Electronic weighing apparatus utilizing surface acoustic waves

• US 5,910,647 A
• Filed: 10/07/1996
• Issued: 06/08/1999
• Est. Priority Date: 06/12/1995
• Status: Expired due to Term

• Current Assignee: Circuits & Systems, Inc.
• Original Assignee: Circuits & Systems, Inc.
• Inventors: Kats, Vyacheslav D., Gordon, Arnold S.
• Primary Examiner(s): Gibson, Randy W.
• Application Number:
  US08/729,752
• Time in Patent Office: 974 Days
• Field of Search: 177/210 R, 177/210 FP, 177/211, 177/225, 177/226, 177/227, 177/228, 177/229, 177/210 C, 73/862.59, 73/778, 73/DIG. 4
• US Class Current: 177/210.0FP
• CPC Class Codes: G01G 3/13 having piezoelectric or piezoresistive properties

Abstract

A weighing apparatus includes a base supporting a cantilevered elastic member bearing a load platform. The interior of the elastic member is hollowed and is provided with first and second piezoelectric transducers mounted on respective opposed posts. The transducers are arranged substantially parallel to each other with a small gap between them and are coupled to an amplifier to form a "delay line" and a positive feedback loop, i.e. a natural oscillator. According to various aspects of the invention, one or both substrates is provided with anti-reflection structure; the transducers are arranged on overlapping substrates to further reduce reflection; the transducers are coupled to a thermal sink to reduce the effects of thermal gradients across the transducers; two pairs of transducers are provided and arranged to provide a pair of oppositely polarized signals which doubles the accuracy of measurements and also compensates for the effects of temperature gradients; a thermal transducer channel is provided on the same substrate to measure the effects of temperature gradients and thereby compensate for temperature effects; a pair of differential transducers is arranged to measure the effects of temperature changes in the same acoustic channel in which displacement measurements are made; and a phase shift is applied to the output of the amplifier(s) to improve gain.

Claims

1. An electronic weighing apparatus, comprising:

a) a displaceable elastic member means for receiving a load and being displaced by the load such that the displacement of said elastic member means is related to the weight of the load, said elastic member having a hollowed central portion;

b) a first piezoelectric transducer having a first substrate and a first surface acoustic wave (SAW) transmitter, said first piezoelectric transducer being coupled to said elastic member within said hollowed central portion;

c) a second piezoelectric transducer having a second substrate and a first SAW receiver, said second piezoelectric transducer being mounted in close proximity to said first piezoelectric transducer such that said displacement of said elastic member causes a corresponding displacement of one of said first and second piezoelectric transducers relative to each other;

d) a first amplifier having an input and an output, said input of said first amplifier being coupled to said first SAW receiver and said output of said first amplifier being coupled to said first SAW transmitter such that said first piezoelectric transducer, said first amplifier, and said second piezoelectric transducer form a first oscillator having a first output frequency; and

e) processor means coupled to said output of said first amplifier, wherein

displacement of said elastic member means causes a displacement of one of said first and second piezoelectric transducers relative to each other and thereby changes said first output frequency, and said first output frequency is used by said processor means to determine an indication of the weight of the load.

2. An electronic weighing apparatus according to claim 1, wherein:

one of said first and second piezoelectric transducers is provided with anti-reflection structure to minimize reflection of surface acoustic waves.

3. An electronic weighing apparatus according to claim 2, wherein:

said anti-reflection structure is one of a surface damper on said substrate, an angled cut on an end of said substrate, and a rounded end on said substrate.

4. An electronic weighing apparatus according to claim 1, further comprising:

f) a third piezoelectric transducer having a second surface acoustic wave (SAW) transmitter, said third piezoelectric transducer being coupled to said elastic member within said hollowed central portion;

g) a fourth piezoelectric transducer having a second SAW receiver, said fourth piezoelectric transducer being mounted in close proximity to said first piezoelectric transducer such that said displacement of said elastic member causes a corresponding displacement of one of said third and fourth piezoelectric transducers relative to each other, said displacement being in a direction opposite to and in an amount substantially equal to said relative displacement of said first and second piezoelectric transducers relative to each other;

h) a second amplifier having an input and an output, said input of said second amplifier being coupled to said second SAW receiver and said output of said second amplifier being coupled to said second SAW transmitter such that said third piezoelectric transducer, said second amplifier, and said fourth piezoelectric transducer form a second oscillator having a second output frequency, said output of said second amplifier being coupled to said processor means, wherein

displacement of said elastic member means causes a displacement of one of said third and fourth piezoelectric transducers relative to each other and thereby changes said second output frequency by an opposite but substantially equal amount as said change in said first output frequency, said first and second output frequencies being used by said processor means to determine an indication of the weight of the load.

5. An electronic weighing apparatus according to claim 1, wherein:

one of said first and second substrates is mounted on a thermal sink.

6. An electronic weighing apparatus according to claim 5, wherein:

both of said first and second substrates are mounted on thermal sinks.

7. An electronic weighing apparatus according to claim 5, wherein:

said thermal sink is mounted on a thermally insulating material.

8. An electronic weighing apparatus according to claim 1, wherein:

one of said first and second transducers includes temperature sensing means on said substrate for determining the temperature of said substrate, said temperature sensing means being coupled to said processing means and said temperature being used by said processing means to determine an indication of the weight of the load.

9. An electronic weighing apparatus according to claim 8, wherein:

said temperature sensing means includes a SAW transmitter and a SAW receiver.

10. An electronic weighing apparatus according to claim 8, wherein:

said temperature sensing means is on said first substrate and comprises a SAW receiver.

11. An electronic weighing apparatus according to claim 8, wherein:

said temperature sensing means is on said second substrate and comprises a SAW transmitter.

12. An electronic weighing apparatus according to claim 9, wherein:

said temperature sensing means is located in the same acoustic channel as said first SAW transmitter.

13. An electronic weighing apparatus according to claim 1, further comprising:

f) phase shift means coupled to said first amplifier for shifting the phase of said first output frequency by approximately 180°

; and

g) one of frequency and gain detection means coupled to said output of said first amplifier and coupled to said phase shift means for activating said phase shift means upon the detection of a predetermined one of frequency and gain.

14. An electronic weighing apparatus, comprising:

a) a displaceable elastic member means for receiving a load and being displaced by the load such that the displacement of said elastic member means is related to the weight of the load;

b) a first piezoelectric transducer having a first substrate and a first surface acoustic wave (SAW) transmitter, said first piezoelectric transducer being coupled to said elastic member;

c) a second piezoelectric transducer having a second substrate and a first SAW receiver, said second piezoelectric transducer being mounted in close proximity to said first piezoelectric transducer such that said displacement of said elastic member causes a corresponding displacement of one of said first and second piezoelectric transducers relative to each other;

d) a first amplifier having an input and an output, said input of said first amplifier being coupled to said first SAW receiver and said output of said first amplifier being coupled to said first SAW transmitter such that said first piezoelectric transducer, said first amplifier, and said second piezoelectric transducer form a first oscillator having a first output frequency; and

e) processor means coupled to said output of said first amplifier, wherein

displacement of said elastic member means causes a displacement of one of said first and second piezoelectric transducers relative to each other and thereby changes said first output frequency, and said first output frequency is used by said processor means to determine an indication of the weight of the load, and wherein

one of said first and second piezoelectric transducers is provided with anti-reflection structure to minimize reflection of surface acoustic waves.

15. An electronic weighing apparatus, comprising:

a) a displaceable elastic member means for receiving a load and being displaced by the load such that the displacement of said elastic member means is related to the weight of the load;

b) a first piezoelectric transducer having a first substrate and a first surface acoustic wave (SAW) transmitter, said first piezoelectric transducer being coupled to said elastic member;

c) a second piezoelectric transducer having a second substrate and a first SAW receiver, said second piezoelectric transducer being mounted in close proximity to said first piezoelectric transducer such that said displacement of said elastic member causes a corresponding displacement of one of said first and second piezoelectric transducers relative to each other;

d) a first amplifier having an input and an output, said input of said first amplifier being coupled to said first SAW receiver and said output of said first amplifier being coupled to said first SAW transmitter such that said first piezoelectric transducer, said first amplifier, and said second piezoelectric transducer form a first oscillator having a first output frequency;

e) processor means coupled to said output of said first amplifier, and

f) a third piezoelectric transducer having a second surface acoustic wave (SAW) transmitter, said third piezoelectric transducer being coupled to said elastic member within said hollowed central portion;

g) a fourth piezoelectric transducer having a second SAW receiver, said fourth piezoelectric transducer being mounted in close proximity to said first piezoelectric transducer such that said displacement of said elastic member causes a corresponding displacement of one of said third and fourth piezoelectric transducers relative to each other, said displacement being in a direction opposite to and in an amount substantially equal to said relative displacement of said first and second piezoelectric transducers relative to each other;

h) a second amplifier having an input and an output, said input of said second amplifier being coupled to said second SAW receiver and said output of said second amplifier being coupled to said second SAW transmitter such that said third piezoelectric transducer, said second amplifier, and said fourth piezoelectric transducer form a second oscillator having a second output frequency, said output of said second amplifier being coupled to said processor means, wherein

displacement of said elastic member means causes a displacement of one of said first and second piezoelectric transducers relative to each other and thereby changes said first output frequency, and said first output frequency is used by said processor means to determine an indication of the weight of the load, and, wherein

displacement of said elastic member means causes a displacement of one of said third and fourth piezoelectric transducers relative to each other and thereby changes said second output frequency by an opposite but substantially equal amount as said change in said first output frequency, said first and second output frequencies being used by said processor means to determine an indication of the weight of the load.

16. An electronic weighing apparatus, comprising:

a) a displaceable elastic member means for receiving a load and being displaced by the load such that the displacement of said elastic member means is related to the weight of the load;

b) a first piezoelectric transducer having a first substrate and a first surface acoustic wave (SAW) transmitter, said first piezoelectric transducer being coupled to said elastic member;

c) a second piezoelectric transducer having a second substrate and a first SAW receiver, said second piezoelectric transducer being mounted in close proximity to said first piezoelectric transducer such that said displacement of said elastic member causes a corresponding displacement of one of said first and second piezoelectric transducers relative to each other;

d) a first amplifier having an input and an output, said input of said first amplifier being coupled to said first SAW receiver and said output of said first amplifier being coupled to said first SAW transmitter such that said first piezoelectric transducer, said first amplifier, and said second piezoelectric transducer form a first oscillator having a first output frequency; and

e) processor means coupled to said output of said first amplifier, wherein

displacement of said elastic member means causes a displacement of one of said first and second piezoelectric transducers relative to each other and thereby changes said first output frequency, and said first output frequency is used by said processor means to determine an indication of the weight of the load, and wherein

one of said first and second transducers includes temperature sensing means on said substrate for determining the temperature of said substrate, said temperature sensing means being coupled to said processing means and said temperature being used by said processing means to determine an indication of the weight of the load.

17. An electronic weighing apparatus according to claim 16, wherein:

said temperature sensing means includes a SAW transmitter and a SAW receiver.

18. An electronic weighing apparatus according to claim 16, wherein:

said temperature sensing means is on said first substrate and comprises a SAW receiver.

19. An electronic weighing apparatus according to claim 16, wherein:

said temperature sensing means is on said second substrate and comprises a SAW transmitter.

20. An electronic weighing apparatus according to claim 17, wherein:

said temperature sensing means is located in the same acoustic channel as said first SAW transmitter.

21. An electronic weighing apparatus, comprising:

a) a displaceable elastic member means for receiving a load and being displaced by the load such that the displacement of said elastic member means is related to the weight of the load;

b) a first piezoelectric transducer having a first substrate and a first surface acoustic wave (SAW) transmitter, said first piezoelectric transducer being coupled to said elastic member;

c) a second piezoelectric transducer having a second substrate and a first SAW receiver, said second piezoelectric transducer being mounted in close proximity to said first piezoelectric transducer such that said displacement of said elastic member causes a corresponding displacement of one of said first and second piezoelectric transducers relative to each other;

d) a first amplifier having an input and an output, said input of said first amplifier being coupled to said first SAW receiver and said output of said first amplifier being coupled to said first SAW transmitter such that said first piezoelectric transducer, said first amplifier, and said second piezoelectric transducer form a first oscillator having a first output frequency;

e) processor means coupled to said output of said first amplifier;

f) phase shift means coupled to said first amplifier for shifting the phase of said first output frequency by approximately 180°

; and

g) one of frequency and gain detection means coupled to said output of said first amplifier and coupled to said phase shift means for activating said phase shift means upon the detection of a predetermined one of frequency and gain, wherein

displacement of said elastic member means causes a displacement of one of said first and second piezoelectric transducers relative to each other and thereby changes said first output frequency, and said first output frequency is used by said processor means to determine an indication of the weight of the load.