DEVICE FOR SENSORLESS CONTROL OF DIRECT CURRENT BRUSHLESS MOTORS

Abstract: A control device for brushless direct current motors (10), adapted to generate a signal (VTH1) which is proportional to the advance in electrical degrees (ATP), function of the speed of rotation of the motor, in which the start of each stage can occur even when the electromotive force (EMF) has not yet lowered below a peak value (EPK).

Description: IMPROVED CONTROL DEVICE FOR DIRECT CURRENT BRUSHLESS MOTORS DESCRIPTION The present invention refers to a control device for brushless direct current motors. Different kinds of brushless direct current motors have been studied and provided, all " of them being intended to reduce production and operation costs as well as to increase, at the same time, the functions and variety of control, piloting speed and the overall reliability. EP 0 316 716 describes, for example, a brushless direct control motor provided with a control electronics of the diphase type with four monopolar windings, where the geometry of the teeth and the magnetic field is such that it generates an electromotive force of the trapezoidal type in each winding, so that the control electronics can detect an optimum point on the oblique side of the trapezoidal wave at which the switching can be effected. In this case, there is no need to use the Hall probes for switching and both phases can be dimensioned in order to achieve conduction on 180 electrical degrees for each winding (that is the same as two parallel monophases). Finally, the two monophases can be also dimensioned in different ways, so that the control electronics can select in the motor the use of three different motors, two of them being of the monophase type and the other one of the diphase type. Due to these conditions the motor in diphase operation can produce a maximum power which is approximately twice as much as the power developed when it is operating in the conventional way; besides, in this way, it is possible to achieve three operating speeds, by means of control power components which always operate in saturation mode and not in "PMW" ("Pulse Width Modulation") or "linear" mode, so that there are advantages related to the fact that losses, electrical and mechanical noises and costs for the design and the dimensioning of the components are lower. In order to achieve these objects, the motor block is associated to a set of sensors which gather different data (speed, power intensity, temperature, position of the rotor) from each phase of the motor, as digital elaboration of the shape of the output waves. Moreover, a circuit advancing and suppressing the transistors is adapted to generate a signal which is proportional to the advance in electrical degrees, function of the speed of rotation of the motor, with which each of the four phases is to be triggered, in relationship with the passage at zero of the respective electromotive force. Said circuit, however, once used on motors with basically trapezoidal electromotive force, allows the achievement of an advanced start which, independently from the speed of rotation, is anyway limited to an ideally possible limit value on a theoretic electronic angle, while the real limit of the electronic angle will be even lower, since it is necessary that the electromotive force lowers below the peak value in order to generate a signal that can switch four comparators located inside a logic intended to detect the position of the rotor in order to adjust the timing of the output signals and therefore provide the switch advancement. It is an object of the present invention to provide a control device for brushless direct current motors that solves the aforementioned inconveniences and that especially allows each phase to be started even when the electromotive force has not yet been lowered. It is another object of the present invention to provide a control device for brushless direct current motors that achieves a dependent extension which is function of the speed of rotation of the motor. It is another object of the present invention to provide a control device for brushless direct current motors the costs of which, thanks to the advantages achieved, are relatively low and the operation of which is highly reliable. These objects are achieved by means of a control device for brushless direct current motors as claimed in claim 1. The theoretical advantageous electrical angle that can be achieved is equal to 90 electrical degrees; since any switching is anyway determined by the respective switching of the comparators located in the logic that detects the position of the rotor, which in turn are controlled by a tension signal coming out from the advance circuit, it is possible to achieve an increase of the maximum allowed switching advance for any speed of rotation of the motor, by means of a sort of alteration of said tension signal. Said alteration is achieved by correctly elaborating the signals themselves which reach the input of the circuit for the advance and suppression of the transistors implemented in the brushless motors of the known type. Additional objects and advantages of the present invention will become more evident from the following description referring to the appended drawings, provided as non-restrictive example, in which: figure 1 is a partial block diagram referring to a portion of a control electronic circuit for a brushless direct current motor of the conventional type; figure 2 shows a detailed electrical circuit of the block diagram in figure 1; figure 3 shows an electrical scheme of a brushless direct current motor, according to the present invention; figure 4 is a variation to the electrical scheme in figure 3, according to the invention; figure 5 shows schematically, by means of a cartesian graph, the qualitative run of the electrical magnitudes which are significant in a control device for brushless direct current motors, according to the present invention; figure 6 shows a functional block diagram of the control device, according to the present invention; figure 7 shows schematically an embodiment of an electronic circuit, developed in the analogue version, of a control device, according to the present invention; figure 8 shows schematically the quality run in time of the input and output wave shape referring to the block diagram in figure 6. With reference to the aforementioned figures, it must be underlined that the present invention refers to an improved control device for motors with no speed control (i.e., a brushless motor used for a motor-fan or for the fuel pump in a motor vehicle). Besides, even if the description of the control device, according to the invention, refers to an operating status in "LOW SIDE" configuration, with four static switches, indicated by II, 12, 13, 14 in figure 3, said device can be applied to an operating status in "HIGH SIDE" configuration, after providing the obvious modifications to the circuits, as shown in figure 4, in which the static switches are indicated by I1H, I2H, I3H, I4H. The motor block, indicated by the reference number 10 in figure 1, is characterised by four windings and present a diphase structure; in practice, the windings, two at a time, represent each one of the two phases and each phase consists of two monopolar windings the winding direction of which is mutually opposite, each one of the windings covering a half-wave" of the phase. Block 1 1 in figure 1 includes a set of sensor means of the motor 10 and is intended to gather information and data from the signal of the phases Ml, M2, M3, M4 (configuration of the motor as in figure 3), or M1H, M2H, M3H, M4H (configuration of the motor as in figure 4), by means of adequate elaboration of the wave shapes at the end of each phase. More in particular, Ml , M2, M3, M4 and the homologues M1H, M2H, M3H, M4H represent differences in voltage between "drain" and "source" of each static switch II , 12, 13, 14, I1H, I2H, 13H, 14H. In particular, reference number 12 indicates a logic circuit that detects speed, reference number 13 indicates a logic circuit intended to detect the current and temperature intensity on each winding Al , A2, A3, A4, or A1H, A2H, A3H, A4H, of the motor 10, while reference number 14 indicates a logic circuit deputed to detect the position of the rotor. The wave shapes present on phases Ml , M2, M3, M4 are processed by circuit 14 which provides a set of signal to block 16, so that it is possible to get useful information about the position of the rotor and the control of the communication in sequence. Reference number 15 indicates a circuit to advance and suppress the transistors; this circuit utilises, in this same block 1 1, the information coming from the logic circuit that detects the speed 12, so that it is possible to control the logic circuit that detects the position of the rotor 14 in order to correct the timing of the output signals and, therefore, provide switching advance. At the same time, a passive filter is generated on the switching transistors, while the output signal SO of the logic circuit that detects the speed 12, is afterwards used to control different logic units on following blocks. The signal SO, in fact, controls a control and start logic block, indicated by reference number 18 in figure 1, including a unidirectional logic of rotation and digital filtering 16, which processes the signals exiting the logic circuit that detects the position of the rotor and in this way provides a set of output control signals for the different phases of the motor 10, so that the motor keeps rotating in the desired direction. Besides, since due to the switching, signals Ml , M2, M3, M4, contain additional spurious signals, a digital filter logic for the transistors is located in the logic block 16; the signal exiting the block 16, controls the power transistors directly through a start oscillator that allows the casual start of one of the two phases, so that the motor 10 is started from the rest position and this movement can thus generate adequate signals on the phases Ml, M2, M3, M4, adapted to provide the right timing for the starts. In order to detect a current signal in each of the windings Al, A2, A3, A4 of the motor 10 and process it for the control, a method based on the direct measurement of the voltage drop in each one of the phases is used in the logic circuit 13, with adequate processing of the information related to the voltages in each single winding Al, A2, A3, A4; to that purpose, if signals Ml, M3 are carried through the divider formed by the resistors R18, R20 (see figure 2), the signal resulting from their sum is the very voltage signal of the conducting phase. In fact, the electromotive force in the two phases always presents, at any moment, in the case of both Ml and M3, opposite sign so that the complete signal is null. The inductive voltages present on the windings and equal to L (di/dt) are also the same, yet being opposite in sign, on the two opposite windings Al, A3, so that these voltage drops to become null at the nodes of the resistors R18, R20; on the contrary, in each one of the half waves only one of the two windings is conducting so the signal Ml or M3, reported on the divider formed by the resistors R18, R20, filtered for fast signals by the capacitor C4 and attenuated by the resistor R22, will provide a voltage signal equal to CL1, when it conduces the signal M3. The same applies to the signals M2 and M4 and therefore each one of the two output signal lines covers 50% of the signal Ml of a winding Al and 50% of the signal M3 of the other winding A3. Since the signals CL1 , CL3 (and, accordingly, CL2, CL4) are equal to the product of the resistance of the respective windings Al, A3, A2, A4 and the current there circulating, and since said resistance varies with the variation of the temperatures of the same, the signals CL1, CL3 (CL2, CL4) tend to increase, not only because of the increase in the intensity of the circulating current, but also because of the increase in resistance occurring as a consequence of the increase in temperature of the windings Al, A2, A3, A4. The logic circuit 14 includes four comparators OA3, OA4, OA5, OA6, presenting at their input the signals Ml, M2, M3, M4, of the analogue type, and providing, at their output, square wave signals with a change of status at the passage of the signals Ml, M2, M3, M4 for the fixed level of the divider formed by the resistors R21 and R3 1 in block 15. The detection on the circuit 14 is then done on a zero setting by the divider formed by R21 and R31 and the signals that can be used for the following logic are the signals indicated by 1, 2, 3, 4 in figures 1 and 2, where signals 3 and 4 are in effect signals opposite to those exiting the comparators OA5, OA6; in this case, it is sometimes preferable to adequately arrange the comparators OA3, OA4, OA5, OA6 (as in figure 2), so that it is not necessary to insert signal inverters or logic NOT. Block 15, in fact, implies the connection of the diodes Dl, D2, D3, D4 in order to prevent the signal Ml, M2, M3, M4 from generating, during the switching, in the interdiction status of the transistors, extra-voltage caused by the presence of leakage inductance, with a consequent damage for the power transistors. According to the present invention, the circuit block C l, Rl, R4, C2, D5, D6, D7, illustrated in detail in figure 2, is replaced by the electronic circuit in figure 7; the function of block 15 is in fact that of generating a signal which is proportional to the advance in electrical degrees, function of the speed of rotation of the motor 10, by which it is intended to start each one of the four phases in relationship with the passage at zero of the respective electromotive force. In practice, according to the diagram in figure 2, block 15, not only filters the extra voltage but, through the capacitor Cl, detects the value of the electromotive force for each phase in relationship to the voltage of the battery VB, since this signal is used for the polarisation of the logic circuit 14 through the resistors R21, R31, providing it with a reference that varies with the speed " of the motor 10. An advance in switching is thus produced because the detection of the square wave on signals 1, 2, 3, 4 is done taking as reference a voltage level which progressively increases together with the speed. The output signal from Dl, D2, D3, D4 consists, in fact, of a communication transient and of a signal of electromotive force which is alternately present on one of the phases; this signal, referred to the voltage value VB, is attenuated in relationship to VB by a value equal to R4/(R1 + R4) and said attenuation is adjusted for one motor at a time, according to the desired switching advance. The signal VTH, attenuated in the ratio R4/(R1 + R4), is the one appearing through the divider formed by the resistors R21 and R31 , while the signal downstream Dl, D2, D3, D4 is filtered by the capacitor C2 to provide a continuous component, representing a signal proportional to the speed of the motor 10. Finally, the logic circuit 12 provides at its output a signal SO, which is proportional to the speed of the motor 10 and is obtained as a difference between the signal VTH present at the ends of the divider R21, R31, read by the operational OAl, and the signal VB, read by the operational OA2, so that VB, which is present also on the divider R21, R31, is nullified by using as a differential the two operationals OAl, OA2; as a consequence of that, a signal proportional merely to the speed of the motor 10 will be present on the resistor R6. However, the use of block 15 on motors 10 of basically trapezoidal electromotive force FEM (see diagram in figure 5) allows the achievement of an advanced start which", independently from the speed of rotation, is limited to the electrical angle ATA, also indicated in figure 5 and located between one end of the flat part of the FEM and the passage at zero, because the voltage VTH, which is constant, can be translated in parallel between the peak EPK and the value VB. Besides, the electrical angle ATA represents the ideal advance limit achievable, while the real limit, indicated by APA in figure 5, is even lower, since in is necessary that the electromotive force FEM lowers a little below the peak value EPK in order to allow block 15 to generate a signal useful to trigger the comparators OA3, OA4, OA5, OA6 of the circuit 14. In order to allow each phase to start even when the electromotive force FEM has not yet lowered, the functional block, schematically shown in figure 6 and illustrated as non-restrictive example, by means of the electronic circuit in figure 7, according to the present invention, is used. The theoretical advance angle achievable according the invention, is equal even to 90 electrical grades and is indicated by ATP in figure 5. In practice, the signal VTH has to be altered in order to achieve the increase of the maximum switching advance achievable for any speed of rotation of the motor 10, since any switching is anyway determined by the triggering of the comparators OA3, OA4, OA5, OA6. Said alteration is achieved, according to the functional block diagram in figure 6, by means of adequate processing of the signals Ml, M2, M3, M4 entering block 15. A main "MASTER" block, indicated by MA in figure 6, implements, either in totally analogue way or in analogue-digital way, a transfer function corresponding to the one of block 15; VTH is the output signal of block MA and its run is represented by the graph in figure 5. An additional block "SLAVE", indicated by SL in figure 6, is basically a ramp generator controlled by signals obtained from the main block MA; it also can be done in totally analogue way or in analogue-digital way. The control signal VC establishes the inclination of the ramp, while VK determines a synchronous zero setting (or "reset") at any switching; more in particular, this last signal VK, being it of the digital type, can be obtained, alternatively, not from block MA but through adequate processing of the four control signals Ml, M2, M3, M4 of the static switches II, 12, 13, 14. VTHR is the output signal of block SK and presents a ramp run. Exits VTH and VTHR of blocks MA and SL respectively, are summed by a summing block SUM, the output of which VTH1-VTH+VTHR is connected to block 14, containing the comparators OA3, OA4, OA5, OA6 (see circuit in figure 2), instead of signal VTH which is normally used in the conventional control devices; the run of signal VTHl is schematically represented by the graph in figure 5 where differences with VTH can be appreciated. The introduction of the ramp, which alters the reference level presented to the comparators OA3, OA4, OA5, OA5, makes it possible to achieve their switching even when the electromotive force FEM has not yet lowered below its peak value EPK. An implementation of the control device according to the present invention, the structure of which is schematically represented in the block diagram in figure 6, is represented by the circuit scheme in figure 7; however, it is to be underlined that said implementation is provided as non- restrictive example of embodiment, in its totally analogue version developed especially to operate in "LOW SIDE" configuration. The part of the circuit including components Dl, D2, D3, D4, Rl, R2, R3, R4, R5, R6, Ql, Q2, Q3, C2 has exactly the function indicated in the description of block MA in figure 6; a current proportional to the difference in voltage present at the ends of the capacitor C2 is injected in the resistor R3, said difference being in turn proportional to the difference in voltage existing between VK and VB representing the run of the four electromotive forces in OR-wired. The signal VTH then increases together with the increase in speed of rotation of the motor 10, thus increasing the advance as the speed of rotation increases. The controlled ramp generator SL is achieved in the implementation here considered, by means of: - a generator of current, the value of which is proportional to the difference in voltage present at the ends of the capacitor C2 (said generator consists of the circuit components R7, R8, R9, Q4, Q5 and charges the capacitor C3), - a switch connected to the ends of the capacitor C3 and controlled at the closing by the difference in voltage present between VK and VB (said switch consists of the components R12, R13, R14, Q8, Q9); the trigger threshold of said switch is fixed in order to cause the closing when peaks due to switching on VK occur (for example, in figure 8, these peaks are shown, being indicated by C in the quality graph of signal K-Ml, where K is a proportionality constant), - a voltage-current converter, consisting of the components RIO, Rl l, Q6, Q7, collecting the difference in voltage present at the ends of the capacitor C3 and injecting the generated current in Q2. The generated current will then present a saw tooth run and by being summed to the one generated by the first part of the circuit already described, increases the VTH level, thus making it possible to achieve the desired increase in the advance in switching, with respect to what allowed only by block MA As stated before, figure 8 shows, as an example, the run of signal Ml (difference in voltage between "drain" and "source" of the static switch II) adequately attenuated, according to the constant K of proportionality, by means of the respective divider of the logic circuit 14 in figure 2, and the respective run of the saw-tooth output voltage VTHl, determining the start of the static switch II connected to Ml when VTHl intersects the signal KMl (point A in figure 8), with the exception of point B of the signal KMl, at which a unidirectional logic rotation circuit 16 (schematically illustrated in figure 1) inhibits the switching which, otherwise, would occur inopportunely. The above description shows clearly the characteristics and the advantages of the control device for brushless direct control motors which is the object of the present invention. It is clear that the control device according to the present invention can be modified in many ways without going beyond the scope of the innovative principles of the invention, and that the practical embodiments of the invention, materials, forms and dimensions of the details illustrated can change according to the needs of the case and the same can be substituted by different ones which are technically equivalent.

Claims: CLAIMS 1. Improved control device for brushless direct current motors (10) of the diphase type with four monopolar windings, including a first logic block (11) which includes electronic circuits (14) to detect at least one position of a rotor of said motor (10), electronic circuits (12) to detect the speed of said motor (10), electronic circuits (13) to detect the current and temperature of said motor (10) and a logic circuit (15), adapted to generate a first reference signal (VTH), proportional to an advance in electrical degrees, function of said speed of rotation of the motor (10), with which each one of the phases can be triggered with respect to at least one zero- setting point of respective electromotive forces (FEM), said electronic detection circuits (12, 13, 14) and said logic circuit (15) being adapted to process voltage signals (Ml , M2, M3, M4, M1H, M2H, M3H, M4H) present at the ends of a plurality of windings of said motor (10), said signals (1, 2, 3, 4) processed by the first logic block (1 1) being sent to a second logic block (18), which includes electronic control and start circuits (16) adapted to generate a sequence of signals, so that it is possible for said motor ( 10) to start and rotate and a series of switchings occurring on each one of said windings, in order to trigger each phase, characterised in that said logic circuit ( 15) includes means (MA, SL, SUM) adapted to generate a second reference signal (VTHl), starting from said voltage signals (Ml , M2, M3, M4, M1H, M2H, M3H, M4H) and starting from a battery voltage (VB), which allows to trigger each phase when said electromotive forces (FEM) are equal to a peak value (EPK), for any value of the speed of rotation of the motor (10). 2. Improved control device as claimed in claim 1, characterised in that said means (MA, SL, SUM) generating said second reference signal (VTHl) include an electronic device (MA) implementing a transfer function, adapted to provide said first reference signal (VTH), and a means (SL) generating at least a ramp signal or a saw-tooth signal (VTHR) controlled by signals (VC, VK) obtained from said electronic device (MA) or through the processing of said voltage signals (Ml, M2, M3, M4, M1H, M2H, M3H, M4H) present on said windings, said first reference signal (VTH) and said ramp signal (VTHR) having been summed by means of a summing device (SUM). 3. Improved control device as claimed in claim 2, characterised in that said control signals (VC, VK) assign a value of inclination for said ramp signal (VTHR) and a command for zero setting said signal (VTHR) synchronous to the triggering of each one of said switchings. 4. Improved control device as claimed in claim 1, characterised in that said second reference signal (VTHl) is sent to the input of a plurality of comparators (OA3, OA4, OA5, OA6) of said electronic circuit detecting (14) the position of said rotor. 5. Improved control device as claimed in claim 2, characterised in that said implementation electronic device (MA) includes analogue or analogue- digital devices. 6. Improved control device as claimed in claim 2, characterised in that said means (SL) generating a ramp signal (VTHR) includes components that can be totally analogue, mixed analogue-digital or totally digital. 7. Improved control device as claimed in claim 3, characterised in that at least one of said control signals (VC, VK) is digital. 8. Improved control device as claimed in claim 1, characterised in that it can be implemented on drives with four static switches (II, 12, 13, 14, I1H, I2H, I3H, I4H) in "LOW SIDE" o "HIGH SIDE". 9. Improved control device as claimed in claim 1, characterised in that said advance is equal to a value included between 0 and 90 electrical degrees, including the limit values. 10. Improved " control device as claimed in claim 8, characterised in that said voltage signals (Ml, M2, M3, M4, MIH, M2H, M3H, M4H) consist of voltage differences between the "drain" and "source" terminals of each one of said static switches (II, 12, 13, 14). 1 1. Improved control device as claimed in claim 2 that can be implemented on drives in "LOW SIDE" configuration, characterised in that said means (MA, SL, SUM) generating said second reference signal (VTHl) can be realised with electronic components of the analogue type, in particular said means (SL) generating at least a ramp signal (VTHR) being realised by means of at least a current generator (R7, R8, R9, Q4, Q5) charging at least a capacitor device (C3), a switch (R12, R13, R14, Q8, Q9) connected to the ends of said capacitor device (C3) and controlled at the closing by a difference in voltage present between at least one of said control signals (VC, VK) and said battery voltage (VB), and a voltage-current converter (RI O, Rl l, Q6, Q7), collecting a difference in voltage present at the ends of said capacitor device (C3) and injecting current in at least a transistor device (Q2). 12. Improved control device as claimed in claim 2, characterised in that said second reference signal (VTHl) presents an intermediate level the value of which is higher than the value of said first reference signal (VTH), so that said switching advance increases with reference to what allowed by said electronic device (MA) for the implementation of said transfer function. 13. Improved control device as claimed in claim 1, characterised in that said voltage signals (Ml, M2, M3, M4, MIH, M2H, M3H, M4H) present on said windings are attenuated according to a predetermined factor connected to the values of a set of dividers of resistors (RIO, Rl l, R12, R13, R14, R15, R16, R17) of said electronic circuit that detects (14) the position of said rotor. 14. Improved control device as claimed in claim 2, characterised in that said ramp signal, or saw-tooth signal, VTHl) causes the triggering of a static switch (II, 12, 13, 14, I1H, I2H I3H, I4H) connected to a respective voltage signal (Ml, M2, M3, M4, MIH, M2H, M3H, M4H) present on said winding, when said ramp signal or saw-tooth signal (VTHl) intersects said voltage signal (Ml, M2, M3, M4, MIH, M2H, M3H, M4H) in at least a first point (A), with the exception of at least a second point (B), where a unidirectional rotation logic circuit (16) inhibits the switching in order to avoid inopportune switching. 15. Improved control device for brushless direct current motor as described an illustrated in the appended drawings.

Inventors: CASATI ROBERTO, DE FILIPPIS PIETRO, FASOLA GIANCARLO, NAVARRA PIETRO, CASATI, ROBERTO, DE FILIPPIS, PIETRO, FASOLA, GIANCARLO, NAVARRA, PIETRO
Applicants: BITRON SPA, CASATI ROBERTO, FASOLA GIANCARLO, FILIPPIS PIETRO DE, NAVARRA PIETRO, BITRON S.P.A., CASATI, ROBERTO, DE FILIPPIS, PIETRO, FASOLA, GIANCARLO, NAVARRA, PIETRO
Application year: 2000
Main classification: H02P6
IPC codes: H02P6