Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 57
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bearings. The Viscous damping control allows you to specify the amount of damping
that the bearings are contributing to the system. This can be used to fine-tune the
observer, to deliver precise estimates of high-frequency velocity.
When Observer gain is set to zero, this must be set to at least 1. Generally speaking,
Viscous damping numbers between 1 and 20 are reasonable.
Observer gain
This sets the gain of the “rigid body observer” used in the Mach-DSP. When this is set
to zero, then the observer is not used, and high-frequency velocity is derived simply as
a result of integrating the coil current. (The implications of this are described below.)
When Observer gain is increased above zero, a servo loop is formed which compares
the predicted (observed) galvanometer position and actual measured galvanometer
position, the gain of this servo loop being controlled by the Observer gain setting. High
amounts of Observer gain will force the predicted position to match the actual position
very closely, and thus force predicted (i.e. observed) velocity to be very accurate.
However, high observer gains may also inadvertently inject noise into the main servo
loop. Therefore, there is a tradeoff between Observer gain and the quality of the
predicted velocity. Generally an Observer gain setting of around 20 is reasonable.
Spring constant (magnetic return to center)
Some galvanometer scanners have a natural tendency to return the rotor to a central
rotation angle when coil current is removed. This is especially true of the ScannerMAX
Compact series of scanners, as well as older moving iron galvanometers that used a
torsion bar. Other galvanometer scanners will not impose any torque on the rotor when
coil current is removed. When constant coil current is required to maintain the position
of a galvanometer (as is the case with the Compact series), this constant coil current
will result in a DC offset that appears in the derived high-frequency velocity signal. For
that reason, the Mach-DSP servo allows you to specify the Spring constant (both
positive and negative) so that this can be taken into account and subtracted from the
coil current, ultimately providing a more accurate derived high frequency velocity.
A final note about observers…
For more than two decades, conventional galvanometer servo drivers have derived
high-frequency velocity information by simply integrating coil current. This involves
simple circuitry and provides reasonable results. However, integrating the coil current
almost always results in a DC offset, as well as low-frequency AC signals riding on the
derived high-frequency velocity. These extraneous signals may extend mirror settling-
time (as is the case with most conventional servo drivers on the market). Setting the
Observer gain higher than zero will engage the rigid-body observer, which in turn
eliminates this DC offset as well as the low-frequency AC signal, thus improving mirror
settling-time. Therefore the use of an observer can be very valuable. However, it will
only work correctly if the Torque-constant to inertia ratio, Viscous damping, and Spring
constant settings are all set correctly. Therefore, although the use of an observer may
provide better settling time if adjusted correctly, it can actually cause more problems if
