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F. Bordry et al.
synchronism and homogeneity in the magnetic field around the circumference.
However this is not always true: in the case of the LHC the main circuits are divided
into eight sectors due to the very high stored magnetic energy and constraints in
the protection of the superconducting magnets. As a consequence, individual sector
currents must be controlled with very high absolute accuracy in amplitude and time
[75] to ensure tracking between all sectors. Requirements for accuracy in current
control are also determined by the type of magnet and its function: the need for
accuracy in the current control for corrector magnets is much less stringent than for
the main quadrupole and dipole magnets.
8.4.4.1 Power Converter Control
In the past, power converter control in accelerators was usually based on analogue
feedback loops using PID (Proportional/Integral/Derivative) control. In such systems, the reference value at the input of the loop is often given by a DAC (Digital to
Analogue Converter) and the output current of the power converter measured by a
precision current transducer which provides the feedback signal for the control loop
[76]. The accuracy achieved with such techniques is limited by errors due to drift,
linearity and temperature dependency. Moreover, adjustment of control parameters
can be cumbersome, as it might require trimming of potentiometers or replacement
of components.
Developments in digital electronics and in particular DSPs, PLDs and Microcontrollers, as well as the need for increased performance in power converter
applications fuelled significant progress in digital control during the last decades.
The first applications using digital control in power converters for accelerators
were implemented in the late 1980s [77]. Amongst the advantages of using digital
control are increased stability and reproducibility, less susceptibility to noise and
thermal effects, easy implementation of different control methods (state-space,
robust, fuzzy) as well as easy loop parameterization. On the other hand, the use of
digital control increases system complexity and introduces new sources of error such
as the ones resulting from ADC measurement uncertainty and limited resolution on
arithmetic calculations, which might lead to arithmetic errors.
When high accuracy is required, one effective power converter control strategy
is to have an external current loop controlling a power supply that works as a
voltage source. In a digitally controlled system, the output current of the converter
is measured by a current transducer connected to an ADC and then compared with
a digital reference. The error is fed into a digital regulator and the result sent to a
DAC that provides an analogue signal to control the voltage source [77].
This solution is implemented in the control of the LHC power converters at
CERN. In this case, the control challenge is even more demanding due to the 8sector powering strategy used for the main dipole and quadrupole circuits. This
powering strategy requires not only an accurate control of the current of each
power converter but also an accurate generation and synchronization of the current
references sent to the converters along the 27 km circumference of the LHC. For this
F. Bordry et al.
synchronism and homogeneity in the magnetic field around the circumference.
However this is not always true: in the case of the LHC the main circuits are divided
into eight sectors due to the very high stored magnetic energy and constraints in
the protection of the superconducting magnets. As a consequence, individual sector
currents must be controlled with very high absolute accuracy in amplitude and time
[75] to ensure tracking between all sectors. Requirements for accuracy in current
control are also determined by the type of magnet and its function: the need for
accuracy in the current control for corrector magnets is much less stringent than for
the main quadrupole and dipole magnets.
8.4.4.1 Power Converter Control
In the past, power converter control in accelerators was usually based on analogue
feedback loops using PID (Proportional/Integral/Derivative) control. In such systems, the reference value at the input of the loop is often given by a DAC (Digital to
Analogue Converter) and the output current of the power converter measured by a
precision current transducer which provides the feedback signal for the control loop
[76]. The accuracy achieved with such techniques is limited by errors due to drift,
linearity and temperature dependency. Moreover, adjustment of control parameters
can be cumbersome, as it might require trimming of potentiometers or replacement
of components.
Developments in digital electronics and in particular DSPs, PLDs and Microcontrollers, as well as the need for increased performance in power converter
applications fuelled significant progress in digital control during the last decades.
The first applications using digital control in power converters for accelerators
were implemented in the late 1980s [77]. Amongst the advantages of using digital
control are increased stability and reproducibility, less susceptibility to noise and
thermal effects, easy implementation of different control methods (state-space,
robust, fuzzy) as well as easy loop parameterization. On the other hand, the use of
digital control increases system complexity and introduces new sources of error such
as the ones resulting from ADC measurement uncertainty and limited resolution on
arithmetic calculations, which might lead to arithmetic errors.
When high accuracy is required, one effective power converter control strategy
is to have an external current loop controlling a power supply that works as a
voltage source. In a digitally controlled system, the output current of the converter
is measured by a current transducer connected to an ADC and then compared with
a digital reference. The error is fed into a digital regulator and the result sent to a
DAC that provides an analogue signal to control the voltage source [77].
This solution is implemented in the control of the LHC power converters at
CERN. In this case, the control challenge is even more demanding due to the 8sector powering strategy used for the main dipole and quadrupole circuits. This
powering strategy requires not only an accurate control of the current of each
power converter but also an accurate generation and synchronization of the current
references sent to the converters along the 27 km circumference of the LHC. For this