Technical Field The present invention relates to a control device for an electric motor, in particular for a brushless synchronous motor with permanent magnets.

Background Art As is known, in general, an electric motor basically consists of an inductor circuit and an induced circuit.

The first is designed to be energised by an electric current flowing through it so as to create a corresponding magnetic flux.

This magnetic flux, operatively connecting to the induced circuit, allows the reciprocal movement of the inductor circuit and the induced circuit.

Depending on the types of motor and the requirements to be satisfied, the inductor circuit may remain stationary, whilst the induced circuit is moved. Alternatively, the induced circuit remains stationary whilst the inductor circuit is moved.

The portion of the motor which remains stationary is called the stator. The mobile portion is called the rotor.

In greater detail, relative to one of the specific fields of application of the present invention, brushless synchronous motors with permanent magnets basically consist of a rotor, designed to rotate about its longitudinal axis, and a stator, connected to the rotor and able to activate rotor movement.

The rotor has at least two magnetic poles, with opposite polarity and preferably in opposite positions on the rotor.

The stator normally has a plurality of energising windings, each positioned around a corresponding core made of ferromagnetic material.

Suitable control means pass an energising current through the above-mentioned windings, generating a magnetic induction flux in the cores.

This flux, acting on the magnetic poles of the rotor, moves the rotor, using the forces of attraction and repulsion created between the rotor poles and the cores of the windings.

The energising current is supplied to the windings in such a way that the magnetic induction flux periodically changes its direction of propagation.

Thanks to its magnetic polarisation, the rotor tends to be positioned in a preferential direction relative to the field. In other words, the rotor follows the variations in the direction of flux propagation, rotating at a predetermined synchronous speed.

In this way, by suitably synchronising the variations in the energising current with the rotor angular position, the rotor rotates at the required speed.

To achieve this synchronisation, the several types of control techniques are known from prior art.

A first technique is the use of a sensor designed to detect the instantaneous angular position of the rotor. In brushless synchronous motors with permanent magnets, use of a Hall effect sensor positioned close to the rotor may be used to good advantage.

Suitable processing means, connected to the sensor element, are used for generating control signals applied to the control means, according to the angular position of the rotor, so as to create variations in the direction of magnetic flux propagation synchronised with the rotation of the rotor.

Control techniques of this type have obvious operating disadvantages if considered relative to the production complexity and high cost of the hardware needed for transfer of the rotor angular position parameter.

Therefore, the technical sector in question has seen the development of sensorless control techniques, that is to say, control techniques which do not involve the use of a sensor to detect the angular position of the rotor.

A very widespread sensorless technique consists of reading the BEMF (Back electro-motive force), that is to say, the electro-motive force induced by the rotor's rotating magnetic field in one of the

stator windings through which the energising current is not passing at that moment.

For example, the case of a three-phase motor may be considered.

It will have three star-connected windings, and the energising current will periodically cross them.

In particular, at any moment, the energising current flows through two of the three windings (and, therefore, they are linked with the magnetic induction flux), whilst the energising current does not flow through the remaining winding.

The above-mentioned BEMF can be detected at this winding.

The BEMF detected in this way normally has a sinusoidal form over time and crosses the voltage at the centre of the star at regular intervals.

The switching points for the energising current are conventionally selected at the moment when the BEMF crosses the above-mentioned voltage at the centre of the star.

Therefore, the control circuits in the prior art synchronise rotor rotation and the energising currents, switching the latter at the moments in time defined when the BEMF detected as described above crosses the voltage at the centre of the star.

An evident disadvantage relative to the control techniques described up to now is the fact that they cannot be applied to single-phase electric motors, that is to say, motors which only have one energising winding, or, similarly, two half-coils connected to one another in series.

Disclosure of the Invention The aim of the present invention is, therefore, to overcome the disadvantages indicated in the prior art described above.

In particular, the aim of the present invention is to provide a control device for an electric motor, which takes a BEMF reading and which, at the same time, can be applied to single-phase motors.

An auxiliary aim of the present invention is to provide a control device for electric motors, in particular applicable to single-phase electric motors, characterised by simple circuitry and low production costs.

These and other aims are substantially achieved by a control device for an electric motor as described in the claims herein.

Brief Description of the Drawings Further features and advantages are more clearly illustrated in the detailed description which follows, with reference to the accompanying drawings, which illustrate a preferred embodiment of a control device for an electric motor without limiting the scope of its application, and in which: Figure 1 is a block diagram of a device made according to the present invention, connected to a single-phase motor; Figures 2,3 and 4 are block diagrams of alternative embodiments of the device illustrated in Figure 1.

Detailed Description of the Preferred Embodiments of the Invention The control device for an electric motor disclosed is labelled 1 as a whole in the accompanying drawings.

Firstly, it must be said that the control device disclosed may be applied to various types of electric motors, irrespective of the characteristics of the motors themselves.

It is sufficient to create a BEMF induced at an electromagnetic element of the motor, by the electromagnetic interaction between the inductor circuit and the induced circuit, for the device 1 to be able to detect this BEMF and suitably synchronise the reciprocal movements of the inductor circuit and the induced circuit and the energising current.

In particular, the device 1, as illustrated in Figures 1 and 2, may be applied to an single-phase, brushless synchronous electric motor 10 with permanent magnets.

This motor 10 comprises a stator 70, which basically constitutes the motor 10 inductor circuit 30, and a rotor 80, which forms the induced circuit 20.

The stator 70 has at least one winding 200, wound around a core 31 and designed to be energised by the current 100 flowing through it.

The winding 200 preferably consists of a first half-coil 4 and a second half-coil 5 which are connected to one another.

When the energising current 100 passes through the winding 200, a primary magnetic flux 9 is generated in the core 31.

The direction of propagation of the primary magnetic flux (p is varied by appropriate switching of the energising current 100. The winding 200 is designed to be driven between a first operating condition and a second operating condition according to the energising current 100,.

In the first operating condition, the primary magnetic flux ç has a direction of propagation which goes from a first end 4a of the first half-coil 4 to a second end 4b of the first half-coil 4. In the second operating condition, the direction of propagation of the primary magnetic flux (p goes from the second end 4b to the first end 4a of the first half-coil 4.

As indicated above, the motor 10 also comprises the rotor 80, electromagnetically connected to the stator 70 and having a first magnetic pole 81 and a second magnetic pole 82, with opposite polarity to the first pole 81. Since it is positioned close to the core 31, the rotor 80 is affected by the presence of the primary magnetic flux p. In particular, as a result of interaction with the primary magnetic flux, the rotor 80 rotates about its longitudinal axis.

Advantageously, to achieve optimum propagation of the primary magnetic flux, the core 31 is basically U-shaped. The core 31 basically consists of a first extended portion 32, a second extended portion 33, arranged parallel with the first portion 32, and a connecting portion 34, inserted between the first and the second portions 32,33.

As illustrated in Figures 1 to 4, the winding 200 is wound around two or more portions of the core 31.

As indicated above, in a preferred embodiment, the stator 70 winding 200 is divided into two half-coils, 4 and 5, the first wound around the portion 32 of the core 31, the second wound around the portion 33 of the core 31.

Both half-coils are designed to be energised by the current 100 flowing through them so as to generate the primary magnetic flux p.

In the same way as the first half-coil 4 described above, the second half-coil 5 is designed to be driven between a first and a second operating condition, according to the above-mentioned energising current 100 and the direction of propagation of the primary magnetic flux p.

In the first operating condition, the direction of propagation of the primary magnetic flux ç is from the first end 5a to the second end 5b of the second half-coil 5, whilst in the second operating condition, the direction of propagation of the primary magnetic flux ç is from the second end 5b to the first end 5a of the second half-coil 5.

Advantageously, the first half-coil 4 and/or the second half- coil 5 may be made with layered winding, to limit the amount of copper used and minimise the flux dispersed.

Conveniently, the first half-coil 4 and the second half-coil 5 are connected to one another in series, and wound in such a way that the fluxes generated are concordant.

In this particular case, it is important that the first half- coil 4 and the second half-coil 5 are connected so that the first operating condition of the first half-coil 4 corresponds with the first operating condition of the second half-coil 5, and the second operating condition of the first half-coil 4 corresponds with the second operating condition of the second half-coil 5.

In light of the above description, it is clear that the primary magnetic flux ç is the magnetic flux generated in the stator 70 core 31 by all of the half-coils in the winding 200 when the energising current 100 flows through the winding.

Otherwise, the BEMF induced in the winding 200 is caused by the magnetic action exerted on the core 31 by the rotating magnetic field created by the rotor 80 as it moves.

The following formula applies to a generic winding around the core 31: d v = Ri+ #s (1) dt

where s is the magnetic flux resulting from the composition of the primary magnetic flux (fp) generated by the winding 200 when the energising current 100 passes through it, and the magnetic flux çm generated by the rotor's permanent magnet. The terms v, i and R represent the voltage, current and resistance of the generic winding.

As indicated above, the device 1 is used to synchronise switching of the energising current 100 with the angular position of the rotor 80.

The device 1 comprises an electromagnetic element 2 designed to have the primary magnetic flux ç passing through it.

A detector circuit 40, connected to the electromagnetic element 2, is used for reading an electric or magnetic parameter 3 characteristic of the above-mentioned electromagnetic element 2 and generating at its output the BEMF 110 induced in the electromagnetic element 2.

The BEMF 110 supplied at output in this way is generated according to the characteristic parameter 3.

Depending on requirements and the specific applications for which the present invention is intended, the characteristic parameter 3 may be the voltage read at the ends of the electromagnetic element 2, or the current flowing in the electromagnetic element 2.

If the characteristic parameter 3 is the voltage read at the ends of the electromagnetic element 2, the formula (1) is conveniently expressed as follows: <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> d<BR> <BR> <BR> v = Ri+ #+e(#) (2) dt <BR> <BR> <BR> <BR> <BR> <BR> showing the term e (0) which represents the BEMF induced in the electromagnetic element 2 by the magnetic action exerted on the core 31 by the rotating magnetic field created by the rotor 80 as it moves. The term 8 represents the angular position of the rotor, contained in the BEMF induced in the electromagnetic element 2.

However, in both cases, a possible algorithm for calculation of the BEMF induced in the electromagnetic element 2 requires a subtraction between the characteristic parameter 3 detected and a predetermined reference value, representative of the primary magnetic flux p through the core 31.

If the characteristic parameter 3 is the voltage at the ends of the electromagnetic element 2, assuming that the current i in the electromagnetic element 2 is zero, the reference value is identified by the term d in formula (2), where ç is the primary dt

magnetic flux.

The detector circuit 40 can process a possible estimate of the term from a mathematical model of the stator circuits, starting with the energising current 100 and the voltage applied to the winding 200.

The dependence of the induced voltage BEMF on the angular position of the rotor 0 is indicated by the formula e (S) = K fs (O) 10 where fs (0) represents a function with form characteristic of the construction of the motor 10, K is a constant and w is the speed of the rotor 80. For the forms of construction of the motor indicated here, the normalised form function fs (6) has a sinusoidal trend with a certain degree of distortion.

The BEMF 110 generated in this way is then processed by a processing block 50, connected downstream of the detector circuit 40 and designed to receive the BEMF 110 at input.

According to the. BEMF 110, the processing block 50 can define the angular position 0 and the speed co of the rotor 80 using mathematical functions applied to the form function fs (6) contained in the BEMF 110, and generate a corresponding command signal 120 at output, which is an expression of the angular position 0 and the speed, for controlling the energising current 100.

In particular, the energising current 100 is regulated in such a way as to correctly synchronise it with the angular position of the rotor 80 and to allow it to be switched at predetermined angular positions of the rotor 80.

To regulate the energising current 100 in this way, the device 1 also has a controller 60, connected to the processing block 50 and designed to receive the command signal 120 at input.

Using this command signal 120, the controller 60 can regulate the energising current 100 as described above.

In particular, a regulating algorithm implemented by the processing unit 50 requires the calculation of a difference between the angular position 0 and the instantaneous speed w of the rotor 80, estimated by the processing unit 50 according to the characteristic parameter 3, and suitable reference values Asp and

cosp. This algorithm also minimises the value resulting from the difference calculation, acting on the energising current 100.

It is important to emphasise how the characteristic parameter 3 of the electromagnetic element 2 is read when the electromagnetic element 2 is connected to the primary magnetic flux p.

In other words, the reading is taken by the circuit 40 when the electromagnetic element 2 is linked to the primary magnetic flux p.

Advantageously, the electromagnetic element 2 consists of a winding 2a, connected to the core 31 and designed to be linked to the primary flux, when the characteristic parameter 3 is read by the detector circuit 40.

In particular, the winding 2a is wound around the core 31 in such a way as to minimise the flux dispersed and to obtain optimum linking with primary magnetic flux (p.

In the embodiment illustrated in Figure 1, the electromagnetic element 2 is designed to be energised by the current 100 flowing through it at least periodically in order to generate the primary magnetic flux p.

The electromagnetic element 2 consists of part of the first half-coil 4 or, alternatively, the whole of the first half-coil 4.

In other words, the electromagnetic element 2 may coincide with the first half-coil 4, the characteristic parameter 3 being read on the first half-coil 4.

The embodiment illustrated in Figure 3 generalises the above so that the electromagnetic element 2 may coincide with the whole of the winding 200, the characteristic parameter 3 being read on the whole winding 200.

In another embodiment illustrated in Figure 4 the electromagnetic element 2 coincides with the first half-coil 4, the characteristic parameter 3 being read on the first half-coil 4, or the electromagnetic element coincides with the second half-coil 5, the characteristic parameter 3 being read on the second half-coil 5, depending on two operating conditions which involve control of the two half-coils by a controller 60 alternately and in such a way as to create opposite directions of propagation of the primary magnetic flux p.

In other words, the controller 60, receiving a suitable command from the processing unit 50, selectively supplies current to the

first and second half-coils 4,5, according to a predetermined time pattern. When the first half-coil 4 is supplied with power, the detector block 40 detects the voltage and/or the current of the second half-coil 5, to obtain the BEMF 110. When the second half- coil 5 is supplied with power, the electrical measurements (voltage and/or current) needed to obtain the BEMF 110 are taken on the first half-coil 4.

In contrast, in the embodiment illustrated in Figure 2, the electromagnetic element 2 is physically separated from the winding 200. However, since it is wound around the core 31, it still allows the primary magnetic flux (p to pass through it, so that the characteristic parameter 3 can be read by the detector circuit 40 when the primary magnetic flux ç is linked to the electromagnetic element 2.

To summarise, from the operating viewpoint, the following steps are basically performed: - firstly, the characteristic parameter 3 of the electromagnetic element 2 is detected during linking of the primary magnetic flux p to the electromagnetic element 2; - then, according to the characteristic parameter 3 detected, the BEMF 110 induced in the electromagnetic element 2 is calculated.

A preferred algorithm calculates the difference between the characteristic parameter 3 and a predetermined reference value, representing the primary magnetic flux p ; - finally, according to the BEMF 110 calculated as described above, the energising current 100 is regulated so as to synchronise the reciprocal movement of the rotor 80 and the stator 70.

As indicated above, the characteristic parameter 3 may be the voltage read at the ends of the electromagnetic element 2, or the current made to flow in the electromagnetic element 2.

In the first case, the step of detecting the characteristic parameter 3 comprises a sub-step of detecting the voltage at the ends of the electromagnetic element 2.

In contrast, in the second case, the step of detecting the characteristic parameter 3 comprises a sub-step of detecting the current flowing in the electromagnetic element 2.

The present invention brings important advantages.

Firstly, it allows the application of a sensorless control system which makes use of BEMF reading to single-phase electric motors.

Moreover, the control device disclosed has very simple hardware and very low production costs.