IMAGE FORMING APPARATUS THAT DETECTS STOPPING POSITION OF ROTOR OF MOTOR

The image forming apparatus includes a motor including at least three coils, first terminals of which are connected to one another; and a motor control unit. The motor control unit detects a stopping position of the motor by executing measuring processing including performing control so that a coil current flows from a first coil to a second coil and measuring a current value. The motor control unit is configured to perform control so that the coil current flows from the first coil toward the second coil in a first period, attenuates the coil current in a second period, and perform control so that a portion of the coil current flows from a second terminal of the second coil, via a second terminal of a third coil, to the third coil in a third period within the second period.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to technology for controlling a motor in an image forming apparatus.

Description of the Related Art

A sensorless DC brushless motor without a sensor for detecting the rotor position is used as a drive source for rotating members in image forming apparatuses. To prevent step-out and reverse rotation upon start up in a sensorless DC brushless motor, the stopping position of the rotor (hereinafter, rotor stopping position) is detected and startup processing in accordance with the rotor stopping position is executed. US-2015-0145454 discloses a configuration in which an excitation current (coil current) flows through each one of a plurality of excitation phases of a motor and the rotor stopping position is detected on the basis of the current value of the coil current flowing when each excitation phase is excited.

In an image forming apparatus, the amount of time required for printing can be reduced by reducing the startup time of the motor. The startup time of the motor can be effectively reduced by reducing the amount of time required to detect the rotor stopping position. To detect the rotor stopping position, it is necessary for a coil current to flow through each one of the plurality of excitation phases and for the current values to be measured. However, measurement of an excitation phase cannot start until the current value of the coil current running through the previous excitation phase has become sufficiently small. Accordingly, the amount of time required to detect the rotor stopping position can be reduced by making the coil current flowing through an excitation phase quickly attenuate.

However, by making the coil current quickly attenuate, the regenerative current to the power supply may become large and the power supply voltage may increase. The regenerative current can be suppressed by increasing the capacity of the electrolytic capacitor connecting to the power supply line, but this increases costs and the size of the image forming apparatus.

SUMMARY OF THE INVENTION

According to an aspect of the present disclosure, an image forming apparatus that forms an image on a sheet conveyed along a conveying path is provided. The image forming apparatus includes: a rotating member; a motor configured to drive the rotating member, the motor including at least three coils, and first terminals of the at least three coils being connected to one another; and a motor control unit configured to control the motor by controlling a potential of second terminals different from the first terminals of the at least three coils, the motor control unit detecting a stopping position of a rotor of the motor by executing measuring processing on each of a plurality of excitation phases indicated by a permutation of two coils from among the at least three coils, the measuring processing including performing control so that a coil current flows from a first coil to a second coil of the at least three coils and measuring a current value of the coil current. The motor control unit is configured to: perform control so that the coil current flows from the first coil toward the second coil in a first period of the measuring processing, perform control so that the coil current flowing from the first coil toward the second coil in the first period attenuates in a second period following the first period, and perform control so that a portion of the coil current flowing from the first coil toward the second coil flows from the second terminal of the second coil, via the second terminal of a third coil different from the first coil and the second coil from among the at least three coils, to the third coil in a third period within the second period.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

FIG.1is a configuration diagram of an image forming apparatus of the present embodiment. The image forming apparatus may be any one of a printing apparatus, a printer, a copy machine, a multi-function peripheral, and a facsimile machine, for example. A sheet stored in a cassette25of the image forming apparatus is conveyed along a conveying path by a feeding roller26and a conveyance roller27. An image forming unit1forms a yellow, magenta, cyan, and black toner image and transfer these toner images onto the sheet conveyed along the conveying path. A fixing device24includes a heating roller and the pressure roller and applies heat and pressure to the sheet with the transferred toner images to fix the toner images to the sheet. After toner image fixing processing is performed on the sheet, the sheet is discharged to the outside of the image forming apparatus. A motor15F is a drive source that rotates the roller of the fixing device24. The motor15F may be a sensorless motor without a sensor for detecting the rotor position. Note that the image forming apparatus illustratedFIG.1is a color image forming apparatus that forms images using toner images of four colors. However, for example, the image forming apparatus may be a monochrome image forming apparatus that forms images using only black toner. Also, the image forming apparatus ofFIG.1is an electro-photographic image forming apparatus. However, the image forming apparatus may be any type of image forming apparatus including an inkjet image forming apparatus or the like.

FIG.2illustrates the control configuration of the image forming apparatus. When image data of an image to be formed is received from a host computer22via a communication controller21, a printer control unit11controls the image forming unit1to form a toner image on a sheet and controls the fixing device24to fix the toner image on the sheet. Also at this time, the printer control unit11controls a motor control unit14to perform rotation control of each motor15including the motor15F. The motors15are drive sources for a roller of the fixing device24, a roller of a conveying unit that conveys the sheet along the conveying path, a rotating member included in the image forming unit1, and the like. Note that the conveying unit includes the feeding roller26and the conveyance roller27. Also, the rotating member of the image forming unit1includes a photosensitive body, a developing roller, and the like, for example. Also, the printer control unit11displays the state of the image forming apparatus on a display unit20. Note that the printer control unit11includes one or more processors and memory, for example. The memory stores various types of control programs and data, and the one or more processors control each unit of the image forming apparatus on the basis of the various types of control programs and data stored in the memory.

FIG.3illustrates an example of the configuration of the motor control unit14. The motor control unit14includes a microcomputer51. The microcomputer51communicates with the printer control unit11via a communication port52. The microcomputer51outputs a pulse width modulation signal (PWM signal) from a PWM port58. In the present embodiment, the microcomputer51, for the coils of the three phases (U phase, V phase, W phase) of the motor15F, outputs a total of six PWM signals, namely high-side PWM signals (U-H, V-H, W-H) and low-side PWM signals (U-L, V-L, W-L). Accordingly, the PWM port58includes six terminals U-H, V-H, W-H, U-L, V-L, W-L. Each terminal of the PWM port58is connected to a gate driver61.

An inverter60includes a total of six switching elements corresponding to the PWM signals output from the PWM port58in a 1-to-1 relationship. In other words, the inverter60includes switching elements corresponding to U-H, V-H, W-H, U-L, V-L, and W-L. An FET can be used as the switching element, for example. The gate driver61controls the on/off of the switching elements corresponding to the PWM signals of the inverter60on the basis of the PWM signals from the PWM port58. In the present embodiment, when the PWM signal is at a high level, the corresponding switching element is turned on, and when the PWM signal is at a low level, the corresponding switching element is turned off. An output62of the inverter60is connected to a second terminal of coil73(U phase),74(V phase), and75(W phase) of the motor. Note that first terminals different from the second terminals of the coils73,74, and75are connected to one another and form a neutral76. The motor15F includes a rotor72.

The inverter60is connected to a power supply terminal66(third terminal) connected to a non-illustrated DC power supply, and the inverter60and the motor15F run on the power from the DC power supply. The potential of a power supply terminal66is +24 V, for example. Also, a bulk electrolytic capacitor65is connected in parallel with the inverter60between the power supply terminal66and a ground terminal67(fourth terminal). The bulk electrolytic capacitor65absorbs the regenerative current and supplies current to the inverter60.

The microcomputer51can controls the voltage applied to each coil73to75by controlling each switching element of the inverter60via PWM signals. In this manner, the microcomputer51adjusts the current value of the coil current flowing through each coil73to75to control the rotation of the motor15F. In other words, the microcomputer51and the inverter60controls the rotation of the motor15F by controlling the potential of the second terminal of each coil of the motor15F.

The coil current flowing through each coil73,74, and75is converted into voltage by a resistance63corresponding to the U phase, the V phase, and the W phase. The voltage of the resistance63corresponding to the U phase, the V phase, and the W phase is input into an AD converter53of the microcomputer51via an amplifier64. The terminal of the resistance63that is not the terminal connected to the inverter60is connected to the ground terminal67. The AD converter53converts the input voltage into a digital value. The microcomputer51detects the current value of the coil current flowing through each coil73,74, and75on the basis of the digital value. In this manner, the resistance63, the amplifier64, and the microcomputer51form a current detection unit that detects the current values of the coil currents. Also, the microcomputer51includes a non-volatile memory55that stores various types of data and the like used in controlling the motor15F and a volatile memory57.

Each coil73,74, and75can be set to a first state, a second state, or a third state via the switching elements of the inverter60. The first state is a state in which the second terminal of the coil is connected to the power supply terminal66of a first potential (for example, +24 V) by only the high-side switching element corresponding to the coil being turned on. The second state is a state in which the second terminal of the coil is connected to the ground terminal67of a second potential (for example, ground potential) lower than the first potential via the resistance63by only the low-side switching element corresponding to the coil being turned on. The third state is a state in which the second terminal of the coil is not connected to either the power supply terminal66or the ground terminal67, meaning that the second terminal is in an open state, by the high-side and low-side switching element corresponding to the coil being both turned off. In this manner, the inverter60functions as a setting unit that sets the state of each coil to the first state, the second state, or the third state and as a switching unit that switches the state of each coil between the first state, the second state, and the third state.

FIGS.4A and4Bare diagrams illustrating examples of the configuration of the motor15F. The motor15F includes a six-slot stator71and the four-pole rotor72. The stator71is provided with the U-phase coil73, the V-phase coil74, and the W-phase coil75. The rotor72is formed of a permanent magnet and includes two N pole and S pole pairs. The stopping position of the rotor72depends on the excitation phase. Note that in the present embodiment, an excitation phase is indicated by a permutation of two coils from among the coils73,74, and75. In other words, there are six excitation phases in total, namely U-V, U-W, V-U, V-W, W-U, W-V Here, exciting the U-V phase means exciting so that a coil current flows from the U-phase coil73toward the V-phase coil74via the neutral76. For example, when the U-V phase is excited, the rotor72stops at the rotational position illustrated inFIG.4A. Note that at this time, the U phase is an N pole and the V phase is an S pole. Next, when the U-W phase is excited, the rotor72stops at the rotational position illustrated inFIG.4B.

If the driving of the motor15F is stopped and the coil current made 0, the force holding the rotor72stops acting, and the rotor72can be rotated by the application of a rotational force from the outside of the rotor72. Thus, when the fixing device24is attached or removed from the image forming apparatus or when a jammed sheet shut in the fixing device24is removed, the rotor72can rotate. At this time, the motor control unit14becomes unable to determine the stopping position of the rotor72. Also, just after power is supplied to the image forming apparatus, the motor control unit14does not determine the stopping position of the rotor72. Thus, in a case where the motor15F is to be rotated, the motor control unit14first executes processing to detect the stopping position of the rotor72.

Here, typically, a coil such as the coils73,74, and75has a configuration in which a copper wire is wound around a core formed from stacked electromagnetic steel sheets. The magnetic permeability of the electromagnetic steel sheets decreases when an external magnetic field is present. The inductance of the coil is proportional to the magnetic permeability of the core. Thus, when the magnetic permeability of the core is decreased, the inductance of the coil is also decreased. For example, since the U-phase coil73inFIG.4Afaces only the S pole of the rotor72, the degree of reduction in the inductance of the U-phase coil73is greater than that of the W-phase coil75facing both the S pole and the N pole of the rotor72. Also, the amount of change in the inductance is different depending on whether the direction of the external magnetic field and the direction of the magnetic field produced by the coil current are the same or opposite direction. Specifically, in the state ofFIG.4A, when the direction of the magnetic field generated by the U-phase coil73is the same as the direction of the magnetic field produced by the opposing S pole of the rotor72, that is, a coil current flows so that the U phase is an N pole, the amount of decrease in the inductance is greater than in a case where a coil current flows in a direction so that the U phase is an S pole. In this manner, the value of the detected inductance is different depending on the stopping position of the rotor72and the excitation phase to be excited. Also, the iron loss of a coil changes depending on changes in the inductance, and thus the resistance component of the coil also changes. Hereinafter, the coil inductance component and the resistance component will be collectively referred to as the coil impedance.

FIG.5illustrates the synthetic impedance observed upon excitation of each excitation phase when the rotor72is stopped at a position where the U-V phase is excited. Hereinafter, the stopping position of the rotor72when the X-Y phase is excited is referred to as the “X-Y phase position”. Since the rotor72is stopped at the U-V phase position, the synthetic impedance when the U-V phase is excited is less than the synthetic impedance when other excitation phases are excited. Accordingly, the position of the rotor72can be determined by determining the relative magnitude relationship of the synthetic impedance of each excitation phase. In the present embodiment, as described below, the measuring processing to excite the excitation phases and measure the coil currents is executed in order on each excitation phase. Then, the stopping position of the rotor72is detected on the basis of the current values of the coil currents measured for each excitation phase.

The measuring processing for each excitation phase is similar, and hereinafter, the measuring processing will be described using the U-V phase as a representative. The U-phase coil73(first coil) and the V-phase coil74(second coil), which are excitation targets in the U-V phase measuring processing will be referred to below as the excitation coils, and the W-phase coil75(third coil) not included in the excitation phase will be referred to below as the non-excitation coil. As illustrated inFIG.6, the time period of the measuring processing for one excitation phase is divided into an A period (first period) and a B period (second period). Also, a C period (third period) is set inside the B period. The A period is a time period in which the inverter60is controlled so that a coil current flows from the U-phase coil73toward the V-phase coil74. The B period is a time period in which the inverter60is controlled so that the coil current flowing from the U-phase coil73toward the V-phase coil74in the A period attenuates. Ideally, the current value of the coil current is 0 at the time when the B period ends, but in practice, the current value of the coil current may be a positive or negative value at the end timing of the B period. Note that in a case where the current value is a positive value, the coil current is flowing from the U-phase coil73toward the V-phase coil74. Note that in a case where the current value is a negative value, the coil current is flowing from the V-phase coil74toward the U-phase coil73.

First the A period will be described. In the A period, the microcomputer51outputs a PWM signal that changes the high-level duty cycle to a sine wave from the U-H terminal. Note that though not illustrated, the microcomputer51outputs a PWM signal with polarity inverted from that of the PWM signal output from the U-H terminal from the U-L terminal (hereinafter, complementary PWM signal). Thus, in the A period, the second terminal of the U-phase coil73is connected to the power supply terminal66while the PWM signal output from the U-H terminal is a high level and connected to the ground terminal67via the resistance63while the PWM signal is a low level. In other words, in the A period, the U-phase coil73is alternately set to the first state and the second state. Note that a setting period (third setting period) from when the U-phase coil73is set in the first state to when the U-phase coil73is set in the second state corresponds to a high-level period of the PWM signal output from the U-H terminal. As illustrated inFIG.6, this setting period increases as time passes in the A period and then decreases.

Also, in the A period, the microcomputer51outputs low level (duty cycle of 0%) from the V-H terminal and the W-H terminal. Also, though not illustrated, in the A period, the microcomputer51outputs high level (duty cycle of 100%) from the V-L terminal. Thus, the second terminal of the V-phase coil74connects to the ground terminal67via the resistance63. Also, though not illustrated, the microcomputer51outputs low level from the W-L terminal. In other words, in the A period, the V-phase coil74is set to the second state, and the W-phase coil75is set to the third state.

In the B period (second period) following the A period, the microcomputer51outputs a PWM signal that changes the high-level duty cycle to a sine wave from the V-H terminal and the complementary PWM signal for the PWM signal output from the V-H terminal from the V-L terminal. In other words, in the B period, the V-phase coil74is alternately set to the first state and the second state. Note that a setting period (first setting period) from when the V-phase coil74is set in the first state to when the V-phase coil74is set in the second state corresponds to a high-level period of the PWM signal output from the V-H terminal. As illustrated inFIG.6, this setting period increases as time passes in the B period and then decreases. Note that in the B period, the microcomputer51outputs low level from the U-H terminal and, though not illustrated, high level from the U-L terminal. In other words, in the B period, the U-phase coil73is set to the second state.

Also, though not illustrated, in the B period, the microcomputer51outputs low level from the W-L terminal. Also, as illustrated inFIG.6, in the B period excluding the C period, the microcomputer51outputs low level from the W-H terminal. In the C period (third period), the microcomputer51outputs a PWM signal that changes the duty cycle to a sine wave from the W-H terminal. In other words, in the B period excluding the C period, the W-phase coil75is set to the third state, and in the C period, the W-phase coil75is set alternately between the first state and the third state. Note that in the C period, a setting period (second setting period) from when the W-phase coil75is set in the first state to when the W-phase coil75is set in the third state corresponds to a high-level period of the PWM signal output from the W-H terminal. As illustrated inFIG.6, this setting period increases as time passes in the C period and then decreases.

In the A period, the coil current flows from the power supply terminal66, through the U-phase high-side switching element, the U-phase coil73, the V-phase coil74, the V-phase low-side switching element, and the resistance63for the V phase, and to the ground terminal67. Note that a portion of the coil current may also be supplied from the bulk electrolytic capacitor65. As illustrated inFIG.6, the current value of the coil current in the A period increases with a delay after an increase in the duty cycle of the PWM signal output from the U-H terminal due to the inductance of the coil. In the B period excluding the C period, the coil current flows from the ground terminal67, through the resistance63for the U phase, the U-phase low-side switching element, the U-phase coil73, the V-phase coil74, and the V-phase high-side switching element, and to the power supply terminal66. In other words, the coil current flows to the non-illustrated DC power supply via the power supply terminal66as a regenerative current. Note that a portion of the coil current flows to the bulk electrolytic capacitor65and not the power supply terminal66. In the C period, since the W-phase high-side switching element is on, a portion of the coil current flowing from the V-phase coil74toward the V-phase high-side switching element flows to the W-phase coil75via the W-phase high-side switching element and not the power supply terminal66. The current flowing to the W-phase coil75then flows to the V-phase coil74via the neutral76. The coil current ofFIG.6indicates the current value (absolute value) of the current toward the inverter60in the A period and the current value (absolute value) of the current from the inverter60toward the power supply terminal in the B period.

In this manner, in the B period for attenuating the coil current, by a portion of the coil current flowing to the non-excitation coil not included in the excitation phase, the current value of the regenerative current flowing into the DC power supply can be decreased. Thus, even in the case of a short B period for hastening the attenuation of the coil current, an increase in the regenerative current flowing to the DC power supply can be suppressed, which in turn suppresses an increase in the power supply voltage. Accordingly, the B period can be shortened to reduce the amount of time required to detect the rotor stopping position.

The duration of the A period and the maximum value of the duty cycle are determined on the basis of the necessary detection accuracy under the condition that the rotor72does not rotate. In the present example, the duration of the A period is 1 ms, and the maximum value of the duty cycle is 65%. The duration of the B period and the maximum value of the duty cycle are set so that the increase value of the power supply voltage due to the regenerative current is kept equal to or less than a predetermined value and the coil current is approximately zero at the end timing of the B period. Note that to make the coil current approximately zero at the end timing of the B period, the time integrated value of the voltage produced in the inductance component of the coil in the A period is made approximately zero. In the present example, the duration of the B period is 1 ms, the same as the A period, and the maximum value of the duty cycle is 24%.

The start timing of the C period, for example, is set to a time period in which the current value of the regenerative current becomes large. The duration of the C period and the maximum value of the duty cycle are set so that the coil current flowing to the W-phase coil75attenuates to approximately zero within the B period. In the present example, the start timing of the C period is set to 0.25 ms after the start timing of the B period, the duration is set to 0.2 ms, and the maximum value of the duty cycle is set to 70%. Note that the relationship between the elapsed time and the duty cycle in the B period may be adjusted by correcting the duty cycle coefficient described below. The maximum value of the duty cycle of the C period is determined taking into account adjustment of the duty cycle of the B period.

Note that the change in the duty cycle of the PWM signal applied to each coil, that is, the shape of the voltage waveform applied to the coil, may be a triangular wave-like shape or a trapezoidal wave-like shape as illustrated inFIGS.7A and7B. More typically, the shape of the voltage waveform applied to the coil can be one that, in each period, increases toward a maximum value and then decreases toward 0 after reaching the maximum value or at any subsequent timing. Also, the change in the duty cycle of the PWM signal applied to the coil can be a rectangular wave-like shape. In other words, in each time period, the duty cycle of the PWM signal applied to the coil can also be made constant. Also, the shapes of the changes in the duty cycles of the PWM signals in the A period, the B period, and the C period do not need to be the same.

In the present embodiment, the duty cycle data is pre-stored in the non-volatile memory55of the microcomputer51. Also, the microcomputer51outputs a PWM signal that changes the duty cycle as illustrated inFIG.6and the like on the basis of the duty cycle data.FIG.8illustrates an example of duty cycle data in a case where the duty cycle of the PWM signal is changed to a sine wave as illustrated inFIG.6. The duty cycle data is data indicating the relationship between the elapsed time and the duty cycle value in each period in a time series. #1 ofFIG.8indicates reference data for the A period and the B period, and #2 indicates reference data for the C period. A duty cycle coefficient for the A period, the B period, and the C period are stored in the non-volatile memory55, and the microcomputer51obtains the actual duty cycle value by multiplying the reference data by the duty cycle coefficient. In the present example, the duty cycle coefficient of the A period, the B period, and the C period is 0.65, 0.24, and 0.7, respectively. Since the maximum value of the duty cycle indicated by each reference data is 100, the maximum value of the duty cycle of the A period, the B period, and the C period is 65%, 24%, and 70%, respectively. Also, the microcomputer51switches the duty cycle each 50 s. Since the A period and the B are 1 ms in duration, the #1 reference data includes 20 time series values. Since the C period is 0.2 ms in duration, the #2 reference data includes 4 time series values.

The microcomputer51detects the current value of the coil current in predetermined cycles for both the A period and the B period and stores the detected current value in the memory57as a measurement value. For example, the predetermined cycle is 50 s. When the B period ends, the microcomputer51determines the maximum value (absolute value) among the measurement values stored in the memory57. As described above, the microcomputer51excites each of the six excitation phases and determines the maximum value for each excitation phase.FIG.9illustrates an example of the maximum values determined for the six excitation phases. According toFIG.9, the maximum value when the U-V phase is excited is the largest. As described above, in a case where the rotor72is stopped at the X-Y phase position, the synthetic impedance measured when the X-Y phase is excited decreases to the smallest, and the current value of the coil current measured when the X-Y phase is excited increases to the largest. Thus, in the example ofFIG.9, the stopping position of the rotor72can be determined to be the U-V phase position.

Next, a method of correcting the duty cycle coefficient will be described. The duty cycle coefficient is determined in advance so that, initially, the coil current at the end timing of the B period approaches 0 in accordance with a standard parameter of the motor15F. However, depending on variation in the characteristics of each motor15F, the optimal duty cycle coefficient may be different for each individual motor15F. Also, the optimal duty cycle coefficient may change over time due to the change over time of the motor15F. Thus, in the present embodiment, the duty cycle coefficient of when a predetermined condition is satisfied is updated and corrected. The predetermined condition may be, for example, configured to be satisfied when the image forming apparatus is powered on or when restoring from a sleep state. Also, the predetermined condition can be a periodic condition or a condition based on the number of sheets an image is formed on. Furthermore, it can be configured so that the predetermined condition is satisfied when the current value (absolute value) of the coil current at the end timing of the B period is greater than a predetermined value. Note that in the present embodiment, three different duty cycle coefficients are used for the A period, the B period, and the C period, but the duty cycle coefficient to be updated is one. In the present embodiment, the duty cycle coefficient of the B period is updated, but the duty cycle coefficient of the A period and the C period may be updated. Note that the duty cycle coefficient is updated so that the current value of the coil current at the end timing of the B period approaches 0. Hereinafter, the current value of the coil current at the end timing of the B period will be referred to as the “final value”.

For example, in a case where the final value is a negative value, that is, the coil current flows from the Y-phase coil toward the X-phase coil at the end timing of the B period with the X-Y phase excited, the duty cycle of the B period becomes too large. Thus, in such a case, the duty cycle coefficient is updated to a lower value than the current value. On the other hand, in a case where the final value is a positive value, that is, the coil current flows from the X-phase coil to the Y-phase coil with the X-Y phase excited, the duty cycle of the B period is too small. Thus, the duty cycle coefficient is updates to a larger value than the current value.

FIG.10is a flowchart of the processing for detecting the rotor stopping position executed by the motor control unit14. In S10, the motor control unit14selects the excitation phase to excite. In S11, the motor control unit14executes the measuring processing on the excitation phase selected in S10. In other words, the motor control unit14outputs the PWM signal as described usingFIGS.6and7A and7Bon the basis of the duty cycle data and the duty cycle coefficient and repeatedly measures the current value of the coil current. The motor control unit14stores the repeatedly measured current value in the memory57as measurement values. When the B period ends, in S12, the motor control unit14determines the maximum value of the current values repeatedly measured. In S13, the motor control unit14waits until the current value (absolute value) of the coil current becomes equal to or less than a threshold. When the current value of the coil current becomes equal to or less than the threshold, in S14, the motor control unit14determines whether or not measurement for all of the six excitation phases has been performed. If not, the processing is repeated from S10. On the other hand, in a case where the measurement for all six excitation phases has been performed, in S15, the motor control unit14determines the stopping position of the rotor72on the basis of the maximum value of the coil current measured for each excitation phase.

Note that the processing ofFIG.10is merely an example, and the processing for detecting the rotor stopping position according to the present embodiment is not limited to following the flowchart illustrated inFIG.10. For example, determining the maximum value of the excitation phases (S12) can be performed after the measurement of all of the excitation phases has been completed. Also, when the B period for excitation of an excitation phase has been completed, the determination of S14may be performed, and in a case where “No” is determined in S14, the motor control unit14waits until the coil current becomes the threshold (S13) and repeats the processing from S10.

FIG.11is a flowchart of the processing for updating the duty cycle coefficient executed by the motor control unit14. Note that the processing ofFIG.11relates to updating the duty cycle coefficient for one excitation phase, and the motor control unit14repeatedly executes the processing ofFIG.11for the six excitation phases. In S20, the motor control unit14outputs the PWM signal as described usingFIGS.6and7A and7Bon the basis of the duty cycle data and the duty cycle coefficient. Note that for the duty cycle coefficient used in S20, the value at this point in time is used. In S21at the end timing of the B period, the motor control unit14stores the final value of the coil current in the memory57as a first determined value. In S22, the motor control unit14waits until the current value (absolute value) of the coil current becomes equal to or less than a threshold.

When the current value (absolute value) of the coil current becomes equal to or less than a threshold, in S23, the motor control unit14outputs the PWM signal as described usingFIGS.6and7A and7Bon the basis of the duty cycle data and a first corrected duty cycle coefficient. The first corrected duty cycle coefficient is, from among the current duty cycle coefficients of the A period, the B period, and the C period, the duty cycle coefficient of the B period to be updated corrected to a value less than the current value. For example, the motor control unit14corrects the duty cycle coefficient of the B period by multiplying the current duty cycle coefficient of the B period by an adjustment value less than 1, for example, 0.95. In S24at the end timing of the B period, the motor control unit14stores the final value of the coil current in the memory57as a second determined value. In S25, the motor control unit14waits until the current value (absolute value) of the coil current becomes equal to or less than a threshold.

When the current value (absolute value) of the coil current becomes equal to or less than a threshold, in S26, the motor control unit14outputs the PWM signal as described usingFIGS.6and7A and7Bon the basis of the duty cycle data and a second corrected duty cycle coefficient. The second corrected duty cycle coefficient is, from among the current duty cycle coefficients of the A period, the B period, and the C period, the duty cycle coefficient of the B period to be updated corrected to a value greater than the current value. For example, the motor control unit14corrects the duty cycle coefficient of the B period by multiplying the current duty cycle coefficient of the B period by an adjustment value greater than 1, for example, 1.05. In S27at the end timing of the B period, the motor control unit14stores the final value of the coil current in the memory57as a third determined value.

Thereafter, in S28, the motor control unit14updates the duty cycle coefficient of the B period on the basis of the first determined value to the third determined value so that the coil current approaches 0 at the end timing of the B period. For example, the motor control unit14obtains the value of the duty cycle coefficient of the B period used in S20, S23, and S26and the relationship between the value of the duty cycle coefficient of the B period and the final value on the basis of the first determined value to the third determined value. The relationship between the duty cycle coefficient of the B period and the final value can be obtained via linear interpolation of the three determined values, for example. Also, the motor control unit14obtains a value of a duty cycle coefficient for making the current value of the coil current 0 at the end timing of the B period on the basis of the obtained relationship and stores this in the non-volatile memory55as the updated value for the duty cycle coefficient of the B period.

Note that the processing for updating the duty cycle coefficient ofFIG.11can be executed as processing independent of the processing for detecting the rotor stopping position ofFIG.10, but the first determined value of S21ofFIG.11can be obtained via the processing for detecting the rotor stopping position ofFIG.10. In other words, the current value of the coil current measured at the end timing of the B period in S11ofFIG.10can be used as the first determined value. In this case, only the processing from S23is executed for the processing for updating the duty cycle coefficient ofFIG.11, and the first determined value uses the latest value determined in the processing for detecting the rotor stopping position.

In this manner, according to the present embodiment, a portion of the regenerative current flows to the non-excitation coil. According to this configuration, even when the inverter60is controlled so that the attenuation time of the coil current is hastened, an increase in the power supply voltage can be suppressed. Thus, by performing control so that the attenuation time of the coil current is hastened and the amount of time required for the measuring processing of each excitation phase is reduced, the amount of time required for rotor stopping position detection can be reduced.

Second Embodiment

Next, a second embodiment will be described, focusing on the points that differ from the first embodiment. In the present embodiment, the change over time of the duty cycle of the PWM signal is different from that of the first embodiment.FIG.12illustrates the change over time according to the present embodiment of the duty cycle of the PWM signal output from the PWM port58in the U-V phase measuring processing. Note that the A period as in the first embodiment.

In the present embodiment, the B period is 0.5 ms, and the duty cycle of the PWM signal output from the V-H terminal changes in an M-like shape as illustrated inFIG.12. In other words, when the B period starts, the first setting period from when the V-phase coil74is set from the first state to the second state is set to the maximum value. Thereafter, the first setting period decreases with the passage of time. Then, from a timing until the B period ends, the first setting period increases with the passage of time. Note that the timing of when the first setting period changes from decreasing to increasing can be, for example, the timing at which half of the B period has elapsed from the start of the B period. By performing control in this manner, the regenerative current flowing out from the second terminal of the V-phase coil74via the V-phase high-side switching element increases after the start of the B period. This can cause the voltage of the bulk electrolytic capacitor65to quickly increase. Thereafter, since the first setting period decreases as the voltage of the bulk electrolytic capacitor65increases, the regenerative current to the DC power supply is not increased. Also, the duty cycle increases as the coil current decreases. With this configuration, the change over time of the regenerative current flowing into the DC power supply in the B period can be reduced, allowing for an increase in the voltage of the DC power supply to be suppressed.

Note that in the C period, the change over time of the duty cycle of the PWM signal output from the W-H terminal is set so that the coil current flowing in the W-phase coil75attenuates to 0 within the B period. In the present example, as illustrated inFIG.12, the C period is a period of 0.1 ms that starts at the same time as the B period, and the duty cycle of the PWM signal output from the W-H terminal changes in a rectangular wave-like shape. In other words, in the C period, the second setting period from when the W-phase coil75is set to the first state to when the W-phase coil75is set to the third state is constant. #1, #2, and #3 ofFIG.13indicate the duty cycle data of the A period, the B period, and the C period, respectively.

With this configuration, the change over time of the regenerative current in the B period can be reduced, allowing for an increase in the voltage of the DC power supply to be suppressed. Thus, by performing control so that the attenuation time of the coil current is hastened and the amount of time required for the measuring processing of each excitation phase is reduced, the amount of time required for rotor stopping position detection can be reduced.

OTHER EMBODIMENTS

This application claims the benefit of Japanese Patent Application No. 2023-132044, filed Aug. 14, 2023, which is hereby incorporated by reference herein in its entirety.