Angular velocity sensor

The present invention relates to a gas rate sensor which is capable of detecting angular velocity.

In general, a gas rate sensor provides an output signal in response to any difference between the output signals supplied from a pair of thermal sensing elements. Such a different appears when the gas flow ejected from a gas nozzle deviates, to flow more on one of the thermal sensing elements than on the other, due to the influence on the gas flow of an applied motion the angular velocity of which is to be determined in terms of its magnitude and direction.

The angular velocity is determined by detecting a small imbalance in the heat dissipation from the pair of thermal sensing elements due to the deviation of the gas flow, and therefore the surrounding temperature change significantly reduces the sensitivity of the gas rate sensor. This necessitates the use of temperature compensating means in the gas rate sensor.

In an attempt to reduce the adverse effects caused by the surrounding temperature the gas rate sensor is subjected to forced heating by using appropriate heaters, and the temperature within the gas rate sensor is detected by appropriate temperature sensors, and the temperature within the gas rate sensor is controlled to keep it constant.

Disadvantageously, the sensitivity of the gas rate sensor and the offset value remain too unstable to provide an accurate output signal until the temperature within the gas rate sensor has reached a stable condition after connecting the electric heater to an associated power supply. In fact, no satisfactory gas temperature control has yet been attained in a gas rate sensor.

With the above in mind the present invention seeks to provide a gas rate sensor which is capable of correcting the gas rate sensor output signal in a way so as to account for an instantaneous temperature change within the gas rate sensor, allowing the temperature within the gas rate sensor to vary.

According to the present invention there is provided a gas rate sensor which provides an output signal in response to any difference between the output signals from a pair of thermal sensing elements due to the force which an angular velocity exerts on the gas flow ejected over the pair of thermal sensing elements, from an associated nozzle, in which said gas rate sensor is equipped with: means to effect temperature compensation of the gas rate sensor output signal by substracting an offset value from the gas rate sensor output signal; means to determine the resistances of the pair of thermal sensing elements; means to detect the situation in which the resistances of the pair of thermal sensing elements increase or decrease simultaneously; means to make a decision as to whether or not the gas rate sensor output signal remains within a predetermined tolerance with respect to the offset signal when such a situation is detected; and means to permit the gas rate sensor output to be used as a new offset value when the gas rate sensor output signal remains within said predetermined tolerance.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

• Figure 1 shows diagrammatically the gas rate sensor equipped with temperature compensation means.
• Figure 2 is a circuit diagram of a resistance detection circuit;
• Figure 3 is a longitudinal section of the gas rate sensor;
• Figure 4 shows how the gas flow deviates when the gas rate sensor is moved at an angular velocity to be determined; and
• Figure 5 is a graph representing the temperature-­to-resistance characteristics of a pair of heating wires.

Referring to Figure 3, the casing 1 of the gas rate sensor is open at one and, and is closed at the other end. The casing 1 has three longitudinal ridges 120 degrees apart from each other on its inner surface. When the gas rate sensor body 4 is put in the casing 1, these longitudinal ridges define three longitudinal channels 3.

As seen from the drawing, the gas rate sensor body 4 is composed of a holder section 5, a neck section 6 and a cylinder section 7. The holder section 5 serves to confine the gas within the casing 1. The holder section 5 has a pump compartment 8, and the pump compartment 8 contains a diaphragm type piezoelectric pump 9. When the pump 9 works, gas is drawn in the longitudinal channels 3 through the inlets 10 of the holder section 5. After passing through a central nozzle 11 and rectifying apertures 12, positioned around the nozzle in the end of the cylinder section 7, the gas is drawn into a sensor compartment 13 in the form of laminar flow. Then, the gas flows over a pair of heating wires 14a and 14b, which are used as thermal sensing elements and are positioned downstream of the sensor compartment 13. Thereafter, the gas flows into the pump compartment 8, where it is directed to the longitudinal channels 3 by pumping. The pair of heating wires 14a and 14b are located symmetrically with respect to the center line o-o of the nozzle 11, as seen from Figure 4. When no force is applied to the gas rate sensor in a lateral direction, the gas is ejected fromthe nozzle 11, flowing straight along the center line o-o, and exposing each of the heating wires 14a and 14b to an equal gas flow rate, and hence cooling each heating wire equally.

When a lateral force is applied to the gas rate sensor to cause it to move at an angular velocity, the gas flow will deviate from the center line o-o as shown in broken lines. The amount of deviation is indicated by "ε". As a result, the gas flows more on heating wire 14a than on heating wire 14b, thus causing unbalanced outputs from the two heating wires. Then, a signal representing the difference between the unbalanced outputs will appear at the output terminal of the gas rate sensor, and the output signal will be amplifed by an amplifier circuit 15. The polarity and amplitude of the amplified signal represents the direction and magnitude of the angular velocity of the gas rate sensor, respectively.

A printed board 16 of the amplifier circuit 15 is attached to the flange 2 of the casing 1 as seen from Figure 3. A hollow cylinder 17 contains the whole structure of the gas rate sensor.

A singal appearing at the output terminal of the gas rate sensor is likely to vary with surrounding temperature. The gas rate sensor output signal x is given by:

X = (R₂ (T)/R₁ (T)) - 1

where R₁ (T) stands from the resistance of the heating wire 14a at temperature T and R₂ (T) stands for the resistance of the heating wire 14b at temperature T.

If two heating wires 14a and 14b have the same temperature-to-resistance characteristic (i.e. R₁ (T) is equal to R₂ (T)), and if the gas rate sensor has no angular velocity, the sensor output signal x will be zero as seen from equation (1). In this ideal case no correction of the gas rate sensor output signal will be required.

However, it is difficult to select and use a pair of heating wires 14a and 14b which have the same temperature-to-resistance characteristic. Usually, two heating wires 14a and 14b have different characteristics as shown in Figure 5. Therefore, even if the gas rate sensor has no angular velocity, the gas rate sensor output signal cannot be zero. Also, an error will be caused in detecting the angular velocity of the gas rate sensor because these heating wires do not have the same temperature-to-resistance characteristic.

Necessary temperature compensation of the gas rate sensor output signal x will be effected according to the present invention as follows:

Figure 1 shows a gas rate sensor system according to one embodiment of the present invention. It comprises a gas rate sensor 18, a resistance detection circuit 19 for detecting the resistances R₁ and R₂ of the heating wires 14a and 14b used in the gas rate sensor 18, and a temperature compensation circuit for effecting a temperature compensation of the gas rate sensor output x in response to the detected heating wire resistances R₁ and R₂ and the gas rate sensor output signal x. Figure 2 shows the structure of the resitance detection circuit 19 comprising: a bridge made from the heating wires 14a and 14b and two known resistances Ra and Rb; another known resistance series-­connected to the bridge; and a constant voltage source 21, producing a voltage E, connected across the bridge and the resistance Rc. An arithmetic processor 22 is connected across the series resistance Rc, and the arithmetic processor 22 uses the voltage E₁ appearing across the series resistance Rc, and the gas rate sensor output signal x to carry out the following arithmetic operation for determining the resitances R₁ and R₂ of the heating wires 14a and 14b;

The following equation hold for the resistance circuit of Figure 2:

E-E₁ = (R₁ + R₂) I₁ = (Ra + Rb) I₂      (2)

I₁ + I₂ = I      (3)

E₁ = I.Rc      (4)

From equations (2), (3) and (4) the following equation is derived:

R₁₊R₂ = Rc(E-E₁)(Ra + Rb)/E₁(Ra + Rb)-Rc(E-E₁)      (5)

If (R₁+ R₂) is represented by y, then

(R₁ + R₂) = y      (6)

The gas rate sensor output signal x is given by Equation (1) as follows:

x = (R₂/R₁) - 1      (7)

Thus, from Equations (5), (6) and (7) R₁ and R₂ are derived as follows:

R₁ = y/(x+2)      (8)

R₂ = y.(x+1)/(x+2)      (9)

By detecting the voltage E₁ across the resistance Rc the resistance R₁ and R2 of the heating wires 14a and 14b can be determined from Equation (8) and (9) in real-time.

The temperature compensation circuit 20 corrects the gas rate sensor output signal x by substracting from the gas rate sensor output signal x an offset value which is initially stored in the temperature compensation circuit 20 in accordance with the characteristics of the heating wires 14a and 14b in the gas rate sensor 18. Then, the temperture compensation circuit 20 makes a decision as to whether or not the gas rate sensor output signal x remains within a predetermined tolerance with respect to the offset value, when the resistances R₁ and R₂ detected by the resistance detection circuit 19 increase or decrease together. If the gas rate sensor output signal x remains within the predetermined tolerance the temperature compendation circuit 20 will carry out correction by using the current gas rate sensor output signal x as a new offset value in place of the old one so that the gas rate sensor output signal x tends to zero.

If the gas sensor output signal x lies outside the predetermined tolerance the temperature compensation circuit 20 will not change the offset value, regarding the gas rate sensor as having an angular velocity.

As apparent from above, a gas rate sensor system according to the present invention determines the resistances of the pair of heating wires of the gas rate sensor in order to detect the temperature change of the surrounding atmosphere of the heating wires in terms of the simultaneous increase or decrease of the pair of heating wire resistances. It is presumed that the gas rate sensor has no angular velocity when the gas rate sensor output signal remains within a given tolerance, such that the current gas rate sensor output signal is nearly equal to the predetermined offset value. The current gas rate sensor output signal is then used as a offset value. Thus, appropriate temperature compensation of the gas rate sensor output signal can be made, to account for the temperature change of the atmosphere surrounding the pair of heating wires.

If a car is equipped with a gas rate sensor system according to the present invention for detecting any change in its direction of movement, the offset value can be updated without stopping the car for that purpose.