Accelerometry

Newton’s 2nd Law The Law of Acceleration Describes how a body moves when an external force is applied “An external force will cause the body to accelerate in direct proportion to the magnitude of the force and in the same direction as the force” F=ma If a body’s mass is assumed to be constant, then a measured acceleration is an indicator of a change in the force applied to the body.

Direct measurement by accelerometry Accelerometers are devices that measure applied acceleration acting along a sensitive axis which can be used to measure the rate and intensity of body movement in up to three planes (anterior–posterior, mediolateral and vertical) Accelerometers can also be used to measure tilt (body posture) making them superior to those devices that have no ability to measure static characteristics.

Direct measurement by accelerometry Although each type of accelerometer uses different mechanisms to measure the accelerations and there are many different designs and manufacturing techniques, conceptually, all use a variation of the spring mass system, shown in figure 1. In this system, when acceleration is applied, a small mass within the accelerometer responds by applying a force to a spring, causing it to stretch or compress. The displacement of the spring can be measured and used to calculate the applied acceleration.

Types of accelerometers Accelerometers included for kinematic studies include piezoelectric, piezeresistive, differential capacitor, all of which implement the same basic principle of the spring mass system. Piezoelectric accelerometers Piezoelectric accelerometers consist of a piezoelectric element with a seismic mass. When the sensors undergo acceleration, the seismic mass causes the piezoelectric element to bend. This change causes a displacement charge to build up on one side of the sensor, which results in a variable output voltage signal that is proportional to the applied acceleration.

Types of accelerometers Piezoresistive accelerometers These accelerometers are typically manufactured from a surface micromachined polysilicon structure. On this sits polysilicon springs (arranged in a Wheatstone configuration) whose electrical resistance changes as the acceleration forces are applied. The acceleration is proportional to the resulting voltage. Differentiable capacitor accelerometers These operate on the principle of change of capacitance is proportional to applied acceleration. They use a differentiable capacitor with central plates attached to the moving mass and fixed external plates. The applied acceleration unbalances the capacitor. This results in the output wave for the accelerometer.

Acquiring the correct accelerometer data The output of an accelerometer worn on the body is dependent on four factors as listed as follows: Position at which it is placed, Its orientation at this location, The posture of the subject and The activity being performed by the subject. If the subject or patient is at rest the output of the accelerometer, is determined by its inclination relative to the gravitational vector. If the orientation of the accelerometer relative to the person is known, then the resulting accelerometer recordings can be used to determine the posture of the subject relative to the vertical or gravitational direction.

Acquiring the correct accelerometer data The main acceleration components are due to the movement of the body. The body-generated linear, centripetal and coreolis accelerations are the main factors that constitute both the translational and rotational body movements. Other spurious resulting accelerations, such as artifact due to soft tissue movement or external vibrations may also be present in the accelerometer signal, but can be minimized through careful instrument placement and signal filtering.

Requirements for accelerometric monitoring of human movements The accelerations generated during human movement vary across the body and depend on the activity being performed. Accelerations increase in magnitude from the head to the ankle, and are generally greatest in the vertical direction (Bhattacharya et al 1980), although the accelerations in the other two directions cannot be neglected (Lafortune 1991).

Medical assessment of human movement Some analyses, such as gait analysis, under controlled laboratory settings can be a useful tool in detecting underlying causes of gait abnormalities; However, Analyses in a laboratory setting do not reveal functional subject variability in everyday activity patterns. Accelerometers have become the preferred choice for continuous, unobtrusive and reliable method in human movement detection and monitoring.