SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
monitoring capability for safety purpose. As illustrated below the DRCU has electrical
interfaces with:
• the FPU
• the DPU
• the PDU (S/C)
• the EGSE (for shutter ground test only)
The DRCU is composed of two subsystems; the Detector Control Unit (DCU) and the FPU Control Unit
(FCU) which are both housed in a single box. Each system has analogue interfaces with the systems in the
CVV and digital interfaces with the DPU for the transfer of scientific data, telemetry and
commands/acknowledge packets. There is no electrical connection between the DCU and FCU.
A common SMPS (with several outputs) is part of the DRCU box. The secondary output lines of this SMPS
supply the sub-systems via distribution electronics whose function it is to monitor the current consumption
of each sub-system and enables switching off in case of over current.
Electrical interfaces with the FPU are located in the front cover of each board while interface with DPU are
located in the rear cover of the DRCU box : this configuration minimizes wiring length and risk of cross-talk
of sensitive analogue signals of the FPU/DRCU interface.
4.1.2.2 Mechanisms Control Unit MCU
The MCU controls and monitors the two mechanisms of the SPIRE instrument, viz.:
(i) the Spectrometer Mechanism (SMEC). The control is typically based on a scan at a configurable
speed, but can be set-up on the basis of a step position control in case on use of the step and
integrate mode of the instrument;
(ii) the chopper and jiggle axis of the Beam Steering Mirror subsystem . The control is a position step
control pattern.
The position and rate control of the 3 axes is performed by an Analog Devices 21020 DSP chip using PID
control architecture. The MCU software is based on a master scheduler using the principle of time sharing
without a specific multi tasking kernel. The tasks to be performed shall be called on the basis of a software
interrupt generated by the inner DSP timer. The software interrupt defines the global sampling time (ie the
computation cycle) of the DSP tasks at a programmable rate of between 100 and 300 µs. During each cycle,
the following tasks are performed :
(i) the SMEC control loop task
(ii) the BSM chop control loop task,
(iii) the BSM jiggle control loop task,
(iv) the communication with the command line and other various internal DSP tasks.
The scan parameters are put in memory for configuration purpose with a command bi-directional serial line.
The MCU receives commands from the DPU via a 32-bit, bi-directional serial interface. The MCU also
sends three types of data back to the DPU:
(i) H/K Data: which consists of the readout of single variables read by the command line at a rate of
about 1 Hz. The H/K variables readout by a get_parameter command from the DPU. The delay
between transmission of a H/K readout request and the response is about 500 µs. Typical H/K
variables include, mean SMEC scan speed, SMEC control status, SMEC encoder status, Chopper
mean position, Jiggle mean position etc.
(ii) Trace Data: The trace data is a buffer in the DSP memory which contains a long data acquisition.
The number of trace variables, the sampling time, the length of the data buffer are programmable.
This buffer can be read off-line, i.e. when the DPU sends a dedicated command for each varaible.
Typically, the trace data mode shall be used for the scanning of large number of samples for
engineering purposes;
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