The GREAT
data acquisition system is installed in three 47U 19inch cabinets. Each cabinet
has a 5KW water based heat exchanger to provide a stable environment for the
electronics. The heat exchangers are mounted on the outside of the cabinets
where they will also work to dry the air as it passes through. There are drains
at the bottom of each one.
The
environment will be monitored to prevent the equipment from over-heating, or
being subject to condensation.
It is intended
the cabinets will all be normally closed to allow maximum stability of the
environment. The main benefit of a stable environment is to minimize
temperature induced gain shifts in the NIM Shaping amplifiers, and the VXI
ADCs.
The
monitoring system will be connected to the VXI control PC mounted in one of the
cabinets. The sensors are distributed around the cabinets.
Each
cabinet will contain some temperature, humidity, and door sensors. To allow for
control of the water based heat exchangers there will be a relay unit in each
cabinet to control the electrically operated cold water valve. The control will
be simply on, or off. The water flow will be stopped if the temperature of the
electronics falls below the dewpoint of the cabinet environment. For example if
a VXI crate power supply fails, and the temperature within the cabinet falls.
The operation of the system is explained in a later section.
Temperature
sensors will be placed at the top and bottom of the VXI crate to measure the
temperature gradient across the VXI cards.
The VXI,
and VME crate power-supplies are connected via a CANbus network to a VME
interface. The connection allows internal temperatures, power, voltages,
currents and fan speed to be monitored. The fan speed can be controlled via
this interface.
Operation:
The
environment monitoring system output will be used by software in the VXI
control PC, and MIDAS to control the fan speeds in the VXI, and VME crates.
The
objective is to ensure the electronics does not overheat, or reach a
temperature below the dewpoint at any time.
Once the
cabinets are closed, i.e. during an experiment, and the environment has
stabilized, it is not the intention of this system to change the fan speeds etc
without requesting permission of the user. This is to minimize the noise
effects that changes of fan speed may have on the electronics. Investigations
are necessary to determine which is better, a stable temperature, or fixed fan
speeds. Day/night ambient temperature fluctuations in the cave may require
intervention.
However if
the temperature drops below the dewpoint,
which will cause condensation and thus damage to the equipment, the
water will be turned off and the user informed.
If the
temperature of the equipment rises to a point where damage will occur , and
there is no more adjustment in the cooling system. The equipment will be turned
off.
The user
will have to then call on engineering assistance to rectify the fault. The
GREAT system is designed to withstand power failures by use of battery back-up
for the master clock, and counter. The environment switch off will be another
form of power failure, and so it will be possible to recover from it and
continue an experiment.
It may be
of use to pass the environmental information to the tape system for storage
with the experimental results.
There is
more work to be done to define the exact parameters for a good working
environment, and to decide on the response of the system to activities like
changing modules during a run. The main requirement of the environmental
control system is to safeguard the electronic equipment from damage.