Abstract:
A position sensor monitors relatively fast moving objects with signal conditioning for reduced power and reduced wiring. A transducer and related circuitry generate a dynamic signal proportional to a position of a moving object and also generate one or more low frequency or static (DC or zero frequency) error signals. The low or zero frequency error signals are removed and a position signal is generated using only two connections to a remote sensor monitor, thus allowing ease in multiplexing and reduced wiring. Circuit options allow placing less circuitry on the sensor itself for small size or more circuitry on the sensor for less control requirement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation-in-part application claiming the benefit of U.S. Provisional Application Ser. No. 60/710,750 for “Position Sensor Including Error Level Compensation” having filing date Aug. 24, 2005, and of U.S. application Ser. No. 10/995,963 for “Offset Compensated Position Sensor and Method” having filing date of Nov. 23, 2004, which itself claims the benefit of U.S. Provisional Application Ser. No. 60/524,799 for “Offset Compensated Position Sensor,” and U.S. Provisional Application Ser. No. 60/524,919 for “Minimized Cross-Section Sensor Package,” both having filing date Nov. 25, 2003, all disclosures of which are herein incorporated by reference in their entirety, and all commonly owned with the instant application. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to sensors, and in particular to position and motion sensors.  
       BACKGROUND OF THE INVENTION  
       [0003]     Many mechanical systems contain moving parts not directly linked through mechanical means whose position, timing, or speed must be monitored and controlled with correction schemes for safe or efficient operation. A prime example is the operation of diesel engine fuel injectors. These injectors are usually controlled either hydraulically through rapid compression of fuel or electrically through operation of a fast moving solenoid valve. In both systems, the timing and speed of the actual injection of fuel into the combustion chamber greatly depends on the characteristics of the fuel being used. This is especially true of biodiesel fuels that contain various entrained organic materials and gases that make the fuel compressible and change its viscosity or other characteristics that affect valve speed or timing.  
         [0004]     Mechanical systems such as internal combustion engines usually contain a significant number of these moving objects. For instance, there are usually multiples of 4, 6, 8 or more cylinders in diesel engines utilizing fuel injectors each containing a moving valve or other object that must be monitored for efficient or safe operation. Each injector requires a separate sensor. The wiring of these sensors to a remotely located engine monitoring and control system must be designed to accommodate extreme temperatures and vibrations and adds cost and weight to the system. A method of reducing the amount of wires should be employed when implementing these position sensors for maximum efficiency and minimum cost. One widely accepted method of reducing the wiring is to provide output signals in the form of changes in current drawn by the sensor that is directly proportional to the position of the object being monitored. This allows the sensor to operate requiring only two wires; one to deliver operating voltage and current to the sensor and another to provide a ground reference and to form a complete path for the current through the sensor. An example is a sensor that draws zero milliAmperes when the object is at rest and draws 5 milliAmperes when the object is closest to the sensor, with intermediate currents being drawn when the object is between these extremes of movement. These sensors operate by drawing their current through an external resistance inline with their connecting wires such that the resistance develops a dropped voltage level that is directly proportional to the current through the sensor. For instance, connecting a 20-Ohm resistor inline with the 5-milliAmpere sensor listed above results in a varying voltage drop of 0 to 100 milliVolts across this inline resistor. This voltage drop is monitored by external devices to convert the current information into voltage information for further processing.  
         [0005]     Mechanical systems such as internal combustion engines also are designed so that the objects that must be monitored are known to be moving within specific limits or windows of timing such that at least some objects are moving at times that other objects are known to be at rest. For instance, the internal combustion engine fuel injectors operate in sequences equally timed in relation to the rotational position of the crankshaft. For instance, injector number one opens between 0 and 25 degrees of rotation, injector number two operates between 50 and 75 degrees, and the like. A method of further reducing the number of wires required for these systems can be employed by multiplexing or connecting all sensors to the same set of wires and a single inline resistor. Since each signal from each individual sensor is known to be occurring within a separate period or window of time, monitoring equipment that also monitors this timing information can know which sensor output is being sampled at any particular time. In the example for the internal combustion engine, a timing signal may be developed from a separate sensor delivering the rotational position of the crankshaft that is used to inform the injector position sensor monitoring system which injector should be operating at any specific rotational position of the crankshaft. This information is used to tag or otherwise mark the pulse train from the monitoring resistor to identify each individual sensor output.  
         [0006]     Position sensors used to monitor these moving objects generate an electrical signal that is proportional to the distance between the moving object and a fixed position. An ideal output signal contains only this information; however, several unwanted electrical signals generally characterized as noise are also usually generated or otherwise transmitted along with the desired position signal. These noise signals are generally divided into either low frequency or into high frequency noise. Higher frequency noise is usually easily filtered out with a low pass filter since the frequency of these noise signals is higher than the frequency of the position signal because moving objects are constrained to velocities that generate signals in or just above the audio or ultrasonic range and because in a well designed sensor these high frequency noise levels are usually several magnitudes in power level below the desired output position signal.  
         [0007]     Most position sensing transducers also generate low frequency noise in the form of a slowly drifting or static DC offset, or error signals that may be a significant portion of the total overall signal. An example of such transducers is a Hall cell where the signal generated is produced by a magnet. The signal from this transducer contains a large DC offset voltage generated by the magnet and a smaller AC signal generated as the target changes the magnetic flux density. Another example is a capacitive or inductive sensor where the slowly changing signal is caused by semiconductor device drift caused by temperature or other changes. This slowly changing or static error signal causes numerous problems in employing two-wire current output position sensors. The generation of any signal current through the sensor causes power to be dissipated inside the sensor. This adds to the temperature of the devices in the sensor, reducing the maximum ambient temperature that the sensor can operate at and reducing overall sensor reliability. The addition of a relatively static or DC current through the output sensing resistor connected to any number of these sensors increases the voltage dropped across the resistor. This leaves less power for the sensors or means that the applied voltage must be increased to generate the required operating voltage for the sensors. This power is wasted and also requires a higher power capability for resistors, by way of example. Also, increased current through the sensor wires means they also must be increased in diameter to accommodate the increased power lost through their series resistance. A further limitation on these type sensors is that especially upon power-up, the sensor should desirably not draw a large amount of current and should automatically calibrate itself so that no excessive current is drawn at any time during its operation. For instance, on vehicles utilizing storage batteries, the initial power-up of these sensors usually occurs at the same time that the battery is being used to crank the engine, reducing the amount of power available to power the sensors.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is directed to sensing position or movement of an object. A position sensor signal conditioner and remotely electrically connected sensor monitoring and control equipment provide a method of multiplexing multiple numbers of sensors on a minimum number of wires with a minimum of energy required from each sensor monitoring system. The sensor and external control function to require a minimum quantity of devices to be physically located on the sensor while enabling full error and temperature compensation. The external control circuit functions by generating and using an external clock to subtract error levels from a transducer signal wherein said error levels are of the same polarity as a desired dynamic signal only when said dynamic signal is known to be absent from the output of said sensor.  
         [0009]     One embodiment of the invention is herein described as a sensor that may comprise a waveform generator and an error correction generator for modifying a sensing signal by removing unneeded power and providing the signal to a remote monitor via two wires useful in multiplexing multiple sensors. The waveform generator is operable for receiving an unconditioned sensing signal from a transducer and modifying the unconditioned sensing signal in response to an error correction signal for providing a conditioned sensing signal. The error correction generator may provide the error correction signal using a comparator for receiving the conditioned sensing signal and determining a value thereof, a controller for providing first and second timing signals responsive to the value of the conditioned sensing signal, and a signal processor for providing the error correction signal responsive to the first and second timing signals.  
         [0010]     The error correction generator determines and eliminates strong static signals and error signals that do not deliver information about a position of an object being sensed, wherein inclusion of the static and error signals would require energy. One embodiment may include a digitally stored offset and error correction closed-loop compensation circuit for constantly comparing a value of the conditioned sensing signal to a desired minimum value and generates a correction signal that is subtracted from the offset and error signal to deliver a sensor signal output that is close to a desired minimum value. The constant comparing of the sensor signal output to the desired minimum value proceeds in a first direction relative to a direction of sensor output signals generated when an object being sensed moves in a relatively slow manner compared to a nominal speed of objects being monitored such that signals are generated as the objects move are not subtracted from the sensor output to a degree significant enough to cause significant variance between a position of the object and a signal level delivered by the sensor indicating the position. Further, the constant comparing of the sensor signal output to the desired minimum proceeds in a second direction relative to the direction of signals generated when the object being monitored moves in a relatively fast manner compared to the speed of objects being monitored so signals generated by errors or from other noise sources are subtracted from the sensor output in a manner sufficient to allow for a deletion of these error or static signals from being a significant portion of the position signal generated by the sensor.  
         [0011]     One embodiment of the invention may include a window reference circuit that constantly compares a desired conditioned sensor signal output to an existing conditioned sensor signal output and adjusts the conditioned sensor signal output if it is above a preset high reference signal or below a preset low reference signal. The signal processor may generate a relatively small reference signal that is large enough to eliminate small values of drift in a negative going direction yet is small enough not to generate a significant amount of signal due to a discrete nature of calibration voltages from a DAC and counter combination employed thereby. The error correction generator may generate a relatively large reference signal that substantially exceeds the largest voltage encountered by the sensor as an object being monitored moves its maximum amount, allowing rapid recalibration due to sudden changes in an offset voltage caused by rapid temperature or other changes. Yet further, the signal processor may include a DAC and counter combination circuit that contains enough resolution such that even if a sensor offset correction signal is generated as a result of a change in sensor output due to a movement of an object being sensed, the error correction signal is not a significant portion of the conditioned sensing signal representative of a position of the object.  
         [0012]     In a second embodiment of the invention, a significant size reduction and an increase in reliability can be achieved by reducing or eliminating the number of semiconductor devices on the chip. In a first embodiment, the semiconductor devices required to implement the clock with both a short and a long alternation, the counter, and the DAC take up significant amounts of space on the die. The clock circuit longer alternation requires a separate counter to implement an increase the time of the alternation. Significant decreases in size could be realized by reducing the number of bits of the counter and DAC H.  
         [0013]     However, a fast-running clock without the longer alternation would rapidly drive the dynamic signal down to the zero reference level or below. Since in the original version, the only purpose of reducing the offset output level is to reduce the power consumed by the sensor and related power supplies, an increase in the step size simply means that more power is consumed. However, any such step increase in a free-running version of the offset elimination circuit means that any changes in level during the dynamic signal level would become objectionable. Decreasing the number of devices in other parts of the circuit that process the dynamic signal would also contribute to a decrease in output quality. It is likely therefore that the only way to decrease die size and increase reliability are to eliminate the longer alternation and to decrease the number of bits of the counter and the DAC and to prevent the offset elimination circuit from operating during the dynamic signal event.  
         [0014]     This could be accomplished easily in most cases by using an external clock only to generate signals that allow compensation only during the time that the dynamic signal is known to be absent. An example application is in the monitoring of diesel fuel injection systems, where the target is known to be at rest a large part of the time, and the actual movement of the target is initiated by a control system so the approximate timing of the dynamic event is known.  
         [0015]     In some of the aforementioned sensors, a quantity of useful information can be obtained about the sensor and its environment if the amount of static offset can be obtained. For instance, the amount of time it takes the sensor offset compensation circuitry to eliminate the offset can be used to determine the status of magnets used to generate magnetic fields if the time taken is proportional to the field detected. Likewise, if these sensors contain an internal clock and offset compensation circuit, the timing and power drawn by the internal clock can be used to determine the temperature of the sensor. Reference is made to pending application, U.S. Ser. No. 10/345,847 for “Multiplexed Autonomous Sensors and Monitoring System and Associated Methods”, the disclosure of which is incorporated herein by reference in its entirety. In these cases, an additional advantage is gained by leaving an internal clock on the sensor and causing this internal clock to provide offset compensation at specific times during which the temperature of the sensor is desired or during which the static magnetic field is desired. The length of time taken to eliminate the offset yields the value of the offset, and the power drawn by the digital pulses yield the sensor temperature. However, to prevent the internal clock from running all the time and possibly reducing the sensor output during a dynamic event, the internal clocked offset compensation system must only respond to events or circumstances that cause the offset to decrease. Provided this requirement is met, there is an advantage to having an internal clock and an internally controlled offset compensation circuit on these sensors, even though an external clock could be used alone to accomplish the same operations. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     For a fuller understanding of the invention, reference is made to the following detailed description, taken in connection with the accompanying drawings illustrating various embodiments of the present invention, in which:  
         [0017]      FIG. 1  is a functional block diagram illustrating one embodiment of a position sensor according to the teachings of the present invention;  
         [0018]      FIG. 2  is a schematic block diagram illustrating one electronic circuit implementation of position sensor of  FIG. 1 ; and  
         [0019]      FIG. 3  is a schematic block diagram illustrating an alternate embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternate embodiments.  
         [0021]     With reference initially to  FIG. 1 , a position sensor  100  is herein described as including a waveform generator  102  operable for receiving an unconditioned sensing signal  104 S from a transducer  104  and modifying the unconditioned sensing signal responsive to an error correction signal  105 S for providing a conditioned sensing signal  102 S. As will herein de described, a window reference circuit may constantly compare an ideal sensor output to the existing sensor output and adjust the output if it is above a preset large reference signal or below a preset small reference signal. An error correction generator  105  is operable with the waveform generator  102  for providing the error correction signal  105 S. The error correction generator  105 , as herein described by way of example, comprises a comparator  105 A for receiving the conditioned sensing signal  102 S and determining a value thereof, a controller  105 B for providing first and second timing signals responsive to the value of the conditioned sensing signal, and a signal processor  105 C for providing the error correction signal  105 S responsive to the first and second timing signals. A sensor monitoring system  200  may be remotely located for providing power to the sensor  100  and receiving the conditioned sensing signal  102 S via two wires, making the sensor  100  most desirable for multiplexing with other sensors.  
         [0022]     Referring now to  FIG. 2 , comparators  116  and  118 , along with a voltage divider network composed of resistors  110 ,  112 , and  114 , comprise a window comparator, the comparator  105 A that, along with the controller  105 B including control logic having OR gates  120  and  134 , inverters  122  and  128 , SR Latch  130 , Clock  126 , and divider  132 , control the count rate and direction of the signal processor  105 C including a counter  108  and digital to analog converter (DAC)  106 . The output  105 S of the DAC  106  is subtracted from the output  104 S of the transducer  104  in the waveform generator, herein presented as differential amplifier  102 . The voltage divider network provides reference voltages to the negative pins of comparators  116  and  118 . The reference to comparator  118  is the low end of the window and the reference to comparator  116  is the high end of the window. When the output of differential amplifier  102  is below the window the comparators  116  and  118  and logic system will cause counter  108  to count down at a high rate, providing, via the DAC  106 , a negative going offset to the negative input of differential amplifier  102 , causing its output to go positive. This output will keep going positive until it passes above the low end of the window at which time the comparators and logic will cause counter  108  to count down at a low rate. Counter  108  will count down at a low rate whenever the window comparator input is inside the window from below. A very high ratio between counting up and counting down around the lower edge of the window keeps the signal baseline right at the lower edge of the window when the signal is a pulse train. If the ratio was 1/1 the average pulse height would seek the lower end of the window. If, for some reason, a transient has driven the signal above the window, the comparators and logic will cause counter  108  to count down at a high rate until part of the signal has gone below the window. Thereafter it will only count down at a low rate.  
         [0023]     Upon a rapid increase in sensor voltage on power-up, preset  124  generates a pulse that causes counter  108  and DAC  106  outputs to go to their highest value and the output of differential amplifier  102  to go to zero thereby lowering the current through resistor  136  to zero. Thus upon startup and initial calibration the sensor draws a minimum of current. Also, the sensor can be recalibrated at any time by external means by simply removing and reapplying power.  
         [0024]     The low end of the window set by resistors  110 ,  112 , and  114  is just high enough in value to compensate for any offset in comparator  118  that ordinarily might not allow the output of differential amplifier  102  to get below the comparator  118  threshold. This divider network also sets the value of the window on the negative pin of comparator  116  to a level substantially higher than the dynamic signal from the transducer  104  and differential amplifier  102  generated when an object moves or when a parameter being monitored by transducer  104  changes.  
         [0025]     With reference to the controller  105 B, logic may operate in the following manner. If the input to the window comparator  105 A is below a preselected window, the resultant low output from comparator  118  is inverted by an inverter  122 , placing a high signal into the lower input of Or gate  134  and forcing its output high which connects the wiper of switch  138  to a FastCLK pin of divider  132 . At the same time, since the inputs to both comparators  116 ,  118  are low, both inputs to Or gate  120  are high which causes counter  108  to count down rapidly, causes the output  105 S of DAC  106  to fall, and causes the output  102 S of differential amplifier  102  to rise. When this output  102 S rises above the lower edge of the window comparator  118 , it goes high forcing the output of Or gate  120  high and the output of inverter  122  low and consequently the output of Or gate  134  low, changing connecting switch  138  to a SlowCLK pin. Counter  108  now counts down at the slow rate until the output of differential amplifier  102  goes below the window and the process continues to cycle. Generally, the slow clock signal will be used for error correction when a transducer output signal is anticipated, and a fast clock signal used for an error correction when noise and only error signals are expected.  
         [0026]     When a sensor system baseline from differential amplifier  102  is in a desired position with all offset corrected, the high end of the window generated by the resistor network is significantly higher in value than a normal dynamic signal from differential amplifier  102  caused by a changing magnetic field.  
         [0027]     As the object or process being monitored increases the output of differential amplifier  102 , the components of the sensor operate to begin increasing the output of the DAC  106  in order to compensate for an increase in value. However, the rate of clock  126  is chosen to be slow enough that a significant number of changes of signal level do not occur during a fast movement of objects being monitored. Also, the number of bits chosen for the operation of the counter  108  and the DAC  106  are such that the increase and decrease in the output  105 S, while the differential amplifier  102  output changes, are not a significant portion of the dynamic signal generated by the transducer  104  when the object being monitored moves. The DAC  106  and counter  108  combination may contain enough resolution such that even if sensor offset correction signal is generated as a result of a change in sensor output due to the movement of the object being sensed, the error correction signal is not a significant portion of the sensor position signal.  
         [0028]     With the sensor  100 , as herein described by way of example, there is a determination and elimination of strong static signals or other error signals that do not deliver information about the position of the object being sensed whose inclusion in the sensor output signal would waste energy. A digitally stored offset and error correction closed-loop compensation may thus constantly compare the sensor output to a desired minimum value and generate a correction that may be subtracted from the offset and error signal to deliver a sensor output that is as close to the desired, an ideal minimum, as is practical without requiring unnecessary circuitry that is typically used for signal conditioning. For the sensor  100 , herein described, the constant comparison of the sensor output  102 S to the desired value, an ideal minimum value, proceeds in a first direction relative to a direction of signals generated when the object (a target) being monitored moves in a relatively slow manner compared to the speed of objects being monitored such that signals are generated as the objects move that are not subtracted from the sensor output to a degree significant enough to cause significant variance between the position of the object and the position signal level delivered by the sensor. The constant comparison of sensor output proceeds in a second direction relative to the direction of signals generated when the object being monitored moves in a relatively fast manner compared to the speed of objects being monitored so that signals generated by errors or from other noise sources are subtracted from the sensor output in a manner sufficient to allow for a deletion of these error or static signals from being a significant portion of the position signal generated by the sensor.  
         [0029]     By way of further example, in operation, the sensor  100  may generate a relatively small reference signal that is large enough to eliminate small values of drift in a negative going direction yet is small enough not to generate a significant amount of signal due to the discrete nature of the calibration voltages from the DAC and counter combination. A relatively large reference signal that may substantially exceed the largest voltage encountered as the object moves its maximum amount is accommodated by allowing rapid recalibration due to sudden changes in offset voltage caused by rapid temperature or other changes.  
         [0030]     With reference again to  FIG. 2 , for the embodiment herein described by way of example, the sensor  100  is connected to the sensor monitoring system  200 , external circuitry through a current-to-voltage converter resistor  202  to a power supply  204 . Upon a rapid increase in sensor voltage caused by an inrush of current upon power-up, a preset  124  generates a signal that causes counter  108  to go to its highest value, driving DAC  106  output  105 S to its highest value. The output  105 S of DAC  106  thus drives differential amplifier  102  output  102 S low. A resistor  136  is connected between the output of differential amplifier  102  and system ground  206  through a sensor lead  142 . Thus, upon startup and initial calibration, the sensor  100  draws a minimum of current. For the embodiment herein described by way of example, the resistor  136  converts the voltage output  102 S of the differential amplifier  102  to a current drawn through sensor leads  140  and  142 . This results in a requirement of only two wires to connect the sensor  100  to the external circuitry of the monitoring system  200 . The sensor  100  thus modulates a current across the pair of wires  140 ,  142  connected to the sensor monitoring system  200  where the modulated sensor current is converted into a modulated sensor signal voltage.  
         [0031]     If system parameters change suddenly and significantly, causing a large and rapid increase in the output  102 S of the differential amplifier  102 , the voltage at the negative input pin of the comparator  116  is set by the values chosen for resistors  110 ,  112 , and  114  to a value higher than the dynamic signal caused by the object moving. In this way movement of the object being monitored does not cause the sensor  100  to attempt a subsequent rapid calibration of the offset level.  
         [0032]     Referring now to  FIG. 3 , a second embodiment is illustrated in a block diagram for an implementation of a sensor that uses an internal clock to compensate for error signals that go below a threshold value, ignores signal excursions due to dynamic responses from a transducer, and uses an external clock to compensate for error signals that go above the threshold value. A control signal  200  is provided to the sensor  100  and signals are generated by external equipment to generate clock signals or to set the preset detector  124 . These signals can be any common methods employed to generate clock signals and generate a preset signal. One example of such signals that is commonly employed is to generate the preset signal by generating a signal with a rapid rise in value to a preset value and to generate a clock signal by a rapid negative-going pulse with short duration. Reference is made to pending application, U.S. Ser. No. 10/995,963 for “Offset Compensated Position Sensor”, and as above indicated has its disclosure incorporated herein by reference in its entirety. The preset detector signal is connected to an input of preset detector  124  which generates a preset signal to counter  108  which presets the counter  108  to a high state. This is connected to DAC  106  which places the output of DAC  106  to a highest analog voltage. This high analog voltage is connected to the minus input pin of amplifier  102 . The output of transducer  104  is connected to the non-inverting input of amplifier  102 . The analog output of DAC  106  is designed to be higher in its maximum value than any possible signal level generated by transducer  104 . This generation of a preset signal by control  200  results in the output of amplifier  102  going to a lowest possible value. The output of amplifier  102  is connected to the positive pin of comparator  118 . A threshold signal  146  is connected to the negative pin of comparator  118 . The output of amplifier  102  is designed to be lower than the threshold  146  when control  200  generates a preset signal. This places the output of comparator  118  in a lowest possible state. This output is connected to the count direction up/down pin of counter  108  and places the direction of the counter into the down direction. The output of comparator  118  is likewise connected to clock selector  132 ′. As long as this output is at its lowest state, clock selector  132 ′ enables the output of internal clock  126  to be connected to the clock input of counter  108 . The output of control  200  is returned to a state where the output of the preset detector  124  does not hold the preset hi section of the counter  108  in a preset condition. The output of clock  126  is a continuous pulse stream at a highest possible frequency. When these pulses are connected to the clock input of counter  108 , they cause the counter  108  to ramp downward at the same rate as the clock. This causes the analog output of DAC  106  to begin to decrease. This state of events continues until the output of amplifier  102  goes just above the value of threshold  146 . When this occurs, the output of comparator  118  goes high, resetting the count direction of counter  108  to a high direction. External control  200  is not at this time generating clock pulses so the output of clock detector  120  is low. This output, combined with the high state of comparator  118  causes clock selector  132 ′ to block the output of internal clock  126 . The output of DAC  106  becomes static at a level required to keep the output of amplifier  102  as close as possible to the value of threshold  146 . The output of transducer  104  during a dynamic signal event is specifically chosen to go in a positive direction in relation to the threshold value. This positive excursion during a dynamic event does not change the output state of comparator  118 , which keeps the output of counter  108  in the UP direction and keeps the clock selector  132 ′ from connecting the output of internal clock  126  to the clock input of counter  108 . Thus the dynamic event signal output of transducer  104  is delivered through amplifier  102  to the output  300  of sensor  100  without being affected by the offset compensation circuitry.  
         [0033]     During operation of sensor  100 , temperature or other environmental factors may occur that cause the output of transducer  104  to go more negative than its value during the initial preset by control  200 . If this occurs, the output of amplifier  102  goes more negative, causing the output of comparator  118  to remain in a low state, keeping the counter  108  direction down and causing clock selector  132 ′ to connect the output of internal clock  126  to the clock input of counter  108 . Each time clock  126  goes through a transition, counter  108  is decremented. The output of DAC  106  then goes lower. This continues until the output of the DAC reaches the signal level of transducer  104 , at which time the output of amplifier  102  again goes above the threshold. This causes the direction of counter  108  to return to the UP direction, and causes the clock selector  132 ′ to block the output of the internal clock  126  from counter  108 . The state of the offset compensation circuitry in sensor  100  will then remain constant.  
         [0034]     If the output level of transducer  104  increases a sufficient amount due to noise or temperature or other effects, to go above the value of threshold  146 , comparator  118  output goes high and sets counter  108  to the UP direction. During the times that the dynamic signal is known to be absent, output  300  is monitored by external control  200  for this increase in signal level relative to the direction of the dynamic signal. If the increase is significant, a clock pulse is generated by control  200 . This clock pulse is connected to clock detector  144  in sensor  100 . Since comparator  114  output is at a high state, the external clock pulse is connected through clock detector  144  through clock selector  132 ′ to the clock input of counter  108 . Each positive alternation of the external clock increments the counter  108  and the DAC  106 , resulting in more voltage being subtracted at amplifier  102 . This brings the output  300  back down in incremental steps to the value of threshold  146 . The external clock signal is stopped, and the output of the sensor  100  remains stable until the next dynamic signal or until environmental or other changes cause a change in the static level.  
         [0035]     Note that in this general description of a block diagram incorporating the methods of using the internal and external clock, the particular selection and arrangement of circuit elements shown is simply that which facilitates an easiest explanation of a means to incorporate the said methods. Anyone practiced in the are will readily see many similar such circuit arrangements that would satisfy the requirements of the claimed methods.  
         [0036]     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.