Patent Application
20150002000
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.
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.
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.
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:
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.
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:
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 {circumflex over (Φ)}{circumflex over (Φ1)}(t), {circumflex over (Φ)}{circumflex over (Φ2)}(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
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
The present invention comprises a plurality of advantages, i.e.:
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13305921.2 | Jun 2013 | EP | regional |
