Description:
Magnetic bearings levitate a rotating object (typically a rotor) with a magnetic field and are unstable in open-loop operation. Position feedback control is required to maintain the rotor in a centered position. Typically, a separate position sensor is used to directly measure the rotor's position for use in this feedback loop. Self-sensing magnetic bearings use the bearing's electrical signals to estimate rotor position.
There are two primary classes of self-sensing methods: state estimation and inductance (displacement) measurement from switching Pulse Width Modulation (PWM) amplifier waveform. State estimation approaches model the bearing as a linear, time-invariant system and treat the rotor position as a state to be estimated as part of linear, time-invariant feedback control. PWM-based approaches rely on effects of the driving switching amplifier to estimate position. The PWM based self-sensing method can be further divided into two categories: current ripple-based approach in which the estimates of position are generated from the relationship between switching voltage and current signals, and differential voltage based method in which the estimates of position are generated from the voltage difference between coaxial pair of electromagnet coils. A major drawback of the current ripple-based approach is a noisy current signal in switching amplifiers.
The current invention is a differential voltage-based method that improves upon existing differential voltage-based methods, and it avoids the drawbacks of current ripple-based methods. This new invention has a novel amplifier structure that can control the current and self-sensing in a time multiplex way and samples only a small segment of time, which reduces switching noise. The logic chip has a novel strategy to avoid noises cause by the power electronic switch. The current invention is also better able to deal with bearings that have a continuous back iron because flux equations are solved in a two-degree-of-freedom plane. Unlike the current ripple method, which usually requires a fixed PWM frequency, this new invention can work with variable frequency PWM amplifiers.
