How do MEMS inertial sensors work? An overview:

Dr. Thomas Frasch
Dr. Thomas Frasch, November 2018

Measuring inclination, acceleration and vibration in a technical application requires high-precision inertial sensors: tiny MEMS (Micro Electromechanical Systems) based on single crystal silicon sensor elements and the latest micromechanical manufacturing processes. 

 

Different micromechanical processes are used for their production, with each technology having different strengths. First Sensor has developed new micromechanical processes for MEMS production and launched inertial sensors that combine the advantages of the others in an innovative product series.

 

The result: superior performance features, an amazing price–performance ratio and new applications, especially in the fields of geoengineering, condition monitoring, navigation and robotics.

 

Good reasons to highlight this MEMS innovation in your own blog post ... 

MEMS – Micro Electromechanical System: what a high-precision accelerometer can do

Miniaturized MEMS (Micro Electromechanical System) sensors can now measure accelerations in all three spatial dimensions.

 

MEMS inertial sensors have shown themselves to be very robust, reliable and fast. These state-of-the-art products are also extremely temperature-stable. And they detect even the smallest changes in position or acceleration. 

 

MEMS inertial sensors

This image shows the inclinometer’s extremely high resolution: Even the deflection of a 10-meter long board by a single human hair with a diameter of 100 µm can be detected. This corresponds to a deflection of only 0.0005° (2 arcsec or 10 µm/M).

The future is digital 

MEMS sensor technology is a key technology for the Internet of Things. As digitalization progresses, miniaturized accelerometers and inclinometers will likewise also continue to develop. In the future, inertial sensors will be intelligently programmed, for example, and will have microcontrollers, miniature batteries or tiny radio chips to send their measurement data online. 

 

Current areas of application for high-precision inertial sensors from First Sensor:

 

• Condition monitoring of buildings

• Monitoring of Oil & Gas on/off-shore infrastructures, nuclear, gas and hydropower plants etc.
• Monitoring of wind power and solar energy plants, high-voltage lines, dams, pipelines, etc.
• Stabilization and alignment systems
• Navigation
• Infrastructure and transport

Mini format, mini production costs, mini operating costs: the technical concept of a capacitive inertial sensor

MEMS inertial sensors for the capacitive measurement of inclination, acceleration and vibration rely on state-of-the-art micromechanical manufacturing processes and on a tiny silicon sensor: a spring-mass system whose structures are silicon webs just a few µm wide. During acceleration, masses suspended on springs are deflected, allowing a change in capacity to be measured. An ASIC reads this change in capacity and transmits the measured value. 

 

During production of the sensors, masses and springs (bear in mind that these are structures often only a thousandth of a millimeter thick) are etched out of the silicon. 

 

MEMS-Inertialsensor
Thanks to their micro format, MEMS can be produced in large quantities. They also consume less energy.

Technical components of an MEMS inertial sensor:

• Monocrystalline silicon sensor
• High-performance ASIC
• Housing for both chips

MEMS performance data from First Sensor:

Measurement ranges
±30°
±3 g, ±8 as well as ±15 g
Noise density
less than 0,0004°/√Hz
less than 30 µg/√Hz
Resolution
less than 0,0015°
less than 40 to 95 µg
Measurement frequency
> 6 Hz
> 6 Hz

 

Download the data sheet for SI/SA inertial sensors (English)!

The heart of the sensor: the monocrystalline silicon sensor

The silicon sensor – the heart of every MEMS – is traditionally manufactured in bulk or in a surface micro-machining process. However, manufacturer First Sensor is now using new manufacturing technologies: the HARMS (High Aspect Ratio Microstructures) and the AIM (Air Gap Insulated Microstructures) processes.


The former enables microstructures with a high aspect ratio and thus minimizes cross sensitivities. The latter minimizes parasitic capacitance by isolating the components using an air gap. The result is MEMS that offer more advantages than all traditionally manufactured inertial sensors combined.

 

Advantages of the First Sensor MEMS inertial sensors in measuring inclination, acceleration and vibration:

 

+ Flexible MEMS design: measurement ranges from 1 to 15 g

+ Four standard sensors: four different measurement ranges, optimum adaptation to the range (spring-mass principle) 
+ Measurement of two axes with one sensor: easy to use
+ Silicon microstructures with high aspect ratio: ultra-low transverse axis sensitivity, fatigue-free, long-term stable sensor
+ Air-gap-isolated microstructures: minimized parasitic capacitance, minimized mechanical stress thanks to missing SiO2 layers, excellent temperature stability, easier calibration

Direct comparison of MEMS technologies

Surface micro-machining
HARMS/AIM technology
Bulk micro-machining
In plane displacvement and out of plane displacement
In plane displacement
Out of plane displacement
Maximum of three axes per chip
Typically two axes per chip
One axis per chip
Small chip
Medium to large chip
Large chip
Low etching depths (cost-effective)
Deep etched structures (cost-intensive)
Deep etched structures (cost-intensive)
Small seismic masses
Medium to large seismic masses
Large seismic masses
Small capacitors (small capacitive detection range)
Large capacitors (large capacitive detection range)
Large capacitors (large capacitive detection range)
Low signal-to-noise ratio
Low noise, high signal-to-noise ratio
Low noise, high signal-to-noise ratio
Low stability
High stability
High stability
Low costs
Medium costs
High costs
For consumer goods such as smartphones and applications in automotive engineering such as airbag deployment etc.
For middle and upmarket market segments such as industrial automation, geoengineering, condition monitoring, navigation, robotics etc.
For upmarket market segments such as aviation and space travel

Sensor brain: The high-performance ASIC

If the silicon sensor is the heart of an MEMS inertial sensor, the ASIC is its brain. The integrated circuit reads the capacitive signals of the sensor element and transmits the measured value digitally.

 

Features of a high-performance ASIC:

 

• Very low-noise capacitive detection

• Optimal support for nominal and differential capacitance range
• High-resolution with high dynamic range
• Digital SPI interface (configuration of the sensor ASIC system, readout of sensor data)
• Flexible signal filter

Sensor housing: The hermetically sealed housing

The housing that encloses the two chips must not only allow the sensor to perform, but must also be cost-effective to produce and implement.

 

Features of the MEMS housing:

 

• In-house design that can be tailored to different applications

• Ceramic substrate
• Hermetically sealed housing
• Cost-effective production of small and medium quantities

Learn about the latest MEMS technology and its many possible uses in our webinar:

 

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