Abstract:
Described is a device and method of measuring the linear position of a piston  002  movable within a hydraulic or pneumatic cylinder barrel  001 . The measuring device includes a photo optical sensing apparatus  011  mounted at the cylinder head. The photo optical sensing apparatus  011  can be located inside or outside of the cylinder  001 . The sensing apparatus  011  design utilizes a typical optical sensing apparatus, and optional functional modules for determining absolute displacement, and communication. Calibration locations, which are used to obtain absolute displacement measurements, are determined by calibration images or separate sensors indicating their presence.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    This invention relates to using an optical photo sensor for measuring mechanical movement of a piston, rotor, joint, or other mechanical apparatus with reliable absolute position measurements obtained through repeated calibration of the optical photo sensor measured relative to the position of the piston, rotor, joint, or other mechanical apparatus. 
         [0003]    2. Description of Prior Art 
         [0004]    Measuring the absolute position of a piston relative to the cylinder is fundamental to control the operation of machinery. Correspondingly, industry has produced a variety of position sensing apparatuses using mechanical, magnetic, acoustic, and optical techniques for determining the instantaneous position of the movable piston or rotor. 
         [0005]    One class of piston position sensing techniques is based on magnetic field sensors. U.S. Pat. No. 6,989,669 B2 forms a magnetically hard layer on the piston rod, and uses sensor to read the magnetic pattern recorded in the layer. However, the piston rod needs to be reproduced in order to form a magnetically hard layer on it, and the harsh environment may accidentally erase or alternate the magnetic pattern, which causes measurement unreliability. U.S. Pat. No. 6,690,160 B2 grooves the housing of the cylinder and mounts two magnetic members in the cylinder housing. Then, a magnetic field sensor generates signals which indicate the relative distance between the two magnetic members. Accordingly, the piston position is determined. This invention needs to groove the cylinder housing, which make the manufacturing and changing of component not easy. U.S. Pat. No. 5,201,838 uses a Hall effect transistor to sense the magnetic field which is generated by a permanent magnet. The sensed signal is used to determine the position of the piston. The accuracy of position measurement depends on the performance of the magnetic field sensor in use, and this class of position sensing technique is vulnerable in a strong magnetic environment. 
         [0006]    Some inventions indirectly measure the piston position using various sensors. For example, U.S. Pat. No. 6,817,252 B2 uses a bi-directional flow sensor; U.S. Pat. No. 3,970,034 uses co-operating pairs of pressure sensors; U.S. Pat. No. 6,848,323 B2 measures the position based on a differential pressure flow sensor. U.S. Pat. No. 6,549,873 B1 senses the speed of ultrasonic wave and records time length. In U.S. Pat. No. 4,523,514, a potentiometric positioning sensing transducer is used, which is immunized to electrical noise. These sensors are readily available. However a complex detector means is required in order to obtain an accurate measurement. Moreover, correction required for accurate measurement requires additional sensors or apparatus, which increase the expenses. Even with complex additional sensors the accuracy of these sensing methods is very limited. 
         [0007]    Resonant frequency techniques have been used in several inventions, such as U.S. Pat. No. 5,471,147, U.S. Pat. No. 5,438,274, U.S. Pat. No. 4,936,143, U.S. Pat. No. 5,617,034. The common feature of this class of position measurement methods is that a RF transmitting section and a receiving section are used to determine the resonant frequency of the cavity, which indicates the piston position. The RF signals in use include radio frequency signals, alternating pressure signals, and electromagnetic waves. Unfortunately oil is an efficient absorber of RF energy, as a result a significant portion of the transmitted RF energy is lost to heating the oil. 
         [0008]    Piston position sensing techniques based on mechanical or electromechanical sensors were designed, for example, U.S. Pat. No. 5,438,261 uses a coil and an oscillator which produces a position signal as the reciprocating movement of the piston. U.S. Pat. No. 6,234,061 B1 and U.S. Pat. No. 6,694,861 B2 both use a non-contacting electromechanical transducer to provide an output signal proportional to the position or motion of the piston. However, these inventions need to mount the measurement apparatus in the cylinder, which makes manufacture and maintenance not easy. Moreover, extra power is needed to transmit and receive signals. 
         [0009]    U.S. Pat. No. 5,977,778 and U.S. Pat. No. 6,722,260 B1 use the reflection of signals to measure the piston position in a cylinder. The signals in use include electromagnetic bursts and microwave pulses. The extension measurement directly depends on the transmitter and receiver. However, in order to obtain a more accurate measurement, more power is needed for signal transmitting and receiving. Moreover, the leaking of electromagnetic bursts or microwave pulses may be harmful to the surroundings, and the cylinder needs to be extensively modified to accommodate the sensing assembly, which causes relatively high complexity and cost, and relatively low reliability, durability, and accuracy. 
         [0010]    Moreover, U.S. Pat. Nos. 4,814,553 and 7,268,341 provides an optical apparatus for determining the absolute position of a point on a surface or along a path including a tablet, scale, or overlay and a movable mouse-type cursor. The optical apparatus relies on markings added to the surface of the moving piston or rotor. These optical markings are costly to produce and are prone to rapid wear. 
       BACKGROUND OF THE INVENTION 
     Objects and Advantages 
       [0011]    Standard commercial photo optical image sensing apparatus such as those used in a computer optical mice are inexpensive, reliable and draw very little power. These photo image sensors can measure relative horizontal displacement on a wide variety of surfaces. The photo image sensors are able to withstand extremely high shock loads and a wide temperature range. These characteristics combined with the photo image sensor low cost, results in the photo image sensors being an attractive alternative to conventional position sensors used for piston or rotary actuators. 
         [0012]    These photo image sensors are however susceptible to airborne and surface contaminants which affect the optical image quality resolved by the photo image sensor. This limitation is overcome by enclosing the photo image sensor inside a protective housing. The protective housing is sealed against the movable piston or rotor surface. This arrangement prevents airborne and surface contaminants from entering into the protected enclosed space housing the photo image sensor. The protective housing also protects the photo image sensor from mechanical damage in the industrial application environment. As a result of the small size of the photo image sensor, the protective housing can be mounted without significant modification to either piston or rotary actuators. For example, on piston actuators, the protective housing enclosing the photo image sensor can be easily mounted at the cylinder head either inside the cylinder body or outside the cylinder body. 
         [0013]    High resolution standard commercial photo image sensors are available with resolutions of 1600 counts per inch or greater. The error distance measured in counts is extremely small. Low cost photo image sensors with error distances of less than 5 counts in 6400 are commonly available. However, if the absolute displacement measurement of the piston or rotor is not corrected, errors will accumulate over time. This limitation is overcome by integrating the calibration positions that reduced the accumulated error. At these calibration positions, the absolute displacement measurement is rectified which zeros the accumulated error distance. The high accuracy of the photo image sensor is maintained by zeroing the accumulated error distance. As a result, the limitations which currently prevent economical mass produced photo image sensors from greater industrial application use are overcome. 
       SUMMARY 
       [0014]    Accordingly, an apparatus to measure the planar movement between surfaces in applications such as a piston within a cylinder includes a photo image sensing apparatus fixed at the cylinder head. The designed photo image sensing apparatus utilizes a typical optical sensing apparatus, and optional functional modules for determining absolute displacement, traveled path distance, and communication. Calibration locations, which are used to obtain absolute displacement measurements, are determined by calibration images or separate sensors indicating their presence. 
     
    
     
       DRAWINGS 
       Figures 
         [0015]    The advantages of this invention may be better understood by reading the following description as well as the accompanying drawings, where numerals indicates the structural elements and features in various figures. The drawings are not necessarily to scale, and they demonstrate the principles of the invention. 
           [0016]      FIG. 1  is a cross section view of a hydraulic cylinder with an attached photo image sensor taken along cutting plane A-A of  FIG. 6 ; 
           [0017]      FIG. 2  is block diagram of a photo image sensing apparatus; 
           [0018]      FIG. 3A  is a side view of a piston rod with recorded calibration pattern; 
           [0019]      FIG. 3B  is a side view of a piston rod with encoded calibration pattern; 
           [0020]      FIG. 3C  is a side view of a piston rod with different calibration patterns at three positions; 
           [0021]      FIG. 4  is a flow diagram of the main control loop of an embodiment of the present invention; 
           [0022]      FIG. 5  is a flow diagram of the positioning subroutine as called by the main control loop of  FIG. 4 ; 
           [0023]      FIG. 6  is an isometric view of hydraulic cylinder with an attached photo image sensor 
       
    
    
     DRAWINGS 
     Reference Numerals 
       [0000]    
       
           001  hydraulic cylinder barrel 
           002  piston 
           003  piston rod 
           004  base stop 
           005  head stop 
           006  seal in cylinder 
           007  hydraulic cylinder head chamber 
           008  hydraulic cylinder base chamber 
           010  sensing apparatus housing 
           011  photo image sensing apparatus 
           012  seals for sensing apparatus 
           018  base contact pressure sensor 
           019  head contact pressure sensor 
           030  sensor board 
           031  microprocessor 
           032  EPROM 
           033  SRAM/Flash 
           034  RAM 
           035  battery 
           036  image sensor 
           037  light emitting diode or laser 
           038  light opening 
           039  USB interfaces 
           040  CAN bus interface 
           051  recorded calibration pattern 
           052  encoded calibration pattern 
           053  calibration pattern at position  1   
           054  calibration pattern at position  2   
           055  calibration pattern at position  3   
           060  reset timer 
           062  initialization, validation and communication 
           064  read main operating state 
           066  operation or calibration state decision 
           068  calibration state 
           070  position measurement operation state 
           072  communication service 
           080  read pixel image 
           082  pattern match between current position pixel image and previous position pixel image 
           084  measure relative displacement 
           085  estimated absolute displacement 
           086  pattern match between current position pixel image and calibration pixel image 
           088  matched calibration image decision 
           090  measure absolute displacement between current position and calibration position 
           091  statistical analysis 
           092  estimate absolute displacement from previous absolute displacement and relative displacement 
           093  reliability analysis 
           094  read contact register 
           096  no contact decision 
           098  no operation 
           100  base contact decision 
           102  reset absolute displacement to minimum 
           104  head contact decision 
           106  reset absolute displacement to maximum 
           108  report error 
       
     
       DETAILED DESCRIPTION 
       [0078]      FIG. 6  is an isometric view of hydraulic cylinder with an attached photo image sensor and shows the cutting line A-A used to obtain the cross section shown in  FIG. 1 . 
         [0079]      FIG. 1  shows a side cross-sectional view of an embodiment of a hydraulic cylinder assembly with a photo image sensing apparatus  011 . The hydraulic cylinder assembly includes a cylinder barrel  001  and a sensing apparatus housing  010 . A piston  002  is arranged within the cylinder barrel  001  for reciprocating motion along an axis in response to hydraulic fluid. The piston  002  partitions the cylinder barrel  001  into two chambers,  007  and  008 . The housing  010  is securely mounted on the cylinder barrel  001 . 
         [0080]    One end of a piston rod  003  is fixed to the piston  002  and extends along the axis of the movement. The other end of the piston rod  003  extends out of the housing  010 . Either or both the cylinder base and outside end of the piston rod  003  maybe connected directly or indirectly with a machine component. 
         [0081]    The cylinder barrel  001  has two openings for the passage of fluid such as oil or water into and out of the chambers  007 ,  008  for moving the piston  002 . Seals  006  within the cylinder barrel  001  are arranged to lie flush with the surface of the piston rod  003  and thus prevent fluid from leaving the chamber  007 . 
         [0082]    The housing  010  encloses a photo image sensing apparatus  011 , which is used to determine the instantaneous position of the piston rod  003 . Seals  012  within the housing  010  are arranged to clean the surface of the piston rod  003  and thus prevent fluid or dirt from contaminating the sensing apparatus  011 . The housing  010  provides protection for the photo image sensing apparatus  011  from the environment and permits easy replacement of the sensing unit. The photo image sensing apparatus  011  is mounted in the housing  010  within proximity of the piston rod&#39;s surface to permit reading of the movement of the piston rod  003 . 
         [0083]    The head contact pressure sensor  019  is mounted at the head stop  005  of the cylinder barrel  001 . The base contact pressure sensor  018  is mounted at the base stop  004  of the cylinder barrel  001 . Together these two contact sensors provide a two-bit digital signal to indicate whether the piston  002  reaches the head stop  005  or the base stop  004 , or neither. Correspondingly when the piston  002  reaches either the head  005  or base stop  004 , the absolute displacement information in storage is adjusted and updated. 
         [0084]    In operation, fluid forced into or removed from the chambers  007 ,  008  at time-varying pressures causes the piston  002  and thus the piston rod  003  to slide back and forth relative to the photo image sensor  011 . The photo image sensor  011  reads the relative displacement of the piston rod  003  and produces a corresponding digital signal. The microprocessor  031  on the sensor board  030  calculates the absolute displacement of the piston rod  003  by matching the calibration pattern and using the relative displacement. The obtained absolute displacement indicates the actual position of the piston rod  003  and piston  002 . 
         [0085]      FIG. 2  is a diagrammatic view of the laser photo optical sensing apparatus  011 , which includes a light emitting diode or laser  037 , a pixel image sensor  036 , a microprocessor  031 , and peripheral electronic circuit. The light emitter  037  projects light, the light beam reflected from the piston rod  003  surface, and the image sensor  036  captures the reflected image. Afterwards the image sensor  036  transfers the captured pixel image to the microprocessor  031 . The microprocessor  031  calculates the relative displacement and the absolute displacement by comparing the current captured pixel image with the stored pixel images. A SRAM or Flash memory  033  stores a recorded calibration pattern  051  of the piston rod  003  at a specific location, and an EPROM  032  stores encoded calibration pattern  052  of the piston rod  003  at a specific location and program used by the microprocessor  031 . A battery  035  is used to supply power for the sensing apparatus  011 . The sensor board  030  provides communication interface, one is USB interface  039  and the other is CAN bus interface  040 . The USB interface  039  is used to communicate with the contact pressure sensors  018 ,  019  within the cylinder barrel  001 , and the CAN bus interface  040  is used to communicate with other units on the machine. The sensor board  030  is quite similar with the electronic board in an optical mouse. Extra functional modules are added to achieve additional calibration and communication functionalities. 
         [0086]      FIG. 3A  and  FIG. 3B  are diagrammatic views of two piston rods  003  with different calibration patterns. In  FIG. 3A , the pattern  051  on the piston rod  003  is an inherent feature of the piston rod  003  at a specific location. In  FIG. 3B , the pattern on the piston rod  003  represents an encoded feature stenciled at a specific location on the piston rod  003 . The pattern shown in  FIG. 3B  is a representative example of one of many possible choices which will uniquely identify the piston rod&#39;s  003  position. The purpose of the encoded pattern  052  is to easily calibrate the absolute displacement. Both of these two calibration patterns can be used to calculate the absolute displacement of the piston rod  003 . 
         [0087]      FIG. 3C  is a diagrammatic view of a piston rod  003  with three different calibration patterns  053 ,  054 , and  055  at three calibration positions. These three calibration patterns can be either recorded ones or encoded ones. The number of calibration patterns is not confined to three. The number and placement of calibration patterns is determined by application requirements. Multiple calibration patterns enables more frequent calculation of the absolute displacement so that the estimated absolute displacement is closer to the actual absolute displacement. Unique calibration patterns make it possible to determine which is the current calibration position based on its calibration pattern. 
         [0088]    The multiple calibration positions can be used to estimate the piston absolute displacement as follows. In order to avoid unnecessary number of comparisons, the current absolute displacement of the piston is used to determine the two calibration positions bordering it. In the case where all the calibration positions are to one side of the piston, only the first calibration position needs to be considered. The observed surface at the current absolute displacement only needs to be compared with the two adjacent calibration patterns. For example, if the piston is located between the calibration position  1  and  2 , then the observed surface absolute displacement only needs to be compared with the calibration patterns  053  and  054 . 
         [0089]    The surface quality or average pixel shade of the piston rod are measured by the laser image sensor  036 . A suddenly change in surface quality or average pixel shade is used to indicate a calibration position. The surface quality or average pixel shade at each calibration position differ such that their unique surface qualities distinguish each from the other. Unique surface qualities or pixel shades of each calibration position are not necessary to calibrate the absolute displacement measurement. The Unique calibration positions ensure that one calibration position is not mistaken for another. The neighbouring calibration positions are determined by the piston&#39;s current estimated absolute displacement. When a calibration position is detected by its suddenly changed surface quality/average pixel shade or by recognizing its specific surface quality/average pixel shade, the piston absolute displacement estimated is corrected. The surface qualities and/or pixel shades at all calibration positions are pre-stored in the Flash memory  033  or EPROM  032  as required. 
         [0090]      FIG. 4  refers to the main control logic of the laser photo optical sensing apparatus  011 . In control block  060 , a timer is reset. The timer is of conventional design and is used to detect if the microprocessor  031  is not executing the designed control logic. The use of a timer is well known in the art and is therefore not further discussed. 
         [0091]    In control block  062 , the system is initialized. The initialization routine includes validating the hardware, and software parameters, testing the communication channels. Any errors detected during this initialization process are reported according to their severity. Critical errors which prevent the initialization process from completing or would prevent the correct operation of the sensing apparatus  011  cause the microprocessor  031  to report a warning error or the microprocessor  031  to exit on critical error. 
         [0092]    In control block  064 , the state of the sensing apparatus  011  is checked. The sensing apparatus  011  has two functional states, one is operation, and the other is calibration. 
         [0093]    If the state is the calibration state, control flow proceeds to the control block  068 . If the state is the operation state, control flow proceeds to the control block  070 . 
         [0094]    In control block  068 , the location of the recorded calibration pattern is precisely measured, and the location and pattern information is stored into the SRAM  033 . 
         [0095]    In control block  070 , the subroutine GETPP is called. As explained below, the GETPP subroutine determines the absolute displacement of the piston rod  003  and a confidence interval of the estimated absolute displacement. 
         [0096]    In control block  072 , the system communications are serviced. This includes reading the absolute displacements from the SRAM/Flash  033 , calculating a checksum for transmission purposes, transmitting the data from the sensor apparatus  011  to other control units, and indicating the reliability of the sensor apparatus  011 . 
         [0097]      FIG. 5  illustrates the operation of the subroutine GETPP, which calculates the absolute displacement of the piston rod  003 . 
         [0098]    In control block  080 , the photo image sensor  036  reads the pixel image of the light reflected from the surface of the piston rod  003 , and then sends the pixel image to the microprocessor  031 . 
         [0099]    In control block  082 , the microprocessor  031  reads the previous pixel image from the RAM  034 , and compares it with the current pixel image received from the photo image sensor  036 . Then, the microprocessor  031  calculates the relative displacement of the piston rod  003  and stores it into the RAM  034 . A movement count is used to record the number of relative displacement measurements taken since previous absolute displacement measurement at the calibration position. The movement count increments by one and is stored into the SRAM/FLASH  033 . 
         [0100]    In control block  084 , the microprocessor  031  reads the mean absolute displacement error and the movement count from SRAM/FLASH  033 , and reads the relative displacement of the piston from RAM  034 . Then, the microprocessor  031  uses this information to correct the relative displacement, and stores the corrected relative displacement into RAM  034 . 
         [0101]    In control block  085 , a corrected absolute displacement is calculated by adding the most recent corrected relative displacement to the previous corrected absolute displacement. The calculated corrected absolute displacement is named as estimated absolute displacement, and it is stored into the SRAM/FLASH  033 . 
         [0102]    In control block  086 , the microprocessor  031  compares the current pixel image with the calibration patterns which are stored in EPROM  032  or SRAM/FLASH  033 , respectively. 
         [0103]    In control block  088 , if the current pixel image matches either of the calibration patterns  051  or  052 , the control goes to control block  090 , Otherwise, control proceeds to control block  092 . 
         [0104]    In control block  090 , the absolute displacement is directly obtained from the precise location of either the calibration patterns  051  or  052 . The absolute displacement is obtained by pattern matching the current pixel image with the stored calibration patterns  051  or  052 . The new absolute displacement is stored into the SRAM/FLASH  033 . 
         [0105]    In control block  091 , statistical analysis is implemented. The absolute displacement measurement error, movement count, and traveled path distance are first calculated and stored into SRAM/FLASH  033 . The absolute displacement measurement error is calculated using the following pseudo code: 
         [0000]      Estimate absolute displacement=Absolute displacement at most recent count+Relative displacement measurements 
         [0106]    Thus, the absolute displacement measurement error is calculated using the following pseudo code: 
         [0000]      Absolute displacement measurement error=Absolute displacement at a calibration location−Estimated absolute displacement at the calibration location
 
         [0107]    The traveled path distance is calculated using the following pseudo code: 
         [0000]      Traveled path distance=Sum of the absolute values of all previous relative displacements 
         [0108]    Then, the microprocessor  031  calculates the mean and variance of the absolute displacement error in relation to movement count. Obviously, the error mean and variance will increase as the movement count increases. 
         [0109]    The mean and variance of the absolute displacement error is stored in the SRAM/FLASH  033  for the correction of the relative displacement in control block  084 . Finally, the movement count is reset to be zero. 
         [0110]    In control block  092 , the absolute displacement is set to be the estimated absolute displacement. 
         [0111]    In control block  093 , reliability of the absolute displacement estimation is analyzed. The relationship between the absolute displacement measurement error and the traveled path distance of the piston rod  003  is determined, where the absolute displacement measurement error is described as a function of the traveled path distance. Basically, the absolute displacement measurement error increases as the traveled path distance increases. Accordingly, the function is used to determine the reliable or confident path distance the piston rod can travel. 
         [0112]    Moreover, the microprocessor  031  calculates a confidence interval of the estimated absolute displacement using its probability density function and movement count. Excessively low confidence in the estimated absolute displacement signals that the optical apparatus for measuring mechanical displacement is not functioning with sufficient accuracy and corrective measures are required. 
         [0113]    Furthermore, the possibility density distribution of the absolute displacement measurement error with respect to the absolute displacement and/or traveled path distance is calculated. The possibility density distribution function is used to optimally determine the number and location distribution of the calibration patterns, which to the greatest extent minimize the absolute displacement measurement error. 
         [0114]    In control block  094 , a register that indicates the states of the two contact pressure sensors  018  and  019  in the cylinder barrel  001  is read by the microprocessor  031 . 
         [0115]    In control block  096 , if the register&#39;s value is 00, the piston  003  has neither reached the base stop  004  nor the head stop  005 , then control goes to control block  098 . Otherwise, control proceeds to control block  100 . 
         [0116]    In control block  098 , no operation and control returns to the main control loop. 
         [0117]    In control block  100 , if the register&#39;s value is 01, the piston  002  has reached the base stop  004 , then control goes to control block  102 . Otherwise, control proceeds to control block  104 . 
         [0118]    In control block  102 , the absolute displacement value is set to its minimum and control returns to the main control loop. 
         [0119]    In control block  104 , if the register&#39;s value is 10, the piston  002  has reached the head stop  005 , then control goes to control block  106 . Otherwise, control proceeds to control block  108 . 
         [0120]    In control block  106 , the absolute displacement value is set to its maximum and control returns to the main control loop. 
         [0121]    In control block  108 , the register&#39;s value must be 11 or uncertain value, which means an error has occurred. In this case, an error is reported and control returns to the main control loop. 
       CONCLUSION, RAMIFICATIONS, AND SCOPE 
       [0122]    Although the invention has been described and shown with reference to specific preferred embodiments, it should be understood by those who are skilled in the art that some modification in form and detail may be made therein without deviating from the spirit and scope of the invention as defined in the following claims. For example, the housing  010  can be mounted within the cylinder barrel  001  in order to avoid shortening the stroke length of the piston  002 . Although the embodiments described above primarily concerns the measurement of piston&#39;s linear extension or rotary movement, the principles of the invention can be used to determine the rotation direction and angle of the piston rod  003 . The sensor apparatus  011  can equally be attached to shaft, or rotating surface of rotary devices. The application of the sensor apparatus  011  needs not be restricted to the described embodiment for measuring a piston&#39;s linear or rotary movement. Alternative optical lens such as a micro-lens is used to modify the working distance between the sensor apparatus  011  and the surface of which the sensor apparatus  011  is measuring movement. 
         [0123]    The sensor apparatus  011  can also measure movement by means of observing a moving surface of hinge, swivel, sliding and spherical joints. When a suitable surface does not exist as part of a joining apparatus, a part with a suitable surface can be attached to the apparatus. By adding an additional part or parts to a joined apparatus, the sensor apparatus  011  can be mounted at different location and measure its displacement with respect to the surface of the added part. 
         [0124]    The advantages provided by the sensor apparatus  011  included in this invention over prior art position sensors are availability of inexpensive, reliable, low power sensors. The sensor apparatus  011  is more easily installed on a wide variety of jointed apparatus than prior art position sensors. And the position and path distance measurement provided by the sensor apparatus makes it easy to integrated with digital electronic control systems. 
         [0125]    Thus the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.