An Educational Virtual Test Rig for the Simulation of Hybrid Stepping Motors

The study plan of the Mechatronics offered by the School of Engineering of the University of Bergamo includes the course of “Drives for mechanical systems" (6 ECTS), delivered in the second semester. The main educational goal of this course is to introduce the student to the basic concepts of electrical motors and drives, whose study is an important aspect of mechatronics education. In particular, this paper focuses on hybrid stepping motors, which operates on a rather simple principle, but exhibit some critical running conditions, mainly due to the loss of synchronization between the stator’s magnetic field and the rotor. Hybrid stepping motors, compared to other drive systems, are low-cost and easy to use devices. For these reasons, and thanks to their robustness and reliability, they are widely used in several fields, mainly for small size automation. Typical applications include small robots and small automated machine, computer peripherals, a wide range of automotive devices like headlights levelling/swivelling, climate flaps adjustments, etc. Furthermore, the peculiar incremental motion of a stepping motor allows to develop open-loop applications without the need of position/velocity transducers. From the educational point of view, such topic is usually explained by means of two complementary approaches: theoretical frontal lectures concerning motor’s running principle, mechanical configuration and modelling, and experimental activities carried out in our Mechatronics and Mechanical Dynamics lab. where a research test rig, for stepping motor characterization, is available. Thanks to this device, students can learn in deep the complex electromechanical behavior of this type of motors. Regrettably, in the last two academic years, on the account of COVID19 outbreak, all lectures were delivered online and no activity was allowed in presence, therefore it was not possible to carry out the usual laboratory activity. In order to overcome these restrictions and to properly replace the experimental activity, we have developed a simulation based virtual test rig. The simulation code has been written in Matlab, and it is based on a simplified electromechanical model of the stepping motor, of its drive and of the coupled load. Moreover, most of the model parameters can be easily drawn from their data-sheets. In particular, the virtual test rig allows to display rotor’s angular position and velocity, motor’s torque, currents flowing in the motor’s phases and can detect the loss of synchronization. Users of this virtual testing environment, in our case graduate students of the Mechatronics curriculum, can simulate the behavior of the stepping motor coupled with the load, varying both motor and drive parameters (for example, drive voltage motor’s phases resistance and inductance, the rated current and the rotor inertia) as well acting on load parameters (e.g. load inertia, friction and applied torque). Students seemed to appreciate this approach and after a very short training on the simulation software they were able to proceed by themselves with a better understanding of stepping motor behavior. In conclusion we can say that the presented virtual test-rig can be efficiently and properly used to carry on “virtual experimental activities” replacing the experimental activity usually carried out on the actual test-rig.