CST – Computer Simulation Technology

Non Linear 3D EM Simulation of a Speedometer for Automotive Applications

Automotive suppliers were among the first to incorporate numerical simulation in the product development process to reduce development cycles and minimize the production costs. Electromagnetic field simulation such as provided by CST EM STUDIO® (CST EMS) helps the engineers to accelerate product development.

This article shows the application of CST EMS to the simulation of a stepper motor found in speedometer applications.

Figure 1: CAD Import of the Stepper Motor

As a result of the very tight integration of simulation in the development process, CAD import is one of the most important requirements to provide geometry data for the simulation environment. The CST STUDIO SUITE® is renowned for its excellent CAD-data import capabilities, automatic healing tools and advanced modification and parameterization features....

This model was imported from Pro/Engineer® and was simulated without any additional geometrical modification or user intervention. The particular interests of the simulation were to obtain the torque vs. angle curve for different rotor positions as well as the distribution of the non-linear permeability in the iron parts.

Another objective was to check whether eddy currents, induced by the sinusoidal coil currents, have any influence to the system's behavior.

Figure 2: Magnetic flux density

The magnetic flux density distribution shown in figure 2 was obtained via a non-linear magnetostatic simulation. The field plot shows where leakage fluxes appear and that some flux bypasses the magnet via the second bracket of the magnetic circuit.

Figure 3: Relative permeability distribution

The permeability distribution can be visualized as shown in figure 3 to detect regions where the material is saturated.

Figure 4: Torque at different rotor angles

The torque vs. angle curve in Figure 4 can be very easily obtained by a parameter sweep. To parameterize the rotor angle of the CAD model in CST EMS just two mouse clicks are necessary. The magnet drives a worm gear with a transmission ratio of 1:50, so that the resulting torque on the pointer is about 4 mN-m.

Figure 5: Coil inductance at different rotor positions

In addition, the same run yields the inductance of the coils. It is very important to consider the coils' inductances in the design of the electronics controling the device.

Figure 6: Eddy current distribution in the stator's plate

The system is controlled by a pulse width modulator in a frequency range of some kHz. So a subsequent frequency domain simulation is performed to obtain the eddy currents induced in the stator's plate. Further inspections of the flux densities in the air gap indicate that the effect of the eddy currents on the torque is negligible.

This article demonstrates how to obtain characteristic parameters of a stepper motor from a 3D EM simulation for a complex CAD model. Torques and inductances can be achieved by a parameter sweep for different rotor positions. Besides the magnetostatics simulation, a frequency domain simulation shows the effect of induced eddy currents. These tasks can be performed completely in the powerful graphical user interface of CST EMS. All steps such as CAD import, parameterization, source definition and postprocessing can be easily and efficiently performed by the user.

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