Control Strategy for the Longitudinal Degree of Freedom of a Complete Vehicle Test Rig

The Institute for Mechatronic Systems in Mechanical Engineering (IMS) designed a concept for a test rig, which enables the simulation of longitudinal, steering and vertical dynamics for a complete vehicle under laboratory conditions. The main part of the test rig concept is a shaft, which contains three constant velocity joints and two ball-spline supported length compensations. It connects the wheel hub of the test car to an electric motor. In addition a linear actuator is mounted to the middle part of the shaft and a hydraulic actuator replaces the suspension strut. These actuators can load the longitudinal, steering and vertical degree of freedom of the test car according to simulated driving maneuvers. A prototype of this concept is being built at the IMS lab. Beginning with a precise explanation of the test rig concept this paper discusses the control strategy for the rotational speed of the wheel hub of the car mounted on the test rig based on a simulation. The following issues have to be considered. The shaft connecting the electric motor to the wheel hub has an elasticity, which cannot be neglected. As the actuator and sensor position are not collocated and this is an unrestrained degree of freedom, a model based control approach like for instance pole placement is required to assure stability. In this context the effect of the placement of the poles on the longitudinal dynamics will be evaluated. Another factor, which has influence on the longitudinal dynamics, is the torque in the vehicle power train, which is regarded as a disturbance. In a vehicle it can only be observed over the bus system with insufficient accuracy. For this reason a disturbance feedforward strategy is explained and the influences of the accuracy of the power train torque are reviewed.