Abstract:
An absolute position detector that interprets, rather than avoids, abnormal sensory states. Different combinations of sensors in an array are activated as a marker travels along a path. The current activation/deactivation state of the array is converted into a digital signal that is reliably indicative of the current absolute position of the marker along the path. In a preferred embodiment, a dynamic magnetic field is provided whose current condition represents the current absolute position of a moving marker. As the marker moves, the condition of the field changes to activate different groups of Hall Effect devices in an array. The activated groups may comprise one or more Hall Effect devices in the array, depending on the condition of the field as created by the position of the marker. The current activation/deactivation state of the array is then converted, advantageously via truth table logic, into a digital signal representative of the current position of the marker.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    In simple systems, an absolute position detector (APD) is generally used to determine the position of a moving component along a path of travel. The APD typically accomplishes this via detection of the current position of a marker affixed to or associated with the component. In more sophisticated systems, the APD is further used to determine the positional state of an assembly of interrelated moving components, such as gears, via resolution of the combined current position of a series of markers on the interrelated moving components.  
           [0002]    APDs are commonly used in connection with an assembly of multiple rotating gears, such as may be found, for example, in valve actuator mechanisms. In a valve actuator, an APD generally detects the positional state of a series of interrelated circular gears, the positional state representative of the degree of openness of the valve. In such cases the APD monitors the paths of circular travel of markers on the interrelated gears.  
           [0003]    Such circular travel is by no means the only type of motion monitored by APDs, however. APDs may be used to monitor position, for example, on other shaped closed loop paths, such as elliptical or irregular closed loop paths. Alternatively, APDs may be used to measure position on straight or curved paths that are not on a closed loop. Travel along such open paths may or may not be reciprocating. For example, APDs can be used to measure the state of a torque sensor in a valve actuator via monitoring of the current position of a member traveling along a pendulum-like path.  
           [0004]    It is also well understood that absolute position detection is a dynamic operation. Successive samples of a current positional state may be resolved into a stream that forms a dynamic control tool.  
           [0005]    The advent of computerized control has increased the need for APDs that generate high speed streams of samples that indicate current positional states in digital format. The speed and capacity of modern computers allow positional samples to be processed at a high speed. There is a particular need for APDs that generate samples at that speed. The faster the stream of samples that is processed, the finer the resolution of monitoring and control of the positional state.  
           [0006]    For this reason, APDs relying on mechanical systems, such as cams to activate a switch or a potentiometer, are fast becoming obsolete. These devices have always suffered from backlash because of the mechanical load on the components. Wear and tear on the mechanical parts has always tended to shorten operational life. On top of these inherent drawbacks, such mechanical APDs tend not to be robust enough to be able to be operated at high speeds.  
           [0007]    The prior art has addressed the problems of mechanical APDs by teaching use of non-contact APDs, such as those described in U.S. Pat. No. 5,640,007. This patent describes an optical encoder in which a plurality of encoder wheels each contain a series of openings that pass by a light emitting device as the gears are turned. As light is shined through these openings and detected, the openings through which light is transmitted encodes the position of the wheels, and this code is compared to a defined code sequence to determine the position of a rotatable shaft functionally connected to the position detector.  
           [0008]    Certain types of detectors have also utilized magnetic fields to encode the position of a device and to determine the position of a rotatable shaft. A magnetic sensor apparatus is described in U.S. Pat. No. 4,728,950. The device described in the &#39;950 patent addresses the problem of automation of reading decimal gears, such as the type normally used in utility meters, in which each higher order digit is displaced in a 10 to 1 ratio relative to the lower digit. The devices described teach use of Hall Effect sensor devices to detect the magnetic field of a magnet attached to each digit gear. In the devices described, each gear has a permanent magnet attached thereto, and an array of 10 Hall Effect sensor devices are placed in a circular array with respect to each gear, so that the magnets pass over the sensor devices in succession as the gears turn. The resulting coded output of the sensor devices is read in a linear fashion, such that the reading of the position of the higher gears are dependent on the readings of the lower gears.  
           [0009]    Both of the &#39;007 and &#39;950 inventions rely exclusively on normal state recognition, and indeed go to great lengths to avoid abnormal states. This reliance on normal state recognition is typical of prior art APDs. The term “normal state” refers to an environment in which preselected individual sensors, and often single individual sensors, are associated with a corresponding number of discrete absolute positions available to a source. Whenever the source occupies one of those positions and activates solely the designated sensor or group of sensors for that position, the APD is in a “normal state.” In contrast, whenever the source is between those positions in its path of travel, or otherwise activates more than one designated sensor or group of sensors at the same time, the APD is in an “abnormal state.” 
           [0010]    Returning to the &#39;007 and &#39;950 patents, these inventions are characteristic of the traditional prior art approach using exclusively normal state position detection. A key feature of these prior art inventions is to ensure that stray or unfocused emissions from the source are not recorded in error by the wrong sensor, i.e. a sensor other than the one designed to be indicative of the current absolute position. In the case of the &#39;007 patent, reflective surfaces are used to focus light through the apertures and onto the light sensing devices. In the case of the &#39;950 patent, a three-pole magnet is used to focus the magnetic field onto single Hall Effect devices in predefined normal states.  
           [0011]    Sole reliance on normal state detection imputes a number of drawbacks and inherent limitations on APDs. First, the resolution of the APD is limited to the number of discrete sensors deployed. The APD can measure no finer a resolution than the predefined number of normal states monitored for. Further, structural or space limitations may dictate that less than an optimal number of sensors can be deployed on a particular moving component.  
           [0012]    Second, while stray and erroneous sensory readings can perhaps be minimized, they can never truly be eradicated. This is especially true for a safe, low-cost sensory medium such as magnetic flux, which exists in more of a field than a directed beam. This is further especially true in high speed environments where the transitions between successive normal states become harder to identify. Thus, APDs relying solely on normal state detection necessarily include a lot of structure minimizing the effect of the inevitable. This structure adds to the cost and complexity of the APD, while possibly detracting from overall robustness.  
           [0013]    This prior art tendency towards “extra structure to minimize the inevitable” is highlighted in U.S. Pat. No. 4,737,710. A complex combination of Hall Effect device placements, stray flux shielding, and signal adaptation circuitry is disclosed to purify detection of normal states in high speed service. It is nonetheless a fact that APDs such as illustrated in the &#39;710 patent exist in an abnormal state most of the time. Except when a source is momentarily directly in communication with a single sensor, the APD is effectively between sensors, and thus by definition in an abnormal state. Sole reliance on normal state detection may therefore not always be the most optimal approach.  
           [0014]    It will therefore be appreciated that an APD that interprets detection of abnormal states, rather than avoiding them, would be highly advantageous. The disadvantages of sole reliance on normal state detection as described above would be obviated. Further, such an APD would further have good application to high speed service.  
           [0015]    Moreover, reliance on detection of both normal and abnormal states presents opportunities for additional novelty in the process of decoding the physical state of an array of sensors into a signal suitable for digital processing. Although decoding techniques known in the art could be used, it would be further advantageous to have a decoding technique available that specifically enhances the performance and error-detection security of an APD interpreting both normal and abnormal states.  
         SUMMARY OF THE INVENTION  
         [0016]    These and other objects and advantages are provided by an absolute position detector that interprets, rather than avoids, abnormal sensory states. The inventive APD allows a source to activate different combinations of sensors in an array as a marker travels along a path. The marker may be the source itself, or alternatively a moving window interposed between the sensors and a substantially stationary source. The current activation/deactivation state of the array is converted into a digital signal that is reliably indicative of the current absolute position of the marker along the path.  
           [0017]    In a first embodiment, arrays of Hall Effect devices monitor a dynamic magnetic field whose current condition represents the current absolute position of a series of magnets traveling on a set of interrelated circular gears. In operation of the first embodiment, the current relative position of the magnets on the gears generates a specific momentary magnetic field, which in turn activates a specific group of Hall Effect devices in the combined arrays. The activated group may comprise one specific or more Hall Effect devices in each of the combined arrays, depending on the condition of the magnetic field. The current activation/deactivation state of the array is sampled, and then converted, advantageously via truth table logic, into a digital signal representative of the current positional state of the gears.  
           [0018]    In a second embodiment, arrays of Hall Effect devices monitor the pendulum-like motion of a member in reciprocating travel along an open path. The position of the member on the path indicates a detected level of torque. Again, magnets are disposed on the member so as to create different activation/deactivation states in the arrays according to different positions of the member. Truth table logic then converts the current activation/deactivation state of the array into a digital signal representative of the detected level of torque associated with the current position of the member.  
           [0019]    As noted, both of the APD embodiments described herein practice the APD invention by detecting and encoding absolute positions via sampling the condition of sensor arrays that may be in normal and abnormal states. The conversion of these encoded absolute positions to a digital signal is actually achieved by decoding the condition of each of the sensors in the sensor array and applying truth table logic. It will be understood that this decoding function may be achieved by any preselected decoding technique that identifies the current state of the sensor array and then converts it to a computer processable code, such as binary code, Gray code, V-scan code, or alternatively a customized or proprietary code.  
           [0020]    The APD invention according to one aspect is therefore embodied in a device for detecting the positional state of an incrementing multi-gear assembly, comprising:  
           [0021]    an assembly of components in geared relationship, each component disposed to rotate, said geared relationship enabling the rotation of one component to increment a next component in sequence;  
           [0022]    a plurality of said components each including a system further comprising:  
           [0023]    a source affixed to the component and traveling along a predefined circular path as said component rotates; and  
           [0024]    an array of at least three sensors deployed equidistantly around the path of the source, different combinations of sensors within the array disposed to become activated and deactivated via sensory communication with the source as the source travels its path, said different combinations ranging from one sensor to multiple sensors activated by the source at different times during travel along the path by the source; and  
           [0025]    a converter, the converter disposed to convert current combined activation/deactivation states of sensors in each system into a digital signal representative of a current overall positional state for the assembly.  
           [0026]    According to a second aspect, the APD invention is further embodied in a method for establishing the position of a marker disposed to travel along a predefined path, the method comprising: (a) deploying an array of at least two sensors along at least a part of the path of the marker, different combinations of sensors within the array disposed to become activated and de-activated via sensory communication with at least one source as the marker travels its path, said different combinations ranging from one sensor to multiple sensors activated by at least one source at different times during travel along the path by the marker; and (b) converting current combined activation/deactivation states of the sensors in the array into a digital signal representative of a current position for the marker along the path.  
           [0027]    According to a third aspect, the APD invention is further embodied in a device for detecting the positional state of a geared assembly, comprising:  
           [0028]    a plurality of moving components in geared relationship;  
           [0029]    a plurality of said moving components each including a system further comprising:  
           [0030]    a marker affixed to the moving component and traveling along a predefined path as said component moves; and  
           [0031]    an array of at least two sensors deployed along at least a part of the path of the marker, different combinations of sensors within the array disposed to become activated and deactivated via sensory communication with at least one source as the marker travels its path, said different combinations ranging from one sensor to multiple sensors activated by at least one source at different times during travel along the path by the marker; and  
           [0032]    a converter, the converter disposed to convert current combined activation/deactivation states of sensors in each system into a digital signal representative of a current overall positional state.  
           [0033]    According to a fourth aspect, the APD invention is further embodied in a positional detector, comprising: a moving component; a marker affixed to the moving component and traveling along a predefined path as said component moves; an array of at least two sensors deployed along at least a part of the path of the marker, different combinations of sensors within the array disposed to become activated and deactivated via sensory communication with at least one source as the marker travels its path, said different combinations ranging from one sensor to multiple sensors activated by at least one source at different times during travel along the path by the marker; and a converter, the converter disposed to convert current combined activation/deactivation states of sensors in the array into a digital signal representative of a current position for the marker along its path.  
           [0034]    According to a fifth aspect, the APD invention is embodied in a device for detecting the position of a rotating member traveling in pendulum motion, the device comprising: a source affixed to the member, the source moving in reciprocating motion along a path described by a circular arc; an array of at least three sensors deployed equidistantly around the path of the source, different combinations of sensors within the array disposed to become activated and deactivated via sensory communication with at least one source as the source travels its path, said different combinations ranging from one sensor to multiple sensors activated by at least one source at different times during travel along the path by the source; and a converter, the converter disposed to convert current combined activation/deactivation states of sensors in the array into a digital signal representative of a current position of the member.  
           [0035]    It will therefore be seen that a technical advantage of the APD invention is that it relies on both normal and abnormal state detection in determining absolute position. The invention further eliminates the need for mechanical structure whose sole purpose in prior art APDs has been to avoid abnormal state detection. Both of these advantages contribute to an APD that is robust and reliable. Moreover, the APD is no longer limited to a best possible detection resolution that is defined by the physical number of sensors in an array. Best possible resolution in the inventive APD is defined by the number of discrete activation/deactivation states of the array that the source can activate during travel of the marker. Using binary state sensors such as Hall Effect devices, for example, it will be seen that there are potentially up to 2″ discrete activation/deactivation states of an array having n sensors.  
           [0036]    A further technical advantage of the APD invention is that it is well suited to high speed digital service. Being non-contact in nature, the inventive APD avoids all the operational speed limitations of mechanical APDs of the prior art. Further, the inventive APD detects current absolute position using a current activation/deactivation state of the sensors as a basis. Hardware and software techniques are well suited to conversion of a stream of such current activation/deactivation states into a digital signal at high speed.  
           [0037]    A yet further technical advantage of the APD invention is that it is scalable. A current activation/deactivation state of multiple interrelated arrays may easily be resolved into a current positional state of a moving assembly. If desired, such overall resolution may be accomplished more efficiently by concurrent hardware or software monitoring of the combined condition of all arrays, as opposed to monitoring individual arrays and then aggregating the results in a separate operation.  
           [0038]    Another technical advantage of the APD invention is that it is bidirectional. This advantage applies whether the invention is embodied on a closed loop or open path system. In either type of system, the inventive APD operates regardless of the direction of travel, since detection of absolute position is based on a monitored current activation/deactivation state of the sensor array. Moreover, it will be appreciated that for the same reason, the inventive APD operates to detect absolute position in the presence of reciprocating motion.  
           [0039]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
         [0041]    [0041]FIGS. 1A and 1B are cutaway views of a valve actuator to which embodiments of the APD invention may be attached via absolute position detector pinion assembly  100  and torque sensor pinion assembly  150 ;  
         [0042]    [0042]FIG. 1C is a plan view of the valve actuator illustrated in FIGS. 1A, 1B and  1 C, shown in cutaway outline form to depict position detector pinion  100  and torque sensor pinion  150  linked to embodiments of the APD invention;  
         [0043]    [0043]FIG. 1D is an enlargement as shown on FIG. 1C;  
         [0044]    [0044]FIG. 1E is a section as shown on FIG. 1D;  
         [0045]    [0045]FIG. 2 is a cutaway view of valve position detector mechanism  200  depicted on FIGS. 1C, 1D and  1 E;  
         [0046]    [0046]FIG. 3 is a sectional view as shown on FIG. 2;  
         [0047]    [0047]FIG. 4 is a further sectional view as shown on FIG. 2; and  
         [0048]    [0048]FIG. 5 is an enlargement of selected parts depicted on FIG. 1E.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0049]    Embodiments of the APD invention are described with respect to an APD monitoring the condition of a valve actuator. The APD in a first embodiment comprises a system of interrelated circular gears that each deploy markers following endless loop paths of travel. This system of gears is coupled to the gear mechanism of the valve actuator that actually opens and closes the valve. The relative positions of the markers as monitored by the APD is thus able to indicate the current state of openness of the valve. The APD in a second embodiment comprises a torque sensing member that reciprocates along an open path of travel converting rotary to pendulum-like motion. This APD senses and indicates the amount of torque currently being delivered by the actuator in opening or closing the valve or mechanism.  
         [0050]    [0050]FIG. 1A is a general arrangement drawing of the exemplary valve actuator in cutaway view. It will be seen that position detector assembly  100  is coupled via linkage including helical gear  110  to the main worm drive gear  120 . As will be illustrated momentarily with reference to FIG. 1D position detector assembly  100  drives a gear mechanism illustrated further on FIGS. 2 through 4. This mechanism operates to provide a valve position detector embodying the APD invention. With further reference to FIG. 1A, worm drive gear  120  opens and closes the valve responsive to rotation of motor  130 . Motor  130  activates worm drive gear  120  to open and close the valve between predefined fully open and fully closed positions. Movement of worm drive gear  120  between those positions thus generates corresponding rotation of position detector assembly  100 .  
         [0051]    [0051]FIG. 1A further shows torque sensor pinion  150  also coupled to worm drive gear  120 . As will be illustrated momentarily with reference to FIGS. 1D and 1E, torque sensor pinion assembly  150  drives a pendulum-like mechanism illustrated further on FIG. 5. This pendulum-like mechanism senses different levels of torque applied by motor  130  to worm drive gear  120 . With further reference to FIG. 1A, it will be seen that torque sensor pinion assembly  150  can sense this torque via main worm shaft  165  working against spring assembly  160 . The torque sensor mechanism of FIG. 5 is a second embodiment of the APD invention and will be discussed in greater detail later.  
         [0052]    [0052]FIGS. 1B is a more detailed cutaway view of FIG. 1A. The interrelation of motor  130 , gear limit position detector assembly  100 , helical gear  110 , worm drive gear  120 , torque sensor pinion assembly  150 , spring assembly  160 , main worm shaft  165 , torque linkage pinion  175  and worm torque rack  170  can be seen in more detail.  
         [0053]    [0053]FIG. 1C is a plan view of the valve actuator in cutaway outline form. Position detector pinion assembly  100  and torque sensor pinion assembly  150  will be evident from comparative reference to FIGS. 1A and 1B. It will be further seen on FIG. 1C that absolute position detector mechanism  200  is operatively linked to position detector pinion assembly  100 , and that torque rotor mechanism  500  is operatively linked to torque sensor pinion assembly  150 .  
         [0054]    [0054]FIG. 1D is an enlargement as shown on FIG. 1C. Absolute position detector mechanism  200  will be discussed in greater detail momentarily. It will also be seen on FIG. 1D how rotation of torque sensor pinion assembly  150  causes corresponding pendulum-like rotation of torque arm  510  in torque sensor mechanism  500 .  
         [0055]    [0055]FIG. 1E is a section as shown on FIG. 1D. Torque arm  510  rotating about axis  515  can be seen as part of torque sensor mechanism  500 , described in greater detail later with reference to FIG. 5. The components of valve position detector mechanism  200  can also be seen as assembled. Selected components will now be described in conjunction with FIGS. 2, 3, and  4 .  
         [0056]    [0056]FIG. 2 is an enlarged cutaway view of absolute position detector mechanism  200 , previously seen from FIGS. 1C and 1D to be driven by position detector pinion assembly  100 . FIGS. 1C and 1D depict input gear  212  on FIG. 2 to be operatively linked to the rotation of position detector pinion assembly  100 .  
         [0057]    Referring now to FIG. 2, input gear  212  rotates about input shaft  210 , and drives lowest digit gears  218  via input drive gear  214 . Digit gears  218 ,  230 ,  232  and  234  rotate independently around shaft  216 , as is known in the art. It will be seen on FIG. 2 that digit gears  218 ,  230 ,  232  and  234  increment in sequence. It will be understood that the linkage between digit gears  218 ,  230 ,  232  and  234  enables counting of the revolutions of shaft  210 . As one digit gear completes all or part of one rotation, a tooth or teeth thereon (see item  220  shown on FIG. 3) advances first incrementing gear  222 , which in turn increments the next digit gear  230  in sequence by a predetermined rotation. A current relative positional state of digits gears  218 ,  230 ,  232  and  234  thus indicates the number of turns that shaft  210  has made since a preselected datum (such as predefined travel limit for the valve). It will be understood with reference to FIGS. 1A through 1D that the absolute position detector mechanism  200  illustrated in FIG. 2 is thus able to monitor and identify, via linkage through position detector pinion assembly  100 , the current degree of openness of the valve.  
         [0058]    [0058]FIG. 3 is a sectional view as shown on FIG. 2. It will be seen that as digit gear  230  makes its rotation, teeth  220  advance incrementing gear  222 . FIG. 3 also depicts magnet  310  disposed on digit gear  230 . Magnet  310  rotates with digit gear  230 . It will be understood from FIG. 2 that FIG. 3 is also representative of the details of corresponding digit gears  218 ,  232  and  234 , in that digit gears  218 ,  232  and  234  also have magnets  310  disposed thereon and rotating therewith.  
         [0059]    [0059]FIG. 4 is a further sectional view as shown on FIG. 2. FIG. 4 depicts a printed circuit board (PCB)  400  typical of PCBs  400  interposed between digit gears  218  and  230 ,  230  and  232 ,  232  and  234 , and  234  and the casing of absolute position detector mechanism  200 . Holes  416  and  422  are large enough to permit unimpeded rotation of the digit gears on shaft  216  and incrementing gears  222  respectively. PCB  400  further provides Hall Effect devices  410  in a circular array deployed to monitor movement of magnet  310  disposed on digit gears  218 ,  230 ,  232  and  234 . Note that digit gears  218 ,  230 ,  232  and  234  are omitted from FIG. 4, but will be understood from FIGS.  2  and  3  to be carrying magnets  310  thereon, and to be further keeping magnet  310  within the sensory range of at least one Hall Effect device deployed in the array on PCB  400 .  
         [0060]    With further reference to FIG. 2, it will be appreciated that, consistent with the depiction of FIG. 4, absolute position detector mechanism  200  further comprises additional PCBs  400 . Each additional PCB  400  deploys additional circular arrays of Hall Effect devices  410  to monitor the movement of a magnet  310  on digit gears  218 ,  230  and  234 . Bus  450  carries the collective signals of Hall Effect devices  410  on PCBs  400  to a converter  460 . This converter  460 , described functionally in more detail below, tracks the evolving combined momentary state of all of the Hall Effect devices  410  throughout the mechanism by taking successive samples thereof. Such sampling enables the overall positional state of digit gears  218 ,  230 ,  232  and  234  to be monitored, which in turn enables the current positional state of the valve actuator to be monitored.  
         [0061]    It will be further appreciated from FIG. 4 that PCB  400  also provides printed circuitry (not illustrated) that connects Hall Effect devices  410  in such a way that signals therefrom can be monitored remotely. In the particular embodiment shown, PCBs  400  provide eight Hall Effect devices  410  each with open collector outputs. Hall Effect devices such as Serial No. DN6848S, available from Panasonic Corporation have been found to operate satisfactorily in this service. The eight outputs from the Hall Effect devices are pulled up to +5V logic supply by 10K Ohm resistors (maximum), 4.7K Ohms typical. The eight outputs of the sensors are input to a transparent latch IC such as a 74HC573 with three-state output. Each latch has a separate output enabled control line to allow multiplexing of the data to an 8-bit bus. All four latch ICs have a common latch control signal to ensure the current position of all digit gears is detected without time delay.  
         [0062]    Of course it will be understood that the foregoing circuitry and component details on PCB  400  are illustrative only, and that the APD invention is not limited to any specific design or functionality of PCB  400 . It will be further understood that the APD invention is not limited to the use of Hall Effect devices as a monitoring device. Moreover, where Hall Effect devices are used to monitor magnetic flux, the present APD invention is in no way limited to the particular Hall Effect device models used in the embodiments described. Those of ordinary skill in the art will be able to select different Hall Effect devices with known performance characteristics, and then deploy same in a chosen spatially-arranged array (or combination of arrays) to suit a specific application of the APD invention.  
         [0063]    It will be appreciated from the above description that in accordance with the APD invention, no special provisions are made in absolute position detector mechanism  200  to identify and monitor predetermined normal states for magnets  310  moving in and out of the sensory range of particular Hall Effect devices  410 . Contrary to the prior art, magnets  310  are free to generate signals in one or more Hall Effect devices as digit gears  218 ,  230 ,  232  and  234  move, regardless of whether momentary positions of magnets  310  generate normal or abnormal states in one Hall Effect device  410 , or in a group thereof. An abnormal state in which more than one Hall Effect device senses the same magnet is still representative of an identifiable current position of the magnet. It will be understood that converter  460  can deduce such magnet positions from identification of the Hall Effect devices that are active, whether in a normal or abnormal state, and with reference if necessary to a prior identified normal position. Advantageously, however, careful design will ensure that the magnets and Hall Effect devices are selected and deployed such that a combination of activated Hall Effect devices indicates a unique position for the magnet.  
         [0064]    The foregoing description of the first embodiment thus illustrates the APD invention as capable of interpreting both normal and abnormal states. It will be seen that Hall Effect devices  410  are disposed so as to encode the current positional state of the absolute position detector mechanism  200 , regardless of whether the detected positional state finds magnets  310  in normal or abnormal states with respect to Hall Effect devices.  
         [0065]    In this regard, it will be appreciated that the APD invention as described with reference to FIGS. 2 through 4 above is configured for magnets  310  to activate either one Hall Effect device  410  or a pair of adjacent Hall Effect devices. When the pair is detected to be activated, the position of the magnet may be identified as physically between the two. In this way, the four PCBs  400  on FIG. 4 each deploying eight Hall Effect devices in combination enable sufficient positional resolution for the APD to generate a 16-bit sample representative of the current overall activation/deactivation state of the sensors. It will nonetheless be appreciated that these parameters are design choices that have been found to be suitable for the specific valve actuator described, and that the APD invention is not limited in this regard.  
         [0066]    The foregoing description also describes monitoring and detection of a positional state using magnetic flux as a communication medium between magnets as sources and Hall Effect devices as sensors. It will be further understood that the APD invention is not limited in this regard either. Other communications systems are available with equivalent enabling effect. For example, sources of visible light radiation, infrared radiation, ultraviolet radiation, ultrasound radiation, radioactive radiation, electrostatic charge, radio frequency radiation or microwave frequency radiation may be substituted to activate corresponding sensor devices.  
         [0067]    The foregoing description further describes the source (i.e. magnet  310 ) moving as a marker within the sensory range of the sensors (i.e. Hall Effect devices  410 ). It will be understood that the APD invention is also not limited in this regard. The APD invention is equivalently enabled on an embodiment in which the source may be substantially stationary. In such embodiments, positional state is monitored and detected by the movement of markers comprising moving windows interposed between the source and the sensors.  
         [0068]    [0068]FIG. 4 in the foregoing description further depicts the array of Hall Effect devices  410  as spatially equidistant. Again, it will be understood that the APD invention is not limited in this regard. In order to establish a desired monitoring and detection of a positional state, it may be desirable to use arrays of sensors spaced irregularly, and whose combined activation/deactivation states are capable of identifying special conditions. The ability of the APD invention to adapt to the use of irregularly-spaced arrays is yet a further benefit of positional monitoring and detection with reference to both normal and abnormal states.  
         [0069]    Converter  460  illustrated in FIG. 4 may be enabled in alternative ways. The basic principle is that current positional state will be deduced via reference to truth table logic available to the converter. Such truth table logic is calibrated to identify the positions of markers, or the overall positional state of a series of markers, according to the currently sensed combined activation/deactivation states of the sensors.  
         [0070]    The converter may be deployed as a processor driven by software. In this case, the processor is advantageously disposed to poll the current activation/deactivation states of the sensors and, via reference to a truth table, to translate same into a group of current individual source positions. The processor is then further disposed to resolve the group of current individual source positions into a digital signal representative of a current overall positional state. The converter according to this deployment may be embodied on software executable on a general purpose computer.  
         [0071]    Alternatively, the converter may be deployed in hardware. In this case, the hardware is advantageously configured to monitor and detect changes in the current activation/deactivation states of the sensors. Then, via reference to truth table logic encoded in the hardware, the hardware uses such logic to convert the detected changes into a digital signal representative of a current overall positional state. The converter according to this deployment may be embodied on any of many commercially available forms of hardware circuitry, such as logic integrated circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), a programmable logic array (PLA), a programmable logic device (PLD), an erasable programming logic device (EPLD), an application specific integrated circuit (ASIC) or other such similar devices.  
         [0072]    The first embodiment has been described above with exemplary reference to an APD monitoring the opening and closing mechanism of a valve actuator. In this embodiment, as shown on FIGS. 2 and 3, magnets  310  are deployed on digit gears  218 ,  230 ,  232  and  234  and rotate in an endless circular loop. It will be appreciated that other embodiments of the APD invention may deploy movement of sources (or markers comprising windows for sources) on endless loops having other shapes.  
         [0073]    Further, the APD invention may be embodied on mechanisms in which sources (or markers comprising windows for sources) travel on open paths. It will be appreciated that such travel will often reciprocate along the open paths, although the APD invention does not require reciprocating travel.  
         [0074]    A second embodiment of the APD invention illustrates this open path alternative, as deployed on torque sensor mechanism  500  illustrated on FIGS. 1C, 1D,  1 E and  5 . By way of background, torque monitoring is a highly advantageous feature in valve actuation for several reasons. As a minimum all valves require a safety mechanism for stopping the motor when excessive torque is detected. Many valves also require that the valve be seated within a certain torque range. Too much torque could damage the valve by producing too much stress on the mechanical components. Too little torque might allow leakage by the seat that would not be desired by the process and could result in valve damage due to erosion of the seat. Preventative maintenance can also be implemented on the valve by analyzing the torque throughout the life of the valve and actuator.  
         [0075]    [0075]FIG. 1B illustrates in detail a mechanism in which the rotation of worm shaft  165  causes main worm drive gear  120  to rotate. Opening the valve causes drive gear  120  to rotate in one direction, and closing the valve causes it to rotate in the other direction. The torque delivered by worm shaft  165  also generates a corresponding axial force against worm drive gear  120  encouraging worm shaft  165  to displace laterally along its axis of rotation against spring assembly  160 . Again, opening the valve causes worm shaft  165  to displace towards spring assembly  160 , and closing the valve causes worm shaft  165  to displace away from spring assembly  160 . The greater the torque, the greater the lateral displacement of worm shaft  165 .  
         [0076]    With further reference to FIGS. 1B and 1C, it will be seen that the lateral displacement of worm shaft  165  causes rack gear  170  to rotate torque linkage pinion  175  on torque sensor pinion assembly  150 . Turning now to FIGS. 1C, 1D, and  1 E, it will be understood that the rotation of pinion  175  responsive to lateral displacement of worm shaft  165  is translated via torque sensor pinion assembly  150  into corresponding rotation of torque arm  510  about axis  515 .  
         [0077]    [0077]FIG. 5 is an enlargement of corresponding parts on FIG. 1E, showing additional detail. It will be appreciated that the resulting motion of torque arm  510  about axis  515  is a pendulum-like movement, reciprocating back and forth along an open path of arc-like travel. It will be further appreciated that the degree of pendulum-like motion of torque arm  510  from a datum position represents a corresponding degree of displacement of worm shaft  165 , which in turn represents a corresponding degree of torque generated by drive  120 . Moreover, the direction of such pendulum-like motion of torque arm  510  from a datum position indicates an increase or decrease in torque from a calibrated datum level associated with the datum position of torque arm  510 .  
         [0078]    With further reference to FIG. 5, it will be understood that torque arm  510  is depicted in a datum position. Upper magnet  520  and lower magnets  525 A and  525 B are disposed on torque arm  510 . During the pendulum-like motion of torque arm  510  about axis  515 , it will be seen on FIG. 5 that magnets  520 ,  525 A and  525 B move in and out of the sensory range of Hall Effect devices  530  deployed in upper array  540  and lower array  545 .  
         [0079]    Applying the principles of the APD invention described above with respect to absolute position detector mechanism  200  on FIGS. 2, 3 and  4 , it will be appreciated that the combined condition of arrays  540  and  545  on FIG. 5 may be monitored, and will identify the current position of torque arm  510  in its pendulum-like path. Specifically, as torque arm  510  displaces clockwise and counterclockwise, magnets  520 ,  525 A and  525 B will also displace clockwise and counterclockwise, tending to activate and deactivate different sets of Hall Effect devices  530  on upper array  540  and lower array  545 .  
         [0080]    As with absolute position detector mechanism  200  described above, converter (not shown on FIG. 5) monitors and detects the changing position of torque arm  510  via sampling of the combined activation/deactivation states of arrays  540  and  545 , followed by reference to truth table logic calibrated to recognize particular combined activation/deactivation states as indicative of particular positions of torque arm  510 . Then, via hardware, software or a combination thereof, the converter may convert such samples into a digital signal representative of the changes in position of torque arm  510  over time. In this way, it will be seen that the second embodiment of the APD invention functions as a torque sensor.  
         [0081]    It will be again seen with reference to FIG. 5 that torque sensor mechanism  500  is unaffected by normal and abnormal states of individual Hall Effect devices  530  that may be activated or deactivated by magnets  520 ,  525 A and  525 B during motion of torque arm  510 . As described above with respect to FIGS. 2, 3 and  4 , if a particular position of torque arm  510  on FIG. 5 activates more than one Hall Effect device  530  in either array  540  or  545 , that fact is accounted for in the truth table logic identifying torque arm  510  position from combined activation/deactivation states of the arrays. As a result, the resolution of the arrays to detect position of member  515  is increased, and the need for mechanical structure to exclude abnormal states (as seen throughout the prior art) is obviated.  
         [0082]    [0082]FIGS. 2 through 5 also demonstrate that the APD invention is both scalable and bidirectional. A current activation/deactivation state of multiple interrelated arrays may easily be resolved into a current positional state of a moving assembly. The number of such interrelated arrays may be scaled up or down. Also, the APD invention operates in bidirectional motion, whether in a closed loop or open path system.  
         [0083]    Moreover, it will be appreciated from reference to the embodiments described in FIGS. 2 through 5 that the APD invention is well suited to high speed digital service. The APD invention is non-contact in nature. The APD invention thus avoids the operational speed limitations of many of its predecessors. Further, the invention operates by monitoring activation/deactivation states of groups of sensors. Hardware or software techniques are well suited to decoding of a stream of such activation/deactivation states into a digital signal at high speed.  
         [0084]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.