Specifications

Doc Number: VIS-ANA-VER-01001-9002
Date: 27 May 2004
Issue: 5.0
Page: 53 of 80
Author: J. Delgadillo
Controls Preliminary Design Report Issue 5.doc
The major subassemblies that make up the Data Acquisition System are listed below:
MANUFACTURER MANUFACTURER
PART NUMBER
DESCRIPTION
CONDOR D.C. POWER
SUPPLIES, INC
HC24-2.4A+ Power Supply
Beckhoff BK5120 CANOpen Bus Coupler
Beckhoff KL3102 2 Channel Analogue
Input Terminal
Beckhoff KL3202 2 Channel Input Terminal
PT100 (RTD)
Pyromation Incorporated R3T1853 Series 3-Wire, Platinum RTD
4.2.5.13 Optical Incremental Tape Encoder
The position encoders to be used are the Heidenhain ERA 880C, distance-coded tape
encoder, for the Azimuth, Altitude and Cassegrain Rotator axes.
The optical encoders contain gold line graduations, which reflect light in a defined direction,
and diffusely reflecting gaps. These Heidenhain tape encoders operate on the principle of
photo-electrically scanning very fine gratings. The photoelectric scanning of a substrate is
the measurement of the change in light intensities using photovoltaic cells. When a line
grating is moved relative to another grating with the same structure, the lines of two
graduations alternately align. The resulting light-dark modulation is sensed by photovoltaic
cells. Electronics in the tape encoder read-heads produces two sinusoidal waveforms that are
shifted from each other by 90°, and have approximate amplitude of 1 V peak-to-peak as the
scanning reticule is moved across the fixed scale tape.
The ERA 880C incremental encoders have a steel scale tape with a grating period of 40
microns. The tape encoders have a second (reference) track that consists of graduated disks
next to the incremental grating. This reference track is distance-coded, which means that the
number of graduations between reference marks is unique. An algorithm is then
implemented to obtain an absolution axis position after traversing two consecutive reference
marks.