Abstract:
A method to trim a monitor circuit for a voice coil motor including the steps of transmitting a test current through a voice coil motor, determining if a linear response is generated by the monitoring circuit, trimming the monitoring circuit if a non-linear response is received; repeating said transmitting step, said delivery step and said trimming step until said trim is achieved.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to control of motors and more particularly to a method and apparatus for accurately controlling the velocity of an actuary or motor member for monitoring the back electromagnetic force (EMF) of an actuator coil.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional actuators, which are sometimes referred to as motors, have a movable supported member and a coil. When the current is passed through the coil, a motive force is exerted on the member. A control circuit is coupled to the coil in order to controllably supply current to the coil. One example of such arrangement is found in a hard disk drive, where the movable member of the actuator supports a read write head adjacent to a rotating main disk for approximate radial movement of the head relative to the disk. There are situations in which is desirable to move the member from one end of its path of travel at a predetermined velocity which is less than the maximum velocity.  
           [0003]    When a current is applied to the coil of the actuator, the member is subjected to a force tending to accelerate the member at a rate defined by the magnitude of the current and at a direction defined by the polarity of the current. Consequently, in order to accelerate or decelerate the member until it is moving at the desired velocity and in the desired direction. It is important to note the actual direction and velocity of the member, In this regard, it is known that the back EMF voltage on the coil of the actuator is representative of the velocity direction of movement of the member. When a current is supplied to the coil of the actuator, the member is subjected to a force tending to accelerate the member at a rate defined by a magnitude of the current, and at direction defined by the polarity of the current. Consequently, in order to accelerate or decelerate the member till it is moving at the desired velocity and in the desired direction, it is important to know the actual direction and the velocity of the member. In this regard, it is known that the back EMF voltage of the coil of the actuator is representative of the velocity and direction of movement of the member. Specifically, the following relationship applies to actuators: 
             V   M   =I   M   ×R   M   +K   E ω 
           [0004]    Where the V M is the voltage across the actuator (motor), in the current through the actuator R M is internal resistance of the actuator, and K E =torque constant of the actuator and ω=velocity of the actuator. The term, K E ωrepresents the back-EMF of the actuator coil.  
           [0005]    It is desirable to have a control circuit for an actuator that actually monitors the back-EMF of the actuator coil and effectively controls the movement of the actuator member under widely varying load conditions.  
           [0006]    Typically, the back-EMF monitor circuits include a first step amplifier to measure the voltage across the voice coil motor by sense resistor. However, in order to output a signal which linearly reflects the back-EMF of the gain of the amplifier the gain of this first stage amplifier must be accurately adjusted. The gain typically equals G=R 2 /R z I which equals the ratio of the VCM coil resistance with respect to the resistance in parallel with the first stage amplifier. It is undesirable if the ratio of R 2  to R 1  does not equal the ratio of the resistance of the VCM to the resistance of the sensory resistor.  
           [0007]    Under the conditions of the ratio of the resistance of R 2 /R 1  is equal to the resistance of the VCM and the resistance of the sensor, there is a current component that varies linearly with the current through the VCM. The circuit to monitor the back EMF voltage should be independent of the current that actually goes through the VCM. Typically, the resistance of the VCM current cannot be manufactured with sufficient accuracy and usually possesses a significant temperature coefficient. The resistor R 1  is usually chosen to have a high degree of accuracy while the resistor R 2  is chosen to match the temperature coefficient of R VCM .  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides an adjustable resistance R 2  to which is automatically adjusted to maintain the proper R 2 ÷R 1  ratio which is achieved by passing a test current through the VCM, by measuring if the test current provides an ideal response from the voice coil motor, the adjusting the resistance R 2  by one increment and repeating the above step until R 2  is chosen to be such that R 2 /R 1 =R VCM /R SENS . As a consequence, the gain of the amplifier is increased by increasing the resistance R 2  one bit at a time until the output of the amplifier is approximately linearly equal to the back EMF. Thus, the gain ratio R 2 ÷R 1  is properly adjusted to R VCM . This operation is performed dynamically during the operation of the voice coil motor so that any variations in temperature which affects the voice coil motor will be compensated by appropriate changes of R 2 .  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 illustrates a back-EMF monitor circuit;  
         [0010]    [0010]FIG. 2 illustrates a detailed chart of the present invention;  
         [0011]    [0011]FIG. 3 illustrates a detailed chart description of R 2 (resistor R 10 );  
         [0012]    [0012]FIG. 4 illustrates a top view of a system of the present invention; and  
         [0013]    [0013]FIG. 5 illustrates a side view of the system.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    [0014]FIG. 1 illustrates a back-EMF monitor of the present invention. As illustrated, the VCM back-EMF monitor circuit is on chip  90 . External to chip  90  is voice coil motor (VCM)  112  connected to in series to a sense resistor  114  which senses the current through the voice coil motor and consequently generates a voltage corresponding to the current. The integrated circuit  90  includes an amplifier  100  to amplify the output voltage of the VCM  112  and a resistor  108  is connected to the first side of sense resistor  114 . The resistor  108  is used to determine the gain for amplifier  100 . Connected to the other end of resistor  108  and connected to the negative input of amplifier  100  is resistor  110 . The resistor  110  is a variable resistor to vary the gain of amplifier  100  such that the ratio of R 2  to R 1  is equal to R VCM  to R SENS . The other end of resistor  110  is connected to the output of amplifier  100 . The other input, namely in this embodiment, the plus input is connected to the other end of sense resistor  114  and connected to one end of the voice coil motor  1   14 . Additionally, offset trimming logic  124  is connected to the plus input to amplifier  100 . The offset trimming logic  124  performs ???????? (we need to expound here). The output of amplifier  100  is a voltage V MEASA1  which is a voltage which indicates the voltage across the voice coil motor  112 . The resistor  110  is a variable resistance resistor and controlled by the resistor timing logic  106 . The resistor trimming logic  106  outputs through transmission gauge  132  a signal to control resistor  110 . One way that resistor  110  can be charged after receiving the signal is to increase by one unit the resistance of resistor  110 . The voltage  MEASA1  provides an indication of the current through the sense resistor  114 .  
         [0015]    Additionally, the VCM back-EMF monitor an integrated circuit  90  includes a second amplifier  102 . The output of the amplifier  100  is connected to a resistor  120 . The other side of resistor  120  is connected to the negative input of amplifier  102 . Additionally, the other end of voice coil motor  112  is connected to resistor  122 . The other end of resistor  122  is connected to the plus input of amplifier  102 . Additionally, resistor  118  is connected to the plus input of amplifier  102  with the other end of resistor  118  being connected to voltage V-hd REF. Resistor  118  is to ? what is the function here? Additionally, the negative input of amplifier  102  is connected to resistor  116 . Resistor  116  is a variable resistor to control the gain of amplifier  102 . The other end of resistor  116  is connected to the output of amplifier  102 . The output of amplifier  102  is a voltage V MEAS . The voltage V MEAS  corresponds linearly to the back-EMF voltage of the VCM. The output of amplifier  102  is connected to comparator  104 . The comparator  104  compares the voltage V MEAS  with the reference voltage V RCF . The output of comparator  104  provides a voltage V MPOLAR  when the two voltages are equal or voltage V MEAS  is greater than the voltage V REF . This indication when the two voltages are equal, namely, voltage V MPOLAR  is input to resistor trimming logic circuit  106  to indicate that the ideal or desired setting for resistor  110  has been reached. Transmission gate  132  controls the output of resistor trimming logic  106  to resistor  110 . Additionally, transmission gate  130  allows the value of resistor  110  to be read outside of integrated circuit  90 . The transmission gate  136  allows the voltage V MEAS  to be measured outside of integrated circuit  90 . The transmission gate  134  allows ?????  
         [0016]    In operation, a test current for example a current in the range of 25 to 100 miliamps are input through the voice coil motor  112 . How? A sense voltage which results from the IR current resistor  114  is sensed across resistor  114 , and this voltage is input to amplifier  100 . The output from amplifier  100  is voltage V MEASA1  and this voltage is amplified by amplifier  102  and outputs an output voltage V MEAS . This voltage corresponds linearly to the back-EMF. This voltage V MEAS  is compared with respect to the reference voltage V REF  at comparator  104 . Assuming voltage V MEAS  is less than the reference voltage V REF  an output voltage from comparator  104  is input to resistor trimming logic  106  to provide a resistance value for R 2  resistance  110  to increase the resistance of resistor  110  incrementally by one. The resistance of resistor  110  is increased by one, and the process is repeated until the comparator  104  finds no difference in voltage between the output voltage of amplifier  102  namely V MEAS  and V REF  at which time the resistor trimming logic ceases incrementing the value of resistance for resistor  110 . If the resistance value is desired to be read outside the integrated circuit  90 , the transmission gate  130  is opened and the transmission gate  132  is closed. Furthermore, if the output of amplifier  102  desires to be measured the transmission gate  136  is open to allow the voltage to be read outside of integrated circuit  90 .