Abstract:
Methods and apparatus for providing a position sensor to sense the position of a ferromagnetic target and generate a target position signal, a scaling block to receive the target position signal, and a digital RC filter to generate an output using bit shifting for dividing by some factor of two, wherein all division for computations in the RC filter are performed by bit shifts.

Description:
SUMMARY 
     The present invention provides method and apparatus for a linear sensor integrated circuit (IC) that provides output signals to control a motor, such as a coil motor, to control a lens in auto focus digital camera applications. The current in the coil to drive the motor changes until the position of the external lens assembly or CCD array results in a sensor, such as a Hall effect sensor, having a voltage that matches the input signal position command. With digital RC filter operation, which enables resource sharing, a cost effective device and compact device is provided. 
     In one aspect of the invention, an integrated circuit comprises: a position sensor to sense the position of a ferromagnetic target and generate a target position signal, a scaling block to receive the target position signal, a digital RC filter to receive the target position signal, a reference position, and a scale factor, and generate an output, the RC filter including: a multiplier to multiply the scale factor and a difference of the reference position and the target position signal, and a bit shifter for dividing by some factor of two, wherein all division for computations in the RC filter are performed by bit shifts, a PID controller coupled to the RC filter to receive the output from the RC filter, and an output driver to provide a position output signal. 
     The integrated circuit can further include one or more of the following features: computations in the RC filter and computations in the scaling block share a multiplier, the target includes a lens having a ferromagnetic material for camera auto focus, the position sensor includes a Hall element, the PID controller receives position information from the position sensor, wherein the digital filter computes y[n+1]+(y[n]*2 z +k*(x−y[n]))/2 z , where x is a position value, k is a scaling value, z is a positive integer, and y is the output to the PID, and/or the output driver is configured to generate the position output signal to an actuator coil. 
     In another aspect of the invention, a system comprises a camera having auto-focus capability, the camera comprising: a position sensor to sense the position of a ferromagnetic target and generate a target position signal, a scaling block to receive the target position signal, a digital RC filter to receive the target position signal, a reference position, and a scale factor, and generate an output, the RC filter including: a multiplier to multiply the scale factor and a difference of the reference position and the target position signal, and a bit shifter for dividing by some factor of two, wherein all division for computations in the RC filter are performed by bit shifts, a PID controller coupled to the RC filter to receive the output from the RC filter, and an output driver to provide a position output signal. 
     The system can further include one or more of the following features: computations in the RC filter and computations in the scaling block share a multiplier, the target includes a lens having a ferromagnetic material for camera auto focus, the position sensor includes a Hall element, the PID controller receives position information from the position sensor, wherein the digital filter computes y[n+1]=(y[n]*2 z +k*(x−y[n]))/2 z , where x is a position value, k is a scaling value, z is a positive integer, and y is the output to the PID, and/or the output driver is configured to generate the position output signal to an actuator coil. 
     In a further aspect of the invention, a system comprises: a camera having auto-focus capability, the camera comprising: means for sensing a position of a ferromagnetic target; a digital RC filter means coupled to the means for sensing a position for generating an output, the digital RC filter including: a multiplier to multiply the scale factor and a difference of the reference position and the target position signal, and a bit shifter for dividing by some factor of two, wherein all division for computations in the RC filter are performed by bit shifts, a PID controller means coupled to the RC filter to receive the output from the RC filter; and an output driver means coupled to the PID controller means. 
     In another aspect of the invention a method comprises: employing a position sensor to sense the position of a ferromagnetic target and generate a target position signal, employing a scaling block to receive the target position signal, employing a digital RC filter to receive the target position signal, a reference position, and a scale factor, and generate an output, the RC filter including: a multiplier to multiply the scale factor and a difference of the reference position and the target position signal, and a bit shifter for dividing by some factor of two, wherein all division for computations in the RC filter are performed by bit shifts, employing a PID controller coupled to the RC filter to receive the output from the RC filter, and employing an output driver to provide a position output signal. 
     The method can further include one of more of: computations in the RC filter and computations in the scaling block share a multiplier, the target includes a lens having a ferromagnetic material for camera auto focus, and/or the digital filter computes y[n+1]=(y[n]*2 z +k*(x−y[n]))/2 z  where x is a position value, k is a scaling value, z is a positive integer, and y is the output to the PID. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which: 
         FIG. 1  is a schematic representation of a position sensor and driver in accordance with exemplary embodiments of the invention; 
         FIG. 2  is a schematic representation of a PID controller; 
         FIG. 3  is a schematic representation of a RC filter and scaling block; 
         FIG. 3A  is a schematic representation of scaling block operation for the implementation of  FIG. 3 ; 
         FIG. 3B  is a schematic representation of RC filter operation for the implementation of  FIG. 3 ; 
         FIG. 4  is a schematic representation of a PID block for the implementation of  FIG. 2 ; and 
         FIG. 5  is an exemplary implementation of a camera having an integrated circuit position sensor and driver. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a system  100  having digital auto-focus in accordance with exemplary embodiments of the invention. A sensor  102 , such as a Hall effect sensor, determines a position of a magnet on the target, e.g., lens, that is provided to a digital proportional-integral-derivative (PID) controller  104 . The PID controller  104  is connected to an output driver  106  that provides output signals  106   a,b  that can be coupled to an actuator coil  10  for moving the lens. A controller  108  controls overall operation of the device and an interface  110  provides serial, for example, communication with a processor, microcontroller or other device. 
     In an exemplary embodiment, the sensor  102  signal is amplified  120 , filtered  122 , and converted by an ADC  124  from an analog to a digital signal, which is input to the PID controller  104 . The output of the PID controller  104  is converted by a DAC  126  from a digital to an analog signal before being sent to the output driver  102 . 
     A series of registers  130  are coupled to the controller  108  and output driver  102  to store various information including position information, PID information, such as coefficients, ADC/DAC information, calibration information, bit shift information, etc. The registers  130  are available for read/write operations via the interface  110  over the serial SDA, SCL lines. An EEPROM and controller  132  are coupled to the registers and to the SCL line for programming operations. It is understood that any suitable memory device can be instead of, or in addition to, an EEPROM. 
       FIG. 2  shows further detail for the PID controller  104  of  FIG. 1 . In general, the PID controller  104  calculates an error value between the Hall effect sensor  102  ( FIG. 1 ) and a target  10 , e.g., lens, location programmed by the user, for example. In exemplary embodiments, coefficients for the PID controller  104  are selected to minimize controller error and reduce settling time. 
     Position information, shown as ten bits, is provided to a calibration scaling block  150 , which provides scaled lens position information to a RC filter  152 . The PID block  154  receives an output from the digital RC filter  152  and processes the filtered signal to provide an output control signal to adjust the lens position. 
     The calibration scaling block  150  receives positive register  152  and negative register  154  information during calibration. The lens can be driven to a first maximum position and the location stored, such as in a PREG register, and then driven to a second maximum position, opposite the first, and this location stored in a DREG register. This aligns the range of travel across the resolution of the device. 
     The scaled position information is provided to a digital RC filter  152 , which outputs filtered and signed position information to the PID block  154 . The RC filter provides a smooth change in the reference position on the PID controller. Information, e.g., voltage, is provided from the Hall sensor to the PID block  154 , which outputs control information that can be used to move the lens. 
       FIG. 3  shows an exemplary implementation of the calibration scaling block  150  and RC filter  152  of  FIG. 2 . A programmable digital approximation of an RC filter is used as the input filter to a ND (proportional, integral, derivative) controller, such as the PID block  154  of  FIG. 2 . An exemplary position update implementation is set forth below:
 
 y[n+ 1]= y[n]+k* ( x[n]−y[n] )
 
where, x is the desired input position to the PID controller, y is the reference position sent to the PID controller, and k is a scale value between 0 and 1. When x is abruptly changed, y will move toward x with an exponential behavior similar to an RC analog filter response to a step input. The effective time constant of this digital RC approximation can be programmed by changing either k and/or the update rate.
 
     In an exemplary embodiment, space expensive division is avoided by modifying the equation to require division only by a factor of two, which can be accomplished with bit shifts. An exemplary transformed equation is set forth below:
 
 y[n+ 1]=( y[n]* 2 z   +k* ( x−y[n] ))/2 z  
 
which provides division by 2^z, Where z is a positive integer and k is an integer between 0 and 2^z. This digital RC could be used as the input filter for any controller, including both analog and digital.
 
     After running calibration, PREG and NREG contain the actual maximum and minimum values needed for full lens travel. In one embodiment, only the most significant 8 bits out of the 12 bit ADC are stored in PREG and NREG, effectively rounding the saved calibration values. In the illustrated embodiment, the user input position value POS is ten bits and should represent the full lens travel range. Thus, a user POS value of 0x0 needs to map to the rounded, twelve bit NREG value and a user POS value of 0x3FF needs to map to the rounded, twelve bit PREG value, To rescale the POS values the following can be used:
 
 X =(( P REG− N REG)×POS)/1024+ N REG
 
where PREG and NREG are 12 bits wide with the top 8 bits being those in the PREG and NREG registers and the bottom 4 bits being zero for minimized multiplier size. Then X is the 12 bit output which is provided as input for the RC filter. In one embodiment, there is a small approximation since POS should be divided by 1023. However, dividing by 1024 allows for the division to be done by a bit shift. The order of operations requires that the multiplication of POS by (PREG−NREG) is done before the division to improve accuracy.
 
     It is understood that the resealing event may only need to occur when a new POS, PREG, or NREG value is loaded. Other than that, the multiplier is an available resource that will be shared with the RC Filter logic described below. 
     There is a need to allow any changes of the POS value to be implemented as an RC curve. In an exemplary embodiment, the following is used:
 
 y[n+ 1]=( y[n]* 2 10   +k* ( x−y[n] ))/2 10  
 
where x is the 12 bit resealed user POS value, k is a scaling value, and y is the output to the PID. The order of operations requires that the downshift be performed at the end allowing for a multiplier to be of a smaller size and still retain the accuracy in the equation.
 
     It should be noted that the effective value of k is scaled down by 1024 due to the ‘2 10 ’ scaling terms in the equation. The selectable scaling ranges allows for k to be represented by a 1 to 8 bit number which allows for easy sharing of the multiplier with the resealing logic, In  FIG. 3 , it is shown that for this embodiment, k was chosen to be 6 bits wide with the top two bits of the 8 bit value going into the multiplier being fixed at zero. 
       FIG. 3A  shows the path during calibration scaling. In accordance with
 
 X= (( P REG− N REG)×POS)/1024+ N REG,
 
NREG( 7 : 0 ) is subtracted from PREG( 7 : 0 ) at summer  302  having an output which passes through a multiplexer  304  to a multiplier  306 , which receives POS( 9 : 0 ) via multiplexer  308 . The multiplier  306  output passes through demultiplexer  310  to summer  312 , which has NREG( 7 : 0 ) as in input. The output of the summer  312  is twelve bit POS scaling information.
 
       FIG. 3B  shows the RC filter implementation path. Scaling value k RC_scale[ 5 : 0 ] passes though multiplexer  304  to the multiplier  306 . Value y[n]*2 z , which is stored from the last update in RC filter register  314  and bit shifted  315  to achieve division by 2^10 is provided to summer  316  for subtraction from the scaled position value (x in the equation). The output of the summer  316  (x-y[n]) is provided to the multiplier  306  via multiplexer  308 . The multiplier  306  output (k*(x-y[n])) passes through demultiplexer  310  to summer  318  for addition with the value in the RC filter register) (y[n]*2 10 ). The summer  318  output is then provided to the RC filter register  314  and then to a bit shift module  320  to provide the RC filter output (y[n+1]), which is sent to the PID block. 
     With this arrangement, the multiplier  306  is used for scaling and RC filter operations. As can be seen, division by a factor of 2 is achieved by bit shifts. By implementing division in bit shift operations, significant space savings are achieved as compared with divisions which cannot be performed with bit shifting alone. 
       FIG. 4  shows an exemplary PID implementation  400  in which the RC filter output  402  and the Hall sensor output  404  are provided as PID inputs. The PID output provides a control signal for the coil to control the target, e.g., lens, movement. It is understood that PID blocks are well known in the art. 
       FIG. 5  shows an exemplary circuit diagram of a camera  500  having a position sensor and driver integrated circuit  502  to sense a position of a target, such as a lens, and to generate a drive signal to actuate a motor until a desired position for the target is achieved. In one embodiment, the sensor is provided as a Hall effect sensor to generate a voltage corresponding to a ferromagnetic target location on a lens in an auto-focus module  504 . In one embodiment, the ferromagnetic target comprises a hard ferromagnetic material, such as a permanent magnet. 
     It is understood that a position sensor can include a variety of magnetoresistive devices, such as giant magnetoresistance (GMR), anisotropic magnetoresistance (AMR), and the like. In one embodiment, the integrated circuit includes a Hall effect sensor and a magnetoresistive sensor. 
     Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.