Delta Tau GEO BRICK LV User Manual
Turbo PMAC User Manual
92
Setting Up Turbo PMAC-Based Commutation and/or Current Loop
Turbo PMAC permits digital closure of the motor current loops, mathematically creating phase voltage
commands from numerical registers representing commanded and actual current values. These numerical
phase voltage commands are converted to PWM format through digital comparison to an up/down
counter that creates a digital saw tooth waveform. The analog current measurements must be converted
to digital form with ADCs before the loop can be closed.
commands from numerical registers representing commanded and actual current values. These numerical
phase voltage commands are converted to PWM format through digital comparison to an up/down
counter that creates a digital saw tooth waveform. The analog current measurements must be converted
to digital form with ADCs before the loop can be closed.
PMAC2 Digital PWM Generation (per phase)
PWM
Command
Range
I900 - 2
[I7m00] - 2
Time
PWM
Top Signal
PWM
Bottom Signal
Servo
Interrupt
generated
here
4 x I900 [I7m00] + 6
PWM_CLK Cycles
Counter Clock is 120 MHz
PWM
Up/Down
Counter
I900 + 1
[I7m00] + 1
PWM
Counter
Range
Ix66
- Ix66
Deadtime
(I904 [I7m04])
New command loaded
at counter turnaround
PWM Command
By directly commanding the on-off states of the power transistors in this manner, Turbo PMAC
minimizes the calculation and transport delays in the servo loop. This permits the use of higher gains,
which in turn permit greater stiffness, acceleration, and disturbance rejection. Also, digital techniques
permit the use of mathematical transformations of the current-loop data, turning measured AC quantities
into DC quantities for loop closure. This technique, explained in the next section, significantly improves
high-speed performance by minimizing high-frequency problems.
minimizes the calculation and transport delays in the servo loop. This permits the use of higher gains,
which in turn permit greater stiffness, acceleration, and disturbance rejection. Also, digital techniques
permit the use of mathematical transformations of the current-loop data, turning measured AC quantities
into DC quantities for loop closure. This technique, explained in the next section, significantly improves
high-speed performance by minimizing high-frequency problems.
Frames of Reference
A very important advantage of the digital current loop is its ability to close the current loops in the field
frame. To understand this advantage, some basic theoretical background is required.
frame. To understand this advantage, some basic theoretical background is required.
In a motor, there are three frames of reference that are important. The first is the stator frame” which is
fixed on the non-moving part of the motor, called the stator. In a brushless motor, the motor armature
windings are on the stator, so they are fixed in the stator frame.
fixed on the non-moving part of the motor, called the stator. In a brushless motor, the motor armature
windings are on the stator, so they are fixed in the stator frame.
The second frame is the rotor frame, which is referenced to the mechanics of the moving part of the
motor, called the rotor. This frame, of course, rotates with respect to the stator. For linear brushless
motors, this is actually a translation, but because it is cyclic, we can think of it as a rotation.
motor, called the rotor. This frame, of course, rotates with respect to the stator. For linear brushless
motors, this is actually a translation, but because it is cyclic, we can think of it as a rotation.
The third frame is the field frame which is referenced to the magnetic field orientation of the rotor. In a
synchronous motor such as a permanent-magnet brushless motor, the field is fixed on the rotor, so the
field frame is the same as the rotor frame. In an asynchronous motor such as an induction motor, the field
slips with respect to the rotor, so the field frame and rotor frame are separate.
synchronous motor such as a permanent-magnet brushless motor, the field is fixed on the rotor, so the
field frame is the same as the rotor frame. In an asynchronous motor such as an induction motor, the field
slips with respect to the rotor, so the field frame and rotor frame are separate.
Working in the Field Frame
The physics of motor operation are best understood in the field frame. A current vector in the stator that
is perpendicular to the rotor field (that is, current in the stator that produces a magnetic field perpendicular
to the rotor magnetic field) produces torque. This component of the stator current is known as quadrature
current. The output of the position/velocity loop servo algorithm is the magnitude of the commanded
quadrature current. For diagnostic purposes on a Turbo PMAC, an “O” command can be used to set a
fixed quadrature current command.
is perpendicular to the rotor field (that is, current in the stator that produces a magnetic field perpendicular
to the rotor magnetic field) produces torque. This component of the stator current is known as quadrature
current. The output of the position/velocity loop servo algorithm is the magnitude of the commanded
quadrature current. For diagnostic purposes on a Turbo PMAC, an “O” command can be used to set a
fixed quadrature current command.