Speaker
Description
Summary
The TPC is the main tracking detector of the ALICE central barrel and, together
with the other central barrel detectors, has to provide charged-particle momentum
measurements. Charged particles traversing the TPC volume ionise the gas along
their path, liberating electrons that drift towards the end plates of the cylinder.
Moving from the anode wire towards the surrounding electrodes, the positive ions
created in the avalanche induce a positive current signal on the pad plane. The
readout of the signal is done by the 570132 pads that form the cathode plane of the
multi-wire proportional chambers located at the TPC end plates.
Under operating conditions the system must safely apply a DC voltage of 100kV to
the central electrode placed half way between the two end plates of the detector.
Four resistor rods supply the proper potential to the field cage.
The Control System was developed following the TPC subsystems requirements,
covering the following features: control and monitoring of the High Voltage power
supply, the current in the resistor rods, the cooling, and some additional
equipment. Voltage must be smoothly applied by a ramping mechanism.
The Power Supply (PS) is a Heinzinger PNC 150000 neg-2, customized for the
experiment needs. It can provide up to 150kV DC. Its main modification is the
addition of an electronic controller that limits the slew rate in case of sudden
power up or down. It is the first time that such a high voltage power supply
integrates this feature. The PS is also controlled by a RS232 interface. A
Programmable Logic Controller (PLC) controls the ramping and several safety
features. Thanks to its simple architecture and its stand-alone characteristic, the
PLC provides more reliability than a common PC. The chosen PLC is a Schneider
Electric P575634M. As it directly controls the PS through the RS232, dedicated
communication routines were implemented. Currents in the resistor rods of the field
cage are monitored as voltage drops over a resistor. Monitoring is performed by two
ELMB reading 16 analog inputs. Each ELMB is connected to the PLC through CAN bus.
The PLC controls that the voltages remain in range. Status levels of relevant
parameters for the operation of the PS are monitored through 13 interlocks (digital
inputs) connected to the PLC. They provide status information about cooling water,
UPS, gas condition and circulation.
In case of any failure in the monitored elements, the PLC reacts by triggering a
ramp down of the PS. Depending on the error level, the ramping down can be slow
(PLC driven) or fast (controlled by the electronic slew rate limiter). The PLC
itself is monitored by a device designed in order to trigger a hardware ramp down
in case of losing the PLC heartbeat.
As the TPC is fundamental for the ALICE experiment to work, the system was chosen
for the PLC reliability. Hence, special effort was put into the development of
communication with the electronic devices that, in conventional systems, are
usually controlled in transparent way by a common PC.
