# Accelerometer

An acceleration sensor (also accelerometer , accelerometer , vibration sensor , vibration sensor , accelerometer , accelerometer , B-knife or G-sensor ) is a sensor that measures its acceleration . This is mostly done by determining the inertial force acting on a test mass . Thus, for. B. determine whether a speed increase or decrease takes place. The acceleration sensor belongs to the group of inertial sensors .

If continuous acceleration measurements are recorded, this series of measurements is called an accelerogram.

## Measurand

The acceleration is measured in the SI unit m · s −2 (meters per second squared). In practice, however, it is often given as a multiple or part of the mean value of the acceleration due to gravity . The mean acceleration due to gravity is denoted by g (small “G” in italics) and is rounded to 9.81 m · s −2 .

${\ displaystyle 1g = \ mathrm {9 {,} 81 {\ frac {m} {s ^ {2}}}}}$ ## Application examples

Acceleration is a mechanical variable that plays a major role in many areas of technology . Accelerometers therefore have a variety of possible uses - for example:

## Measurement principles

The first of these measuring instruments had a so-called “sensitive axis ” on which the seismic mass was arranged so that it could be displaced with springs and which operated a sliding resistor with a sliding contact, for example . These so-called gyrometers were - in connection with gyroscopic instruments - the basis of many control methods and inertial navigation until around 1970 .

Later they were largely replaced by more precise systems with flexible quartz rods (“Q-Flex”) or magnetically stabilized masses. Miniaturized sensors are usually built with piezoelectric sensors or as MEMS (Micro-Electro-Mechanical System). Many technical applications require full three-dimensional measurements, for example in mechanical engineering , for controlling robots or in space travel . Here is miniaturization is an important prerequisite - alongside insensitivity to temperature, vibration and other effects. Numerous applications manage with 2D sensors , however , when it is mainly about movements in one plane.

Small sensors with a mass of a few grams have measuring ranges from a few g to tens or even hundreds of g and are robust against impacts. The resolution reaches 0.01m g .

Precision instruments with a mass of several kilograms provide accuracies of 10 −9 g .

In principle, most of the acceleration sensors used today are based on Newton's law of inertia:

${\ displaystyle F = m \ cdot a}$ In the event of acceleration, the spring-suspended mass changes its position relative to the surrounding sensor housing, which is further evaluated inside the sensor.

### Piezoelectric acceleration sensors

A piezoceramic sensor plate converts dynamic pressure fluctuations into electrical signals that can be processed accordingly. The pressure fluctuation is generated by a (seismic) mass attached to the piezoceramic and acts on the piezoceramic when the entire system is accelerated. This system is z. B. used in wheel balancing machines, where each imbalance of the wheel generates a corresponding signal in the piezoceramic. It detects the tire imbalance within seconds.

### Microsystems

In recent years, miniaturized acceleration sensors have become increasingly important. These are micro-electro-mechanical systems (MEMS) and are mostly made of silicon . These sensors are spring-mass systems in which the “springs” are silicon webs only a few μm wide and the mass is also made of silicon. Due to the deflection during acceleration, a change in the electrical capacitance can be measured between the spring-mounted part and a fixed reference electrode . The entire measuring range corresponds to a change in capacitance of approx. 1  pF . The electronics for evaluating this small change in capacitance are accommodated on the same integrated circuit (IC).

There are also variants in which piezoresistive resistors are attached to the bending beam by means of ion implantation , which change their resistance according to the bending and thus allow conclusions to be drawn about the acceleration.

To manufacture these miniaturized sensors, the mass and the small silicon springs (silicon pins) are etched out of the silicon using photolithography . In order to obtain a self-supporting structure, an underlying layer of silicon dioxide is also removed by etching.

This type of acceleration sensors has the advantage of relatively low unit costs (mass production) and high reliability (some such sensors can withstand accelerations up to a thousand times the measuring range without damage). Because of their small size, they are also characterized by high measuring speed. You are therefore z. B. used to trigger airbags in vehicles.

Sensors in MEMS technology are produced not only for measuring the (linear) acceleration, but also for measuring the angular velocity , so-called rotation rate sensors or gyroscopes .

### More acceleration sensors

• Strain gauges : Another way of determining the force on the test mass by determining the deformation of the attachment (e.g. a rod) using strain gauges (especially suitable for lower frequencies).
• Magnetic induction : When the test mass is suspended from a spring, an electrical voltage is induced by a magnet in a coil, similar to a dynamic microphone ( moving coil microphone ).
• The Ferraris sensor measures the relative acceleration without a test mass using eddy currents. It is used to analyze and control highly dynamic drives.