Abstract:
An accelerometer signal processor is disclosed comprising a sensor for generating A, B, C and D sensor signals in response to a sensor excitation. Variable oscillators convert the sensor signals into oscillating signals, and the oscillating signals up-count and down-count counters. The outputs of the counters represent a linear acceleration along at least two axes, as well as a rotational acceleration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of commonly owned patent application Ser. No. 10/004,433, filed Oct. 31, 2001, for MULTI-AXIS ACCELEROMETER COMPRISING A MASS SUSPENDED BY SPRIGS ABOVE AN OPTICAL SENSOR. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to accelerometers. More particularly, the present invention relates to an accelerometer signal processor comprising variable oscillators and counters. 
     2. Description of the Prior Art 
     Accelerometers are employed in various applications to detect and compensate for disturbances, such as a shock or periodic vibration. An example is a disk drive which employs an accelerometer to detect disturbances affecting an actuator arm while attempting to maintain a head over a centerline of a track. The output of the accelerometer is used as a feed-forward compensation signal in a servo control system to effectively reject the disturbance. 
     Conventional accelerometers comprise a sensor for detecting accelerations, and signal processing circuitry for converting the sensor signals into acceleration signals representing a direction and magnitude of the detected accelerations. The acceleration signals are typically generated as digital signals for processing by other control circuitry, such as a servo controller in a disk drive. A conventional Analog-to-Digital converter (ADC) is typically employed to convert the analog sensor signals into digital sensor signals, and the digital sensor signals processed to generate the digital acceleration signals. However, a conventional ADC implemented with analog circuitry is relatively expensive due to its size and complexity. 
     There is, therefore, a need to reduce the cost of the signal processing circuitry used to convert the analog sensor signals of an accelerometer into digital acceleration signals representing a direction and magnitude of detected accelerations. 
     SUMMARY OF THE INVENTION 
     The present invention may be regarded as an accelerometer signal processor for use in processing sensor signals generated by an accelerometer. The accelerometer comprises a sensor for generating A, B, C and D sensor signals in response to a sensor excitation. The accelerometer signal processor comprises an A variable oscillator (VO) for generating an A oscillating signal in response to the A sensor signal, a B VO for generating a B oscillating signal in response to the B sensor signal, a C VO for generating a C oscillating signal in response to the C sensor signal, and a D VO for generating a D oscillating signal in response to the D sensor signal. A first axis counter comprising an up count input responsive to the A oscillating signal and a down count input responsive to the D oscillating signal generates on output signal indicative of an acceleration along a first axis. A second axis counter comprising an up count input responsive to the B oscillating signal and a down count input responsive to the C oscillating signal generates on output signal indicative of an acceleration along a second axis. A rotation counter comprising an up count input responsive to the A and D oscillating signals and a down count input responsive to the B and C oscillating signals generates an output signal indicative of a rotational acceleration. 
     In one embodiment, the accelerometer signal processor comprises a first summing circuit for summing frequencies of the A and D oscillating signals to generate an output signal applied to the up count input of the rotation counter, and a second summing circuit for summing frequencies of the B and C oscillating signals to generate an output signal applied to the down count input of the rotation counter. 
     In one embodiment, the sensor comprises an optical sensor and the sensor excitation comprises light. In one embodiment, the accelerometer comprises an AGC counter responsive to the A, B, C and D oscillating signals and a reference oscillating signal. The AGC counter for generating an AGC signal for controlling the sensor excitation. 
     The present invention may also be regarded as a method of processing sensor signals generated by an accelerometer comprising a sensor for generating A, B, C and D sensor signals in response to a sensor excitation signal. The method comprises the steps of generating an A oscillating signal in response to the A sensor signal, generating a B oscillating signal in response to the B sensor signal, generating a C oscillating signal in response to the C sensor signal, and generating a D oscillating signal in response to the D sensor signal. The method further comprises the step of up-counting a first-axis counter in response to the A oscillating signal and down-counting the first-axis counter in response to the D oscillating signal, the first axis counter for generating an output signal indicative of a first linear acceleration. The method further comprises the step of up-counting a second axis counter in response to the B oscillating signal and down-counting the second axis counter in response to the C oscillating signal, the second axis counter for generating on output signal indicative of a second linear acceleration. The method further comprises the step of up-counting a rotation counter in response to the A and D oscillating signals and down-counting the rotational counter in response to the B and C oscillating signals, the rotation counter for generating an output signal indicative of the rotational acceleration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a multi-axis accelerometer according to an embodiment of the present invention comprising a mass suspended above a sensor by a plurality of springs. 
     FIG. 2 illustrates how the optical sensor of FIG. 1 is illuminated when in the steady state. 
     FIGS. 3A and 3B illustrate how the optical sensor of FIG. 1 is illuminated when subjected to linear X-axis and Y-axis accelerations. 
     FIGS. 4A and 4B illustrate how the optical sensor of FIG. 1 is illuminated when subjected to rotational accelerations. 
     FIG. 5 shows a signal processor for processing the sensor signals generated by the optical sensor of FIG. 1 to generate the detected acceleration signals according to an embodiment of the present invention. 
     FIG. 6A shows an amplifier circuit for amplifying the signal output by the optical sensor of FIG.  1 . 
     FIG. 6B shows further details for the circuitry of FIG. 5 using the amplifier circuit of FIG. 6A according to an embodiment of the present invention. 
     FIG. 6C shows further details of one of the counter circuits shown in FIG. 6B, including the circuitry for calibrating the counter circuit to account for offsets in the optical sensor and other circuitry. 
     FIG. 7 shows a disk drive comprising a multi-axis accelerometer according to another embodiment of the present invention. 
     FIG. 8A shows an exploded view of a multi-axis accelerometer according to an embodiment of the present invention wherein the optical sensor and mass are enclosed in a housing to form a compartment that is filled with a fluid such as oil to provide a damping effect. 
     FIG. 8B shows an assembled view of the multi-axis accelerometer of FIG.  8 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 shows an accelerometer signal processor implemented in CMOS circuitry for detecting acceleration signals according to an embodiment of the present invention. The accelerometer signal processor comprises an A variable oscillator (VO)  16 A for generating an A oscillating signal  18 A in response to an A sensor signal  20 A, a B VO  16 B for generating a B oscillating signal  18 A in response to a B sensor signal  20 B, a C VO  16 C for generating a C oscillating signal  18 C in response to a C sensor signal  20 C, and a D VO  16 D for generating a D oscillating signal  18 D in response to a D sensor signal  20 D. A first axis (e.g., X-axis) counter  22 A comprising an up count input responsive to the A oscillating signal  18 A and a down count input responsive to the D oscillating signal  18 D, the first axis counter  22 A generates on output signal  24  indicative of an acceleration along the first axis. A second axis (e.g., Y-axis) counter  22 B comprising an up count input responsive to the B oscillating signal  18 B and a down count input responsive to the C oscillating signal  18 C, the second axis counter  22 B generates an output signal  26  indicative of an acceleration along the second axis. A rotation counter  28  comprising an up count input responsive to the A and D oscillating signals ( 18 A and  18 D) and a down count input responsive to the B and C oscillating signals ( 18 B and  18 C), generates an output signal  30  indicative of a rotational acceleration. 
     The embodiment of FIG. 5 comprises a first summing circuit (XOR circuit  32 A) for summing frequencies of the A and D oscillating signals ( 18 A and  18 D) to generate an output signal  34 A applied to the up count input of the rotation counter  28 , and a second summing circuit (XOR circuit  32 B) for summing frequencies of the B and C oscillating signals ( 18 B and  18 C) to generate an output signal  34 B applied to the down count input of the rotation counter  28 . 
     In the embodiment of FIG. 5, the VOs ( 16 A- 16 D) perform an analog-to-digital conversion of the sensor signals ( 20 A- 20 D), wherein the frequency of the oscillating signals ( 18 A- 18 D) represents the magnitude of their respective sensor signals ( 20 A- 20 D). Over a selected number of sample periods, the count value of the first axis counter  22 A represents the difference between the integrated output of the A and D sensors: 
     
       
         X-axis= A−D   
       
     
     This difference represents an acceleration along the X-axis. Similarly, the count value in the second axis counter  22 B represents the difference between the integrated output of the B and C sensors: 
     
       
         Y-axis= B-C   
       
     
     This difference represents an acceleration along the Y-axis. Finally, the count value in the rotation counter  28  represents the difference between the integrated output of the A+D sensors and the integrated output of the B+C sensors: 
     
       
         Rotation= A+D−C−B   
       
     
     This difference represents a rotational acceleration. The sign of the detected acceleration signals represents the direction of movement (left or right, up or down, clockwise or counterclockwise). 
     FIG. 6A shows an amplifier circuit  31 A according to an embodiment of the present invention for amplifying the output of the A sensor  8 A to generate sensor signal  20 A. A similar amplifier circuit is used to generate the sensor signals  20 C- 20 D. In this embodiment, the sensor comprises an optical sensor  4  (FIG. 1) including photo diodes  36  which are run in a shorted-junction configuration driving into a low impedance such that they operate like a current source generating a current signal Is  38 A. Transistors  42 A and  42 B form a differential pair, which forces transistor  44  to supply whatever current is necessary to hold the gate of transistor  42 A at an externally supplied reference voltage Vref  46 . Resistor  40  serves as a noise limiter and isolates parasitic capacitances in the photodiode  36  from the differential pair  42 A and  42 B to control stability. When the photodiode  36  is illuminated with light  6 , the resulting current is balanced in the drain of transistor  44  and mirrored in the drains of transistors  48 A and  48 B. One drain generates an output Iagc  50 A for use in an automatic gain control circuit as described below with reference to FIG. 6B, and the other drain generates the sensor signal Iout  20 A. 
     FIG. 6B shows further details of the circuitry of FIG. 5 according to an embodiment of the present invention. In this embodiment, the Iagc currents ( 50 A- 50 D) output by the sensor signal amplifiers ( 31  A- 31 D) are summed and the resulting summed signal is mirrored as an input signal to a reference VO  52 . The oscillating signal  54  output by the reference VO  52  drives the clock input of a sample time counter  56 . The sample time counter  56  determines the integration interval for the sensor signals ( 20 A- 20 D), and a sample input signal  58  initiates the integration interval by clearing counters  56 ,  22 A,  22 B and  28 , and by enabling the VOs  52  and  16 A- 16 D through NOR gates  60 A and  60 B. At the end of the integration interval, the output  62  of the sample time counter  56  disables the VOs  52  and  16 A- 16 D through NOR gates  60 A and  60 B, and the contents of the counters  22 A,  22 B and  28  are extracted via bus  64 . 
     The embodiment of FIG. 6B also comprises an automatic gain control circuit (AGC) which maintains the parameters of the signal processor within predetermined bounds, including operation of the optical sensor  4 , linearity of the VOs, as well as to compensate for aging, drift and nonlinearities in the light source. The AGC operates by controlling the light source (e.g., a light emitting diode) so as to constrain the frequency of oscillating signal  54  approximate to the frequency of externally supplied reference signal Fmax  66 . The oscillating signal  54  is applied to the down input signal of counter  68 , and the reference signal Fmax  66  is applied to the up input signal of counter  68  (divided by two through D register  70 ). If the frequency of the oscillating signal  54  is too high, then the BO output  71  of counter  68  will disable the light source by turning off transistor  72  through OR gates  74 A and  74 B. With the light source disabled, the sum of the Iagc currents  50 A- 50 D will decrease causing the frequency of the oscillating signal  54  to decrease. When the frequency of the oscillating signal  54  decreases below the threshold determined by Fmax  66 , the CO output  76  of counter  68  will enable the lights source by turning on transistor  72  through OR gates  74 A and  74 B. The result is a hysteretic on/off duty cycle that cycles the LED on and off as necessary to maintain the conversion frequency within reasonable limits. The capacitance of the sensors acts as an integrator and stores enough charge so that the average output frequency of the VO circuits  50 A- 50 D is fairly constant. 
     In one embodiment, the counters  22 A,  22 B and  28  are periodically calibrated to compensate for offsets in the optical sensor  4  due to drift, aging, and other changes in the circuitry. FIG. 6 shows further details for one of the counters  22 A,  22 B and  28  for carrying out the calibration according to an embodiment of the present invention. The counter comprises a Run/Cal input signal  78  for reversing the up and down input signals  80  and  82  during a calibration session by controlling switches  84 A- 84 D. To start the calibration, the counters  22 A,  22 B and  28  are cleared and the light source is disabled. A brief pause in counting is made to allow the optical sensor current to drop to its dark current level. The sample time counter  56  is then enabled for the integration interval and the counters  22 A,  22 B and  28  count up to an offset value that corresponds to all circuit offsets. This offset value is then loaded from cascaded counters  86 A- 86 D into preload registers  88 A and  88 B. During normal operation, the counters  22 A,  22 B and  28  are preloaded with the values stored in registers  88 A and  88 B, thereby correcting for dark current and circuit offsets. The output of the counter is read through buffer registers  90 A and  90 B when a Read signal  92  is asserted. It may be desirable to run the calibration at some larger fraction of the normal current, due to nonlinearities in the VO circuitry. 
     FIG. 1 shows a suitable multi-axis accelerometer  2  for generating the sensor signals processed by the accelerometer signal processor of FIG.  5 . The multi-axis accelerometer comprises at least one optical sensor  4  for generating a sensor signal in response to light  6  illuminating the optical sensor  4 , a mask ( 8 A- 8 D) positioned over the optical sensor  4  for covering a first area of the optical sensor  4 , at least one spring ( 10 A- 10 D), and a mass  12  suspended above the optical sensor  4  by the spring ( 10 A- 10 D). The mass  12  comprises at least one mass aperture ( 14 A- 14 D) for allowing the light  6  to pass through the mass aperture ( 14 A- 14 D) and illuminate a second area of the optical sensor  4  not covered by the mask ( 8 A- 8 D). When the multi-axis accelerometer  2  accelerates causing the mass  12  to move, a corresponding movement of the mass aperture ( 14 A- 14 D) alters the illumination of the optical sensor  4  such that the sensor signal is indicative of the acceleration. 
     Any suitable optical sensor may be employed in the multi-axis accelerometer, including a charged couple device (CCD) or a photodiode. The SPOT Series segmented photodiode manufactured by UTD Sensors, Inc. in Hawthorne, Calif. is a suitable optical sensor for use in the embodiments of the present invention 
     In the embodiment of FIG. 1, the optical sensor  4  and mask ( 8 A- 8 D) are integrally formed. The at least one mass aperture comprises a plurality of linear apertures ( 14 A- 14 D), and the mask comprises a corresponding plurality of opaque lines ( 8 A- 8 D) formed on the surface of the optical sensor  4 . Also in this embodiment, the at least one spring comprises a plurality of vertical wires ( 10 A- 10 D) each having a first end connected to the mass  12  and a second end connected to the optical sensor  4 . The length of the vertical wires ( 10 A- 10 D) determines the shadow effect of the optical sensor  4  as well as the spring constant of the spring. The diameter of the vertical wires ( 10 A- 10 D) also affects the spring constant as well as the vertical support. The diameter of the vertical wires should be selected to provide sufficient vertical support for the mass while providing sufficient sensitivity. As described below, in one embodiment the accelerometer comprises a housing to provide a compartment between the optical sensor  4  and the mass  12 . The compartment is filled with a fluid, such as oil, to provide a damping effect. 
     The optical sensor  4  in the embodiment of FIG. 1 comprises an A sensor, a B sensor, a C sensor and a D sensor. The A sensor generates the A sensor signal  20 A, the B sensor generates the B sensor signal  20 B, the C sensor generates the C sensor signal  20 C, and the D sensor generates the D sensor signal  20 D. A first linear acceleration is detected relative to the A sensor signal  20 A and the D sensor signal  20 D, a second linear acceleration is detected relative to the B sensor signal  20 B and the C sensor signal  20 C, and a rotational acceleration is detected relative to the A, B, C and D sensor signals ( 20 A- 20 D). 
     The operation of the multi-axis accelerometer in detecting acceleration is understood with reference to FIG. 2, FIGS. 3A-3B and FIGS. 4A-4B. FIG. 2 shows the multi-axis accelerometer at rest, that is, not subjected to any acceleration. The opaque lines (e.g., lines  8 A 0 - 8 A 4 ) forming the mask are offset from the linear mass apertures (e.g. mass apertures  14 A 0 - 14 A 4 ) such that half of the light passing through the mass apertures illuminates the optical sensor  4 . FIG. 3A shows how the illumination of the optical sensor  4  changes when the multi-axis accelerometer  2  is subjected to an X-axis acceleration. The force in this example is to the left, such that the mass  12  moves toward the right with respect to the optical sensor  4 . The light illuminating the A quadrants increases and that illuminating the D sensor decreases, while the light illuminating the B and C quadrants remains unchanged. If the acceleration is in the reverse direction, the light illuminating the A quadrant decreases and that illuminating the D sensor increases. When the acceleration is along the Y-axis in the upward direction as shown in FIG. 3B, the mass  12  moves down with respect to the optical sensor  4 . The light illuminating the C quadrants increases and that illuminating the B sensor decreases, while the light illuminating the A and D quadrants remains unchanged. If the acceleration is in the reverse direction, the light illuminating the C quadrant decreases and that illuminating the B sensor increases. 
     Referring now to FIG. 4A, when the multi-axis accelerometer  2  is subject to a rotational acceleration in the counter-clockwise direction, the mass  12  rotates in a clockwise direction with respect to the optical sensor  4 . The light illuminating the A and D quadrants increases, while the light illuminating the B and C quadrants decreases. If the acceleration is in the clockwise direction such that the mass  12  moves in the counter-clockwise direction as shown in FIG. 4B, then the light illuminating the B and C quadrants increases, while the light illuminating the A and D quadrants decreases. 
     Therefore in the embodiment of FIG. 1, an acceleration along the X-axis is detected relative to the A and D sensor signals, an acceleration in along the Y-axis is detected relative to the B and C sensor signals, and a rotational acceleration is detected relative to the A, B, C and D sensor signals. In one embodiment, the X-axis acceleration is detected by computing a difference between the A and D sensor signals, and the Y-axis acceleration is detected by computing a difference between the B and C sensor signals. The rotational acceleration is detected by computing a first sum generated by summing the A and D sensor signals, a second sum generated by summing the B and C sensor signals, and by computing a difference between the first sum and the second sum. 
     Any suitable multi-axis accelerometer which generates A, B, C, and D sensor signals may be employed in the embodiments of the present invention. For example, a suitable capacitive or inductive multi-axis accelerometer which generates A, B, C, and D sensor signals may be employed. 
     FIG. 7 shows a disk drive  100  according to an embodiment of the present invention. The disk drive  100  comprises a disk  102 , a head  104 , an actuator  106  for actuating the head  104  radially over the disk  102 , and a multi-axis accelerometer  108  for generating an acceleration signal  110  representing a vibration affecting the actuator, the acceleration signal  110  for controlling operation of the disk drive  100 . The multi-axis accelerometer  108  comprises a mass suspended by springs above an optical sensor (e.g., FIG. 1) and signal processing circuitry for processing the sensor signals to generate the acceleration signal  110  (e.g., FIG.  5 ). 
     In the embodiment of FIG. 7, a servo controller  112  processes the acceleration signal  110  to generate appropriate control signals applied to a voice coil motor (VCM)  114 . The servo controller  112  processes the acceleration signal  110  as a feed-forward signal in a servo control loop for controlling the motion of the actuator  106  through the VCM  114 . The disk drive  100  of FIG. 7 further comprises a spindle motor  116  for rotating the disk  102  and a disk controller  118  for communicating with a host system. The disk controller  118  provides user data received from the host to a read/write channel  120  over line  121 . The read/write channel  120  encodes the user data to generate write data  122  written to the disk  102  via a preamp  124 . During a read operation, the preamp amplifies the signal from the head  104  to generate a read signal  126  supplied to the read/write channel  120 . The read write channel  120  comprises suitable circuitry for demodulating the read signal  126  into the recorded user data which is transmitted over line  121  to the disk controller  118  and ultimately to the host. The read/write channel  120  also demodulates embedded servo data  128  for use by the servo controller  112  in maintaining proper centerline tracking during read and write operations. A disturbance, such as a shock or periodic vibration, causing a linear or rotational acceleration of the disk drive  100  is detected by the multi-axis accelerometer  108  and substantially rejected by the servo controller  112  in response to the acceleration signal  110 . 
     If a large enough shock causes the disk drive  100  to accelerate faster than the servo controller  112  can respond, other measures may be taken to prevent damage to the disk drive  100 . In one embodiment, the disk drive  100  disables the write current if a large shock is detected while writing data to the disk  102  during a write operation. In another embodiment, when a large shock is detected the disk drive  100  parks the head  104  to ameliorate damage due to the head  102  “slapping” the surface of the disk  104 . The head  104  may be parked on a ramp, or on a landing zone of the disk  102 . When parked on a landing zone, the disk  104  is spun down so that the head  104  “sticks” to the surface of the disk  102 . 
     FIG. 8A shows an exploded view of a multi-axis accelerometer according to an embodiment of the present invention wherein the optical sensor  4  and mass  12  are enclosed in a housing  130 . In this embodiment the mask ( 8 A- 8 D) is formed integrally with the optical sensor  4 . The mass  12  is suspended above the optical sensor  4  by a spring formed from horizontal wires ( 132 A- 132 D) having a first end connected to the mass  12  and a second end connected to a base  134  of the housing  130 . The housing  130  comprises a receptacle  136  for receiving the light source (e.g., an LED) and a channel  136  for directing the light over the mass  12 . The compartment formed by the housing  130  between the mass  12  and optical sensor  4  is filled with a fluid (e.g., oil) to provide a damping effect. FIG. 8B shows an assembled view of the multi-axis accelerometer of FIG.  8 A.