptable increase in the cycle time of the operation. The distance between the robot flange and the centre of mass may also be important and this should be kept to a minimum.

Supply of services

With robot end effectors loose cable or hoses are something to be avoided. They increase the size of the tool (making it harder to reach into confined spaces) and cause many failures due to rubbing or snagging on other parts of the mechanism. It is therefore preferable to route all services or signal connections through the robot arm and then internally through the tool.

Enviromental capabilities

End effectors are often required to work in hostile enviroments. High temperatures, dust or the presence of chemicals will require special materials or designs to be used.

Sensor capabilities

For certain applications some degree of sensory feedback from the gripper is necessary. This may be measurement of insertion or gripping forces or may simply be a proximity sensor to say if anything is between the jaws of the gripper. Some standard grippers are provided with feedback to show the seperation of the jaws but most grippers have no feedback.

Others

Other factors to be considered include the speed of the gripper jaws and the range of sizes of component they can grip. The amount of maintenance required is also important though most modern mechanisms require little or no maintenance. For some situations the behaviour of the gripper on power failure may be critical. Some but not all use either springs to apply the gripping force or non return valves to ensure that pressure is maintained.