Liquid level control with capacitive sensors

• US 5,238,369 A
• Filed: 07/28/1992
• Issued: 08/24/1993
• Est. Priority Date: 11/26/1990
• Status: Expired due to Fees

• Current Assignee: Tecumseh Products Company
• Original Assignee: Tecumseh Products Company
• Inventors: Farr, James B.
• Primary Examiner(s): Bertsch, Richard A.
• Assistant Examiner(s): Scheuermann, David W.
• Application Number:
  US07/920,832
• Time in Patent Office: 392 Days
• Field of Search: 417/18, 417/32, 417/36, 417/44
• US Class Current: 417/18
• CPC Class Codes: F04B 49/025 by means of floats
• CPC Class Codes: F04D 15/0218 the condition being a liquid level or a lack of liquid supply
• CPC Class Codes: G01F 23/265 for discrete levels
• CPC Class Codes: G01F 23/266 measuring circuits therefor
• CPC Class Codes: G05D 9/12 characterised by the use of electric means

Abstract

The present invention is a liquid level control system for selectively activating and deactivating a pump according to the liquid level indicated by capacitive sensors. Activating circuitry starts the pump motor when both upper and lower sensors indicate the presence of liquid. The pump motor is latched on by the activating circuitry and continues operation until the lower capacitive sensor indicates the absence of liquid, in which case the pump is shut off. A high frequency oscillator operates reliably over a wide range of operating conditions to maintain relatively constant charging time for the capacitive sensors.

Claims

1. An apparatus for controlling liquid levels, said apparatus comprising:

a pump;

a motor drivingly connected to said pump;

first sensor means for detecting the presence of liquid and mounted at a first vertical position, said first sensor means including a capacitive circuit having two capacitive electrodes, said first sensor means producing a first signal indicative of the presence of liquid at said first position;

second sensor means for detecting the presence of liquid and mounted at a second vertical position, which is different than said first vertical position, said second sensor means including a capacitive circuit having two capacitive electrodes, said second sensor means producing a second signal indicative of the presence of liquid at said second position;

means for activating said motor, said activating means operably coupled to said first and second sensor means, said activating means starting said motor to drive said pump when both of said first and second signals are produced by said first and second sensors, said activating means for continuing operation of said motor while at least one of said first and second signals are being produced, and said activating means stopping said motor when both said first and second signals are no longer produced; and

heatsink means for absorbing heat generated by said activating means whereby the temperature of said activating means is maintained relatively constant so that variations in response time of said activating means is minimized.

2. The system of claim 1 wherein said pump further includes an outer case, and one of said electrodes for each of said first and second sensor means includes said pump outer case.

3. The system of claim 1 wherein said activating means includes means for stopping said motor when the temperature of said system exceeds a predetermined level.

4. The system of claim 1 wherein said activating means includes means for minimizing the occurrence of false triggering caused by high temperatures.

5. The system of claim 1 wherein said first sensor means includes a monostable multivibrator.

6. The system of claim 1 wherein said second sensor means includes a monostable multivibrator.

7. The system of claim 1 wherein at least one of said electrodes for one of said first and second sensor means is vertically moveable.

8. The system of claim 7 wherein said pump includes mounting means for adjustably positioning said vertically moveable electrode.

9. The system of claim 1 further comprising a dielectrically isolated box attached to said pump and said heatsink means is disposed within said box.

10. The system of claim 9 wherein said activating means includes power switching means for providing electric power to said motor, and said power switching means is disposed in thermal contact with said heatsink means.

11. The system of claim 9 wherein said heatsink means includes a conductive plate disposed in thermal contact with said box.

12. The system of claim 11 wherein one of said capacitive electrodes for said first sensor means includes said conductive plate.

13. The system of claim 1 further including latching means for coupling said motor to a power source, said latching means including first, second, and gate terminals, said first and second terminals coupled in a series circuit relationship with said motor and the power source, said gate terminal coupled to said activating means, whereby said latching means closes said series circuit when activated by said activating means.

14. The system of claim 13 wherein said latching means further includes a switch means for maintaining said latching means in a closed state, said switch means coupled to said activating means whereby after said latching means closes said series circuit, said switch means maintains said latching means in said closed state when said activating means indicates at least one of said first and second signals are being produced.

15. The system of claim 14 wherein said latching means further includes enabling means for enabling the closure of said latching means, said enabling means coupled to said activating means whereby after neither of said first and second signals are produced, said enabling means allows the closure of said series circuit when said activating means indicates the higher vertically mounted one of said first and second sensor means are producing one of said first and second signals.

16. The system of claim 14 wherein said activating means includes means for adjusting to ambient temperature changes, said adjusting means coupled to said switch means whereby the switching of said switch means is relatively unaffected by temperature or manufacturing variations.

17. The system of claim 16 wherein said adjusting means includes a divider having at least one non-unitary temperature coefficient resistor, said divider coupled to said switch means whereby the switching of said switch means is relatively unaffected by temperature changes of said activating means.

18. An apparatus for controlling liquid levels, said apparatus comprising:

a pump;

a motor drivingly connected to said pump;

first sensor means for detecting the presence of liquid and mounted at a first vertical position, said first sensor means including a capacitive circuit having two capacitive electrodes, said first sensor means producing a first signal indicative of the presence of liquid at said first position;

second sensor means for detecting the presence of liquid and mounted at a second vertical position, which is different than said first vertical position, said second sensor means including a capacitive circuit having two capacitive electrodes, said second sensor means producing a second signal indicative of the presence of liquid at said second position;

means for activating said motor, said activating means operably coupled to said first and second sensor means, said activating means starting said motor to drive said pump when both of said first and second signals are produced by said first and second sensors, said activating means for continuing operation of said motor while at least one of said first and second signals are being produced, and said activating means stopping said motor when both said first and second signals are no longer produced;

said activating means including means for charging said capacitive sensors for a predetermined time period, said charging means including high frequency oscillator means for cycling the operation of said charging means at a frequency substantially exceeding line current.

19. The apparatus of claim 18 wherein said pump further includes an outer case, and one of said electrodes for each of said first and second sensor means includes said pump outer case.

20. The apparatus of claim 18 wherein said activating means includes means for stopping said motor when the temperature of said system exceeds a predetermined level.

21. The apparatus of claim 18 wherein said first sensor means includes a monostable multivibrator.

22. The apparatus of claim 18 wherein said second sensor means includes a monostable multivibrator.

23. The apparatus of claim 18 wherein said at least one of said electrodes for one of said first and second sensor means is vertically moveable.

24. The apparatus of claim 23 wherein said pump includes mounting means for adjustably positioning said vertically moveable electrode.

25. The apparatus of claim 18 further comprising a dielectrically isolated box attached to said pump and heatsink means for dissipating heat of said activating means which is disposed within said box.

26. The apparatus of claim 25 wherein one of said electrodes of said first sensor means includes a conductive plate, and said heatsink means includes said conductive plate as well.

27. The apparatus of claim 25 wherein said activating means includes power switching means for providing electric power to said motor, and said power switching means is disposed in thermal contact with said heatsink means.

28. The apparatus of claim 18 further including latching means for coupling said motor to a power source, said latching means including first, second, and gate terminals, said first and second terminals coupled in a series circuit relationship with said motor and the power source, said gate terminal coupled to said activating means, whereby said latching means closes said series circuit when activated by said activating means.

29. The apparatus of claim 28 wherein said latching means further includes a switch means for maintaining said latching means in a closed state, said switch means coupled to said activating means whereby after said latching means closes said series circuit, said switch means maintains said latching means in said closed state when said activating means indicates at least one of said first and second signals are being produced.

30. The apparatus of claim 28 wherein said activating means includes means for adjusting to ambient temperature changes, said adjusting means coupled to said switch means whereby the switching of said switch means is relatively unaffected.

31. The apparatus of claim 30 wherein said adjusting means includes a divider having at least one non-unitary temperature coefficient resistor, said divider coupled to said switch means whereby the switching of said switch means is relatively unaffected by temperature changes of said activating means.