Motor drive control device, motor unit, and motor drive control method

Abstract

The present invention suppresses an increase in power consumption of a motor. The control circuit (1) is provided with: a commutation mode setting unit (11) that selects, as a commutation mode, a first commutation mode in which commutation of a coil (41) of a motor (4) is performed on the basis of a detection result of zero crossing of a counter electromotive force voltage of the coil (41), or a second commutation mode in which commutation of the coil (41) is performed for a target conduction time; a drive control signal generation unit (12) that generates a drive control signal (Sd) on the basis of the set commutation pattern; a current limit value setting unit (14) that sets a current limit value; and a current limiting unit (16) that instructs the drive control signal generation unit (12) to stop excitation of the coil (41) when the current flowing through the coil (41) reaches a current limiting value. The current limit value setting unit (14) sets the current limit value to an upper limit value (Ith1) during a period in which the commutation mode is the first commutation mode, and sets the current limit value to a lower limit value (Ith2) smaller than the upper limit value (Ith1) during at least a part of a period in the second commutation mode.

Description

Technical Field

[0001] The present invention relates to a motor drive control device, a motor unit and a motor drive control method, for example, to a motor drive control device for driving a stepping motor. Background Art

[0002] In the past, a motor drive control device has been known that has a current limiting function for limiting the current flowing through the coil of the motor (hereinafter also referred to as "coil current") so that it does not exceed a predetermined value (hereinafter also referred to as "current limiting value") (for example, refer to Patent Document 1). Furthermore, there is known a motor drive control device that detects the zero crossing of the back electromotive force voltage of the coil of the motor and performs commutation of the coil (for example, refer to Patent Document 2). Prior art literature Patent Literature

[0003] Patent Document 1: Japanese Patent Application Publication No. 2018-207607 Patent Document 2: Japanese Patent Application Publication No. 2018-38213 Summary of the invention Problems to be solved by the invention

[0004] The inventor of the present application has studied a new motor drive control technology before the present application. Specifically, the inventor of the present application has studied the following motor drive control technology: the motor is controlled in such a way that the coil current of the motor does not exceed the current limit value, and when the motor speed is lower than the threshold, the coil is commutated based on the zero crossing of the back electromotive force voltage of the coil, and when the motor speed is above the threshold, the coil is commutated according to the pre-set commutation time. However, the inventor of the present application knows that the above motor drive control technology has the following problems.

[0005] That is, according to the above motor drive control technology, as the load on the motor becomes lighter, the motor speed increases, and when the motor speed reaches a threshold, the motor is controlled so that the speed becomes below the threshold. On the other hand, regardless of the motor speed, the coil current will rise to the current limit value. Therefore, sometimes when the load on the motor becomes lighter, a coil current higher than necessary flows, and the power consumption of the motor becomes higher.

[0006] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to suppress an increase in the power consumption of a motor. Solutions for solving problems

[0007] A motor drive control device according to a representative embodiment of the present invention is characterized in that it comprises: a control circuit that generates a drive control signal for controlling the drive of the motor; and a drive circuit that drives the coil of the motor based on the drive control signal, wherein the control circuit has: a commutation mode setting unit that selects a first commutation mode or a second commutation mode as a commutation mode based on the rotation speed of the motor, wherein the first commutation mode commutates the coil based on the detection result of the zero crossing of the back electromotive force voltage of the coil, and the second commutation mode commutates the coil according to the target power-on time; and a drive control signal generating unit that generates a drive control signal based on the rotation speed of the motor. The drive control signal is generated in the commutation mode set by the commutation mode setting unit; a current limit value setting unit is set to a current limit value that serves as a reference for limiting the current flowing through the coil; and a current limiting unit instructs the drive control signal generating unit to stop excitation of the coil when the current flowing through the coil reaches the current limit value, the current limit value setting unit sets the current limit value to a first value during the period when the commutation mode is the first commutation mode, and sets the current limit value to a second value that is smaller than the first value during at least a portion of the period in the second commutation mode. Effects of the Invention

[0008] According to the motor drive control device of the present invention, it is possible to suppress an increase in the power consumption of the motor. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a block diagram showing the configuration of a motor unit according to the embodiment. Figure 2 It is a diagram schematically showing the structure of a motor. Figure 3 It is a diagram showing the configuration of a control circuit in the motor drive control device according to the embodiment. Figure 4 This is a timing chart showing an example of the operation of the motor and the motor drive control device accompanying a change in load. Figure 5 1 is a timing chart showing an example of changes in coil current when the motor is driven by the motor drive control device according to the embodiment. Figure 6 1 is a flowchart showing the flow of a current limit value switching process according to the embodiment. Figure 7 This is a flowchart showing the flow of the commutation mode setting process (step S1). DETAILED DESCRIPTION

[0010] 1. Overview of implementation methods First, a typical embodiment of the invention disclosed in this application will be described in outline. Note that, in the following description, as an example, reference numerals on the drawings corresponding to the components of the invention are indicated in parentheses.

[0011] [1] A motor drive control device (3) according to a representative embodiment of the present invention is characterized in that it comprises: a control circuit (1) for generating a drive control signal (Sd) for controlling the drive of a motor (4); and a drive circuit (2) for driving the coils (41A, 41B) of the motor based on the drive control signal, wherein the control circuit has: a commutation mode setting unit (11) for selecting a first commutation mode or a second commutation mode as a commutation mode based on the rotation speed of the motor, wherein the first commutation mode commutates the coil based on the detection result of the zero crossing of the back electromotive force voltage of the coil, and the second commutation mode commutates the coil according to the target power-on time; and a drive control signal generating unit (12) for driving the coils (41A, 41B) of the motor based on the drive control signal. A generating unit (12) for generating the drive control signal based on the commutation mode set by the commutation mode setting unit; a current limit value setting unit (14) for setting a current limit value that serves as a reference for limiting the current flowing through the coil; and a current limiting unit (16) for instructing the drive control signal generating unit to stop excitation of the coil when the current flowing through the coil reaches the current limit value, the current limit value setting unit setting the current limit value to a first value (Ith1) during the period when the commutation mode is the first commutation mode, and setting the current limit value to a second value (Ith2) that is smaller than the first value during at least a portion of the period in the second commutation mode.

[0012] [2] In the motor drive control device described in [1] above, the commutation mode setting unit may set the commutation mode to the first commutation mode when the rotational speed of the motor has not reached the first threshold value (Sth1), and set the commutation mode to the second commutation mode when the rotational speed of the motor reaches the first threshold value; when the commutation mode is the second commutation mode, the commutation mode is set to the first commutation mode when the rotational speed of the motor decreases to a second threshold value (Sth2) smaller than the first threshold value.

[0013] [3] In the motor drive control device described in [1] or [2] above, it may also be that after the commutation mode is switched from the first commutation mode to the second commutation mode, the current limiting value setting unit causes the current limiting value to change stepwise from the first value to the second value when the zero crossing of the back electromotive force voltage cannot be detected.

[0014] [4] In the motor drive control device described in any one of [1] to [3] above, the current limiting value setting unit may also change the current limiting value in a step-like manner from the second value to the first value when the commutation mode is the second commutation mode and when the zero crossing of the back electromotive force voltage can be detected.

[0015] [5] In the motor drive control device described in any one of [1] to [4] above, it may also be that the ratio of the change in the current limiting value relative to time when the current limiting value switches from the second value to the first value is greater than the ratio of the change in the current limiting value relative to time when the current limiting value switches from the first value to the second value.

[0016] [6] In the motor drive control device described in any one of [1] to [5] above, the current limit value setting unit may periodically determine whether the back electromotive force voltage is detected to have crossed zero, and when the back electromotive force voltage crosses zero, increase the current limit value, and when the back electromotive force voltage crosses zero, decrease the current limit value.

[0017] [7] A motor unit (5) according to a representative embodiment of the present invention is characterized by comprising: the motor (4); and a motor drive control device (3) as described in any one of [1] to [6] above.

[0018] [8] A method according to a representative embodiment of the present invention is a motor drive control method for controlling the drive of a motor by a motor drive control device, wherein the motor drive control method comprises: a first step (S1), wherein the motor drive control device sets a first commutation mode or a second commutation mode as a commutation mode based on the rotation speed of the motor, wherein the first commutation mode commutates the coil based on the detection result of the zero crossing of the back electromotive force voltage of the coil of the motor, and the second commutation mode commutates the coil according to the target power-on time; a second step, wherein the motor drive control device generates a drive control signal for controlling the drive of the motor; and a third step (S2 to S9) , the motor drive control device sets a current limit value that serves as a reference for limiting the current flowing through the coil; and a fourth step, the motor drive control device generates the drive control signal for stopping the excitation of the coil when the current flowing through the coil reaches the current limit value, and the third step includes: a fifth step (S4~S7), the motor drive control device sets the current limit value to a first value during the period when the commutation mode is the first commutation mode; and a sixth step (S5~S9), the motor drive control device sets the current limit value to a second value that is smaller than the first value during at least a portion of the period in the second commutation mode.

[0019] 2. Specific examples of implementation methods Hereinafter, specific examples of the embodiments of the present invention will be described with reference to the drawings. It should be noted that in the following description, the same reference numerals are given to the common components in each embodiment, and duplicate descriptions are omitted.

[0020] Implementation Method Figure 1 It is a block diagram showing the configuration of the motor unit 5 according to the embodiment.

[0021] like Figure 1 As shown, the motor unit 5 includes a motor 4 and a motor drive control device 3 for driving the motor 4. The motor unit 5 can be applied to various devices that use a motor such as an actuator that can be used in HVAC (Heating Ventilation and Air-Conditioning) as an air conditioning unit for vehicle use as a power source.

[0022] The motor 4 is, for example, a stepping motor. In the present embodiment, the motor 4 is, as an example, a two-phase stepping motor.

[0023] Figure 2 It is a diagram schematically showing the structure of the motor 4.

[0024] like Figure 2 As shown, the motor 4 as a two-phase stepping motor has an A-phase coil 41A, a B-phase coil 41B, and a rotor 42. The coils 41A and 41B are wound around a stator yoke (not shown) and together with the stator yoke, form a stator. It should be noted that in this embodiment, when the coils 41A and 41B are not distinguished, they are sometimes simply represented as "coil 41".

[0025] like Figure 2 As shown, the rotor 42 includes permanent magnets with multi-pole magnetization in such a manner that the S poles 42S and the N poles 42N are alternately arranged along the circumferential direction. Figure 2 In FIG. 1 , a case where the rotor 42 has two poles is shown as an example, but the number of poles of the rotor 42 is not particularly limited.

[0026] By passing a current through the coil 41A of the A phase, the stator yoke of the A phase is excited, and by passing a current through the coil 41B of the B phase, the stator yoke of the B phase is excited. The phases of the currents flowing through the coils 41A and 41B are periodically switched, thereby rotating the rotor 42. An output shaft (not shown) is connected to the rotor 42, and the output shaft is driven by the rotational force of the rotor 42, thereby realizing the function of the above-mentioned actuator, for example.

[0027] The motor drive control device 3 is a device for driving the motor 4. The motor drive control device 3 controls the energization state of the coils 41A and 41B of each phase of the motor 4 based on the drive command signal Sc from the host device 6, for example, to control the rotation and stop of the motor 4.

[0028] like Figure 1 As shown, the motor drive control device 3 includes a control circuit 1 and a drive circuit 2 . The drive circuit 2 energizes the coils 41A and 41B of the motor 4 to drive the motor 4. The drive circuit 2 excites the coils 41A and 41B of the motor 4 based on the drive control signal Sd output from the control circuit 1 to rotate the rotor 42 of the motor 4.

[0029] The drive circuit 2 has, for example, an inverter circuit 21A for exciting the coil 41A of phase A, an inverter circuit 21B for exciting the coil 41B of phase B, a current detection circuit 20A for detecting the current of the coil 41A of phase A, and a current detection circuit 20B for detecting the current of the coil 41B of phase B.

[0030] The inverter circuits 21A and 21B are composed of, for example, an H-bridge circuit including four switching elements (e.g., transistors). Hereinafter, the inverter circuit 21A is also referred to as "H-bridge circuit 21A", and the inverter circuit 21B is also referred to as "H-bridge circuit 21B". In addition, when the H-bridge circuit 21A and the H-bridge circuit 21B are not distinguished, they are simply expressed as "H-bridge circuit 21".

[0031] The H-bridge circuit 21A and the H-bridge circuit 21B have, for example, the same circuit configuration as each other. Although not shown, the H-bridge circuit 21 includes a plurality of switching elements (eg, transistors) whose on / off is controlled by a drive control signal Sd.

[0032] It should be noted that the drive circuit 2 may include a pre-drive circuit for driving the switching elements of the H-bridge circuits 21A and 21B based on the drive control signal Sd.

[0033] like Figure 2 As shown, the H-bridge circuit 21A is connected to the positive terminal AP of the coil 41A and the negative terminal AN of the coil 41A. The H-bridge circuit 21B is connected to the positive terminal BP of the coil 41B and the negative terminal BN of the coil 41B.

[0034] The H-bridge circuits 21A and 21B energize the coils 41A and 41B by applying voltages to the terminals AP, AN, BP, and BN. Specifically, the H-bridge circuit 21 selectively turns on / off the switching elements constituting the H-bridge circuit 21 according to the drive control signal Sd supplied from the control circuit 1, thereby switching the coil 41 to be excited and switching the energization direction of the coil 41.

[0035] For example, Figure 2 As shown, during the "A phase (+) excitation period" in which the current +Ia flows from the terminal AP of the A phase coil 41A to the terminal AN, the H-bridge circuit 21A applies a voltage of "+Va" to the terminal AP of the coil 41A, for example. On the other hand, during the "A phase (-) excitation period" in which the current -Ia flows from the terminal AN of the A phase coil 41A to the terminal AP, the H-bridge circuit 21A applies a voltage of "-Va" to the terminal AP of the coil 41A, for example. During the "B phase (+) excitation period" in which the current +Ib flows from the terminal BP of the B phase coil 41B to the terminal BN, the H-bridge circuit 21B applies a voltage of "+Vb" to the terminal BP of the coil 41B, for example. During the “B phase (−) excitation period” in which the current −Ib flows from the terminal BN of the B phase coil 41B to the terminal BP, the H bridge circuit 21B applies a voltage of “−Vb” to the terminal BP with respect to the terminal BN of the coil 41B.

[0036] The current detection circuit 20A is connected to the H-bridge circuit 21A, detects the current flowing through the coil 41A, and outputs a current detection signal Sia corresponding to the magnitude of the detected current. The current detection circuit 20B is connected to the H-bridge circuit 21B, detects the current flowing through the coil 41B, and outputs a current detection signal Sib corresponding to the magnitude of the detected current.

[0037] The current detection circuits 20A and 20B include, for example, shunt resistors. The shunt resistors are connected in series with the H-bridge circuits 21, for example, between the power supply voltage and the ground voltage of the H-bridge circuits 21, and the voltage generated at both ends of the shunt resistors is output as current detection signals Sia and Sib. It should be noted that the current detection circuits 20A and 20B can adopt various known circuit configurations that can detect the current flowing through the coils 41A and 41B of the motor 4, and are not limited to the above-mentioned circuit configuration including the shunt resistors.

[0038] The control circuit 1 is a circuit that performs overall control of the motor drive control device 3 . The control circuit 1 is, for example, a program processing device (for example, a microcontroller) having a structure in which a processor such as a CPU (central processing unit), various storage devices such as a RAM (random access memory), a ROM (read-only memory), a timer (counter), an A / D conversion circuit, a D / A conversion circuit, and peripheral circuits such as an input / output I / F circuit are interconnected via a bus. In the present embodiment, the control circuit 1 is packaged as an IC (integrated circuit), for example, but is not limited to this. It should be noted that the control circuit 1 and the drive circuit 2 can also be packaged as one.

[0039] The control circuit 1 has a function of controlling the driving of the motor 4 by, for example, generating a drive control signal Sd and supplying the signal to the drive circuit 2 .

[0040] Figure 3 1 is a diagram showing a configuration of a control circuit 1 in a motor drive control device 3 according to the embodiment.

[0041] like Figure 3 As shown, the control circuit 1, as a functional unit for realizing the above-mentioned functions, includes a commutation mode setting unit 11, a drive control signal generating unit 12, a storage unit 13, a current limit value setting unit 14, a current value acquiring unit 15, a current limiting unit 16, a zero crossing detecting unit 17 and a rotation speed calculating unit 18.

[0042] For example, in the program processing device (microcontroller) as the above-mentioned control circuit 1, the processor performs calculations according to the program stored in the storage device using various parameters stored in the storage device, and controls peripheral circuits such as the A / D conversion circuit and the timer to realize these functional units. It should be noted that part or all of these functional units can also be realized by dedicated circuits (hardware).

[0043] The storage unit 13 is a functional unit for storing various data required for the motor drive control performed by the control circuit 1. For example, the storage unit 13 stores current limit value information 130 including an upper limit value (an example of a first value) Ith1, a lower limit value (an example of a second value) Ith2, a unit increase amount ΔIa, and a unit decrease amount ΔId of the current limit value described later.

[0044] The commutation pattern setting unit 11 is a functional unit for setting a commutation pattern that specifies a commutation method of the motor 4 .

[0045] The motor drive control device 3 of this embodiment has at least two modes as commutation modes. One is a first commutation mode for commutating the coils 41A and 41B based on the detection result of the zero crossing of the back electromotive force voltage of the coils 41A and 41B, and the other is a second commutation mode for commutating the coil 41 according to the target energization time. Here, the target energization time refers to the target value of the length of the period during which one energization mode (excitation state of the coil 41) continues.

[0046] For example, the commutation mode setting unit 11 obtains a drive command signal Sc for the motor 4 input from the outside of the motor drive control device 3 (for example, the host device 6). The drive command signal Sc includes, for example, information specifying the rotation position of the motor 4, information indicating the stop of the rotation of the motor 4, and the like. The drive command signal Sc is, for example, a PWM (Pulse Width Modulation) signal. The commutation mode setting unit 11 obtains information on the target rotation position of the motor 4 (target rotation position) by analyzing the drive command signal Sc, and on the basis of setting the commutation mode to the first commutation mode or the second commutation mode, instructs the drive control signal generating unit 12 to drive the motor 4. It should be noted that the details of the setting method of the commutation mode based on the commutation mode setting unit 11 will be described later.

[0047] The zero-crossing detection unit 17 is a functional unit for monitoring the voltage of the coils 41A and 41B and detecting the zero-crossing of the back electromotive force voltage generated in the coils 41A and 41B. Here, the zero-crossing of the back electromotive force voltage refers to the state in which the back electromotive force voltage generated at both ends of the unexcited coil 41 switches from positive to negative or from negative to positive.

[0048] The zero-crossing detection unit 17 monitors the voltage between terminal AP and terminal AN as the voltage of coil 41A, and monitors the voltage between terminal BP and terminal BN as the voltage of coil 41B. When the zero-crossing of the back electromotive force voltage of coils 41A and 41B is detected, a zero-crossing detection signal Sz indicating that the zero-crossing is detected is output.

[0049] The speed calculation unit 18 is a functional unit that calculates the speed of the motor 4. When the zero crossing of the back electromotive force voltage of the coil 41 is periodically detected, the speed calculation unit 18 calculates the speed of the motor 4 based on the zero crossing detection signal Sz output from the zero crossing detection unit 17. For example, the speed calculation unit 18 calculates the speed of the motor 4 based on the period of the output zero crossing detection signal Sz in the first commutation mode. On the other hand, when the zero crossing of the back electromotive force voltage is not detected in the second commutation mode, the speed calculation unit 18 calculates the speed of the motor 4 based on the target power-on time pre-set in the second commutation mode. The speed calculation unit 18 generates an output signal So including information on the calculated speed. The output signal So is output, for example, to the outside of the motor drive control device 3 (for example, the host device 6), and is input to the commutation mode setting unit 11.

[0050] The drive control signal generating unit 12 generates a drive control signal Sd based on the commutation pattern set by the commutation pattern setting unit 11. The drive control signal Sd is a signal for controlling the on / off of each switching element of the H-bridge circuits 21A and 21B, and is, for example, a PWM signal.

[0051] In order to move the rotor 42 to the target rotation position or the target standby position, the drive control signal generating unit 12 generates and outputs the drive control signal Sd in a manner that excites the A-phase coil 41A and the B-phase coil 41B at a predetermined timing based on a predetermined excitation method. Here, the predetermined excitation method can be any excitation method that can detect the back electromotive force voltage of the coil 41, such as a well-known single-phase excitation method or a 1-2 phase excitation method.

[0052] When the drive control signal Sd is generated by a single-phase excitation method, the drive control signal generating unit 12 generates and outputs the drive control signal Sd, for example, so as to switch the power-on state of the coils 41A and 41B of the A phase and the B phase in the order of "A phase (+) excitation period", "B phase (+) excitation period", "A phase (-) excitation period", and "B phase (-) excitation period".

[0053] At this time, switching of each excitation period, that is, commutation of the coils 41A and 41B is performed according to the commutation pattern set by the commutation pattern setting unit 11 . Specifically, when the first commutation mode is designated as the commutation mode, the drive control signal generating unit 12 commutates the coils 41A and 41B each time the zero crossing of the back electromotive force voltage is detected by the zero crossing detecting unit 17. As in the above example, when the motor 4 is driven by a single-phase excitation method, for example, when the zero crossing detecting unit 17 outputs the zero crossing detection signal Sz during the "A phase (+) excitation period", the drive control signal generating unit 12 switches from the "A phase (+) excitation period" to the "B phase (+) excitation period".

[0054] Furthermore, when the second commutation mode is designated as the commutation mode, the drive control signal generating unit 12 commutates the coils 41A and 41B based on the target energization time.

[0055] For example, the drive control signal generating unit 12 has a timer (counter), which starts measuring time through the timer when the "A phase (+) excitation period" starts. When the measured time reaches the target power-on time, it switches from the "A phase (+) excitation period" to the "B phase (+) excitation period".

[0056] Here, the information of the target energization time may be pre-stored in the drive control signal generating unit 12 or the storage unit 13, or may be calculated by the drive control signal generating unit 12. For example, the information of the target rotation speed of the motor 4 and the information of the conduction angle corresponding to the excitation method indicating the size of the electrical angle for continuously energizing the coil of one phase in one direction may be pre-stored in the storage unit 13, and the drive control signal generating unit 12 may calculate the target energization time based on the information of the target rotation speed and the information of the conduction angle read from the storage unit 13.

[0057] It should be noted that, in the case where the back electromotive force voltage is detected to be zero-crossing in the second commutation mode, the drive control signal generating unit 12 may also perform commutation of the coils 41A and 41B based on the detection of the back electromotive force voltage to be zero-crossing (input of the zero-crossing detection signal Sz) in the same manner as in the first commutation mode. That is, in the case where the back electromotive force voltage is not detected to be zero-crossing in the second commutation mode, the drive control signal generating unit 12 may perform commutation of the coils 41A and 41B based on the target power-on time, and in the case where the back electromotive force voltage is detected to be zero-crossing in the second commutation mode, even in the second commutation mode, regardless of the target power-on time, the drive control signal generating unit 12 may perform commutation of the coils 41A and 41B based on the detection of the back electromotive force voltage to be zero-crossing.

[0058] The current value acquisition unit 15 is a functional unit that acquires the value of the current flowing through the coils 41A and 41B of each phase of the motor 4. The current detection signals Sia and Sib output from the current detection circuits 20A and 20B of the drive circuit 2 are input to the current value acquisition unit 15. The current value acquisition unit 15 includes, for example, an A / D conversion circuit, and converts the voltage as the current detection signal Sia into a digital value through the A / D conversion circuit, and outputs it as the current value of the coil 41A of the A phase. Similarly, the current value acquisition unit 15 converts the voltage as the current detection signal Sib into a digital value through the A / D conversion circuit, and outputs it as the current value of the coil 41B of the B phase.

[0059] The current limit value setting unit 14 is a functional unit for setting a current limit value. The current limit value is a value that serves as a reference for limiting the current flowing through the coil 41 of the motor 4 , in other words, a value that determines the upper limit of the current of the coil 41 .

[0060] The current limit value setting unit 14 sets the current limit value based on the information of the commutation mode set by the commutation mode setting unit 11, the zero-crossing detection signal Sz output from the zero-crossing detection unit 17, and the current limit value information 130 stored in the storage unit 13. It should be noted that the details of the setting method of the current limit value will be described later.

[0061] The current limiting unit 16 is a functional unit that monitors the current flowing through the coil 41 of the motor 4 and controls the current so that it does not exceed the current limit value. The current monitoring by the current limiting unit 16 is performed for each phase of the motor 4. For example, during the "A phase (+) excitation period" and "A phase (-) excitation period" when the coil 41A of the A phase is excited, the current (current detection signal Sia) of the coil 41A of the A phase is monitored, and during the "B phase (+) excitation period" and "B phase (-) excitation period" when the coil 41B of the B phase is excited, the current (current detection signal Sib) of the coil 41B of the B phase is monitored.

[0062] When the current flowing through the coil 41 reaches the current limit value, the current limiting unit 16 instructs the drive control signal generating unit 12 to stop the excitation of the coil 41. For example, the current limiting unit 16 compares the current value (absolute value) of the coil 41A output from the current value acquiring unit 15 with the current limit value (absolute value) set by the current limit value setting unit 14 during the period when the coil 41A of the A phase is excited. When the current value of the coil 41A is equal to or greater than the current limit value, the current limiting unit 16 outputs a signal instructing to stop the excitation of the coil 41A, that is, instructing to turn off each switching element of the H-bridge circuit 21A based on the drive control signal Sd, for a predetermined period. The current limiting unit 16 also limits the current of the coil 41B of the B phase by the same method during the period when the coil 41B of the B phase is excited.

[0063] The drive control signal generating unit 12 generates a drive control signal Sd to stop the excitation of the coil 41 during a period in which a signal instructing to stop the excitation of the coil 41 is output from the current limiting unit 16. For example, during the "A phase (+) excitation period", when the current limiting unit 16 outputs a signal instructing to stop the excitation of the coil 41A of the A phase, the drive control signal generating unit 12 generates a drive control signal Sd to turn off each switching element in the H-bridge circuit 21A of the A phase. As a result, the power supply to the coil 41A of the A phase is stopped, and therefore, the current of the coil 41A of the A phase is reduced and limited to not exceed the current limit value. The current of the coil 41B of the B phase is also limited in the same way.

[0064] Here, a method of setting the commutation mode is described.

[0065] The commutation mode setting unit 11 selects the first commutation mode or the second commutation mode as the commutation mode based on the rotation speed of the motor 4 . For example, when the load of the motor 4 is heavy and the rotation speed of the motor 4 is relatively slow, it is easy to detect the zero crossing of the back electromotive force voltage of the coil 41. Therefore, when the rotation speed of the motor 4 does not reach the first threshold value Sth1, the commutation mode setting unit 11 sets the commutation mode to the first commutation mode.

[0066] By setting the commutation mode to the first commutation mode, commutation of the coil 41 is performed based on detection of the zero crossing of the back electromotive force voltage, so that the motor 4 can be rotated at an appropriate rotation speed and torque that follow the magnitude of the load.

[0067] On the other hand, when the coil 41 is commutated by the first commutation mode based on the detection of the zero crossing of the back electromotive force voltage, the lighter the load of the motor 4 is and the higher the rotation speed of the motor 4 is, the more difficult it is to detect the zero crossing of the back electromotive force voltage. Therefore, when the rotation speed of the motor 4 reaches the first threshold value Sth1, the commutation mode setting unit 11 sets the commutation mode to the second commutation mode.

[0068] By setting the commutation mode to the second commutation mode, the coil 41 is commutated according to the target energization time, thereby avoiding the motor 4 from losing steps and reliably rotating the motor 4. At this time, since the motor 4 rotates at a speed corresponding to the target energization time, the increase in speed can be suppressed.

[0069] It should be noted that, as described above, when the back electromotive force voltage zero crossing is detected in the second commutation mode, the coil 41 can be commutated based on the detection of the back electromotive force voltage zero crossing, as in the first commutation mode. In this case, the motor 4 rotates at a rotation speed corresponding to the magnitude of the load.

[0070] Furthermore, when the commutation mode is the second commutation mode, the commutation mode setting unit 11 sets the commutation mode to the first commutation mode when the rotation speed of the motor 4 decreases to the second threshold value Sth2. Here, the second threshold value Sth2 is a value different from the first threshold value Sth1, for example, the second threshold value Sth2<the first threshold value Sth1.

[0071] Thus, for example, when the load of the motor 4 increases and the rotation speed decreases, the second commutation mode can be automatically returned to the first commutation mode, and the motor 4 can be driven with an appropriate rotation speed and torque corresponding to the load. In addition, regarding the threshold of the rotation speed of the motor 4, by setting the second threshold value Sth2 and the first threshold value Sth1 to different values, it is possible to prevent the commutation mode from being frequently switched due to changes in the rotation speed (load) of the motor 4.

[0072] Next, a method of setting the current limit value will be described.

[0073] As described above, when the load on the motor 4 becomes lighter and the motor 4 rotates in the second commutation mode, the coil current rises to the current limit value regardless of the rotation speed of the motor 4, so that sometimes more coil current than necessary flows, and power consumption increases. Therefore, the motor drive control device 3 of this embodiment has a function of making the current limit value variable.

[0074] Specifically, the current limit value setting unit 14 sets the current limit value to an upper limit value Ith1 which is a first value during the period when the commutation mode is the first commutation mode, and sets the current limit value to a lower limit value Ith2 (<upper limit value Ith1) which is a second value smaller than the first value during at least a portion of the period in the second commutation mode.

[0075] For example, the current limit value setting unit 14 periodically determines whether or not a zero crossing of the back electromotive force voltage is detected, and increases the current limit value when a zero crossing of the back electromotive force voltage is detected, and decreases the current limit value when a zero crossing of the back electromotive force voltage is not detected. The switching process of the current limit value is described in detail below.

[0076] Figure 4 1 is a timing chart showing an example of the operation of the motor 4 and the motor drive control device 3 in response to a change in load.

[0077] exist Figure 4 In the figure, the horizontal axis represents time t. Figure 4 , the magnitude of the load, the rotation speed of the motor 4, the current limit value, the commutation mode, and the detection state of the zero crossing of the back electromotive force voltage are respectively shown from the upper side to the lower side of the paper.

[0078] For example, Figure 4As shown, at time t0, the rotation speed of the motor 4 does not reach the first threshold value Sth1. Therefore, the commutation mode setting unit 11 sets the commutation mode to the first commutation mode. In addition, the current limit value setting unit 14 sets the current limit value to the upper limit value Ith1 as the first value. As a result, the motor 4 is controlled to rotate at a rotation speed corresponding to the load, and the coil current does not exceed the upper limit value Ith1. It should be noted that the information of the upper limit value Ith1 is stored in the storage unit 13 as the current limit value information 130, for example, and the current limit value setting unit 14 reads and uses the information from the storage unit 13.

[0079] After time t0, the motor 4 is similarly driven and controlled by the first commutation mode, so the rotation speed of the motor 4 increases with the reduction of the load. And at time t1, when the rotation speed of the motor 4 reaches the first threshold value Sth1, the commutation mode setting unit 11 switches the commutation mode from the first commutation mode to the second commutation mode. Thus, the drive control signal generating unit 12 commutates the coil 41 based on the preset target energization time, so that the motor 4 rotates at a rotation speed corresponding to the target energization time.

[0080] After the commutation mode is switched from the first commutation mode to the second commutation mode, when the current limit value setting unit 14 determines that the zero crossing of the back electromotive force voltage cannot be detected, it starts the process of switching the current limit value from the upper limit value Ith1 as the first value to the lower limit value Ith2 as the second value.

[0081] For example, Figure 4 As shown, at time t1, after the commutation mode is switched from the first commutation mode to the second commutation mode, the zero crossing of the back electromotive force voltage cannot be detected at time t2. At this time, the current limit value setting unit 14 changes (decreases) the current limit value in steps from the upper limit value Ith1 to the lower limit value Ith2.

[0082] For example, the current limit value setting unit 14 changes the current limit value in steps in synchronization with the determination of whether the back electromotive voltage zero crossing is detected. Specifically, the current limit value setting unit 14 periodically determines whether the back electromotive voltage zero crossing is detected, and decreases the current limit value by a unit amount according to the determination result.

[0083] For example, the current limit value setting unit 14 determines whether the zero-crossing detection signal Sz is output during a certain period (whether the zero-crossing is detected), and when the zero-crossing detection signal Sz is not output during the certain period, the current limit value is reduced by the unit reduction amount ΔId. It should be noted that the information of the lower limit value Ith2 and the information of the unit reduction amount ΔId are stored in the storage unit 13 as the current limit value information 130, and the current limit value setting unit 14 reads and uses this information from the storage unit 13.

[0084] After time t2, since the zero crossing of the back electromotive force voltage cannot be detected, the current limit value setting unit 14 reduces the current limit value by the unit reduction amount ΔId at a certain period. Then, at time t3, the current limit value reaches the lower limit value Ith2. At this time, the current limit value setting unit 14 stops the process of reducing the current limit value. As a result, after time t3, the motor 4 is controlled to rotate at a speed corresponding to the target energization time, and the coil current does not exceed the lower limit value Ith2.

[0085] When the commutation mode is the second commutation mode, the current limit value setting unit 14 starts a process of switching the current limit value from the second value lower limit value Ith2 to the first value upper limit value Ith1 when the zero crossing of the back electromotive force voltage is detected.

[0086] For example, Figure 4 As shown in FIG. 1 , after time t3, the load begins to increase, and at time t4, the back electromotive force voltage begins to cross zero. At this time, the current limit value setting unit 14 changes (increases) the current limit value from the lower limit value Ith2 to the upper limit value Ith1 in a step-like manner. Specifically, the current limit value setting unit 14 periodically determines whether the back electromotive force voltage crosses zero, and increases the current limit value by a unit amount according to the determination result.

[0087] For example, the current limit value setting unit 14 determines whether the zero-crossing detection signal Sz is output during a certain period (whether the zero-crossing is detected), and increases the current limit value by the unit increase amount ΔIa each time the zero-crossing detection signal Sz is output. It should be noted that the information of the unit increase amount ΔIa is stored in the storage unit 13 as the current limit value information 130, for example, and the current limit value setting unit 14 reads and uses the information from the storage unit 13.

[0088] After time t4, the current limit value setting unit 14 increases the current limit value by the unit increase amount ΔIa each time the back electromotive force voltage is detected to be zero-crossing. When the current limit value reaches the upper limit value Ith1 at time t5, the current limit value setting unit 14 stops increasing the current limit value.

[0089] After time t4, although the commutation mode is the second commutation mode, the zero crossing of the back electromotive force voltage is detected. Therefore, the drive control signal generating unit 12 can also perform commutation of the coil 41 based on the detection of the zero crossing of the back electromotive force voltage without relying on the set commutation mode (second commutation mode). As a result, during the period after time t4, the rotation speed of the motor 4 decreases with the increase of the load.

[0090] Then, at time t6, the rotation speed of the motor 4 decreases to the second threshold value Sth2. At this time, the commutation mode setting unit 11 switches the commutation mode from the second commutation mode to the first commutation mode. As a result, the motor 4 is controlled to rotate at a rotation speed corresponding to the load again, and the coil current does not exceed the upper limit value Ith1.

[0091] Note that, at this time, since the zero crossing of the back electromotive force voltage is periodically detected according to the rotation speed of the motor 4 , the current limit value setting unit 14 maintains the current limit value at the upper limit value Ith1 which is the first value.

[0092] Here, if Figure 4 As shown in FIG. 1 , the ratio (absolute value) of the change in the current limit value relative to time when the current limit value is switched from the lower limit value (second value) Ith2 to the upper limit value (first value) Ith1 and the ratio (absolute value) of the change in the current limit value relative to time when the current limit value is switched from the upper limit value Ith1 to the lower limit value Ith2 may also be different from each other. For example, the unit increase amount ΔIa and the unit decrease amount ΔId may also be different values ​​from each other.

[0093] Preferably, if Figure 4 As shown, the ratio (absolute value) of the change in the current limit value relative to time when the current limit value is switched from the lower limit value Ith2 to the upper limit value Ith1 is greater than the ratio (absolute value) of the change in the current limit value relative to time when the current limit value is switched from the upper limit value Ith1 to the lower limit value Ith2. For example, the unit increase ΔIa>unit decrease ΔId.

[0094] In other words, it is preferred that Figure 4 As shown in FIG. 1 , the time for the current limit value to reach the upper limit value Ith1 from the lower limit value Ith2 (the period from time t4 to time t5) is shorter than the time for the current limit value to reach the lower limit value Ith2 from the upper limit value Ith1 (the period from time t2 to time t3). This is because the load increases when the current limit value switches from the lower limit value Ith2 to the upper limit value Ith1. Therefore, as described above, by shortening the time for the current limit value to reach the upper limit value Ith1 from the lower limit value Ith2, the current limit value is increased rapidly, thereby avoiding the shortage of the rotation torque caused by the increase of the load.

[0095] Figure 5 1 is a timing chart showing an example of changes in coil current when the motor 4 is driven by the motor drive control device 3 according to the embodiment.

[0096] exist Figure 5 In the figure, the horizontal axis represents time t and the vertical axis represents current. Figure 52 shows the temporal changes of the current of the A-phase coil 41A and the current of the B-phase coil 41B when the current limit value is reduced from the upper limit value Ith1 to the lower limit value Ith2 when the motor 4 is driven by a single-phase excitation method. Figure 5 The time t0 to t3 in corresponds to Figure 4 Time t0~t3 in the timing diagram.

[0097] It should be noted that in Figure 5 In the figure, for the convenience of illustration, the current of coil 41A of phase A (phase A current) and the current of coil 41B of phase B (phase B current) are represented by the same solid line, and a description of coil 41 of the phase through which the current flows is added near the solid line.

[0098] like Figure 5 As shown, for example, during the period from time t0 to time t1 when the motor 4 is driven and controlled in the first commutation mode, the drive control signal generating unit 12 switches the coil 41 to be excited according to the detection of the zero crossing of the back electromotive force voltage. During the excitation of each phase, when the current of the coil 41 is less than the absolute value of the upper limit value |Ith1|, the drive control signal generating unit 12 continues the unidirectional excitation of the coil 41. When the current of the coil 41 reaches the absolute value of the upper limit value |Ith1|, the drive control signal generating unit 12 stops the excitation of the coil 41 according to the instruction from the current limiting unit 16, and repeatedly restarts the excitation of the coil 41 after a certain period of time.

[0099] After that, the load of the motor 4 gradually becomes lighter, and at time t1, when the rotation speed of the motor 4 reaches the first threshold value Sth1, the commutation mode is switched from the first commutation mode to the second commutation mode. After the commutation mode is switched to the second commutation mode, when the back electromotive force voltage of the coil 41 is no longer detected to be zero-crossing at time t2, the current limit value is switched from the upper limit value Ith1 to the lower limit value Ith2.

[0100] The current limit value decreases stepwise from the upper limit value Ith1 after time t2, and reaches the lower limit value Ith2 at time t3. During the period from time t2 to time t3, the current limit value changes stepwise as described above, so Figure 5 As shown, the current of the coil 41 also changes in a step-like manner.

[0101] Although not shown, when the commutation mode changes from the second commutation mode to the first commutation mode, the current limit value changes from the lower limit value Ith2 to the upper limit value Ith1, and thus the current of the coil 41 also changes in a stepwise manner.

[0102] Next, the flow of the current limit value switching process will be described.

[0103] Figure 61 is a flowchart showing the flow of a current limit value switching process according to the embodiment.

[0104] For example, first, the control circuit 1 sets the commutation mode (step S1). The commutation mode setting process in step S1 is performed, for example, as follows: Figure 7 The process shown is executed.

[0105] Figure 7 This is a flowchart showing the flow of the commutation mode setting process (step S1).

[0106] In the commutation mode setting process, first, the commutation mode setting unit 11 determines whether the current commutation mode is the first commutation mode and whether the rotation speed of the motor 4 has reached the first threshold value Sth1 (step S11 ).

[0107] When the commutation mode is the first commutation mode and the rotation speed of the motor 4 reaches the first threshold value Sth1 (step S11 : Yes), the commutation mode setting unit 11 sets the commutation mode to the second commutation mode (step S14 ).

[0108] On the other hand, in step S11, when the commutation mode is not the first commutation mode and / or the rotational speed of the motor 4 has not reached the first threshold value Sth1 (step S11: No), the commutation mode setting unit 11 determines whether the commutation mode at this time is the second commutation mode and whether the rotational speed of the motor 4 is below the second threshold value Sth2 (step S12).

[0109] When the commutation mode is the second commutation mode and the rotation speed of the motor 4 is equal to or lower than the second threshold value Sth2 (step S12 : Yes), the commutation mode setting unit 11 sets the commutation mode to the first commutation mode (step S13 ).

[0110] On the other hand, in step S12, when the commutation mode is not the second commutation mode and / or the rotation speed of the motor 4 is not less than the second threshold value Sth2 (step S12: No), the commutation mode setting unit 11 does not change the commutation mode.

[0111] After the above-mentioned commutation mode setting process (step S1), Figure 6 As shown, the control circuit 1 determines whether the commutation mode is the second commutation mode (step S2).

[0112] When the commutation mode is not the second commutation mode, that is, when the commutation mode is the first commutation mode (step S2 : No), the control circuit 1 increases the current limit value (step S4 ). Specifically, the current limit value setting unit 14 calculates a value obtained by adding the unit increase amount ΔIa to the current limit value at that point in time.

[0113] Next, the current limit value setting unit 14 determines whether the value calculated in step S4 is greater than the upper limit value Ith1 (step S6). If the value calculated in step S4 is not greater than the upper limit value Ith1 (step S6: No), the current limit value setting unit 14 sets the value calculated in step S4 as the current limit value, and the control circuit 1 repeats the processing of steps S1 to S9.

[0114] On the other hand, in step S6, if the value calculated in step S4 is greater than the upper limit value Ith1 (step S6: Yes), the current limit value setting unit 14 sets the upper limit value Ith1 as the current limit value (step S7). Thereafter, the control circuit 1 repeats the processing of steps S1 to S9.

[0115] On the other hand, in step S2, when the commutation mode is the second commutation mode (step S2: yes), the control circuit 1 determines whether the zero crossing of the back electromotive force voltage is detected (step S3). When the zero crossing of the back electromotive force voltage is detected (step S3: yes), the control circuit 1 performs the processing of the above-mentioned steps S4 to S7.

[0116] When the back electromotive force voltage is not detected to be zero-crossing (step S3: No), the control circuit 1 decreases the current limit value (step S5). Specifically, the current limit value setting unit 14 calculates a value obtained by subtracting the unit decrease amount ΔId from the current limit value at that time.

[0117] Next, the current limit value setting unit 14 determines whether the value calculated in step S5 is less than the lower limit value Ith2 (step S8). If the value calculated in step S5 is not less than the lower limit value Ith2 (is greater than the lower limit value Ith2) (step S8: No), the current limit value setting unit 14 sets the value calculated in step S5 as the current limit value, and the control circuit 1 repeats the processing of steps S1 to S9.

[0118] On the other hand, in step S8, if the value calculated in step S5 is smaller than the lower limit value Ith2 (step S8: Yes), the current limit value setting unit 14 sets the lower limit value Ith2 as the current limit value (step S9). Thereafter, the control circuit 1 repeats the processing of steps S1 to S9.

[0119] As described above, in the motor drive control device 3 of the present embodiment, the control circuit 1 sets the current limiting value to a first value (upper limit value Ith1) when the commutation mode is the first commutation mode, and sets the current limiting value to a second value (lower limit value Ith2) smaller than the first value during at least a portion of the second commutation mode.

[0120] Thus, for example, when the load is lightened and the rotation speed of the motor 4 is reduced and the drive control of the motor 4 is performed in the second commutation mode, the current of the coil 41 can be prevented from being unnecessarily increased and the power consumption of the motor 4 can be prevented from being increased.

[0121] In addition, after the commutation mode is switched from the first commutation mode to the second commutation mode, the control circuit 1 changes (decreases) the current limit value in a step-like manner from the first value (upper limit value Ith1) to the second value (lower limit value Ith2) when the zero crossing of the back electromotive force voltage cannot be detected.

[0122] Thus, when the commutation mode changes from the first commutation mode to the second commutation mode, it is possible to prevent the current in the coil 41 from suddenly decreasing and thus preventing the operation of the motor 4 from becoming unstable.

[0123] Furthermore, when the commutation mode is the second commutation mode, the control circuit 1 changes (increases) the current limit value in steps from the second value (lower limit value Ith2) to the first value (upper limit value Ith1) when the zero crossing of the back electromotive force voltage can be detected.

[0124] Thus, the rotation speed of the motor 4 decreases due to the increase in load, and the coil current can be increased before the commutation mode is completely changed from the second commutation mode to the first commutation mode, thereby preventing the motor 4 from losing steps due to insufficient coil current in the first commutation mode. In addition, by increasing the current limit value in a stepwise manner, it is possible to prevent the current of the coil 41 from increasing sharply and causing the operation of the motor 4 to become unstable.

[0125] In addition, as described above, it is preferred that the ratio of the change in the current limiting value relative to time when the current limiting value switches from the second value (lower limit value Ith2) to the first value (upper limit value Ith1) is greater than the ratio of the change in the current limiting value relative to time when the current limiting value switches from the first value (upper limit value Ith1) to the second value (lower limit value Ith2).

[0126] Based on the above, when the load is reduced, the increase of unnecessary coil current to the motor 4 can be suppressed more quickly, and when the load increases, the coil current required for driving the motor 4 can be supplied more quickly, thereby achieving a good balance between the suppression of power consumption and the stability of the operation of the motor 4.

[0127] Alternatively, the control circuit 1 may periodically determine whether or not a zero crossing of the back electromotive voltage is detected, and increase the current limit value when a zero crossing of the back electromotive voltage is detected, or decrease the current limit value when a zero crossing of the back electromotive voltage is not detected. Thus, the current limit value is adjusted according to the change in the load of the motor 4 , so that the motor 4 can be driven more stably.

[0128] 《Expansion of implementation methods》 As mentioned above, although the invention completed by the present inventors was specifically described based on the embodiment, the present invention is not limited to this, and various changes can be made within the scope not departing from the gist of the invention.

[0129] For example, the number of phases of the stepping motor as the motor 4 in the above embodiment is not limited to two phases. In addition, the motor 4 in the above embodiment is not limited to a stepping motor. For example, the motor may also be a brushless DC motor.

[0130] In addition, in the above-mentioned embodiment, an example is given of a case where the current limit value is changed in a step-like manner in synchronization with the determination of whether or not the zero crossing of the back electromotive force voltage is detected, but the present invention is not limited thereto. For example, the current limit value may be changed in a step-like manner per unit time. For example, in the case where the zero crossing of the back electromotive force voltage cannot be detected in the second commutation mode, the current limit value may be decreased in a step-like manner from the upper limit value Ith1 to the lower limit value Ith2 per unit time, and in the case where the zero crossing of the back electromotive force voltage can be detected in the second commutation mode, the current limit value may be increased in a step-like manner from the lower limit value Ith2 to the upper limit value Ith1 per unit time.

[0131] In addition, the above flowchart shows an example for explaining the operation, and is not limited to this. That is, the steps shown in each figure of the flowchart are specific examples and are not limited to the process. For example, the order of some processing can be changed, other processing can be inserted between each processing, and some processing can be performed in parallel. Description of Reference Numerals

[0132] 1: control circuit; 2: drive circuit; 3: motor drive control device; 4: motor; 5: motor unit; 6: upper device; 11: commutation mode setting unit; 12: drive control signal generating unit; 13: storage unit; 14: current limit value setting unit; 15: current value acquisition unit; 16: current limit unit; 17: zero crossing detection unit; 18: rotation speed calculation unit; 20A, 20B: current detection circuit; 21, 21A, 21B: H-bridge circuit (inverter circuit); 42: rotor; 42S: S pole; 42N: N pole; 41, 41A, 41B: coils; 130: current limit value information; AP, AN, BP, BN: terminals of coils 41A, 41B; Ith1: upper limit value (an example of the first value); Ith2: lower limit value (an example of the second value); ΔIa: unit increase; ΔId: unit decrease; Sc: drive command signal; Sd: drive control signal; Sia, Sib: current detection signal; So: output signal; Sth1: first threshold value; Sth2: second threshold value; Sz: zero crossing detection signal.

Claims

1. A motor drive control device, comprising: A control circuit that generates a drive control signal for controlling the drive of the motor; as well as a drive circuit that drives the coil of the motor based on the drive control signal, The control circuit has: a commutation mode setting unit, which selects a first commutation mode or a second commutation mode as a commutation mode based on a rotation speed of the motor, wherein the first commutation mode commutates the coil based on a detection result of a zero crossing of a back electromotive force voltage of the coil, and the second commutation mode commutates the coil according to a target energization time; a drive control signal generating unit that generates the drive control signal based on the commutation mode set by the commutation mode setting unit; a current limit value setting unit that sets a current limit value that serves as a reference for limiting a current flowing through the coil; as well as a current limiting unit that, when the current flowing through the coil reaches the current limit value, instructs the drive control signal generating unit to stop excitation of the coil; The current limit value setting unit sets the current limit value to a first value while the commutation mode is the first commutation mode, and sets the current limit value to a second value smaller than the first value during at least a portion of the second commutation mode.

2. The motor drive control device according to claim 1, wherein: The commutation mode setting unit sets the commutation mode to the first commutation mode when the rotation speed of the motor does not reach the first threshold value; sets the commutation mode to the second commutation mode when the rotation speed of the motor reaches the first threshold value; and when the commutation mode is the second commutation mode, the commutation mode is set to the first commutation mode when the rotation speed of the motor decreases to a second threshold value smaller than the first threshold value.

3. The motor drive control device according to claim 2, wherein: The current limit value setting unit changes the current limit value stepwise from the first value to the second value when a zero crossing of the back electromotive force voltage cannot be detected after the commutation mode is switched from the first commutation mode to the second commutation mode.

4. The motor drive control device according to claim 3, wherein: When the commutation mode is the second commutation mode, the current limit value setting unit changes the current limit value in a stepwise manner from the second value to the first value when a zero crossing of the back electromotive force voltage can be detected.

5. The motor drive control device according to claim 4, wherein: A ratio of a change in the current limit value relative to time when the current limit value is switched from the second value to the first value is greater than a ratio of a change in the current limit value relative to time when the current limit value is switched from the first value to the second value.

6. The motor drive control device according to claim 2, wherein: The current limit value setting unit periodically determines whether a zero crossing of the back electromotive force voltage is detected, and increases the current limit value when a zero crossing of the back electromotive force voltage is detected, and decreases the current limit value when a zero crossing of the back electromotive force voltage is not detected.

7. A motor unit comprising: the motor; and A motor drive control device as claimed in any one of claims 1 to 6.

8. A motor drive control method, for controlling the drive of a motor by a motor drive control device, the motor drive control method comprising: In a first step, the motor drive control device sets a first commutation mode or a second commutation mode as a commutation mode based on the rotation speed of the motor, wherein the first commutation mode commutates the coil based on a detection result of a zero crossing of a back electromotive force voltage of the coil of the motor, and the second commutation mode commutates the coil according to a target energization time; In a second step, the motor drive control device generates a drive control signal for controlling the drive of the motor; In a third step, the motor drive control device sets a current limit value that serves as a reference for limiting the current flowing through the coil; as well as In a fourth step, the motor drive control device generates the drive control signal for stopping excitation of the coil when the current flowing through the coil reaches the current limit value, The third step comprises: In a fifth step, the motor drive control device sets the current limit value to a first value during a period in which the commutation mode is the first commutation mode; as well as In a sixth step, the motor drive control device sets the current limit value to a second value smaller than the first value during at least a portion of the second commutation mode.

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