Design of an analog-digital PI controller with gain scheduling for laser tracker systems

Laser trackers are important devices in position metrology. A moving reflector is tracked by a laser beam to determine its position in space. To ensure a proper function of the device the feedback control loop is an essential part. An analog PI controller with online parameter adaptation and absolute distance measurement ability is used to guarantee an optimal dynamic system. The feedback controller is connected to a quadrant detector which serves as the sensor element in the control loop. The position of an incoming laser beam is measured by the quadrant detector and the controller provides the input signals for a subsequent actuator. The control variable is the deviation of the laser beam from the centre of the diode which should ideally be zero. The actuator consists of two axes and each one is equipped with a rotatable mirror. The task of the controller is to rotate the mirrors in such a way so that the laser beam follows the movements of the reflector. To design an optimal controller linear, time-invariant models of the actuator and the position sensor are developed to optimize its parameters. The gain of the plant correlates with the distance between the reflector and the laser tracker. To achieve the optimal dynamic performance the controller is automatically adapted to the distance during operation. A method based on oscillation injection to measure the absolute distance is developed. Due to higher dynamic demands a standard analog PI controller is implemented with the controller gain tuned by digital potentiometers. A microcontroller is used to adjust the parameters and to estimate the distance. During the power up sequence and in case of a beam loss the system is completely controlled by the digital part.