Method for detecting the field position of an electric motor

Abstract

To provide a field position detection method for a motor which can be operated while surely detecting a field position of the motor during the operation even if a duty ratio of a drive voltage is changed when operating a three-phase brushless motor with 120° electrification, under PWM control and in a sensor-less drive manner.SOLUTION: In the case of operation while changing a duty ratio of a drive voltage to be applied to a three-phase coil via an inverter circuit 52, an MPU 51 selects a duty ratio optimal for sensing a non-electrified phase coil voltage at a predetermined drive voltage. In the case of driving with the optimal duty ratio in an electrification phase of one cycle, the drive voltage is applied in an on-duty phase of the optimal duty ratio. When applying a drive voltage based on a duty ratio exceeding the optimal duty ratio, the on-duty phase of one cycle is divided into a plurality of on-duty phases including the optimal duty ratio and the drive voltage is applied.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to a method for detecting the field position of an electric motor when the electric motor is driven at low speed by sensorless driving. [Background technology]

[0002] Traditionally, brushed DC motors have been used for small direct current motors, but problems with brush noise, electrical noise, durability, etc. have led to the emergence of brushless DC motors.More recently, sensorless motors, which do not have position sensors, have been attracting attention from the perspective of being smaller, lighter, more robust, and less expensive, and were first adopted in hard disk drives and other devices in the information equipment field, but with the development of vector control technology, they have also begun to be used in home appliances and automotive applications.

[0003] In sensorless drive, the rotor position is detected from the induced voltage, but since no induced voltage occurs when the motor is stationary, the rotor position cannot be determined and starting is not possible.To detect the rotor position when the motor is stationary, one method is to provide a coil current sensor and current detection circuit, and use an inverter to pass a sine wave coil current through the coil using PWM drive, thereby estimating the position from the current response. The rotor position when stationary can be detected from the inductance deviation using the methods described above. Alternatively, the rotor can be rotated by forced commutation without position sensing, and the position can be determined.

[0004] However, once starting begins, the motor begins to rotate, making it difficult to detect rotor position from inductance deviation using sensing pulses. For example, detecting inductance deviation by superimposing a high-frequency current on the excitation current is considered, but this requires extensive hardware and software. Furthermore, the effects of magnetic saturation and induced voltage must be considered, as well as factors that are difficult to estimate, such as inherent errors in the motor and drive circuit. For this reason, the ramp-start method, which forcibly positions the rotor using fixed excitation without position detection and then gradually increases the rotation speed while achieving synchronization, is widely used. However, this method requires a long time to position the rotor and can even cause reverse rotation. Furthermore, synchronization is achieved using open-loop control, which requires time for acceleration and is prone to loss of synchronization due to load fluctuations. To avoid this, a large current is used for starting, reducing efficiency and requiring a large DC power supply. Load fluctuations cause loss of synchronization, limiting its applications. It cannot be used for reciprocating mechanisms, externally driven rotation, or applications with viscous or fluctuating loads.

[0005] Therefore, as a field position detection method that aims to reduce costs using simple hardware and software and can detect the rotor position in excitation interval units at 120° current conduction without generating sensing noise at startup, a method has been proposed in which, when a voltage is applied to a three-phase brushless motor driven at 120° current conduction, the induced voltage generated in the non-current conducting phase changes depending on the position of the permanent magnet field (rotor) to detect the rotor position (Patent Document 1; JP 2019-17235 A). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-17235 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when the motor is energized with 120° energization, the induced voltage generated in the non-energized phase changes depending on the magnitude of the drive voltage applied to the motor. In particular, in the case of PWM drive, it changes depending on the magnitude of the duty ratio (energization time) of the applied voltage. 12 is a graph showing the change in induced voltage generated in the non-energized phase for each duty ratio of a certain motor according to the rotor rotation position. This graph shows that the magnitude of the induced voltage changes significantly depending on the magnitude of the duty ratio. Fig. 13 shows an example of a drive voltage waveform diagram when the duty ratio is changed in PWM drive. In the case of the motor shown in Fig. 12, when the duty ratio is 20%, the change in the positive and negative induced voltages generated in the non-energized phase is noticeable and easy to measure, but at other duty ratios, the level of the induced voltage decreases, making the voltage change slow and difficult to measure.

[0008] In order to solve this problem, Patent Document 1 proposes a method of multiplying a correction coefficient according to the duty ratio. However, depending on the motor, even if the induced voltage is multiplied by a correction coefficient, position detection becomes difficult at low or high duty ratios above a certain level. Therefore, only a limited range of duty ratios can be used. This limits the control range of the motor torque, making it impossible to achieve the desired rotation speed and torque of the motor. [Means for solving the problem]

[0009] The present invention has been made to solve these problems, and its object is to provide a method for detecting the field position of an electric motor that can reliably detect the field position of the electric motor and enable low-speed operation when a three-phase brushless motor is operated at 120-degree conduction using PWM control and sensorless drive, even if the duty ratio of the drive voltage is changed.

[0010] a control means for storing current-angle information and current-pattern information for each 60° current-carrying section in which continuous rotation is possible, and for switching the current-carrying state by controlling the switching of the output means; and a measurement means for performing A / D conversion of the three-phase coil voltages and sending the A / D conversion to the control means, wherein the control means periodically performs 120° current-carrying through the output means, the current-carrying section having a current-carrying pattern in which the position at which self-excitation is stopped by two-phase fixed current-carrying through the output means coincides with the start position of a 60° current-carrying section, and the measurement means measures current-carrying phase voltages and non-current-carrying phase voltages in the on-cycle of PWM current-carrying, thereby detecting the field position of the motor, thereby performing sensorless driving of the motor, the control means measuring the current-carrying phase voltages and non-current-carrying phase voltages in the on-cycle of PWM current-carrying, the control means varying the duty ratio of the predetermined drive voltage applied to the three-phase coils through the output means, , at a given drive voltage The optimum duty ratio for sensing the non-energized phase coil voltage is selected, and when driving at the optimum duty ratio during the energization period of one cycle, a drive voltage is applied during the on-duty period of the optimum duty ratio, and when applying a drive voltage based on a duty ratio exceeding the optimum duty ratio, the on-duty period of one cycle is divided into a plurality of on-duty periods including the optimum duty ratio and the drive voltages are applied.

[0011] a control means for storing current-angle information and current-pattern information for each 60° current-carrying section in which continuous rotation is possible, and for switching the current-carrying state by controlling the switching of the output means; and a measurement means for performing A / D conversion of the three-phase coil voltages and sending the A / D conversion to the control means, wherein the control means periodically performs 120° current-carrying through the output means, the current-carrying section having a current-carrying pattern in which the position at which self-excitation is stopped by two-phase fixed current-carrying through the output means coincides with the start position of a 60° current-carrying section, and the measurement means measures current-carrying phase voltages and non-current-carrying phase voltages in the on-cycle of PWM current-carrying, thereby detecting the field position of the motor, thereby performing sensorless driving of the motor, the control means measuring the current-carrying phase voltages and non-current-carrying phase voltages in the on-cycle of PWM current-carrying, the control means varying the duty ratio of the predetermined drive voltage applied to the three-phase coils through the output means, , at a given drive voltage The optimum duty ratio for sensing the non-energized phase coil voltage is selected, and when driving at the optimum duty ratio during the energization period of one cycle, a drive voltage is applied during the on-duty period of the optimum duty ratio, and when applying a drive voltage based on a duty ratio that does not exceed the optimum duty ratio, the time of one cycle is extended or a drive voltage including the optimum duty ratio is applied over multiple cycles.

[0012] In this way, when changing the duty ratio of the drive voltage that PWM energizes the three-phase coil of a sensorlessly driven motor, if the drive is to be performed at an optimal duty ratio during the current conduction period of one cycle, the drive voltage is applied during the on-duty period of that optimal duty ratio; if a drive voltage based on a duty ratio exceeding the optimal duty ratio is to be applied, the on-duty period of one cycle is divided into multiple on-duty periods including the optimal duty ratio and the drive voltage is applied; if a drive voltage based on a duty ratio that does not exceed the optimal duty ratio is to be applied, the time of one cycle is extended or a drive voltage including the optimal duty ratio is applied over multiple cycles. As a result, when changing the duty ratio of the drive voltage applied by PWM to the three-phase coils of a sensorless-driven motor, regardless of which controllable duty ratio is selected, it becomes possible to detect the field position of the motor during the low-speed drive period from the non-energized phase coil voltage by applying a drive voltage based on at least the optimal duty ratio, thereby enabling smooth continuous operation of the motor at low speeds. [Effects of the Invention]

[0013] When operating an electric motor at low speeds without a sensor using PWM control with 120-degree energization, a method for detecting the field position of the electric motor can be provided, which can reliably detect the field position of the electric motor and operate it at low speeds even when energizing with a changed duty ratio. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 10 is a waveform diagram of inductance and voltage of a non-energized phase coil during UV excitation. [Figure 2] FIG. 10 is a waveform diagram of inductance and voltage of a non-energized phase coil during UW excitation. [Figure 3] FIG. 10 is a waveform diagram of inductance and de-energized phase coil voltage during VW excitation. [Figure 4] FIG. 10 is a waveform diagram of inductance and voltage of a non-energized phase coil during VU excitation. [Figure 5] FIG. 10 is a waveform diagram of inductance and voltage of a non-energized phase coil during WU excitation. [Figure 6] FIG. 10 is a waveform diagram of inductance and non-energized phase coil voltage during WV excitation. [Figure 7] FIG. 10 is a diagram showing the measured waveform of a non-energized phase coil voltage. [Figure 8] FIG. 1 is a block diagram of a drive circuit for a three-phase DC brushless motor. [Figure 9] This is a diagram showing the configuration of a star-connected three-phase brushless DC motor. [Figure 10] FIG. 10 is a drive voltage waveform diagram for each duty ratio in a two-cycle conduction section in a timing chart of 120° conduction. [Figure 11] FIG. 10 is a drive voltage waveform diagram for each duty ratio in a two-cycle conduction section in a timing chart of 120° conduction. [Figure 12] FIG. 10 is a waveform diagram of an induced voltage induced in a non-energized phase coil voltage when the duty ratio is changed in PWM drive. [Figure 13] FIG. 10 is a diagram showing the waveform of a driving voltage when the duty ratio is changed in PWM driving. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of a method for detecting the field position of an electric motor according to the present invention will be described with reference to the accompanying drawings. The present invention will be described using, as an example of an electric motor, a sensorless motor having a permanent magnet field in the rotor, windings in the stator arranged with a phase difference of 120° in a star connection, and phase ends connected to a motor output means.

[0016] As an example, a method for detecting the permanent magnet field position of a sensorless motor that drives a three-phase DC brushless motor without using a sensor will be described below, along with the configuration of a sensorless motor drive device. An embodiment of a three-phase brushless DC motor according to the present invention will be shown with reference to FIG. 9. As an example, a three-phase brushless DC motor equipped with a two-pole permanent magnet rotor and a stator 4 with three slots will be shown. The motor may be either an inner rotor type or an outer rotor type. Furthermore, the permanent magnet field may be either an interior permanent magnet (IPM) motor or a surface permanent magnet (SPM) motor.

[0017] In Figure 9, a rotor 2 is integrally provided on a rotor shaft 1, and a two-pole permanent magnet 3 is provided as a field magnet. Pole teeth U, V, and W are arranged on a stator 4 with a phase difference of 120°, facing the permanent magnet 3. Windings u, v, and w are provided on each of the pole teeth U, V, and W of the stator 4, and the phases are star-connected with a common C, forming a three-phase brushless DC motor that is wired to a motor drive device (described later). Note that the common wire is omitted because it is not necessary.

[0018] Next, an example of a drive circuit for a three-phase DC brushless motor is shown in FIG. The drive method assumed at startup is 120° energization bipolar square wave excitation. The MOTOR is a three-phase sensorless motor. The MPU 51 is a microcontroller (control means). The MPU 51 stores six energization patterns for the three-phase coils (U, V, W) and field position information that specifies the 120° energization excitation switching intervals (intervals 1 to 6) corresponding to each energization pattern, and controls the switching of the output means in response to the rotation command RUN from the upper controller 50 to arbitrarily switch the excitation state.

[0019] The inverter circuit 52 (INV: output means) energizes the three-phase coils and performs switching operations such as excitation phase switching or PWM control to control the motor torque. The inverter circuit 52 includes diodes connected in antiparallel to the switching elements, and is provided with half-bridge switching circuits for three phases that can be arbitrarily connected to the positive power supply line and the ground power supply line. The A / D conversion circuit 53 (ADC: measuring means) is connected to the coil output terminals U, V, and W, and simultaneously samples the coil voltages of each of the three phases in response to a conversion start signal from the MPU 51, sequentially converts them from analog to digital, and sends the conversion results to the MPU 51. The ADC 53 is normally built into the MPU 51, and when using the built-in ADC 53, it is desirable to provide a voltage divider circuit using resistors because the maximum input voltage is low. In this way, according to this proposal, the drive circuit can be configured very simply.

[0020] The inductance change (space harmonics) due to rotor angle θ is approximated as ΔL = -cos(2θ), and is known to have two periods per electrical angle.On the other hand, when two-phase current is applied to a three-phase coil using square wave PWM current, it is known that two-period voltage fluctuations centered on the neutral point potential according to θ are observed in the non-current-carrying phases.

[0021] Figure 1 shows the theoretical waveforms of the voltage change waveform ΔVw of the non-energized phase, the inductance changes (ΔLu, ΔLv) of the U and V phases, and the combined inductance change ΔLu-v of the two phases when rotated by one electrical angle while UV excitation is performed with PWM energization. Note that the voltage change waveform has the polarity of the combined inductance change waveform reversed, and swings positive and negative around a neutral point potential that is half the voltage applied to the coil.

[0022] Figure 7A shows the measured waveform of the coil voltage of the non-energized phase using an inner rotor motor. The theoretical voltage waveform of the non-energized phase was assumed to be reverse polarity, reflecting the inductance, and the waveform is nearly identical, demonstrating that this assumption is correct. Furthermore, when using square-wave energization, ringing occurs in the induced voltage, but measurements have shown that the ringing time is very short, converging within the measurement error range of several microseconds to several tens of microseconds for various motors. This means that the induced voltage can be accurately detected even with square-wave PWM energization pulses used to drive the motor.

[0023] When a large current flows through the three-phase coil, magnetic saturation occurs and the inductance stops changing, which is particularly noticeable in small outer rotor motors. When magnetic saturation occurs, the two-periodic inductance change waveform retains its peak and bottom adjacent to the setup position where self-excitation stops due to two-phase fixed current flow, but the other peak and bottom disappear, resulting in a single period. Figure 7B shows an example of an inductance waveform that has become periodic due to magnetic saturation. The motor used for the measurement was a small outer rotor motor, which is different from the motor used in Figure 7A. The setup position when UV is energized is 150°, and in the ΔVw waveform, only the peak and bottom adjacent to the setup position are clearly observed.

[0024] The setup position where self-excitation stops due to two-phase fixed current is the inductance zero cross point and also the induced voltage zero cross point, and the setup point and the adjacent peaks and troughs are stable against magnetic saturation. As can be seen from Figures 7A and 7B, the voltage fluctuations of the non-energized phase reflect the rotor angle θ, and moreover, monotonicity is guaranteed within the interval, so even when no induced voltage is generated at rest, it is possible to estimate the rotor position by passing an excitation current. The voltage fluctuation range is 10% or more of the voltage applied to the coil, and is on the order of several volts, which is overwhelmingly advantageous when considering that conventional methods detect induced voltages on the order of millivolts at start-up.

[0025] As explained above, by detecting inductance changes in the non-energized phase coil voltage using square wave PWM control for motor drive and using only the inductance changes near the setup position, stable position detection is possible from a stationary state to low speed rotation ranges. This simplifies the sensing procedure, improves efficiency as no power is required for sensing, and reduces noise as no sensing noise is generated.

[0026] The angle and conduction pattern for each conduction section of 120° conduction is summarized in the table below. In the table, CW conduction is a conduction pattern in which the angle rotates in the direction that increases, and CCW conduction is a conduction pattern in which the angle rotates in the direction that decreases. Setup conduction is a conduction pattern in which self-excitation stops at the angle shown in parentheses in the box in the table, and both the start and end points are listed for each section. For each conduction pattern, the phase connected to the + power supply side is listed first, and the phase connected to the GND side is listed after a hyphen.

[0027] (The rest is blank) [Table 1]

[0028] The rotation direction in which the section number in Table 1 increases is defined as CW, and the rotation direction in which it decreases is defined as CCW. The section end position is the boundary point with the adjacent + side section when rotating in the CW direction, and the boundary point with the - side section when rotating in the CCW direction. For example, in the case of section 1, the boundary point with section 2 is 90° when rotating in the CW direction, and the boundary point with section 6 is 30° when rotating in the CCW direction.

[0029] In Figure 1, the start point of the energized section during clockwise rotation with UV excitation is shown as Point A, and the end point of the energized section is shown as Point B. The setup point is Point C, and the bottom section where Point B is located has a stable phase and can be used for position detection. Therefore, threshold values ​​Vth on the positive and negative sides with a specified potential difference from the neutral point potential are set in advance, and the non-energized phase coil voltage ΔVw is compared with the threshold value Vth for each measurement, and if the threshold value is exceeded, it can be detected that the end point of the section has been exceeded. Refer to Figure 4, as VU current is applied during CCW rotation. The rotor rotates from the 90° electrical angle side towards the 30° electrical angle side. Therefore, the end point of the interval is at an electrical angle of 30°. Because the setup point is at an electrical angle of 330°, the bottom part on the 30° electrical angle side has a stable phase and can be used for position detection. Therefore, just as with CW rotation, the end point of the interval can be detected by comparing the voltage of the non-energized phase W with the threshold value Vth.

[0030] In section 2, which is from an electrical angle of 90° to an electrical angle of 150°, UW excitation is selected. Figure 2 shows the change in inductance and the change in the coil voltage of the non-energized phase during UW excitation. The waveform is shifted 60° and the polarity is inverted from that of Figure 1, the non-energized phase is the V phase, and the setup position C point is an electrical angle of 210°. When the rotor is in section 2 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 150 electrical degrees. If point B is detected, switching to section 3 will enable continuous rotation.

[0031] In section 3, which is from electrical angle 150° to electrical angle 210°, VW excitation is selected. Figure 3 shows the change in inductance and the change in the coil voltage of the non-energized phase during VW excitation. The waveform is shifted by 60° and the polarity is inverted from that of Figure 2, the non-energized phase is the U phase, and the setup position, point C, is at an electrical angle of 270°. When the rotor is in section 3 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 210 electrical degrees. If point B is detected, switching to section 4 will enable continuous rotation.

[0032] In section 4, which is from electrical angle 210° to electrical angle 270°, VU excitation is selected. Figure 4 shows the change in inductance and the change in the coil voltage of the non-energized phase during VU excitation. The waveform is shifted by 60° and the polarity is inverted from that of Figure 3, the non-energized phase is the W phase, and the setup position, point C, is at an electrical angle of 330°. When the rotor is in section 4 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 270 electrical degrees. If point B is detected, switching to section 5 will enable continuous rotation.

[0033] In section 5, which is from electrical angle 270° to electrical angle 330°, WU excitation is selected. Figure 5 shows the change in inductance and the change in the coil voltage of the non-energized phase during WU excitation. The waveform is shifted by 60° and the polarity is inverted from that of Figure 4, the non-energized phase is the V phase, and the setup position, point C, is at an electrical angle of 30°. When the rotor is in section 5 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 330 electrical degrees. If point B is detected, switching to section 6 will enable continuous rotation.

[0034] In section 6, which is from electrical angle 330° to electrical angle 30°, WV excitation is selected. Figure 6 shows the change in inductance and the change in the coil voltage of the non-energized phase during WV excitation. The waveform is shifted by 60° and the polarity is reversed from that of Figure 5, the non-energized phase is the U phase, and the setup position, point C, is at an electrical angle of 90°. When the rotor is in section 6 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 30 electrical degrees. If point B is detected, switching to section 1 will enable continuous rotation.

[0035] In this way, the section end point located at the peak or bottom portion adjacent to the setup position can be detected using a preset threshold. When the de-energized phase coil voltage exceeds the threshold, the section number is increased by +1 for clockwise rotation and decreased by -1 for counterclockwise rotation, allowing for continuous rotation.

[0036] As before, the rotation direction in which the section number increases is defined as CW, and the rotation direction in which the section number decreases is defined as CCW. The section start position is the boundary point with the adjacent - (negative) side section when CW, and the + (positive) side section when CCW. For example, in the case of energized section 1, the boundary point with energized section 6 is 30° when CW, and the boundary point with energized section 2 is 90° when CCW.

[0037] In Figure 1, the start point of the section when rotating in the CW direction with UV energization is shown as point A. During normal operation, the motor rotates in the desired direction, so there is no need to detect the start point of the section, but when an external force is causing the motor to rotate at low speed in the opposite direction to the desired direction, start point detection is required to switch excitation correctly. When rotating at high speeds, it is necessary to apply the brakes to slow down the motor, so it is thought that detection of the start point position is only necessary when rotating at low speeds. When rotating in the opposite direction, induced voltage becomes a problem. In Figure 1, the induced voltage of the non-energized phase W has a zero-cross point at the electrical angle of 60° in the middle of the interval, and when rotating in the forward direction, the gradient due to the inductance change and the gradient of the induced voltage match, making it possible to reliably detect the end of the interval. However, when rotating in the reverse direction, the gradients of the two are opposite, and the waveform due to the inductance change cancels out, making it difficult to detect the start of the interval. Furthermore, in the case of a motor that has a periodicity due to magnetic saturation, it is nearly impossible to detect the start of the interval.

[0038] Therefore, if we focus on point C, the setup position for WU excitation in Figure 5, point C passes through an electrical angle of 30°, so by comparing the neutral point potential with the non-energized phase V-phase voltage ΔVv, if ΔVv becomes smaller than the neutral point potential, it can be detected that the electrical angle has exceeded 30° and that the rotor has rotated toward section 6. Therefore, if UV excitation is performed in section 1 and the non-energized phase V-phase voltage is measured by momentarily switching to WU excitation, it is possible to determine whether the electrical angle is just before or has passed 30°. If measurements are repeated periodically until the electrical angle passes 30°, the start of the section, i.e., the excitation switching position, can be detected.

[0039] When detecting the section start point electrical angle of 90° during CCW rotation, the 90° electrical angle position can be detected by performing WV excitation with reference to Figure 6 and measuring the voltage of the non-energized U phase, just as when rotating in the CW direction. Around the setup positions of 30° electrical angle or 90° electrical angle, the voltage change gradient is steep, making it easy to determine whether the voltage is positive or negative, and there is little phase shift, so position detection can be performed reliably. Although it is small, some power is consumed for sensing, so it is desirable to lengthen the sensing cycle.

[0040] Similarly, for current conduction sections 2 to 6, the start of the section can be detected by selecting the current conduction pattern that serves as the setup point and periodically detecting the inductance zero crossing points. If the start point is detected, it means that the motor is rotating in the reverse direction, so if you reverse the section number, continuous rotation will be possible.

[0041] If the rotor is rotating in the opposite direction to the desired direction at an extremely low speed due to an external force, it is necessary to detect the start of the interval and switch excitation in order to return to normal rotation. The start of the interval can be detected by setting a start threshold. For example, in Figure 1, if the motor is located in section 1, the potential at point A can be set as the start-point threshold. Similarly, for sections 2 to 6 in Figures 2 to 6, the potential at point A for each section can be set as the start-point threshold. Therefore, a start-point threshold Vth2 with a predetermined potential difference from the neutral point potential is set in advance, and the non-energized phase coil voltage ΔV is compared with the start-point threshold Vth2 for each measurement. If the start-point threshold is exceeded, it can be detected that the section start point has been exceeded. The gradient of the non-energized phase coil voltage ΔV can also be determined, and if the gradient is opposite to that during forward rotation, it can be detected that the motor is in a reverse rotation state.

[0042] Therefore, if the start point is detected while the rotor is in reverse rotation, the excitation interval can be reversed by one interval, generating forward rotation torque, i.e., braking, to suppress reverse rotation and return to forward rotation. However, because the polarity of the induced voltage during reverse rotation is opposite to that during forward rotation, the de-energized phase coil voltage ΔV at the start of the interval is smaller than during forward rotation and does not exceed the start-point threshold Vth2. In this case, it is possible to estimate the induced voltage through calculation and correct the start-point threshold Vth2. Alternatively, start-point detection may be limited to extremely low rotation speeds where errors due to induced voltage can be ignored.

[0043] This method allows the start point to be detected in the drive excitation state without any special field position detection excitation. Therefore, there is no reduction in current conduction efficiency and no electromagnetic noise caused by sensing current conduction. In addition, by detecting the start point, it is possible to apply the brakes from the reverse state and return to forward rotation.

[0044] Changes in the non-energized phase coil voltage are due to inductance when the rotor is stationary, but when the rotor rotates, an induced voltage is superimposed, and in the case of PWM control, changing the duty ratio can cause the induced voltage to change significantly, making it difficult to detect the field position within the measurement interval. Therefore, the following method of detecting the field position of the motor is adopted.

[0045] When operating the rotor 2 by changing the duty ratio of a predetermined drive voltage applied to the three-phase coil to accelerate its rotation, an optimum duty ratio for sensing the non-energized phase coil voltage is selected for the on-duty section, and the on-duty section in one energized period is divided into multiple on-duty sections including the optimum duty ratios, and the drive voltage is applied.

[0046] 10 and 11 are drive voltage waveform diagrams for each duty ratio in two energization periods in a timing chart for 120° energization. As shown in Fig. 10, MPU 51 selects a duty ratio (optimum duty ratio; for example, 20%) suitable for sensing the on-duty period of one energization period through inverter circuit 52, enabling detection of the non-energized phase coil voltage. When applying a drive voltage based on a duty ratio exceeding this optimal duty ratio, MPU 51 divides one on-duty period into multiple energization periods including the optimal duty ratio and applies the drive voltage. When applying a drive voltage based on a duty ratio that does not exceed the optimal duty ratio, MPU 51 extends the duration of one period or applies a drive voltage including the optimal duty ratio over multiple periods.

[0047] To explain a specific example, in Figure 10, the predetermined drive voltage with a duty ratio of 20% shown in Figure 10B is set as the optimal duty ratio suitable for sensing that can detect the non-energized phase coil voltage. For example, if one cycle of PWM control is 25 μsec, the on-duty interval with an optimal duty ratio of 20% is 5 μsec. This on-duty interval of 5 μsec will be referred to hereafter as the optimal on-duty interval. By applying the predetermined drive voltage to this optimal on-duty interval at other duty ratios, it becomes easier to measure changes in the positive and negative induced voltages generated in the non-energized phase coil even at duty ratios other than the optimal duty ratio.

[0048] First, let us consider an example in which the motor is driven at a duty ratio of 10%, which does not exceed the optimal duty ratio of 20%. As mentioned above, if the optimal duty ratio is 20% and the optimal on-duty interval is 5 μsec, then if the duty ratio is 10%, and one cycle is also 25 μsec, the on-duty interval will be 2.5 μsec. Therefore, it is not possible to apply a drive voltage with an optimal on-duty interval of 5 μsec. Therefore, as shown in Figure 10A, a duty ratio of 10% can be achieved by doubling the time of one cycle from 25 μsec to 50 μsec and applying a drive voltage that includes the optimal on-duty interval of 5 μsec at the optimal duty ratio of 20%. Note that while Figure 10A shows an example of extending the time of one cycle, this can also be achieved by applying a drive voltage that includes the optimal on-duty interval of 5 μsec at the optimal duty ratio of 20% over multiple cycles. For example, if this is to be achieved in two cycles, a control operation may be repeated in which a drive voltage with a duty ratio of 20% is applied for an optimum on-duty interval of 5 μsec in the first cycle, and the drive voltage is set to zero in the second cycle.

[0049] Next, we will explain an example of driving a motor at a duty ratio exceeding the optimal on-duty period of 5 μsec when the duty ratio is 20%, which is optimal for sensing. In this case, the on-duty period of one cycle is divided into multiple sections, with a drive voltage including a 5 μsec on-duty period with an optimal duty ratio of 20% and a drive voltage for an on-duty period based on a duty ratio exceeding the optimal duty ratio of 20%. For example, when driving the motor by increasing the duty ratio from the optimal duty ratio of 20% to 30%, a drive voltage is applied during the 5 μsec optimal on-duty period including the optimal duty ratio of 20% during the conduction period of one cycle, followed by an off-duty period and a 2.5 μsec on-duty period with a duty ratio of 10%, which is the portion exceeding the optimal duty ratio of 20%. In other words, the total on-duty period is 7.5 μsec, which is the sum of the optimal on-duty period of 5 μsec and the on-duty period of 2.5 μsec that exceeds the optimal on-duty period of 5 μsec. Since one cycle is 25 μsec, the total on-duty period of 7.5 μsec has a duty ratio of 30%.

[0050] Furthermore, when the motor is controlled to be driven with a duty ratio increased to 50%, a drive voltage is applied for an optimal on-duty section of 5 μsec that includes an optimal duty ratio of 20% in one conduction period, followed by an off-duty section and then a drive voltage for an on-duty section of 7.5 μsec with a duty ratio of 30%. Note that the conduction period of one cycle may be divided into two or more on-duty sections.

[0051] Note that an off-duty period must always be set at the end of one PWM drive cycle. For example, when drive control is performed with the above duty ratio of 50%, the next cycle will begin after an optimal on-duty period of 5 μsec, an off-duty period, an on-duty period of 7.5 μsec, and another off-duty period. This clarifies the range of one cycle and makes control easier.

[0052] In this way, when changing the duty ratio of the drive voltage applied by PWM to the three-phase coils of a sensorlessly driven motor, regardless of which controllable duty ratio is selected, it is possible to detect the field position of the motor during the low-speed drive period from the non-energized phase coil voltage in the energization section based on the on-duty section (e.g., 5 μsec) of at least the optimal duty ratio (e.g., 20%), thereby enabling smooth continuous operation of the motor in the low-speed range.

[0053] 10B and 11B differ in the timing of application of the remaining on-duty interval drive voltage, which is applied following the drive voltage based on the optimal on-duty ratio (e.g., 20%) at which the non-energized phase coil voltage can be detected, during the energization interval of one cycle. FIG. 10B illustrates an example in which a longer off-duty interval is applied after application of a drive voltage based on an optimal on-duty interval (e.g., 5 μsec) for an optimal on-duty ratio (e.g., 20%), and a drive voltage for an on-duty interval exceeding the optimal on-duty interval is applied. FIG. 11B illustrates an example in which a drive voltage based on an optimal on-duty interval (e.g., 5 μsec) for an optimal on-duty ratio (e.g., 20%) is applied, and the off-duty intervals before and after application of the drive voltage for the on-duty interval exceeding the optimal on-duty interval are equal.

[0054] Furthermore, it is preferable that the field position of the motor is detected by changing the duty ratio of the drive voltage applied to the three-phase coil between 10% and 80%. If the duty ratio is below 10% or above 80%, the off-duty period in the current-carrying period of one cycle becomes shorter, making it difficult to capture fluctuations in the induced voltage and thus difficult to detect the field position of the motor. This allows the motor to be operated at low speed while detecting the field position, even if the duty ratio of the drive voltage applied to the motor coil is changed over a wide range, improving the controllability of the motor.

[0055] Below, an example of the field position detection operation at startup by the MPU 51 is explained. First, the detection operation for forward rotation is described. The threshold value Vth is set in advance as appropriate. The initial speed and direction of rotation are measured. Normally, a stationary state is detected. If the motor is rotating, the motor transitions to rotation operation. If the motor is stationary, the initial position is detected using any method. As a result, for example, the motor is positioned in section 1. An excitation pattern UV energization that is compatible with CW rotation in energization section 1 is selected.

[0056] In Figure 8, the inverter circuit 52 energizes UV for only one pulse using PWM control, and the three-phase coil voltages are A / D converted by the ADC 53 during the on cycle. The MPU 51 then calculates the neutral point potential by dividing (U phase voltage + V phase voltage) by 2. Next, it determines whether the non-energized W phase voltage - neutral point potential exceeds Vth. If it does not, it returns to PWM control and repeats energization and measurement. If it does exceed Vth, it is the end of the section, and the section number is incremented. Thereafter, an excitation pattern is selected in the same way as in energization section 1, and energization is repeated using PWM control, resulting in continuous rotation.

[0057] When the duty ratio of the drive voltage is increased beyond the optimum duty ratio (e.g., 20%) for detecting the non-energized W-phase coil voltage in order to accelerate the motor, the current conduction section of one cycle of PWM current is divided into multiple on-duty sections including the optimum duty ratio (e.g., 20%) and the drive voltage is applied. For example, when drive control is performed at a duty ratio of 50%, a drive voltage with a duty ratio of 20% is applied during the current conduction section of one cycle, and then a drive voltage with a duty ratio of 30% is applied via an off-duty section.

[0058] In the above embodiment, the case where the motor is accelerated from a low speed rotation has been described, but the same applies to the case where the motor is decelerated to a low speed rotation. [Explanation of symbols]

[0059] 1 Rotor shaft 2 Rotor 3 Permanent magnet 4 Stator 50 Host controller 51 MPU 52 Inverter circuit (INV) 53 A / D converter (ADC)

Claims

1. an electric motor including a rotor having a permanent magnet field, a stator having three-phase coils, output means for bidirectionally energizing the three-phase coils via a half-bridge inverter circuit, control means for PWM-controlling the coil output in response to a command from a host controller, storing energization angle information and energization pattern information for each 60° energization section that allows continuous rotation, and switching control of the output means based on these to switch energization states, and measurement means for A / D-converting the three-phase coil voltages and sending them to the control means, a current control means for controlling the motor through the output means to periodically perform 120° energization, including off-cycles, in a current pattern in which the position at which self-excitation stops by two-phase fixed energization coincides with the start position of a 60° energization interval, and the measurement means measures energized phase voltages and non-energized phase voltages during on-cycles of PWM energization to detect the field position of the motor; and the control means, when operating the motor by varying the duty ratio of a predetermined drive voltage applied to the three-phase coils through the output means, selects an optimum duty ratio for sensing the non-energized phase coil voltage at the predetermined drive voltage, and, when driving at the optimum duty ratio within one current interval, applies a drive voltage during an on-duty interval of the optimum duty ratio, and when applying a drive voltage based on a duty ratio exceeding the optimum duty ratio, divides the on-duty interval of one cycle into a plurality of on-duty intervals including the optimum duty ratio and applies the drive voltage.

2. an electric motor including a rotor having a permanent magnet field, a stator having three-phase coils, output means for bidirectionally energizing the three-phase coils via a half-bridge inverter circuit, control means for PWM-controlling the coil output in response to a command from a host controller, storing energization angle information and energization pattern information for each 60° energization section that allows continuous rotation, and switching control of the output means based on these to switch energization states, and measurement means for A / D-converting the three-phase coil voltages and sending them to the control means, a current control means for controlling the motor through the output means to periodically perform 120° energization, including off-cycles, in a current pattern in which the position at which self-excitation stops by two-phase fixed energization coincides with the start position of a 60° energization interval, and the measurement means measures energized phase voltages and non-energized phase voltages during on-cycles of PWM energization to detect the field position of the motor; and the control means, when operating the motor by varying the duty ratio of a predetermined drive voltage applied to the three-phase coils through the output means, selects an optimum duty ratio for sensing the non-energized phase coil voltage at the predetermined drive voltage, and, when driving at the optimum duty ratio during one current interval, applies a drive voltage during an on-duty interval of the optimum duty ratio, and, when applying a drive voltage based on a duty ratio that does not exceed the optimum duty ratio, extends the time of one cycle or applies a drive voltage including the optimum duty ratio over multiple cycles.

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