1 Introduction
Reliability refers to a product's ability to perform its intended function under specified conditions and within a specified time. It is a measure of a product's ability to operate without failure when put into use. The level of product reliability indicates the likelihood that the product will perform its intended function under specified conditions and within a specified time. Reliability is typically expressed using reliability percentage, failure rate, and MTBF. The reliability level of components such as sensors is usually expressed using failure rate, while equipment reliability is often expressed using indicators such as reliability percentage, MTBF, and availability.
Pressure is one of the most fundamental parameters for measurement and control in production processes and scientific experiments. Pressure sensors, which measure pressure and convert it into electrical signals according to certain rules, are crucial components for measuring this signal and are widely used in production and research in industries such as industry, petroleum, chemical engineering, biomedicine, navigation, aerospace, and aviation. The reliability of sensors is extremely important, directly affecting the performance of the system using them , influencing the quality and speed of production and construction, and even potentially leading to serious personal safety issues. In the use of sensors, reliability can be defined as fault-free operation, that is, the overall performance that guarantees the performance within the limits required by technical specifications. When a sensor fails to transform a specific physical quantity, it is clear that the mechanical integrity of its components and the deviation of the output measurement parameters have been compromised, causing the entire product equipped with the sensor to fail to complete its task. Therefore, in the sensor manufacturing process, to ensure the accuracy, stability, and consistency of pressure signal measurement and transmission by the pressure sensor, the reliability of the pressure sensor testing process must be analyzed.
2. Pressure sensor testing principle
Pressure sensors include piezoresistive pressure sensors manufactured using the piezoresistive effect, piezoelectric pressure sensors manufactured using the piezoelectric effect, and strain gauge pressure sensors manufactured using the strain effect. This article will only take the piezoresistive pressure sensor as an example to explain its testing principle.
The piezoresistive pressure sensor core is shown in Figure 1. R1, R2, R3, and R4 are four resistive elements. When subjected to an external force, the resistance of elements R1 and R3 increases due to tensile stress, while the resistance of R2 and R4 decreases. Thus, the external force F causes a change in the resistance value of the four resistive elements. The resistors R1, R2, R3, and R4 on the sensor are connected in a DC bridge circuit as shown in Figure 2. The terminals cd are connected to a regulated power supply E, and the terminals ab are the bridge voltage output terminals, with an output voltage of U0, as shown in Figure 2.
Equation (2) is the bridge balance condition. The sensor has four identical resistors attached. Ideally, assuming they have the same resistance value, we have...
R1=R2=R3=R4=R（3）
Therefore, when the sensor is not subjected to external force, the bridge circuit satisfies the equilibrium condition, and the voltage U0 output at terminals a and b is 0. When the beam is subjected to a load F, as shown in Figure 3, the bridge circuit becomes unbalanced, and then...
From equation (6), it can be seen that the unbalanced voltage U0 output by the bridge is proportional to the change in resistance ΔR, which is the working principle of the unbalanced bridge. Obviously, the measured value of U0 can reflect the magnitude of the external force F. In addition, from equation (6), it can also be seen that if a larger output voltage U0 is obtained, a higher power supply voltage E can be used, which also shows that an unstable power supply voltage will bring errors to the measurement results. In the actual production test process, the four resistors of the bridge arm cannot be exactly the same, and the environmental conditions applied during the test will also affect the output of the sensor. Thus, when the environmental conditions change, their increments ΔR are not exactly the same, which ultimately leads to a deviation between the test results and the ideal results.
3. Pressure sensor testing process
The testing of pressure sensors mainly refers to temperature output characteristic testing and pressure output characteristic testing. Temperature output characteristic testing mainly refers to the effect of different temperature conditions on the sensor output, mainly referring to zero-point temperature drift and sensitivity temperature drift; pressure output characteristic refers to the output characteristics reflected by different pressures under the same environmental conditions of the sensor, which mainly measures the static characteristic indicators of the sensor.
The pressure sensor measurement system mainly consists of four parts: pressure source, pressure sensor under test, measurement circuit, and display and recording. Their relationship is shown in Figure 4.
Under certain environmental conditions (provided by a high and low temperature environmental test chamber) and powered by a power supply, the pressure sensor under test measures the signal generated by the pressure source (piston pressure gauge or digital pressure controller). The signal generated after measurement is finally transmitted to the display device (computer) for display and recording through the measurement circuit (multi-channel data acquisition device).
4. Factors affecting the reliability of pressure sensor testing equipment during testing.
The reliability indicators of pressure sensor testing are constantly changing with design modifications, different usage conditions, and performance degradation during operation. Therefore, it is necessary to consider multiple aspects.
4.1 Specify the conditions for use
Every product is developed under specified operating conditions. These conditions include working conditions (such as functional mode, operating method, load conditions, power source, and maintenance conditions) and environmental conditions (such as temperature, humidity, air pressure, and vibration). The reliability of the same product varies under different working and environmental conditions. Equipment used to test pressure sensors is no exception.
Environmental conditions affecting the reliability of the testing process generally include the following: climatic conditions, mechanical conditions, biological conditions, chemical conditions, electrical and electromagnetic conditions, radiation conditions, system connection conditions, and human factors. Here, we primarily consider climatic conditions, chemical conditions, and system connection conditions. Climatic conditions include factors such as temperature, humidity, air pressure, wind, rain, frost, and dust storms. Chemical conditions include factors such as power supply instability, interference signals from conductive wiring systems, interference from electromagnetic fields, lightning, corona discharge, and other electrical disturbances. System connection conditions include factors such as the connections between functional units within a large system, and the connection of a system or piece of equipment with other equipment. All three of these factors affect equipment reliability, but the degree of influence varies depending on the specific equipment.
4.2 Usage Time
The reliability of sensor testing equipment is a function of time, decreasing over time. Even after component screening and system aging and repair, the failure rate remains constant when the equipment operates within the random failure zone. However, the distribution of equipment reliability over time conforms to the pattern shown in Figure 5: the longer the equipment is used, the lower its reliability.
4.3 Functions that can be achieved
The term "function" refers to a product's main performance indicators and technical requirements. This ensures the product fulfills its designated tasks and functions. Product unreliability specifically refers to whether the product meets its specified performance indicators and technical requirements. Higher performance indicators and technical requirements mean a smaller allowable range of variation, reducing the likelihood of the product fulfilling its "designated function," thus lowering its reliability. Conversely, lower performance indicators and technical requirements mean a wider allowable range of variation, increasing the likelihood of the product fulfilling its "designated function," thus higher its reliability. For sensor testing equipment, higher accuracy requirements generally result in lower reliability, but more accurate data obtained from the sensor. Conversely, lower accuracy requirements for pressure sensors lead to higher reliability, but also a greater impact on obtaining accurate sensor data.
It should also be particularly pointed out that, from the perspective of using testing equipment, assuming that the basic performance indicators meet the usage requirements, the most important indicator of the equipment is its reliability. If the equipment is unreliable, its performance is unstable, and it breaks down frequently, then even if the initial performance of the testing equipment is excellent, it is meaningless, let alone obtaining the true data from the sensors.
4.4 Reliability Analysis
Assuming that, at a certain confidence level, the reliability of the pressure source is R1(t), the reliability of the environmental supply equipment is R2(t), the reliability of the measurement circuit is R3(t), and the reliability of the power supply system is R4(t), then the reliability R(t) of obtaining the true sensor data must be related to R1(t), R2(t), R3(t), and R4(t), i.e.
R (t) = f [R1 (t), R2 (t), R3 (t), R4 (t)] (7)
From equation (7) and the measurement system in Figure 4, it can be seen that
R(t)=K? R1(t)? R2(t)? R3(t)? R4(t)(8)
In the formula, K is a coefficient that is closely related to the usage time and conditions of various devices.
Taking the testing of pressure sensors using our unit's sensor testing equipment as an example: Under the same testing conditions, zero-point temperature tests were conducted on 10 uncompensated pressure sensors using two different environmental devices. The parameter specifications of the two ovens are shown in Table 1, and the sensor test data are shown in Table 2.
Table 2 shows that, under identical conditions, different temperature chambers yield different test results. The most important thing for us is to determine which set of data is more reliable and closer to the true performance of the sensor. Table 1 shows that temperature chamber #1 has been in operation for nearly two years, while temperature chamber #2 has been in operation for over ten years. Based on the reliability distribution of the equipment over time, the reliability of equipment #1 is higher than that of equipment #2. Therefore, the sensor data obtained using equipment #1 is more reliable.
4.5 Measures to increase the reliability of the testing process
① Strengthen the management of pressure sensor testing equipment, conduct regular inspections and maintenance, and extend the service life of the equipment while ensuring that its performance meets the requirements.
② Strictly control environmental conditions such as temperature, humidity, and cleanliness of the testing site; shorten the turnaround time between each process and improve the testing efficiency of the sensor.
③ Regularly test and calibrate various testing equipment to ensure that the performance of the testing equipment meets the requirements for sensor testing, thereby ensuring the reliability of the obtained test data.
④ We must persist in summarizing, calculating, and analyzing the test data of the sensors, identify the factors that affect the sensor output during the testing process, and solve them, so as to lay a solid foundation for improving the quality of sensor products.
5 Conclusion
Based on the reliability of pressure sensors, this paper proposes the concept of reliability in the pressure sensor testing process, elaborates on the various factors affecting the pressure sensor output during the testing process, proposes measures to increase the reliability of the sensor testing process, and provides a new approach for in-depth research on pressure sensors.
