User's Manual

PMAC User Manual
Closing the Servo Loop 105
Torque-mode amplifiers are popular for several reasons. Since they do not need a tachometer or analog
velocity-loop electronics, they can be simpler and less expensive. Because the current loop gains are
dependent only on motor properties, and not on the load, they can be tuned by the manufacturer for the
particular motor that is being used. No retuning is required by the machine builder when the motor is
connected to the load.
In addition, torque-mode amplifiers generally work better in applications with rapid accelerations and
decelerations. They do not depend heavily for their performance on error integrators, which introduce
time lags and slow response to velocity changes. As such, they are often preferred over velocity-mode
amplifiers, even without the cost advantage.
Voltage-Mode Amplifiers
Voltage-mode amplifiers are the simplest and least costly amplifiers. A voltage command input causes a
larger proportional voltage output to the motor. No feedback of any kind is used in the amplifier itself.
However, they are usually unsuitable for industrial position control applications for several reasons.
First, a voltage command to the motor must overcome the motor’s L/R electrical time constant to cause
current in the motor. This can create delays in motor response. Second, the voltage command to the
motor that can produce current even after the delay is only that voltage above the back EMF of the motor;
therefore the torque produced is dependent on the motor speed. For both of these problems, it is up to the
slower digital position and velocity loops to try to correct.
Finally, controlling current directly is the best way of preventing damaging overcurrent conditions. Most
industrial servomotors work with a supply voltage that could burn them up if they were exposed fully to it
for just several milliseconds without substantial back EMF. If current sensors are required anyway for
protective overcurrent shutdown, they can also be used to close current loops at very little additional cost.
The behavior of voltage-mode amplifiers is somewhere between velocity-mode amplifiers and torque-
mode amplifiers. At low loads, the speed is limited by the back EMF of the motor; at low speeds, the
current is limited by the resistance of the armature. From the controller's point of view, the motor gets
some damping from its own back EMF, but not enough for most positioning applications, so it must be
supplemented with the controller's derivative gain.
Sinusoidal-Input Amplifiers
A relatively new type of amplifier for brushless motors, both permanent magnet and induction, that does
not do the commutation itself, relying on a controller such as PMAC to do it, expects two analog phase
current commands from the controller. At a constant velocity and load, these commands will be
sinusoidal waveforms, so these amplifiers are sometimes called sinusoidal-input amplifiers.
The amplifier still closes current loops on these phases, and it generates the third and fourth phase
commands if necessary through simple balance loops. To the PMAC servo loop, this type of amplifier
looks like a torque-mode amplifier; the magnitude of the sinusoids is proportional to the torque command.
The servo algorithm produces a single torque command; however, instead of writing this command value
directly to an analog output, PMAC processes it through the commutation algorithm to produce two
analog outputs.
There are several advantages to this type of amplifier. First, it permits use of the PMAC high-
performance commutation algorithms, which often provide superior performance to amplifier
commutation. Second, for synchronous motors (permanent magnet and switched reluctance), it allows the
use of less expensive incremental position sensors, because of the PMAC power-on phasing search
capabilities. Third, it reduces wiring, because only the controller needs position feedback, not the
amplifier.