Table 3 provides data on geometric and inertial character- istics of the components of the test rig.

Fig. 2. Belt Drive lumped-parameter model

2.2 Control system

The control architecture of the test rig is composed by a high voltage control section for the two electric motors and a low voltage ECU that computes the reference speed to give as an input to the control loops of the two electric motors and of acquiring and analyzing the data read by the sensors mounted on the test rig. The power supply (HMV01.1R-W0065-A-07 by BoschRexroth) powers both the inverters that are connected onto the same DC BUS. The main feature of such supply unit is the possibility of power regeneration at the mains, meaning that the power produced by one of the two motors acting as  alternator is given back to the network. The modular power section is completed by two BoschRexroth inverters, HMS01.1N- W0210 for Motor 1 and HMS01.1N-W0070 for Motor 2. The control sections (CSH01.1C-CO-ENS-EN2-MEM-L2-

S) are the same for both inverters and provide three nested control loops: current, speed and position control. In the studied application the position control is not of interest, so such loop is not considered in the following analysis. The two inverters give the possibility of closing the control loops inside their firmwares but also outside by means of a dedicated control unit. The control laws implemented onto

Fig. 3. Power and information exchanges of the system. In blue the information exchanges area, in red the bond-graph representation of the power transmission area

the inverters’ firmware consisit of a Proportional-Integral (PI) compensator for each control loop. In Fig. 3 there are indicated the possible inputs that can be provided to the two inverters through the master communication and CANOpen Protocol. The two control sections allow the utilization of some optional features for analog and digital inputs and outputs, encoders acquisition, serial interface towards commissioning tool on PC, emulation of encoder data towards ECU, master communication with ECU through CANOpen Protocol.

The ECU is built around a Control Card based  on a Texas Instruments floating point Digital Signal Processor (DSP). The Control Card is equipped with 16 ADC channels, capture and quadrature encoder modules and an asynchronous RAM which allows the real time acquisitions of the variables measured on the test rig by the sensors described above. Besides, the communication peripherals allow the use of the CANOpen protocol for communication towards the two inverters. The ECU is used to control the operations of the two motors, actuating the automatic belt tensioner and monitoring the parameters measured on the system.

3. SYSTEM MODEL AND SPECIFICATIONS DEFINITION

As shown in Fig. 3, the system is  composed  by two main subsystems: one characterized by a power exchange, including the electromechanical system comprising the power transmission and the two electric motors, the other by an information exchange, including the control ar- chitecture. The system operates in two main conditions: crankshaft driving the transmission, and BSG driving the transmission.

In the design phase of the test rig, the performance spec- ifications were defined by means of a model of the  BDS

3.1 Dynamic Model

The overall power transmission structure can be pided into 8 main components as shown in Fig. 3: belt, electric motors pulleys, A/C compressor, idler pulley, automatic tensioner pulley and electric motors. These subsystems are included in the red perimeter of Fig. 3, their interac- tion is described with the corresponding power variables. The causal form (input/output relations in each   subsys-