Motor drive device and motor drive method

Abstract

To provide a small and inexpensive motor drive device that can prevent overcurrent due to saturation of an output inductor in advance.SOLUTION: A motor drive device 1 includes a switching circuit 10 that outputs a drive current Idrive for driving a motor M, an output inductor that smoothes the drive current Idrive, a shunt resistor R0 that detects the drive current Idrive between the output inductor and the motor M, and a control device 30 that PWM-controls the switching circuit 10 on the basis of a difference between the detected drive current Idrive and a current command value Sc from the outside, and the control device 30 sets a time until the drive current Idrive reaches a saturation current Is of the output inductor from the lower limit value Iu of a ripple current of a predetermined rated current Ir when the switching circuit 10 is controlled with the maximum duty as a time limit Tm, and stops the PWM control before the duration of the maximum duty reaches the time limit Tm.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a motor drive device and a motor drive method.

Background Art

[0002] A motor drive device that outputs a drive current for a motor by configuring a full-bridge circuit (H-bridge) with four switching elements is widely used. For example, in the prior arts described in Patent Document 1 and Patent Document 2, each switching element in the motor drive device is PWM-controlled, so that the direction and magnitude of the drive current supplied to the motor can be controlled according to the duty of PWM within the range of a predetermined rated current. More specifically, these prior arts feedback the drive current as the output current of the full-bridge circuit, and adjust the duty for each switching element so that the current command value input from the outside matches the drive current.

[0003] By the way, in such a motor drive device, by increasing the duty of PWM, not only does the drive current supplied to the motor increase, but also the followability of the drive current with respect to the current command value can be improved to increase the response speed of the motor. Therefore, the motor drive device may control the duty to 100% for the purpose of improving the response speed. However, if the state where the duty is 100% continues, the drive current flowing through the output inductor of the full-bridge circuit will eventually reach the saturation current and become an overcurrent. Therefore, it is necessary to provide an overcurrent protection function such as stopping the PWM control when the drive current exceeds a predetermined threshold value.

[0004] However, when performing such overcurrent protection, the current on the output side of the motor drive device is detected and compared with a predetermined threshold value. When it is determined that an overcurrent state exists, the PWM control of the full-bridge circuit is stopped. Therefore, at the timing of stopping the PWM control, it is possible that the drive current already exceeds the saturation current.

[0005] Therefore, a general motor drive device detects the current on the input side of a full-bridge circuit by providing a shunt resistor in a switching element as described in Patent Document 3, for example, to perform overcurrent protection and take measures before the drive current reaches the saturation current.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, when detecting current with a shunt resistor as in the prior art described in Patent Document 3, in order to detect a high-speed signal, it is necessary to select a relatively expensive non-inductive resistance element as the shunt resistor, which may lead to an increase in cost. In addition, it is necessary to ground the shunt resistor connected to the switching element as described above, and this does not always match the ground level of the control device. For this reason, the motor drive device requires an insulation circuit for mutually insulating the ground levels of the shunt resistor and the control device, and there is a risk that miniaturization is hindered due to an increase in circuit scale.

[0008] The present invention has been made in view of such a situation, and an object thereof is to provide a small and inexpensive motor drive device and a motor drive method capable of preventing in advance an overcurrent associated with saturation of an output inductor.

Means for Solving the Problems

[0009] The motor drive device according to the present invention includes a switching circuit that outputs a drive current for driving a motor, an inductor that smoothes the drive current, a current detection unit that detects the drive current between the inductor and the motor, and a control device that performs PWM control on the switching circuit based on the difference between the drive current detected by the current detection unit and a current command value input from the outside. The control device sets, as a limit time, the time from when the drive current reaches the saturation current of the inductor from a predetermined rated current when the switching circuit is controlled at the maximum duty, and stops the PWM control before the duration of the maximum duty reaches the limit time.

[0010] The motor drive device adjusts the drive current output to the motor, and thus feedbacks the drive current from the current detection unit to perform PWM control on the switching circuit. Here, an inductor is provided in the motor drive device to smooth the drive current. Further, the motor drive device sets, as a limit time, the time from when the drive current reaches the saturation current of the inductor from a predetermined rated current when the switching circuit is controlled at the maximum duty. Then, the motor drive device counts the duration of the control at the maximum duty and stops the PWM control before the set limit time is reached.

[0011] Thereby, the motor drive device can prevent, in advance, an overcurrent caused by the drive current exceeding the saturation current by predicting in advance the limit time for the drive current to rise from the rated current to the saturation current and stopping the PWM control. Further, the motor drive device does not need to provide a non-inductive shunt resistor that requires grounding in the switching circuit for overcurrent protection, and can prevent an increase in the circuit scale. Therefore, according to the motor drive device, an overcurrent associated with saturation of the output inductor can be prevented in advance in a small size and at low cost.

Effects of the Invention

[0012] According to the present invention, it is possible to provide a small and inexpensive motor drive device that can prevent an overcurrent associated with saturation of an output inductor in advance.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the content described below, and can be arbitrarily modified and implemented without changing the gist thereof. Also, the drawings used in the description of the embodiments schematically show the configurations, and may perform partial emphasis, enlargement, reduction, or omission for better understanding, and may not accurately represent the scales, shapes, etc. of the constituent members in some cases.

[0015] <First Embodiment> FIG. 1 is a circuit diagram showing the overall configuration of the motor drive device 1. The motor drive device 1 forms a drive current Idrive for driving the motor M by converting an input voltage Vin input to a pair of input terminals including a first input terminal Tin1 and a second input terminal Tin2, and outputs it to the motor M via a pair of output terminals including a first output terminal Tout1 and a second output terminal Tout2. It is a power supply circuit.

[0016] Here, the motor M is, for example, a single-phase DC motor in this embodiment, and an equivalent circuit composed of a motor inductor Lm and a motor resistance Rm connected in series is shown in FIG. 1. Further, the motor M may be a rotary motor or a linear motor. And the driving direction and driving speed of the motor M are controlled according to the direction and magnitude of the driving current Idrive supplied from the motor driving device 1.

[0017] The motor driving device 1 mainly includes a switching circuit 10, an output filter circuit 20, a shunt resistor R0 as a "current detection unit", a control device 30, and a gate driver 40.

[0018] The switching circuit 10 has a circuit configuration called a full bridge or an H bridge, and includes first switches S1 to fourth switches S4 each composed of a semiconductor switching element. Note that the semiconductor switching element is exemplified as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) in this embodiment, but may be another type of field effect transistor, or may be a bipolar transistor or an IGBT (Insulated Gate Bipolar Transistor).

[0019] Here, the first switch S1 to the fourth switch S4 each composed of a MOSFET have their gates connected to a gate driver 40 described later, and are turned ON / OFF by being PWM controlled (Pulse Width Modulation). The drain of the first switch S1 is connected to the first input terminal Tin1 together with the drain of the third switch S3, and the source is connected to the drain of the second switch S2. The drain of the fourth switch S4 is connected to the source of the third switch S3, and the source is connected to the second input terminal Tin2 together with the source of the second switch S2.

[0020] The output filter circuit 20 includes a first output inductor Lout1, a second output inductor Lout2, and an output capacitor Cout. One end of the first output inductor Lout1 is connected to the connection point between the third switch S3 and the fourth switch S4, and the other end is connected to one end of the output capacitor Cout. One end of the second output inductor Lout2 is connected to the connection point between the first switch S1 and the second switch S2, and the other end is connected to the other end of the output capacitor Cout. Thereby, the output filter circuit 20 smoothes the output current output from the switching circuit 10 and supplies a drive current Idrive to the motor M via the first output terminal Tout1 and the second output terminal Tout2.

[0021] In this embodiment, one end of the shunt resistor R0 is connected to the other end of the first output inductor Lout1, and the other end is connected to the first output terminal Tout1, thereby detecting the drive current Idrive between the first output inductor Lout1 and the motor M.

[0022] The control device 30 is composed of, for example, a known microcomputer control circuit, and includes a current control unit 31, a PWM control unit 32, and a limit time calculation unit 33 as functional modules. Then, the control device 30 controls the overall operation of the motor drive device 1 so as to form a drive current Idrive suitable for driving the motor M.

[0023] The current control unit 31 detects the drive current Idrive via the shunt resistor R0, and a current command value Sc is input from the outside. Then, the current control unit 31 calculates the difference between the two so that the switching circuit 10 can be PWM-controlled so that the drive current Idrive and the current command value Sc match.

[0024] The PWM control unit 32 calculates the duty of the PWM corresponding to the difference calculated by the current control unit 31, generates a PWM signal with the duty, and outputs it to the gate driver 40.

[0025] When the input voltage Vin is input from the first input terminal Tin1, the limit time calculation unit 33 stores in advance the output inductor in the output filter circuit 20 and the motor inductor Lm of the motor M. As will be described in detail later, the limit time calculation unit 33 sets a limit time Tm during which the switching circuit 10 can continue to operate at the maximum duty. Note that although the path through which the drive current Idrive is input from the shunt resistor R0 is shown in FIG. 1, it is not necessary to obtain the drive current Idrive in the present embodiment.

[0026] Based on the PWM signal input from the PWM control unit 32, the gate driver 40 generates the gate voltages of the first switch S1 to the fourth switch S4 to perform PWM control on each switch. More specifically, the gate driver 40 controls the pair of the first switch S1 and the fourth switch S4 and the pair of the second switch S2 and the third switch S3 to be complementarily turned on and off in an inverted phase at a duty corresponding to the pulse width of the PWM signal. Note that in the present embodiment, it is assumed that the operating frequency of the PWM control is constant.

[0027] With the above configuration, the motor drive device 1 forms a drive current Idrive corresponding to the current command value Sc input from the outside and supplies it to the motor M, thereby controlling the drive state of the motor M within a range where the drive current Idrive does not exceed a predetermined rated current Ir.

[0028] By the way, the motor drive device 1 may improve the followability of the drive current Idrive with respect to the current command value Sc and increase the response speed of the motor M by controlling the duty of the PWM control for the switching circuit 10 to 100%, that is, the maximum duty. However, if the state where the duty is 100% continues, there is a risk that the drive current Idrive flowing through the output inductor of the switching circuit 10, that is, the first output inductor Lout1 and the second output inductor Lout2 in the present embodiment, will eventually reach the saturation current Is and become an overcurrent.

[0029] Therefore, the motor drive device 1 sets a limit time Tm that can continue the maximum duty state by a motor drive method described in detail below, and stops the PWM control for the switching circuit 10 before the drive current Idrive reaches the saturation current Is of the output inductor.

[0030] FIG. 2 is a timing chart showing the change in the drive current Idrive and the control of the switching circuit 10 when the PWM control shifts to the maximum duty from the state where the drive current Idrive is the rated current Ir. That is, FIG. 2 schematically shows the change in the drive current Idrive detected by the shunt resistor R0 when the horizontal axis is time, and the control states of the first switch S1 to the fourth switch S4 at that time.

[0031] The control device 30 basically performs PWM control of the switching circuit 10 within the range where the drive current Idrive is equal to or less than the rated current Ir. Specifically, for example, when the duty for maintaining the drive current Idrive at the rated current Ir is 80%, as shown in the period from time t1 to time t2 in FIG. 2, the second switch S2 and the third switch S3 are 80% and the first switch S1 and the fourth switch S4 are 20% in one cycle of the PWM control. The switching circuit 10 is controlled so as to have an on-duty. Here, the rated current Ir is defined with respect to the average value of the ripple current of the drive current Idrive that varies with the duty ratio of the PWM control.

[0032] In addition, the control device 30 sets a limit time Tm in the limit time calculation unit 33 until the drive current Idrive reaches the saturation current of the output inductor from the rated current Ir in case a current command value Sc corresponding to the maximum duty of the PWM control is continuously input. More specifically, the limit time calculation unit 33 calculates the limit time Tm by the following formula when the total inductance value of the output filter circuit 20 and the motor M is L, the difference current value between the lower limit value Iu of the ripple current of the rated current Ir and the saturation current Is, and the input voltage input to the switching circuit 10 is Vin. Tm = L × Im / Vin ··· Formula (1)

[0033] Here, the total inductance value L is a known fixed value expressed as L = 2×L0 + L1 in the present embodiment, where the inductance values of the first output terminal Tout1 and the second output terminal Tout2 are L0 respectively, and the inductance value of the motor inductor Lm is L1. Also, the differential current value Im is a known fixed value calculated from the lower limit value Iu of the ripple current of a predetermined rated current Ir and the saturation current Is as a characteristic of the output inductance.

[0034] On the other hand, the input voltage Vin in the above formula (1) is a time-series signal sequentially acquired during the execution of PWM control in the present embodiment, and the value thereof changes to update the limit time Tm. That is, even when the input voltage Vin increases, the risk of overcurrent can be reduced by setting the limit time Tm shorter.

[0035] In addition, when the input voltage Vin can be regarded as constant without fluctuation, by storing the limit time Tm calculated from the above formula (1) as a fixed value in the control device 30 in advance, it is not necessary to sequentially update the limit time Tm, and it is not necessary to acquire the input voltage Vin either. Therefore, it can contribute to the miniaturization and cost reduction of the motor drive device 1.

[0036] Then, by setting the limit time Tm as described above, the control device 30 measures the duration of the maximum duty by an internal timer (not shown) and compares it with the limit time Tm when the PWM control is switched to the maximum duty as shown at time t2 in FIG. 2, for example. For this reason, the control device 30 can avoid the overcurrent state without the drive current Idrive reaching the saturation current Is of the output inductor by stopping the PWM control at a timing before the duration reaches the limit time Tm as shown at time t3 in FIG. 2, for example.

[0037] Further, after the control device 30 stops the PWM control based on the limit time Tm, as shown at time t4 in FIG. 2, the control device 30 resumes the PWM control on the condition that the drive current Idrive has decreased to the rated current Ir. Thereby, even when the duty remains 100% again after time t4, the control device 30 can perform overcurrent protection based on the limit time Tm set as described above.

[0038] As described above, the control device 30 does not perform overcurrent protection after the drive current Idrive detected on the output side reaches the saturation current Is. Instead, the control device 30 predicts in advance the limit time Tm during which the drive current Idrive rises from the lower limit value Iu of the ripple current of the rated current Ir to the saturation current Is and stops the PWM control, thereby preventing in advance an overcurrent caused by the drive current Idrive exceeding the saturation current Is. Further, the motor drive device 1 does not need to provide a non-inductive shunt resistor that requires grounding in the switching circuit 10 for overcurrent protection, and can prevent an increase in the circuit scale. Therefore, according to the motor drive device 1, an overcurrent associated with saturation of the output inductor can be prevented in advance in a small and inexpensive manner.

[0039] Also, the motor drive device 1 according to the present embodiment can predict the duration of the maximum duty relatively accurately by calculating the limit time Tm based on the known saturation current Is of the output inductor by the above formula (1) in the limit time calculation unit 33. Further, the motor drive device 1 according to the present embodiment monitors the input voltage Vin input to the switching circuit 10 during the execution of the PWM control, and updates the limit time Tm according to the input voltage Vin, so that even when the input voltage Vin changes, overcurrent protection can be performed in response to the change.

[0040] <Second Embodiment> Next, the motor drive device 1 according to the second embodiment will be described. The motor drive device 1 according to the second embodiment differs from the motor drive device 1 of the first embodiment in the calculation method of the limit time Tm. Hereinafter, the differences from the first embodiment will be described, and the same reference numerals will be given to the components common to the first embodiment, and detailed descriptions thereof will be omitted.

[0041] In the motor drive device 1 according to the second embodiment, when the difference current value between the lower limit value Iu of the ripple current of the rated current Ir and the saturation current Is is Im, and the slope of the ripple current of the drive current Idrive detected by the shunt resistor R0 is ΔIL (see FIG. 2), the limit time Tm is calculated by the following equation. Tm = Im / ΔIL ··· Equation (2)

[0042] Here, for ΔIL, for example, by being acquired for each cycle of PWM control, Equation (2) can be updated, and thereby, even when the input voltage Vin changes, the limit time Tm can be set corresponding to the change. Further, the motor drive device 1 can set the limit time Tm even when the motor inductor Lm is unknown.

[0043] Then, the motor drive device 1 according to the second embodiment stops the PWM control at the timing before the duration of the maximum duty reaches the limit time Tm based on the limit time Tm set by Equation (2), so that the drive current Idrive can avoid an overcurrent state without reaching the saturation current Is of the output inductor.

Explanation of Reference Numerals

[0044] 1 Motor drive device 10 Switching circuit 20 Output filter circuit 30 Control device 31 Current control unit 32 PWM control unit 33 Limit time calculation unit 40 Gate driver M Motor R0 shunt resistor S1 to S4 First switch to Fourth switch Lout1, Lout2 First output inductor, Second output inductor Lm Motor inductor Ir Rated current Iu Lower limit value of the ripple current of the rated current Is Saturation current Tm Limit time ΔIL Slope of the ripple current

Claims

1. A switching circuit that outputs a drive current for driving a motor, an inductor that smoothes the drive current, a current detection unit that detects the drive current between the inductor and the motor, and a control device that performs PWM control on the switching circuit based on the difference between the drive current detected by the current detection unit and a current command value input from the outside. The control device sets, as a limit time, the time from when the drive current reaches the saturation current of the inductor from a predetermined rated current when the switching circuit is controlled with a maximum duty, and stops the PWM control before the duration of the maximum duty reaches the limit time. A motor drive device.

2. When the total inductance value of the inductor and the motor is L, the differential current value between the lower limit value of the ripple current of the rated current and the saturation current is Im, and the input voltage input to the switching circuit is Vin, the limit time is calculated as Tm = L × Im / Vin. The motor drive device according to claim 1.

3. The control device monitors the input voltage input to the switching circuit during the execution of the PWM control, and updates the limit time according to the input voltage. The motor drive device according to claim 2.

4. When the differential current value between the lower limit value of the ripple current of the rated current and the saturation current is Im, and the slope of the ripple current of the drive current detected by the current detection unit is ΔIL, the limit time is calculated as Tm = Im / ΔIL. The motor drive device according to claim 1.

5. After stopping the PWM control based on the limit time, the control device resumes the PWM control on the condition that the drive current has decreased to the rated current. The motor drive device according to any one of claims 1 to 4.

6. A motor drive method for driving a motor with a drive current obtained by smoothing the output current of a switching circuit with an inductor, wherein the switching circuit is PWM controlled based on the difference between the drive current detected between the inductor and the motor and a current command value input from the outside. A motor driving method, wherein when the switching circuit is controlled at a maximum duty ratio, the time from when the drive current reaches the saturation current of the inductor from a predetermined rated current is set as a limit time, and the PWM control is stopped before the duration of the maximum duty ratio reaches the limit time.

7. The motor driving method according to claim 6, wherein the limit time is calculated as Tm = L × Im / Vin, where L is the total inductance value of the inductor and the motor, Im is the differential current value between the lower limit value of the ripple current of the rated current and the saturation current, and Vin is the input voltage input to the switching circuit.

8. The motor driving method according to claim 7, wherein the input voltage input to the switching circuit is monitored during the execution of the PWM control, and the limit time is updated according to the input voltage.

9. The motor driving method according to claim 6, wherein the limit time is calculated as Tm = Im / ΔIL, where Im is the differential current value between the lower limit value of the ripple current of the rated current and the saturation current, and ΔIL is the slope of the ripple current of the detected drive current.

10. The motor driving method according to any one of claims 6 to 9, wherein after the PWM control is stopped based on the limit time, the PWM control is restarted on the condition that the drive current has dropped to the rated current.

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