How Do Accelerometers Work?
Jun 12 2009
Anatomy Of An Accelerometer
This document describes the physical mechanics of the ACCELEROMETER, a Micro Electro-Mechanical System (MEMS) used in popular applications commonly seen in most consumer’s technological tool and toy boxes. The category of device, MEMS, describes the initial physical feature implicitly with the use of the word micro. While nano-technology ( 1 x 10-9 ) is a common buzzword used for current and future of electronic developments, micro-technology (1 x 10-6 ) is evident around us and can actually be seen by the human eye. The additional term electro-mechanical is used to describe the device as having electronic as well as mechanical operating characteristics.
FUNCTION – The function of an accelerometer is to provide accurate response to movement in measurable increments small enough to be converted to useful signals. This is accomplished by converting physical measurement of movement from a mass into electronic impulses that can be interpreted by a separate electronic device and measured compared to gravity or 1 g-force as the basic unit. Movement direction and degree of force can be determined with an accelerometer and can be used a 3-axis (x, y, and z-planes) configuration to provide a 3-dimensional response to be measured. Common uses of this can be seen in the user feedback feature of the Nintendo Wii video-game controller, in which speed, rotation, and force is measured from the user and provided as input data to the game console to influence movements on the monitor or television screen. Another example commonly encountered is the ability of Personal Digital Devices (PDA’s) such as cell phones and Apples I-Pod line to automatically re-orientate the screen to always be readable. The application of strain gauges that measure the forces applied to a mechanical structure is another popular and advantageous use of accelerometers.
APPEARANCE – The physical attributes of the accelerometer are replicable to the commonly seen Integrated Circuit (IC) design as seen in Figure 1 below. Slightly bigger than a dime,
source: www.directindustry.com
Figure 1 – Size Comparison and Physical Appearance
the overall dimensions are quite small and can be used in many applications. The outer casing is made of a silicon-type material that houses the internal MEMS. The manufacturer’s name and logistical markings are printed on the outer casing in white or black letters. The output is signaled via a wire bundle that exits the silicon portion and is held tight by a machine crimped steel coupling. The output wires are individually insulated as well as channeled through a separate insulation creating a one-wire transit for the data to be provided to the receiving device. There are many other designs styles and size ranges but the attributes described above are found in most common accelerometer applications.
OPERATION – The accelerometer operates mechanically and then produces an electrical signal as an output. As previously stated in the functionality description, the force of movement measured by the accelerometer is accomplished mechanically by using a free floating mass and determining how hard and fast it moves relative to gravity. Early accelerometers used a large scale mechanism compared to the modern MEMS type accelerometers in which a mass was linked between two equally matched springs and the movement was measured relative to the resting position of the mass. In Figure 2 we can see the modern configuration of the accelerometer.
source: R.J. Noriega-Manez – Stanford University
Figure 2 – Physical Model of MEMS Accelerometer
v Substrate – The material or portion of the MEMS that the movement of the sensing unit will measure against to determine the direction and magnitude of force applied to the system.
v Anchor – The connecting element of the substrate to the rest of the sensing system to provide a rigid attachment point.
v Resistor – Usually a piezo or pressure changing resistor is used to sense deflection and cause a change in resistance and subsequently a change in current. This can be received analogous or digitally to a sensing or measuring device.
v Cantilever – This provides the physical means to allow deflection to be recorded based on its configuration of one side being connected rigidly and the other being unattached causing a spring-like effect to any movement or outside force encountered.
v Test Mass – This calibrated to allow zero movement at rest and adds sensitivity to the system by allowing the slightest of force applied externally to the system to cause the cantilever to deflect and the resistor to change value and produce measureable results. This again can be communicated analogous or digitally to the sensing device and measured to provide input to a system or feedback to a measuring device (e.g. strain gauge). Depending on the sensitivity and complexity of the accelerometer system forces can be sensed and measured in very small increments to provide accurate feedback on forces that can’t be detected by humans.