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DIGITAL LN2 CONTROL SYSTEM FOR SUPERCONDUCTING LINAC
AT PLF, MUMBAI
J.N. Karande, P. Dhumal, A.N. Takke, S. Pal, V. Nanal and R.G. Pillay
Pelletron LINAC Facility, TIFR, Mumbai – 400005.
Abstract
The superconducting LINAC at PLF, Mumbai has a
modular structure with eight cryostats divided in two
groups. Each cryostat has a LN2 vessel of ~ 40L capacity,
which serves as a thermal shield for helium vessel. A
coaxial capacitance type sensor is developed in house to
measure the LN2 level. Monostable multi-vibrator based
readout electronics is designed to measure the change in
capacitance due to LN2 level change. The sensor
capacitance with a fixed value resistor decides the ‘ON’
time of the monostable multi-vibrator. A 16 bit timer of
microcontroller 8051 is used to measure the pulse width.
The change in pulse width is converted into a LN2 level
and displayed on a 4*20 LCD screen. The control unit is
also equipped with automatic control of LN2 filling
valves triggered by levels. The digital LN2 control can be
operated locally or remotely via a serial RS232
communication line. Two control stations are designed
and developed to take care of eight cryostats.
INTRODUCTION
The superconducting LINAC at PLF, Mumbai has a
modular structure with eight cryostats divided in two
groups. Each cryostat has a LN2 vessel of ~ 40L capacity,
which serves as a thermal shield for helium vessel. A
capacitive level transducer is developed for measuring
LN2 level inside the cryostat or a storage dewar.
Capacitive measurement is chosen for better accuracy and
also due the fact that it is unaffected by the magnetic
field. This paper presents design and development of a
continuous readout type capacitive level transducer and
control electronics for automatic filling of Liquid
Nitrogen (LN2) in modular cryostats.
LN2 LEVEL SENSOR
The basic principle of capacitive type level
measurement is the change of capacitance between the
electrodes due to the presence of liquid Nitrogen, which
acts as a dielectric medium. The capacitance of two
coaxial tubes of diameters d (inner tube) and D (outer
tube) and height H is given by,
𝐶=
in the sensor. Nylon fishing thread is used to avoid the
shorting between two tubes. Figure 1 shows a picture of a
coaxial capacitance type sensor and sensor electronics
developed in-house. The sensor electronics is mounted
directly on top of the sensor, to eliminate additional
capacitance of connecting cables.
Figure1: A coaxial capacitance type LN2 level sensor and
sensor electronics developed in-house
LN2 CONTROL ELECTRONICS
LN2 cryo-control station design is based on Silicon Lab
C8051F20 microcontroller board. LN2 control station is
developed for auto LN2 filling in LINAC cryostats. One
control station is capable of reading upto 6 level sensors
and of controlling a maximum of 8 valves. The PC
interface has been developed for remote control and
monitoring.
Monostable multi-vibrator based readout electronics is
designed to measure the change in capacitance due to
LN2 level change [1]. The ‘ON’ time of the monoshot
output pulse is decided by the Capacitance (C) of the
transducer and a fixed value resistor (R). The monoshot
output is inverted and then connected to the
microcontroller. The falling edge of the level sensor
output pulse initiates the 16 bit counter in the
microcontroller. When the pulse changes its state from
low to high, the timer stops the counting action. Thus, the
width of the output pulse is proportional to the level of
LN2 in the cryostat. Figure 2 shows a typical snapshot of
the level sensor output indicating two different LN2
levels in the cryostat.
2𝜋𝜀0 𝜀𝑟
𝐻
ln(𝐷/𝑑)
where 𝜀0 is the permittivity of the free space and 𝜀𝑟 is the
dielectric constant of the medium between the electrodes.
The sensor is made of two closely spaced concentric
0.5 mm thick SS tubes of 8 and 10mm diameter. Special
care is taken during construction and alignment of tubes
Figure 2: Output waveforms of LN2 Level sensor
readout
The pre-determined calibrated value of change in
Capacitance per unit length (pf/cm) is used to convert the
measured time into percentage level (0 to 100%) and is
displayed on 4*20 LCD display.
The LN2 filling valve is a solenoid operated valve and
requires a 24V DC voltage. These valves can be operated
either in manual or in auto mode. In manual mode, any
valve can be opened or closed using 4*4 matrix keyboard.
In auto mode, the valve operation depends on the
measured level. The valve is designed to close when level
exceeds 90% and opens when level falls below 20%. The
default mode for the control station is set for remote
operation. An additional keyboard as well as display is
provided for local control and readout. Figure 3 shows a
picture of the assembled LN2 cryo-control station. Two
control stations for LINAC Phase I and Phase II have
been installed and tested.
level. In the present design, since the change in the
capacitance per unit length is determined, only the full
level value suffices for calibration.
Calibration feature is included in the software and a
CALIB key is provided for this purpose. When pressed,
this key issues a command to read the LN2 levels for all
six sensors corresponding to full value (100%). The
program calculates the empty level value (0%) using
change in capacitance per unit length (pf/cm) which is
already supplied in the software. Using these two values
for 0 to 100%, mapping is done for entire range. In order
to prevent accidents, the CALIB key needs to be pressed
for two seconds for initiating calibration action.
SUMMARY
A coaxial capacitance type sensor is developed in house
to measure the LN2 level. Monostable multi-vibrator
based readout electronics is designed to measure the
change in capacitance due to LN2 level change. LN2
control station is developed for auto LN2 filling in
LINAC cryostats. One control station is capable of
reading upto 6 level sensors and of controlling a
maximum of 8 valves. Two control stations have been
designed and installed in LINAC accelerator. The digital
LN2 control can be operated locally or remotely via a
serial RS232 communication line. LN2 levels of eight
cryostats and two storage dewars are available for
monitoring in the control room.
ACKNOWLEDGMENT
Figure 3: LN2 Sensor and control electronics
AUTO CALIBRATION
Auto calibration is one of important features of the
present design. For calibration of each sensor, one needs
to know readings corresponding to empty and full level
inside the vessel. In operating condition the full level
value can be easily obtained, while it is not always
possible to get the reading corresponding to an empty
We would like to thank Mr. L.V. Kamble for valuable
assistance during the fabrication of sensors and the PLF
staff for their support.
REFERENCES
[1] I.V. Velichokov et al., “Capacitive level meters for
cryogenic liquids with continuous read-out”, Cryogenics 30,
(1990) 538.