Mach-DSP User’s Manual
Document Number: MACH-DSP-9021
Page 27
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5.3
Servo architecture and digitizing resolution
The Mach-DSP is essentially a digital implementation of an analog servo driver, but one
that has much greater flexibility and utility. An overview of the architecture, including
block diagrams is presented further below.
Digitizing resolution and implications
The ±10V differential analog input command signal is filtered in the analog domain, and
then digitized at 16-bits. For most applications, this input signal range corresponds to an
optical deflection of either 40 optical degrees or 60 optical degrees peak-to-peak. Thus,
from a command input standpoint, the corresponding digitized command resolution is:
40 optical degrees / 65,536 = 0.00061 degrees per bit (11 microradians)
60 optical degrees / 65,536 = 0.00092 degrees per bit (16 microradians)
The scanner’s position signal is also digitized at 16-bits. However, in order to
accommodate applications that require scan angles of 80 degrees optical or even
greater, generally speaking, the 16-bit full scale digitizing range corresponds to around
90-degrees on the position signal. The corresponding digitized position resolution is:
90 optical degrees / 65,536 = 0.00138 degrees per bit (24 microradians)
The numbers above, in microradians, represent the best-case resolution that is possible
with any digital servo driver that uses 16-bit converters with these full-scale signal
ranges. Certain noise sources (including quantizing noise) may effectively reduce this
resolution even further. However, the Mach-DSP does incorporate a “Resolution
Enhancement Filter”, which averages several samples of the Command and Position
signals, thus providing better precision than the figures above would suggest.
Nevertheless, for applications requiring very fine positioning and low dither, testing will
need to be performed using various settings of the Resolution Enhancement Filter, to
determine whether the Mach-DSP can deliver the precision required by the application.
Servo architecture overview and block diagram
A complete block diagram of the Mach-DSP servo is shown on the next two pages. This
diagram can also be accessed on the HELP menu of the Mach-DSP program.
Test Points on the diagram
The “TP” numbers are test points that can be accessed in the Oscilloscope portion of
the Mach-DSP program. These test points can also be routed to any of six separate
user-defined analog outputs on the Mach-DSP servo driver. The Mach-DSP program,
oscilloscope function, and analog outputs are all discussed in Chapter 9.
