Accelerometer

An accelerometer is a sensor (sensor ) that measures the acceleration by mostly acting on a test mass inertia force is determined. Thus it can be determined, for example, whether a speed increase or decrease occurs. The acceleration sensor is in a group of inertial sensors.

It is also called the accelerometer or accelerometers to continue B knife and G sensor. If continuous acceleration measurements recorded, this is regarded as a series of measurements Akzelerogramm (Analog to seismogram, which is recorded by a seismometer ).

Measure

The acceleration is in the SI unit m · s-2 (meters per second squared ) were measured. 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 doing with g denotes (small "G" in italics ) and is rounded to 9.81 m · s -2.

→ Main article g - force

Application Examples

The acceleration is a mechanical parameter, which plays a large role in many areas of technology. Acceleration sensors therefore have a variety of applications - for example:

• Measurement of (linear) accelerations ( accelerometers ).
• Measurement of vibrations of buildings and machines.
• Active suspension systems in vehicles.
• Alarm systems for movable goods, or as a touch sensor.
• Protection against head crash with hard disks (for example, Apple's Sudden Motion Sensor or Lenovo's Active Protection System).
• Health care applications, health care and monitoring.
• In crash tests, the dummies and vehicles.
• Sensors in digital cameras (eg for automatic switching from portrait image on wide square image and image stabilization ).
• Sensors in mobile phones (such as the Apple iPhone or HTC Dream).
• Damage investigations in transporting the goods.
• In acceleration recorders and seismographs in the field of seismology and earthquake monitoring.
• Tilt measurement in static systems (that is, as long as other accelerations are negligible compared to the acceleration due to gravity ).
• Active speakers.
• Together with gyroscopes for attitude control or stabilization of aircraft such as helicopters or UAVs.
• To control video games (Wii Remote, Playstation Move)
• In mining and technology, the control of elevators was performed by accelerometers early, in which case a one-dimensional measurement system was sufficient. At least since the publication of the ISO 18738 "Measurement of lift ride quality" in 2003, the three-dimensional acceleration sensor but has found its way in the lift.
• Also for the satellite and rocket technology and the analysis of vehicle movements or the car electronics the acceleration measurement is indispensable.

• Precision Accelerometers are partly used for measurements in the Earth's gravity - see gravimetry and gradiometry, as well as ESA's GOCE.
• Positioning with inertial navigation systems (including inertial navigation system ). INS are now superseded especially in the aviation industry increasingly by GPS.
• Sleep phase alarm clock; wake them to the awakening person on at a time, to which it moves. This ensures that the person does not wake up in the REM phase, which usually leads to greater fatigue later in the day. Here also comply with motion sensors.

Measurement principles

The first of these instruments have a so-called "sensitive ( photosensitive ) axis", in which the proof mass was arranged. They were until about 1970 - in conjunction with centrifugal technology - the basis of many control methods and inertial navigation; later they were largely replaced by more accurate systems with flexible quartz rods ( "Q -Flex" ) or magnetically stabilized masses. Miniaturized sensors are usually constructed with piezoelectric sensors or MEMS (Micro - Electro-Mechanical System). Many technical applications require full three-dimensional measurements such as mechanical engineering, for controlling robots or in space. This miniaturization is a key requirement - in addition to insensitivity to temperature, vibration and other effects. However, numerous applications come out with 2D sensors when it comes mainly movements in one plane.

Have small sensors with a mass of a few grams ranges from a few grams up to dozens or even hundreds of g and are robust to shocks. The resolution reached 0.01 mg.

Precision instruments, with a mass of several kilograms deliver accuracies of 10 - 9g.

Piezoelectric accelerometers

A piezoelectric ceramic sensor platelets converts dynamic pressure fluctuations into electrical signals that can be processed accordingly. The pressure fluctuation is generated by a piezoelectric ceramic attached to the ( seismic ) mass and contributes to acceleration of the whole system on the piezoelectric ceramic. This system is used for example in Radauswuchtungsmaschinen where any unbalance of the wheel generates a corresponding signal in the piezoceramic. It detects within seconds the unbalance on the tires.

Microsystems

In recent years, miniaturized accelerometers have increasingly gained importance. These are micro-electro - mechanical systems ( MEMS) and are usually made ​​of silicon. These sensors are spring-mass systems, where the " springs " are only a few microns wide silicon webs and the mass is made ​​of silicon. Due to the deflection during acceleration can be measured between the sprung suspended part and a fixed reference electrode, a change in the electrical capacity. The entire range is defined as a change in capacitance of only about 1 pF, so the electronics must be immediately integrated on the same semiconductor chip for evaluation of this small change in capacitance.

There are also variants where on the bending beam piezoresistive resistors are fitted by ion implantation, which change according to the bending their resistance and leave it back close to the acceleration.

The mass and the small silicon springs ( silicon legs ) are etched by photolithography from the silicon for the production of these miniaturized sensors. 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 of such sensors can still accelerations up to a thousand times the measuring range without damage survive ). Because of the small size, they are also characterized by high measurement speed. They are therefore used for example for triggering airbags in vehicles.

Sensors in MEMS technology are made not only for the measurement of the (linear) acceleration, but also for the measurement of the angular acceleration ( so-called gyro sensors ).

Further acceleration sensors

• Strain gauges: Another way to determine the force on the test mass by the deformation of the fixture is determined (e.g., a rod ) by means of strain gauges ( particularly suitable for low frequencies).
• Magnetic induction: In the movement of the suspended proof mass to a spring, an electric voltage is induced by a magnet into a coil, similar to a dynamic microphone ( moving coil microphone ).
• The Ferraris sensor measures the relative acceleration without test mass with the aid of eddy currents. It is used for analysis and control of highly dynamic drives.