Specifications

Doc Number: VIS-ANA-VER-01001-9002
Date: 27 May 2004
Issue: 5.0
Page: 75 of 80
Author: J. Delgadillo
Controls Preliminary Design Report Issue 5.doc
Appendix C - AZ/ALT and CASS Central Control Unit
Refer to the Figure 4.2.2.1.D, AZ/ALT PDU CCU Block Diagram and Figure 4.2.2.2.D,
CASS PDU CCU Block Diagram. Only the CCU components that are considered to be test
equipment (not required to meet the requirements of AD01 and AD02) are discussed in this
section.
Logic Power is supplied to the off-the-shelf Pentium motherboard from a universal input
300W switching power supply. The power supply supplies power to the CCU Motherboard
and to the boards that are plugged in to it.
The Digital Signal Processor (DSP) board has an 80 MHz processor and dual-ported RAM
which provides a very high speed PC bus communications path to the CCU Interface board.
The CCU Interface board has a watchdog timer that resets an out of control computer
microprocessor.
The CCU has the main function to execute the required position loop algorithms when the
MCU is in control of the mount. The CCU receives the position commands from the MCU
via a serial RS-232 link. The CCU Interface board also serves as the link between the CCU
computer and the Control board.
The CCU firmware is also capable of compensating for repeatable mount mechanical errors
(such as foundation tilt, azimuth bearing wobble, non-orthogonality, etc.).
The Differential/Receiver Driver board shown on the block diagrams, conditions the
Heidenhain tape encoder reference pulses to be of the appropriate amplitude for interfacing to
the Delta Tau Accessory 51P High Resolution Interpolator card. Since the Heidenhain tape
encoders are incremental, after powering up the CCU, an encoder calibration procedure must
be performed in order to calculate the absolute axis position. A Heidenhain provided formula
is used to compute the axis absolute position after traversing two distance-coded reference
marks. The computed axis absolute positions are sent to the MCU and to the PMU for
display.
The CCU Interface board firmware, flash EPROM, is responsible for the interface to the
Azimuth, Altitude and Cassegrain relay boards. These boards provide fault/status
information to the LCUs via dry-contact closure. This ensures that the required galvanic
isolation between VertexRSI and LCU units is maintained. The fault/status signals provided
to the LCUs by the relay boards is shown below on Table 1 (AZ), Table 2 (ALT) and Table 3
(CASS).