Actuators help robots achieve machining accuracy

The Fraunhofer IPA has developed a compensating actuator system based on piezo actuators from Physik Instrumente (PI) to enable industrial robots to achieve the accuracy necessary for machining.

Accurate machining is usually carried out with machine tools or machining centres.

This means high costs and workpiece sizes limited by geometrical factors.

It would be cheaper and more flexible if industrial robots could be used instead, but their inaccuracy has so far made this impossible.

This inaccuracy arises from the long serial kinematics chain with only low stiffness.

A different approach is now possible.

It has not been necessary to develop new robots because standard versions have been combined with an external actuator system to increase the absolute accuracy.

The Fraunhofer IPA in Stuttgart developed a separate compensating actuator system for the milling head.

The inaccuracies that occur with robotic milling units are measured online while the machining is in progress and are corrected directly where they occur in real time by the compensating actuator system.

The workpiece is not firmly clamped to a table, as is the case when machining with CNC machines.

With this system, the workpiece is gripped by the robot, which guides it as it is machined by the milling head.

This means that only one machine is required for handling and machining, investment costs decrease and flexibility increases.

In addition, this type of robotic solution can be incorporated into fully automated production lines.

The compensation kinematics, including the actuator technology and mechanical setup, were developed at the Fraunhofer IPA and are based on piezo actuators from PI.

The piezo actuators operate without wear and tear, are friction free and have no slip.

They can be accelerated with up to 10G and are suitable for the high frequencies that are required to compensate for the inaccuracies in the robot's trajectory.

Since piezo actuators only operate with small displacements on principle; the IPA combined them with flexures.

This made it possible to realise paths up to 690um in the application described.

The trajectory accuracy of the kinematics in all three axes is in the nanometre range.

The flexures that transfer the power and the motion also operate without wear and tear and require little maintenance.

They are said to be lighter, quieter, stiffer, more dynamic and more accurate than conventional compensation mechanisms.

The closed-loop robotic solution is intended to open up new possibilities for the milling of metals and plastics.

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