Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 58
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not adjusted correctly. For that reason, these controls should be adjusted and tuned
using the built-in oscilloscope, while monitoring the Observed Velocity (TP63),
Observed Position (TP36) and Observer Error (TP40).
Coil temperature calculator
As current is driven into the scanner to create beam deflection, the temperature of the
coil and that of the inside the scanner may rise. The actual temperature rise depends on
the waveform of the command input signal, as well as the scanner tuning. The
temperature inside the scanner also depends on the X-Y mount and other factors that
are external to the scanner (including the ambient temperature).
Galvanometer scanners are generally designed to operate with their coil having a
maximum temperature of around 100 degrees C, while the outside of the scanner body
having a maximum temperature of around 50 degrees C. If the coil temperature is
allowed to exceed the maximum specified on the datasheet, permanent damage to the
scanner may result. Because of this, the Mach-DSP servo includes a Coil Temperature
Calculator, and systems to make sure that the coil temperature is not exceeded.
On the Pangolin-only tab, there are adjustments that control the manner in which the
Coil Temperature Calculator will react. The normal manner is to greatly impede scanner
acceleration (and thus drive current) when the motor’s coil temperature has approached
the maximum, and then to temporarily shut down the servo if the maximum temperature
is greatly exceeded.
The Mach-DSP servo driver implements two modes in which the coil temperature may
be calculated by the servo. “Mode 1” is to measure only the coil’s RMS drive current,
and then estimate the temperature of the coil, given assumptions about the scanner’s
mounting configuration and environment. This is essentially an open-loop approach.
“Mode 2” is to actively measure both the coil’s instantaneous drive current and drive
voltage, and derive the coil’s actual resistance, and then calculate the actual
temperature of the coil. This is essentially a closed-loop approach.
For decades, manufacturers of conventional servo drivers have used RMS current
alone to estimate coil temperature. This is simple and effective, but possibly error-
prone, because assumptions are made that the scanner is mounted to a compression-
style X-Y mount that is bolted to an infinite heat sink, and that the scanner’s body will
never exceed 50 degrees C. When this is not the case, these simple coil temperature
calculators will underestimate the actual coil temperature – and in a worst-case
scenario, this will result in scanner destruction. On the other hand, the Mode 2 approach
of actively measuring the coil temperature using both coil voltage and coil current may
result in a very accurate estimation of coil temperature – and one which does not rely on
any assumptions about the X-Y mount or other environmental factors. However, as was
the case with the velocity observer described above, this mode requires additional
information to be entered (cable resistance, coil inductance, and Back EMF voltage)
and if these are not entered correctly, the coil temperature calculator will not estimate
coil temperature correctly. This second mode also requires more computational power
