MEMS-based accelerometers

From WikID

Introduction

Figure 1: MEMS-based accelerometer[5]

MEMS (Micro-Electro Mechanical System)-based accelerometers are devices that measure the proper acceleration. In relativity theory, proper acceleration is the physical acceleration experienced by an object. [1] The psychical acceleration is measurable by sensors. These sensors are part of the sensing cluster of ubiquitous technologies. Sensing technologies make use of physical parameters from the environment, such as temperature, pressure, force and light. An accelerometer measures weight per unit of mass, a quantity also known as specific force, or g-force. [2] Measuring g-forces allows users to for instance interact with products by means of gesture recognition.

What can the technology do?

MEMS-based accelerometers are available in 1-, 2- and 3-axis configurations, with analog or digital output, in low-g or high-g sensing ranges. [3]

Low g-force MEMS accelerometers:

  • Output: analog and digital
  • Number of axes: 1 - 3 axes
  • Range of g-force: (+/-) 0 - 16 g
  • Typical bandwidth: 1.6 - 2.5 kHz
  • Voltage supply: 1.7 - 6 V
  • Supply current: 145 µA – 700 µA
  • Temperature range: -40 - 125°C

High g-force MEMS accelerometers:

  • Output: analog and digital
  • Number of axes: 1 - 2 axes
  • Range of g-force: (+/-) 0 - 70 g
  • Typical bandwidth: 0.4 - 22 kHz
  • Voltage supply: 3 - 6 V
  • Supply current: 1.3 mA – 5 mA
  • Temperature range: -40 - 105°C

Application fields

MEMS-based accelerometers are one of the simplest but also most applicable micro-electromechanical devices. They are widely used in cost sensitive, low power, motion- and tilt-sensing applications like mobile devices, gaming systems, disk drive protection, image stabilization and sports and health devices. [4] The best known applications are the Wii remote of Nintendo and Apple's iPhone

The working principle

MEMS-based accelerometers are available with different technologies. The most common are based on capacitors and gas chambers.

MEMS-based accelerometer with capacitors is typically a structure that uses two capacitors formed by a moveable plate held between two fixed plates. Under zero net force the two capacitors are equal but a change in force will cause the moveable plate to shift closer to one of the fixed plates, increasing the capacitance, and further away from the other fixed reducing that capacitance. This difference in capacitance is detected and amplified to produce a voltage proportional to the acceleration. The dimensions of the structure are of the order of microns.

Figure 2: Capacitor technology[5]

Another design is based on the distribution of hot gas in a chamber and is typically used to measure static tilt. The accelerometer has a chamber of gas with a heating element in the center and four temperature sensors around its edge. Just as hot air rises and cooler air sinks, the same applies to hot and cool gasses. The figure below on the left illustrates the situation when the accelerometer is held level. The hot gas rises to the top-center of the accelerometer's chamber, and all four temperature sensors measure the same temperature. The hot gas rises to the top-center of the accelerometer’s chamber, and all four temperature sensors measure the same temperature. When tilted, as is shown in the figure below on the right, the hot gas will now collect closer to one, or possibly two, of the temperature sensors and the difference in the four temperatures being measured will give a measure of the tilt. [5]

Figure 3: Gas chamber technology[5]


Inputs, outputs and performance characteristics

The main component of MEMS accelerometers with a capacitor is a cantilever beam with a proof mass (also known as seismic mass). Damping results from the residual gas sealed in the device. As long as the Q-factor is not too low, damping does not reduce sensitivity. External accelerations make the proof mass deflect from its neutral position. This deflection is measured in an analogue or digital manner. Most commonly, the capacitance between a set of fixed beams and a set of beams attached to the proof mass is measured. This method is simple, reliable, and inexpensive[2].


Sources


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