Hot wire anemometer gas flow sensor having improved operation and compensation

• US 6,470,741 B1
• Filed: 06/23/2000
• Issued: 10/29/2002
• Est. Priority Date: 06/23/2000
• Status: Expired due to Fees

• Current Assignee: Instrumentarium Dental Incorporated (Envista Holdings Corporation)
• Original Assignee: Instrumentarlum OY
• Inventors: Fathollahzadeh, Kiomars
• Primary Examiner(s): Fuller, Benjamin R.
• Assistant Examiner(s): Mack, Corey D.
• Application Number:
  US09/599,903
• Time in Patent Office: 858 Days
• Field of Search: 732/041.5, 732/041.8, 738/618.5, 732/041.1
• US Class Current: 73/204.15
• CPC Class Codes: G01F 1/6965 comprising means to store calibration data for flow signal calculation or correction
• CPC Class Codes: G01F 1/698 Feedback or rebalancing circuits, e.g. self heated constant temperature flowmeters

Abstract

A hot wire gas flow rate sensor having a bridge circuit with a sensing resistor avoids saturation of the signal processing circuitry at high gas flow rates by increasing a bias signal to a differential amplifier in the signal processing circuitry in a step-like manner as the flow rate responsive signal to the differential amplifier from the sensing resistor increases. The differential input signal to the amplifier is thus maintained at a level that avoids saturation of the amplifier. Appropriate scaling is provided to the output circuitry of the sensor in accordance with changes in the bias signal so that the sensor output correctly indicates gas flow rates. Compensation is provided for changes in gas composition and when the sensing resistor is changed by determining the voltage drop across a first sensing resistor with a gas of known composition at a zero gas flow condition. The voltage drop across a second sensing resistor at the zero gas flow condition for the gas is also determined. When sensing the flow rate with the second sensing resistor, the ratio of these voltage drops is applied to the flow responsive component of the voltage drop across the second sensing resistor. This compensates the flow responsive voltage drop component for changes in sensing resistor resistance. The voltage drop across the sensing resistor is determined at zero gas flow when the gas composition changes and ratioed to the voltage drop obtained with the known composition gas to compensate for changes in gas composition.

Claims

1. Improved circuitry for a hot wire gas flow rate sensor, the sensor having a bridge circuit containing a hot wire sensing resistor exposed to the gas, the flow rate of which is measured, control means coupled to the output of the bridge circuit for energizing the input of the bridge circuit to provide a current through the sensing resistor which maintains the resistance of the sensing resistor at a desired value as gas flows past the sensing resistor, the voltage drop across the sensing resistor being an indication of the flow rate of the gas and the voltage drop increasing as the gas flow rate increases, said circuitry comprising:

a differential amplifier operable by a differential input signal and providing an output signal responsive thereto, said differential amplifier being driven into a saturated state when said differential input signal is of a predetermined magnitude, said differential amplifier receiving a signal corresponding to the sensing resistor voltage drop at a first input thereof;

bias signal means for providing a bias signal to a second input of said differential amplifier to form, with said voltage drop signal, the differential input signal to said differential amplifier;

means responsive to the magnitude of the voltage drop signal supplied to said first input of said differential amplifier for establishing the magnitude of the bias signal provided to said second input at a level that limits the differential input signal formed by the difference between the bias signal and the voltage drop signal to one that does not exceed the predetermined magnitude over a range of the voltage drop signal; and

output circuitry receiving the output of said differential amplifier for providing an indication of the flow rate of the gas.

2. The improved gas flow rate sensor circuitry according to claim 1 wherein said bias signal magnitude establishing means is further defined as changing the magnitude of the bias signal responsive to the magnitude of the voltage drop signal so that the differential input signal to said differential amplifier does not exceed said predetermined magnitude as the magnitude of the voltage drop signal varies.

3. The improved gas flow rate sensor circuitry according to claim 1 wherein said bias signal magnitude establishing means is further defined as including a comparator for comparing the magnitude of the voltage drop signal to at least one reference value to establish the magnitude of the bias signal.

4. The improved gas flow rate sensor circuitry according to claim 3 wherein said bias signal magnitude establishing means is further defined as comprising a comparator for comparing the magnitude of the voltage drop signal to a plurality of reference values for providing a bias signal at a level that limits the differential input signal to the differential amplifier to a value not greater than the predetermined magnitude differential input signal.

5. The improved gas flow rate sensor circuitry according to claim 4 wherein said comparator is further defined as coupled to means providing a plurality of signals of differing magnitudes, and wherein said bias signal magnitude establishing means includes means for selecting a desired one of said plurality of signals for provision to said differential amplifier as a bias signal.

6. The improved gas flow rate sensor circuitry according to claim 5 wherein said selecting means comprises a logic circuit interposed between said comparator and said signal providing means.

7. The improved gas flow rate sensor circuitry according to claim 5 wherein said bias signal magnitude establishing means provides a scaling signal to said output circuitry in accordance with the magnitude of the bias signal for causing the output circuit to provide a signal correctly indicating the flow rate of the gas.

8. The improved gas flow rate sensor circuitry according to claim 5 further including an input amplifier having an input receiving said voltage drop signal and an output connected to said first input of said differential amplifier.

9. The improved gas flow rate sensor circuitry according to claim 1 wherein the output signal of said differential amplifier is provided to an analog to digital converter for supply to said output circuitry.

10. The improved gas flow rate sensor circuitry according to claim 1 wherein said bias signal magnitude establishing means provides a scaling signal to said output circuitry in accordance with the level of the bias signal for causing the output circuit to provide a signal correctly indicating the flow rate of the gas.

11. The improved gas flow rate sensor circuitry according to claim 1 further including an input amplifier having an input receiving said voltage drop signal and an output connected to said first input of said differential amplifier.

12. The improved gas flow rate sensor circuitry according to claim 1 further defined as circuitry for a breathing gas flow rate sensor.

13. The improved gas flow rate sensor circuitry according to claim 12 further defined as circuitry for an expired breathing gas flow rate sensor.

14. A method for measuring the flow rate of a gas passing a sensing resistor provided with current to maintain the resistance of the resistor at a constant value, said method comprising the steps of:

supplying a voltage drop signal from the resistor to a first input of a differential amplifier, said differential amplifier being driven into a saturated state by a differential input signal of a predetermined magnitude;

providing a bias signal to a second input of the differential amplifier to form, with the voltage drop signal, the differential input signal to the differential amplifier;

establishing the magnitude of the bias signal at a level that limits the differential input signal to not greater than the predetermined magnitude over a given range of the voltage drop signal; and

changing the magnitude of the bias signal as the magnitude of the voltage drop signal varies so that the differential input signal does not exceed the predetermined magnitude,

an output signal of said differential amplifier providing an indication of the gas flow rate of the gas.

15. The method according to claim 14 wherein the step of changing the magnitude of the bias signal is further defined as changing the magnitude in at least one discrete step.

16. A method according to claim 15 wherein the step of changing the magnitude of the bias signal is further defined as comparing the voltage drop signal with a reference to provide a bias signal sufficient to prevent the differential input signal to the differential amplifier from exceeding the predetermined magnitude.

17. A method according to claim 16 wherein the step of changing the magnitude of the bias signal is further defined as comparing the voltage drop signal with a plurality of reference values and providing a bias signal determined in accordance with the comparison to the second input of the differential amplifier.

18. A method according to claim 14 further defined as scaling the output of the differential amplifier circuit for providing a correct indication of the flow rate of the gas.

19. A method according to claim 14 further defined as amplifying the voltage drop signal prior to supplying same to the first input of the differential amplifier.

20. A method according to claim 14 further defined as subjecting the output signal of the differential amplifier to an analog-digital conversion.

21. A method of compensating a bridge circuit gas flow rate sensor when replacing a first hot wire sensing resistor with a second hot wire sensing resistor, said method comprising the steps of:

(a) at zero gas flow, operating the bridge circuit containing the first sensing resistor (40₁) to establish a balanced condition in the bridge circuit at a desired current through the first sensing resistor;

(b) determining the voltage drop (V_w1(0)) across the first sensing resistor at such conditions;

(c) at zero gas flow, operating the bridge circuit containing the second sensing resistor (40₂) to establish a balanced condition in the bridge circuit with the desired current through said second sensing resistor;

(d) determining the voltage drop (V_w2(0)) across the second sensing resistor at such conditions; and

(e) establishing a ratio of the voltage drops so determined across the first and second sensing resistors which, when applied to the flow component V_w2(f) of the voltage drop across said second sensing resistor when sensing gas flow, compensates the sensor for the replacement of said first sensing resistor with said second sensing resistor.

22. The method according to claim 21 wherein the first and second sensing resistors are exposed to the same gas at zero gas flow.

23. The method according to claim 21 wherein the steps of operating the bridge circuit containing the first resistor at zero gas flow and determining the voltage drop across the first sensing resistor at such conditions are carried out as an initial calibrating procedure.

24. The method of claim 21 further defined as a method of compensating a gas flow rate sensor for changes in the composition of gas, the flow rate of which is being measured, said method further comprising the steps of:

(f) at zero gas flow, operating the bridge circuit with the second sensing resistor exposed to a first gas of a first composition to establish a balanced condition in the bridge circuit at a desired current through the sensing resistor;

(g) determining the voltage drop (V_w′

(0)) across the second sensing resistor at such conditions for the first gas;

(h) at zero gas flow conditions, operating the bridge circuit with the second sensing resistor exposed to a second gas of a second composition;

(i) determining the voltage drop (V_w″

(0)) across the second sensing resistor at such conditions for the second gas; and

(j) establishing a ratio of the voltage drops so determined across the second sensing resistor which, when applied to the flow component V_w″

(f) of the voltage drop across said sensing resistor when sensing the flow of the second gas, compensates the sensor for the change in the composition of the gas, the flow rate of which is being measured.

25. The method according to claim 24 wherein steps (c) and (d) of claim 21 and steps (f) and (g) of claim 24 comprise common steps.

26. The method of claim 24 wherein steps (h) through (j) are periodically repeated.

27. A method of compensating a gas flow rate sensor for changes in the composition of gas, the flow rate of which is being measured, said gas sensor having a hot wire sensing resistor in a bridge circuit, said method comprising the steps of:

(a) at zero gas flow, operating the bridge circuit with the sensing resistor exposed to a first gas of a first composition to establish a balanced condition in the bridge circuit at a desired current through the sensing resistor;

(b) determining the voltage drop (V_w′

(0)) across the sensing resistor at such conditions for the first gas;

(c) at zero gas flow, operating the bridge circuit with the sensing resistor exposed to a second gas of a second composition;

(d) determining the voltage drop (V_w″

(0)) across the sensing resistor at such conditions for the second gas; and

(e) establishing a ratio of the voltage drops so determined across the sensing resistor which, when applied to the flow component V_w″

(f) of the voltage drop across said sensing resistor when sensing the flow of the second gas, compensates the sensor for the change in the composition of the gas, the flow rate of which is being measured.

28. The method according to claim 27 wherein steps (c) through (e) are periodically repeated.