8 Accelerator Engineering and Technology: Accelerator Technology
419
tracking between different transducers. Such is the case in the LHC at CERN,
where the main dipole and quadrupole circuits are divided into eight sectors. In
this case, calibration of the current measurement chain using the same reference
ensures that all sectors track each other, allowing the beam to circulate around the
machine without “seeing” any difference in the magnetic field between sectors.
The periodicity of calibrations must be set according to the accuracy requirements of the power converter. Some devices include automatic calibration mechanisms: a common practice is to use a multiplexer that connects the ADC input to
a voltage reference for calibration. However, the references themselves might need
periodic calibration, so human intervention might be unavoidable.
Calibration procedures usually involve the use of dedicated equipment which
must be previously characterised in a laboratory using well known reference
devices. The complexity of the necessary calibration infrastructure depends on the
accuracy one is trying to achieve. Some of the methods employed to calibrate
DCCTs and ADCs and required calibration equipment are described below.
DCCTs can be calibrated through different methods:
1. The reference DCCT method, where the output of the DCCT being calibrated is
compared against the output of a “reference” DCCT measuring the same current.
2. The output stage method, which involves injecting a reference current in the
burden resistor of the DCCT and measuring its output with a calibrated ADC or
with a DVM. The value of the error is then memorised by the digital controller.
This method has the disadvantage that it only calibrates the DCCT output stage
(burden resistor + precision amplifier) and it requires a precision current source
to generate the calibration current.
3. The calibration winding method, which involves injecting a reference current in
an auxiliary winding in order to produce an Ampere-Turn value equivalent to the
one produced by the primary current hence simulating real primary current. The
output of the DCCT is measured with a calibrated ADC or with a DVM and the
value of the error is memorised by the digital controller.
The latter method requires a precision current source to generate the calibration
current. At CERN, a programmable current reference has been developed for the
calibration of DCCTs equipped with calibration windings [89]. It can produce DC
currents ranging from −5 A to 5 A with sub-ppm accuracy. In the LHC, in-situ
calibration systems equipped with these devices are installed close to the main
power converters. They are housed in temperature controlled racks and can be
remotely controlled via an ethernet connection in order to calibrate one or several
DCCTs in the main dipole and quadrupole power converters.
The remotely controlled calibration system used in the LHC is depicted in Fig.
8.54.
In applications where a reference current source is not available, DCCTs
are usually calibrated using the reference DCCT method. However, since in-situ
deployment of a reference device is not always easy, the calibration procedure
often requires bringing the DCCT to a laboratory. As removal and transport of the
magnetic heads is a difficult process, the calibration in the laboratory is normally
419
tracking between different transducers. Such is the case in the LHC at CERN,
where the main dipole and quadrupole circuits are divided into eight sectors. In
this case, calibration of the current measurement chain using the same reference
ensures that all sectors track each other, allowing the beam to circulate around the
machine without “seeing” any difference in the magnetic field between sectors.
The periodicity of calibrations must be set according to the accuracy requirements of the power converter. Some devices include automatic calibration mechanisms: a common practice is to use a multiplexer that connects the ADC input to
a voltage reference for calibration. However, the references themselves might need
periodic calibration, so human intervention might be unavoidable.
Calibration procedures usually involve the use of dedicated equipment which
must be previously characterised in a laboratory using well known reference
devices. The complexity of the necessary calibration infrastructure depends on the
accuracy one is trying to achieve. Some of the methods employed to calibrate
DCCTs and ADCs and required calibration equipment are described below.
DCCTs can be calibrated through different methods:
1. The reference DCCT method, where the output of the DCCT being calibrated is
compared against the output of a “reference” DCCT measuring the same current.
2. The output stage method, which involves injecting a reference current in the
burden resistor of the DCCT and measuring its output with a calibrated ADC or
with a DVM. The value of the error is then memorised by the digital controller.
This method has the disadvantage that it only calibrates the DCCT output stage
(burden resistor + precision amplifier) and it requires a precision current source
to generate the calibration current.
3. The calibration winding method, which involves injecting a reference current in
an auxiliary winding in order to produce an Ampere-Turn value equivalent to the
one produced by the primary current hence simulating real primary current. The
output of the DCCT is measured with a calibrated ADC or with a DVM and the
value of the error is memorised by the digital controller.
The latter method requires a precision current source to generate the calibration
current. At CERN, a programmable current reference has been developed for the
calibration of DCCTs equipped with calibration windings [89]. It can produce DC
currents ranging from −5 A to 5 A with sub-ppm accuracy. In the LHC, in-situ
calibration systems equipped with these devices are installed close to the main
power converters. They are housed in temperature controlled racks and can be
remotely controlled via an ethernet connection in order to calibrate one or several
DCCTs in the main dipole and quadrupole power converters.
The remotely controlled calibration system used in the LHC is depicted in Fig.
8.54.
In applications where a reference current source is not available, DCCTs
are usually calibrated using the reference DCCT method. However, since in-situ
deployment of a reference device is not always easy, the calibration procedure
often requires bringing the DCCT to a laboratory. As removal and transport of the
magnetic heads is a difficult process, the calibration in the laboratory is normally