# Patent

## Title
Active magnetic bearings control system

## Abstract

A control device for controlling the position of a rotor supported by active magnetic bearings is provided. The control device includes a trajectory planning module for generating a requested position, speed and acceleration; a feedback unit for generating a position feedback value and a speed feedback value; a first correction circuit for generating a first command signal according to the difference between the requested position and speed and the position and speed feedback value respectively; a feed-forward controller for generating a second command signal; an adder for adding first and second command signals and delivering a third command signal for a non-linear inversion circuit connected to the adder for generating flux command signals for the electromagnets, and a second correction circuit for generating voltage command signals for the power amplifiers which control the current flowing in the electromagnet coils of the active magnetic bearings.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to European Patent Application No. EP13305921.2 filed Jun. 28, 2013, the contents of which are hereby fully incorporated by reference.
TECHNICAL FIELD
The present invention relates to magnetic bearings and more particularly to an axial magnetic bearing having improved dynamic performances due to a motion planning control.
BACKGROUND
Magnetic bearings are used in different rotating machines such as electric motors, compressor, turbines or the like in order to maintain the axial or/and radial positions of a rotating shaft by means of magnetic fields acting on a rotor of the machine.
Axial magnetic bearings are often used to reject axial disturbances coming from the industrial environment (pressure waves and oscillations) that can create unwanted effects on the behaviour of the rotor of the turbine or the compressor such that limit cycles, vibrations, instabilities.
The axial bearing force is built using a pair of electromagnets connected to power amplifiers (classically one power amplifier per electromagnet) for which the control voltage is adapted by a controller. However, the unlaminated nature (an unlaminated bearing contributes to the eddy currents creation) of the thrust magnetic bearing limits the controller action. The dynamic of the actuator is considerably reduced (an axial bearing cannot reject disturbances located outside of a specific bandwidth frequencies) and the iron losses are increased.
So, as a known solution, when it is possible, the axial bearing design is changed (introduction of slots or use of a laminated design) to break or at least limit the eddy current creation and to reduce losses. Other solutions based on using specific material, for example insert made of ferromagnetic powder, are also identified.
However, all of those known solutions cannot be generalized because they are expensive and increase the magnetic bearing cost. Moreover, they cannot be used for each bearing due to mechanical limitations such as reduction of the mechanical resistance or reduction of the available force for example.
SUMMARY OF THE INVENTION
The present invention aims to eliminate the above disadvantages by dealing with the eddy current using a motion planning control (flatness based control) preferably without changing the hardware or the magnetic bearing design. With this solution we also minimize the losses.
For this, the control device for controlling the position of a rotor supported by active magnetic bearings supplied through power amplifiers whose outputs are connected to electromagnet coils of the active magnetic bearings, according to the invention is of the type comprising:

    a trajectory planning module for generating a requested position, speed and acceleration,
    a feedback unit for generating a position feedback value and a speed feedback value from measurements of at least a position Z(t) of the rotor and current intensities I1(t), I2(t) in the electromagnet coils,
    a first correction circuit connected to the trajectory planning module and to the feedback unit for generating a first command signal {umlaut over (Z)}feedback(t) according to the difference between the requested position and speed and the position and speed feedback value respectively,
    a feedforward controller connected to the trajectory planning module for generating a second command signal {umlaut over (Z)}feedback(t) according to a computation of the requested position, speed and acceleration,
    an adder connected to the feedforward controller and to the first correction circuit for adding the first and second command signals and delivering a third command signal {umlaut over (Z)}com(t),
    a non-linear inversion circuit connected to the adder for generating flux command signals Φ1com(t), Φ2com(t) for the electromagnets from the third command signal, and
    a second correction circuit connected to the non-linear inversion circuit and to the feedback unit for generating voltage command signals U1com(t), U2com(t) for the power amplifiers which control the current flowing in the electromagnet coils of the active magnetic bearings according to the difference between the flux command signals and feedback flux values Φ1com(t), Φ2com(t).


According to another feature of the invention, the feedback position and speed values are observed position {circumflex over (Z)}(t) and observed speed {dot over ({circumflex over (Z)}(t) delivered by an observer receiving at least the measured position Z(t) issued by position sensors and the current intensities I1(t), I2(t) issued by current sensing elements. The observer can further receive measured voltages U1(t), U2(t) applied to the electromagnet coils or measured flux obtained from flux sensing elements and delivers observed flux for the second correction circuit.
Preferably, the first correction circuit comprises a proportional-integral controller on the position of the rotor and a proportional controller on the speed of the rotor and the second correction circuit comprises a proportional controller on the flux of the electromagnet coils.
Advantageously, the trajectory planning module further delivers a requested acceleration.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood and further details and advantages thereof will appear more clearly in the following description with reference to the accompanying drawings illustrating embodiments of the invention, and in which:
FIG. 1 illustrates a schematic diagram of a control device of the position of a rotor of an active magnetic bearing according to the invention,
FIG. 2 shows a flow chart of the successive steps executed by a trajectory planning module of FIG. 1,
FIGS. 3 and 4 represents the position and the speed of the thrust disc of an axial magnetic bearing for a classical biased control or a control according to the invention respectively,
FIGS. 5 and 6 represents the current intensity in each electromagnet of an axial magnetic bearing for a classical biased control or a control according to the invention respectively, and
FIGS. 7 and 8 represents the voltage applied to each electromagnet of an axial magnetic bearing for a classical biased control or a control according to the invention respectively.



DETAILED DESCRIPTION
FIG. 1 shows a block diagram of a control device according to the invention for controlling the position of a rotor 10A supported by an active magnetic bearing (thrust bearing 10) along a predetermined axis Y-Y′.
The set point (i.e. the desired position) constitutes the input of a trajectory planning module 12 which delivers requested position Zreq(t), requested speed Żreq(t) and requested acceleration {umlaut over (Z)}req(t) for a feedforward generation circuit 14 which delivers a feedforwarded acceleration signal {umlaut over (Z)}feedforward(t).
The feedforwarded acceleration signal {umlaut over (Z)}feedforward(t).is applied to the first input of an adder 16 which receives on a second input a feedback acceleration signal {umlaut over (Z)}feedforward(t) from a correction circuit 18.
The correction circuit comprises a proportional-integral (PI) controller 18A receiving the requested position Zreq(t) from the trajectory planning module 12 and an observed position {circumflex over (Z)}(t) and a proportional controller 18B receiving the requested speed Żreq(t) from the trajectory planning module 12 and an observed speed {dot over ({circumflex over (Z)}(t). The created {umlaut over (Z)}feedforward(t) is the sum of 18A and 18B outputs.
The observed position and speed are delivered by an observer 20 which receives different measurements from different sensors. More particularly, the observer receives a measured position of the rotor Z(t) from detectors of the rotor position (position sensors 8A, 8B; 9) located along the Y-Y′ axis, measured currents I1(t), I2(t) in first and second coils 22A, 22B of an electromagnet of the active magnetic bearing from current sensing elements 24A, 24B and measured voltages U1(t), U2(t) at the first and second coils 22A, 22B. However, it can be noted that only the measured position and the measured currents are necessary for the observer 20 which can reconstruct the speed from these two measurements and compute the feedback. On the contrary, the availability of the measured voltages is not necessary (they can be estimated from the command U1com and U2com instead) or can be substituted by measured flux obtained from flux sensing elements (not shown).
The feedforwarded acceleration signal {umlaut over (Z)}feedforward(t) and the feedback acceleration signal {umlaut over (Z)}feedforward(t) are added in the adder 16 for delivering a rotor acceleration command {umlaut over (Z)}com(t) applied to a nonlinear inversion circuit 26 which delivers flux command signals Φ1com(t), Φ2com(t) for two inputs of a flux proportional controller 28 which also receives on two other inputs observed flux (t), (t) in the first and second electromagnet from the observer 20. The flux proportional controller 28 outputs the voltage commands U1com(t), U2com(t) for the power amplifiers 30A, 30B.
With the invention, the command signal (rotor acceleration command {umlaut over (Z)}com(t)) is the sum of a feedback term and another term (named motion planning control) where an anticipated dynamic control is computed from a realistic desired dynamic. The feedback term can be computed with a linear controller such a PID or a state feedback control. This feedback term tries to follow the trajectory planning instead of the final value of the set point. The motion planning control term is an anticipated control which generates a dynamic command corresponding to a desired trajectory. More particularly, the motion planning control term can be adapted with respect to some constraints, such as the minimization of the eddy current for example, and the limitations of the system (amplifiers+actuators+rotor) could be easily implemented in the motion planning control term.
The trajectory can be built for the prescribed magnetic bearing force, the magnetic flux in the bearing, the current in the assembly (i.e. amplifier+electromagnet), the axial motion of the rotor.
The trajectory computed by the trajectory planning module will be used for two reasons, as illustrated in the flow chart of FIG. 2, the settling position trajectory at the start-up of the system and the trajectories of return to nominal position when big disturbances move the rotor above a maximum position error.
Indeed, after a first step of initialisation 100, a test 102 of convergence of the observers is made. If a negative response results from this test, an alarm 104 is set on and the test is pursued. On the contrary, if a positive response to this test is obtained the voltage commands U1com(t), U2com(t) for the power amplifiers are created (i.e. setting trajectory in a step 106) and an alarm is set on (step 108) as long as Z≠Zreq (test of step 110). When Z≈Zreq (response YES at the test of step 110), the position Z is read in a further step 112 as long as this position is below a maximum position error (response NO at the test of step 114). On the contrary, when the position Z is above the maximum position error (response YES at the test of step 114), the voltage commands U1com(t), U2com(t) for the power amplifiers are created (i.e. rejection trajectory in a step 116) and an alarm is set on (step 118) as long as Z≠Zreq before to return to the step 112 of reading of the Z position.
For a better understanding of the invention, results of tests which show the comparison of the behaviour of a classical biased control (PID controller) and the control according of the invention during the starting phase of the control of the position of an axial active magnetic bearing are illustrated on FIGS. 3 to 8. It must be noticed that the current in the amplifiers is limited to only positive values. So the negative value of voltage can only command the rate current decrease: a maintained negative voltage leads to null current. The axial position of the thrust disc attached on the rotor of the axial magnetic bearing is set from the initial position to the nominal position.
FIG. 3 represents the position of the thrust disc Z(t) with a classical control 120 and a control according to the invention 122. The third curve 124 illustrates Zreq(t) with the control according to the invention. Due to the flatness nature of the curves in opposition to the classical shape, the control according to the invention is named by the inventors as a flatness-based control (FBC).
FIG. 4 represents the speed of the thrust disc with a classical control 130 and a control according to the invention 132. The third curve 134 illustrates the observed speed {dot over ({circumflex over (Z)}(t) with the control according to the invention.
FIG. 5 represents the current intensity I1(t) in the first electromagnet with a classical control 140 and a control according to the invention 142 and FIG. 6 the current intensity I2(t) in the second electromagnet with a classical control 150 and a control according to the invention 152.
FIG. 7 represents the voltage U1(t) applied to the first electromagnet with a classical control 160 and a control according to the invention 162 and FIG. 8 the voltage U2(t) applied to the second electromagnet with a classical control 170 and a control according to the invention 172.
The present invention comprises a plurality of advantages, i.e.:

    
    
        The possibility to control the magnetic thrust bearing without bias (flux/current) such that the energy consumed using this controller can be much lower than controllers with a classical biased control,
        The possibility to easily perform a control with good performances not only at the nominal position but at any position in the air gap,
        The possibility for the rotor to follow any reachable position trajectory differentiable enough within the air gap, and
        The possibility to take into account the physics limitations of the system without compromise on the controller (feedback control term).
    
    


The invention is particularly suitable for axial magnetic bearings of important machines like chiller or turbo-expander for example as for axial magnetic bearings within smaller systems like HVAC for cars or trucks.
Although preferred embodiments have been shown and described, it should be noted that any changes and modifications may be made therein without departing from the scope of the invention as defined in the appended claims. For example, if the control circuit has been explained with one pair of opposite electromagnets, it is clear that it is generalized to four pairs of electromagnet.

## Claims

1. A control device for controlling the position of a rotor supported by active magnetic bearings supplied through power amplifiers whose outputs are connected to electromagnet coils of the active magnetic bearings, the control device comprising:
a trajectory planning module for generating a requested position and a speed,
a feedback unit for generating a position feedback value and a speed feedback value from measurements of at least a position (Z(t)) of the rotor and current intensities (I1(t), I2(t)) in the electromagnet coils,
a first correction circuit connected to the trajectory planning module and to the feedback unit for generating a first command signal ({umlaut over (Z)}feedforward(t)) according to the difference between the requested position and speed and the position and speed feedback value respectively,
a feedforward controller connected to the trajectory planning module for generating a second command signal ({umlaut over (Z)}feedforward(t)) according to a computation of the requested position and a speed,
an adder connected to the feedforward controller and to the first correction circuit for adding the first and second command signals and delivering a third command signal ({umlaut over (Z)}com(t)),
a non-linear inversion circuit connected to the adder for generating flux command signals (Φ1com(t), Φ2com(t)) for the electromagnet coils from the third command signal, and
a second correction circuit connected to the non-linear inversion circuit and to the feedback unit for generating voltage command signals (U1com(t), U2com(t)) for the power amplifiers which control the current flowing in the electromagnet coils of the active magnetic bearings according to the difference between the flux command signals and feedback flux values (Φ1com(t), Φ2com(t)).

2. The device of claim 1, wherein the feedback position and speed values are observed position {circumflex over (Z)}(t) and observed speed {dot over ({circumflex over (Z)}(t) delivered by an observer receiving at least the measured position Z(t) issued by position sensors (8A, 8B; 9) and the current intensities (I1(t), I2(t)) issued by current sensing elements.

3. The device of claim 2, wherein the observer further receives measured voltages (U1(t), U2(t)) applied to the electromagnet coils or measured flux obtained from flux sensing elements and delivers observed flux for the second correction circuit.

4. The device of claim 1, wherein the first correction circuit comprises a proportional-integral controller on the position of the rotor and a proportional controller on the speed of the rotor.

5. The device of claim 1, wherein the second correction circuit comprises a proportional controller on the flux of the electromagnet coils.

6. The device of claim 1, wherein the trajectory planning module further delivers a requested acceleration.