Patent Publication Number: US-2023147064-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, 2021-183533, filed on Nov. 10, 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 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. 
     There are two types of driver circuits for motors that are driven by a constant current, i.e., analog type driver circuits and digital type driver circuits. Analog type driver circuits are difficult to design because they require phase compensation for an error amplifier. 
     On the other hand, digital type driver circuits require a high-bit A/D converter for converting a detected signal indicating a current flowing through a motor into a digital signal. 
     SUMMARY 
     Some embodiments of the present disclosure provide a digital type driver circuit that can employ a low-bit A/D converter. 
     According to one embodiment of the present disclosure, a motor driver circuit includes: an error detection amplifier configured to receive a current feedback signal indicating a drive current of a motor as an object to be driven and an analog command signal indicating a target amount of the drive current, and generate an analog error signal indicating an error between the drive current and the target amount of the drive current; an A/D converter configured to convert the analog error signal generated by the error detection amplifier into a digital error signal; a digital compensator configured to generate a digital control amount based on the digital error signal output by the A/D converter; a D/A converter configured to convert the digital control amount into an analog control signal; and an output stage configured to supply a drive signal according to the analog control signal to the motor. 
     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 a diagram showing input and output characteristics of the motor driver circuit of  FIG.  1   . 
         FIG.  3    is a block diagram of a motor driver circuit according to a comparative technique. 
         FIG.  4    is a diagram for explaining an operation of the motor driver circuit of  FIG.  3   . 
         FIG.  5    is a circuit diagram showing a configuration example of a motor driver 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: an error detection amplifier configured to receive a current feedback signal V FB  indicating a drive current of a motor and an analog command signal V DAC  indicating a target amount of the drive current, and generate an analog error signal V ERR  indicating an error between the drive current and the target amount of the drive current; an A/D converter configured to convert the analog error signal V ERR  generated by the error detection amplifier into a digital error signal D ERR ; a digital compensator configured to generate a digital control amount based on the digital error signal D ERR  output by the A/D converter; a D/A converter configured to convert the digital control amount into an analog control signal; and an output stage configured to supply a drive signal according to the analog control signal to the motor. 
     With this configuration, instead of the current feedback signal V FB  indicating the drive current of the motor, the analog error signal V ERR  that changes linearly with respect to the error between the drive current I DRV  and the target amount I REF  of the drive current I DRV  is input to the A/D converter. In a state where a feedback loop is stabilized, since a current error I REF −I DRV  converges to near zero, a gain of the error detection amplifier can be set high. This makes it possible to employ an A/D converter with a low number of bits. 
     Further, the error detection amplifier should not be confused with an error amplifier used in an analog method. The error detection amplifier in the present disclosure is an amplifier having a gain of about several times or several tens of times and includes a feedback path of a direct current, whereas the analog error amplifier has a gain of theoretically infinite, actually several hundred to several thousands of times, and does not include a feedback path of a direct current. The analog error amplifier includes only a feedback path for phase compensation of an alternating current and differs in function and configuration from the error detection amplifier in the present disclosure. 
     In one embodiment, the motor driver circuit may further include: a sense resistor provided on a path of the drive current of the motor; and a current sense amplifier connected to the sense resistor and configured to generate a current feedback signal that changes linearly with respect to a current sense signal which is a voltage drop across the sense resistor. 
     In one embodiment, the current sense amplifier may include: a first operational amplifier; a first resistor connected between an inverting input of the first operational amplifier and one end of the sense resistor; a second resistor connected between the inverting input of the first operational amplifier and an output of the first operational amplifier; a third resistor connected between a non-inverting input of the first operational amplifier and the other end of the sense resistor; and a fourth resistor having one end configured to receive a predetermined level of voltage and the other end connected to the non-inverting input of the first operational amplifier. 
     In one embodiment, the error detection amplifier may include: a first input node configured to receive the current feedback signal; a second input node configured to receive the analog command signal; an output node; a second operational amplifier having an inverting input, a non-inverting input configured to receive a predetermined level of voltage, and an output; a fifth resistor connected between the inverting input of the second operational amplifier and the first input node; a sixth resistor connected between the inverting input of the second operational amplifier and the second input node; and a seventh resistor connected between the output of the second operational amplifier and the inverting input of the second operational amplifier. 
     In one embodiment, the error detection amplifier may further include: an eighth resistor connected between the output of the second operational amplifier and the output node; and a capacitor connected to the output node. The eighth resistor and the capacitor constitute an anti-alias filter for the AID converter in a subsequent stage. 
     In one embodiment, the output stage may include: a first amplifier configured to amplify the analog control signal in a non-inverting manner; and a second amplifier configured to amplify the analog control signal in an inverting manner. 
     In one embodiment, the motor may be 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 both of a case where all circuit elements are formed on a semiconductor substrate and a case where main elements of the circuit are integrated on the semiconductor substrate. In addition, some resistors, capacitors, and the like for adjusting a circuit constant may be provided outside the semiconductor substrate. By integrating the 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 circuit is 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 embodiment is presented by way of example only, and is not intended to limit the present disclosure, and any feature or combination thereof described in the embodiment 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  generates position control data POS indicating a target position of the linear motor  102  and transmits the position control data POS to the motor driver circuit  200 . The host controller  104  is composed of, for example, a microcontroller, an FPGA (Field Programmable Gate Array), or an ASIC (Application Specific Integrated Circuit). 
     The motor driver circuit  200  receives the position control data POS and supplies a drive current I DRV  of an amount corresponding to the position control data POS to the linear motor  102 . The linear 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 linear motor  102 . 
     Subsequently, a configuration of the motor driver circuit  200  will be described. The motor driver circuit  200  includes a current command generator  210 , an error detection amplifier  220 , an A/D converter  230 , a digital compensator  240 , a D/A converter  250 , an output stage  260 , and a current sense amplifier  270 , and is a functional IC (Integrated Circuit) integrated on one semiconductor substrate. 
     The current command generator  210  generates an analog command signal V DAC  indicating a target amount of the drive current I DRV  supplied to the linear motor  102 . For example, the current command generator  210  includes an interface circuit  212 , a logic circuit  214 , and a D/A converter  216 . The interface circuit  212  is connected to the host controller  104  and receives control data. The interface circuit  212  may be, for example, an I2C (Inter IC) interface circuit or an SPI (Serial Peripheral Interface) circuit. For example, the control data from the interface circuit  212  includes a code indicating a target position of the mover of the linear motor  102 . The logic circuit  214  outputs a control code based on the received code to the D/A converter  216 . The control code may be the same as the code received from the host controller  104  or may be another code obtained by calculating the received code. The D/A converter  216  converts the control code generated by the logic circuit  214  into the analog command signal V DAC . 
     The configuration of the current command generator  210  is not limited those described above, and the current command generator  210  may be configured to directly receive the analog command signal V DAC  from the outside. 
     The current sense amplifier  270  generates a current feedback signal V FB  indicating the drive current I DRV  flowing through the linear motor  102 . For example, the current feedback signal V FB  is represented by the following equation, where k and V CMREF  have arbitrary constant values. 
     
       
      
       V 
       FB 
       =k×I 
       DRV 
       +V 
       CMREF  
      
     
     The error detection amplifier  220  receives the current feedback signal V FB  and the analog command signal V DAC  and generates an analog error signal V ERR  indicating an error between the drive current I DRV  and its target amount I REF . 
         V   ERR =( I   REF   −I   DRV )× g , where g is a finite gain.
 
     The A/D converter  230  converts the analog error signal V ERR  generated by the error detection amplifier  220  into a digital error signal D ERR . 
     The digital compensator  240  generates a digital control amount D CTRL  based on the digital error signal D ERR  output by the A/D converter  230 . The digital compensator  240  includes a PI (Proportional-Integral) compensator or a PID (Proportional-Integral-Derivative) compensator. The PI compensator multiplies the digital error signal D ERR  by a proportional gain K P , multiplies an integral value of the digital error signal D ERR  by an integral gain K I , and adds them together to generate the digital control amount D CTRL . 
     The PID compensator multiplies the digital error signal D ERR  by a proportional gain K P , multiplies an integral value of the digital error signal D ERR  by an integral gain K I , multiplies a derivative value of the digital error signal D ERR  by a derivative gain K D , and adds them together to generate the digital control amount D CTRL . The PI compensator and the PID compensator are also called a PI controller and a PID controller, respectively. The PI compensator and the PID compensator may be selected according to characteristics of an object to be controlled. 
     The D/A converter  250  converts the digital control amount D CTRL  into an analog control signal V CTRL . 
     The output stage  260  applies a drive signal to both ends of the linear motor  102  according to the analog control signal V CTRL . The drive signal may be a voltage signal or a current signal. 
     The configuration of the positioning device  100  has been described above. Subsequently, an operation of the positioning device  100  will be described.  FIG.  2    is a diagram showing input and output characteristics of the motor driver circuit  200  of  FIG.  1   . By the feedback control by the digital compensator  240 , feedback works so that the error between the feedback signal V FB  and the analog command signal V DAC  approaches zero. Therefore, in a state where a feedback loop is stable, the following equation is established. 
     
       
      
       V 
       FB 
       =k×I 
       DRV 
       +V 
       CMREF 
       =V 
       DAC  
      
     
     In the steady state where the above equation is established, the drive current I DRV  is stabilized at the target level I REF  represented by the following equation. 
         I   REF =( V   DAC   −V   CMREF )/ k    
     The operation of the motor driver circuit  200  has been described above. The motor driver circuit  200  does not require an analog phase compensation circuit as compared with an analog system, and therefore is easy to design. 
     A further advantage of the motor driver circuit  200  becomes clear by comparison with a comparative technique.  FIG.  3    is a block diagram of a motor driver circuit  200 R according to a comparative technique. This motor driver circuit  200 R is similar to that of the embodiment in that the motor driver circuit  200 R is configured digitally but differs in the position of an A/D converter  230 R. In the comparative technique, the A/D converter  230 R converts the feedback signal V FB  generated by the current sense amplifier  270  into a digital feedback signal D FB . Thereafter, an error D ERR  between the digital feedback signal D FB  and a digital command value D REF  is generated by a subtractor  232  functioning as an error detector. 
     Attention is paid to the feedback signal V FB  which is an input of the A/D converter  230 R. Since the error D ERR  is zero in a steady state where a feedback loop is stabilized, a relationship of D REF =D FB  is established. That is, when the digital command value D REF  is changed, the feedback signal V FB , which is the input of the A/D converter  230 R, is changed so that a digital feedback signal D FB  matching the changed digital command value D REF  is generated.  FIG.  4    is a diagram for explaining an operation of the motor driver circuit  200 R of  FIG.  3   . In the comparative technique, the feedback signal V FB  is changed over a wide range in response to a change in the digital command value D REF . Therefore, it is necessary to select a high-bit A/D converter as the A/D converter  230 R. 
     Advantages of the motor driver circuit  200  over this comparative technique will be described. Attention is paid to the input of the A/D converter  230  in the motor driver circuit  200  of  FIG.  1   . The input of the A/D converter  230  is the analog error signal V ERR . In the steady state where the feedback loop is stabilized, the analog error signal V ERR  is substantially zero regardless of the target level I REF  of the drive current I DRV , that is, regardless of a magnitude of the analog command signal V DAC . Therefore, a variation range of the input voltage of the A/D converter  230  is narrower than that in the comparative technique. This allows the A/D converter  230  to adopt a lower bit A/D converter than the A/D converter  230 R in the comparative technique. By reducing the number of bits of the A/D converter  230 , a chip cost and power consumption of the motor driver circuit  200  can be reduced. 
     In addition, in the motor driver circuit  200 , a gain of the error detection amplifier  220  can be increased. The reason is because, regardless of the gain, in the steady state, the error detection signal V ERR , that is, the input of the A/D converter  230 , has a voltage level corresponding to zero. Increasing the gain of the error detection amplifier  220  produces the same effect as increasing the number of bits (resolution) of the A/D converter  230 . Therefore, the motor driver circuit  200  can reduce the number of bits of the A/D converter  230  for this reason as well. 
     For example, when the comparative technique requires a 16-bit A/D converter  230 R, the number of bits of the A/D converter  230  can be reduced to 12 bits in the present embodiment. A SAR (Successive Approximation) DAC can be used as the low-bit A/D converter  230 , but other forms may be employed. 
       FIG.  5    is a circuit diagram showing a configuration example of the motor driver circuit  200 . The linear motor  102  and a current sense resistor R S  are connected in series between output pins AOUT and BOUT of the motor driver circuit  200 . A voltage drop proportional to the drive current I DRV  occurs across the current sense resistor R S . The voltage drop across the current sense resistor R S  is fed back as a current detection signal V CS  between current sense pins ISNS and KSNS of the motor driver circuit  200 . 
     
       
      
       V 
       CS 
       =R 
       S 
       ×I 
       DRV  
      
     
     The current sense amplifier  270  generates the feedback signal V FB , which changes linearly with respect to the current detection signal V CS  and becomes a predetermined level V CMREF  when V CS =0 (that is, I DRV =0). 
     The current sense amplifier  270  includes a first operational amplifier OA 1 , a first resistor R 1 , a second resistor R 2 , a third resistor R 3 , and a fourth resistor R 4 . 
     The first resistor R 1  is connected between an inverting input (−) of the first operational amplifier OA 1  and one end (INS pin) of the sense resistor R S . The second resistor R 2  is connected between the inverting input (−) of the first operational amplifier OA 1  and an output of the first operational amplifier OA 1 . The third resistor R 3  is connected between a non-inverting input (+) of the first operational amplifier OA 1  and the other end (KSNS pin) of the sense resistor R S . The fourth resistor R 4  has one end that receives a predetermined level of voltage V CMREF , and the other end connected to the non-inverting input (+) of the first operational amplifier OA 1 . The feedback signal V FB  corresponds to an output voltage of the first operational amplifier OA 1 . 
     When a relationship of R 1 =R 3  and R 2 =R 4  is established, the following equation is established. 
         V   FB   =R 2/ R 1 ×V   CS   +V   CMREF    
     The error detection amplifier  220  includes a first input node n 1 , a second input node n 2 , an output node n 3 , a second operational amplifier OA 2 , a fifth resistor R 5 , a sixth resistor R 6 , a seventh resistor R 7 , an eighth resistor R 8 , and a capacitor C 1 . The feedback signal V FB  is input to the first input node n 1 , and the analog command signal V DAC  is input to the second input node n 2 . 
     The second operational amplifier OA 2 , the fifth resistor R 5 , the sixth resistor R 6 , and the seventh resistor R 7  constitute an adder circuit. The second operational amplifier OA 2  receives a reference voltage V CMREF  at a non-inverting input (+) thereof. The fifth resistor R 5  is connected between an inverting input (−) of the second operational amplifier OA 2  and the first input node n 1 . The sixth resistor R 6  is connected between the inverting input (−) of the second operational amplifier OA 2  and the second input node n 2 . A voltage command signal V EAOUT  may correspond to an output voltage of the second operational amplifier OA 2 . The seventh resistor R 7  is connected between the inverting input (−) and an output of the second operational amplifier OA 2 . 
     A gain g of the adder circuit with respect to the feedback voltage V FB  is R 7 /R 5 . 
     The eighth resistor R 8  is connected between the output of the second operational amplifier OA 2  and the output node n 3  of the error detection amplifier  220 . The capacitor C 1  is connected to the output node n 3 . The eighth resistor R 8  and the capacitor C 1  constitute a low-pass filter. This low-pass filter functions as an anti-alias filter for the A/D converter  230  in a subsequent stage. 
     The output stage  260  includes a first amplifier  262  and a second amplifier  264 . The first amplifier  262  amplifies the analog control signal V CTRL  in a non-inverting manner. The second amplifier  264  amplifies the analog control signal V CTRL  in an inverting manner. 
     The first amplifier  262  includes a third operational amplifier OA 3  and ninth to twelfth resistors R 9  to R 12 . A configuration of the first amplifier  262  is similar to that of the current sense amplifier  270 . When a relationship of R 9 =R 11  and R 10 =R 12  is established, an output voltage V AOUT  of the first amplifier  262  is represented by the following equation. 
         V   AOUT   =R 10 /R 9×( V   CTRL   −V   CMREF )+ HVPWR  
 
     The second amplifier  264  includes a fourth operational amplifier OA 4  and thirteenth to sixteenth resistors R 13  to R 16 . A configuration of the second amplifier  264  is similar to those of the current sense amplifier  270  and the first amplifier  262 . When a relationship of R 13 =R 15  and R 14 =R 16  is satisfied, an output voltage V BOUT  of the second amplifier  264  is represented by the following equation. 
         V   BOUT   =R 14 /R 13×( V   CTRL   −V   CMREF )+ HVPWR  
 
     In addition, a relationship of R 10 /R 9 =R 14 /R 13  is satisfied. 
     The configuration of the motor driver circuit  200  has been described above. According to this motor driver circuit  200 , the number of bits of the A/D converter  230  can be reduced. 
     Application 
       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  (or  200 A) according to the embodiment is incorporated in the motor driver circuit  920  and drives the seek motor  912 . 
     The seek motor  912  positions the head  906  via the swing arm  904 . Low noise is required during a read and write period of a hard disk, that is, when the head  906  is located in a particular region. Therefore, it is possible to automatically suppress noise during the read and write period by defining x a  and x b  of the seek motor  912  so that: (i) the seek motor  912  is in a first state φ1 when the seek motor  912  is positioned to locate the head  906  in an effective region on the platter  902 ; and (ii) the seek motor  912  is in a second state φ2 when the seek motor  912  is positioned to locate the head  906  in the other regions. 
     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 can also be applied to a lens positioning mechanism of a camera and the like. 
     According to the present disclosure in some embodiments, it is possible to employ a low-bit AID converter as a motor driver circuit. 
     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.