Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 52
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Low Frequency Damping filter
Within the Mach-DSP, low frequency damping provides gain over a Velocity term that is
derived from differentiating the position signal. Differentiating an electrical signal
essentially is a high-pass filter and thus, this tends to exacerbate noise in the system.
For that reason, this velocity term is passed through a low-pass filter.
As was the case with the Position-proportional filter, you can choose to bypass the filter,
or you may choose a single-pole low-pass or two-pole low-pass filter. Bypassing is not
recommended. As the cutoff frequency is reduced, noise in the servo is also reduced,
but an additional phase delay is also introduced into the servo, which could affect the
quality of motion. Normally this phase delay is compensated using High Frequency
damping, and appropriately adjusting the High Frequency damping filter. Nevertheless,
the filter frequencies must be adjusted carefully and there is a tradeoff between noise
that exists in the servo and the quality of motion that results.
High Frequency Damping filter
Within the Mach-DSP, high frequency damping provides gain over a Velocity term that
is derived from integrating the current flowing through the scanner coil. (This integrated
current may optionally be combined with the scanner’s actual position so that the Mach-
DSP provides true observed velocity.) Although this provides a very good estimate of
velocity, and one that does not contain noise, this kind of velocity is normally not usable
for very low velocities. For that reason, this velocity term is passed through a high-pass
filter.
As was the case with the Low Frequency damping filter, you can choose various filter
types and cutoff frequency to optimize and tradeoff noise in the servo and position
quality. Best results are usually obtained when the cutoff frequency of the High
Frequency Damping filter is set somewhat higher than that of the Low Frequency
Damping filter.
Output filter 1 and Output filter 2
The approach presently used in galvanometer scanning is to place a mirror on the end
of a motor shaft. This creates a two-mass, one-spring mechanical system (with one
mass being the scanner’s moving magnet rotor, another mass being the mirror, and the
spring being the shaft and mirror mount that connects the rotor to the mirror). This type
of mechanical system will always have multiple resonances. In the case of small mirrors
(for example 3mm or 4mm aperture), these resonances may have a sufficiently high
frequency and sufficiently low Q that they will not be noticeable. But generally when the
mirrors become large enough to accommodate a 5mm or larger aperture, these
resonances will cause instability in the servo, especially at high servo gains.
(Note that the actual frequencies of these resonances can be determined using the
Dynamic Signal Analyzer feature within the Mach-DSP program.)
