Method for detecting the position of a rotor of a DC motor and relative circuit

Abstract: The present invention refers to a method for detecting the position of a rotor of a DC motor and to the relative circuit. According to an embodiment the method for detecting the position of a rotor of a DC motor (SM) with N phases having a plurality of windings (AV1, AV2, AV3), comprising the steps of: connecting at least two of said windings (AV1, AV2, AV3) between a first (Vcc) and a second (Gnd) prefixed voltage according to a first current path for a prefixed time (Tact); allowing the current stored in said two windings (AV1, AV2, AV3) to discharge through a second current path; comparing the voltage at the ends of one of said two windings (AV1, AV2, AV3) with a third prefixed voltage and providing a control signal when said voltage is smaller in absolute value than said third prefixed voltage; performing the above steps for each of the winding (AV1, AV2, AV3) couples of said motor (SM); detecting the position of said rotor on the base of the control signals obtained.

Description: The present invention refers to a method for detecting the position of a rotor of a DC motor and to the relative circuit. A DC motor comprises a permanent magnet that constitutes the rotor and a stator having a plurality of windings. This type of motors is commonly used to operate the computer hard disks, the DVDs (Digital Video Disk), the CDs etc. In most cases the DC motors have three phases and the driving circuit generally comprises integrated circuits whose output stages, typically half-bridges, drive the windings in a cyclic way. For the starting of a DC motor it is necessary to supply the windings in the correct combination of phases, and in order to do so it is necessary to know in which position the rotor is. The detection of the position of the rotor is based normally on the fact that the winding impedance depends on the position of the rotor. For this reason a small current is provided sequentially in each of the combinations of the windings and the voltage at the heads of an external resistance placed in series with each of the windings combinations is measured. Particularly for a motor with three phases U, V and W a current is made flowing sequentially between the following couples of phases U-V, U-W, V-W, V-U, W-U, W-V, and the time, for each couple of phases, that the voltage at the heads of the resistance takes to arrive at a reference value is measured. Or, alternatively, the voltage at the heads of the resistance after a prefixed time is measured. The value of the resistance changes normally from 100 milliohm to 500 milliohm and cannot be integrated inside of the integrated circuits and therefore it is placed outside with consequent increasing of the number of pins of the integrated circuit, bulk on the printed circuit board and increasing of the associated cost. Besides, the resistance causes a limit on the maximum voltage that can be furnished to the motor and has a not negligible power dissipation, in particular such drawbacks take a remarkable importance in mobile applications and for devices supplied at low voltage (5 V and 3,3V). US 5,254,914 discloses an apparatus for detecting the rotor position of a brushless DC motor. More specifically, the position at start is determined by the injection of short current pulses in different motor phases, each phase or pair of phases being energised first by a pulse of one polarity and of the opposite polarity. The sign of the time difference between the decay times of the induced voltages in the un-energised phase is detected. By performing a succession of these tests on different phases or pairs of phases of the standing motor, a table of results is established which clearly defines the position of the rotor relative to the motor phases. In particular each phase is supplied by the current until the voltage across the undriven phase, becomes smaller than the comparator reference voltage. US 4,520,302 discloses a driving circuit for stepping motors which comprises means for derive an indication of rotor position by measuring the current flow. The current flow characteristic that is measured may be current rise time, current decay time or chopping frequency. The current characteristic being monitored may be the characteristic in an energized winding or else in an unenergized winding at low current level. In view of the state of the art described, it is an object of the present invention to provide a solution completely integrable in the integrated circuit. According to the present invention, these and other objects are achieved by means of a method for detecting the position of a rotor of a DC motor according to claim 1. Such objects are also achieved by means of a circuit for detecting the current of a DC motor with 3 phases, according to claim 12. Thanks to the present invention it is possible to determine the position of a rotor of a DC motor eliminating the external resistance and therefore improving the performances of the motor. Particularly, not existing any voltage drop on the resistance, it is possible to supply a greater voltage to the motor and therefore there are a greater starting current and a greater maximum speed, with a lower total dissipated power. The features and the advantages of the present invention will be evident from the following detailed description of an embodiment thereof, illustrated as a non-limiting example in the annexed drawings, wherein: Figure 1 represents the partial scheme of a driving circuit of a three-phase motor according to the known art; Figure 2 represents the partial scheme of a driving circuit of a three-phase motor according to the present invention. In Figure 1, that represents the partial scheme of a driving circuit of a three-phase motor according to the known art, there are three substantially identical driving circuits at half-bridge S1, S2 and S3. Each of the half-bridges is constituted by a high side transistor and a low side transistor, each one with the associated reflow diode, respectively indicated with MHU, MLU and DHU, DLU for the half-bridge S1; MHV, MLV and DHV, DLV for the half-bridge S2; and MHW, MLW and DHW DLW for the half-bridge S3. The high side transistors have their drain connected to the positive supply voltage Vcc, the low side transistors have their source connected together and connected to the ground Gnd through a resistance Rs. The source of each high side transistor is connected to the drain of each low side transistor, the point of connection of the half-bridge S1 corresponds to the U phase and is connected to a terminal of the winding AV1, the point of connection of the half-bridge S2 corresponds to the V phase and it is connected to a terminal of the winding AV2, and the point of connection of the half-bridge S3 corresponds to the W phase and it is connected to a terminal of the winding AV3. The other terminals of the windings AV1, AV2 and AV3 are connected together. Such windings represent a 3-phase motor SM schematically. The gate of the transistors are connected to a control circuit, not shown, that allows to supply the phases U, V and W of the motor SM in an opportune way. The positive terminal is connected to the resistance Rs of a comparator C1, a voltage reference Vref is applied to the negative terminal of the comparator C1. The output of the comparator C1 is connected to the control circuit, not shown, that is able to determine the position of the rotor. For the detection of the position of the rotor a current is made to flow between one of the couples of phases U-V, U-W, V-W, V-U, W-U, W-V, controlling in the right way the transistors of the half-bridges S1, S2 and S3, and the time intervened between the activation of the transistors and the time in which the voltage at the ends of the resistance Rs overcomes the reference voltage Vref and switches the comparator C1 is measured. The operation is repeated for all the couples of remaining phases and therefore the control circuit effects the opportune calculations for determining the position of the rotor. In Figure 2, that represents the partial scheme of a driving circuit for a three-phase motor according to the present invention, the elements equal or similar to those of Figure 1 have the same indicative references. With respect to Figure 1 the resistance Rs is missing, and the positive terminal of the comparator C1 is connected to a reference voltage that in this case corresponds to the positive supply Vcc. The negative terminal of the comparator C1 is connected, through a selector Sel composed of three switches, to the three phases U, V, W of the motor SM. The output of the comparator C1 is connected to a control circuit Cont, which also provides the control to the selector Sel and to the gate of the transistors of the three half-bridges S1, S2 and S3. According to an embodiment of the present invention, a first couple of phases, for example U-V, is supplied for a prefixed time Tact (that can be programmable to make the system flexible), the selector is set so that the negative terminal of the comparator C1 is connected to the phase V. Therefore the output stage, that is the transistors of the half-bridges S1, S2 and S3, are set at high impedance and the necessary time for the decay of the current to zero is measured. More precisely, if the U-V phase is fed, by activating the transistors MHU and MLV and leaving the others switched off, for a Tact time the current in the phases U and V, and therefore in the windings AV1 and AV2 of the motor SM, will start to flow along the continuous line path of Figure 2. At the end of the Tact time the two transistors MHU and MLV are switched off activating a condition of high impedance (for all the six transistors of the half-bridges S1, S2 and S3). At this point the current stored in the two windings AV1 and AV2 starts to reflow through the diodes DHV and DLU according to the dotted line of Figure 2, starting the decay phase. During all the time of this phase the voltage of the V phase will have a greater value than the supply voltage Vcc, while the voltage of the U phase will have a smaller value of the ground voltage Gnd. Once the decay phase of the current is ended the two diodes DHV and DLU are de-activated and the voltage of the V phase will have a smaller value than the supply voltage Vcc, while the voltage of the U phase will have a greater value than the ground voltage Gnd. By the change of the value at the output of the comparator C1, that is at the reaching of the reference voltage, and therefore at the reaching of the supply voltage Vcc, it is possible to determine the decay time of the current stored in the windings AV1 and AV2. Particularly, in the previous case by measuring the voltage of the phase V, the value of output of the comparator C1 will be at low level for all the reflow time and it will be at high level at its end. By repeating the same steps for each of the six possible configurations of the phases it is possible therefore to measure the six different times necessary for the decay of the current and therefore to allow the control circuit Cont to determine the position of the rotor by means of operations well known to the technician skilled in the art. Hereinbelow a table (TABLE 1) is reported that points out for each couple of phases how the negative terminal of the comparator C1 must be connected through the selector Sel. Couple of phases supplied Phase to be measured U-V V U-W W V-W W V-U U W-U U W-V V According to another embodiment of the present invention, the comparator C1 can have the positive terminal connected to the ground voltage Gnd, the value of the output voltage will be at high level for all the reflow time and it will be at low level at its end. Accordingly, the table that points out for each couple of phases how the negative terminal of the comparator C1 must be connected through the selector Sel, will be modified as from the TABLE 2 which is below reported. Couple of phases supplied Phase to be measured U-V U U-W U V-W V V-U V W-U W W-V W However, the positive terminal of the comparator C1 can have as reference voltage any voltage comprised between Vcc and Gnd. According to a further embodiment of the present invention, instead of activating a condition of high impedance for all three half-bridges S1, S2 and S3, it is possible to activate the low side transistor of the couple of phases opposed to the supplied one. For example if the couple of phases U-V, that is the transistors MHU and MLV, is supplied for a Tact time, and then the transistor MLU is actived (setting all the others at high impedance) the current will reflow through the transistor MLU and the DHV diode. In a further variant, the transistor MHV can be activated (setting all the others at high impedance) and the current will reflow through the transistor MHV and the DLU diode. Or, also in an alternative way, for the current reflow both the transistors of the phase opposed to the supplied one can be activated. For example, if the couple of U-V phases, that is the transistors MHU and MLV is supplied for a Tact time, and then the transistors MLU and MHV are activated (setting all the others at high impedance), the current will reflow through the aforesaid transistors. The only difference of these variants, with respect to the first embodiment is that at the terminal of the comparator C1 a different voltage will appear according to the cases.

Claims: Method for detecting the position of a rotor of a DC motor (SM) with N phases having a plurality of windings (AV1, AV2, AV3), comprising the steps of: connecting at least two of said windings (AV1, AV2, AV3) between a first (Vcc) and a second (Gnd) prefixed voltage according to a first current path for a prefixed time (Tact); disconnecting said at least two of said windings (AV1, AV2, AV3); allowing the current stored in said two windings (AV1, AV2, AV3) to discharge through a second current path; comparing the voltage at the ends of one of said two windings (AV1, AV2, AV3) with a third prefixed voltage and providing a control signal when said voltage is smaller in absolute value than said third prefixed voltage; performing the above steps for each of the winding (AV1, AV2, AV3) couples of said motor (SM); detecting the position of said rotor on the base of the control signals obtained. Method according to claim 1 characterized in that said N phases are three. Method according to claim 1 characterized in that said first prefixed voltage is the supply voltage (Vcc) and said second prefixed voltage is ground (Gnd). Method according to claim 1 characterized in that said third prefixed voltage is the supply voltage (Vcc). Method according to claim 1 characterized in that said third prefixed voltage is ground (Gnd). Method according to claim 1 characterized in that said second current path connects said at least two of said windings (AV1, AV2, AV3) between said second (Gnd) and said first prefixed voltage (Vcc). Method according to claim 1 characterized in that the step of allowing the current stored in said two windings (AV1, AV2, AV3) to discharge through a current path consists of setting the N phases at high impedance. Method according to claim 1 characterized in that said first current path is formed by the activation of two transistors. Method according to claim 1 characterized in that said second current path is formed by the activation of two transistors. Method according to claim 1 characterized in that said second current path is formed by activation of a transistor and of a diode. Method according to claim 1 characterized in that said second current path is formed by activation of two diodes. Circuit for detecting the current of a DC motor (SM) with three phases, having a plurality of windings (AV2, A2, AV3), comprising means for connecting at least two of said windings (AV1, A2, AV3) between a first (Vcc) and a second (Gnd) prefixed voltage according to a first current path for a prefixed time (Tact); means for disconnecting said at least two of said windings (AV1, AV2, AV3); means for allowing the current stored in said two windings (AV1, AV2, AV3) to discharge through a second current path; means for comparing the voltage at the ends of one of said two windings (AV1, AV2, AV3) with a third prefixed voltage and providing a control signal when said voltage is smaller in absolute value than said third prefixed voltage; means for performing the above steps for each of the winding (AV1, AV2, AV3) couples of said motor (SM); means for detecting the position of said rotor on the base of the control signals obtained.

Inventors: BOSCOLO MICHELE, BRAMBILLA MASSIMILIANO, GALBIATI EZIO, BOSCOLO, MICHELE, BRAMBILLA, MASSIMILIANO, GALBIATI, EZIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 2001
Main classification: H02P6
IPC codes: H02P6, G11B19