Control of a brushless motor containing asymmetries

p0001The present invention relates to brushless motors management and particularly applies to the system described in the application <patcit id="pcit0001" dnum="EP0801463A"><text>EP-A-0801463</text></patcit> the applicant whose <figref idrefs="f0001 f0002">Figures 1 to 4</figref> are reproduced here.

p0002The <figref idrefs="f0001">figure 1</figref> represents a sectional view of a typical DC motor without collector. The motor comprises a permanent magnet rotor 12 and a stator 14 provided with a number of windings (A, B, C shown in<figref idrefs="f0001">2</figref>). The windings are arranged in a plurality of slots 18. In the illustrated motor, the rotor 12 is housed in the stator 14. Similarly, the stator 14 may be housed in the rotor 12. The rotor 12 is magnetized and rotates permanently to align its magnetic flux on that produced by the windings.

p0003The <figref idrefs="f0001">2</figref> represents a wiring diagram of the stator of such an electric motor and power control circuits used. Often such engines include three phases A, B and C. These can be connected in a star with a common node N (as in the figure) or delta. The free end of each winding is connected through a pair of switches XSA, XGA; XSB, XGB; XSC, XGC to the Vs power supply and to ground, respectively. A reverse-biased diode DSA, DGA; DSB, DGB; DSC, DGC is placed in parallel on each switch. The opening and closing of the switches can be controlled by a microcontroller.

p0004As shown in <figref idrefs="f0002">3</figref>, The switches are controlled by a sequence of steps. The diagram shows the voltage applied to each winding to the common node N. The figure shows the case of a motor having phases A, B and C, controlled by six steps s1, s2, s3, s4, s5, s6 correspondent each to a particular configuration of magnetic flux in the motor. At each of these six steps, one of the phases A, B, C is cut and the other two are polarized oppositely. The rotor align its magnetic flux on the stator and thus rotate synchronously with this flow as it rotates as a result of the switching of the six steps.

p0005When the rotor rotates, its rotation induces a voltage (electromotive force against or back EMF) in each of the motor windings. For a loaded motor, the back emf generated in one winding is substantially in phase with the current flowing in this winding.

p0006The <figref idrefs="f0002">4</figref> represents the six stages of the VA voltage applied to the winding A of the BEMF (BemfA) produced in the winding A by the rotor load, and current IA in the winding A. Periods of current increase and current reduction are respectively called feeding period (pe) and energization period (pd). Between the period of energization and the feeding period is scheduled td a monitoring period during which the back emf can be monitored.

p0007The voltage actually supplied to each of the motor windings is controlled by pulse-width modulation (PWM) of the DC supply voltage. The frequency of the pulse width modulation signal is usually high with respect to the engine rotation frequency, for example about 10 kHz. The switches are controlled by a microcontroller as a function of current to supply the motor for switching between the steps s1 to s6 and to ensure the PWM control.

p0008During periods between PWM low-level pulses, the motor freewheels; the motor kinetic energy as it rotates is converted into electrical energy by its rotation in one maguétique field. The engine does not slow during these times because the high values ​​of the PWM frequency, the inertia of the motor and its load make these changes undetectable.

p0009So that the motor works properly, the existing flow in the stator must always be slightly ahead of the rotor to continue to pull the engine forward. Also, the flux in the stator behind the rotor is preferably an own polarity to repel the rotor to assist the rotation. However, we should never lose synchronization between the movement of the rotor and the rotation of the rotor flux as may stop spinning or achieve poor performance. Accordingly, to maximize engine performance, switching the windings from one stage to the other must be controlled according to the actual rotor position. For this, in a mode of self-commutated engine operation, one monitors the back emf induced in the windings and more particularly to zero crossing point of the back EMF to determine the rotor position at a given instant.

p0010Demand for <patcit id="pcit0002" dnum="EP0801463A"><text>EP-A-0801463</text></patcit> provides a control circuit of an electric motor without a commutator in which the transition from one stage to the next is determined by a zero crossing of the back emf in an unpowered winding during this step.

p0011More particularly, this application <patcit id="pcit0003" dnum="EP0801463A"><text>EP-A-0801463</text></patcit> provides a method for controlling a brushless motor having a plurality of windings each having a first end connected to a common node and a second end connectable to supply voltages, comprising the steps of:<ol><li>a) applying an upper supply voltage to a second end of each of a first subset of windings, applying a lower supply voltage at a second end of each of a second subset of windings, the second end of at least one winding being disconnected from the supply voltages; applying a supply voltage to at least one of the first and second subsets being periodically interrupted;</li><li>b) detecting the presence of a current in the disconnected winding and detecting a current cessation time;</li><li>c) beginning at the instant of cessation, monitoring the value of a force against electromotive induced in the disconnected winding and detecting a zero crossing point of the back emf with respect to the upper power supply voltage and / or less;</li><li>d) counting a predetermined delay from the detected zero crossing point; and</li><li>e) after expiry of the predetermined delay, removing the supply voltages from the second ends of the first and second subsets of windings and applying the upper supply voltage to a second end of each of a third subset of windings; applying the lower supply voltage to a second end of each of a fourth subset of windings; the second end of at least one remaining winding not connected to the supply voltages.</li></ol>

p0012According to the application of <patcit id="pcit0004" dnum="EP0801463A"><text>EP-A-0801463</text></patcit>, The predetermined delay is a predetermined fraction of the time taken to perform a preceding cycle of steps b) to e), this predetermined fraction being modifiable during operation of the motor.

p0013The Applicant has noted that, in the case where the present engine asymmetries at or magnets is magnetically deformed or if the mechanical symmetry of the stator is not exactly the same as that of the rotor, the electric position information corresponding to the zero crossing of the back emf are not regular in time that the rotor speed is regular.

p0014In case one chooses for each stage a delay T proportional to the time interval between the immediately preceding zero crossings, it is an aggravated dissymmetry.

p0015The present invention aims to overcome this disadvantage.

p0016The document <patcit id="pcit0005" dnum="DE4231226A"><text>DE-A-42 31 226</text></patcit> discloses a switching system of a brushless motor providing a correction of the switching delay.

p0017More particularly, the present invention provides a self-commutated type of a brushless motor control method having a plurality of windings switched according to a determined cycle of steps, wherein each transition between steps is determined at a moment which follows a zero crossing of the electromotive force against a determined delay. Said delay is selectively chosen depending on the particular stage as a function of the duration of a determined intervals in between preceding zero passages, the method comprising the steps of:<ul><li>determine to a constant speed engine running the intervals between zero crossings of the back emf,</li><li>forward the longest interval of a delay proportional to the duration of the last the shortest interval,</li><li>to follow the shortest interval of a delay proportional to the duration of the last longest interval, and</li><li>to follow the average duration of intervals of a delay proportional to the duration of the last interval of medium duration.</li></ul>

p0018According to one embodiment of the present invention, this method consists of following each zero crossing detection of the back emf of a delay proportional to the duration of a interval shifted from the current step of a number of determined steps.

p0019These objects, features and advantages, and others of the present invention will be discussed in detail in the following description of specific embodiments in non-limiting in connection with the accompanying drawings:<ul><li>the <figref idrefs="f0001">figure 1</figref> shows a sectional view of a motor without collector known;</li><li>The <figref idrefs="f0001">2</figref> shows a known circuit for controlling the motor of the <figref idrefs="f0001">figure 1</figref>;</li><li>The <figref idrefs="f0002">3</figref> represents voltages applied to elements of the circuit of <figref idrefs="f0001">2</figref>;</li><li>the <figref idrefs="f0002">4</figref> shows currents and voltages in the windings of a motor;</li><li>the <figref idrefs="f0003">Figures 5A and 5B</figref> represent, upon rotation of a motor, respectively, passage detection instants zero and moments of switching the windings of a motor, according to the prior art; and</li><li>the <figref idrefs="f0003">6A and 6B</figref> respectively correspond to <figref idrefs="f0003">Figures 5A and 5B</figref> in the case of operation according to the present invention.</li></ul>

p0020Although this is not described in detail, the operation control signals for the various circuit elements according to the invention are produced by a host microcontroller, such as a microcontroller or the ST72xx ST9xxx family manufactured by SGS-Thomson.

p0021When a crossing point zero of the emf is detected, a delay is imposed from this point to the next stage of transition. This variable delay is selected by the host microcontroller based on the instantaneous speed of the engine for maximum performance and optimum motor torque. Measuring back emf is performed during the disconnection periods of the PWM.

p0022Thus, in a self-commutated mode, the detection of the transition point to zero of the back emf in a given winding is used to trigger the current point of initiation in the same winding. This command is applied with a delay T / x after the point of zero crossing, T being the duration of the step, and x is a constant set to get the best possible engine operation. The constant x is adjusted in real time to reflect changes in the mechanical load of the engine, the required acceleration, stability and performance required.

p0023The <figref idrefs="f0003">Figures 5A and 5B</figref> illustrate a series of steps for operating a motor if it has mechanical asymmetries. In<figref idrefs="f0003">5A</figref>, The successive steps are shown operating as detected by the successive zero crossings of the back emf. It is assumed that the motor has a mechanical dissymmetry of order three. It appears then unequal intervals T1, T2, T3 between successive zero crossings of the back emf, whereas if the engine is operating at constant speed and that everything is properly set, these three intervals T1, T2, T3 should be identical. the interval T3 to have normal value and the interval T1 significantly longer than interval T2 is shown. In the conventional case, one starts from zero crossings of the back emf, corresponding to the end of each of the intervals T1, T2, T3, and triggers a transition of the switching mode of the various motor windings after a delay T / x, where T is the period of the immediately preceding interval. Then, as illustrated in<figref idrefs="f0003">5B</figref>It is seen that the longest interval T1 is followed by a long delay T1 / x, while the end of the shortest interval T2 is followed by a short delay T2 / x. Thus, during subsequent operating cycles, T'2 phase will be even shorter than the previous stage T2 while the T'3 phase corresponding to T3 will be elongated. Accordingly the existence of a mechanical dissymmetry in the engine structure resulting in a worsened asymmetry of the switching steps and there is an increase of defects.

p0024One result is that after a short interval the dead time required for detection of the back emf will be greatly reduced which limits the maximum possible engine speed. It will also result that the rotation of the stator flux will not be regular although the rotor rotates regularly. This can cause vibration, torque blows, noise and yield reduction.

p0025Thus, the present invention aims to address the irregularities of an engine, that is to say irregularities zero crossings of the back emf for a constant speed rotary motor to compensate for these mechanical errors by a trigger adapted transitions of various switches on the motor windings.

p0026As schematically represent <figref idrefs="f0003">6A and 6B</figref>We aim to trigger the motor commutation with a short delay <u>at</u> inmediately after a long interval T1 and with a longer delay <u>b</u> immediately after a short interval T2. Thus, T'2 time between transitions corresponding to a short interval will be extended from the time T2 not undervalued as was the case in<figref idrefs="f0003">5A to 5B</figref>.

p0027Those skilled in the art will find various ways to achieve these results, these methods may be taken into account by the control microcontroller using an appropriate program. For example, is rotated a first time constant speed motor and it is checked the allocation of intervals T1, T2, T3. In the particular example of<figref idrefs="f0003">6A</figref>, We can calculate the duration <u>at</u> as corresponding to T2 / x, duration <u>b</u> as corresponding to T1 / x and duration <u>c</u> as corresponding to T3 / x. of course be appreciated, as was explained above, the durations T1, T2, T3 vary according to the engine rotational speed. therefore be selected in the particular case illustrated in<figref idrefs="f0003">6A and 6B</figref> for the duration <u>at</u> duration (T2 / x) corresponding to an interval twice prior to the step that has just ended (T1) for the duration <u>b</u> duration (T1 / x) corresponding to an interval immediately prior to the step that has just ended (T2) and for the duration <u>c</u> duration (T3 / x) corresponding to the interval just ended (T3). also can be in other cases, for example but not exclusively in case of asymmetry of order two, always choose to use as a basis the duration of an interval preceding a step step that just finish. Any other combination may be chosen depending on the type of asymmetry and the desired sophistication of the correction method.

p0028Thanks to the invention, the asymmetry in the step switching is attenuated, the maximum engine speed is increased, the rotational speed of the rotor flux becomes regular, that is to say, the torque ripple is reduced and so the noise while the yield is increased. In addition, the electrical operation and the engine performance is less influenced by mechanical or magnetic defects and dimensional tolerances used in the construction of the engine.

p0029The example was for a motor connected in star. The same process can be used to control a connected motor in triangle. Although described in connection with a motor with three coils, the method according to the invention just fits to engines with more or fewer coils.

p0030Although the present invention has been described in the particular case of processing at the request of <patcit id="pcit0006" dnum="EP0801463A"><text>EP-A-0801463</text></patcit>It will be appreciated that it applies generally to any system for controlling a brushless self-commutated collector wherein a transition between steps excitation of the motor windings is determined by adding a delay at the instant zero crossing of the back emf in an unpowered motor winding.