Abstract:
A method and apparatus for calibrating the output signal of a linear position detector without accessing the interior of the detector housing is provided. According to one exemplary embodiment, a magnet is selectively movable toward and away from the exterior of the electronics housing, and a sensor is provided within the housing for sensing the presence of the magnet. According to this embodiment, the linear position detector is calibrated by setting a movable marker at the desired position and pushing the magnet toward the housing. The sensor then detects the presence of the magnet, and a processor saves the position of the marker as a reference point. All future positions of the marker can then be scaled based upon the reference point. Thus, the linear position detector can be calibrated without the need for opening the electronics housing and potentially exposing the electronics components to moisture, contaminants, and/or static electricity. Preferably, the magnet is connected to a push button on a base, and the base includes an attachment mechanism, such as a clip for example, to attach the base to the position detector.

Description:
TECHNICAL FIELD 
     The present invention relates to methods and apparatus for calibrating the output signal of linear position detectors, and, in one preferred embodiment, to a calibration system for defining reference points in a linear position detector in which energy, such as magnetic energy, is provided on the exterior of the detector housing and received by a sensor within the housing, so that the housing need not be opened during calibration, thereby preserving the moisture resistant housing seal. 
     BACKGROUND OF THE INVENTION 
     A magnetostrictive linear position detector typically includes a magnetostrictive waveguide wire which is housed in a protective waveguide housing about which a magnet is slidingly engaged. A current pulse is sent through a wire near the waveguide (or through the waveguide itself), and this pulse interacts with the magnetic energy of the magnet to induce a torsional strain wave in the magnetostrictive waveguide at the location of the magnet. The strain wave travels along the length of the waveguide and passes through a mode convertor, such as a pickup coil, which converts the mechanical wave into an electrical signal. To obtain the location of the magnet, the time between the transmission of the current pulse and the reception of the signal from the coil can be measured and converted to a distance, because the speed that the torsional wave will travel along the waveguide is known. Accordingly, when the magnet is connected to a movable mass, such as a liquid level quantity in a storage tank, or a movable element in a machine tool for example, the exact position of the mass can be measured and monitored. 
     In more advanced magnetostrictive linear position detectors, the ability to set reference points along the measurement stroke is provided. For example, in some such sensors, the magnet can be positioned at any location along the waveguide housing, and a button or buttons can be pressed to save the current position of the magnet in memory so that this position can be used as a reference point. In some systems, the output signal to be provided at this reference point car be assigned, such as by manipulating the programming buttons. Additional reference points can also be assigned and utilized in a similar manner. 
     Such calibration systems allow the output of the transducer to be changed from the original setup provided by the manufacturer. For example, while the manufacturer might configure the transducer to provide a 0 volt output when the magnet is at one end of the waveguide housing and a 10 volt output when at the opposite end, the ultimate user of the transducer may desire different settings. With such a calibration system, the user could assign any possible voltage output to any of the possible magnet positions. For example, the user may wish that a position 2 inches (50.8 mm) from the first end provides a 0 volt output, and that a position 3 inches (76.2 mm) from the opposite end provides a 10 volt output. By using such a calibration system, these reference points can be assigned the desired outputs. Once the reference points are assigned, the system can then be set up to scale all subsequent magnet positions based upon the reference points. 
     Accordingly, the programmability or adjustability of reference points allow the user to customize the sensor to provide the desired output range over the desired measurement stroke. Thus, a reference point can be, for example, an endpoint of the stroke. 
     However, such calibration systems are not without disadvantages. For example, such systems can expose the electronics to potential damage. More specifically, to access the programming buttons, screws or other covers on the electronics housing must be removed, and the buttons can then be depressed by extending a pin or screw driver through the resulting access openings. However, providing such access openings, even when sealed off by screws and the like, can compromise the ability of the housing to seal off moisture and other contaminates which can damage the delicate electronic components inside. In many applications, an excellent watertight seal is required, such an IP67 rated seal, and access openings generally diminish the ability of the housing to achieve and maintain such a seal. 
     Moreover, if the screw or cover is lost or is not properly replaced over the access opening after the desired programming has been conducted, the seal is again compromised or lost. In addition, the device used to depress the programming buttons, such as a screwdriver, pin, or finger, can carry electrostatic charge which can itself damage electronic components within the housing. 
     Accordingly, it is desirable to provide a system and method for calibrating a linear position detector which does not affect the ability of the transducer&#39;s electronics housing to protect against undesirable ambient and external moisture, contaminates, and electrostatic discharge, and which does not require opening and closing or other physical access through the housing for programming the detector. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to obviate the above-described problems. 
     It is another object of the present invention to provide a linear position detector having adjustable output capability while also providing good protection from moisture and contaminants. 
     Another object of the present invention is to provide a system and method for calibrating the output signal of a linear position detector which eliminates the need to provide access openings to the interior of the transducer housing. 
     Yet another object of the present invention is to provide a system and method for calibrating the output signal of a linear position detector which minimizes the risk of damaging electronic components. 
     It is another object of the invention to provide a linear position detector with adjustable output capability that includes a simpler electronics housing design. 
     Yet another object of the present invention is to provide a linear position detector which can be calibrated in a simpler and more efficient manner. 
     To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as described above, a calibration system for a linear position detector having a movable marker is provided. According to the present invention, the system comprises a housing having a wall and an energy source located exterior to the housing. An energy sensor is located within the housing and arranged for selective communication with the energy source through the housing wall. A processor is also located within the housing and arranged in communication with the sensor. The processor is adapted to define a reference point based upon the current position of the marker when the energy sensor receives a predetermined energy signal from the energy source. Preferably, the energy source is a magnet and the energy sensor is a Hall effect sensor, although other sources of energy could be utilized, such as sources of electromagnetic or electrical energy. 
     The energy source could be coupled to a base having an attachment mechanism adapted to attach the base adjacent the exterior of a housing of a linear position detector. In addition, the energy source could be selectively movable from a home position to a selection position. 
     According to the present invention, a method for calibrating the output signal of a linear position detector having a housing and movable marker is provided. The method comprises selectively providing energy from a location exterior of the housing, sensing the presence of the energy, and, upon sensing the energy, defining a reference point based upon the current position of the movable marker. 
     A calibration system for a linear displacement detector having a movable marker is provided according to the principles of the present invention. The system comprises a housing, an activator located exterior to the housing, and a processor located within the housing. The activator is adapted to selectively apply an energy signal from an energy source through the housing. The processor is adapted to define a reference point based upon the current position of the marker when the energy signal is received within the housing. The activator can comprise a button or switch, for example, and the energy source can comprise a source of magnetic or electrical energy, for example. 
     Still other aspects of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described preferred embodiments of this invention, simply by way of illustration, as well as a best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other different aspects and embodiments without departing from the scope of the invention. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not as restrictive in nature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the invention, it is believed that the same will be better understood from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a perspective view of a calibration apparatus adapted for selective clipping adjacent the housing of a linear position detector, according to one exemplary embodiment of the present invention; 
     FIG. 2 is a side view of the apparatus of FIG. 1, with the button and magnet of the apparatus shown in cross section; 
     FIG. 3 is a front view of the apparatus of FIG. 1; 
     FIG. 4 is a bottom view of the apparatus of FIG. 1; 
     FIG. 5 a  is a partial, enlarged front view of the tube of the apparatus of FIG. 1, with the button removed from the tube for clarity; 
     FIG. 5 b  is a side view of one of the two buttons included in the apparatus of FIG. 1; 
     FIG. 6 is a partial perspective view of the calibration apparatus of FIG. 1 and a linear position detector having an exemplary electronics housing for receiving the calibration apparatus, according to the principles of the present invention; 
     FIG. 7 is a diagram illustrating an exemplary interface between the linear position detector and the calibration apparatus of FIG. 6, and showing the electronics housing in cross-section; and 
     FIG. 8 is a block diagram illustrating an exemplary calibration system for adjusting the output of a linear position detector, according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail, wherein like numerals indicate similar elements throughout the views, FIGS. 1-6 illustrate a preferred embodiment of an external calibration device  10  for use with a linear position detector, according to the principles of the present invention. The calibration device  10  includes a base  12  having a back side  14 , a front side  16  located generally opposite the back side, a top  18 , and a bottom  20  generally oppositely disposed from the top. Preferably, the base  12  is made of a rigid material such as plastic, although a number of materials could be utilized, such as aluminum, fiberglass, carbon fiber, or steel for example. 
     Preferably and as best shown in FIGS. 1 and 3, an inner surface  22  of the base  12  defines an opening, such that the base is generally hook, loop, or “C” shaped. Accordingly, any connectors or protrusions on the linear position sensor about which the device  10  is attached may conveniently extend through the opening in the base  12  unencumbered, as described further below. 
     For attachment of the device  10  to or adjacent to a linear position sensor, it is preferred that base  12  include a pair of clips  24  near its top  18  and bottom  20 . These exemplary attachment mechanisms  24  can be connected to the base in a number of manners, although it is preferred that they be integrally connected with the base. More specifically, as best shown in FIGS. 1 and 3, the base  12  preferably includes a pair of gaps  26  formed near the top  18  and bottom  20  of the base  12 , the gaps being bridged by a pair of spans  27 . Accordingly, the spans  27  can integrally connect with the clips  24 . Such an arrangement provides the clips  24  with a desirable amount of flexibility and/or rotatability, so that they may accommodate slight misalignments and the like and can be more easily attached and removed from the position sensor as desired. 
     As shown best in FIG.  1  and FIG. 2, each clip  24  preferably includes a tooth or extension  25  formed at one end of the clip. Preferably, each tooth  25  extends generally inwardly, toward the center of the device  10 . Such extension or tooth  25  is provided to engage a corresponding slot formed in the housing of the sensor, as will be described in more detail below with respect to FIG.  6 . 
     The base  12  also includes a pair of generally hollow tubes or channels  28  which extend from the lower section of the base, as shown in FIG. 1, FIG. 2, and FIG.  4 . Preferably, these tubes  28  are integrally formed with the base  12 , although other connections or attachments could equally be utilized. While tubes  28  are shown as generally tubular structures, it is contemplated that other structures, such as open channels, cages or guide rings could similarly be utilized. 
     In addition to the base  12 , the calibration device  10  also preferably includes a pair of push buttons  30  which are each slidingly mounted at least partially within a respective hollow tube  28  of the base  12 . Each push button  30  preferably includes a head  34  and a tip  31 , with the head and tip being connected by a middle portion  35 , as is best seen in FIG.  2 . Each of these parts of the button  30  can be made from a rigid material such as plastic or nylon, similar to that of base  12 , although it should be understood that any of a variety of materials could be utilized without departing from the scope of the invention. 
     The tip  31  of each button  30  is preferably hollow, so as to allow for the mounting of a magnet  32  therewithin. A variety of mechanisms can be used to secure each magnet  32  adjacent each tip  31 . For example, each magnet  32  can be adhesively mounted at least partially within the tip  31 , sealed within the tip  31 , or otherwise placed snugly adjacent the end of the tip, such as through an interference fit, adhesive, etc. Each magnet  32  provides magnetic energy and thereby acts as a source of energy which can be sensed by an energy sensor, such as a Hall effect sensor, within the housing of the linear position transducer. When the magnet  32  is placed in sufficient proximity of the energy sensor, such as by pushing the button toward the housing of the transducer, sufficient magnetic flux can be provided to the sensor to trip the sensor such that it provides a reception signal. The operation of this exemplary embodiment is described in further detail below with respect to FIGS. 6-8. 
     FIG. 2 includes a cross-sectional view of the button  30 . As shown in this figure, a rod  40  is mounted in the hollow head  34  of the button  30  and extends into the hollow middle section  35  of the button. Coiled about the rod  40  is a spring  42  for biasing the button to a predetermined non-extension or home position. When the button  30  is mounted in the tube  28 , the spring  42  extends between the head  34  of the button and a retainer or plug  44  which extends into the middle of the hollow interior of the tube  28 . As shown in FIG. 5 a , each hollow tube  28  preferably includes a plug  44  which includes a pair of cross-spans  45  for integral connection of the plug  44  with the interior wall of the tube  28 . The plug  44  provides a surface upon which the spring  42  can be compressed and also helps to retain button  30  in assembled condition with device  10 . In this regard, as also shown in FIG. 5 a , the plug  44  does not completely close off the tube  28 . Rather, a pair of passages  47  are defined by the plug  44 , and these passages  47  allow the button to be captively retained within tube  28  and selectively reciprocated in an axial direction along tube  28 . For example, as button  30  is pushed inwardly through the tube  28 , the head  34  of the button approaches the tube, while the spring compresses between the head and the plug  44 . 
     More specifically, the middle portion  35  of each button  30  is provided with a slot  46  on each side, as best shown in FIG.  1  and FIG. 5 b . In a preferred embodiment, middle portion  35  can be provided as an integral, skeletal or cage-like structure which can receive and retain rod  40  and the spring  42  telescoped thereover, while accommodating portions of plug  44  (e.g. its cross spans  45 ) to allow captive reciprocation. Each slot  46  is of a sufficient width such that each cross span  45  can extend outwardly through the slot, when the button  30  is mounted in the tube  28 . Accordingly, each slot  46  slidingly engages a cross span  45  as the button  30  is pushed inwardly, as well as when the button springs back to its home position when the button is released. Likewise, the upper and lower portions of the middle section  35 , which define the slot  46 , slidingly engage the passages  47 . Plug  44  simultaneously provides a “stop” at the inner end of spring  42 . 
     Preferably, the middle section  35  of the button  30  is integrally connected with the tip  31 , but is detachably connected to the head  34 . With this configuration, the button can be assembled by sliding the upper and lower portions of the middle section  35  through the passages  47  in the tube  28 . Then, the spring  42  can be placed about the rod  40 , and the rod connected to the head  34 . Finally, the head  34  can be connected to the middle section  35 , such as by providing threads or notches in the head with which the portions of the middle section can frictionally engage, so as to establish a “snap” in fit. Accordingly, as best shown with respect to FIG. 2, the button  30  can slide within the tube  28  while the spring  42  is compressed between the head  34  and the plug  44 . When the button is pushed inwardly, it is prevented from being pushed out of the tube because the outer diameter of the head  34  is larger than the inner diameter of the tube  28 . When the button  30  is released and the spring  42  biases it back to the home position, the button is prevented from being forced all the way out of the tube  28  because the inner face of the tip  31  contacts the plug  44 . While this particular captive and reciprocating arrangement is preferred, it should be understood that reciprocable association of the button and its magnetic flux source with device  10  can be accomplished by any of a variety of other arrangements. For example, other spring loaded bayonet-type assemblies might be substituted. 
     FIG. 6 illustrates a preferred manner in which the exemplary calibration device of FIGS. 1-5 could engage a linear position detector. More specifically, a linear position detector  50  can be provided having a waveguide housing  52 , a magnetic marker  54  which slidingly moves along the housing  52 , and an electronics housing  56  which is connected to the waveguide housing. In this example, the magnetostrictive waveguide wire as well as the support and damping members for the wire are housed within the waveguide housing  52 , while the pulse generator, position measurement circuits, and other electronic components for determining the position of the marker  54  are preferably housed within the electronics housing  56 , as is known in the art. A flange connector  58  is preferably provided adjacent to the housing  56  for connecting the electronics with the various input and output devices with which the detector will be used. Pins  60  are provided on the connector  58  for connecting to the various inputs, such as the voltage source and ground inputs, as well as to provide the output which is relative to position, such as a 0-10 volt analog output, or a 0-20 mA current output for example. Details of one possible configuration for a linear position detector are described in U.S. Pat. No. 3,898,555, the entire disclosure of which is hereby incorporated herein by reference. 
     According to one aspect of the present invention, the electronics housing  56  is provided with a groove or other locking device  62  on its top surface. A similar groove or locking device (not shown) is preferably also provided on the bottom surface of the housing  56 . As can be understood, the grooves  62  allow the calibration device  10  to be detachably mounted adjacent to the detector  50  in a uniform and reliable manner. More specifically, as the calibration device  10  is brought near the housing  56 , inward forces F 1  and F 2  can be directed near the outer ends  23  of the clips  24  and toward the center of the device  10 , such that the clips are adjusted (e.g. rotated or flexed) slightly about the cross-spans  27 . These forces cause the tooth  25  of the top clip  24  to move upwardly slightly and the tooth of the bottom clip  24  to move downwardly slightly, as the outer ends  23  of the clips are forced inwardly and closer to the center of the device  10 . This bending or flexing of the clips provides sufficient clearance so that the clips  24  can fit over the top and bottom outer surfaces of the electronics housing  56 . Appropriate forces can be provided to the clips  24 , for example, by squeezing the clips  24  between the thumb and forefinger. Stops  36  can be provided to prevent the clips  24  from being flexed too far and from breaking off. 
     As the clips slide over the housing  56  and toward the grooves  62 , the tips  31  of the buttons  30  should be aligned with inlets  64  provided in the housing. Once the tooth  25  reaches the groove  62  and the force on the clip  24  is released, the tooth will engage the groove, providing a “snap” in fit or temporary positioning arrangement between calibration device  10  and the linear position detector. Accordingly, the calibration device  10  is securely held adjacent to the electronics housing  56 . At this point, the tips  31  of the buttons  30  are preferably situated at least adjacent or partially within the recesses or inlets  64 . In addition, the flange connector  58  preferably extends through the middle opening  22  of device  10 . 
     While the clips  24  are an example of a particular attachment mechanism for attaching the calibration device  10  to the housing  56 , it should be understood that any number of other mechanisms for situating or attaching the device adjacent to the housing and linear position sensor could be provided without departing from the invention. For example, screws, pins, hooks, snaps, corresponding engageable parts, or other mechanisms could be utilized. 
     The electronics housing  56  can be formed of a hard aluminum alloy or steel material. The grooves  62  and inlets  64  are preferably formed within the material and do not provide a passageway into the housing interior. Accordingly, an excellent seal from moisture and contaminants can be maintained at all times. To help maintain this seal, it is preferred that the flange connector  58  and the waveguide housing  52  tightly connect with the electronics housing  56 , such as by using appropriate screw-type fittings as well as sealing material and devices. 
     Because the tips  31  of the buttons  30  do not completely fill the recesses  64 , they can be pushed further into the inlets by pressing the button heads  34 . As best shown in the close-up view of FIG. 7 wherein the electronics housing wall is shown in cross-section, because the inlets  64  are sealed from the interior of the housing, the tips  31  remain exterior to the housing, although they may make contact with or otherwise approach the housing wall  70  inside of the inlet by pressing the button heads  34 . 
     On the opposite side of wall  70 , magnetic field sensors  74  are provided within the housing interior near each inlet  64 , such as on a circuit board. Such a sensor  74  can detect the presence of a magnetic field. For example, a Hall effect sensor could be utilized wherein a voltage is produced which is proportional to the strength of the field. In a typical Hall effect sensor, if the voltage exceeds an adjustable threshold voltage level, a switch will be thrown and an output signal provided indicating that the magnetic energy has been sensed. An exemplary Hall effect sensor is the HAL 115 manufactured by ITT Semiconductor. Other magnetic field sensors which provide an output signal and/or which switch a switching device in the presence of a magnetic field (and/or which otherwise indicate the presence of a magnetic field) could be used as well. For example, the KMZ10A magnetic field sensor, manufactured by Philips Semiconductors, could be utilized. 
     Accordingly, when it is desired to define a reference point, such as an end point of the measurement stroke, the button head  34  can be pressed forcing the button tip  31  closer to the housing wall  70 . This movement causes the flux producer or magnet  32  which is arranged adjacent the tip  31  to also move closer to the housing wall  70 , and, accordingly, closer to the sensor  74 . Because the magnet  32  is closer to the sensor  74 , the magnetic field received by the sensor is stronger, causing the sensor  74  to provide the output signal and/or to trip a switch. 
     As is known in the art, the sensitivity of the sensor  74  can be adjusted such that the magnet  32  does not trip the sensor  74  when it is in the home position at some initial distance away from the sensor  74 , and also such that the magnet  32  does trip the sensor when it is in a “selection” position, some distance closer to the sensor. If a Hall effect sensor is utilized, this tuning of the sensitivity of the sensor can be achieved by adjusting the threshold voltage. An alternative way to assure that the magnet trips the sensor at the appropriate position of the button  30  would be to experiment with different energy sources or, for example, magnets having stronger or weaker magnetic properties until the appropriate switching is obtained. Preferably, the magnet comprises a ferrox material. Other magnetic materials might be used as well. 
     An additional way to adjust the sensitivity would be to change the distance that a sensor  74  is positioned with respect to the electronic housing wall  70  and/or to adjust the depth of the inlet  64 . In one preferred embodiment, the magnet  32  has a diameter of approximately 0.17 inches (4.4 mm) and a depth of approximately 0.21 inches (5.3 mm). In this embodiment, the buttons  34  are configured such that the button tips  31  protrude about 0.10 inches (2.5 mm) into the inlets  64  when in the home position and protrude approximately 0.35 inches (9.0 mm) into the inlets  64  when in the fully extended (select) position. Each inlet  64  of this exemplary embodiment is about 0.40 inches (10.2 mm) deep, and has a diameter of about 0.26 inches (6.6 mm), and each Hall effect sensor  74  is placed adjacent to the housing wall  70 . The housing wall  70  is preferably about 0.059 inches (1.5 mm) thick at the end of the inlet  64 , such that a relatively thin layer is provided between each magnet  32  and each Hall effect sensor  74 . Other configurations, arrangements, and dimensions are also possible and may be dependent upon the type and size of magnet and type and sensitivity of sensor utilized. 
     Once the button  30  is released, the tip  31  moves away from the housing wall  70  and returns to the home position due to the bias of the spring  42 . The sensor  74  then ceases providing the output and/or switches to the non-detection state because the tip  31  has moved away from the sensor  74 , thereby reducing or eliminating the magnetic energy which permeates the sensitive component or components in the sensor. Other alternative configurations and/or movements of the buttons  30  could be used as well. For example, rather than using a linear movement, the button could be configured as a swing arm to arc toward and away from the sensor housing. 
     FIG. 8 is a block diagram illustrating an exemplary calibration system according to the principles of the present invention. As shown, two or more energy sources  100  are preferably positioned exterior to the housing  102 . Within the housing  102  are also positioned two corresponding sensors  104 , each sensor being appropriately adapted and/or positioned for receiving energy, such as magnetic energy for example, from one of the sources  100  at selected times. Accordingly, each sensor  104  is in communication with a source  100 . (When a first device is described herein as being “in communication with” a second device, it is meant that the first device is adapted to transmit energy to the second device and/or receive energy from the second device. That communication can be accomplished by any of a variety of structures, such as hard wiring, wireless transmission or the like.) 
     For example, each source  100  could provide a magnetic field, such as described with respect to the exemplary embodiment of FIGS. 1-7. Alternatively, each source  100  could comprise a radio frequency transmitter for transmitting electromagnetic waves. The sensor  104  would then comprise a device capable of detecting the presence of the signal transmitted. For example, an audible tone generator and receiver could be used, as could transmitters and receivers used in radio communication and the telecommunications industries. Another alternative might be to provide a light emitting device, such as a light emitting diode, as the source  100 . Then, provided that the housing  102  includes transparent portions to allow in the emitted light, the sensors  104  could comprise photosensitive devices such as phototransistors, for example. Another alternative would be to place a switch on the exterior of the housing and to connect the switch to a source, such as a voltage source, which can be located either on the exterior or interior of the housing  102 . The switch could then be selectively activated to apply the energy signal from the source through the housing and to a sensor or processor within the housing, such as through a wired conductive connection, to indicate that a reference point is to be defined. Accordingly, it is contemplated that a variety of devices could be provided to serve as the source  100  and the sensor  104 , without departing from the scope of the invention. 
     When a sensor  104  detects energy from its corresponding source  100 , the sensor can provide an output signal to a microprocessor  106  indicating the reception of the energy. In one embodiment, such a microprocessor then receives the position of the magnetic marker from the position measuring circuit  108 . As is known in the art, this circuit controls a signal generator  110  for providing an interrogation signal through the conductor near the waveguide. The resulting torsional strain wave in the waveguide is detected by a signal receiver  112 , which can comprise a pickup coil, transducer, mode convertor, or the like. The time between the transmission of the interrogation signal by the pulse generator  110  and the reception of the resulting signal at the signal receiver  112  is measured and converted to a position by the circuit  108 , as is known in the art. The microprocessor  106  can then save this position in a memory location, such as the nonvolatile memory  114 . A similar process could be used with the other of the two sources  100  to define a second reference point, such as a second endpoint of the measurement stroke. 
     In addition to saving the current position, the microprocessor  106  could also assign predetermined voltages to the two saved reference points when defining these points. For example, the microprocessor  106  could be programmed such that the position saved when the first sensor  104  is tripped could be assigned a value of 0 volts and the position saved when the second sensor  104  is tripped could be assigned a value of 10 volts. Then, the microprocessor  106  could scale all future measurements based upon these reference points, such as by using a linear interpolation equation. Then, when circuit  108  measures a point between the two reference points, the microprocessor  106  would adjust the output that could otherwise be provided based upon the saved reference points. For example, for a point that is measured as being halfway between the two reference points, an output of 5 volts could be provided by the microprocessor  106 . This output could then be converted from the digital format to an analog format by the digital-to-analog converter  116  and then amplified by the amplifier circuit  118  to be output to the user. Alternatively, the user could be provided with the digital output directly from the microprocessor  106 . 
     In addition, instead of programming predetermined voltage values (e.g., 0 and 10 volts) for the positions that are measured when the sensors  104  are tripped, the output voltage could be user-definable during the defining of the reference points. For example, after the first sensor  104  receives the signal from the source  100 , the microprocessor  106  could save the position measured by the circuit  108  and then allow the user to select a voltage to be assigned to this position. The two sources  100  could be utilized to move a displayed number up or down until the desired value is reached. Then both sources  100  could be activated simultaneously to indicate that the value should be saved, and used as the output for the saved position. 
     An alternative method of defining the reference points would be to utilize the saved positions to define a scaling equation based upon the positions that are detected by the position measuring circuit  108  when the sensors  104  are tripped. 
     The following is an exemplary scaling equation that could be used: 
     
       
         output=(position−low ref)*(volt range/(high ref−low ref))  
       
     
     wherein 
     low ref is the smaller value of the two positions saved during calibration; 
     high ref is the larger value of the two positions saved during calibration; 
     volt range is the range of voltages that is to be output between the two reference points (e.g. 0-10 volts); and 
     position is the position measured by the position measuring circuit. 
     For example, consider the situation where the microprocessor  106  is initially configured to provide 0 volts at a detected position of 0 inches along the waveguide housing and 10 volts at a detected position of 10 inches (254 mm). If the circuit  108  indicates a position of 2 inches (the low ref) when the first sensor  104  is tripped, and a position of 8 inches (the high ref) when the second sensor  104  is tripped, the resolution would become 10 volts per 6 inches, rather than 10 volts per 10 inches. Then, for all positions detected between 2 and 8 inches, 2 inches could be subtracted, and the result multiplied by 10/6. For example, a detected position of 6 inches would result in: (6−2)*10/6=6.67 volts. This value of 6.67 volts would then be output from the processor  106  and converted to an analog signal by the digital to analog converter (DAC)  116 . Positions detected outside of the 2 to 8 inch range could produce a predetermined error signal or alarm. Other methods for defining reference points are possible as well. 
     It is also contemplated that a timing requirement could be utilized with the system to prevent unintentional tripping of the sensors. For example, it could be required that the sensor  104  receive the signal from the source  100  for a certain amount of time before the microprocessor  106  will define the reference point. For example, in the example of FIG. 6, the microprocessor  106  could be programmed such that the button  30  must be pressed for two seconds before the microprocessor will take action. This can be achieved by programming the microprocessor  106  with a counter or clock routine to determine the length of time that a signal is received from the sensor  104 . 
     In addition, other lockout or security mechanisms could be utilized. For example, the microprocessor  106  could be programmed such that a particular sequence and/or repetition of button presses must take place before allowing programming. For example, it could be required that the following code be received before programming is allowed: 1-2-1-1-2-2, where 1 represents pushing button 1, and 2 represents pushing button 2. 
     As described above, it is preferred that two energy sources are provided for defining two separate reference points. However, it should be understood that fewer or more sources could be utilized for defining additional reference points and/or to accomplish other functions. For example, a single magnet could be mounted on a single button, and the first press of the button could cause the first reference point to be defined, the second press of the button could cause the second reference point to be defined, and so on. Also, if the processor within the housing is already equipped to detect the energy from the source, then a separate sensor would not be necessary. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings. 
     Thus, it should be understood that the embodiments were chosen and described in order to best illustrate the principals of the invention and its practical application to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited for the particular use contemplated. Accordingly, it is intended that the scope of the invention be defined by the claims appended hereto.