State space controller for actuator control of an active jerk decoupled feed drive

For machining operations with multiple directional changes, e.g. machining of sculptured surfaces, a fast attainment of the maximum axis acceleration is essential to obtain short machining times. Machine tools with highly dynamic linear direct drives are often used for these purposes. However, a high jerk (time derivative of the axis acceleration) causes a vibration excitation of the machine structure [1]. The vibrations lead to a reduced workpiece quality and may cause damage to machine components. Therefore, the jerk is often limited in numerical control [2]. However, by limiting the axis dynamics, the potential of linear direct drives is not fully utilized. An approach to increase axis dynamics and to simulataniously reduce machine vibrations is passive jerk decoupling (JDC) [3]. However, the main disadvantage of passive JDC is an additional resonance in the low-frequency domain due to the mechanical low pass filter characteristics. To address this, the concept of active JDC was developed [4] and a first test rig was set up [5]. The active JDC is an improvement of the passive JDC by adding actuators and sensors between the drive and the machine frame. A state space controller is implemented to selectively damp the vibrations of the machine by means of the actuator force. For this, the dominant vibrations are modelled as state-space variables. The vibrations that are not detectable by the sensor system are estimated using an observer. In this paper, the controller and observer synthesis for parameterising the control system for the active JDC is described. A new simulation-based method for pole placement of the closed loop's Eigenvalues is presented. The potential of the active JDC is demonstrated by showing the vibration response of the machine structure after a positioning step with a high jerk. The vibration and positioning response of the test rig due to a position step of X = 30 mm is shown in Figure 1. Compared to a stiff fixation without active JDC, the position overshoot is reduced by 95% at a maximum actuator force that is up to 58% less than the vibration-exciting motor force.