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Electro-Thermal Simulation of a Voice Coil Actuator using EMS inside SOLIDWORKS

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Problem description

In this article, a voice coil actuator will be studied [1]. A series of simulations will be performed using the optimal actuator parameters determined in [1]. Transient magnetic study will be used  to compute the current at different DC voltage excitations. Simulation results will be compared to experimental data published in [1]. Temperature evolution of the voice coil actuator will be studied using an electrothermal analysis with EMS.

EMS and SOLIDWORKS will be used to carry out the different simulations of the studied voice coil actuator. Figures 1a), 1b) and 1c) show full and cross section views of the studied voice coil device. A stranded coil, made of copper with 760 turns, is free to move axially along the airgap zone created between the ferromagnetic shell and the permanent magnet. The neodymium permanent magnet N42, which is stationary and axially magnetized, is a second source of energy. both the orientor and shell are made of soft iron characterized by its high permeability. The flux orientor is used to guide the field and improve the magnetic circuit path. A bobbin is inserted to support of the coil.  

The studied voice coil actuator, a) full model, b) cross section view, c) agenda
Figure 1 - The studied voice coil actuator, a) full model, b) cross section view, c) agenda

Simulation and Results

Transient Simulation using EMS – Current and force calculation for different voltages

In this section, electromagnetic simulation is performed using Transient module of EMS. The coil is supplied with a range of DC voltages.  Magnetic fields, current and Lorentz force are computed. Figure 2 shows the magnetic flux density when the steady state is achieved.

Figure 2 - Cross section view of the magnetic flux density
 

Figure 3 illustrates the calculated current response versus different applied voltage rates. It shows that the steady state is reached in 2ms.  This can help to improve the time response of the system. The computed currents are 2A and 11A when 10V and 60V are respectively applied to the stranded coil. Figure 4 contains several plots of the force results at the same static position (12mm). The plotted curves of the force measured by experimental tests contain few oscillations before achieving the steady state. A force of 4.5N is obtained when a DC voltage of 10V is applied while it is 26N when the applied DC voltage is 60V. The higher the voltage the higher the force is.

 Current results
Figure 3 - Current results
 
 Force results at static position
Figure 4 - Force results at static position
 

Electrothermal Analysis – Winding loss and temperature calculation of the voice coil actuator

This analysis will focus on the electromagnetic losses and the temperature generated in the voice coil actuator. Since eddy effect is neglected, copper losses (winding losses) are the main electromagnetic losses in this actuator. Due to the Joule law, current flowing the winding conductors generates a heat -called Joule heat. Hence, the temperature of the voice actuator will rise quickly and proportionally to the applied voltage.

Transient electrothermal analysis is carried on using EMS to compute winding losses and temperature evolution versus time into a static position of the coil. EMS allows thermal coupling analysis without any export/import data. Electromagnetic time constant is too small compared to thermal time constant, i.e the steady state of the electromagnetic solution will be reached in very small time while achieving the steady state regime of the thermal analysis will take much longer time. Therefor, different simulation end times/time step sizes should be used to accelerate the analysis.

Figure 5 shows the winding loss results for two different voltages. Copper losses in the voice coil are about 800W and 8W when 200V and 20V DC voltages are applied respectively. 

 Winding losses results
Figure 5 - Winding losses results
 

Figures 6a) and 6b) display the final temperature of the voice coil actuator for 20V and 200V, respectively. The temperature is generated inside the coil, due to copper loss, then it propagates to the whole system by conduction.

The temperature reachs a peak value of 314 K after 60s in the case of 20V while it rises from 300 K ambiant temperature to 429 K in 5 seconds. Figure 7 contains the temperature results in case of 200V at 5s. It confirms that the coil is the source of heat in this case. Temperature evolution for the whole system is shown in Figure 8.

Temperature results, a) 20V, b) 200V
Figure 6 - Temperature results, a) 20V, b) 200V
 
 Temperature inside the coil (200V)
Figure 7 - Temperature inside the coil (200V)
 
Temperature evolution versus time (200V) 
Figure 8 - Temperature evolution versus time (200V)

Conclusion

The voice coil actuator studied in this application showed many advantages such as constant force along the stroke, high speed and acceleration, etc. Computer simulation results demonstrates very good agreement with experimental and testing data. This will allow to create and build innovative voice coil actuators with improved efficiency and lower cost.

References

[1]: Vahid Mashatan. Design and Development of an Actuation System for the Synchronized Segmentally Interchanging Pulley Transmission System (SSIPTS). Department of Mechanical and Industrial Engineering University of Toronto 2013