Kyoto University Graduate School of Engineering Department of Micro Engineering
Machining Measurement and Control Laboratory

Parallel-link Machine Tools

Parallel-link machine tools have a completely different structure from conventional machining centers. We have been working on various issues related to the servo control of parallel-mechanism machine tools.

Most of conventional machine tools are driven by feed drive axes that are orthogonal to each other. A parallel-link machine tool has a completely different structure; it is driven by parallel links and rotary joints. Numerous efforts have been devoted on the research and development of parallel-link machine tools since the 90s. Parallel-mechanism machine tools are, however, not widely accepted in today's market. Compared to conventional serial mechanism machine tools, parallel mechanism in general have critical and inherent problems in the motion accuracy and the stiffness. To address these problems, our group has been conducting a research on issues related to the motion control of parallel-kinematic machine tools.

Kinematic calibration under the cancellation of gravity-induced errors Kinematic calibration under the cancellation of gravity-induced errors
To optimize the motion accuracy of a parallel kinematic machine, we showed that it is crucial to separate the error caused by the calibration error and that caused by the gravity. By applying the proposed new calibration method, the motion error was reduced to the target level almost all over the machine's workspace. (March, 2003)

Compensation of Gravity-induced Motion Errors Compensation of Gravity-induced Motion Errors
Unlike conventional serial kinematic feed drives of orthogonal axes, the gravity imposes a significant effect on the positioning accuracy of a parallel kinematic feed drive. We proposed a methodology to compensate motion control errors on a Hexapod-type parallel mechanism machine tool.

Disturbance Observer for Parallel Kinematics Disturbance Observer for Parallel Kinematics
For a parallel kinematic machine, a disturbance observer to estimate the external disturbance such as cutting forces was proposed from armature current of six servo motors. (March, 2003)

A Calibration Method to Evaluate Positioning Errors on Global Coordinates A Calibration Method to Evaluate Positioning Errors on Global Coordinates
We proposed to use a specialized jig to fix a ball of a DBB device exactly in the given location in order to evaluate the machine's positioning error in global coordinates. A significant improvement in the positioning accuracy of the parallel kinematic machine was obtained. (March, 2003)

Calibration of Kinematic Parameters by using DBB Tests Calibration of Kinematic Parameters by using DBB Tests
For high-accuracy control of parallel kinematic machines, it is crucial to identify various parameters such as the length of struts or the location of joints. A kinematic calibration method of these parameters is proposed based on DBB circular tests. (March, 2001)