Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 26
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5.2
Servo configurations possible with the Mach-DSP
The Mach-DSP provides control over the gain of five fundamental signals, which are
ultimately fed into a summing node, including:
•
A signal proportional to the error between the commanded and actual position
(Error Proportional gain)
•
A signal proportional to the integral of the position error
(Error Integrator gain)
•
Two separate signals proportional to the velocity of the scanner
(Low Frequency Damping and High Frequency Damping gain)
•
A signal proportional to the position of the scanner
(Position-proportional gain)
By adjusting the level of each gain control described above, the following servo
configurations are possible with the Mach-DSP servo driver:
PD (Proportional and Derivative):
Error Proportional (servo) gain, used along with one
or both Damping terms. PD is the fastest type of servo configuration and the easiest to
tune. Because of this, it is usually used for laser displays and laser light shows, since
these applications consider speed to be the most important criteria in scanning.
PID (Proportional, Integral and Derivative):
The PD configuration as described
above, plus a small amount of Error Integrator gain. PID adds a little bit of integrator,
which will reduce the error (difference between commanded position and actual
position) to zero. To control overshoots, the Error Integrator must be used along with a
small amount of Position Proportional. This PID configuration allows the integrator to
force the mirror to its final target position, but due to the low integrator gains used in this
configuration, the error integrator is only used for relatively low frequency and slow
moving signals. PID could be a good choice for certain point-and-hold applications.
PDF (Pseudo Derivative Feedback):
Error Proportional is not used at all. Instead, the
Error Integrator gain is used exclusively to create acceleration. To prevent the servo
from going unstable, Position Proportional is also used (in similar proportions) which
acts as a strong “spring” against the Error Integrator. One or both damping terms are
also used with PDF. The result is that the integrator is always operational and always
correcting the servo, driving the error to zero. Thus, PDF provides the greatest accuracy
of the servo configurations possible with the Mach-DSP. However, since the only signal
path from the command signal to the power amplifier is through the integrator, this
generally results in slower operation than is possible by PD or PID. PDF is also much
more difficult to tune than PD or PID. Some manuals refer to a servo configuration that
uses Error Integrator, Velocity and Position Proportional as a “third-order servo”.
PDFF (Pseudo Derivative Feedback, with Feed Forward):
The PDF configuration
described above, but additionally the Error Proportional term is also used. Since Error
Proportional (i.e. “Servo Gain”) is used in this configuration, it provides a more direct
(i.e. “feed-forward”) path from the command signal to the power amplifier. Thus the
PDFF configuration is faster than PDF, and is a great alternative when both high speed
and high accuracy are needed. It is also the hardest to tune of all configurations.
