Real-Time Control of Magnetic Bearings Using RTLinux

RTLinux meets one of the engineering world's most demanding control requirements, without requiring separate controller hardware and costing substantially less than comparable proprietary systems.

Figure 3. Oscilloscope Trace Showing Both the Test Signal (Yellow) and Analog Output (Blue)

A more intuitive way to test the operation of the system is to hook the analog output to an audio amplifier and speaker and input a sine wave from an external signal generator. I did this, and the tone was clean and steady. The original analog controller is a simple lead-lag filter on each of the four bearing axes, which is duplicated with the difference equation:

[ y(n) = 0.7467*y(n-1 )+ 4.6380*x(n) - 4.5189*x(n-1)]

Figure 4 shows the digital and analog controller responses to a mechanical impulse on the shaft. The responses are virtually identical. In this figure, the digital controller response is shown at the top, and the analog controller response is at the bottom. The time shown in the figure is 100 milliseconds. The digital loop is operating at 10kHz in a MIMO configuration, which has five input channels and eight output channels.

Figure 4. Oscilloscope Trace Comparing Digital (Upper) to Analog (Lower) Controller Response to a Mechanical Impulse