Voltage Controlled Suspension

About voltage control

A magnetic bearing can be configured as current control or voltage control. While current control is being widely used, we have made most of our designs using voltage control. Since many years ago we have been using voltage control, in which we have received a lot of experiences. Compared with current control, voltage control has many advantages, which is the reason why we use voltage control.

We also experienced weaknesses of voltage control. A major problem is that the magnetic bearing model is dependent on coil copper resistance which, in turn, is dependent on coil temperature. This is not the case in current control. But we have come up with simple and effective solutions for this problem.

Advantages of voltage control

Voltage control is not new, but its advantages may not be fully recognized. To see how voltage control differs from current control we have carried out a lot of researches. Many results have been published as academic papers. Below are some of the advantages of voltage control as compared to current control.

Lower hardware complexity − Voltage control is simple in hardware electronics and complicated in control algorithm. In voltage control, the power amplifier does not need to force the current to track the control signal, which makes the power amplifier simple and low-cost. However, the PID controller, which is used in current control, is not enough for voltage control. A higher-order controller has to be used.

Allowing for lower bandwidth − The bandwidth refers to the speed in which a magnetic bearing responses to external disturbance forces. The higher the bandwidth, the faster the speed. But a higher bandwidth is not always better. In fact, a lower bandwidth is better in all aspects except the disturbance responsiveness. The best situation is that the bandwidth is just enough. At a given magnetic bias, the lowest bandwidth that still maintains a robust suspension is much lower for voltage control than for current control. As a result, current control may have to have a bandwidth that is higher than enough.

Less sensitive to sensor noise − In current control the power amplifier has a local controller for the current to track the control signal. The controller gain has to be high for an acceptable tracking performance. Then, the overall gain from the displacement sensor to the power amplifier output (it is actually a voltage) may excessively amplify the sensor noise, producing acoustic noise at the bearing body. In voltage control, however, there is not such a local controller, and the overall gain is much lower.

Facilitating voltage bias − When a magnetic bearing is electrically biased, either a current bias (maintaining a biasing current) or a voltage bias (maintaining a biasing voltage) can be used. In both current and voltage control, the voltage bias is found to give better magnetic bearing performances. However, implementation of the voltage bias is difficult in current control, and much easier in voltage control.

Facilitating clean-voltage control − For attenuation of the mechanical vibration caused by rotor imbalance, the most common method is to maintain a clean current (clean means free from the synchronous component). Maintaining a clean voltage will achieve the same objective, and has been shown to be more effective. But implementation of the clean-voltage control is difficult in current control, and much easier in voltage control.

Less sensitive to eddy current − In current control the eddy current in the magnet cores has a strong negative influence, thus lamination is adopted whenever possible. In voltage control, however, the influence is much weaker, allowing for better performances when solid cores are used. The axial bearing almost always uses solid cores because of manufacturing reasons. Using solid cores for the radial bearing (only for the stator) is also preferable.