User's Manual

PMAC User Manual
104 Closing the Servo Loop
Amplifier Types
PMAC can interface to a variety of different types of amplifiers. The type of amplifier used for a
particular motor has important ramifications for the tuning of the servo loop. Each of the common types
is explained below.
Velocity-Mode Amplifiers
Many amplifiers accept a velocity command from the controller and a velocity feedback signal from a
motor sensor – usually from a tachometer or synthesized from a resolver. Motors using these amplifiers
have their velocity loops closed in the amplifier, and should not require use of the derivative gain of the
PMAC position loop, provided that the velocity loop is well tuned. In these systems, the PMAC analog
output represents a velocity command. These amplifiers also close current loops internally, and if the
motor is brushless, they perform the phase commutation.
The key virtue of velocity-mode amplifiers is that in closing an analog velocity loop, they are not subject
to the quantization errors and sample-rate limitations of a digital velocity loop. Therefore, they can often
achieve higher velocity-loop gains, resulting in higher stiffness and better disturbance rejection. For this
reason, they are used widely in machine tool cutting applications, maintaining accuracy against high
cutting forces.
Because of these high gains, the position-loop proportional gain in PMAC tends to be much lower for
these amplifiers than for other types. However, much of the velocity-loop stiffness comes from velocity
integral gain. Integral gain in a loop creates a lag, and this makes response to external commands
sluggish. Therefore, these amplifiers are not well suited to applications with quick starting and stopping,
as many indexing applications use.
As processor speeds increase with DSPs and higher clock rates, helping to overcome sample rate
limitations, and digital velocity estimation techniques improve with methods such as 1/T, reducing
quantization errors, velocity-mode amplifiers are being used less and less.
Before tuning the PMAC position loop, it is important that the velocity loop of a velocity-mode drive be
well tuned with the load that it will drive. Because the velocity-loop tuning is load dependent, the
amplifier manufacturer cannot do the final tuning; the machine builder must tune the loop. The velocity
step response must not have any significant overshoot or ringing; if it does, it will not be possible to close
a good position loop around it with PMAC. The PMAC Executive Program’s tuning section has a
function called Open-Loop Tuning that can be used to give velocity command steps to the amplifier and
to observe the response plotted on the screen. This makes it easy to tune the amplifier, or simply to
confirm that it has been well tuned.
Torque-Mode Amplifiers
Another popular type of amplifier is the torque-mode amplifier, in which the analog voltage from the
controller represents a torque command for the motor (a force command for a linear motor). Since the
basic motor equation shows that torque is proportional to motor current, these are often called current-
mode or current-loop amplifiers (a current loop must be closed to ensure that the torque out is
proportional to the voltage in). Another name occasionally used for these types of amplifiers is the
transconductance amplifier, signifying that a voltage input results in a proportional current output. If the
motor is brushless, these amplifiers also perform the motor phase commutation.
Newton's second law states that torque or force is proportional to rotary or linear acceleration,
respectively, so the commands into these amplifiers are effectively acceleration commands. No velocity
loop is closed in the amplifier; so it is up to PMAC to close the velocity loop itself to get enough damping
for a stable system. With the standard PID filter, this is done with the derivative gain, so it is important to
have enough derivative gain in these systems for stable response.