Speed control device for brushless motor

Abstract: A control device for a brushless motor comprises a first section which stores a data table provided to minimize a difference between an actual rotation speed of the motor and a target rotation speed of the same. The difference is inevitably produced when the motor is rotated. The data table provides a plurality of rotation speed ranges each having data for a rotation speed of the motor. The control device further comprises a second section which compares the target rotation speed and the data table to select data which corresponds to the rotation speed range to which the target rotation speed belongs. The control device further comprises a third section which corrects the target rotation speed based on the selected data and drives the brushless motor in accordance with the corrected target rotation speed.

Description: The present invention relates to a control device for controlling a brushless motor to minimize a difference between an actual rotation speed of the motor and a target rotation speed of the same, said difference being inevitably produced when the motor is rotated. For driving blower fans of automotive air conditioners and cooling fans for radiators of motor vehicles, brushless motors have been widely used. For controlling the brushless motors, a control device including a microcomputer is usually employed. In order to clarify the task of the present invention, one automotive air conditioner having a brushless motor controlled by a known control device will be described before making a detailed explanation of the present invention. Fig. 7 is a block diagram of the known automotive air conditioner which employs a brushless motor 4 for driving a blower fan 4a. Denoted by numeral 2 is an air conditioning control unit for centrally controlling the air conditioner. Denoted by numeral 3 is a control panel for permitting a driver or passenger to input instructions to the control unit 2 for setting the temperature in a vehicle cabin. In the illustrated case, the control panel 3 has a power switch and a temperature control knob. Denoted by numeral 1 is a control device for controlling the motor 4. The control device 1 comprises a processing unit 12, a drive section 13 which includes a driver circuit and resistors, and six field effect transistors (MOSFET) 14 each two being applied to one winding of the motor 4. Denoted by numeral 5 is a battery and 6 is a switch circuit for feeding when ON an electric power of the battery 5 to the motor 4. The switch circuit 6 is actuated by an ignition switch (not shown). In the following, a voltage of the electric power supplied from the battery 5 through the switch circuit 6 will be referred to as "VCC voltage". The control panel 3 provides the air conditioning control unit 2 with an information signal representing the position of the temperature control knob, that is, a temperature desired by the passenger. In addition, information signals issued from various sensors are fed to the control unit 2, which are signals representing the temperature in the vehicle cabin, outside temperature, quantity of solar radiation and so on. Based on the information signals received, the control unit 2 issues to the processing unit 12 a voltage signal representing a desired rotation speed of the motor 4. Because the blower fan 4a is driven by the motor 4, the rotation speed of the motor 4 controls the air amount blown from an air outlet to the vehicle cabin. By means of an analogue/digital converter (A/D converter), the voltage signal from the control unit 2 is converted to a digital data and led into the processing unit 12 for calculation of a target rotation speed value of the motor 4. The target rotation speed value calculated by the processing unit 12 is a data representing a duty (which will be referred to as "input value" (Din). The data is subjected to an after-described correction and finally becomes a pulse signal value used for driving the motor 4. Thus, controlling of the brushless motor 4 is carried out by allowing the air conditioning control unit 2 to change the target rotation speed value (viz., input value) led into the control device 1. In order to allow the brushless motor 4 to run accurately at the target rotation speed, it is necessary to precisely control the rotation speed of the motor 4 in response to the input value (Din) led into the control device 1. For dealing with a fluctuation of the VCC voltage caused by a temperature change of the battery 5, a so-called "proportional control" is used for controlling the motor 4. That is, the voltage appears at both ends of each winding of the motor 4 when each field effect transistors 14 is ON, and the voltage fluctuation tends to make a fluctuation of current in the windings. In the proportional control, the input value (Din) is corrected based on a deviation between the sensed VCC voltage and a reference voltage. This correction will be referred to as "VCC voltage correction" hereinafter. In the processing unit 12, the input value (Din) is derived, and the VCC voltage is subjected to sampling to obtain VCC voltage value "Vi". And, in a VCC correction factor calculating section 12a of the processing unit 12, a VCC correction factor "Ma" is calculated by using the following equation (1). VCC correction factor "Ma" = Vt/Vi wherein: Vt : reference VCC voltage (=12.5V) stored in a reference voltage generating section 12b. The VCC correction factor "Ma" is led into a VCC correction section 12c of the processing unit 12, and by using the following equation (2), a VCC voltage correctton is carried out in the section 12c to obtain a medium value (Dm). medium value = input value (Din) x Ma After correcting the VCC voltage, a linearity correction is carried out to cause the rotation speed of the motor 4 to have a linearity to the medium value (Dm). That is, when the medium value (Dm) is led into a linearity correcting section 12e of the processing unit 12, a linearity correction factor "Mc" corresponding to the medium value (Dm) is looked up from a linearity correction table 12d, and by using the following equation (3), an output value (Dout) of pulse signal directed to the drive section 13 is calculated. output value (Dout) = Dm x Mc Upon receiving the pulse signal with the output value through the drive section 13, the six field effect transistors 14 are turned ON in given order thereby to feed the windings of the motor 4 to drive the same. Accordingly, in the processing unit 12, the voltage signal of the target rotation speed value for the motor 4 is processed for controlling the rotation speed of the motor 4. With this, the rotation speed of the brushless motor 4, that is, the amount of air blown from the air outlet is controlled to a target amount for contributing to the air conditioning in the vehicle cabin. As is described hereinabove, in the known control device for the brushless motor employed in the automotive air conditioner, in order to drive the brushless motor 4 at a target rotation speed, both a proportional control based on the deviation of the power voltage and a correction for improving the linearity of the rotation speed of the motor 4 are practically carried out. Recently, with increased demand for amenity in the vehicle cabin, it has become necessary to control the temperature in the vehicle cabin with a much higher accuracy and thus, the control for the brushless motor of a blower fan has needed a higher accuracy. However, the known control device fails to provide the brushless motor 4 with finely controlled rotation due to a mechanical energy loss of the motor 4 and a ripple on the source voltage of the motor 4. In fact, the dispersion is inevitably produced in accordance with the rotation speed of the motor 4. For solving the drawback, various measures have been proposed. However, almost all have failed to exhibit a satisfied result. Prior art document EP 0 739 085 A2 teaches a control means for a switch reluctance motor wherein an actual speed signal and a voltage output signal is used to generate a digital compensated speed signal used for controlling the motor. In particular a digital circuitry is provided to compensate variations in DC link voltage transmitted to a switched reluctance machine. The digital voltage compensation system periodically samples the DC link voltage and actual rotor speed, then supplies the samples in digital form to a microcontroller that derives a compensated speed signal to compensate for changes in DC link voltage. Furthermore, prior art document US 4,893,942 teaches an electric food mixer with a membrane potentiometer control which is responsive to a user's touch to select the speed of the mixer's motor. The output of the membrane potentiometer is coupled to a microprocessor by means of an analogue-to-digital converter. The membrane potentiometer is capable of providing an infinite number of speed selection signals within the operation range of the potentiometer. A lock-up table is provided for storing a finite amount of speed data. The data representing mixer motor speed stored in the lock-up table is associated with respective ranges of motor speed selection signals from the membrane potentiometer. Thus, the infinite number of speed selection signals, as provided by the membrane potentiometer, is converted to a finite amount of data by associating a specific speed signal stored in the lock-up table with a specific range of motor speed signals from the potentiometer. Prior art document US 5,161,634 teaches an electric vehicle that runs at a speed controlled by adjusting power supply to an electric motor in response to a speed setting signal from a speed setting device including an accelerator lever and a change speed lever. This vehicle comprises a first control unit for producing a target speed value based on the speed setting signal, a second control unit for controlling the electric motor, and a rotary encoder for detecting an actual vehicle speed from rotation of the electric motor and feeding the vehicle speed back to the first and second control units. The first control unit compares the target speed value and actual vehicle speed and, when a difference there between exceeds a predetermined value, varies the target speed value for application to the second control unit. The second control unit compares the target speed value and actual vehicle speed and determines an amount of power supply to the electric motor. Prior art document FR 2644015 relates principally to a control for brushless DC motor and to a device including such a control. Said device is provided for controlling a brushless DC motor including a module for correcting the speed control signals as a function of the supply voltage. Thus, a brushless DC motor is produced whose speed of rotation is independent of the supply voltage. The device applies whenever the supply voltage to a brushless DC motor might vary. It therefore applies more particularly to a device including brushless DC motors supplied from batteries, cells, dynamos or alternators associated with rectifiers. The device applies very particularly to infrared image capture devices including a pick-up cooled by a cryogenic device including a compressor which is driven by a brushless DC motor. It is an objective of the present invention to provide a control device for controlling a brushless motor to minimize a difference between an actual rotation speed of the motor and a target rotation speed of the same, wherein said control device can accurately control the rotation speed of a brushless motor throughout a wide speed range. Said objective is solved by a control device for controlling a brushless motor to minimize a difference between an actual rotation speed of the motor and a target rotation speed of the same according to independent claim 1. Preferred embodiments are laid down in the dependent claims. Hereinafter, the present invention is illustrated and explained in further detail by means of a preferred embodiment in conjunction with the accompanying drawings. In the drawings wherein: Fig. 1 is a functional block diagram of a control device for a brushless motor, which is an embodiment; Fig. 2 is a table showing conversion factors used in the control device of the embodiment; Fig. 3 is a map showing a value correction factor "Mb" in view of an input value "Din" and a VCC voltage value "Vi"; Fig. 4 is a graph showing a relationship between a medium value and the input value "Din" in various values of the VCC voltage "Vi"; Fig. 5A is a graph depicting an advantageous characteristic of the present embodiment in terms of a relationship between the input value "Din" and an actual rotation speed of brushless motor; Fig. 5B is a graph similar to Fig. 5A, but depicting a characteristic of the known control device; Fig. 6 is a flowchart showing programmed operation steps executed in a computer used in the present embodiment; and Fig. 7 is a block diagram of a known automotive air conditioner which employs a brushless motor for driving a blower fan. Referring to Figs. 1 to 6, there is shown a control device for controlling a brushless motor, which is an embodiment. Fig. 1 shows a functional block diagram of a processing unit 11 employed in the embodiment. It is to be noted that this processing unit 11 is practically applied to the automotive air conditioner of Fig. 7 substituting for the above-mentioned known processing unit 12. Similar to the above-mentioned processing unit 12, the processing unit 11 employed in the present embodiment comprises a reference voltage generating section 12b, a VCC correction factor calculating section 12a, a section 12d for storing a linearity correction table and a linearity correcting section 12e. In addition, the processing unit 11 further comprises a conversion factor deriving section 11a which selects a conversion factor used for converting the VCC. correction factor "Ma" based on the input value "Din", a section 11b which stores the data for the conversion factor, a correction factor converting section 11c which converts the VCC correction factor "Ma" based on the conversion factor selected, and a value correcting section 11d which, based on the input value (Din), derives the medium value (Dm) directed to the linearity correcting section 12e. In the following, operation will be described. In the present embodiment, similar to the above-mentioned known control device, a proportional control is carried out based on the fluctuation of the VCC voltage value "Vi". That is, in the processing unit 11, the VCC voltage value "Vi" is subjected to analogue/digital conversion and then sampling, and then in the VCC correction factor calculating section 12a, calculation of VCC correction factor "Ma" is carried out based on the reference voltage 12b and the VCC voltage value "Vi". At the same time, from a voltage signal issued from the air conditioning control unit 2, there is derived an input value "Din" which represents a target rotation speed value of the brushless motor 4. The input "Din" thus derived is subjected to after-mentioned processes to become a duty value of a pulse signal for driving the motor 4. In the present embodiment, the input value "Din" is led into the conversion factor selecting section 11a. In this section 11a, based on the input value "Din", a conversion factor needed for converting the VCC correction factor "Ma" is picked up from the conversion factor storing section 11b. The conversion factor storing section 11b stores such a table as shown in Fig. 2. In fact, such table is stored in ROM (read only memory) of the processing unit 11. In the table, "n" denotes an identification data "ID", and "Dmin" and "Dmax" represent the minimum and maximum duty values of each range of the input value "Din". Denoted by "An" is a primary conversion factor which is multiplied to the VCC correction factor "Ma", and denoted by "Bn" is a secondary conversion factor which is added to the multiplied result. It is to be noted that these primary and secondary factors "An" and "Bn" have been so determined that the actual rotation speed characteristic can finally approach a rotation speed characteristic which would be exhibited when the VCC voltage is 12. 5V. For this determination, correction of duty is repeatedly carried out by repeatedly rotating the motor 4. In the conversion factor selecting section 11a, a judgment is carried out as to which range of the conversion factors of the table of Fig. 2, the input value "Din" is included with reference to the "Dmin" and "Dmax", and based on the result, the identification data "ID" corresponding to the range is determined. Upon determination of the ID, the primary and secondary conversion factors "An" and "Bn" corresponding to the ID are selected and fed to the correction factor converting section 11c. In the section 11c, the VCC correction factor "Ma" is converted to a value correction factor "Mb" by using the following equation (4): value correction factor "Mb" = Ma x An + Bn That is, when the value correction factor "Mb" is represented as a linear function of the VCC correction factor "Ma", the primary and secondary conversion factors "An" and "Bn" represent gradient and intercept components respectively. Fig. 3 is a three dimensional map showing a relationship between the value correction factor "Mb", the input value "Din" and the VCC voltage value "Vi". That is, this map shows various value correction factors "Mb" with the input value "Din" and the VCC voltage value "Vi" being variables. In the duty correcting section 11d, the value correction factor "Mb" and the input value "Din" are multiplied together to derive a medium value "Dm" by using the following equation (5): medium value "Dm" = Din x Mb Fig. 4 is a graph showing a relationship between the input value "Din" and the medium value "Dm" in various values (viz., 10V, 12. 5V and 16V) of the VCC voltage "Vi". For comparison, the relationship in case of the known control device is also shown. It is to be noted that when the VCC voltage "Vi" is 12.5V, the present embodiment and the known control device show the same relationship. As is seen from this graph, in both the present embodiment and the known control device, due to function of the proportional control, when the VCC voltage "Vi" is relatively high, that is, 16V, the input value "Vi" is so corrected as to decrease the medium value "Dm", while, when the VCC voltage "Vi" is relatively low, that is, 10V, the input value "Vi" is so corrected as to increase the medium value "Dm". In the present embodiment, however, the primary and secondary conversion factors "An" and "Bn" are used for converting the VCC correction factor "Ma" for each range (see Fig. 2). Thus, in both a higher speed range of the "Vi" being 16V and a lower speed range of the "Vi" being 10V, the medium value "Dm" is somewhat increased as compared with that of the known control device, and in both a high speed range of the "Vi" being 10V and a lower speed range of the "Vi" being 16V, the medium value "Dm" is somewhat decreased as compared with that of the known control device. That is, in the present embodiment, throughout the rotation speed of the brushless motor 4, the duty for indicating the target rotation speed of the motor 4 can be finely converted. Upon the medium value "Dm" being derived in the above-mentioned manner, a linearity correction is carried out in the linearity correcting section 12e. That is, the medium value "Dm" is led into the section 12e, and a linearity correcting factor "Mc" corresponding to the medium value "Dm" is looked up from the linearity correction table storing section 12d. Then, by using the following equation (6), output value "Dout" is derived. output value "Dout" = Dm x Mc From the linearity correcting section 12e, there is issued a pulse signal of the output duty "Dout" for driving the brushless motor 4 through the drive section 13 (see Fig. 7). As is described hereinabove, in the present embodiment, the series of operations from the sampling of the voltage signal (which represents a target rotation speed of the motor 4) from the air conditioning control unit 2 to the actual driving of the motor 4 is continuously carried out by after another. Thus, the rotation speed of the brushless motor 4 can be finely controlled throughout a widened speed range. In the present embodiment, the primary and secondary conversion factors "An" and "Bn" of the conversion factor storing section 11b are previously stored as data for minimizing an inevitable difference between the target rotation speed of the motor 4 and the actual rotation speed of the same. Accordingly, in the present embodiment, there is no need of thinking out a measure for removing or at least minimizing an inevitable dispersion of the actual rotation speed of the motor 4, which would be caused by a mechanical energy loss of the motor 4 and a ripple on the source voltage. This induces improved control of the brushless motor 4 and thus induces improved control for the air amount blown from an air outlet to the vehicle cabin. Since the present embodiment practically uses the conversion factor table which has two zones provided for the primary and secondary conversion factors "An" and "Bn", combination of them is easily achieved and thus an appropriate conversion factor is easily derived. In the present embodiment, the entire range of the input value "Din" of the conversion factor table is evenly divided. Thus, In the following, advantageous effect possessed by the present embodiment will be described with reference to Fig. 5A. For ease of understanding, Fig. 5B is also presented which shows an effect possessed by the known control device of Fig. 7. As is seen from Fig. 5B, in the known control device, the actual rotation speed of the brushless motor does not finely correspond to the input value "Din". This is because of an inevitable ripple on the VCC voltage "Vi". While, as is seen from Fig. 5A, in the present embodiment, the actual rotation speed of the brushless motor corresponds accurately to the input value "Din", especially within a certain range from the input duty of 10% to that of 70%. That is, even when the VCC voltage "Vi" varies in the range between 10V, 12.5V and 16V, the actual rotation speed of the motor 4 does not show dispersion in the certain range. That is, in accordance with the present embodiment, actual rotation speed of the brushless motor 4 can finely correspond to the input value "Din" which represents the target rotation speed of the motor 4. It is to be noted that the conversion factors "An" and "Bn" of each range of the table of Fig. 2 are so determined that when the VCC correction factor "Ma" is 1 (one), that is, when the VCC voltage "Vi" is equal to the reference voltage "Vt", the value correction factor "Mb" is 1 (one). That is, 12.5V is used as a reference voltage for the VCC voltage, and the conversion factors "An" and "Bn" are so determined as to match the rotation speed characteristic possessed by the VCC voltage. Accordingly, as is seen from Fig. 5A, even if the VCC voltage "Vi" varies, a rotation speed set when the VCC voltage "Vi" is equal to the reference voltage "Vt" is finally obtained. Accordingly, in the air conditioning control unit 2, based on the characteristic set when the reference voltage "Vt" is 12. 5V, it is only necessary to derive an input value "Din" which represents the target rotation speed of the motor 4. In the following, programmed operation steps executed in the processing unit 11 will be described with reference to the flowchart shown in Fig. 6. First, at step S1, based on the VCC voltage "Vi" and the reference voltage "Vt", the VCC correction factor "Ma" is derived. Then, at step S2, based on the input value "Din" issued from the air conditioning control unit 2, the identification data "ID" is picked up from the table (see Fig. 2) stored in the conversion factor storing section 11b. At step S3, based on the identification data "ID" thus picked up, the primary and secondary conversion factors "An" and "Bn" are selected. At step S4, from the "Ma", "An" and "Bn", the value correction factor "Mb" is calculated by using the equation (4). At step S5, from "Din" and "Mb", the medium value "Dm" is calculated by using the equation (5). At step S6, from the linearity correction table in the section 12d, the linearity correcting factor "Mc" corresponding to the medium value "Dm" is looked up. Then at step S7, from "Dm" and "Mc", the output "Dout" is calculated by using the equation (6). Based on the output value "Dout" thus provided, a pulse signal is produced for driving the brushless motor 4 through the drive section 13. It is to be noted that in the processing unit 11, the VCC correction factor calculating section 12a, the conversion factor deriving section 11a, the correction factor converting section 11c, the duty correcting section 11d and the linearity correcting section 12e are provided by a software executed by a corresponding computer. The reference voltage in the section 12b, the conversion factor table in the section 11b and the linearity correction table in the section 12d are stored in ROM (viz., read only memory) of the computer. With this arrangement, the number of logic circuits of the computer is reduced thereby to allow the motor 4 to have a simple wiring and thus a satisfied heat radiation. When a brushless motor controlled by the control device is applied to a radiator cooling fan of an automotive engine, the cooling effect of the fan can be accurately controlled. As is described hereinabove, in the control device of the present embodiment target rotation speed of the brushless motor is corrected by practically using a conversion factor data which is provided to minimize an inevitable difference between the target rotation speed of the brushless motor and the actual rotation speed of the same. Thus, in the present embodiment, an inevitable dispersion of the actual rotation speed of the brushless motor, which would be caused by a mechanical energy loss of the motor and a ripple on a source voltage, is minimized, so that the actual rotation speed of the motor is finely and actually controlled in a widened rotation speed range.

Claims: A control device for controlling a brushless motor to minimize a difference between an actual rotation speed of the motor and a target rotation speed of the same, said difference being inevitably produced when the motor is rotated, said control device comprises: a first section (11b) which stores a data table providing a plurality of rotation speed ranges and data being associated to each of the rotation speed ranges; a second section (11a) for comparing said target rotation speed and said plurality of rotation speed ranges to select data which corresponds to a rotation speed range to which said target rotation speed belongs; a third section (11c, 11d, 12e, 12d) for correcting said target rotation speed based on said selected data for driving said brushless motor in accordance with a corrected target rotation speed; fourth section (12a) which calculates, based on a ripple on a source voltage of said brushless motor, a correction factor (Ma) used for a proportional control, wherein said data selected by the second section (11a) comprises a primary conversion factor (An) and a secondary conversion factor (Bn) and the third section (11c, 11d, 12e, 12d) is provided for correcting said target rotation speed based on said correction factor (Ma), said primary conversion factor (An) and said secondary conversion factor (Bn) for driving said brushless motor in accordance with the corrected target rotation speed. A control device according to claim 1, wherein said primary conversion factor (An) is multiplied to said correction factor (Ma) used for the proportional control. A control device according to claim 1 or 2, wherein said secondary conversion factor (Bn) is added to said correction factor (Ma) used for the proportional control. A control device according to one of the claims 1 to 3, wherein the sum of said primary and secondary conversion factors (An, Bn) is constant. A control device according to one of the claims 1 to 4, wherein the plurality of rotation speed ranges of the data table are constituted by evenly dividing a rotation speed band of the brushless motor. A control device according to one of the claim 1 to 5, wherein the brushless motor is employed in a motor vehicle, and said correction factor is a VCC correction factor (Ma) and said fourth section is provided for deriving the VCC correction factor (Ma) based on a VCC voltage and a reference voltage, said VCC voltage being a voltage of electric power supplied from a battery mounted on the vehicle; said target rotational speed is represented by an input value (Din) and the second section (11a) is provided for picking up an identification data from the table with reference to the input value (Din) issued from an air conditioning control unit; said second section (11a) is provided for selecting said primary and secondary conversion factors (An, Bn) with reference to the picked up identification data; said third section (11c, 11d, 12e, 12d) comprises means (11c) for calculating a correction factor (Mb) from said VCC correction factor (Ma) and said selected primary and secondary conversion factors (An, Bn), and means (11b) for calculating a medium value (Dm) from said input value (Din) and said correction factor (Mb), and means (12d) for looking up a linearity correcting factor (Mc) from a linearity correction table, with reference to said medium value (Dm), and means (12e) for calculating an output value (Dout) from said medium value (Dm) and said VCC correction factor (Ma), said output value (Dout) being used for producing a pulse signal representing the corrected target rotation speed to drive the brushless motor. A control device according to claim 6, wherein said fourth section is provided for deriving the VCC correction factor (Ma) by dividing the reference voltage by said VCC voltage. A control device according to claim 6 or 7, wherein said means (11c) for calculating the correction factor (Mb) is using the following equation: Mb = Ma x An + Bn wherein: Mb: correction factor Ma: VCC correction factor An: primary conversion factor Bn: secondary conversion factor A control device according to one of the claims 6 to 8, wherein said means (11 b) for calculating a medium value (Dm) is using the following equation: Dm = Din x Mb wherein: Dm: medium value Din: input value Mb: correction factor A control device according to one of the claims 6 to 8, wherein said means (12e) for calculating an output value (Dout) is using the following equation: Dout = Dm x Mc wherein: Dout: output value Dm: medium value Mc: linearity correcting factor

Inventors: ARAKI FUTOSHI, SEKINE TAKESHI, SUNAGA HIDEKI, TAKAHASHI EIJI, ARAKI, FUTOSHI, SEKINE, TAKESHI, SUNAGA, HIDEKI, TAKAHASHI, EIJI, Araki, Futoshi, c/o Calsonic Corporation, Sekine, Takeshi, c/o Calsonic Corporation, Sunaga, Hideki, c/o Calsonic Corporation, Takahashi, Eiji, c/o Calsonic Corporation
Applicants: CALSONIC KANSEI CORP, CALSONIC KANSEI CORPORATION
Application year: 1999
Main classification: H02P7
IPC codes: H02P6, H02P29, H02P25