Patent Publication Number: US-2023163708-A1

Title: Motor driver circuit for linear motor, positioning device using the same, and hard disk device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present invention claims priority under 35 U.S.C. § 119 to Japanese Application, 2021-189194, filed on Nov. 22, 2021, the entire contents of which being incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a driver circuit for a linear motor. 
     BACKGROUND 
     Linear motors (linear actuators) that position target objects are used in various electronic apparatuses and industrial machines. A voice coil motor is one of the linear motors and can control a position of a mover according to a supplied drive current. A drive circuit for the voice coil motor feedback-controls a current flowing through the voice coil motor so as to approach a target current that defines a target position. 
     In an actuator driver that positions a head of a hard disk, it is possible to switch between a constant current control that stabilizes a current at a target value and a control that stabilizes a back electromotive force at a target value. There is known a circuit that detects a back electromotive force by subtracting a voltage proportional to a current of a motor from a voltage across the motor. 
     For applications such as hard disks, etc., a power supply voltage of 12 V system is used. Therefore, in the known circuit described above, it is necessary to configure a circuit block for detecting the back electromotive force with high-withstand voltage elements having a withstand voltage higher than 12 V. In particular, in a case where a variable gain is used for detecting the back electromotive force, a circuit area increases when gain switching is implemented with the high-withstand voltage elements. 
     SUMMARY 
     Some embodiments of the present disclosure aim to reduce an area of a circuit that detects a back electromotive force. 
     According to one embodiment of the present disclosure, a motor driver circuit includes: a current detection circuit configured to generate a current detection signal according to a drive current of a motor as an object to be driven; a first amplifier configured to amplify the current detection signal; a second amplifier configured to multiply a voltage across the motor by a gain smaller than 1 and output the multiplied voltage; and a third amplifier configured to generate a back electromotive force detection signal according to a difference between an output of the first amplifier and an output of the second amplifier 
     Arbitrary combinations of the above constituent elements and mutual replacement of the constituent elements and expressions among methods, devices, systems, etc. are also effective as embodiments of the present disclosure. Furthermore, the description in this section (SUMMARY) does not provide all the essential features of the present disclosure, and thus sub-combinations of those described features can also constitute the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the present disclosure. 
         FIG.  1    is a block diagram of a positioning device including a motor driver circuit according to an embodiment. 
         FIG.  2    is an equivalent circuit diagram of a motor. 
         FIG.  3    is a block diagram of a back electromotive force detection circuit according to Example 1. 
         FIG.  4    is a block diagram of a back electromotive force detection circuit according to Example 2. 
         FIG.  5    is a circuit diagram showing a configuration example of a back electromotive force detection circuit. 
         FIG.  6    is a view showing a hard disk device including a motor driver circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one of ordinary skill in the art that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, systems, and components have not been described in detail so as not to unnecessarily obscure aspects of the various embodiments. 
     Overview of Embodiments 
     An overview of some exemplary embodiments of the present disclosure is described. This overview presents, as a prologue to the detailed description which will be presented later, some concepts of one or more embodiments in simplified forms for the purpose of basic understanding of the embodiments, but it is not intended to limit the scope of the disclosure. This summary is not a comprehensive overview of all possible embodiments, and it is intended to neither identify key elements of all embodiments nor delineate the scope of some or all aspects. For the sake of convenience, “an embodiment” may be used to refer to one embodiment (example or modification) or a plurality of embodiments (examples or modifications) disclosed herein. 
     A motor driver circuit according to an embodiment includes: a current detection circuit configured to generate a current detection signal according to a drive current of a motor as an object to be driven; a first amplifier configured to amplify the current detection signal; a second amplifier configured to multiply a voltage across the motor by a gain smaller than 1 and output the multiplied voltage; and a third amplifier configured to generate a back electromotive force detection signal according to a difference between an output of the first amplifier and an output of the second amplifier. 
     A large voltage near a power supply voltage can be generated across the motor. When this large voltage is input to a subtraction amplifier, it is necessary to configure the subtraction amplifier with high-withstand voltage elements. With the above-described configuration, by providing the second amplifier and compressing a DC bias included in the voltage across the motor, a voltage input to the third amplifier, which is the subtraction amplifier, can be reduced. As a result, the third amplifier can be configured with low-withstand voltage elements, thereby reducing an area of the motor driver circuit. 
     In one embodiment, the second amplifier may include: a first operational amplifier; a first resistor connected between a first input of the first operational amplifier and a first end of the motor; a second resistor connected between a second input of the first operational amplifier and a second end of the motor; a third resistor connected between the first input of the first operational amplifier and an output of the first operational amplifier; and a fourth resistor having one end connected to the second input of the first operational amplifier and the other end receiving a reference voltage. 
     In one embodiment, the third amplifier may include: a second operational amplifier; a fifth resistor connected between a first input of the second operational amplifier and the output of the second amplifier; a sixth resistor connected between a second input of the second operational amplifier and the output of the first amplifier; a seventh resistor connected between the first input of the second operational amplifier and an output of the second operational amplifier; and an eighth resistor having one end connected to the second input of the second operational amplifier and the other end receiving a reference voltage. 
     In one embodiment, the motor may a linear motor. 
     In one embodiment, the linear motor may be a voice coil motor. 
     In one embodiment, the motor driver circuit may be integrated on one semiconductor substrate. The term “integrated” is intended to include a case where all circuit elements are formed on a semiconductor substrate or a case where main elements of the circuit are integrated on the semiconductor substrate. In addition, some resistors, capacitors, and the like for adjustment of a circuit constant may be provided outside the semiconductor substrate. By integrating a circuit on one chip, a circuit area can be reduced and characteristics of the circuit elements can be kept uniform. 
     A positioning device according to an embodiment includes: a linear motor; and any one of the above-described motor driver circuits, wherein the motor driver circuits are configured to drive the linear motor. 
     A hard disk device according to an embodiment includes the above-described positioning device. 
     Embodiment 
     An embodiment will be now described with reference to the drawings. Like or equivalent components, members, and processes illustrated in each drawing are given like reference numerals and a repeated description thereof will be properly omitted. Further, the embodiments are presented by way of example only, and are not intended to limit the present disclosure, and any feature or combination thereof described in the embodiments may not necessarily be essential to the present disclosure. 
     In the present disclosure, “a state where a member A is connected to a member B” includes a case where the member A and the member B are physically and directly connected or even a case where the member A and the member B are indirectly connected through any other member that does not substantially affect an electrical connection state between the members A and B or does not impair functions and effects achieved by combinations of the members A and B. 
     Similarly, “a state where a member C is installed between a member A and a member B” includes a case where the member A and the member C or the member B and the member C are indirectly connected through any other member that does not substantially affect an electrical connection state between the members A and C or the members B and C or does not impair function and effects achieved by combinations of the members A and C or the members B and C, in addition to a case where the member A and the member C or the member B and the member C are directly connected. 
     In addition, the vertical and horizontal axes of a waveform diagram and a time chart shown in the present disclosure are enlarged or reduced as appropriate for ease of understanding, and each waveform shown is also simplified for ease of understanding. 
       FIG.  1    is a block diagram of a positioning device  100  having a motor driver circuit  200  according to an embodiment. The positioning device  100  includes a linear motor  102 , a host controller  104 , and the motor driver circuit  200 . 
     The host controller  104  comprehensively controls the positioning device  100 . The host controller  104  is configured with, for example, a microcontroller, an FPGA (Field Programmable Gate Array), or an ASIC (Application Specific Integrated Circuit). 
     The motor driver circuit  200  receives a control command from the host controller  104  and supplies a drive current I DRV  of an amount corresponding to the control command to the motor  102 . The motor  102  is, for example, a voice coil motor and has a mover displaced by an amount corresponding to the drive current I DRV  flowing through the motor  102 . 
     Subsequently, a configuration of the motor driver circuit  200  will be described.  FIG.  1    shows blocks related to a speed control for stabilizing a speed of the linear motor  102  at a target value. 
     The motor driver circuit  200  includes a current detection circuit  210 , a back electromotive force detection circuit  220 , a feedback controller  230 , an output stage  240 , an internal logic  250 , and an interface circuit  260 . 
     The current detection circuit  210  generates a current detection signal V CS  according to the drive current I DRV  flowing through the motor  102  as an object to be driven. 
     The back electromotive force detection circuit  220  generates a back electromotive force detection signal V BEMF  indicating a back electromotive force (BEMF) of the motor  102  based on the current detection signal V CS  and a voltage V M  across the motor  102 . The back electromotive force is proportional to the speed of the motor  102 . 
     The internal logic  250  outputs a current command V REF . In a position control mode, the current command V REF  changes linearly with respect to a target position of the motor  102 . In a speed control mode, the current command V REF  is generated so that the back electromotive force detection signal V BEMF  approaches a speed command. 
     The interface circuit  260  can communicate with the host controller  104 . The interface circuit  260  may receive information instructing the current command V REF  in the position control mode from the host controller  104 . 
     Calculation of the current command V REF  in the speed control mode may be performed in the internal logic  250 . Alternatively, the interface circuit  260  may be used to transmit electromotive force information back to the host controller  104 , and the host controller  104  may generate information instructing the current command so that the back electromotive force approaches the speed command, and send the generated information back to the internal logic  250 . 
     The feedback controller  230  generates, in the position control mode and the speed control mode, a voltage command V CTRL  so that the current detection signal V CS  approaches the current command V REF . 
     The output stage  240  generates a drive signal according to the voltage command V CTRL  and supplies the drive signal to the motor  102 . For example, the output stage  240  applies a voltage signal, which is obtained by multiplying the voltage command V CT mL by a gain, to the motor  102 . 
     Next, the speed control mode will be described in detail. As described above, in the speed control mode, it is necessary to detect information on the back electromotive force indicating the speed of the motor  102 . Therefore, detection of the back electromotive force in the back electromotive force detection circuit  220  will be described. 
       FIG.  2    is an equivalent circuit diagram of the motor  102 . The motor  102  is represented by a coil inductance L, a DC resistance r, and a voltage source  103 . The voltage source  103  generates a back electromotive force e proportional to the number of revolutions of the motor  102 . 
     When it can be supposed that a constant drive current I DRV  flows through the motor  102 , a voltage (electromotive force) across the inductance L is zero and a voltage drop across the resistance r is r×I DRV . Therefore, a voltage V M  across the motor  102  is represented by V M =I DRV ×r−e. 
     The back electromotive force detection circuit  220  multiplies the current detection signal V CS  by an appropriate coefficient to generate a voltage Vr corresponding to I DRV ×r. Thereafter, by subtracting the voltage Vr from the voltage V M  across the motor  102 , the back electromotive force detection signal V BEMF  indicating the back electromotive force e is generated. 
     Reference is made back to  FIG.  1   . The back electromotive force detection circuit  220  includes a calibration circuit  222 , a first amplifier AMP1, a second amplifier AMP2, and a third amplifier AMP3. 
     The first amplifier AMP1 amplifies the current detection signal V CS  with a gain g 1 . The second amplifier AMP2 multiplies the voltage across the linear motor  102  by a gain g 2  smaller than 1 and outputs the result. For example, the gain g 2  may be set to be smaller than ⅛ times. 
     The third amplifier AMP3 generates the back electromotive force detection signal V BEMF  according to a difference between an output V A1  of the first amplifier AMP1 and an output V A2  of the second amplifier AMP2. 
     The calibration circuit  222  is active in a calibration mode, and adjusts a circuit constant in the back electromotive force detection circuit  220  so that the back electromotive force detection signal V BEMF  becomes zero in a state where a stator of the linear motor  102  is not moved, that is, in a state where the back electromotive force e is zero. For example, the calibration circuit  222  adjusts the gain g 1  of the first amplifier AMP1. 
     The configuration of the motor driver circuit  200  has been described above. Next, an operation of the motor driver circuit  200  will be described. 
     In the calibration mode, feedback by the feedback controller  230  becomes invalid, and the output stage  240  outputs a sufficiently large drive current I DRV  so that the mover of the linear motor  102  can be held down on the mechanical end. As a result, the back electromotive force e becomes zero. 
     Suppose an internal resistance of the linear motor  102  to be r. In the calibration mode, the output V A1  of the first amplifier AMP1 is given by Equation (1). A 0  is a gain of the current detection circuit  210 . 
         V   A1   =I   DRV   ×A   0   ×g   1   (1)
 
     Since the back electromotive force is zero in the calibration mode, the voltage across the linear motor  102  is r×I DRV  and the output voltage V A2  of the second amplifier AMP2 is given by Equation (2). 
         V   A2   =g   2   ×r×I   DRV   (2)
 
     In the calibration mode, the back electromotive force detection signal V BEMF , which is an output of the third amplifier AMP3, is represented by Equation (3). 
         V   BEMF   =g   3 ×( V   A2   −V   A1 )  (3)
 
     g 3  is the gain of the third amplifier AMP3. 
     In the calibration mode, in order to make the back electromotive force detection signal V BEMF  to be zero, a relationship of V A2 =V A1  should be established. That is, the gain g 1  is adjusted so that Equation (4) is established. 
         I   DRV   ×A   0   ×g   1   =g   2   ×r×I   DRV   (4)
 
     That is, the gain g 1  after the adjustment satisfies Equation (5). 
         g   1   =g   2   ×r/A   0   (5)
 
     Suppose that, in a normal operating state after the calibration, the back electromotive force e is generated in the linear motor  102 . The output V A2  of the second amplifier AMP2 at this time is represented by Equation (6). 
         V   A2   =g   2   ×r×I   DRV   −e   (6)
 
     The back electromotive force detection signal V BEMF , which is the output of the third amplifier AMP3 at this time, is represented by Equation (7). 
         V   BEMF   =g   3 ×( V   A2   −V   A1 )= g   3 ×( g   2   ×r×I   DRV   −e−I   DRV   ×A   0   ×g   1 )  (7)
 
     Here, when g 1  is adjusted so as to satisfy Equation (5) by pre-calibration, the back electromotive force detection signal V BEMF  is represented by Equation (8). 
         V   BEMF   =g   3 ×( g   2   ×r×I   DRV   −e−I   DRV   ×A   0   ×g   1 )=− g   3   ×e   (8)
 
     As described above, the back electromotive force detection circuit  220  of the motor driver circuit  200  can generate the back electromotive force detection signal V BEMF  proportional to the back electromotive force e. 
     The operation of the motor driver circuit  200  has been described above. Next, a specific configuration example of the back electromotive force detection circuit  220  will be described. 
     Example 1 
       FIG.  3    is a block diagram of the back electromotive force detection circuit  220  of  FIG.  1   .  FIG.  3    shows a power supply voltage of each block and a withstand voltage of a transistor forming each block. LV indicates that a block is configured with low-withstand voltage elements, and HV indicates that a block is configured with high-withstand voltage elements. For example, LV indicates a device capable of operating within a range of 0 V to 5 V, and HV indicates a device capable of operating within a range of 5 V to 15 V. In Example 1, the power supply voltage V DD  is 1.5 V. 
     The gain of the second amplifier AMP2 is g 2 = 1/16, for example. When the voltage V M  across the motor varies from −10 V to +10 V, the voltage across the motor is compressed to −0.6 V to +0.6 V in the output voltage V A2  of the second amplifier AMP2. The second amplifier AMP2 is configured with low-withstand voltage elements. 
     The current detection circuit  210  includes a sense resistor R S  provided on a path of the drive current I DRV , and an amplifier that converts a voltage drop across the sense resistor R S  into the current detection signal V CS . When the gain of this amplifier is 1, the gain A 0  of the current detection circuit  210  is equal to R S . 
     For example, when R S =0.22 ohm and the driving current I DR V is 0.78 A to 4.55 A, the current detection signal V CS  can fall within a range from 0.17 V to 1.0 V. The first amplifier AMP1 that amplifies the current detection signal V CS  is configured with low-withstand voltage elements. The gain g 1  of the first amplifier AMP1 is adjusted by calibration. The gain g 1  after the adjustment satisfies the following equation. 
     
       
      
       g 
       1 
       =g 
       2 
       ×r/R 
       S  
      
     
     The gain g 1  corresponds to the internal resistance r of the motor  102 . Supposing that r is within a range of 2.1 ohm to 12.2 ohm, the gain g 1  is within a range of 0.6 to 3.48. The output voltage V A1  of the first amplifier AMP1 is approximately 0.6 V. 
     The third amplifier AMP3 is configured with low-withstand voltage elements. The gain g 3  of the third amplifier AMP3 can be eight times, for example. 
     Example 2 
       FIG.  4    is a block diagram of a back electromotive force detection circuit  220  according to Example 2. In Example 2, the power supply voltage V DD  is 5 V. The gain of the second amplifier AMP2 is g 2 =¼, for example. When the voltage V M  across the motor varies from −9 V to +9 V, the voltage across the motor is compressed to −2.25 V to +2.25 V in the output voltage V A2  of the second amplifier AMP2. The second amplifier AMP2 is configured with low-withstand voltage elements. 
     When R S =0.22 ohm and the driving current I DRV  is 0.73 A to 4.26 A, the current detection signal V CS  can fall within a range of 0.16 V to 0.94 V. The first amplifier AMP1 that amplifies the current detection signal V CS  is configured with low-withstand voltage elements. The gain g 1  of the first amplifier AMP1 is adjusted by calibration. The gain g 1  after the adjustment satisfies the following equation. 
     
       
      
       g 
       1 
       =g 
       2 
       ×r/R 
       S  
      
     
     The gain g 1  follows the internal resistance r of the motor  102 . Supposing that r is within a range of 2.1 ohm to 12.2 ohm, the gain g 1  is within a range of 2.4 to 13.92. The output voltage V A1  of the first amplifier AMP1 is 2.25 V, which is approximately four times that of Example 1. 
     The third amplifier AMP3 is configured with low-withstand voltage elements. The gain g 3  of the third amplifier AMP3 can be doubled, for example. 
     In Example 1, the gain g 2  of the second amplifier AMP2 is 1/16. When the gain g 2  is small, accuracy of detecting the back electromotive force is significantly affected by an error of the current detection signal V CS . In Example 2, the gain g 2  of the second amplifier AMP2 is ¼, which is four times that of Example 1. Thus, the detection accuracy of the back electromotive force is less likely to be affected by the error of the current detection signal. 
     The configuration of the back electromotive force detection circuit  220  has been described above. In this back electromotive force detection circuit  220 , the third amplifier AMP3 can be configured with low-withstand voltage elements. This makes it possible to reduce the area of the back electromotive force detection circuit  220  and, in turn, the area of the motor driver circuit  200 . 
     There are cases where it is desired to make the gain g 3  of the third amplifier AMP3 variable according to a platform or use of the linear motor  102 . In order to change the gain, a resistor network and a plurality of switches are required. When the third amplifier AMP3 is configured with high-withstand voltage elements, it is necessary to configure the switches with high-withstand voltage transistors, which causes an increase in the area of the third amplifier AMP3. 
     In the back electromotive force detection circuit  220  of  FIG.  3  or  4   , even when the gain of the third amplifier AMP3 is variable, the switches can be configured with low-withstand voltage transistors. Thus, the circuit area can be reduced. Further, although the back electromotive force detection circuit  220  is additionally provided with the second amplifier AMP2 configured with high-withstand voltage elements, the gain of the second amplifier AMP2 may be fixed. Thus, a switch for switching the gain is unnecessary. Therefore, the effect of reducing the area of the third amplifier AMP3 exceeds the increase in area by the second amplifier AMP2. 
       FIG.  5    is a circuit diagram showing a configuration example of the back electromotive force detection circuit  220 . The first amplifier AMP1 includes a third operational amplifier OA3, a ninth resistor R9, and a tenth resistor R10. A reference voltage V CMREF  is input to one end of the ninth resistor R9, and the other end of the ninth resistor R9 is connected to an inverting input terminal of the third operational amplifier OA3. The tenth resistor R10 is connected between the inverting input terminal and an output of the third operational amplifier OA3. The output voltage V A1  of the first amplifier AMP1 is represented by equation (9). 
         V   A1   =V   CS ×( R 9+ R 10)/ R 9+ V   CMREF   (9)
 
     The second amplifier AMP2 includes a first operational amplifier OAT and first to fourth resistors R1 to R4. The first resistor R1 is connected between a first input (−) of the first operational amplifier OAT and a first end (AOUT) of the linear motor  102 . The second resistor R2 is connected between a second input (+) of the first operational amplifier OAT and a second end (BOUT) of the linear motor  102 . The third resistor R3 is connected between the first input (−) of the first operational amplifier OAT and an output of the first operational amplifier OA1. The fourth resistor R4 has one end connected to the second input (+) of the first operational amplifier OAT and the other end receiving the reference voltage V CMREF . The output voltage V A2  of the second amplifier AMP2 is represented by Equation (10). Here, it is supposed that R1=R2 and R3=R4. 
         V   A2   =V   M   ×R 3/ R 1+ V   CMREF   (10)
 
     The third amplifier AMP3 includes a second operational amplifier OA2 and fifth to eighth resistors R5 to R8, and is configured similarly to the second amplifier AMP2. When a relationship of R5=R6 and R7=R8 is established, the output voltage V BEMF  of the third amplifier AMP3 is represented by Equation (11). 
         V   BEMF =( V   A1   −V   A2 )× R 7/ R 5+ V   CMREF   (11)
 
     The configuration example of the back electromotive force detection circuit  220  has been described above. 
     Applications 
       FIG.  6    is a view showing a hard disk device  900  including the motor driver circuit  200 . The hard disk drive  900  includes a platter  902 , a swing arm  904 , a head  906 , a spindle motor  910 , a seek motor  912 , and a motor driver circuit  920 . The motor driver circuit  920  drives the spindle motor  910  and the seek motor  912 . 
     The seek motor  912  is a voice coil motor. The motor driver circuit  200  according to the embodiment is incorporated in the motor driver circuit  920  and drives the seek motor  912 . 
     In the present disclosure, the configuration and type of the linear motor as an object to be driven are not particularly limited. For example, the present disclosure can be applied to drive a spring return type voice coil motor and other linear actuators. Alternatively, the motor as an object to be driven may be a spindle motor. 
     In addition, the application of the positioning device  100  is not limited to hard disk devices, and the positioning device  100  can also be applied to lens positioning mechanisms of cameras and the like. 
     According to the present disclosure in some embodiments, it is possible to reduce an area of a circuit that detects a back electromotive force. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures.