Electronic weighing apparatus utilizing surface acoustic waves

• US 6,448,513 B1
• Filed: 02/02/2001
• Issued: 09/10/2002
• Est. Priority Date: 02/02/2001
• 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:
  US09/775,748
• Publication Number:
  US 20020104689A1
• Time in Patent Office: 585 Days
• Field of Search: 738/625.9, 735/79, 735/80, 73D/IG.4, 177/210.R, 177/210.FP, 331/65, 331/66, 331/47
• US Class Current: 177/210.00R
• CPC Class Codes: G01G 3/13 having piezoelectric or piezoresistive properties
• CPC Class Codes: Y10S 73/04 Piezoelectric

Abstract

A weighing apparatus includes a SAW oscillator and a “push oscillator” to force the SAW oscillator into a desired mode of operation. A SAW temperature oscillator and a thermistor are also provided. The frequency of the “push oscillator” is made immune to temperature changes by generating it via a mixer mixing the SAW temperature oscillator with an adjustable fixed frequency oscillator. Long term stability of the SAW temperature oscillator is achieved by periodic calibration with the thermistor.

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;

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

c) a second piezoelectric transducer having a second substrate and the other of said first SAW transmitter and said 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 push oscillator coupled to said first SAW receiver for altering said first output frequency, 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, further comprising:

g) switching means for periodically activating said push oscillator.

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

g) a second SAW receiver on one of said first and second substrates;

h) a second SAW transmitter on the same substrate as the second SAW receiver;

i) 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, said second SAW amplifier output being indicative of temperature.

4. An electronic weighing apparatus according to claim 3, further comprising:

j) an adjustable oscillator having an output;

k) a mixer having a first input, a second input, and an output, said first input being coupled to said output of said adjustable oscillator, said second input being coupled to said output of said second SAW amplifier, wherein

said output of said mixer provides an index frequency for said push oscillator.

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

said push oscillator includes,

i) an adjustable oscillator having an output,

ii) a mixer having a first input, a second input, and an output, said first input being coupled to said output of said adjustable oscillator, said second input being coupled to said output of said second SAW amplifier, and

iii) a modulator having an input and an input, said input of said modulator being coupled to said mixer and said output of said modulator being the output of said push oscillator.

6. An electronic weighing apparatus according to claim 3, further comprising:

j) a thermistor having an output indicative of temperature; and

k) comparison means coupled to said thermistor and said second SAW amplifier for comparing the temperature indication of said thermistor with the temperature indication of said second SAW amplifier.

7. An electronic apparatus for measuring displacement, comprising:

a) a first piezoelectric transducer having a first substrate and one of a first surface acoustic wave (SAW) transmitter and a first SAW receiver;

b) a second piezoelectric transducer having a second substrate and the other of said first SAW transmitter and said first SAW receiver, said second piezoelectric transducer being mounted in close proximity to said first piezoelectric transducer;

c) 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;

d) processor means coupled to said output of said first amplifier; and

e) a push oscillator coupled to said first SAW receiver for altering said first output frequency, wherein

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 magnitude of the displacement.

8. An electronic apparatus according to claim 7, further comprising:

f) switching means for periodically activating said push oscillator.

9. An electronic weighing apparatus according to claim 7, further comprising:

f) a second SAW receiver on one of said first and second substrates;

g) a second SAW transmitter on the same substrate as the second SAW receiver;

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, said second SAW amplifier output being indicative of temperature.

10. An electronic apparatus according to claim 9, further comprising:

i) an adjustable oscillator having an output;

j) a mixer having a first input, a second input, and an output, said first input being coupled to said output of said adjustable oscillator, said second input being coupled to said output of said second SAW amplifier, wherein

said output of said mixer provides an index frequency for said push oscillator.

11. An electronic apparatus according to claim 9, wherein:

said push oscillator includes,

i) an adjustable oscillator having an output,

ii) a mixer having a first input, a second input, and an output, said first input being coupled to said output of said adjustable oscillator, said second input being coupled to said output of said second SAW amplifier, and

iii) a modulator having an input and an input, said input of said modulator being coupled to said mixer and said output of said modulator being the output of said push oscillator.

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

i) a thermistor having an output indicative of temperature; and

j) comparison means coupled to said thermistor and said second SAW amplifier for comparing the temperature indication of said thermistor with the temperature indication of said second SAW amplifier.

13. A method for improving the phase linearity of a SAW wave oscillator in an electronic weighing apparatus having a first amplifier having an input coupled to a first SAW receiver and an output coupled to a first SAW transmitter, said method comprising:

periodically injecting an RF signal into the first SAW receiver in order to change the mode of oscillation to the mode having the best phase linearity.

14. A method according to claim 13, wherein:

the RF signal is approximately ten times stronger than the signal produced by the first amplifier.

15. A method according to claim 13, wherein:

the RF signal is injected for approximately 0.01 second every time a weight measurement is made.

16. A method according to claim 13, wherein the weighing apparatus has a second amplifier having an input coupled to a second SAW receiver and an output coupled to a second SAW transmitter with the output of the second amplifier being indicative of temperature, said method further comprising:

generating the RF signal by mixing the output of the second amplifier with the output of a tunable oscillator.

17. A method according to claim 16, further comprising:

calibrating the RF signal by tuning the tunable oscillator.

18. A method according to claim 16, further comprising:

calibrating the output of the second amplifier to the output of a thermistor.

19. A method for improving the phase linearity of a SAW wave oscillator in an electronic apparatus for measuring displacement having a first amplifier having an input coupled to a first SAW receiver and an output coupled to a first SAW transmitter, said method comprising:

periodically injecting an RF signal into the first SAW receiver in order to change the mode of oscillation to the mode having the best phase linearity.

20. A method according to claim 19, wherein:

the RF signal is approximately ten times stronger than the signal produced by the first amplifier.

21. A method according to claim 20, wherein:

the RF signal is injected briefly every time a weight measurement is made.

22. A method according to claim 19, wherein the apparatus has a second amplifier having an input coupled to a second SAW receiver and an output coupled to a second SAW transmitter with the output of the second amplifier being indicative of temperature, said method further comprising:

generating the RF signal by mixing the output of the second amplifier with the output of a tunable oscillator.

23. A method according to claim 22, further comprising:

calibrating the RF signal by tuning the tunable oscillator.

24. A method according to claim 22, further comprising:

calibrating the output of the second amplifier to the output of a thermistor.