The Collection and Interpretation of Accelerometer Data in Parachute Systems

An excerpt from
Parachute Data Acquisition Techniques and Methods
by Dr. Jean Potvin and Gary Peek
Parks College Parachute Research Group

Accelerometers can be used to measure the force in gravity's (G's) and can include devices oriented on all three axis, "X", "Y", and "Z". Devices are available that generate a voltage that increases and decreases as the acceleration increases and decreases, and the number of G's can be determined by the manufacturer's calibration data, in "volts per G". Most accelerometers have a maximum acceleration, which if exceeded, will destroy the sensor, so they must be capable of withstanding the highest force expected.

Accelerometers must be aligned precisely in the center of the load, near its center of gravity (CG) if information on parachute drag or payload acceleration is sought. Doing otherwise will lead to the accelerometer picking up the payload's possible tumbling motions in addition to payload center of gravity motion. Such data is very difficult to interpret unless the payload container's attitude is precisely known via video or other means.

Similarly, extra motions will be picked up if the accelerometers are mounted in cushioning material in an effort to prevent its destruction during hard landings. Again, unless the jolt response of the cushion is precisely known, the measurements are almost impossible to interpret.

Having said all this, useful information can still be obtained from the acceleration component parallel to the payload-parachute axis (normally the Z-axis) during the inflation stage. This is because the tumbling of the payload container is greatly reduced by the pull of the parachute. The other acceleration components may also reveal useful information during inflation, given that the lower end of the risers act as a point about which the payload CG rotates.

The discomfort felt by jumpers during parachute deployments is often not only the rapid deceleration in the Z axis, but also, motions felt in the other axis'. X and Y acceleration measured at these times may provide useful information as to the cause of the discomfort, or may provide other useful information.

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