Patent Publication Number: US-2015061562-A1

Title: Motor drive unit

Description:
The present application is based on Japanese patent application No. 2013-182325 filed on Sep. 3, 2013, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a motor drive unit and, in particular, to a motor drive unit that allows the failure detection of a current feedback system without using a separate failure detection circuit. 
     2. Related Art 
     A motor drive unit is known that is provided with a motor drive circuit to perform a constant current control of a current flowing through a load of a motor etc. by means of a current feedback system and is provided with an overcurrent detection circuit (see e.g. JP-A-2013-062721). 
     The motor drive unit has a shunt resistor for detecting a current flowing through the motor as a voltage value and a comparator for comparing the detected voltage with threshold voltage. The threshold voltage is generated by a threshold generation circuit which is configured to increase the threshold voltage with an increase in AC changes of motor current/power supply voltage value and also to increase the threshold voltage with a decrease in temperature of the motor. Thus, desired hysteresis characteristics can be maintained and it is possible to stably detect a failure and to recover from the overcurrent detected state. 
     SUMMARY OF THE INVENTION 
     The conventional motor drive unit is not configured to use a part thereof to detect a failure such as overcurrent and it is thus necessary to use a failure detection circuit separate from the motor drive unit. This may cause an increase in circuit space and the number of components. 
     It is an object of the invention to provide a motor drive unit that allows the failure detection of the current feedback system by using a part of the motor drive unit without using a separate failure detection circuit. 
     (1) According to one embodiment of the invention, a motor drive unit comprises: 
     a motor drive circuit that drives a motor by controlling on/off of current; 
     a control unit generating a drive command signal for driving the motor; 
     a current feedback circuit that comprises a current detection resistor and a comparator connected in series with the motor and outputs a comparison output signal based on comparison between a current detection signal of a motor current and a target value signal; 
     a latch circuit that latches a current detection result based on the drive command signal and the comparison output signal; and 
     a gate circuit for driving the motor drive circuit based on the drive command signal and a latch output signal output from the latch circuit, 
     wherein the control unit detects a failure of a current feedback system comprising the current feedback circuit and the latch circuit according to a state of the latch output signal inputted after a time measured based on the drive command signal reaches a predetermined time. 
     In the above embodiment (1) of the invention, the following modifications and changes can be made. 
     (i) The predetermined time is set to be a time T2 to reach a target current to be set by the target value signal during normal operation. 
     (ii) When a failure is detected, the control unit changes the drive command signal to turn off the current flowing though the motor. 
     (iii) The control unit comprises a single chip microcomputer and 
     wherein the time measured based on the drive command signal is measured by a timer unit built in the single chip microcomputer. 
     (iv) The control unit determines a failure when the latch output signal is not inverted after the measured time after the measured time reaches the time T2. 
     (v) The control unit terminates an operation of the failure detection when the latch output signal is inverted after the measured time after the measured time reaches the time T2. 
     (vi) The control unit outputs at least once the drive command signal in a motor drive command time T1 greater than the time T2. 
     EFFECTS OF THE INVENTION 
     According to one embodiment of the invention, a motor drive unit can be provided that allows the failure detection of the current feedback system by using a part of the motor drive unit without using a separate failure detection circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Next, the present invention will be explained in more detail in conjunction with appended drawings, wherein: 
         FIG. 1  is a schematic block diagram illustrating a motor drive unit in an embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating the motor drive unit in the embodiment of the invention; 
         FIG. 3  is a diagram illustrating signal waveforms of respective portions during normal operation of the motor drive unit in the embodiment of the invention; 
         FIG. 4  is a flowchart showing an operation to detect a failure of a current feedback system in the motor drive unit; and 
         FIG. 5  is a diagram illustrating signal waveforms of respective portions when a failure occurs in the motor drive unit in the embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Overall Structure of Motor Drive Unit  1   
       FIG. 1  is a schematic block diagram illustrating a motor drive unit in the embodiment of the invention and  FIG. 2  is a circuit diagram illustrating the motor drive unit in the embodiment of the invention. 
     The motor drive unit  1  in the embodiment of the invention has a motor drive circuit  100  which drives a motor  110  by controlling on/off of current, a microcomputer  300  as a control unit generating a drive command signal V s  for driving the motor  110 , a current feedback circuit  400  which is composed of a shunt resistor  410  as a current detection resistor and a comparator  420  connected in series with the motor  110  and outputs a comparison output signal V c  based on comparison between a current detection signal V i  of a motor current I m  and a target value signal V ref , a latch circuit  500  which latches a current detection result based on the drive command signal V s  and the comparison output signal V c , and a gate circuit  600  for driving the motor drive circuit  100  based on the drive command signal V s  and a latch output signal V r  output from the latch circuit  500 . 
     In this configuration, the motor drive circuit  100  has a bridge circuit  150  controlling on/off of the current I m  flowing through the motor  110  and a drive circuit  200  for driving the bridge circuit  150 . Meanwhile, the microcomputer  300  is configured to determines the state of the latch output signal V r  on the basis of time measured based on the drive command signal V s  and thus to detect a failure of a current feedback system which is composed of the current feedback circuit  400  and the latch circuit  500 . 
     The bridge circuit  150  is composed of four MOSFETs and has a bridge configuration in which the motor  110  is connected between a FET  1  and a FET  3  and between a FET  2  and a FET  4 . By turning on the FETs  1  and  4  and turning off the FETs  2  and  3 , a motor current flows in a direction of I m  shown in  FIG. 2  and the motor  110  runs forward. Conversely, by turning off the FETs  1  and  4  and turning on the FETs  2  and  3 , the motor current flows in a reverse direction and the motor  110  runs backward. The rotation of the motor  110  is controlled by a combination and timing of ON/OFF of the MOSFETs. Note that, the combination and timing of ON/OFF of the MOSFETs are controlled by predetermined on/off signals input from the drive circuit  200  and the gate circuit  600 . 
     The drive circuit  200  is connected to the gate circuit  600  on the input side and is connected to the bridge circuit  150  on the output side. Switching of each FET is controlled based on driving signals V d1  and V d2  output from the gate circuit  600  and an electric current then flows from supply voltage 12V to the bridge circuit  150  and the motor  110 . 
     The microcomputer  300  is a single chip microcomputer as a control unit which is provided with a drive command signal generator  310  generating the drive command signal V s  for driving the motor  110 , Pch direction instruction portions  320 , Nch FET control signal generators  330 , a latch output signal input portion  340 , a current command value generator  350  and a timer unit  360 , etc. 
     The drive command signal generator  310  generates the drive command signal V s  as a PWM (Pulse Width Modulation) signal for driving the motor  110 . Using, e.g., a PWM function, etc., of the microcomputer  300 , the drive command signal V s  during motor drive command time T1 is generated by inverting an output between Hi-level and Lo-level at regular intervals. The drive command signal V s  as an output is connected to the latch circuit  500  and the gate circuit  600 . 
     The Pch direction instruction portions  320  output motor drive direction control signals V m1  and V m2  which are signals for controlling the rotation direction of the motor  110 . The motor drive direction control signals V m1  and V m2  are input to AND circuits  610  of the gate circuit  600 . 
     The Nch FET control signal generators  330  output Nch FET control signals V F1  and V F2  which are signals for controlling the rotation direction of the motor  110 . Hi or Lo-level of the Nch FET control signals V F1  and V F2  are input to the FETs  3  and  4  at a predetermined timing to control ON/OFF of the MOSFETs. The bridge circuit  150  is controlled by a combination of the Nch FET control signals V F1  and V F2  with the motor drive direction control signals V m1  and V m2 , thereby controlling the rotation direction of the motor  110 . 
     The latch output signal V r  output from the latch circuit  500  is input to the feedback input portion  340 . The input latch output signal V r  (=a current feedback signal) is further input to the timer unit  360  in the microcomputer. 
     The target value signal V ref , which is DC voltage signal to the current feedback circuit  400  and is a reference voltage (threshold) of the comparator  420 , is input to the comparator  420  from the current command value generator  350 . By adjusting the target value signal V ref , it is possible to control the motor current I m  and thereby to adjust a steady rotation speed of the motor. 
     The timer unit  360  is built in the microcomputer  300 . The latch output signal V r  is input to the timer unit  360  which then measures time elapsed from the rise of the drive command signal V s . 
     The current feedback circuit  400  and the latch circuit  500  described below form the current feedback system from the motor  110  to the microcomputer  300 . 
     The current feedback circuit  400  is composed of the shunt resistor  410  and the comparator  420  which are connected in series with the motor  110 . The shunt resistor  410  is connected to the comparator  420  via a low-pass filter (LPF)  430 . Thus, unstable operation caused by noise or voltage ripple, etc., is suppressed. The comparison output signal V c  is output based on comparison between the current detection signal V i  of the motor current I m  and the target value signal V ref . The output of the current feedback circuit  400  is connected as the comparison output signal V c  to the input of the latch circuit  500 . When, for example, the bridge circuit  150  is driven by turning on the drive circuit  200  and a current flows through the motor  110 , the same current (I m ) as that flowing through the motor  110  also flows through the shunt resistor  410  which is connected to the motor  110 . Due to the motor current I m  flowing through a resistor Ra of the shunt resistor  410 , the current detection signal V i  (=Ra×I m ) is generated at both ends of the shunt resistor  410 . The comparator  420  compares the current detection signal V i  with the target value signal V ref  generated by the current command value generator  350 , the comparison output signal V c  is inverted and output when the target value signal V ref  becomes less than the current detection signal V i , and this comparison output signal V c  is input to the latch circuit  500 . 
     The latch circuit  500  receives the drive command signal V s  from the microcomputer  300  as well as the comparison output signal V c  from the current feedback circuit  400 , and latches (holds) a current detection result based on the drive command signal V s  and the comparison output signal V c . The latch output signal V r  is input to the latch output signal input portion  340  and is also input to the gate circuit  600  via a drive stopping Tr  620 . 
     The gate circuit  600  outputs the driving signals Val and Vat based on the drive command signal V s  as well as the latch output signal V r  output from the latch circuit  500 . Since the rotation direction of the motor is also controlled in the present embodiment, the AND circuits  610  output the driving signals V d1  and V d2  as the logical AND of the drive command signal V s  and the latch output signal V r  or the motor drive direction control signals V m1 , V m2 . The drive circuit  200  is driven by the driving signals V d1  and V d2  and the motor  110  is powered on. Then, the rotation direction of the motor is controlled by the Nch FET control signal generators  330  in combination with gate control using the motor drive direction control signals V m1  and V m2 . 
     Normal Operation of Motor Drive Unit  1   
       FIG. 3  is a diagram illustrating signal waveforms of respective portions during normal operation of the motor drive unit in the embodiment of the invention. The normal operation (constant-current control operation) of the motor drive unit  1  will be described in order of the following (1) to (9) along the waveforms at main points during normal operation shown in  FIG. 3 . 
     (1) In the motor drive command time T1, the output of the drive command signal V s  becomes Hi-level (ON signal V on ). 
     (2) The driving signals V d1  and V d2  are output from the gate circuit  600  to turn on the FETs  1  and  4  and off the FETs  2  and  3 . 
     (3) The current I m  flows through the motor  110  and the current detection signal V i  is generated by the shunt resistor  410 . 
     (4) The latch output signal V r  becomes Hi-level when the current detection signal V i  becomes more than the target value signal V ref . 
     (5) Accordingly, the FETs  1  and  4  are turned off, the motor  110  stops running and the current detection signal V i  becomes 0. 
     (6) After a predetermined period of time, the drive command signal V s  is switched to Lo-level (switched from the ON signal V on  to a latch-clear signal V clr ). 
     (7) At the switching edge of the drive command signal V s  from Hi to Lo-level (when switched to the latch-clear signal V clr ), the holding state of the latch circuit  500  is released. 
     (8) The driving signal V d  continues staying Lo-level during the Lo-level period of the drive command signal V s  (during the latch-clear signal V clr  period). 
     (9) After a predetermined period of time, the drive command signal V s  is switched to Hi-level (switched to the ON signal V on ). 
     From this onward, the operations of (1) to (9) are repeated in the motor drive command time T1. As shown in  FIG. 3 , in the periods of (3) and (5), the motor  110  is driven with a constant current at a current value which is determined by the current feedback circuit  400  and corresponds to the target value signal V ref , while an actual motor current repeats increase and decrease. 
     Operation of Motor Drive Unit  1  in Failure 
     In  FIG. 3 , a motor current waveform during operation of the motor exponentially increases according to time constant. That is, when the motor is operated, a current flows through the motor  110  and then gradually increases due to resistance and coil component. The motor current I m  is represented by the following formula: 
       Motor current  I   m =(supply voltage 12V/armature resistance)×(1− e   −t   /τe )
 
     where τe is a ratio of inductance to armature resistance. Time T2 to reach the target current set by the target value signal V ref  during the operation of the motor is derived from the above formula. 
     In the present embodiment, when T2, which is derived as a period of time in which the current flows through the motor  110 , gradually increases due to the coil component and then reaches the target current (current feedback), satisfies the relation of Motor drive command time T1&gt;T2, T2 is set as current feedback detection time. 
     That is, when the output of the latch output signal V r  is not inverted after the current feedback detection time T2 is elapsed since the drive command signal V s  is output from the microcomputer  300  to drive the motor  110  (during the motor drive command time T1), it is judged that the circuit is malfunctioning (a failure of the current feedback system) and the motor  110  is stopped by switching the drive command signal V s  to Lo-level. 
       FIG. 4  is a flowchart showing an operation to detect a failure of a current feedback system in the motor drive unit. A failure detection method using the microcomputer  300  will be described below based on the flowchart. 
     Firstly, the microcomputer  300  determines whether or not the rise of the drive command signal V s  is detected (Step  1 ). The process proceeds to Step  2  when the rise of the drive command signal V s  is detected. Step  1  is repeated when the rise of the drive command signal V s  is not detected. 
     In Step  2 , the timer unit  360  starts counting. The count is started in a state that an internal counter value TCNT is reset. 
     The timer unit  360  determines whether or not time T corresponding the counter value TCNT, i.e., T(TCNT), reaches the current feedback detection time T2 (Step  3 ). Step  3  is repeated after adding 1 to the counter value TCNT each time when T(TCNT) has not reached T2. The process proceeds to Step  4  when T(TCNT) reaches T2. 
     In Step  4 , the timer unit  360  stops counting. 
     The microcomputer  300  determines whether or not the relation of T2&lt;Motor drive command time T1 is satisfied (Step  5 ). The failure determination flow is ended when the relation is not satisfied (NO). The process proceeds to Step  6  when the relation is satisfied (YES). 
     The microcomputer  300  determines whether or not the latch output signal V, is inverted and becomes Hi-level (Step  6 ). The failure determination flow is ended in the case of YES. The process proceeds to Step  7  in the case of NO. The microcomputer  300  controls the drive command signal V s  to Low level and thereby stops the motor  110 . In other words, the microcomputer  300  determines that the current feedback system has failed, and then stops the drive command signal V s  as a PWM signal at constant frequency which is then switched to Low level (Step  7 ). This stops the motor  110  and provides safety in the event of failure. 
       FIG. 5  is a diagram illustrating signal waveforms of respective portions when a failure occurs in the motor drive unit in the embodiment of the invention. The operation of the motor drive unit  1  when a failure occurs will be described in order of the following (1) to (5) along the waveforms at main points shown in  FIG. 5 . 
     (1) In the motor drive command time T1, the output of the drive command signal V s  becomes Hi-level (ON signal V on ). 
     (2) The driving signals V d1  and V d2  are output from the gate circuit  600  to turn on the FETs  1  and  4  and off the FETs  2  and  3 . 
     (3) The current I m  flows through the motor  110  and the current detection signal V i  is generated by the shunt resistor  410 . 
     (4) A failure occurs in the current feedback system. In other words, a failure occurs in the current feedback circuit  400  or the latch circuit  500 . Due to this failure, the latch output signal V r  is not inverted even after the current feedback detection time T2 is elapsed. 
     (5) The microcomputer  300  controls the drive command signal V s  to Low level. 
     Accordingly, the driving signals V d1  and V d2  become Low level, the FETs  1  and  4  are turned off and the motor  110  stops running.
 
The motor  110  is stopped by the operations (1) to (5) when a failure occurs in the current feedback system.
 
     Effects of the Embodiment 
     The motor drive unit  1  configured as described achieves the following effects. 
     (1) In the motor drive unit  1  in the embodiment of the invention, time elapsed from the rise of the drive command signal V s  is counted by the timer unit of the microcomputer  300 , detection of the inverted output of the latch output signal V r  is carried out after elapsing the current feedback detection time T2, and a failure of the current feedback system is thereby detected. As a result, it is possible to provide a motor drive unit which can detect a failure of the current feedback system using a portion of the motor drive unit per se without separately providing a failure detection circuit. 
     (2) An electric current flows through the motor  110  even when a failure occurs in the current feedback system (the current feedback circuit  400 , the latch circuit  500 ). Therefore, there is a risk that an overcurrent flows through the motor  110  in the event of failure. However, it is possible to prevent the overcurrent to the motor  110  by the present embodiment. 
     (3) The current feedback detection time T2 can be changed depending on the operating environment such as temperature and it is possible to appropriately detect a failure by setting the optimum T2. In addition, by setting the current feedback detection time T2 to be longer than the calculated time to reach the target current (calculated current feedback), it is possible to add a failure detection function without impairing normal current control operation. 
     (4) It is not necessary to separately provide a failure detection circuit. Therefore, it is possible to reduce space for circuit and this allows a substrate to be downsized and space for substrate to be effectively used. 
     (5) The reduction in the space for circuit, the downsizing of the substrate and the effective use of space for substrate described above provide an effect of improving the product cost. 
     Although the embodiment of the invention has been described, the embodiment is merely an example and the invention according to claims is not to be limited thereto. Although the motor drive unit which drives a motor by controlling on/off of a current has been described above, the present embodiment is applicable not only to the motor and is also applicable as a load driving device as long as activation of the load can be controlled by controlling on/off of the current. It is applicable to electromagnetic coils and heaters, etc., as the load other than motor, as long as it can be driven by an electric current. 
     In addition, this new embodiment and modifications thereof may be implemented in various other forms, and various omissions, substitutions and changes, etc., can be made without departing from the gist of the invention. In addition, all combinations of the features described in the embodiment are not necessary to solve the problem of the invention. Further, the embodiment and modifications thereof are included within the scope and gist of the invention and also within the invention described in the claims and the range of equivalency.