Dew point sensor using mems

• US 6,126,311 A
• Filed: 11/02/1998
• Issued: 10/03/2000
• Est. Priority Date: 11/02/1998
• Status: Expired due to Fees

• Current Assignee: Watlow Electric Manufacturing Company
• Original Assignee: Claud S. Gordon Company
• Inventors: Schuh, William C.
• Primary Examiner(s): Bennett, G. Bradley
• Assistant Examiner(s): VERBITSKY, GAIL KAPLAN
• Application Number:
  US09/184,359
• Time in Patent Office: 701 Days
• Field of Search: 374/21, 374/28, 374/16, 374/118, 73/31.1, 73/579
• US Class Current: 374/21
• CPC Class Codes: G01N 25/68 by varying the temperature of a condensing surface
• CPC Class Codes: G01N 5/025 for determining moisture content

Abstract

A dew point sensor includes a micro-cantilever beam formed on a substrate. A cooling device and a temperature sensor are in thermal contact with the micro-cantilever beam and a control circuit is coupled to the micro-cantilever beam, the cooling device and the temperature sensor. The control circuit controls and monitors a resonance of the micro-cantilever beam, controls a temperature of the cooling device and receives a temperature indication from the temperature sensor. Using the above information, the dew point temperature is determined by identifying the temperature of the micro-cantilever beam when its resonance changes due to a change in mass caused by the formation of dew on the micro-cantilever beam when the dew point is reached. A method of determining the dew point is also disclosed. The method includes exciting a micro-cantilever beam in a vibratory mode and decreasing a temperature of the micro-cantilever beam. When the temperature of the micro-cantilever beam reaches the dew point, dew forms thereon, thus changing its mass which causes a resonance of the micro-cantilever beam to change. The resonance of the beam is then monitored to detect the dew point.

Claims

1. A dew point sensor, comprising:

a micro-cantilever beam formed on a substrate;

a cooling device in thermal contact with the micro-cantilever beam;

a temperature sensor in thermal contact with the micro-cantilever beam; and

a control circuit coupled to the micro-cantilever beam, the cooling device and the temperature sensor, wherein the control circuit is operable to monitor a resonance of the micro-cantilever beam, control a temperature of the cooling device and receive a temperature indication from the temperature sensor, thereby identifying the temperature of the micro-cantilever beam when the resonance of the micro-cantilever beam changes due to a change in mass caused by dew on the micro-cantilever beam when the dew point is reached.

2. The dew point sensor of claim 1, further comprising a heating device in thermal contact with the micro-cantilever beam, thereby providing for a temperature cycling of the micro-cantilever beam in response to the control circuit.

3. The dew point sensor of claim 1, further comprising a conversion circuit for converting a dew point temperature to relative humidity.

4. The dew point sensor of claim 1, wherein the cooling device comprises a thermoelectric chiller.

5. The dew point sensor of claim 1, wherein the temperature sensor is integrated with the micro-cantilever beam on the substrate.

6. The dew point sensor of claim 5, wherein the temperature sensor includes one of a resistive temperature device, a thermocouple and a semiconductor device.

7. The dew point sensor of claim 1, wherein the control circuitry is integrated with the micro-cantilever beam on the substrate.

8. The dew point sensor of claim 1, wherein the micro-cantilever beam comprises a pair of opposing electrodes having a piezoelectric material therebetween and wherein the micro-cantilever beam vibrates at a frequency dictated by a voltage potential across the electrodes.

9. The dew point sensor of claim 8, wherein a portion of the substrate under the pair of electrodes is etched away, thereby providing improved sensitivity in the resonance of the micro-cantilever beam.

10. The dew point sensor of claim 9, further comprising an insulating layer between the pair of electrodes and the substrate, and wherein the insulating layer forms an etch stop for the removal of the portion of the substrate under the pair of electrodes.

11. The dew point sensor of claim 8, wherein the cooling device comprises a thermoelectric chiller and is integrated with the micro-cantilever beam on the substrate.

12. A dew piont sensor comprising:

a micro-cantilever beam formed on a sustrate;

a cooling device in thermal contact with the micro-cantilever beam; and

a control circuit coupled to the micro-device beam, the cooling device and the temperature sensor, wherein the control circuit is operable to monitor a resonance of the micro-cantilever beam, control a temperature of the cooling device and receive a temperature indication from the temperature sensor, thereby identifying the temperature of the micro-cantilever beam when the resonance of the micro-cantilever beam changes due to a change in mass caused by dew on the micro-cantilever beam when the dew point is reached;

wherein the micro-cantilever beam vibrates at a frequency dictated by a voltage potential across the electrodes;

the cooling device comprises a thermoelectric chiller and is integrated with the micro-cantilever beam on the substrate; and

wherein the thermoelectric chiller is formed on the micro-cantilever beam, wherein the thermoelectric chiller comprises;

an insulating layer formed on the micro-cantilever beam; and

a plurality of thermoelectric structures formed of two types of material, and wherein alternating material type thermoelectric structures contact one another forming a cold junction adjacent a first end of the micro-cantilever beam and a hot junction adjacent a second end of the micro-cantilever beam at end.

13. The dew point sensor of claim 12, wherein the cold junctions are positioned over at least a portion of the micro-cantilever beam for lowering a temperature of the micro-cantilever beam to the dew point.

14. The dew point sensor of claim 12, wherein the plurality of structures comprise doped bismuth telluride alloys or doped semiconductor thermoelectric materials.

15. A method of determining the dew point, comprising the steps of:

exciting a micro-cantilever beam in a vibratory mode;

decreasing a temperature of the micro-cantilever beam, wherein when the temperature reaches the dew point, dew forms on the micro-cantilever beam and changes the mass of the micro-cantilever beam which causes a resonance of the micro-cantilever beam to change; and

monitoring the resonance of the micro-cantilever beam, wherein a change in the resonance determines the dew point.

16. The method of claim 15, wherein exciting the micro-cantilever beam into a vibratory mode comprises applying an electrical signal to a piezoelectric material which is coupled to the micro-cantilever beam, wherein the electrical signal causes the piezoelectric material to deform, thus causing the micro-cantilever beam to vibrate at a resonant frequency.

17. The method of claim 15, wherein decreasing the temperature of the micro-cantilever beam comprises the step of thermally coupling a cooling device to the micro-cantilever beam.

18. The method of claim 15, wherein decreasing the temperature of the micro-cantilever beam comprise the steps of:

forming a plurality of thermoelectric elements in thermal proximity to the micro-cantilever beam; and

driving the thermoelectric elements to cool the micro-cantilever beam to a temperature at which the dew point is reached.

19. The method of claim 15, further comprising the step of identifying a temperature of the micro-cantilever beam at which the resonance changes.

20. The method of claim 19, wherein identifying the temperature comprises thermally coupling a temperature sensor to the micro-cantilever beam.

21. The method of claim 20, wherein the temperature sensor comprises one of a resistive temperature device, a thermocouple and a semiconductor device and thermally coupling the temperature sensor to the micro-cantilever beam comprises forming the temperature sensor on or near the micro-cantilever beam.

22. The method of claim 15, further comprising the step of increasing the temperature of the micro-cantilever beam to thereby cycle the temperature of the micro-cantilever beam for multiple dew point measurements.