Abstract
For conventional automobile development, vehicle evaluations are conducted by using completed vehicles on chassis dynamometers or test tracks. However, chassis dynamometer tests are incapable of evaluating transient conditions such as tire slip; one of evaluation items required for hybrid vehicles (HVs) with engines and motors. In addition, test conditions on test tracks are not only limited but also affected by weather. On the other hand, a difficulty with simulation evaluations using Computer-Aided Engineering (CAE) is that it requires highly accurate component models. To resolve these evaluation-related issues, the power train development group of Toyota Motor Corporation has developed a system bench called Virtual and Real Simulator (VRS). This is a power train test bench which simulates vehicle operation conditions by using actual (i.e., "real") components of power train to be evaluated, and integrating them with models with "virtual" dynamic characteristics of vehicles. This article describes the development of VRS for HV evaluation.
In the conventional configuration of a VRS bench, the object for evaluation does not possess hydraulic brakes, and the VRS control unit commands the dynamometers to generate braking torque. The problem with this configuration was that it did not generate torque from regenerative braking which is one of the characteristics of HVs. Besides, the control response at sudden changes of motor torque caused by wave roads (meaning bouncy corrugated roads) must be evaluated. To resolve the issues described above, a virtual brake system model was added to the conventional VRS bench with low-inertia and high-response dynamometers. The added virtual brake system model computes the data required for vehicle control replacing an actual brake electronic control unit (ECU) and issues commands to evaluation objects. As a result, this enabled a simulation of regenerative brake as well as slip control of motor, which are characteristic of HVs. When a vehicle is driven on wave roads, vertical forces act on the vehicle, and slip-and-grip cycle is repeated at the point of contact between the tires and the road surface. Computation of vertical forces on wave roads using a vehicle model and vehicle test on wave roads using an actual vehicle are conducted in order to simulate this condition on the VRS bench. Based on the result from the computation and the actual vehicle test proves that virtual wave road drive can be simulated on the VRS bench by adding the vertical forces generated during the actual vehicle test to the longitudinal forces absorbed by dynamometers.
In the result, the regenerative braking as well as slip control of a motor equivalent to actual vehicle conditions has been successfully simulated. Moreover, VRS can perform evaluation with road surface specifications such as the pitch, amplitude, and lateral phase difference of wave roads. Since the introduction of VRS for HV evaluation, these merits have been utilized to perform precise drive performance evaluations from the initial stages of HV development.
Keywords:hybrid vehicle, VRS, regenerative braking, slip control, wave road