Magnetic Bearing Controller
How the magnetic bearing control box works:
- Inputs include sensor coils used to measure the position
of the rotor
- Outputs include actuator coils used to place force on
the rotor
- Serial Interface for Graphical User Interface (GUI) used
for development and remote control of the Magnetic Bearing
Controller
- LCD Display w/ Keypad used for control of the Magnetic
Bearings System from the control box
- Mean Time Between Failures (MTBF) of the Controller is
currently 14 years
General
 The
magnetic bearing control box contains all of the elements
necessary to operate a magnetic bearing system. Key components,
shown in the Figure, are sensor demodulation electronics,
a digital signal processor (DSP) board, power electronics
and DC power supplies. The control box interacts with the
magnetic bearing actuators and shaft position sensors, which
are located in the machine. The control box receives shaft
position signals from sensor heads located with the magnetic
bearing actuators. This information is demodulated on the
sensor card, and then processed on the DSP board. The output
of the DSP is a command signal for the power amplifiers, which
drive the required control current through the actuator coils.
The power supplies convert AC line power to low DC voltages
for the signal electronics and higher DC voltages for the
power amplifiers.
Sensor Electronics Board
The sensor electronics board has driver/demodulation circuitry
for the position sensor heads. The function of the board is
similar to the signal processor box in a commercial eddy current
probe system, except that five channels (for the five bearing
axes) are processed on the same card instead of just one channel.
The board has driver circuitry to excite the coils in the
sensor heads at 15-50 kHz. Motion of the rotating shaft modulates
this signal, which is picked up and demodulated on the sensor
board to produce a voltage proportional to shaft displacement.
DSP Board
The DSP board runs the magnetic bearing control program.
The main function of the program is to produce the control
signal for the magnetic bearings, but it also performs communication,
diagnostic and monitoring functions as well. The control signal
is produced by a fast digital filter routine that executes
an enhance PID compensator. Single Input/Single Output (SISO)
control is usually used, but MIMO control is occasionally
used for highly gyroscopic machines such as energy storage
flywheels. Many special features of the program take advantage
of the characteristics of magnetic bearings to produce very
smooth running rotating machinery.
One key feature is Adaptive Synchronous Cancellation (open
loop cancellation), which can be in operated in two different
ways depending on the machine requirements:
- Current minimization - minimizes or eliminates the
synchronous control current allowing a machine to operate
with very low synchronous load required and very small transmitted
vibration. This allows smooth operation with larger residual
rotor imbalance than would be possible with conventional
bearings.
- Position minimization - minimizes synchronous rotor
displacement, allowing tighter machine clearances than would
otherwise be possible.
Another key feature of the magnetic bearing control is gain
scheduling. With gain scheduling, the compensation is changed
as rotor speed increases to allow more effective control of
gyroscopic modes of the rotor.
Power Amplifiers
The power amplifiers produce a control current in the magnetic
bearing actuator coils based on the command signal from the
DSP. The power amplifiers use pulse-width modulation and internal
current feedback to produce the required current efficiently
from a fixed DC overhead voltage. The required overhead voltage
varies with the required dynamic load capacity of the bearing/rotor
system.
|
