# Accelerometer: Types, Applications and Selection Tips

An accelerometer or acceleration sensor is a device used for measuring the acceleration of an object. When a change takes place in a linear or angular motion, it results in an acceleration and this is followed by a force. This force is then measured by some mechanical arrangement. By measuring the gravity components, tilt can also be sensed by the accelerometers.

Types of Accelerometers:
• Capacitive
• Piezoelectric
• Piezoresistive
• Thermal conduction accelerometer
• Electromagnetic accelerometer
• Servo accelerometer

### Capacitive accelerometer

It is a type of accelerometer where change in capacitance is employed for measuring the acceleration. A proof mass is suspended between the two plates of a capacitor. Whenever there is an acceleration, the suspended mass swings thereby changing its distance from the plates of the capacitor. The proof mass is itself a metallic structure which makes two capacitors in series. The change in capacitance is responsible for change in capacitance of the arrangement.

Capacitance Principle: The capacitance C between two conductive plates is given by the formula:

where:

• ϵ (epsilon) is the permittivity of the dielectric material between the plates
• A is the area of the plates
• d is the distance between the plates

Capacitive (MEMS) accelerometer

Key Features:

• Capacitive accelerometers measure acceleration by detecting the change in capacitance between a fixed and a movable electrode, caused by an applied force due to motion.
• High sensitivity, small size, low power consumption are some of the advantages of capacitive accelerometers.
• Capacitive accelerometers are used in gesture detection and automatic screen rotation in consumer electronics such as smartphones, tablets, and game controllers.
• Humidity, temperature and long-lasting vibrations can affect the performance of these sensors. So, routine calibration is required for reliable measurements.

Applications:

• Smartphones and Tablets: Screen rotation, gaming controls, and motion detection in consumer electronics like smartphones, smartwatches, gaming controls, etc.
• Airbag Systems: Detect sudden deceleration in locomotives to trigger airbag expansion.
• Tilt Sensing: Used in construction and agricultural equipment to ensure proper operation on uneven terrain
• Rehabilitation Devices: Monitor patient movements during physical therapy.
• Inertial Navigation Systems: Used in aircraft and spacecraft to measure acceleration and help with navigation when GPS is not available.

When to use Capacitive Accelerometers:

• High Sensitivity Requirement: When small accelerations are needed to be measured with high accuracy, capacitive accelerometers can be employed.

• Low Frequency Acceleration: Capacitive accelerometers are suitable for applications where both static (e.g., gravity) and dynamic accelerations need to be measured.

• When Power Efficiency is Critical: Due to their low power consumption, capacitive accelerometers are an excellent choice in portable or wearable devices

• When Compact Size is Necessary: Capacitive accelerometers are very compact and lightweighted making them ideal for applications with limited space like smartphones, tablets, or medical implants.

DigiKey carry some capacitive accelerometers from NXP USA Inc. with part numbers KITSTBLITE2EVM, KITSTARTER1EVM and KITSTARTER2EVM.

### Piezoelectric Accelerometer

These are the accelerometers where piezoelectric effect serves as the basis for acceleration, vibration and shock measurements. The mechanical stress results in the induced charges on opposite surfaces of the object under consideration which, then, can be detected as voltage by a voltmeter.

In a piezoelectric accelerometer, a piezoelectric element is used to connect a known quantity of mass, commonly referred to as the proof mass, to the accelerometer body.

When the sensor frame experiences an acceleration, the proof mass exerts a force on the surface onto which it is placed due to its inertia. This exerted force produces stress in the piezoelectric material and produces a voltage. This induced voltage is utilized to estimate the acceleration.

Piezoelectric (MEMS) Accelerometer

Features

• High Frequency Response: They are used in vibration analysis where high frequencies ranging from a few hertz to several kilohertz are involved.

• High Sensitivity: They are highly sensitive to dynamic forces, making them ideal for detecting small vibrations or high-frequency signals.

• Wide Dynamic Range: Piezoelectric accelerometers can measure a wide range of accelerations, from very low to extremely high, making them versatile for different types of applications.

• Self-Generating Signal: The piezoelectric effect generates a signal without requiring an external power source for the sensing element itself, though signal conditioning electronics may still require power.

Applications

• Vibration Analysis and Monitoring: To monitor vibrations in machines in industries to predict maintenance needs and to prevent mechanical failure or breakdowns.

• Flight Testing: Measure vibrations and accelerations in aircraft during testing to ensure structural integrity and performance.

• Earthquake Detection: Monitor ground movements and vibrations in seismology to detect and analyze earthquakes.

• Camera Stabilization: Some high-end cameras use piezoelectric accelerometers to detect and compensate for small vibrations, improving image stability.

Some Piezoelectric Accelerometers available with DigiKey:
DK Part # Mfr Part # Mfr
2770-780988-01-ND 780988-01 NI
2770-780989-01-ND 780989-01 NI
V22B-ND V22B Mide Technology Corporation
V22BL-ND V22BL Mide Technology Corporation
V21B-ND V21B Mide Technology Corporation
V21BL-ND V21BL Mide Technology Corporation
V20W-ND V20W Mide Technology Corporation
V25W-ND V25W Mide Technology Corporation

### Piezoresistive Accelerometer

In Piezoresistive accelerometer the electrical resistance changes whenever it experiences a mechanical stress which is caused by an acceleration. The seismic mass is attached to one end of the piezoresistive devices. When the sensor experiences acceleration, the inertia of the seismic mass applies stress to the piezoresistive devices.

Piezoresistive (MEMS) Accelerometer

Features:

• High Sensitivity: Piezoresistive accelerometers are highly sensitive and can detect small accelerations. This is why they are particularly useful in applications where precision is critical.

• Dynamic and Static Acceleration Measurement: They are capable of measuring both dynamic and static accelerations. Unlike some other accelerometer types, piezoresistive accelerometers can measure gravity or steady-state forces, shocks and vibrations, etc.

• Wide Operating Temperature Range: They can operate over a wide range of temperatures, though the sensitivity may change with temperature.

• Power Requirement: Unlike piezoelectric accelerometers they require a continuous power supply for their operation.

Applications

• Automotive Crash Testing: Used extensively in automotive crash testing to measure the acceleration and forces experienced by vehicles and occupants during collisions.

• Vibration Monitoring: Monitor vibrations in equipment and structures to detect potential faults or maintenance needs, particularly at low frequencies.

• Aerospace and Defense: Measure acceleration and vibration forces on aircraft and spacecraft during testing and operation. Provide acceleration data for missile and projectile guidance systems, ensuring accurate targeting and flight path control.

• Medical: In medical devices like cardiac monitors they are used for detecting heartbeats and motion.

### Thermal Conduction Accelerometer

In thermal conduction accelerometers heat transfer in the cavity of the sensor serves as the basis for the acceleration measurement. This sensor involves no such mechanical movements as involved in other accelerometers. A heater, a temperature sensitive resistor (thermocouple), a sealed chamber and a working gas are the main components of a thermal conduction accelerometer. The temperature sensitive resistor (thermocouple) is distributed within the sealed gas chamber to measure the changes in gas temperature.

Features:

• Sensitivity: They are excellent to use for low-frequency acceleration measurements including static and quasi-static forces.

• Stable Performance: They are highly stable over a wide range of temperatures.

• Highly Durable and Reliable: As there is no such moving mechanical parts which are in contact, they become highly durable and suitable in harsh environments.

• Power Consumption: Power consumption is very less, so they are suitable where energy saving is critical.

Applications:

• Tilt or inclination of heavy machineries are measured with these accelerometers.
• They are used as inclinometers where precise measurement of angle or tilt is required.
• They can be used for monitoring vibrations in large structures such as bridges, buildings, or towers where low frequency vibrations are concerned.
• In assessment of vehicle stability, such as in rollover detection where low frequency acceleration data is required, these sensors can be employed.
• They can be used in defense and aerospace applications where static accelerations need to be accurately measured.

### Electromagnetic Accelerometer

In Electromagnetic accelerometers, the principle of electromagnetic induction serves as the basis for the measurement of acceleration. It consists of a seismic mass attached to a coil of wire suspended in a magnetic field. As the sensor experiences acceleration, the seismic mass moves and the coil also moves. This coil movement is relative to the magnetic field. This relative motion changes the magnetic flux through the coil, inducing a voltage across the coil terminals. A magnet is fixed inside the sensor to produce a stable magnetic field, and the spring is used to return the seismic mass to its initial position and provide restoring force.

Features:

• They are highly sensitive for low-frequency accelerations. They are used in seismic monitoring or structural health monitoring, as they require low-frequency measurements.

• Electromagnetic accelerometers can be used for measurement of very low to very high range of accelerations.

• They are used in applications where precision is required for long as they are highly durable.

• No external power is required for the operation. The coil itself generates power by relative motion between seismic mass and the magnetic field.

Applications:

• They are widely used in seismometers to detect and measure the magnitude of earthquakes.

• They are used in low-frequency acceleration measurements, for example to monitor vibrations in machineries.

• They are used in transportation systems to monitor vehicle dynamics. For example, to monitor the conditions of tracks by detecting low frequency vibrations caused by the trains.

• They can be used in geophysical research and environmental monitoring to detect low frequency ground movements.

### Servo Accelerometer

A servo accelerometer is a high-precision acceleration measurement device that uses a servo mechanism to balance the displacement of a seismic mass, thereby measuring acceleration. A seismic mass, a spring, a detection device (such as an optical sensor or capacitive sensor), a servo mechanism, and a feedback circuit are the main components of a servo accelerometer.

Whenever there is an acceleration, the seismic mass displaces due to inertia. The spring takes it back to its original position. An optical or capacitive detection device measures the displacement of seismic mass which is proportional to the acceleration.

A feedback loop maintains the force balance within the system. The output signal from the position sensor (detection device) is amplified and sent to a torque generator, which generates a counteracting force to restore the mass to its equilibrium position. The output of the torque generator is directly proportional to the acceleration experienced by the sensor, allowing it to provide highly accurate measurements.

Servo Accelerometer

Features:

• Sensitivity: Servo accelerometers provide exceptional linearity and high sensitivity.

• Frequency Range: These sensors respond to a wide range of frequencies. Static acceleration like gravity and high frequency accelerations can be easily monitored.

• Stability: Due to their minimal drift, these sensors ae suitable for the applications which require consistent performance over long durations.

• Signal to Noise Ratio: Due to the involvement of feedback loop, they offer high signal to noise ratio.

Applications:

• They are used in navigation systems in aircrafts, spacecrafts, marines and missiles.
• They are used to monitor tilts and vibrations in large structures like bridges, buildings, etc.
• These sensors are used in crash testing in automotive industries to measure the forces experienced by the vehicles during crashes.
• They are used in weapon stabilization to ensure accurate fire control.

### Key Considerations and Selection Tips for Accelerometers

As there are many types of accelerometers are available, you should choose the best and optimum accelerometer for the specific application to ensure maximum performance and reliability.

• First, you have to identify the minimum and maximum acceleration measurement range. If you select the sensor with long range, resolution may be compromised and in case of short range, the data values may exceed the maximum capacity of the sensor.

• Sensitivity is one of the most important factors in choosing the best accelerometer. For high values of accelerations low sensitivity accelerometers are suitable, while for low values of accelerations, high sensitivity sensors are best to be used.

• Power consumption of the sensor/accelerometer must be kept in mind while making a choice for sensor selection for a particular application.

• Sensor’s size and weight are one of the deciding factors. Lighter and smaller sensors are suitable for portable devices.

• Choose sensors that are easy to install and integrate to reduce installation and maintenance costs.

DigiKey supports various accelerometers and vibration sensors:

Accelerometers available with DigiKey

Vibrations Sensors available with DigiKey