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6.1 Overview
For the servo-based chipsets (MC2100, MC2300, MC2800, MC3110, MC3310,
MC58000) a positional servo loop is used as part of the basic method
of determining the motor command output. Chapter 4 made references to
the PID loop that exists in PMD’s servo based products. It was
mentioned that the output of the PID represents a desired motor torque.
The function of the servo loop is to match as closely as possible the
commanded position, which comes from the trajectory generator, and the
actual motor position.
To accomplish this the profile generator’s commanded value is
combined with the actual encoder position to create a position error,
which is then passed through a digital PID-type servo filter. The scaled
result of the filter calculation is the motor command (also referred
to as the torque signal), which is output as either a PWM signal to
the motor amplifier, or a 16-bit input to a D/A Converter. In the context
of multiphase motors, the motor command is broken down into components
(phases) based on the current commutation angle before being sent to
the amplifier.
6.2 PID loop algorithm
The servo filter used with the servo-based chipsets is a proportional-integral-derivative
(PID) algorithm, with velocity and acceleration feed-forward terms and
an output scale factor. An integration limit provides an upper bound
for the accumulated error. An optional bias value can be added to the
filter calculation to produce the final motor output command. A limiting
value for the filter output provides additional constraint. This limit
is set using the command SetMotorLimit.
All filter parameters, the motor output command limit, and the motor
bias are programmable, so that the filter may be fine-tuned to any application.
The parameter ranges, formats and interpretations are shown in the following
table:
| Term |
Name |
Representation & Range |
| Ilim |
Integration Limit |
unsigned 32 bits (0 to
2,147,483,647) |
| KI |
Integral Gain |
unsigned 16 bits (0 to 32767) |
| Kd |
Derivative Gain |
unsigned 16 bits (0 to 32767) |
| Kp |
Proportional Gain |
unsigned 16 bits (0 to 32767) |
| Kaff |
Acceleration feedforward |
unsigned 16 bits (0 to 32767) |
| Kvff |
Velocity feed-forward |
unsigned 16 bits (0 to 32767) |
| Kout |
Output scale factor |
unsigned 16 bits (0 to 32767) |
| Bias |
DC motor offset |
signed 16 bits (-32768 to 32,767) |
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Motor command limit |
unsigned 16 bits (0 to 32767) |
6.3 Motor bias
When an axis is subject to a net external force in one direction (such
as a vertical axis pulled downward by gravity), the servo filter can
compensate for it by adding a constant DC bias to the filter output.
The bias value is set using the host instruction SetMotorBias. It can
be read back using the command GetMotorBias.
6.4 Output scaling
The Kout parameter can be used to scale down the output of the PID
filter in situations that require it. It does this by multiplying the
filter result by Kout/65536. It has the effect of increasing the usable
range of Kp, which is typically programmed in the 1 to 150 range when
no output scaling is done. The Kout value is set using the host instruction
SetKout. It can be read back using the command GetKout.
6.5 Output limit
The motor output limit prevents the filter output from exceeding a
boundary magnitude in either direction. If the filter produces a value
greater than the limit, the motor command takes the limiting value.
The motor limit value is set using the host instruction SetMotorLimit.
It can be read back using the command GetMotorLimit. The motor limit
applies only in closed-loop mode. It does not affect the motor command
value set by the host in open-loop mode.
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