A Brief Discussion on Selecting Pressure Sensor Cores in the IoT Era
In the IoT era, measuring pressure parameters for liquids and gases is increasingly common in applications such as smart fire hydrants, smart water meters, smart homes, and 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. A lack of understanding about the advantages and disadvantages can lead to decisions based on hearsay.
To assist in selecting pressure sensors for engineering applications, we draw from our company's extensive experience in automation and IoT sensors. Below is 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.
Different Ranges and Industry Characteristics Determine Sensor Type
The differentiation between high, medium, and low pressure can often be ambiguous. In the industrial sector, the following classifications are common:
Low Pressure: 0-100 kPa to 500 kPa
Medium Pressure: 5 bar to 600 bar
High Pressure: Above 600 bar
Each industry has its own specific needs for pressure sensor use. For example, ceramic pressure sensors are widely utilized in the automotive industry, while high-pressure applications like hydraulic construction machinery often rely on sputtered films. When selecting a sensor, it’s crucial to communicate with industry professionals to understand their selection logic.
Low-Pressure Applications
In low-pressure applications, such as medical ventilators, MEMS and diffused silicon sensor chips are commonly used. However, in industries like food production where hygiene is paramount, or in inkjet printers that require corrosion resistance, larger ceramic capacitive or resistive sensors may be preferred. For instance, products from E+H can measure within the ±7 kPa range for level and pressure measurement, with sensor core diameters around 32 mm.
Medium-Pressure Applications
For medium-pressure applications, where pressures do not exceed three times the burst pressure, standard concave meniscus ceramic resistive pressure sensors are often adequate. Over 95% of air compressors utilize this type. Recent advancements have led to flat membrane ceramic resistive sensors that can withstand burst pressures exceeding ten times the nominal range. With high-temperature and corrosion resistance, these sensors are cost-effective choices in medium-pressure measurements.
High-Pressure Applications
In high-pressure scenarios, such as construction machinery and injection molding machines, sensors must withstand hydraulic shocks. Metal elastic bodies are preferred for their superior toughness compared to ceramics. Metals like 17-4PH provide better reliability regarding burst pressure.
For high-pressure applications, sensors primarily use sputtered thin film and strain gauges as pressure sensor chips, typically outputting signals in the range of 1-2 mV/V. Our company is developing a thick-film metal high-pressure sensor, which follows the same principle as ceramic resistive technology, with an output signal of 2-3 mV/V. Utilizing advanced processes like laser trimming and active temperature compensation, these sensors will outperform existing products in the market.
Principles for Selecting Measurement Accuracy
When selecting accuracy, the goal is not always to choose the highest possible accuracy, but rather what suits the application. High-accuracy pressure sensors can be expensive, and many high-accuracy claims come with limitations on their conditions of use. Carefully review the datasheet to avoid misunderstandings.
For sensor output signals at the same pressure range, MEMS and diffused silicon products typically have a full-scale output of 5-20 mV/V, thick-film ceramic sensors output 2-4 mV/V, while sputtered thin film and strain gauges output 1-2 mV/V. Although MEMS and diffused silicon appear superior, they are highly affected by temperature variations, necessitating adequate temperature compensation and calibration for optimal performance.
With advancements in integrated circuits, backend amplification ICs and ASICs now offer up to 24-bit ADC processing at reduced costs. While ceramic resistive pressure sensors have slightly lower output values than MEMS, stable output signals, combined with high-bit ADC conversion, can match or exceed the accuracy of diffused silicon. This improvement is facilitating the gradual replacement of diffused silicon products in various industrial and civilian applications.
Measuring Medium and Use Limits
Measured media are classified into gases and liquids, with gases further divided into clean gases and those containing water or oil. The primary differences lie 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. In contrast, ceramic resistive pressure sensors are corrosion-resistant and unaffected by the dielectric constant of the medium. Unlike ceramic capacitive pressure sensors, which struggle to measure the pressure of water or oil with water content without isolation, ceramic resistive sensors perform reliably.
In certain fields, the response speed and environmental resistance of MEMS and diffused silicon limit their use above 120 degrees Celsius. Consumer-grade products may experience temperature drift above 80 degrees. Thus, each MEMS and diffused silicon product requires compensation and calibration at various temperatures, increasing costs. In contrast, ceramic thick-film pressure sensors have a temperature coefficient of resistance below 100 ppm and a sensitivity temperature coefficient below 10 ppm. With our active temperature compensation and laser adjustment technology, they can achieve zero temperature drift within a specific accuracy range from -40 to 125 degrees.
Comprehensive Cost
In the IoT era, pressure sensors must be mass-produced, highly reliable, low-cost, and accurate to meet application needs. Cost considerations include material expenses, calibration costs, maintenance, procurement channels, replaceability, and delivery times.
Typically, MEMS and diffused silicon chips require secondary oil filling and packaging, with module prices ranging from 60 to 200 yuan. Assembly and calibration expenses can raise the market price to around 300-400 yuan.
Recently, domestic ceramic capacitive cores cost between 10-20 yuan. While affordable, the ASIC for processing the backend capacitive circuit can be expensive. These chips are primarily controlled by companies like Japan's Renesas, SENSATA, and Melexis, leading to total costs around 30 yuan. They are commonly used in automotive and air conditioning pressure sensors, but their reliance on external sources can result in high production costs.
Sputtered thin-film cores are costly, and welding expenses are high. Strain gauge products face high adhesive costs and instability, making them unsuitable for mass production and better suited for small-scale applications.
The only type of pressure sensor that meets the requirements for mass production, high reliability, low cost, and adequate precision—while utilizing domestically produced conditioning chips—is the ceramic piezoresistive pressure sensor. These sensors offer the lowest comprehensive cost.
Our company has established the largest automated production line for ceramic piezoresistive pressure sensors in the country. We employ highly stable circuits and active temperature compensation techniques. Calibration is only needed for pressure, significantly reducing costs. By integrating domestic conditioning chips from companies like Jiuhao Electronics and Naxon Microelectronics, we can achieve autonomy and control over core components, enabling low-cost, large-scale production with promising application prospects.
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