Control of an automotive electromagnetic suspension system

The main research goal of this thesis is to determine what performance gains can be achieved with a high bandwidth electromagnetic active suspension. As a baseline vehicle a BMW 530i is used, for which a retrofit electromagnetic suspension consisting of a spring and tubular permanent magnet actuator (TPMA) is designed. To design a control system for this actuator, a model of the BMW has been created, which consists of a quarter car model with variable sprung mass, damping coefficient and tire stiffness. As input to this model a road disturbance is used, that was modeled as a white noise source filtered by a first order low-pass filter. To test the performance of the actuator and controllers a full size quarter car test setup is used. As control objectives minimization of the sprung acceleration and dynamic tire compression are used with constraints on the suspension travel and RMS actuator force. The sprung acceleration is used as an indication for ride comfort and the dynamic tire compression is used as an indication for handling quality. To account for human sensitivity to vibrations, the ISO2631-1 standard is used to filter the sprung acceleration. The suspension travel of the controlled system is limited to the maximum value that the BMW achieved with its spring and damper settings over a given road. Furthermore, the maximum RMS actuator force of 1000 N results from thermal limits. Two control approaches are considered, linear quadratic optimal control and robust control. For the former, a controller is found using a linearized quarter car model. By choosing three weighting factors either comfort or handling can be emphasized. Variations of the plant are accounted for by using robust control. Using frequency dependent weighting, certain frequencies can be emphasized. For instance, human sensitivity to vertical vibrations is incorporated using an approximation of ISO2631-1. By varying this weighting together with the other weighting filters either comfort or handling can emphasized, similar to the linear quadratic control. Measurements on the quarter car setup show that an improvement in comfort of 35 % can be achieved with linear quadratic control. This differs 55 % from the value predicted by simulations. However, this deviation can be explained by friction in the test setup and actuator as well as by uncertainties that were not modeled when designing the LQ controller. In case of the handling controller, measurements do match the simulations better on the smooth road. Dynamic tire compression is stability issues of the controller. With robust control an improvement of 48 % in comfort can be achieved on the setup at the cost of an increase of 99.3 % in dynamic tire compression. In terms of handling, an improvement of 17.7 % is achieved, worsening comfort by 10.7 %. Frequency weighting clearly has a desirable effect, as comfort decreases by 6 % for the handling controller on rough road whereas sprung acceleration worsens by 75 %. This means that all vibrations occur outside of the human sensitivity range. Deviations of the measurements from the simulations can be explained by stick slip friction in the suspension actuator as well as vibrations passing through the test setup.