JPH09178590A - Method for deciding go/no-go of pressure sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for estimating the vacuum leakage and electric circuit defect of a strain gauge type pressure sensor being employed as a vacuum sensor accurately in safety without utilizing radioactivity and without causing any pollution over a wide range of discharge and electric output characteristics. SOLUTION: Upon elapse of about 10 days after manufacture of a sensor, a DC 600V voltage is applied between a metal case 6 and the terminal where all electrodes 2 are short-circuited. If a discharge current flows, it means vacuum leakage at a rate higher than maximum allowable rate and thereby the sensor is rejected otherwise a DC 6V voltage is applied to the electrode 2 and the output voltage is recorded. Upon elapse of another 10 days, output voltage is measured and if the voltage difference is higher than several mV, a decision is made that the maximum allowable vacuum leakage rate is exceeded and the sensor is rejected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pass / fail judgment method for vacuum leakage of a strain gauge type pressure sensor.

[0002]

2. Description of the Related Art A strain gauge type pressure sensor is indispensable as a vacuum sensor for improving the efficiency of an automobile engine, for example. A normal pressure sensor of this type detects the pressure difference between the high vacuum region inside and the pressure introducing pipe from the outside by means of a piezoelectric element that is vacuum insulated between the two and detects the detected value. It is amplified and taken out as an electric signal.
The internal vacuum degree during manufacturing is usually 6.7 Pa (0.05 Tor)
r) or less, and the durability as a vacuum sensor is 20
More than a year. That is, the maximum permissible vacuum leak rate of this type of pressure sensor that has been put to practical use is 1.0 × 10 ^−5.
It is said that it is necessary to have Pa · cc / sec or less.

In this sensor structure, when the internal vacuum degree drops to a certain value, the output electric signal of the piezoelectric element becomes non-proportional to the pressure change from the outside, so that the vacuum leakage is fatal to the pressure sensor. For this reason, various inspections have been carried out, but the current general inspection method is to seal the pressure sensor in a pressurized vessel of radioactive material for several days, and if a vacuum leak occurs, suck the radioactive gas. Since it will be stored, take out it and measure the radioactivity intensity with a Geiger counter etc.
The quality is determined and the vacuum leakage rate is determined.

However, the above-mentioned conventional quality determination method has a problem of pollution due to radioactive contamination. That is, it is very difficult to handle radioactive substances, and the work environment and the workability of workers are very poor. The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a simple, safe, accurate, pollution-free and economical method for determining the quality of a pressure sensor.

Further, even if the vacuum degree inside the pressure sensor is maintained, there are cases in which there is an electric circuit defect and the product is defective. Accordingly, it is another object of the present invention to provide a method for determining the quality of a pressure sensor, which can easily and quickly inspect whether or not there is a defect in an electric circuit.

[0006]

The present invention proposes a method of estimating from a discharge phenomenon in a region where the leak rate of a vacuum is very small, and from an electrical output signal in a region where the leak rate is higher than that.

In order to achieve the above-mentioned object, the method for judging the quality of a pressure sensor according to the present invention is a strain for measuring the pressure by a strain voltage generated from the difference between the degree of vacuum in the case and the pressure applied to the pressure introducing portion. This is a method for judging the quality of vacuum leakage of a gauge type pressure sensor, and after a predetermined time has elapsed from the sensor manufacturing date, a high DC voltage of a predetermined value is applied between the terminals with all the electrodes short-circuited and the case. ,
If a discharge current flows, it is determined that a vacuum leak has occurred and the product is defective.If no discharge occurs, a DC bias voltage of a predetermined value is applied to the electrode and the output voltage is recorded. Further, after a lapse of a predetermined time, a DC bias voltage of exactly the same value is applied again to measure the output voltage, and if the measured voltage difference is equal to or more than a predetermined value, it is determined that a vacuum leak has occurred and the product is defective. And

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION First, the principle of the present invention will be described.
The measurement of the vacuum leakage rate using the discharge phenomenon will be described. Paschen's Law, which states that the minimum voltage required to cause a spark discharge is defined as a function of the product of the distance between electrodes and the pressure of gas if the electric field is uniform and the temperature and humidity of gas are constant. Since the vacuum pressure P1 can be estimated from the discharge start voltage or the discharge current caused by the difference in the vacuum pressure, the difference with the vacuum pressure P0 when the pressure sensor is sealed is known, and therefore, from the time of sealing to the time of measurement. From time T, the average vacuum leak rate R in that period can be roughly calculated by the following formula 1.

[Formula 1] R = (P1-P0) · v / T [Pa ·
cc / sec] In the formula, v is the internal volume of the pressure sensor.

Since the internal pressure at the time of vacuum containment is a considerably high vacuum, for example, 6.7 Pa as described above, an appropriate voltage between the metal case of the sensor and each electrode,
For example, even if several hundreds V is applied, no discharge occurs, but if vacuum leakage occurs and the degree of vacuum is appropriately lowered, discharge according to Paschen's law occurs. The degree of vacuum can be estimated from the discharge voltage or discharge current at this time.

Of course, it is not possible to directly measure the degree of vacuum inside the actual pressure sensor, and therefore, as shown in FIGS. 1 (A) and 1 (B).
With respect to the pressure sensor 1 having the structure shown in FIG. 2, a vacuum device and a voltage application system as shown in FIG.

First, in the pressure sensor 1, a plurality of electrodes 2, ... And a pressure introducing pipe 3 are provided on a base 1a so as to be electrically insulated from each other, and a metal diaphragm 4 is attached to the tip of the pressure introducing pipe 3 to form a diaphragm surface. The upper piezoelectric element 5 is provided, wiring is provided between the piezoelectric element 5 and the electrode 2, and the upper portions thereof are covered with the metal case 6. Then, a pipe 8 is arranged between the pressure introducing pipe 3 and the rotary pump 7 of the pressure sensor 1 as described above so that pressure can be applied.
In the middle of, the Pirani vacuum gauge 9 and the pressure adjusting valve 10 are connected, the terminals of all the electrodes 2 of the pressure sensor 1 are short-circuited, and the power supply 11 for voltage application and the voltmeter 12 are connected.

In such a system, when a discharge voltage of an appropriate value of about several hundreds of volts is applied by the power supply 11, the pressure sensor 1 is discharged with a discharge voltage of an appropriate magnitude within a certain range of vacuum. Occurs, and the degree of vacuum can be estimated from the discharge current. However, since the discharge does not occur on the higher or lower vacuum side than the above range, the degree of vacuum in the pressure sensor 1 cannot be determined.

That is, the discharge does not occur if the pressure sensor 1 is a good product and the inside is kept at a predetermined vacuum degree, or even if the discharge does not occur at a speed exceeding a predetermined vacuum leakage rate range. Is there a vacuum leak? Therefore, a direct current voltage of about several volts is applied to the electrodes 2 of the pressure sensor 1, and the output voltage is recorded.
The output voltage is measured again after being left for about a day, and if the voltage difference is several mV or more, it can be determined that the product is defective because it leaks at a speed that greatly exceeds the maximum allowable vacuum leak rate. The output voltage in this case is proportional to the pressure difference between the inside and the outside of the pressure sensor 1 (external pressure-internal pressure) as shown in FIG.
The pressure applied from the pressure introducing pipe 3 is set so that this difference is always positive.

[0014]

EXAMPLE An example of the present invention will now be described with reference to FIG. 3 showing the measurement results of the discharge start voltage Vd and the discharge current Id when the pressure P1 in the pressure sensor 1 is changed, and again with reference to FIG. Explain. The maximum value of the discharge voltage is set to 600V (The IC circuit for amplifying the strain voltage built in the pressure sensor 1 is destroyed by a separate test because the discharge voltage is about 1000V. When the present invention was carried out, the pressure range in which discharge occurred was about 20 Pa to 800 Pa. In this range, the pressure P1 at that time can be known almost accurately from the discharge current value,
As described above, the degree of vacuum in the pressure sensor 1 could be estimated based on Paschen's law.

For example, assuming that the pressure P0 of the pressure sensor 1 during vacuum containment is P0 = 6.7 Pa, the pressure 10 days after the containment is P1 = 20 Pa, and the internal volume v of the sensor is v = 0.5 cc, the vacuum is calculated from the above formula 1. Leak rate is R = 0.77 ×
It becomes 10 ⁻⁵ (Pa · cc / sec), and in this case, it is barely less than the maximum allowable vacuum leakage rate Rm. Conversely, P1 =
If it is 800 Pa, R = 4.6 × 10 ⁻⁴ (Pa · cc /
sec), which means that the vacuum leak occurs at a speed of about 50 times the maximum allowable vacuum leak rate Rm.

On the other hand, as described above, since discharge does not occur in the pressure range of 20 Pa or less and the pressure range of 800 Pa or more,
The degree of vacuum in the pressure sensor 1 cannot be estimated. Therefore, in the system of FIG. 2, the output voltage Vo was measured by the voltmeter 12 while applying the DC bias voltage Vcc = 6V from the power supply 11 to gradually increase the measurement terminal pressure Px of the Pirani vacuum gauge 9. The measurement result is shown in FIG. In the same figure, the electric output when the measuring terminal pressure Px is changed in the pressure sensor of the non-defective product is shown.

The output of the defective pressure sensor is the pressure Px.
Is a constant value close to the DC bias voltage regardless of the change of, the output of the pressure sensor of a good product is a small value of about 10 to 20% of the bias voltage up to near atmospheric pressure (4 × 10 ⁴ Pa). However, it increases slightly as the pressure increases, and rapidly increases to the DC bias voltage value when approaching atmospheric pressure. This pressure increase is due to the internal pressure P1 of the pressure sensor.
Is a high vacuum, the pressure difference (Px-P
1) increase. If the pressure Px applied to the pressure introducing pipe 3 from the outside is kept constant, this slight voltage change is caused by the pressure change P1 inside the sensor, so that the vacuum leak rate can be estimated from the above-mentioned formula 1. As aforementioned,
If the vacuum leak is large and no discharge occurs, it is at least about 50 times the maximum allowable vacuum leak rate and 10
There is a pressure increase of 1000 Pa or more if left for a day (for example, if leaking at a speed 50 times faster, the pressure after 10 days becomes 1600 Pa (= 12 Torr)), and the voltage change with respect to this pressure increase is several mV. Therefore, the pressure change during that time, that is, the vacuum leakage rate can be roughly estimated by the mathematical formula 1 by measuring the voltage increase.

From the above results, considering a practical vacuum leak inspection system for a pressure sensor, first, about 10 days after the sensor is manufactured, 600 V is applied between the terminals where the metal case 6 forming the outer wall and all the electrodes 2 are short-circuited. If a DC voltage is applied and a discharge current flows, at least 0.77 × 10 ^-5
Vacuum leakage occurs at a speed of Pa · cc / sec or more, so it is removed as a defective product, and then vacuum is generated at a speed of 4.6 × 10 ⁻⁴ Pa · cc / sec or more even if no discharge occurs. Since there is a possibility that leakage has occurred, a direct current voltage of 6 V is applied to the electrode 2 of the pressure sensor 1 and the output voltage is recorded, and the output voltage is measured after leaving it for about 10 days. If the value is several mV or more, this is a defective product because it leaks at a speed 50 times or more the maximum allowable vacuum leak rate. In this case, the output voltage is proportional to the pressure difference (Px-P1) between the inside and the outside of the pressure sensor 1, so that the difference (Px-P1) is always positive in the pressure introducing pipe 3, that is, as Px. Its maximum value 4 × 10 ⁴
It suffices to set Pa.

[0019]

As described above, the method of judging the quality of the pressure sensor according to the present invention is the same as that described above. Therefore, instead of the conventional method using radioactivity, the discharge and electric output characteristics of vacuum leakage and electric circuit defects are eliminated. It is possible to estimate a wide range from both of the above, and it is simple and safe, accurate, pollution-free, and economical.

[Brief description of the drawings]

FIG. 1 is a plan sectional view (A) and a side partial sectional view (B) showing a structure of a pressure sensor to be measured.

FIG. 2 is a conceptual diagram showing a measurement system of the pressure sensor of FIG.

FIG. 3 is a diagram showing a discharge result by applying a discharge voltage in the apparatus of FIG.

4 is a diagram showing a measurement result by applying a DC bias voltage in the apparatus of FIG.

[Explanation of symbols]

 1 Pressure Sensor 1a Base 2 Electrode 3 Pressure Introducing Tube 4 Metal Diaphragm 5 Piezoelectric Element 6 Metal Case 7 Rotary Pump 8 Piping 9 Pirani Vacuum Gauge 10 Pressure Control Valve 11 Power Supply 12 Voltmeter

Claims (1)

[Claims] 1. A method for determining the quality of a vacuum leak in a strain gauge type pressure sensor for measuring a pressure by a strain voltage generated from a difference between a degree of vacuum in a case and a pressure applied to a pressure introducing portion, After a lapse of a predetermined time from the sensor manufacturing date, a DC high voltage of a predetermined value was applied between the terminals with all the electrodes short-circuited and the case, and if discharge current flowed, it was determined that vacuum leakage had occurred and the product was defective. If it is judged that no discharge occurs, a DC bias voltage of a specified value is applied to the electrode and the output voltage is recorded, and then a DC bias voltage of the same value is applied again after a specified period of time to output. A method for determining the quality of a pressure sensor, which comprises measuring a voltage and determining that a vacuum leak has occurred if the measured voltage difference is equal to or more than a predetermined value, as a defective product.