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Actuator in VHDL-AMS with SystemVision
Gallery spotlightActuators
Modeling an electromagnetic actuator in a systems context is sometimes required to accurately simulate the dynamic interaction between drive circuit, actuator, and load. Co-simulation, where a systems simulator and MagNet run parallel transient solves, is one method that can be used. An alternative approach, Response Surface Modeling (RSM), creates a functionally equivalent model of the actuator by performing a large number of static analyses at different currents and positions. Presented here is an example of an RSM embedded in a VHDL-AMS model of the actuator, used in conjunction with the SystemVision system simulator from Mentor Graphics. The procedure is automated with the System Model Generator which drives MagNet to execute its static solver and then generates the VHDL-AMS code which implements the RSM. VHDL-AMS is an IEEE standard (1076.1) supported by many circuit and system simulators.
METHODS and RESULTS
SystemVision circuit diagram
Shown here is the SystemVision circuit diagram of the actuator and drive circuit. In this example, the coil is driven by a capacitor charged to 12 V. A spring holds the plunger against the upper stop. At time t=0, a switch closes to connect the charged capacitor to the coil. On the mechanical side the spring, viscous damper, and upper and lower stops are modeled using appropriate mechanical components which, in fact, are themselves described using VHDL-AMS. This system is simple enough to be simulated in the Educational Version of SystemVision, which is a free download.
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MESH of the ACTUATOR MODEL
The MagNet mesh of the actuator model is shown here. Since this device has axial symmetry, the MagNet Static 2d solver is used to calculate force and flux-linkage at different positions and currents. (Note - This solver is also available in the MagNet Trial Edition, which is a free download.) The solution setup to create the VHDL-AMS Response Surface Model (RSM) is automated with the use of the System Model Generator.
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RSM of the FORCE as a function of COIL CURRENT and PLUNGER POSITION
The RSM of the force as a function of coil current and plunger position is shown here. The response surface models for both flux and force were created using the System Model Generator and required a total of 460 Magnetostatic 2d solutions. The code to evaluate these RSM's is embedded in the VHDL-AMS file.
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POSITION of the PLUNGER as a function of time
This graph shows the position of the plunger as a function of time. The complicated behaviour arises from the transfer of energy between the potential energy of the capacitor, the energy stored in the magnetic field, the potential energy in the spring, and the kinetic energy of the plunger.
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SIMULATING an IDEAL BOUNCE
This system can be modeled directly in MagNet's Transient with Motion solver, as shown here. The position is slightly different because MagNet can simulate an ideal bounce (instant velocity reversal), whereas a very large spring constant was used in the VHDL-AMS implementation of the spring in SystemVision. This small error at each bounce is amplified over time. Although the transient solution time is comparable to the time required to generate the RSM, the RSM allows changes in the drive circuit to be made and the result simulated almost instantly in SystemVision.
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Number of time steps: 600
Transient solution time: 6 minutes 12 seconds
Number of static solutions in RSM: 460
Static solution time (total): 5 minutes 10 seconds
SystemVision simulation time: 1.234 seconds
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