Research On Performance And Key Technology Of Inertia Simulation In Brake Dynamometer Testing

Brake dynamometer adopts the brake assembly as the specimen to test the brake efficiency, thermal stability, wear of brake linings and fatigue property by simulating the actual brake process of vehicle according to the specified regulation. Compared with the sample testing, the brake conditions and test results of dynamometer testing is more reliable, for which the latter becomes the authority test of brake assemblies and friction materials.As one of the most prominent superiorities of brake dynamometer over other laboratory test, the former performs inertia braking using the actual brake assembly, the key technology of which lies in the accurate simulation of inertia (to provide the friction work consumed by the brake in the braking process). The traditional inertia simulation method adopts inertia disk mounted on the shaft as the energy storage component to provide the friction work consumed by the brake in the braking process. There are many inherent deficiencies of this method limit its application in new brake dynamometers such as inherent inertia step, overlength of the shaft, oversize of the mainframe, inconvenience of changing inertia and incapability in compensating inertia error.As the most commonly used transforming element of electric power and mechanical power, motor is convenient in torque and speed control. It could be employed to provide part of the braking energy together with the inertia disk so as to make up the deficiencies of puer mechanical inertia. If the brake consumes the same amount of friction power and has the same test data in brake torque, deceleration and distance between the dynamometer testing and the state-of-the-art braking process, the motor represents inertia successfully. This kind of inertia represented by the motor is called electric inertia. Compared with the mechanical inertia, electric inertia has many advantages:①It can represent inertia consecutively in the whole inertia range, eliminating the inherent inertia step of mechanical inertia.②The number of inertia disks is reduced dramatically, which simplifies the design of inertia disks and enables a smarter layout of brake dynamometer.③The disk operation is simplified since a series of inertia can be represented by a specified disk.④The inertia error induced by resistance can be compensated, which is one of the most significant advantages of electric inertia.⑤Road test can be simulated by brake dynamometer with electric inertia as far as the resistance model of the real vehicle is given.⑥The inertia difference between the light and heavy load can be simulated by the electric inertia without ceasing the test.⑦The shaft can accelerate in a more rapid speed when the motor represents positive inertia. It can be forecasted that hybrid inertia will become the mainstream of inertia simulation in brake dynamometer testing.Up to now, the domestic research on electric inertia is still at the theoretical analysis level, while the research abroad is seldom published due to the request of enterprise secret. It is difficult to confirm their solution to inertia simulation. According to the domestic research, the most obvious solution to represent inertia with motor is speed control method and torque control method. The former realize electric inertia through controlling the output speed of the motor to keep the braking process with electric inertia the same as that of the state-of-the-art brake dynamometer. Speed control method is hard to realize, since the fast speed response requirement is difficult to be satisfied on a large inertia system, and the share of the motor control unit brings obstacles to parameter adjustment between speed control and inertia simulation as well. Torque control method realizes the target braking process by controlling the electromagnetic torque of the motor. Being the primary method of electric inertia, torque control method has advantages in fast response, comprehensive adaptability of all the braking mode and avoidance of accumulative error. However, as to the minor electric inertia and small braking deceleration, that is, when the electromagnetic torque of the motor is tiny, the braking time is usually very long, thus the inertia simulation precision can hardly be guaranteed.In order to make up the deficiency of torque control method, energy compensation method is proposed based on the compensation of friction work of the motor. The rule of electromagnetic troque, start point and period of compensation is discussed on the basis of detail analysis on friction work, braking distance and braking deceleration. Compared with troque control method, energy compensation method shows nice performance in medium and small inertia simulation while it might fail in simulating large inertia. Combination of the two methods with torque control method as the primary one and energy compensation method as the supplementary can achieve higher simulation precision with the two methods switch on the medium and small electrotorque of the motro. As to the NT11 brake dynamometer, the switch point is 200 Nm.The inertia system of the new type brake dynamometer with electric inertia is designed. Three disks with inertia of 30kgm2, 45kgm2 and 45kgm2 are selected, where 30kgm2 is the sum of the fixed disk inertia and the shaft inertia. The achievable mechanical inertia thus becomes 30 kgm2, 75 kgm2 and 120 kgm2. According to the design of the typical parameters of three vehicle types and the 1g deceleration limit, the fixed inertia plus electric inertia can satisfy the demand of minicar, the 75 kgm2 mechanical inertia plus electric inertia can satisfy the demand of small vehicle, the 120 kgm2 mechanical inertia plus electric inertia can satisfy the demand of medium vehicle.Both hardware and software is regulated on NT11 brake dynamometer to meet the control requirement of electric inertia: a switch unit is added to switch the motor controller between speed mode and torque mode to implement electric inertia; the torque loading system of the motor is calibrated; electric inertia related instructions are added to the previous software to input the necessary parameters of electric inertia. Experiment under various braking conditions between torque control method and energy compensation method is conducted, which proves the feasibility and scope of the two methods as well as the correctness of the idea of the combination of the two methods.As to the brake dynamometer without error compensation, the simulated inertia often shows a significant inertia error compared with the target inertia as the brake condition varies due to air resistance, bearing friction, etc. An indirect loss model regression method using free braking data is proposed. With the free braking data collected by the software, speed-time model can be regressed by SPSS software and the resistance torque-speed model can be deduced with it. Loss model of the 32 kgm2 disk is deduced as an example. The inertia error compensation rule of the two electric inertia methods is discussed with an emphasis on energy compensation method: the work of the resistance torque can be calculated by integration in the three braking stage; the motor output extra work in the compensation period to make up this work error. The motor control strategy of energy compensation method with attention to inertia simulation and error compensation is given. Inertia error compensation experiment on the two methods under various braking conditions and different electric inertia is conducted on NT11 brake dynamometer, which proves the correctness and feasibility of the inertia error compensation strategy.The final method applied to the new brake dynamometer is the combination of torque control method and energy compensation method with error compensation. The AK-Master regulation from European Union is selected to check the efficiency of inertia simulation. Experiment shows that the simulated inertia following the target inertia precisely without the fluctuation by resistance torque. The inertia error range±2 kgm2 much better than the pure mechanical inertia brake dynamometer, which puts the electric inertia of brake dynamometer into practice.