The miniature coplanar interface differential pressure sensor is a pressure sensor primarily used to measure and control the pressure difference between two oil circuits. This product utilizes SOI silicon wafer material to fabricate a silicon piezoresistive pressure-sensitive chip. The sensing element operates within a temperature range of -55 to 300°C. Through a dual-core structure, it converts the pressure difference signal from the oil circuit into an electrical signal. A sophisticated signal processing circuit then amplifies the weak electrical signal to achieve a standard analog signal output.
1 Introduction
The small coplanar interface differential pressure sensor (hereinafter referred to as differential pressure sensor) is small in size, compact in installation, high in accuracy, high in reliability and strong in environmental adaptability. In particular, the sensing element can adapt to the working environment of -55 to 300℃. It is a key component for measuring and controlling the oil circuit pressure difference of actuators. By expanding the range and modifying the structure, it can be applied to other similar environments.
2. Working principle of differential pressure sensor
The working principle of the differential pressure sensor is based on the diffused silicon piezoresistive mechanism. The diffused silicon piezoresistive (SIP) pressure sensor is the most mature technology among piezoresistive pressure sensors. It utilizes the pressure-sensitive effect of the resistivity of diffused silicon to form four diffused silicon resistors in a Wheatstone bridge, as shown in Figure 1.
The sensor senses the pressure signal of the measured medium through the pressure interface and transmits it to the sensitive chip. When the diffused silicon resistor on the chip senses the pressure, the resistance of its bridge arm changes accordingly. Under the action of the excitation power supply, this change in sensor resistance is converted into a change in electrical signal. A dedicated signal circuit is designed to convert the difference between the two independent pressure signals into a standard electrical signal output, the principle of which is shown in Figure 2.
3. Selection of Sensitive Elements
The differential pressure sensor developed in this project requires a sensitive element operating within a temperature range of -55 to 300°C, making it a high-temperature resistant pressure sensor. If conventional single-crystal silicon wafers are used to fabricate the sensitive chip, the differential pressure sensor can only achieve an operating temperature of 125°C. Therefore, a suitable high-temperature resistant silicon wafer material must be selected. SIMOX material is an advanced SOI material. The pressure-sensitive chip fabricated using SIMOX employs a bulk resistive structure, achieving electrical isolation between resistors through an insulating layer, rather than pn junction isolation. This avoids a sharp increase in reverse leakage current under high-temperature conditions, thus preventing sensor failure.
In conclusion, SIMOX material has been selected as the silicon wafer material for this project.
4 Design
4.1 Circuit Design
The differential pressure sensor outputs an analog voltage. The circuit design uses a precision differential amplifier to linearly amplify the pressure-sensitive signal, modulating the sensor's zero-point and full-scale output values to the nominal values. The principle block diagram is shown in Figure 3.
The design of the signal modulation circuit for the silicon piezoresistive pressure sensor has multiple functions:
① In order to improve the temperature characteristics, the signal circuit and the temperature compensation network of the sensitive device are matched and combined to compensate for the thermal zero drift and thermal sensitivity drift of the sensor. The compensation circuit of the sensor is shown in Figure 4.
② The pressure-sensitive signal is linearly amplified using a precision differential amplifier (typically tens of millivolts/5V excitation), and the zero-point output value and full-scale output value of the sensor are modulated to the nominal value specified in the project for a "normalization" function.
③ Due to the sensitivity and nonlinear discreteness of the sensor, the differential pressure circuit must first modulate the output signals of the two sensitive devices separately, and then perform differential amplification to ensure the characteristic indicators of the differential pressure sensor, namely the symmetry indicators.
The circuit design principle of the differential pressure sensor is shown in Figure 5.
4.2 Structural Design
The differential pressure sensor's shape is designed based on project requirements, while also considering environmental adaptability and reliability. Particular attention is paid to the requirements of symmetry and high overload capacity (the differential pressure sensor's ultimate overload requirement should be able to withstand ±75MPa, 3 times the rated pressure). Therefore, the differential pressure sensor is designed as a dual-core, all-solid-state, symmetrical structure, utilizing dual sensitive chips to measure the pressure in the positive and negative chambers respectively. The core refers to the sensor element with the sensitive chips encapsulated. The dual-core design is specifically designed for measuring large pressure differential signals. After rigorous screening and symmetry selection, the components are matched and then assembled into the sensitive element. The internal structure of the differential pressure sensor is shown in Figure 6.
4.3 Reliability Design
First, the differential pressure sensor's sensing chip uses a SIMOX structure substrate, which enhances its adaptability to ambient temperature; the structure adopts an all-solid-state design, which enhances the sensor's environmental adaptability and stability.
Secondly, precision instrumentation amplifiers and thermistor network technology are used in the circuit to adjust parameters such as linearity and temperature of the sensor. All components in the circuit are surface-mount packaged as much as possible, avoiding the use of movable potentiometers. After assembly, the circuit board undergoes full-temperature range testing and stress screening, and is treated with "three-proof" measures (proof, protection, and dustproofing). The circuit board is mounted and fixed using potting reinforcement measures, providing resistance to vibration, impact, salt spray, and mold.
In addition, EMC design was specifically added to the circuit design. The sensor housing was encapsulated with a seamless structure, the mounting hole gaps were treated with an overlapping method, the housing had a built-in shielding foil, the cable shielding mesh was connected to the housing, and the circuit grounding point was connected at a common point to shield the electromagnetic interference to the sensor. Appropriate filtering devices were used between electrical connections to reduce the sensitivity of the circuit and filter out interference.
5. Conclusion
This product utilizes key technologies such as SOI pressure-sensitive chip fabrication, high-temperature resistant sensitive core packaging, low-drift modulation, and dual-core symmetry technology. It features high and wide temperature range tolerance, miniaturization, high overload resistance, high impact resistance, low drift, strong functionality, and strong environmental adaptability. Breakthroughs have been achieved in the use of SOI pressure-sensitive chips, high-temperature composite electrode fabrication, high-temperature resistant pressure-sensitive core packaging, low-drift modulation, and miniaturized symmetrical structure design.
Repeated tests have proven that the differential pressure sensor can operate continuously for 1000 hours under high-temperature conditions. The sensor's linearity reaches 0.10%FS, hysteresis and repeatability reach 0.03%FS, and thermal zero-point drift and thermal sensitivity drift both reach over 0.008%FS/℃.
