Unraveling the Basics of MEMS Pressure Sensors for Industrial Applications
Pressure sensors are used in a wide range of applications, especially in industrial settings. With that in mind, learn about types of pressure sensors and their use cases.
Pressure sensors are used across a wide variety of applications in industrial markets. Continuous improvements are allowing these sensors to enable new applications, beyond their use in traditional applications such as steam and fluid pressure detection.
Most recently, the availability of new lower power sensor technologies, with reduced size, more integrated capabilities, better economics, and wider operating supply voltages are enabling innovators to deploy sensors for Internet of Things (IoT) applications. In addition, these sensors contribute to the renewed effort to create more sustainable products that consume less power and embed additional innovative features.
In focusing on the applications, this article explains the methods and technologies that detect pressure in industrial applications. We also discuss micro-electro-mechanical sensor (MEMS) technology, an example shown in Figure 1.
Figure 1. Example of a MEMS pressure sensor. Image used courtesy of STMicroelectronics
Many modern pressure sensors use MEMS and we present a deep dive into its full capabilities. We also review the advantages and limitations of each principal attribute and the applicability for various industrial use cases.
Understanding Pressure for Pressure Sensing
Pressure expresses the force exerted onto a surface with a given unit area. Common units for reporting pressure include the “Pascal,” “Bar,” and pounds per square inch (“PSI).” The sensor type or application typically defines the units to use. For example, in water-level applications, bars or millibars typically indicate a pressure value. In automotive tire pressure applications, PSI is more common.
Pressure can also be used to measure vertical distance, or altitude, by measuring the barometric air pressure. Using the altitude at sea level as a reference, the standard sea level is 1013.25 mb (millibars). Moving to higher or lower altitudes from sea level changes the air pressure. This is because the mass of the column of air above the sensor depends on where the sensor is located. Higher altitudes have lower air mass overhead and therefore lower pressure.
Sensors can also measure the pressure of liquids, gases, and solids to measure depth, regulate flow, or provide feedback for many other reasons.
Types of Pressure Sensors
Gauge pressure references measurements to atmospheric or ambient pressure, which is typically 14.7 PSI or 1013.25 mb. “Positive” pressure is the pressure above ambient, and “negative” pressure is below ambient. Gauge pressure is most useful in applications that measure pressure over long periods of time, with little or no additional calibration.
This type of pressure is measured against an absolute vacuum. A full vacuum has an absolute pressure of zero PSI. Absolute pressure sensors are useful in applications that must detect pressure below atmospheric pressure. For example, an altimeter is a type of absolute pressure sensor used to detect altitude above or below sea level.
This pressure type is the difference between measured pressure and a second, reference pressure. The reference pressure may be atmospheric pressure (gauge) or another pressure that can be higher or lower than the pressure being measured. Differential pressure sensors are especially useful when measuring flow rate.
Utilizing Pressure Sensors in Industrial Applications
There are many use cases for pressure sensors in industrial applications and several known methods have been used for decades. The trend, however, is to develop smaller, smarter, and more energy-conscious sensors that benefit the end applications.
Using a MEMS Pressure Sensor for Fluid-level Detection
One application in broad use today is water- or fluid-level detection in a tank.
Figure 2 is a block diagram of a liquid-level measurement system using a MEMS pressure sensor.
Figure 2: Water-level measurement with a single absolute pressure sensor
This system is common in many applications, including consumer or industrial washing machines, respiratory equipment, coffee makers, and other level-detection systems.
This MEMS-sensor approach is especially scalable to IoT applications where measurements need to be automated. For example, applications, where the liquid is inaccessible and therefore impossible to measure using a dip-stick or visual float, are well suited to these sensors.
A MEMS pressure sensor detects water level using a single absolute (barometric) pressure sensor that is connected to a hose or gum tube inserted into the fluid. The MEMS sensor must be located above the fluid to be measured.
The fluid partially fills the gum tube, replacing air with liquid and forcing the air toward the pressure sensor. The sensor does not need to contact the liquid and measures it from the pressure of air in the tube, which can easily be correlated to the amount of liquid in the container. The more liquid in the container, the higher the pressure in the tube.
Using Absolute Pressure Sensor for Fluid Level Measurement
When a single absolute pressure sensor is used for fluid level measurement, the application software should manage the zero offset. The zero offset sets the zero-pressure reference before filling the container with the liquid. Subsequent pressure measurements are then referenced to this zero reference.
Some applications may set the zero reference using the microcontroller unit (MCU) and a dedicated software routine.
Smart sensors with built-in Bluetooth connectivity would typically do this to allow monitoring the sensor wirelessly. In these systems, remote access is mandatory, and a second pressure sensor may be the reference essentially creating a differential sensor. Figure 3 shows a second absolute pressure sensor as a zero reference effectively emulating a differential sensor.
Figure 3: Water-level measurement using two absolute pressure sensors
The second sensor constantly measures the ambient air pressure, which can then be subtracted from the pressure value to obtain the gauge pressure in the tube.
Using Pressure Sensors for Drone and Robotics Applications
Drone and robotics applications are other key applications for pressure sensors. Drones use barometric pressure sensors to detect altitude, which is extremely useful for Altitude Hold.
Thanks to the sensor's small size, low power and noise, high accuracy, and immunity to vibration, modern pressure sensors can detect an absolute altitude of <3 m, with a minimum detectable change in altitude of just a few cm.
To avoid collisions, the altitude or z-axis (shown in Figure 4) component is extremely important in cases where the drone must fly in permitted altitudes or ‘slots.'
Figure 4: Z-axis (altitude) dimension in a drone application
Similar to altimeter applications in UAVs, pressure sensors can detect airplane take-offs and landings in telematics and asset-tracking applications.
The pressure sensor detects instantaneous pressure changes (Figure 5) within the plane when, for example, the cabin doors close before take-off.
Figure 5. Using a pressure sensor to detect airplane take-offs and landings
The sensor can turn off an asset tracker GPS on take-off, to extend the system’s battery life. Controlling the GPS activity is also important in markets where local government regulations dictate GPS radio usage in flight.
Industrial pressure sensors for flow rate and leak detection for air, gases, and liquids are a growing application. Chip manufacturers pre-calibrate most modern pressure sensors and these already produce highly accurate outputs across the full temperature range. Using two absolute pressure sensors to measure the flow rate of gas or air is an emerging low-cost and elegant method that can be realized (Figure 6).
Figure 6. Flow rate using two absolute pressure sensors
This approach puts two absolute pressure sensors in a large and small cavity and detects the pressures in each. With any airflow, the pressure is higher in the small section and lower in the larger cavity. Measuring the difference in pressure across the pipe allows the calculation of the flow rate.
Leveraging MEMS Technology for Pressure Sensing
All in all, there are many different types of pressure sensors, and the progress of MEMS technology has helped enabled the semiconductor industry to make economic pressure sensors in high volumes.
The small size, low power consumption, embedded compensation, and robust packaging allow MEMS pressure sensors to be used in many industrial applications that were possible before.
Modern industrial systems use a mixture of sensor technologies to deploy systems that run more efficiently with less wasted energy.
All images used courtesy of STMicroelectronics
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