Range: First, since the accelerometer measures acceleration, it is important that the range of the accelerometer should be well within the range of the activity that we need to measure. What is the range of human acceleration? As the last article pointed out, the fastest runner in the world can generate nearly 1g whereas a fast car that accelerates from 0-100 km/hr in 2 sec generates about 1.55g and 5g causes fainting in humans. Most human motions would be adequately covered with a range of about 2G but when we start measuring the rotational acceleration then the maximum capability of the accelerometer needs to be higher. For example, if we attach the accelerometer to the tip of our finger and rotate our hand around the shoulder very quickly, the g forces that act on the accelerometer is much higher and a accelerometer that is capable of measure 10-20g might be required.
Note: Acceleration can also be measured as angular rotation acceleration and in those cases the units of measurements are degrees/second-squared. This is different from the acceleration along any one axis and is more complex to translate along any 1 axis.
Number of axis of measurement: Since humans move bodies in 3 dimensions, it is always beneficial to have a 3 degree axis.
Sensitivity: This is the resolution of the accelerometer. In general, the greater the resolution, the more it can resolve. However, getting a very fine resolution also means that it starts measuring a significant amount of noise. Most human activities (except for eye saccades) are not very fast and hence having a greater resolution adds a noise floor to the data that will need to be filtered. The resolution is expressed in various means. One way that manufacturers of accelerometer’s measure it is in tilt accuracy. For example 0.1 degree tilt is difficult to measure but 1 degree tilt is easy. There is some error from the instrumentation and logic circuits but that is very minimal.
Temperature stability: Many accelerometers nowadays compensate for a change in temperature automatically but it is possible that some may not have the circuits built into them and may require external compensation
Frequency of measurements or “bandwidth” The accelerometer can measure acceleration at any instant but needs time to reset for the next measurement. It should be capable of measuring at least 2X times the fastest movement that needs to be recorded. How fast does the data need to be gathered per second for a human? The human neuron transmits at 300Hz to 1kHz but the muscles move much slower. For example the range for tremor is in the 4-8 Hz although a muscle twitch is much faster under zero load. In the presence of load this response slows down considerably and it does depend on the activity that is being performed. A reasonable measure for an accelerometer is in the 50-100Hz range.
There are other characteristics that go into design of the accelerometer – for example does the accelerometer send analog data or digital data. What is the range of that data? Is it 8-bit number that varies from 0-255 or a 16 bit number that varies from 0-65536? These are design consideration for the wearable technology manufacturers and not many of them state these numbers explicitly in their documentation.