Oct.2024 21
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A Brief Discussion on Selecting Pressure Sensor Cores in the IoT Era

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In the IoT era, measuring pressure parameters for liquids and gases is becoming increasingly common in applications such as smart fire hydrants, smart water meters, smart homes, as well as the automotive and home appliance industries. Engineers often face challenges when selecting pressure sensor chips due to the variety of principles and product series available. They may not clearly understand the advantages and disadvantages, leading to decisions based on hearsay.

To address the issue of selecting pressure sensors for engineering applications, and drawing from our company's extensive experience in automation and IoT sensors, we offer a concise overview of the key criteria for selecting different types of pressure sensors based on measurement range, accuracy requirements, media type, and overall cost in various applications:

Different Ranges and Industry Characteristics Determine Sensor Type

The differentiation between high, medium, and low pressure often holds ambiguity. In the industrial sector, 0-100Kpa to 500Kpa is considered low pressure, 5bar-600bar is medium pressure, and above 600bar is high pressure. Each industry has its own characteristics for pressure sensor use. For example, ceramic pressure sensors are widely used in the automotive industry, while high-pressure industries like hydraulic construction machinery often use sputtered films. When selecting a sensor, it's crucial to communicate with industry professionals to understand their selection logic and reasoning.

In low-pressure applications, such as medical ventilators, MEMS and diffused silicon sensor chips are commonly used. However, in specific industries like food production where hygiene is important, or inkjet printers needing corrosion resistance, larger ceramic capacitive or resistive sensors might be chosen. For instance, products from E+H can be used in the ±7Kpa range for level and pressure measurement; ceramic resistive products can also measure within this range, with sensor core diameters around 32mm.

In medium-pressure applications, where there’s no need to withstand pressure over three times the burst pressure, standard concave meniscus ceramic resistive pressure sensors are more than sufficient. For example, over 95% of air compressors use this type. Recent structural improvements in ceramic resistive sensors, similar to ceramic capacitive designs, have led to the development of flat membrane ceramic resistive sensors with burst pressure resistance exceeding ten times the nominal range. Coupled with ceramics' high-temperature and corrosion resistance and unaffected by the dielectric constant of the measuring medium, these sensors are the preferred choice in the medium-pressure measurement field for their high cost-effectiveness.

In high-pressure scenarios like construction machinery, die casting machines, and injection molding machines, sensors need strong resistance to hydraulic shocks. Metal elastic bodies are generally preferred due to their superior toughness compared to ceramics. Metals, usually made from 17-4PH, offer better reliability in terms of burst pressure than ceramics.

    1.High-Pressure Applications:

For high-pressure scenarios, sensors primarily use sputtered thin film and strain gauges as pressure sensor chips, with signal outputs typically in the range of 1-2mV/V. Our company is leveraging its material and process technology advantages to develop products suitable for high-pressure applications. We are introducing a thick-film metal high-pressure sensor, which follows the same principle as ceramic resistive technology, with an output signal of 2-3mV/V. Utilizing unique processes like laser trimming and active temperature compensation, these sensors will outperform existing sputtered thin film and metal strain gauge products. This innovative high-accuracy, anti-creep, mass-producible metal thick-film sensor is an industry first. Stay tuned!

  1. Principles for Selecting Measurement Accuracy:

When selecting accuracy, it’s not about always choosing the highest possible accuracy, but what suits the application. High-accuracy pressure sensors are often expensive, and many products with high claimed accuracy have limitations on their conditions of use. Carefully review and verify the stated conditions on the datasheet to avoid misleading the engineers.

Regarding sensor output signals at the same pressure range, MEMS and diffused silicon products have a full-scale output of 5-20mV/V, thick-film ceramic sensors 2-4mV/V, while sputtered thin film and strain gauges output 1-2mV/V. Thus, MEMS and diffused silicon appear higher, ceramic resistive follows, and sputtered thin film and strain gauges are lower. However, due to the semiconductor process used in MEMS and diffused silicon, sensitive resistors are highly affected by temperature. Temperature effects mean that comprehensive accuracy is not as straightforward; adequate temperature compensation and calibration during use are essential to achieve optimal values.

With advancements in high-precision analog and digital integrated circuits, backend amplification ICs and ASICs now provide up to 24-bit ADC processing, with significantly reduced per-unit costs. Although ceramic resistive pressure sensors have slightly lower output values than MEMS, as long as the output signal is stable, using 128x or even 256x upstream amplification and high-bit ADC conversion circuits in the downstream circuit , the comprehensive measurement accuracy matches or surpasses diffused silicon. This can meet actual needs. The accuracy of ceramic resistive pressure sensors is improving, gradually replacing diffused silicon products in widespread industrial and civilian applications.

  1. Measuring Medium and Use Limits:

Measured media are classified into gases and liquids. Gases can be subdivided into clean gases and those containing water/oil, while liquids include oil and water media. The primary difference is in conductivity, dielectric constant, and chemical composition. Generally, MEMS and diffused silicon cannot come into direct contact with actual air or liquids and require oil-filled silicon or other gels for isolation. Benefiting from using ceramic diaphragms, ceramic resistive pressure sensors are corrosion-resistant and unaffected by the dielectric constant of the medium. Unlike ceramic capacitive pressure sensors, which can’t accurately measure the pressure of water or oil with water content if not isolated at the contact surface, ceramic resistive sensors are unaffected. 

In certain fields, the response speed and environmental resistance of MEMS and diffused silicon are limited by their materials, preventing use above 120 degrees.

Consumer-grade products may experience temperature drift above 80 degrees. Each MEMS and diffused silicon product therefore requires compensation and calibration at various temperatures, leading to high costs. In contrast, ceramic thick-film pressure sensors have a temperature coefficient of resistance below 100 ppm and sensitivity temperature coefficient below 10 ppm. With our unique active temperature compensation and laser adjustment technology, they can achieve zero temperature drift within a specific accuracy range from -40 to 125 degrees.

  1. Comprehensive Cost

In the IoT era, pressure sensors need to be mass-produced, highly reliable, low-cost, and accurate to meet application needs. When selecting sensors, cost considerations are crucial, including material costs, calibration expenses, maintenance costs, procurement channels, replaceability, delivery times, etc.

Typically, MEMS and diffused silicon chips require secondary oil filling and packaging, with module prices ranging from 60 to 200 yuan. The subsequent assembly and calibration expenses are also high, bringing the market price to around 300-400 yuan.

Recently, apart from SENSATA, domestic ceramic capacitive cores cost between 10-20 yuan. Although affordable, the ASIC for handling the backend capacitive circuit is pricey. These chips are mainly controlled by companies like Japan's Renesas, SENSATA, and Melexis, with total costs around 30 yuan. They are mostly used in automotive and air conditioning pressure sensors. Using this technology could lead to high production costs if reliant on external sources.

Sputtered thin-film cores are expensive, and welding costs are high. Strain gauge products face issues with high adhesive costs and instability, making them unsuitable for mass production and better suited for small-scale production.

The only type of pressure sensor that meets the requirements for mass production, high reliability, low cost, and adequate precision, and can utilize domestically produced conditioning chips, is ceramic piezoresistive pressure sensors. These offer the lowest comprehensive cost.

Our company has established the largest automated production line for ceramic piezoresistive pressure sensors in the country. We use highly stable circuits and active temperature compensation techniques. Calibration is only needed for pressure, significantly reducing calibration costs. By integrating domestic conditioning chips from companies like Jiuhao Electronics and Naxon Microelectronics, we can achieve autonomy and control over core components and chips, enabling low-cost, large-scale production and application prospects.