Design of Digital Controllers for Electromagnetic Force Compensated Balances Focused on the Disturbance Transfer Function

The main objective of the paper is to propose a new possible design process for controllers in electromagnetic force compensated balances (EMFC). Controllers used in EMFC balances demand a high precision of the measured and generated electrical quantities. Regarding to the achievable uncertainty in static measurements, EMFC balances are state of the art, but in future applications the importance of dynamic applications will probably increase. Therefore the controlled system should possess a high measurement bandwidth to reduce the measurement time in dynamic applications. For digital controller concepts this results in ambitious requirements on the controller hardware especially on the digital-to-analog and analog-to-digital converters. The paper will illuminate the limitations caused by commercial off-the-shelf standard hardware and propose a possible alternative hardware concept. A design process will be presented, which is strongly focused on the disturbance transfer function of the control loop. This can be achieved by building a detailed model of the balance behavior, in particular by modeling the disturbance sensitivity. Based on the consideration of these additional system characteristics in the controller design process, the measurement time of the balance is reduced significantly compared to conventional PID controllers. By using a specially developed controller hardware environment, this is also achievable with a low static uncertainty of the controlled balance.