Abstract:
A weighing apparatus which includes a SAW oscillator ( 120 b,  122 b) and a “push oscillator” ( 216, 218, 220 ) to force the SAW oscillator into a desired mode of operation. The device adjusts to temperature by using a temperature sensing SAW transducer assembly ( 124, 126 ). Long term temperature stability of the SAW frequency oscillator is achieved by periodic calibration with a thermistor ( 224 ) signal.

Description:
[0001]    This application is related to allowed co-owned application Ser. No. 09/327,707 filed Jun. 9, 1999, Ser. No. 08/729,752 filed Oct. 7, 1996, now U.S. Pat. No. 5,910,647, and Ser. No. 08/489,365 filed Jun. 12, 1995, now U.S. Pat. No. 5,663,531, the complete disclosures of which are hereby incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to electronic weighing devices. More particularly, the invention relates to an electronic weighing device which employs surface acoustic waves to measure weight.  
           [0004]    2. State of the Art  
           [0005]    Precision electronic weighing devices are widely known in the art and there are many different technologies utilized in these electronic weighing devices. Laboratory scales or “balances” typically have a capacity of about 1,200 grams and a resolution of about 0.1 gram, although scales with the same resolution and a range of 30,000 grams are available. The accuracy of these scales is achieved through the use of a technology known as magnetic force restoration. Generally, magnetic force restoration involves the use of an electromagnet to oppose the weight on the scale platform. The greater the weight on the platform, the greater the electrical current needed to maintain the weight. While these scales are very accurate (up to one part in 120,000), they are expensive and very sensitive to ambient temperature. In addition, their range is relatively limited.  
           [0006]    Most all other electronic weighing devices use load cell technology. In load cell scales, the applied weight bends an elastic member which has strain gauges bonded to its surface. The strain gauge is a fine wire which undergoes a change in electrical resistance when it is either stretched or compressed. A measurement of this change in resistance yields a measure of the applied weight. Load cell scales are used in non-critical weighing operations and usually have a resolution of about one part in 3,000. The maximum resolution available in a load cell scale is about one part in 10,000 which is insufficient for many critical weighing operations. However, load cell scales can have a capacity of several thousand pounds.  
           [0007]    While there have been many improvements in electronic weighing apparatus, there remains a current need for electronic weighing apparatus which have enhanced accuracy, expanded range, and low cost.  
           [0008]    The previously incorporated applications disclose an electronic weighing apparatus having a base which supports a cantilevered elastic member upon which a load platform is mounted. The free end of the elastic member is provided with a first piezoelectric transducer and a second piezoelectric transducer is supported by the base. Each transducer includes a substantially rectangular piezoelectric substrate and a pair of electrodes imprinted on the substrate at one end thereof, with one pair of electrodes acting as a transmitter and the other pair of electrodes acting as a receiver. The transducers are arranged with their substrates substantially parallel to each other with a small gap between them and with their respective electrodes in relatively opposite positions. The receiver electrodes of the second transducer are coupled to the input of an amplifier and the output of the amplifier is coupled to the transmitter electrodes of the first transducer. The transducers form a “delay line” and the resulting circuit of the delay line and the amplifier is a positive feedback loop, i.e. a natural oscillator. More particularly, the output of the amplifier causes the first transducer to emit a surface acoustic wave (“SAW”) which propagates along the surface of the first transducer substrate away from its electrodes. The propagating waves in the first transducer induce an oscillating electric field in the substrate which in turn induces similar SAW waves on the surface of the second transducer substrate which propagate in the same direction along the surface of the second transducer substrate toward the electrodes of the second transducer. The induced waves in the second transducer cause it to produce an alternating voltage which is supplied by the electrodes of the second transducer to the amplifier input. The circuit acts as a natural oscillator, with the output of the amplifier having a particular frequency which depends on the physical characteristics of the transducers and their distance from each other, as well as the distance between the respective electrodes of the transducers.  
           [0009]    When a load is applied to the load platform, the free end of the cantilevered elastic member moves and causes the first transducer to move relative to the second transducer. The movement of the first transducer relative to the second transducer causes a change in the frequency at the output of the amplifier. The movement of the elastic member is proportional to the weight of the applied load and the frequency and/or change in frequency at the output of the amplifier can be calibrated to the displacement of the elastic member. The frequency response of the delay line is represented by a series of lobes. Each mode of oscillation is defined as a frequency where the sum of the phases in the oscillator is an integer multiple of 27. Thus, as the frequency of the oscillator changes, the modes of oscillation move through the frequency response curve and are separated from each other by a phase shift of 2X. The mode at which the oscillator will most naturally oscillate is the one having the least loss. The transducers are arranged such that their displacement over the weight range of the weighing apparatus causes the oscillator to oscillate in more than one mode. Therefore, the change in frequency of the oscillator as plotted against displacement of the transducers is a periodic function. There are several different ways of determining the cycle of the periodic function so that the exact displacement of the elastic member may be determined.  
           [0010]    It is generally known in the art of SAW technology that the frequency range in which the losses are the lowest is not necessarily the frequency range in which the oscillator exhibits the best phase linearity. From the teachings of the previously incorporated applications, those skilled in the art will appreciate that in a SAW displacement transducer such as disclosed in the previously incorporated applications, better phase linearity provides a more linear relationship between frequency and displacement. In the case of a weighing apparatus using a SAW displacement transducer as described in the previously incorporated applications, better phase linearity will result in a more linear relationship between weight and frequency.  
           [0011]    It is known in the art of SAW oscillators that changing the topology of the oscillator transmitter and receiver can cause a broader bandwidth of the delay line and that a broader bandwidth results in better phase linearity. It is also known that using a smaller frequency range provides better linearity and that a smaller frequency range can be obtained with a longer delay line. Although these known methods can increase phase linearity in a SAW oscillator, the frequency range in which the best linearity is achieved for a particular oscillator is still not necessarily the range with the lowest losses.  
           [0012]    From the foregoing, those skilled in the art will appreciate that in order to enhance the accuracy of a SAW displacement transducer such as that used in a weighing device, it would be desirable to cause the SAW oscillator to oscillate in the range having the best phase linearity.  
           [0013]    As disclosed in the previously incorporated applications, weighing accuracy is affected by temperature. The previously incorporated applications disclose a SAW temperature oscillator having a transmitter and receiver on the same substrate. The temperature sensitivity of the load cell disclosed in the previously incorporated applications is approximately 500 ppm of the weight reading per 1° C. temperature change. Accuracy of 100 ppm of the weight reading can be achieved if temperature is measured to within 0.2° C. which represents a shift of about 1 kHz of the SAW temperature sensor. This shift is easy to measure in the short term. The resolution of the SAW temperature sensor is on the order of 0.001° C. However, the long term stability of the SAW temperature sensor can drift more than 1 kHz due to many factors including humidity  
           [0014]    It will also be appreciated that temperature changes can make determination of mode of oscillation more difficult. In the previously incorporated applications, mode determination was determined by switching the phase ±π and noting the frequency change. However, factors such as temperature and non-linearity can make the frequency change very small thereby making the mode determination unreliable  
         SUMMARY OF THE INVENTION  
         [0015]    It is therefore an object of the invention to provide an electronic weighing apparatus which is accurate.  
           [0016]    It is also an object of the invention to provide an electronic weighing apparatus which uses surface acoustic waves and is accurate over a broad range of weights.  
           [0017]    It is another object of the invention to provide an electronic weighing apparatus which is compact and easy to construct.  
           [0018]    It is a further object of the invention to provide an electronic weighing apparatus which is inexpensive to manufacture.  
           [0019]    It is another object of the invention to provide an electronic weighing apparatus which utilizes surface acoustic waves and which has enhanced phase linearity.  
           [0020]    It is still another object of the invention to provide an electronic weighing apparatus which utilizes surface acoustic waves and which is oscillates in the mode of best phase linearity.  
           [0021]    It is yet another object of the invention to provide an electronic weighing apparatus which utilizes surface acoustic waves and has long term temperature stability as well as short term temperature stability.  
           [0022]    In accord with these objects which will be discussed in detail below, the improved weighing apparatus of the present invention includes a base which supports a cantilevered elastic member upon which a load platform is mounted. The interior of the elastic member is hollowed and is provided with first and second piezoelectric transducers which are mounted on respective opposed posts. Each transducer includes a substantially rectangular piezoelectric substrate and a pair of electrodes imprinted on the substrate at one end thereof, with one pair of electrodes acting as a transmitter and the other pair of electrodes acting as a receiver. The transducers are arranged with their substrates substantially parallel to each other with a small gap between them and with their respective electrodes in relatively opposite positions. The receiver electrodes of the second transducer are coupled to the input of an amplifier and the output of the amplifier is coupled to the transmitter electrodes of the first transducer. The transducers form a “delay line” and the resulting circuit of the delay line and the amplifier is a positive feedback loop, i.e. a natural oscillator. More particularly, the output of the amplifier causes the first transducer to emit a surface acoustic wave (“SAW”) which propagates along the surface of the first transducer substrate away from its electrodes. The propagating waves in the first transducer induce an oscillating electric field in the substrate which in turn induces similar SAW waves on the surface of the second transducer substrate which propagate in the same direction along the surface of the second transducer substrate toward the electrodes of the second transducer. The induced waves in the second transducer cause it to produce an alternating voltage which is supplied by the electrodes of the second transducer to the amplifier input. The circuit acts as a natural oscillator, with the output of the amplifier having a particular frequency which depends on the physical characteristics of the transducers and their distance from each other, as well as the distance between the respective electrodes of the transducers.  
           [0023]    According to the invention, when a load is applied to the load platform, the cantilevered elastic member bends and causes the first transducer to move relative to the second transducer. The movement of the first transducer relative to the second transducer causes a change in the frequency at the output of the amplifier. The bending movement of the elastic member is proportional to the weight of the applied load and the frequency and/or change in frequency at the output of the amplifier can be calibrated to the displacement of the elastic member.  
           [0024]    According to one aspect of the invention, a “push oscillator” is coupled to the delay line for injecting a strong RF signal at a frequency in the middle of the oscillation mode which exhibits the best phase linearity. The frequency of the “push oscillator” is determined experimentally when the scale is calibrated. The RF signal is injected periodically in short bursts.  
           [0025]    According to a second aspect of the invention, the “push oscillator” frequency is generated by mixing the temperature oscillator with an adjustable fixed frequency oscillator. This immunizes the “push oscillator” from the affects of temperature.  
           [0026]    According to a third aspect of the invention, a thermistor is provided for long term temperature stability. The SAW temperature sensor is periodically calibrated to the thermistor.  
           [0027]    According to a fourth aspect of the invention, the SAW oscillators are not hermetically sealed and the SAW temperature sensor is used to correct the displacement sensor for changes in humidity.  
           [0028]    According to a fifth aspect, a single amplifier is used to power two SAW sensors (e.g. a weight sensor and a temperature sensor) which oscillate at different frequencies.  
           [0029]    According to a sixth aspect of the invention, a crystal oscillator is used to compensate the push oscillator for temperature changes.  
           [0030]    According to a seventh aspect of the invention, two SAW weight sensors are provided where each oscillates at a different frequency.  
           [0031]    Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is a schematic side elevation view of an exemplary embodiment of the invention;  
         [0033]    [0033]FIG. 1 a  is an enlarged schematic plan view of a first transducer;  
         [0034]    [0034]FIG. 1 b  is an enlarged schematic plan view of a second transducer;  
         [0035]    [0035]FIG. 2 is an enlarged schematic side elevation view of a transducer having anti-reflection structure according to the invention;  
         [0036]    [0036]FIG. 3 is an enlarged schematic side elevation view of a pair of transducers according to the invention;  
         [0037]    [0037]FIGS. 4 and 5 are graphs of a portion of a frequency response curve for a delay line according to the invention showing modes of oscillation and phase shifting according to the invention;  
         [0038]    [0038]FIG. 6 is a simplified schematic diagram of circuits used in the weighing device according to the invention;  
         [0039]    [0039]FIG. 7 is a simplified schematic diagram of a circuit wherein two SAW sensors are powered by a single amplifier;  
         [0040]    [0040]FIG. 8 is a simplified schematic diagram of a circuit used to compensate the push oscillator for temperature;  
         [0041]    [0041]FIG. 9 illustrates the frequency shift of two displacement sensors having different frequencies; and  
         [0042]    [0042]FIG. 10 illustrates the frequency shift of two displacement sensors having different frequencies and arranged in a differential manner. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Referring now to FIGS. 1, 1 a , and  1   b , an electronic weighing apparatus  10  according to the invention includes a base  12  which supports a cantilevered elastic member  14  having a cut-out  15 , and upon which a load platform  16  is mounted. The cut-out  15  is provided with two opposed posts  17 ,  19  upon which are respectively mounted a first piezoelectric transducer  20  and a second piezoelectric transducer  22 . The posts  17 ,  19  serve to locate the transducers  20 ,  22  at the center of the elastic member  14  and to mechanically couple the transducers to opposite ends of the elastic member  14 .  
         [0044]    The first transducer  20  includes a substantially rectangular piezoelectric substrate  20   a  and a pair of electrodes  20   b  imprinted on the substrate at the upper end thereof. The second transducer  22  includes a substantially rectangular piezoelectric substrate  22   a  and a pair of electrodes  22   b  imprinted on the substrate at the lower end thereof. The substrates are preferably made of Lithium Niobate. The transducers are arranged with their substrates substantially parallel to each other with a small gap “g” between them. The electrodes  22   b  of the second transducer  22  are coupled to the input of an amplifier (not shown) powered by a power source (not shown) and the output of the amplifier is coupled to the electrodes  20   b  of the first transducer  20 . The circuit arrangement is the same as shown in the previously incorporated application Ser. No. 08/489,365, previously incorporated herein by reference. The resulting circuit is a positive feedback loop natural oscillator, a “delay line”. The output of the amplifier generates an alternating voltage in the electrodes  20   b  of the first transducer  20  which generates a surface acoustic wave (“SAW”)  26  which propagates along the surface of the first transducer substrate  20   a  away from its electrodes  20   b . Since the substrate  20   a  of the first transducer  20  is relatively close to the substrate  22   a  of the second transducer  22 , an oscillating electric field which is induced as a result of the SAW waves  26  in the piezoelectric substrate  20   a  is able to in turn induce similar SAW waves  28  on the surface of the second transducer substrate  22   a  which propagate in the same direction along the surface of the second transducer substrate toward the electrodes  22   b  of the second transducer  22 . The induced waves  28  in the second transducer  22  cause the electrode  22   b  of the second transducer  22  to produce an alternating voltage which is provided to the input of the amplifier. As long as the gain of the amplifier  24  is larger than the loss of the system, the circuit acts as a natural oscillator with the output of the amplifier having a particular frequency which depends on the physical characteristics of the transducers and their distance from each other, as well as the distance between the respective electrodes of the transducers. In particular, the frequency of the oscillator is directly related to the time it takes for the SAW to propagate from the electrodes  20   b  to the electrodes  22   b.    
         [0045]    According to presently preferred embodiments of the invention, described in more detail below, the SAW  26  has a wavelength of approximately 100-200 microns at 20-50 MHz. In order to limit loss in the system, the gap “g” between the substrates of the first and second transducers is kept small. In one preferred embodiment described below, the gap is 10-20 microns. With such a gap, an oscillating system can typically be generated if the amplifier  24  has a gain of at least approximately 25 dB. It will be appreciated that when a load (not shown) is applied to the load platform  16 , the free end of the cantilevered elastic member  14  moves down and causes the second transducer  22  to move relative to the first transducer  20 . In particular, it causes the electrodes  22   b  of the second transducer  22  to move away from the electrodes  20   b  of the first transducer  20 . This results in a lengthening of the “delay line”. The lengthening of the delay line causes an decrease in the frequency at the output of the amplifier. The displacement of the elastic member is proportional to the weight of the applied load and the frequency or decrease in frequency at the output of the amplifier can be calibrated to the distance moved by the elastic member.  
         [0046]    It will be appreciated that locating the transducers at the center of the elastic member compensates for any torque on the member which would exhibit itself at the free end of the member. This results in an improved accuracy as compared to the weighing instrument disclosed in U.S. Pat. No. 5,663,531. Depending on the application (e.g. maximum load to be weighed), the elastic member is made of aluminum or steel. The presently preferred elastic member exhibits a maximum displacement of 0.1 to 0.2 mm at maximum load.  
         [0047]    Reflected waves may occur on both piezo substrates. Reflected waves interfere with the received signal. The interference causes an increase in non-linearity. FIGS. 2 and 3 show presently preferred anti-reflection structures according to the invention.  
         [0048]    Turning now to FIGS. 2 and 3, presently preferred transducers  120 ,  122  are shown. FIG. 2 illustrates the features of transducer  120  which is substantially identical to transducer  122 . FIG. 3 illustrates the transducers mounted on the posts  17 ,  19  of the elastic member  14  of FIG. 1. As shown in FIG. 2, the transducer  120  includes a lithium niobate substrate  120   a  with electrodes  120   b  imprinted thereon. The ends  120   c ,  120   d  of the substrate are tapered and polyurethane dampers  121   a ,  121   b  are placed at the ends to minimize reflection of the SAW waves.  
         [0049]    As mentioned above and in the previously incorporated application, the delay lines according to the invention may oscillate in more than one mode and within each mode, the gain will vary as the frequency changes. Referring now to FIGS. 4 and 5, in the idle state, with no weight applied to the scale, the delay line will oscillate at a frequency “f” which is shown in FIG. 4 as the point having the most gain (least loss). The optimal gain area of the graph of FIG. 4 is shown in the shaded area surrounding f and represents a range of ±100 Khz, for example. This area is considered optimal because it is the area of least loss. However, it is not necessarily the area of best phase linearity. After experimenting, it may be discovered that oscillation in a different mode, e.g. the shaded area of FIG. 5, will produce better phase linearity. According to one aspect of the invention, the oscillator is forced to oscillate in the mode of best phase linearity by injecting a strong RF signal having a frequency at the midpoint of the desired mode of oscillation. The RF signal is injected by a “push oscillator” coupled to the SAW wave receiver as described in more detail below with reference to FIG. 6. According to the presently preferred embodiment, the RF signal has a strength of approximately 100 mv as compared to the SAW oscillator&#39;s strength of approximately 10 mv. The RF signal is preferably injected for a short time (as short as 0.01 seconds) before each weight measurement.  
         [0050]    As mentioned above, and described in detail in the previously incorporated applications, the effects of temperature can be further corrected by providing a separate SAW temperature sensor on the same substrate as one of the displacement transducers. According to the presently preferred embodiment, the SAW displacement oscillator operates at 55 MHz and the SAW temperature oscillator operates at 57 MHz. According to another aspect of the invention, described in more detail below with reference to FIG. 6, the temperature oscillator is used in conjunction with an adjustable 2 MHz oscillator and a mixer to produce the “push oscillator” frequency and automatically adjust the “push oscillator” frequency for temperature changes.  
         [0051]    Turning now to FIG. 6, an exemplary circuit  200  according to the invention includes the displacement SAW transducer formed by the transmitter  122   b  on the substrate  122  and the receiver  120   b  on the substrate  120  coupled to each other by the amplifier  202 . In addition, the circuit includes a temperature SAW transducer formed by the transmitter  124  and receiver  126  on the substrate  122  coupled to each other by the amplifier  204 . The output of amplifier  202  is a frequency Fw which varies according to displacement of the substrates relative to each other, which is an indication of weight when the transducers are arranged as shown in FIG. 1. According to the presently preferred embodiment, the frequency Fw is nominally 54 MHz. Fw will also vary according to temperature. The output of amplifier  204  is a frequency Ft which varies only according to temperature and humidity and which is nominally 57 MHz. The frequencies Fw and Ft are mixed (subtracted) at the mixer  206  to produce a nominal frequency of 3 MHz which varies according to weight and which is temperature compensated. The output frequency of the mixer  206  is input to a microprocessor  208  which calculates weight as described in the previously incorporated applications and displays the weight on display  210 . According to the presently preferred embodiment, the output Ft of amplifier  204  is also mixed via mixer  212  with a 54 MHz signal from oscillator  214  to produce a signal which is nominally 3 MHz and which varies only with temperature and humidity. The signal Fw-Ft provides a temperature adjusted weight signal which accounts for the affects of temperature on the SAW oscillators. It does not compensate for temperature effects on the Youngs modulus of the elastic member ( 14  in FIG. 1). The signal output from mixer  212  is a pure temperature indicator and is used to adjust the weight calculation for the effects of temperature on the Youngs modulus of the elastic member.  
         [0052]    According to one aspect of the present invention, a “push oscillator” is formed from an adjustable oscillator  216 , a mixer  218 , and a modulator  220 . The oscillator  216  has a nominal frequency of 2 MHz which is mixed via the mixer  218  with the output of amplifier  204  to produce an output frequency Fi which is (Ft—approx. 2 MHz). This frequency Fi is used to index the modulator  220  which produces the “push oscillator” output to the input of amplifier  202 . As shown in FIG. 6, the modulator  220  and the oscillator  216  are both coupled to the microprocessor  208 . The microprocessor  208  is programmed to periodically activate the modulator  220  to inject the push frequency as described above. In addition, the microprocessor advantageously is utilized to adjust the oscillator  216  to determine the frequency of the “push oscillator”. The oscillator  216  may be initially adjusted via a simple variable resistor or variable capacitor. However, it is further adjusted by the microprocessor during operation of the scale. One of these advantages is that the microprocessor can adjust the oscillator  216  to produce the phase shifting described in the previously incorporated applications. In addition, it can be used to produce much larger frequency shifts than were possible in the previously incorporated applications. This results in more accurate determinations of which weight range the scale is in. As described in the previously incorporated applications, the oscillator operated as a periodic function where the same frequencies were repeated over different weight ranges. A phase shift of ±π was used to determine which weight range the scale was operating in. As the weight increased, the same phase shift produced a larger frequency shift (because of the increased length of the delay line) and the frequency shift could be used to determine the weight range. However, under some circumstances, the phase shift resulted in a frequency shift which was too small to accurately determine. The push oscillator of the present invention can be used to produce ±4π phase shifts.  
         [0053]    As mentioned above, the oscillator  216  is preferably initially adjusted with a variable resistor or variable capacitor to ensure oscillation on the mode of best phase linearity. Initial calibration is performed as follows: Known weights are placed on the scale and the frequency of the oscillator output is determined for different weights and the modes of oscillation are noted. The push oscillator is tuned to operate in one mode and experiments are conducted to measure linearity. The experiments are repeated for each mode. The push oscillator is then tuned to push to the mode of best linearity.  
         [0054]    According to another aspect of the invention, a thermistor  224  is coupled via an analog to digital converter  226  to the microprocessor  208 . The thermistor is used to provide long term calibration of the SAW temperature transducer. At first calibration measurements are taken from both the SAW temperature sensor and the thermistor assuming that the true temperature is the thermistor reading. Measurements are taken over a range of 20 or 40° C. and the slope of the SAW temperature sensor output is calculated in HzI° C. This slope is nominally approximately 5,000 Hz/° C. using an oscillator having a 57 MHz central frequency. Periodically, this calibration is repeated to account for the long term instability of the SAW temperature sensor.  
         [0055]    Although one of the earlier applications proposed hermetically sealing sensors, it has been determined that the effects of humidity are accurately accounted for with the SAW temperature sensor. Therefore, it has been determined to be desirable to expose both the displacement transducer and the temperature transducer to the temperature and humidity of the ambient atmosphere.  
         [0056]    As described above and in the previously incorporated related patents, a scale according to the invention will typically use two or more SAW devices. One may be a temperature sensor and the other a weight sensor. Normally, each has its own amplifier to form an oscillator. The temperature sensor is typically be used to compensate for temperature and other changes that occur with the weight sensor. However, the temperature sensor can not compensate for differences caused by differences in the amplifier and other devices in the oscillator.  
         [0057]    Referring now to FIG. 7, it is possible to connect two SAW sensors in parallel to a single amplifier. As shown in FIG. 7, a weight sensor having transducers  300 ,  302  is coupled in parallel with a temperature sensor having transducers  304 ,  306  to a single amplifier  308 . Each sensor oscillates at a somewhat different center frequency, such as 50 Mhz and 60 Mhz, respectively. A push oscillator  310  controlled by a processor control  312  is arranged to force the oscillator to either 50 Mhz or 60 Mhz. When it forces the oscillator to 50 Mhz, and is then shut off, the first SAW sensor will control the frequency of oscillation output at  314 . When the push oscillator forces a frequency of 60 Mhz and then shuts off, the second SAW sensor will control the frequency of oscillation output at  314 . As the same amplifier is shared by both SAW sensors, errors caused by differences in the amplifiers used by weight and temperature sensors will be automatically compensated for.  
         [0058]    As described above, a push oscillator can be used to force the SAW oscillator to operate in the preferred area of oscillation. However, as the ambient temperature changes, the preferred frequency of oscillation can also change. It would therefore be advantageous to change the frequency of the push oscillator as the temperature changes. One method of doing this is to measure the temperature, either with a SAW temperature sensor or some other temperature sensor and adjust the push oscillator, if the push oscillator is a phase locked loop or some other controllable type.  
         [0059]    Another method, illustrated in FIG. 8, is to use the SAW temperature sensor  406 ,  404 ,  408  along with a fixed oscillator, such as a crystal oscillator  410 . If the frequencies are properly chosen, and the two outputs are mixed with a mixer  412 , the resultant frequency output at  414  can be used as the push oscillator for the SAW weight sensor. For example, with the SAW temperature sensor center frequency at 54 Mhz, and the SAW weights sensor center frequency at 50 Mhz, the fixed crystal oscillator should have a 4 Mhz frequency. When the temperature oscillator and the crystal oscillator outputs are mixed in a mixer, the output is centered at 50 Mhz, but will vary with temperature.  
         [0060]    As described above and in the previously incorporated patents, in a typical scale the SAW device(s) will be displaced by more than one wavelength. Thus, there will be certain weights which result in identical frequencies of oscillation. A number of methods have been described as to how to determine which “zone” the oscillator is operating in. One method was to use a completely different “coarse” sensor, such as strain gages, or inductive or capacitive sensors. These gives a coarse indication of weight, and therefore determines which “zone” the SAW sensor was operating in.  
         [0061]    According to the present invention two weight sensors are provided, each oscillating at a different frequency. Although both sensors will be displaced by more than one wavelength, they will differ in wavelength. FIG. 9 illustrates the frequency change of two weight sensors operating at different frequencies of oscillation, one shown in a solid line and the other shown in a dashed line. Those skilled in the art will appreciate that at any given weight, the pair of frequency values will be unique.  
         [0062]    As described above and in the previously incorporated patents, a differential pair of SAW weight sensors may be used to improve resolution. When weight increases on the scale platform, one sensor will have an increased distance between the transmitter and receiver, and the other will have a decreased distance. This is best shown in FIGS.  9 - 11  of previously incorporated U.S. Pat. No. 5,910,647. According to the present invention, this arrangement can also be used to locate the zone of oscillation by using different frequencies for each sensor.  
         [0063]    A new method according to the invention is to use different frequencies for each of two or more SAW weight sensors in a differential format. FIG. 10 illustrates the change in frequency of two differential oscillators operating at different frequencies.  
         [0064]    There have been described and illustrated herein several embodiments of an electronic weighing apparatus utilizing surface acoustic waves. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while particular frequencies have been disclosed, it will be appreciated that other frequencies could be utilized. Moreover, while particular configurations have been disclosed in reference to the location of transmitting and receiving electrodes, it will be appreciated that the respective locations of transmitters and receivers could be reversed. Furthermore, while several different aspects of the invention have been disclosed as solving various problems, it will be understood that the different aspects of the invention may be used alone or in combination with each other in configurations other than those shown. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as so claimed.