Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 33
www.ScannerMAX.com
6.0 Command Input waveforms and implications
The Mach-DSP command signal may come from the built-in test signal generator, but
more commonly it will come from the external analog command input connector. The
type of waveform and amplitude of the command signal affects how the servo should be
configured and tuned for optimal performance. This chapter gives an overview of the
types of waveforms that the servo may encounter, and implications of each type of
waveform.
6.1
Large-Signal and Small-Signal waveforms
The first things that must be understood are the terms “small-signal” and “large-signal”.
Although optical scanners are used in a wide variety of applications, the command
waveforms can be grouped into two broad categories:
small-signal
and
large-signal
.
This terminology does not refer to the angular excursion of the mirror, but rather to
signal levels internal to the servo amplifier, and particularly, whether or not the power
amplifier saturates while moving the scanner. The power amplifier will saturate if the
commanded acceleration exceeds the system’s linear range. Thus, it is power amplifier
saturation that defines the transition between small- and large-signal commands.
A full-field step would be deemed a “large-signal” command, if the acceleration required
to follow the waveform can not be accommodated. On the other hand, the same full-
field step would be deemed a “small-signal” command, if the waveform were structured
such that the maximum linear acceleration limit of the scanner/mirror/driver combination
is not exceeded. These are illustrated in Figure 6.1:
∞
∞
α
α
t
t
t
t
θ
θ
Command
Input
to Servo
Acceleration
required
Large Signal
Small Signal
Power Amp Saturation Level
Figure 6.1: Large Signal and Small Signal Commands