Tilting Measurement and Compensation in Robotic Joints

Methods for improving the accuracy of industrial robots have been the focus of many research projects. A large fraction of the position error at the tool is caused by the flexibility of the joints. For the joint stiffness, the gear and the output bearing are the crucial parts. The position error caused by the gear can be measured and compensated by using an additional output side position measuring device. The online measurement and compensation of the tilting angle resulting from the bearing's elasticity has not been addressed so far. This paper presents a sensor system, which can determine the tilting angle of the joint based on two position measurements. The system is first tested by applying weights on a lever and verifying the measured values against a comparator reference. In a second test, the sensor system is mounted in a three-axes robot. The measured tilting angles are used to compensate the resulting displacement by adjusting the reference position based on a forward kinematics model using the tilting angles.