GB2097123A - Absolute pressure sensor - Google Patents

Abstract

An absolute pressure sensor applies repetitive vibrational impulses 85 to a gaseous volume maintained at the average pressure of the ambient. The differential pressure between the compressed and non-compressed conditions of the gaseous volume is proportional to the absolute ambient pressure. Thus, absolute ambient pressure can be determined without a need to maintain a reference pressure. The vibrational impulses may be provided by a high frequency speaker, a piezoelectric element or a piston. The measurements may be made in the transmission mode as shown or resonances may be detected using a resonant cavity (fig. 7). <IMAGE>

Description

SPECIFICATION Absolute pressure sensor This invention relates generally to a transducer assembly for measuring absolute pressure and a method of measuring absolute pressure.

Recently, motor vehicle applications have required the use of pressure sensors for measuring both atmospheric and subatmospheric pressure levels. In internal combustion engine applications, fine control of fuel metering has required that the rapid fluctuation of pressure levels within the intake manifold of the engine be measured as well as the less rapid fluctuation of ambient pressure levels. Sensors able to measure these pressures reliably and with adequate response time have been difficult to obtain and are relatively expensive.

Known capacitive transducers provide a variation in electrical capacitance as a function of pressure.

The pressure can act upon the "plates" of the capacitive transducer, the aligned areas of such plates, the positions of the plates as a function of mechanical movement, the type of dielectric material between the capacitive plates, and combinations of these.

For example, there are known variable capacitance sensors having a fixed ceramic element on which is deposited a metal film. A second and very thin ceramic plate or diaphragm also has a metal film deposited on it and is attached with the glass frit or the like to the thicker, fixed ceramic element. The metal films on the ceramic element constitute the plates of a capacitor. The thin ceramic diaphragm responds to pressure variations to produce a small change in capacitance that is used to vary the frequency produced by an electronic oscillator.

Sensors or transducers of the type described in the preceding paragraph are expensive because of their large size and because they must be manufactured individually. Also, a ceramic pressure transducer of this type may not be tested until it has been assembled with its associated electronic circuitry. If it is found to be defective, the entire assembly then must be discarded.

Further, such capacitive transducers need a reference atmosphere. A reference atmosphere, reference pressure, or vacuum is also a characteristic of various other pressure sensors such as those with a diaphragm. Typically, the reference pressure is established on one side of the diaphragm and the pressure to be measured is on the other side of the diaphragm. Movement of the diaphragm causes a physical indication of the pressure to be measured.

Although the use of a reference pressure is prevelant in absolute pressure sensors, it is undesirable because it adds to the cost of the sensor. A cost increase is particularly undesirable when many absolute pressure sensors must be produced. As noted, current automobile vehicles with advanced fuel metering systems use one absolute pressure sensor to measure ambient pressure and one to measure intake manifold pressure. Increased future usage is anticipated. In addition to the cost of initially establishing the reference pressure, reliably maintaining the pressure sensor reference can be a problem. Any break in the integrity of the container or walls defining the reference atmosphere would cause malfunctioning of the output pressure sensor.

Such malfunction can, of course, result in customer dissatisfaction. These are some of the problems this invention overcomes.

According to one aspect of the invention, there is provided a method for measuring absolute ambient pressure including the steps of maintaining an average pressure equal to the ambient pressure within a defined volume, applying a compression force to the defined volume, measuring the variation in pressure caused by the compression force, and establishing a relationship between the measured variation in pressure and the absolute ambient pressure so that the absolute ambient pressure can be determined without the need for a reference atmosphere.

According to a second aspect of the invention, there is provided an absolute ambient pressure sensor comprising a speaker driver means for supplying electrical power, a speaker means coupled to said speaker driver for applying vibration impulses to a gas, a microphone means for receiving acoustic signals from said speaker means, and a signal level indicator means coupled to said speaker for indicating the magnitude of the acoustic signals received by said microphone means.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an absolute pressure sensor in accordance with a first embodiment of this invention using a piston for compressing the ambient atmosphere within a cylinder, the piston being shown in a position before compression begins.

Figure 2 is a block representation of the apparatus of Figure 1 with the piston shown in a compression position.

Figures 3,4 and 5 are simplified representations of consecutive positions of a piston creating compressed gaseous regions traveling to the right, the arrows indicating the pressure difference to be measured which is indicative of absolute pressure.

Figure 6 is a graphical representation ofthe relationship between pressure and the voltage level output of a microphone receiving the traveling waves indicated in Figures 3,4 and 5.

Figure 7 is a schematic block representation of a second embodiment in accordance with this invention.

Figure 8 is a schematic block representation of a third embodiment in accordance with this invention.

In accordance with one embodiment of this invention, compression of a fixed volume of gas (air) results in a pressure change which, in turn, measures the ambient pressure. No reference volume with a static stable pressure is required. Figure 1 illustrates an absolute pressure sensor 10 which determines the absolute ambient pressure as a function of the change in pressure of a certain volume of air. A piston 11 is positioned within a cylinder 12 and can be driven longitudinally in cylinder 12 by an actuator 13. Cylinder 12 has an orifice 16 for providing communication between the interior of cylinder 12 and the ambient atmosphere. A pressure transducer 14 is coupled to the end of cylinder 12 opposing piston 11. A measuring circuit 15 is connected to transducer 14to measure change in pressure.The initial position of piston 11 is such that orifice 16 is not covered and provides communication to the interior of cylinder 12.

Referring to Figure 2, piston 11 has been moved to the right by actuator 13, decreasing the volume within cylinder 12 between piston 11 and transducer 14 thus increasing the pressure within cylinder 12.

The difference in pressure detected by measuring circuit 15 from the condition shown in Figure 1 (uncompressed) and the condition shown in Figure 2 (compressed) is proportional to the absolute ambient pressure. If the movement of piston 11 remains a fixed distance, then changes in the measured difference pressure indicates proportional changes in the absolute ambient pressure. Measuring device 15 can be calibrated using a known ambient pressure.

That is, air initially at a pressure P1 and volume V, is compressed to a smaller volume V2 and larger pressure P2. Transducer 14 measures the differential or "Gauge" pressure T = P2- P1. If the compression is rapid, adiabatic conditions apply and

where = 1.4 is the specific heat ratio for air which does not vary rapidly with either pressure ortemp- erature. For example, if V1N2 = 10, a = 25, the peak reading of the gauge will be To = a -P1 = (a- 1)P1.

If the atmospheric pressure changes to P1 = P1 - A and the compression cycle is repeated, the new peak gauge reading will be T = (cr- 1 )P1,so that the change in gauge readings will beT-T0 = (a-1) 4 where A represents the change in pressure. Therefore, both the absolute pressure and changes in absolute pressure can be directly measured and no reference pressure is required. The piston arrangement of Figures 1 and 2 is for illustration. A vibrating diaphragm or piston can be fitted on a small chamber with a leak to keep the average pressure at atmospheric pressure. The alternating pressure signal detected by a microphone in the wall ofthe chamber will produce a signal proportional to the absolute pressure.The main requirement for stability of calibration is that the compression ratio on each cycle remains constant for long periods of time.

The compression in the examples given is believed to be adiabatic in nature because of its rapidity. This means that there is not time for heat to flow in or out ofthe compressed volume. Thus, the response will be linear with changes in ambient pressure. Also, it is anticipated that variation in temperature will not cause a large change in operating characteristics. If the compression is not adiabatic, the response is still expected to be substantially linear.

Referring to Figures 3,4 and 5, there is shown a sequential action of a piston 31 acting on an ambient atmosphere to produce traveling compressed waves 32. Figure 5 indicates by arrows the pressure differential which is measured to give an indication of the absolute ambient pressure.

Referring to Figures 7 and 8, pressure waves are generated by vibrating diaphragms 71 and 81 of a high frequency speaker (tweeter) or by a vibrating PZT piezoelectric element. Both are driven by constant voltage amplitude sine wave generators 72 and 82. Thus the amplitude of movement of diaphragms 71 and 81 is established as a constant. The pressure is measured by standard microphones 73 and 83 placed nearby. The microphone signal is amplified by amplifiers 78 and 88, rectified by rectifiers 79 and 89, and recorded by d.c. level meters 79 and 84.

Two types of measurements are made. First, referring to Figure 7, a resonant measurement method uses a speaker covered with a plate 75 having a small hole 76 in it. A resonant cavity 77 is formed and resonances from 14to 52 kHz can be found. At a resonance (where the sound output peaks) the sound input to microphone 73 is measured as the ambient pressure is varied from 0 to 900 mm Hg (about 0 to 1 atmosphere). Over this range, the output level is found to be linear with pressure, to within the reliability of the measurement. The range of pressure covered is limited by the chamber used and can extend to measurement of many atmospheres of pressures.

Second, referring to Figure 8, a transmission measurement method is similar to the resonant measurement method, but sound waves 85 produced by speaker diaphragm 81 or PZT crystal are transmitted through air for a short distance (e.g. 5 cm or less) to microphone 83 and their intensity measured as before. Substantially the same results can be expected from the two methods. Potentially, the resonance method has a signallnoise practical advantage.

The sensitivity of either measurement is very good and in practice there are no fundamental noise limitations. The main requirement for useful operation is achieving a stable diaphragm vibration cycle. Also, crosstalk due to transmission of sound through any support structure must be minimized. The potential for miniaturization and for integration with silicon integrated circuit technology is very good for this device. The vibrating diaphragm can be bonded to a silicon chip using known techniques. The rest of the device, including pickup transducer, amplifier, rectifier and driver circuitry can be included on the same piece of silicon.

Various modifications and variations will no doubt occur to those skilled in the various arts to which this invention pertains. For example, the particular mechanism for forming the zone of compressed atmosphere may be varied from that disclosed herein. These and all other variations which basically rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention.

Claims (9)

1. A method for measuring absolute ambient pressure including the steps of: maintaining an average pressure equal to the ambient pressure within a defined volume; applying a compression force to the defined vol ume; measuring the variation in pressure caused by the compression force; and establishing a relationship between the measured variation in pressure and the absolute ambient pressure so that the absolute ambient pressure can be determined without the need for a reference atmosphere.

2. A method as recited in Claim 1 wherein the step of maintaining an average ambient pressure with a defined volume includes: establishing a resonant cavity with an orifice; establishing communication between the resonant cavity and the ambient; and measuring the variation in pressure by placing a microphone adjacent the orifice.

3. A method as recited in Claim 2 wherein establishing said resonant cavity includes defining at least part of the boundary of said resonant cavity by a speaker diaphragm.

4. A method as recited in Claim 1 wherein the step of applying a compression force includes: applying a vibrational impulse; and establishing a parameter having a known proportionality to the amplitude of vibration of the vibrational pulse.

5. A method as recited in Claim 4 wherein the step of establishing the parameter includes: measuring an electrical current which is proportional to the amplitude of vibration.

6. An absolute ambient pressure sensor comprising: a speaker driver means for supplying electrical power; a speaker means coupled to said speaker driver for applying vibration impulses to a gas; a microphone means for receiving acoustic signals from said speaker means; and a signal level indicator means coupled to said speaker for indicating the magnitude of the acoustic signals received by said microphone means.

7. An absolute ambient pressure sensor as recited in Claim 6, further comprising: a cover means coupled to said speaker means to form a resonant cavity, said cover means including an orifice for providing a communication path between said resonant cavity and the ambient.

8. A method of measuring absolute pressure substantially as herein described with reference to and as illustrated in the accompanying drawings.

9. An absolute pressure sensor substantially as herein described with reference to and as illustrated in the accompanying drawings.