Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 34
www.ScannerMAX.com
6.2
Step waveforms
The servo tuning can be adjusted to optimize the step time to the settling accuracy and
step size (sizes) required by a particular application. Servo response to a step
command can be characterized as under-damped, critically damped, or over-damped.
See Figure 6.2 below.
With the scanner operating at the required step angle, the settling performance can be
monitored on the position test point with the aid of an oscilloscope, or using the
oscilloscope built into the Mach-DSP Servo Control application.
Rise times can be minimized by increasing the servo gain control. Typically the
smoothness of settling can be improved by increasing the Low-Frequency Damping and
High-Frequency Damping. For applications requiring settling to very small tolerances
(200 microradians or less) an over-damped response frequently gives the fastest overall
settling, even though the rise time is slower, because an over-damped tuning is gentler
to the scanner and tends to excite fewer high-frequency resonances, allowing the tight
settling to be achieved more readily.
Since large step commands may tend to saturate the power amplifier of the servo
controller, it may be difficult to optimize the tuning for more than one large-signal step
size. Therefore if consistent performance is required for a variety of different large
steps, we recommend that you enable the slew rate limiter as well as possibly using the
Error Signal Modifier. These help to eliminate the non-linearity’s associated with
saturating the power amplifier. Alternatively, you may structure (also known as micro-
vector) the command waveform.
Figure 6.2: Types of Step Responses
Angle
Command
Overdamped
Critically Damped
Underdamped
Time
