Patent Publication Number: US-5627380-A

Title: Fluid gauging apparatus using integral electrical sensor and a stick gauge

Description:
This application is a continuation-in-part of application Ser. No. 08/069,263 now U.S. Pat No. 5,399,875 for &#34;Liquid Gauging Apparatus With Remote Sensor Interrogation&#34; filed on May 28, 1993. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates generally to apparatus and methods for fluid gauging, and more particularly to apparatus and methods for detecting fluid levels using magnetostrictive sensors and manual stick gauges. 
     Many types of liquid quantity and level sensors are known, including capacitive sensors, resistive sensors, acoustic sensors and so forth. Passive sensors generally operate on the basis of a sensor element that exhibits a parameter, e.g. capacitance, that varies with the liquid level. Active sensors such as acoustic sensors operate on the basis of producing a signal, e.g. an acoustic pulse, that can be used to detect the liquid level by parametric analysis such as echo ranging. 
     Such systems further include an electronic circuit that detects the parametric value of interest and converts that value to a signal that corresponds to the liquid level. 
     A common application for such liquid level sensors is for fuel level and quantity detection in aircraft fuel tanks. However, due to the volatile nature of fuel, it is desirable to minimize the connection of electrical energy to the sensors which may be disposed in the fuel. It is further desirable to minimize the amount of electrical energy stored in the sensors or used by the sensors. 
     A known approach for minimizing the coupling of electrical energy into a fuel tank is described in U.S. Pat. No. 4,963,729, issued to Spillman et al., and owned in common by the assignee of the present invention. In this system, optical energy is coupled to the sensors via optic fibers. This optical energy is then converted to electrical energy for energizing the sensors. The sensors detect the liquid level and then transmit another optical signal back to a detector via the optic fibers. The detector then converts the second optical signal into an output that corresponds to liquid level in the tank. 
     For aircraft applications, on board readings often need to be verified by ground crews, either during routine turn around or to confirm an error reading. The optical fiber link to the internal sensor in the above system prevents as a practical matter interrogation of the sensor by ground crews, other than via the same optic fiber link which may in fact be the cause of a fault reading. 
     A commonly used fuel level sensor in commercial aircraft particularly is a dripstick sensor, which is used as a backup fuel gauging apparatus to the on-board electronic fuel level sensors. For example, dripstick verification may be needed when a refueling truck gauge disagrees with the aircraft fuel gauge, if the on-board fuel gauges appear to be inaccurate or inoperative, or simply by request of the flight crew, among other possible reasons. 
     The dripstick includes a linear body that extends vertically into the fuel tanks. Often there is a plurality of such dripsticks in each wing of the aircraft. A magnetic float is disposed on the dripstick body like a collar that floats at the fuel surface. The dripstick is read by the ground crew by manually withdrawing the dripstick from the wing until a magnetic tip at the upper end of the sensor body engages the float. The operator can feel the resistance of the tip against the magnetic float and stop pulling on the dripstick. The dripstick body includes a series of markings which visually indicate to the operator the fuel level based on how far the dripstick was withdrawn from the tank. Although dripstick designs may vary somewhat, the basic operation of manual access and visual interrogation is the same for the ground crew. 
     Various problems are associated with using the conventional dripsticks, especially the time involved for the ground crew to walk around to all the sensors and manually/visually determine the readings. The mechanic climbs a ladder or uses a lifting device to gain close access to the underside of the wing, withdraws the dripstick until the engagement is detected, records the reading and then replaces the dripstick into the tank. Dripstick design is further complicated by the need for minimal fuel leakage. This entire process must be repeated for each sensor, which adds substantially to the refueling operation and turnaround time for aircraft flight readiness. 
     Commercial air carriers have long identified the need for a dripstick-like backup system, but one that is easier to use. A system that can be interrogated from the ground would eliminate the need for lifting equipment and provide easier reading of difficult access dripsticks. Overall reduction in refueling and fuel verification delays could then be realized. 
     Although alternative fuel gauging systems are thus desirable, it is expected that both commercial and military aircraft customers will want to retain the manual dripstick design due to its familiarity and simple design. This use of redundant systems presents a problem because there is not an unlimited space available to simply add on additional sensor systems, nor can fuel tanks be modified without substantial downtime and cost. 
     The objectives exist, therefore, for simple and reliable apparatus and methods to interrogate liquid gauging sensors from a remote, preferably ground level, location without coupling electrical energy into a volatile container. Such an arrangement should also be compatible with current dripstick sensor configurations if desired for a particular application. 
     SUMMARY OF THE INVENTION 
     To the accomplishment of the foregoing objectives, the invention contemplates in one embodiment apparatus for detecting fluid level in a container, such apparatus including a housing adapted to be disposed in the container; a float with an associated magnetic element, the float position corresponding to fluid level in the container; a magnetostrictive sensor disposed inside the housing; and a stick gauge disposed inside the housing; the magnetostrictive sensor and the stick gauge each being operable to determine the float position. The invention further contemplates the methods of detecting fluid level as embodied in the use of such apparatus. 
     These and other aspects and advantages of the present invention will be readily understood and appreciated by those skilled in the art from the following detailed description of the preferred embodiments with the best mode contemplated for practicing the invention in view of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a simplified representation of a liquid quantity gauging system according to the invention, such system being generally shown in an exemplary manner for use with aircraft fuel gauging; 
     FIG. 1B is an enlarged simplified view of an optics arrangement for the exemplary use depicted in FIG. 1A; 
     FIG. 2 is a system functional block diagram for a sensor interrogation system and method that uses a remote hand held interrogation unit and electronic control circuit according to a preferred embodiment of the invention; 
     FIG. 3 is a simplified flow chart for operational functions of the system illustrated in FIG. 2; 
     FIG. 4 is a partial cross-sectional view of a dripstick sensor suitable for use with the present invention; 
     FIG. 5 is a schematic diagram of an energy conversion and control circuit suitable for use with the present invention, and FIG. 5A is a schematic of an alternative and simpler circuit suitable for use in many applications; 
     FIGS. 6A, 6B and 6C is a schematic diagram of sensor interface and encoder circuits suitable for use with the invention, and FIG. 6D shows another suitable embodiment for an encoder circuit; 
     FIG. 7 is a timing diagram showing representative pulses produced by an encoder circuit suitable for use with the present invention; 
     FIG. 8 is a schematic diagram of an LED/Driver circuit for the present invention; 
     FIGS. 9A and 9B illustrate in schematic form another embodiment of the invention that incorporates a magnetostrictive sensor and stick gauge; 
     FIG. 10 is a detailed illustration in perspective of the embodiment of FIGS. 9A and 9B; 
     FIGS. 11A-11C illustrate one embodiment of a self-contained dripstick assembly in accordance with the invention; 
     FIG. 12 is a schematic diagram of a magnetostrictive sensor suitable for use with the invention; 
     FIGS. 13A and 13B illustrate one embodiment of a stick gauge suitable for use with the invention; 
     FIG. 14 is an enlarged view in section of the cap assembly used in an embodiment of the invention; and 
     FIG. 15 is an electrical schematic diagram of another suitable control circuit for the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1A, aircraft, particularly large commercial aircraft, have a plurality of fuel tanks 10 internal to the wing structures. Although the invention is described herein with particular reference to commercial aircraft, this is for purposes of explanation only and should not be construed in a limiting sense. Those skilled in the art will readily appreciate that the invention can conveniently be used with any aircraft or other vehicles or structures that have liquid containers. The invention also is not limited to fuel tank containers of volatile fuel, but can be used for sensing liquid quantity and levels of many fluid types in virtually any container. The particular sensor used for detecting the fuel level/quantity, such as, for example, a magnetoresistive dripstick as described in an exemplary manner herein, is also a matter of design choice. Those skilled in the art will readily appreciate that the advantages and benefits of the invention can be realized using any sensor that produces an electrical output or produces an output or exhibits a characteristic that can be interpreted by electronic circuits including capacitive, resistance, acoustic and so on to name Just a few. The terms &#34;liquid level&#34; and &#34;liquid quantity&#34; as used herein are intended to be understood in their broadest sense and are essentially interchangeable terms. As is well known, liquid level data can easily be converted to liquid quantity data, and vice-versa, when the tank or container dimensional characteristics are known. The invention is directed in a broader sense to apparatus and methods for remotely interrogating sensors that produce outputs; and is especially useful with sensors that produce outputs corresponding to liquid quantity and/or level in the fluid container. 
     In the embodiment described herein, each fuel tank or liquid container 10 includes one or more quantity/level sensors 12. For example, large commercial aircraft such as the Boeing 747 may use nineteen such sensors. Other applications and containers may use different numbers of sensors or only one sensor. 
     Each sensor 12 can be of any convenient design that is preferably electrically interrogated. In other words, the particular sensor selected is optional for the designer, but preferably will be a sensor that produces (or is connected to a transducer that produces) an electrical output signal that corresponds to the quantity/level of liquid in the container. One example of many of a commonly used sensor is a capacitive sensor in which a capacitive element changes capacitance value in relation to the percent immersion of the element in the liquid. Such a sensor is described, for example, in U.S. Pat. No. 4,841,227 issued to Maier and commonly owned by the assignee of the present invention, the entire disclosure of which is fully incorporated herein by reference. 
     The present invention is particularly well suited for use with a dripstick sensor 12 that is commonly used to mechanically read fuel levels in aircraft fuel tanks. In a conventional dripstick design, the dripstick sensor 12 includes an elongated linear body 14 that is vertically mounted in the fuel tank. For bottom mounted dripsticks, a magnetic or ferrous tip 16 is placed at the upper end of the dripstick. A magnetic float 20 in the form of a collar is slidably placed around the elongate body and floats on or near the fuel surface 18. When the ground crew pulls the dripstick body out of the tank, the mechanic can physically sense the position when the tip 16 engages the floating ring 20. Calibrated markings on the dripstick body then provide a visual reading of the fuel level and hence volume in the tank, which information can be used as a confirmation of on-board fuel level readings. 
     The present invention provides a simple and accurate way to interrogate such a dripstick sensor without the need to mechanically withdraw the dripstick from the fuel tank (as will be more fully explained hereinafter with respect to a sensor such as illustrated in FIG. 4.) The modified dripstick described herein, however, can also be dual configured in a conventional manner for manual withdrawal in case a redundant manual backup system is desired. 
     More generally, though, the invention contemplates apparatus and methods for interrogating many different types of sensors using a remote hand held unit. 
     As illustrated in greater detail in FIG. 1B, the sensor 12 (which in this case will be the modified dripstick described hereinafter) is electrically coupled to a sensor electronic control circuit 40. The control circuit 40 may be disposed in a unit that also is used to mount the sensor 12 in the tank. The control circuit 40 includes a circuit that receives electromagnetic energy 32 (FIG. 1A) from a remote control preferably hand held unit 34 and converts this energy to electrical power for the control circuit 40. Thus, on-board electrical power does not need to be connected into the fuel tank, thereby reducing electrical energy in the tank. 
     The control circuit 40 further includes a circuit that detects the variable parameter of the sensor 12 (e.g. the resistance value or capacitance value as a function of percent immersion) and emits electromagnetic energy 36 having a characteristic that is modulated in relation to the fluid quantity and/or level detected by the sensor 12. For example, the control circuit 40 may emit electromagnetic pulses having a duty cycle or time base modulation that corresponds to the liquid quantity detected. Other modulation and encoding schemes can, of course, be used. The encoded data may further include information in addition to the liquid quantity. For example, the sensor control circuit 40 can be used to encode information such as the type of sensor used, the type of aircraft it is installed in, which sensor corresponds to the particular circuit 40 and so forth. Sensors that detect fuel characteristics other than quantity can also be interrogated if desired. Each sensor control circuit 40 preferably also emits a digital code or protocol so that the data signals can be properly detected and identified. 
     According to an important aspect of the present invention, the sensor control circuit 40 is remotely energized and, in combination with the remote control unit 34, interrogates the sensor 12. Preferably, the remote unit 34 is a small portable hand held unit easily used by ground crews. When activated, the hand held unit transmits electromagnetic energy 32 to the sensor electronics 40 wherein it is converted and stored as useful electrical energy. This stored electrical energy provides the power needed to energize the sensor 12 and also to energize the circuitry needed to transmit the sensor data and identification back to the hand held unit 34 via the modulated electromagnetic energy 36. A detector circuit in the hand held unit detects the modulated beam 36, demodulates the signal to produce electrical signals containing the desired sensor information and fuel data, and if desired presents the information on a visual display 38 or in another output form. 
     A system level approach to such apparatus is illustrated in FIGS. 2 and 3. A sensor control circuit is generally indicated with the numeral 40. The sensor control circuit 40 includes one or more signal control lines 42 that connect the control circuit 40 to the sensor 12 (or alternatively a plurality of sensors when the circuit 40 is configured to operate in a multiplexed mode.) The signal control lines 42 may, for example, be used to supply electrical power to a sensor or to receive an output signal from a sensor, or simply to connect a sensor element (such as a variable component) into a detector loop within the circuit 40. The specific nature of the interface of the sensor 12 to the circuit 40 will be determined by the type of sensor being used. In the described embodiment of this invention, the sensor output is essentially a voltage or current that corresponds to the resistance value of the dripstick magnetoresistive element. The sensor 12 output could just as easily be the actual resistance itself of the sensor connected by the leads 42 to an impedance sensitive detector in the control circuit 40. As another example, the circuit 40 could be optically coupled to the sensor 12, or magnetically coupled to the sensor. A direct hard wired connection is not essential depending on the type of sensor being interrogated and the method of interrogation selected. The important aspect is that the circuit 40 is configured to interrogate or determine the sensor 12 output condition after electrical power has been input to the circuit 40 by operation of the hand held unit 34, which condition corresponds to the fuel level or quantity in the tank. Until electrical energy is delivered to the circuit 40 via activation of the remote unit 34, the sensor 12 and the circuit 40 are preferably and substantially de-energized. 
     As further represented in FIG. 2, the remote unit 34 is remotely connected to the control circuit 40 via the transmitted and received electromagnetic energy signals 32,36. The remote unit 34 circuitry is appropriately configured to transmit electromagnetic energy, such as for example, photoelectric energy such as infrared light (32) to the control circuit 40, and to receive photoelectric energysuch as infrared light (36) from the control circuit 40. The remote control unit 34 is optically linked through air to the control circuit via the light transmission alone, without the use of optic fibers or other light conduits, or electrical connections thereby maximizing the flexibility, portability and simplicity of use of the hand held unit 34. Thus, the term &#34;remote&#34; as used herein refers to electromagnetic coupling without the use of optical or electrical connections between the remote unit and the sensor electronics. 
     The control circuit 40 may be designed to perform any number of functions depending on the number and type of sensors being interrogated with each circuit 40. However, generally the control circuit 40 will include at least the following functional circuits. An energy receiver, such as a photodiode array 44 is used to receive the electromagnetic energy transmitted from the hand held unit 34, and to convert that energy into useful electrical energy. The type of devices used in the array 44 will, of course, depend on the spectral content of the electromagnetic energy used to link the hand held unit 34 to the control circuit 40. Although infrared light is one of the preferred options it certainly is not the only option available. Broadband radiation such as from white tungsten lamps, radio frequency waves and other spectral bands can be used for the electromagnetic energy. Whatever bandwidth is selected will determine the specific detector 44 used, which must be responsive within the spectral band of the electromagnetic energy transmitted to the control circuit. 
     In the described example, the diode array 44 produces electrical energy in response to the received light 32. A power storage circuit 46 receives and stores this electrical energy for use in powering the sensor 12 and the circuit 40 under the control of a power control sequencer circuit 48. The sequencer circuit 48 is designed to sequentially control the application of power to the sensor 12 (or sensors in a multiplexed design) via a sensor interface circuit 50. Depending on the overall complexity of the sensor 12 and the circuit 40, the interface circuit 50 may be as simple as a switch, a voltage level shifter, or more involved such as a multiplexer, transducer and so on. Whatever functions are selected for the circuit 40 will be determined primarily by the amount of electrical energy that can be stored and retrieved from the storage circuit 46, and how much of a load the circuit 40 applies to the storage circuit. 
     The sensor output information is input to an encoder circuit 52 that in turn is connected to an electromagnetic source 54, such as an LED array and driver circuit. In general, the overall electrical load of the circuit 40 is primarily dependent on the power consumption in the LED circuit, especially if the circuit 40 is expected to transmit the sensor data over a long distance to the hand held unit 34. Therefore, sensor data is preferably encoded in a digital manner so that the source 54 can be pulsed rather than continuously operated. 
     The hand held unit 34 also includes an electromagnetic energy source 56 which for convenience may be an LED and associated driver circuit similar to the control circuit light source circuit 54. However, the remote hand held unit could transmit electromagnetic energy for purposes of energizing the sensor using a different spectral band than is used for the encoded energy transmitted back to the remote unit. A detector circuit 58 is used to receive the encoded signals from the sensor control circuit 40 and to convert those pulses into electrical signals that can be processed and interpreted. The detector circuit 58 can be a simple photodiode or photodiode array or other transducer, and may include signal conditioning circuits for amplification, filtering and other signal processing functions well known to those skilled in the art. 
     The detector 58 signals are input to a central processing circuit 60 which for convenience may be a control circuit utilizing a microprocessor or similar controller. Discrete logic and signal processing, of course, could also be used. The controller 60 operates a power control circuit 62 that switches the light source 56 on and off at appropriate times. The source 56 is used, as stated, to supply electrical energy to the sensor circuit 40. The source 56 can be designed to be operated continuously during a sensor interrogation operation, or can be turned off when the hand held unit is ready to receive the encoded light pulses. 
     The controller 60 is programmed in a conventional manner to interpret the received data from the sensor 12 and convert that data into an output, such as the visual display 38 used to display fuel level, quantity, sensor identification, plane identification or any other data transmitted back to the remote unit 34. The controller 60 also monitors an operator interface circuit 64, which may be realized in the form of an alphanumeric keypad optionally provided with special function keys and so forth as is well known. The operator interface can be used, for example, to control actuation of the light source 56, as well as to input control data such as the type of plane, which sensor is being accessed, and so on when such information is not transmitted by the on-board sensor. 
     With reference now to FIG. 3, a suitable overall system functional flow diagram for the apparatus illustrated in FIG. 2 is provided. Again, this flow diagram is only intended to be exemplary, the actual functions and steps being performed by the fuel gauging system being ultimately determined by the complexity and data requirements specified for a particular application. 
     At step 70, the controller 60 turns on the energy source 56 as instructed by the mechanic by activation of the hand held unit 34. The source 56 transmits electromagnetic energy towards one of the dripstick sensors by means of the mechanic aiming the hand held unit at the associated sensor control circuit 40. The detector array 44 is exposed to the incident energy, such as through an optical window (not shown) that seals the electronics from the environment exterior to the wing or tank. The control circuit 40 converts the received electromagnetic energy to electrical energy and interrogates the sensor 12. The sensor output is then encoded and transmitted back to the hand held unit via the modulated source 54. Thus, at step 72 the detector circuit 58 receives the encoded signal 36, processes the signals as required, and at step 74 the controller 60 determines the pulse timing characteristics to interpret the sensor 12 outputs. The controller 60 also decodes the data signals with the encoded information such as the dripstick number, aircraft type and so on. At step 76 the controller, by means of appropriate lookup tables and algorithms converts the decoded signals into the desired information such as fuel volume, level, sensor number and so on, and at step 78 displays the requested information to the mechanic via the display 38, which may be a visual display, printed data, recorded data and so forth. 
     With reference next to FIG. 4, there is shown a dripstick sensor that has been modified for use with the remote interrogation or direct interrogation by on-board electronics approaches of the present invention. For clarity, FIG. 4 only illustrates the operative portion of the sensor in accordance with our invention that detects the fuel height in the tank. Other parts of the dripstick can be conventional in design and are well known. The sensor 12 includes the elongated member 14 which may be, for example, an aluminum tube 80. Within the tube 80 is a precision wire wound resistor 82 which extends substantially along the entire portion of the tube 80 used for detecting the fuel height or level. The float 20 is disposed around the tube 80 as a collar, and retains an actuation magnet 84. As the float moves up and down in relation to the fuel level 18, the magnet 84 causes spring fingers 86 that are part of a conductive strip 88 to contact the wire resistor 82. A magnetic piece 89 may be provided to insure the fingers 86 return out of contact from the resistor 82 when the float is not aligned with the fingers. One end of the resistor 82 serves as a sensor terminal 90 and the conductive strip 88 serves as another sensor terminal 92. Thus, the resistance between the terminals 90,92 directly corresponds to the percent immersion of the tube 80 in the fuel, or stated another way, directly corresponds to the fuel level/quantity in the tank. An insulative layer 94 can be provided between the tube wall 96 and the internal parts of the magnetoresistive sensor to prevent the tube 80 from electrically short circuiting the resistor 82. This particular sensor 12 can easily be interrogated by simply applying a voltage or current across the terminals 90,92 and determining the resistance value. A particular advantage of this modified dripstick sensor is that it can also be used manually by the ground crew for verifying the fuel level readings as with a conventional dripstick. Thus, the modified dripstick can be used as a direct field replacement for conventional dripsticks. Also, the magnetoresistive element can advantageously be hardwired to the on-board electronics if such monitoring of the dripstick readout is desired. 
     In FIG. 5, there is shown one example of a circuit that can conveniently be used for energy conversion and power control in the sensor control circuit 40 with exemplary component values being provided. In this particular arrangement, the detector array 44 is positioned near an optional lens 100 that collimates light energy 32 received from the remote hand held unit 34 through a window 102 which may be flush mounted with the wing underside. The array 44 preferably includes a plurality of photocells 104 that convert incident light energy into voltage and current. The cells could be used in the photovoltaic, photocurrent or both modes. The number of photocells 104 used will be determined by the characteristics of the photocell used, as well as the voltage and charging requirements of the overall control circuit as dictated by the circuit&#39;s power requirements needed to interrogate the sensor 12 and transmit encoded light signals back to the hand held unit 34. 
     The photocells 104 are connected to a circuit charge storage capacitor 106 through a rectifying diode 108. The array 44 is also connected to a LED/Driver (54) storage capacitor 110 through a second rectifying diode 112. Separate storage capacitors are preferred for the LED and control circuit functions due to their respective load effects on the overall charging requirements. For example, the circuit storage capacitor 106 can be smaller in many applications and thus charged quickly for fast access to the sensor 12. Also, by having the LEDs operate from a separate capacitor, the load effects of the LEDs will have less influence on the operation of the control circuit components used to interrogate and encode the sensor 12 data. The LED capacitor 11 stores a DC supply voltage V +  114 at an output node 116, which is connected to the LED/Driver circuit 54 (FIG. 2). Note that in the preferred embodiment, the DC supply to the LED/Driver 54 is maintained for as long as the capacitor is charged by light received from the remote unit 34, or until the capacitor is discharged by operation of the LED circuit 54. The circuit storage capacitor 106 stores a DC supply voltage that appears at node 118 with respect to the common return 120. A power switch circuit 122 connects the stored energy from the capacitor 106 to a V +  terminal and V -  terminal 126 which are connected to the sensor interface circuit 50 (FIG. 2). The circuit 122 operates in a conventional manner to switch on the output transistor 128 when the voltage stored on the capacitor 106 reaches about +5 VDC, and switches the transistor 128 off when the capacitor 106 discharges to about +2.2 VDC. The switch circuit 122 provides a low impedance output for the circuit voltage supply, and also prevents operation of the control circuitry until sufficient energy is stored in the capacitor 106 to assure accurate data can be transmitted back to the remote unit 34. 
     In FIG. 5A there is shown another and simpler circuit that can be used for energy conversion and storage when isolated LED and control circuit supplies are not needed (with like components being given like reference numerals followed by a prime&#39;). In this circuit, the detector array 44&#39; converts the electromagnetic energy into electrical energy that is stored in the main storage capacitor 106&#39; via a rectifying diode 108&#39;. When the stored voltage reaches a predetermined threshold, an output transistor 128&#39; switches on and the supply voltage appears at the V +  and V -  terminals 124&#39;, 120&#39; and is connected to the LED/Driver circuit 54&#39; and the sensor interface circuit 50&#39;. 
     Referring next to FIGS. 6A, 6B and 6C there is shown a set of circuits (A, B and C) that can conveniently be used to detect the dripstick sensor 12 output and to encode the output for transmission to the remote control unit 34. The V +  and V -  supplies shown in FIG. 6 are connected to the output nodes 124,120 of the supply circuit in FIG. 5. The top circuit in FIG. 6 is a simple voltage scaling circuit that connects the V +  and V -  supplies across a precision reference resistor 140. A reference resistor is used in this case because the sensor 12 of this example is a magnetoresistive sensor having a resistance value that is the parameter of interest. The voltage across the reference resistor 140 produces a reference current that is converted into a negative reference voltage, -V ref , by an inverting amplifier 142. A second inverting amplifier 144 configured for unity gain produces a positive reference voltage, +V ref . 
     The middle circuit in FIG. 6 realizes the sensor interface circuit 50 (FIG. 2) and is used to transduce the sensor 12 output into a useful electrical signal. The V +  and V -  supplies are connected across the sensor resistance via the signal lines 42 (FIGS. 2 and 4). This produces a sensor current, I sensor , which is converted to a voltage, V sensor , by another amplifier 146. 
     The voltages +V ref , -V sensor  and -V ref  are input to the lower circuit in FIG. 6, which realizes the encoder circuit 52 (FIG. 2). The basic function of this particular encoding circuit and encoding scheme is to produce a series of pulses having a time displacement between pulses that corresponds to the sensor 12 output reading. Such a circuit and encoding scheme are fully described in U.S. Pat. Nos. 5,075,631 and 5,077,527 both issued to Patriquin and commonly owned by the assignee of the present invention, the entire disclosures of both patents being fully incorporated herein by reference. Reference to these patents should be made for a detailed explanation of the encoder circuit. Essentially, a ramp generator 150 is used to produce a voltage ramp signal 152. The ramp generator is conveniently realized in the form of an integrator amplifier. The ramp signal 152 is input to one input 154 of a comparator 156. The other input 158 to the comparator 156 is connected to a multiplexer or signal switcher circuit 160 controlled by a timing control circuit 162 which can be realized with a counter. The control circuit 162 sequentially applies the +V ref , -V sensor  and -V ref  to the comparator input 158. The particular sequence only requires that the sensor voltage be applied temporarily between the two reference voltages. The comparator 156 output changes state when the ramp voltage reaches the level of the applied signal to the other comparator input. After the comparator changes state, the control circuit switches in the next signal to the comparator input 158 so that the comparator 158 output is a series of short pulses. The comparator output 162 is connected to the LED/Driver circuit 54 (FIGS. 2 and 8.) 
     With reference to FIG. 7, there is shown a timing diagram for typical pulses output from the encoder circuit of FIG. 6. In this case, the controller 60 (FIG. 2) turns off the light source 56 after sufficient time has passed (as at 170) to charge the storage capacitors 106,110 (FIG. 5.) At time T1, the comparator 156 outputs a first pulse that has a leading edge determined by the value of the +V ref  signal. The second pulse has a leading edge at time T2 and this corresponds to the value of the signal V sensor . The third pulse has a leading edge at time T3 that is determined by the value of the -V ref  signal. Because the ramp voltage V Ramp  is a stable constant, and the reference voltages +V ref  and -V ref  are stable constants, the time delay between the leading edges at T1 and T3 should be a fixed reference time. The time occurrence of leading edge T2 which corresponds to the sensor data will vary with the sensor 12 resistance value and thus correspond to the fuel quantity/level in the tank. Thus, the ratio (T1-T2)/(T1-T3) defines the fractional immersion of the sensor 12 in the fuel. 
     It will be appreciated by those skilled in the art that in the specific example described herein, leading edge timing is used for encoding. However, other timing sequences can be used just as conveniently. 
     With reference to FIG. 8, there is shown a simple circuit for interfacing the encoder 52 to the LED/Driver circuit 54. The V +  and V -  supplies are connected to the corresponding terminals 116,120 from the storage circuit in FIG. 5. An LED, or alternatively an array of LEDs, are series connected with a switch 182. The switch 182 has a control input 184 that is connected to the output 162 of the encoder comparator 156. Thus, the switch 182 is pulsed on and off by the encoded pulses from the comparator 156, thus correspondingly pulsing the LED 180 on and off. A lens 186 collimates the light from the LED 184 and transmits the light pulses through a window 188 towards the hand held unit. The lens and window are, of course, optional depending on the particular application. 
     With reference to FIG. 6D, there is shown another embodiment (with like components being given like reference numerals followed by a prime&#39;) for an encoder circuit suitable for use with our invention. In this example, an output comparator 156&#39; is used to produce a series of pulses having a time domain displacement that corresponds to the sensor 12 output data. A control circuit 162&#39; is used to switch sequentially to the comparator (by means of a switching circuit 160&#39;) signals from a pair of one shot timers 190,192. The first one shot produces a pulse edge at a time that corresponds to a reference resistor 140&#39;, and the second one shot 192 produces a pulse edge that corresponds to the sensor 12 resistance value. The first one shot may be armed from a signal produced by the energy storage circuit 46 (FIG. 5), and the second one shot armed by the pulse produced by the reference one shot. As in the other described embodiment, the comparator output 162&#39; is a series of pulses encoded with the sensor 12 reading. 
     As described hereinbefore, the hand held unit 34 includes appropriate circuitry for transducing the encoded light received from the control circuit 40 by means of the detector circuit 58. The controller 60 can be easily programmed to calculate the ratio (T1-T2)/(T1-T3) to determine the fuel level and quantity. 
     Although not shown in detail in the Figures, the encoder circuit 52 may include a second or more ramp generator(s) and associated circuitry to generate a pulse series that encodes identification data such as the sensor 12 number, location, plane identifier and so on. Alternatively, the encoder could transmit one or more digital words (for example by use of a programmable ROM or other memory device) to the hand held unit 34 before the sensor data is transmitted, with the digital words being encoded data of interest to the ground crew. 
     With reference to FIGS. 9A and 9B, there is shown in simplified form another embodiment of the invention that includes a fully integrated magnetostrictive fluid level sensor and manually operated stick gauge. An advantage of this embodiment is the use of the magnetostrictive sensor that can be interrogated electrically to determine the position of a float. The magnetostrictive sensor is contained within a small envelope that can accommodate a stick gauge as well, thus providing in a single unit both an electrically readable sensor as well as a manually readable stick gauge. As with the earlier described embodiments herein, the magnetostrictive sensor can be interrogated by hardwired connection (such as by the on-board electronics for example in aircraft applications), and/or remotely by use of the remote hand held unit. The entire assembly of the magnetostrictive sensor, stick gauge and electronics (for remote or hardwired interrogation) are self-contained in a single package that can fit within the dimensions of a conventional fluid gauge housing, thus permitting simple and direct form, fit and function replacement of a conventional fluid level probe. 
     In general, the apparatus 300 includes a main housing 302 that is installed in the fluid container (not shown). The housing 302 typically is made of aluminum, stainless steel or other suitable non-magnetic material that will not be damaged by the fluid in the container, such as jet fuel. 
     An annular float assembly 304 is disposed around the housing 302 as a collar and slides along the housing at the level of fluid in the container. The float assembly 304 holds a magnet assembly 306 which provides a mechanism for detecting the position of the float as will be described hereinafter. The magnet assembly may include one or more ring magnets or other suitable arrangement of magnets. 
     A dripstick assembly unit 308 is disposed within the housing 302, and in this embodiment includes a magnetostrictive sensor 350 and stick gauge 318 that are integrated into a self-contained unit. The sensor/gauge dripstick unit 308 is adapted to slide easily within the main housing 302 so that the magnetostrictive sensor can be partially or fully withdrawn from the housing 302. The stick gauge 318 is also adapted to slide easily with respect to both the housing 302 and the magnetostrictive sensor 350. 
     The unit 308 includes a cap assembly 310 that can be mated to a mounting member 312 such as a flange for example that can be part of the main housing or part of the container structure. Thus, when the assembly 308 is fully inserted into the housing 302 (as illustrated in FIG. 9B), it is held in place by the mated cap assembly and mounting 312. This can be, for example, a threaded engagement so that when a manual reading of the stick gauge is desired, the cap assembly is unscrewed from the housing and the magnetostrictive sensor can be withdrawn from the housing (in most cases the sensor will be substantially withdrawn but not completely removed from the housing) so that the stick gauge can be accessed and visually read. 
     The dripstick assembly unit 308 is a self-contained unit that includes the magnetostrictive sensor, a stick gauge and the electronics for operating the magnetostrictive sensor as well as for the remote interrogation when included. The electronics also includes the circuits described with respect to the other embodiments for converting received electromagnetic energy into useful electrical energy to power the magnetostrictive sensor. In this manner, electrical energy is present in the assembly 308 only during actual readings. When the apparatus 10 is optionally hardwired to an on-board fuel gauging system, provision can be made to apply power to the magnetostrictive sensor only as needed during interrogation of the sensor. 
     With reference to FIG. 10, we show in perspective a more detailed illustration of the apparatus 300. The housing 302 and float assembly 304 are disposed inside the container or tank &#34;C&#34; (partially shown). The housing 302 is rigidly mounted in the container such as to a mounting flange or wall of the container, and in a generally vertical orientation. Although the exemplary embodiment herein shows a bottom mounted arrangement, those skilled in the art will readily appreciate that the invention can be conveniently adapted for a top mounted arrangement. 
     The housing 302 is provided with a male threaded boss 314 that threadably mates with corresponding female threads in the mounting flange or other suitable structure on the container. The cap assembly 310 is at the lower end of the dripstick assembly unit 308, which is shown in a partially withdrawn position in FIG. 10. The cap assembly 310 includes a male threaded ring 315 that mates with a female threaded receptacle on the container mounting flange or the housing, for example. This allows the dripstick assembly 308 to be fully inserted into the container and locked into place by the threaded engagement. The unit 308 can be withdrawn from the container by simply unscrewing the cap assembly from the mounting flange and sliding the unit out of the main housing 302. 
     The dripstick assembly unit 308 generally includes the cap assembly 310, an elongated, rigid and slidable tube assembly 316 and a stick gauge 318. The tube assembly 316 contains an elongated magnetostrictive sensor therein as well as all the electronics needed for operation of the sensor including a transceiver circuit for remote interrogation (not shown in FIG. 10). If needed for a particular application, the electronics can be located external the assembly 308. The cap assembly 310 includes an end piece 320 that provides access to a photocell array and LED for wireless communication between the dripstick assembly 308 and a remote control hand-held unit such as described hereinbefore. The dripstick assembly 308 is thus a completely self-contained fluid level sensor that can be remotely interrogated, hardwired interrogated or manually interrogated to determine the position of the float assembly 304 and hence the fluid level in the container. 
     In this embodiment, the magnetostrictive sensor 350 and the stick gauge 318 both function with respect to the same magnetic float assembly. Those skilled in the art will understand that the rigid tube 316 could also function as a manual dripstick such as illustrated in FIGS. 9A, B (in an arrangement similar to the earlier described embodiment in which the magnetoresistive sensor was disposed inside a housing that functioned also as a conventional manual dripstick) by providing the tube 316 with gradations, so that the separate stick gauge 318 could be omitted. As a further alternative, the stick gauge can be integral with or permanently affixed to the tube 316 for the same purpose of using the rigid tube as a stick gauge. However, in some applications, the weight of the assembly 308 will be sufficient as to preclude the use of such an arrangement (as will be explained hereinafter, the manual stick gauge operates during a manual reading by having the gauge captured at the upper end thereof by the magnetic float assembly--excess weight can cause the assembly to drop past the float preventing an accurate reading). 
     With reference to FIGS. 11A-C, the dripstick assembly 308 is shown in elevation in FIG. 11A, in longitudinal cross section in FIG. 11b (rotated 90 degrees about the longitudinal axis compared to FIG. 11A), and a partial top end view in FIG. 11C (to show generally the &#34;D&#34; shape of the tube 330). The assembly includes an outer tube 330 which, as best shown in FIG. 11C, is in the shape of a &#34;D&#34; to provide a generally flat surface 332. It will be noted that in this embodiment, the entire unit shown in FIGS. 11A-C is slidable within the main housing 302. 
     The D-tube 330 is connected at one end to a swivel ball mechanism 334. The mechanism includes a generally tubular cup 336 that is attached at one end or sensor end to the D-tube and at the other end to a swivel ball 338. The ball 338 is pivotally captured inside a ball socket extension 340 on the cap assembly 310. The ball and socket arrangement is provided to allow for mounting flanges on the container that are not exactly transverse to the longitudinal or vertical axis of the D-tube 330. The swivel ball 338 is provided with an axial bore 342 through which wires can be routed to the magnetostrictive sensor assembly and electronics circuit board from the photodiode array and LED (as explained in greater detail hereinafter). 
     The magnetostrictive sensor assembly 350 (for clarity shown in a simplified manner in FIG. 11B) is an elongated unit that is generally disposed axially within the D-tube 330. The sensor assembly 350 includes an elongated body 352 and a receiver unit head 354. The receiver unit 354 contains the various components used to apply an electric current pulse in the magnetostrictive element and for detecting the arrival of an acoustic pulse generated by the interaction of the current pulse magnetic field and the magnetic field associated with the float 304. As shown in FIGS. 11A and 11B, the upper distal end 356 (distal from the sensor end) of the magnetostrictive sensor body 352 extends somewhat beyond the upper end of the D-tube 330. A spring load retainer pin arrangement 358 disposed at the upper end 356 can be used, for example, to position the upper end of the D-tube in the housing 302. 
     In further accordance with the invention, the elongated D-tube also retains an elongated printed circuit board and associated electronics 360. The width of the elongated circuit board 360 can be sized to just fit within the D-tube diameter. The circuit board carries all the electronics needed for operation of the magnetostrictive sensor, as well as the interface circuits for the wireless transceiver link with a remote unit. After all the components are positioned inside the D-tube, the entire assembly of the sensor 350 and printed circuit board 360 can be potted inside the D-tube using conventional potting materials. 
     The cap assembly 310 is provided with a key slot 362 so that a screwdriver or other suitable tool can be used to unscrew the cap assembly from the housing, whereby the operator can grasp the cap assembly and pull the entire dripstick assembly 308 out of the housing (or simply allow the assembly to drop under the force of gravity). The retainer pin assembly 358 can conveniently be adapted to prevent the unit 308 from completely falling out of the housing onto the ground, by engaging with the mounting flange or other suitable structure on the container, while at the same time allowing for complete removal of the unit from the housing if so desired. 
     The magnetostrictive sensor assembly 350 is an integral unit that includes the elongated sensor 352 and a receiver head 354 located at one end of the sensor body 352. The particular type of magnetostrictive sensor selected is a matter of design choice. In the exemplary embodiment of FIG. 12, the sensor 350 includes a magnetostrictive waveguide 364 coaxially disposed inside a stainless steel sheath 366. The waveguide can be supported inside the sheath by a number of spacers 368. The distal end of the sheath is closed by a brass plug 370 that also serves to electrically connect the waveguide 364 to the sheath 366. In this manner, a current pulse can be applied to the waveguide by the electronics, and is conducted by the waveguide 364 to its distal end and returned to the electronics via the sheath 366. A magnetic field is produced by the current pulse along the length of the waveguide. 
     When the magnetic field associated with the current pulse interacts with the magnetic field of the permanent magnet 306 in the float 304 (which essentially occurs concurrently with the current pulse), a torsional acoustic pulse is initially induced at the point of interaction by the waveguide 364, and this acoustic pulse travels back along the waveguide 364 to the receiver head 354 where it is detected by an acoustic pick-up 372. The time delay from application of the current pulse to detection of the first acoustic pulse thus corresponds to the position of the float relative to the receiver head 354. 
     Simultaneous with this direct acoustic return pulse, an induced acoustic pulse also travels towards the distal end of the waveguide where it is reflected back towards the receiver head and is detected by the acoustic pickup 372. The total time delays from application of the current pulse to receipt of the direct and reflected acoustic pulses can then be used to calculate the relative position of the float along the length of the waveguide sensor body 352 based on the fact that the total distance travelled by the reflected pulse and the direct pulse is equal to two times the length of the waveguide. The speed of sound along the waveguide can be easily omitted from the calculation by taking a ratio of the direct pulse time measurement to the sum of the time measurements for the reflected or second pulse time measurement and the direct pulse time measurement. This ratio is thus inherently self-compensated, for example, for waveguide speed of sound. 
     The electronics on the circuit board 360 can be used to apply current pulses to the waveguide 364 using conventional timing circuits and pulse shaping circuits. The waveguide is connected to the circuit board by jumper wires inside the D-tube 330 (not shown). The receiver 354 converts the detected acoustic pulses into corresponding electrical signals that can be sent to an on-board fuel gauging system, or can be used as the sensor input signals to the wireless remote interrogation apparatus previously described herein (see FIGS. 1A and 2 and the discussion related thereto). 
     The particular design of the magnetostrictive sensor 350, including the receiver unit 354, can be conventional and will depend on each particular application. These types of sensors, however, are well known, such as described for example, in U.S. Pat. No. 3,898,555 issued to Tellerman. Suitable magnetostrictive sensors, including the receiver unit 354, are available from MTS Systems Corporation, Sensors Division, Research Triangle Park, N.C. In the exemplary embodiment herein, the waveguide 364 is used for both the current pulse and the torsional acoustic pulses, but this is but one example. Many different types of magnetostrictive sensors can be used. 
     A particular advantage of the exemplary design is that it can easily interface with the remote interrogation apparatus 34, 40 because the electrical signals produced by the receiver unit 354 in response to the detected acoustic pulses are simply separated in time in relation to the position of the float. These signals from the magnetostrictive assembly 350 can thus be used directly to pulse the energy source 54 (see FIG. 2) such as an LED, and the remote control unit 34 can detect the pulses and determine the time delay between the received electromagnetic pulses, all without the need for special timing logic circuits in the sensor electronics, if so desired. In other words, the output signal pulses from the magnetostrictive assembly 350 inherently contain, in their time domain spacing, the data for determining the float 304 relative position and hence the fluid level, quantity, volume, mass and so forth. These pulses can thus be coupled to an LED to pulse the LED in a similar manner for detection by the remote unit. 
     A reference electromagnetic pulse can be produced concurrent with the application of the current pulse to the waveguide if so desired. In this arrangement, the time delay between receipt of the reference electromagnetic pulse and receipt of the electromagnetic pulse that corresponds to the reflected acoustic signal provides an indication of the sensor 352 length and thus could be used as part of a sensor identification code. 
     With reference to FIGS. 13A and 13B, the stick gauge 318 is an elongated member that has a relatively flat stick-like body 380. The stick is attached at one end to a side of a spool 382. The spool 382 carries a ferromagnetic band 384 that is magnetically captured by the float magnet 306 when the stick 318 is allowed to drop down from its stored position (when the dripstick assembly 308 is fully installed in the housing 302). 
     The spool 382 includes a through bore 386 that is laterally offset from the stick 380 terminus on the spool. The bore is appropriately sized to slidably receive the upper distal end 356 of the magnetostrictive sensor 350. The lateral offset between the bore 386 and the stick 380 permits the stick 380 to lie on or closely adjacent the flat surface 332 of the D-tube 330 (FIG. 11A). 
     As shown in FIG. 10, the free end 388 of the stick gauge extends to the lower end of the D-tube assembly 316, and for convenience can slide into a small gap in the cup 336. As further illustrated in FIG. 10, the stick body 380 is provided with a series of markings or gradations 378 that provide a visual indication of the float level inside the container. The markings can be read, for example, with respect to the container edge where the stick exits the container, or alternatively for example, with respect to a reference mark placed on the D-tube 330. 
     With reference to FIG. 14, the cap assembly 310 includes a shallow cup extension 322 in which is disposed a photocell array 324 and an LED 326. Those skilled in the art will appreciate that, although in this exemplary embodiment the array 324 and LED 326 are disposed in the cap assembly 310, this is a matter of convenience. The optical elements could be located in other areas, of a wing or aircraft for example, as desired for a particular application. Furthermore, the photocell array 324 need not be disposed at the same location as the LED. The energization of the sensor 350 electronics could alternatively be accomplished, for example, from another location, or even from an on-board supply (without an optical converter as described herein). Typically, although not necessarily, the LED 326 (or other transmitting device as desired) will be disposed near the sensor 350 output to reduce loss of signal strength and noise. 
     The photocell array 324 is one example of a detector 44 (FIG. 2) that detects electromagnetic energy received from the remote control unit 34 and converts it into useful electrical energy that is used to energize the magnetostrictive sensor 350 and associated electronics, as well as the data encoding circuits associated with the sensor control circuit 40 as previously described herein. The LED 326 can be used in this case as the electromagnetic energy source 54 to transmit data back to the remote control hand-held unit 34. As shown in FIG. 10, these photosensitive devices are positioned outside the container so that a remote unit can transmit and receive electromagnetic energy such as from the ground (in applications such as wing mounted fuel tanks, for example). Wires 390 connect the photocell array 324 and the LED to the electronics on the circuit board 360 located inside the D-tube 330. These wires can be routed through a bore 392 in the cap assembly and the axial bore in the swivel ball 338 previously described herein. 
     With reference to FIG. 15, we show an electrical schematic diagram of a suitable transceiver control circuit 400 for the magnetostrictive sensor 350. Those skilled in the art will readily appreciate that this circuit is but one example of many types of control circuits that can be used, the design of which will be application specific. The circuit of FIG. 15 can be conveniently implemented with the overall data transfer circuits with the remote control unit 34 earlier described herein if so desired. 
     In FIG. 15, the photocell array 324 produces electrical energy in response to detecting electromagnetic energy received from the remote unit 34 (or alternatively from ambient light, for example, when enabled by a signal from the remote unit). This electrical energy is stored in capacitor C3 for supply to the electronics over a positive rail 401 (V supply ) and a negative rail 403. A conventional multivibrator 402 when energized periodically pulses an FET switch Q2 on. Those skilled in the art will readily appreciate that more intricate control circuits can be implemented, although in some applications power dissipation may be a design factor depending on how much electrical energy is stored (alternatively as described herein the electrical supply can be provided throughout the interrogation period). 
     The waveguide 364 is connected electrically in series with the LED 326 (D1) and the positive supply rail, as well as in series with the FET switch Q2 and the negative supply rail. Thus, when the switch Q2 is pulsed on, a current pulse is applied to the waveguide and a reference light pulse is produced by the LED 326 which can be detected by the remote control unit 34. 
     The magnetostrictive sensor receiver unit 354 detects the acoustic pulses and produces electrical pulses that correspond to amplitude and duration of the direct and reflected acoustic signals. These electrical signals are amplified by a conventional amplifier circuit 404 and detected by a comparator circuit 406. The comparator output is used to pulse a second switch Q1 at each time an acoustic signal is detected above a detection threshold level by the comparator circuit 406. This produces pulses from the LED 326 that can be detected by the remote unit 34 corresponding in time to the acoustic pulse detections. 
     The remote unit 34 can be provided with appropriate timing circuits for determining the time periods between the detected LED pulses and hence therefrom calculate the relative position of the float 304. From the calculated float position, the remote unit 34 electronics can further determine the fluid quantity from look-up tables stored in memory that relate fuel height measurements at one or more float sensors to volume, as described hereinbefore. Alternatively, when the magnetostrictive sensor is interrogated by on-board circuitry (such as an aircraft fuel management system), the amplified pulses can be transmitted to the fuel management system by hardwired connections including electrical conductors or optic fibers, for example, for further processing to determine fuel height and quantity. 
     As another alternative, the sensor electronics of FIG. 15 could further include logic circuits for determining the float position based on the time delay between the received acoustic pulses, and encode this data for wireless transmission to the remote unit 34. Other alternative embodiments of the control electronics and signal processing functions will be apparent to those skilled in the art, with the embodiments of FIGS. 2 and 15 being intended to be exemplary in nature and not otherwise limiting. 
     In the exemplary embodiment herein, manual reading of the stick gauge 318 can be carried out as follows. The cap assembly 310 is unscrewed from the housing or container mounting structure, and the assembly 308 is either pulled down or allowed to drop down until its upper end engages the mounting structure via retainer 358 to prevent the entire assembly from dropping out of the housing 302. As the dripstick assembly 308 is lowered, the stick gauge 318 also drops until the ferromagnetic band on the spool 382 aligns and is engaged by the magnetic flux of the float magnet 306. When the spool is magnetically captured, the stick gauge 318 disengages from free end 356 of the magnetostrictive sensor 350 which continues to drop until retainer 358 engages. As the sensor 350 is lowered, the free end 388 of the stick gauge also slides out of the cup 336 of the D-tube assembly 330, and becomes free of the sensor 350 so that the manual reading thereof can then be taken. Due to close tolerances between the D-tube and the housing 302, the stick 318 will tend to stay adjacent the D-tube during a reading, but should not hang-up and can be manually moved or readjusted in height until the operator feels the spool engages the float magnet 306. After the reading is taken, the D-tube assembly can be pushed back up into the housing 302. As the D-tube is reinserted, the free end 356 of the magnetostrictive sensor 350 will slide back into the bore 386 in the spool 382 and the stick will reengage with the cup 336, and the stick gauge will move integrally with the D-tube back up into the housing 302. The stick gauge reading can also be taken with the assembly 308 completely removed from the housing 302 if so desired. 
     While the invention has been shown and described with respect to specific embodiments thereof, this is for the purpose of illustration rather than limitation, and other variations and modifications of the specific embodiments herein shown and described will be apparent to those skilled in the art within the intended spirit and scope of the invention as set forth in the appended claims.