Onset HOBO User Manual

HOBO® Pendant® G Data Logger (UA-004-64)
White Paper

This white paper explains the operation of the HOBO Pendant G data logger (UA-004-64).

Operating Principles

The accelerometer used in the Pendant G logger is an Analog Devices ADXL330 model. Inside the accelerometer,
tethered beams are anchored at fixed locations. The beam is micro-machined with center plates that mesh in
between fixed outer plates causing a capacitance. As the beam moves, the center plate displacement causes a
change in capacitance proportional to the applied acceleration. A circuit inside the accelerometer takes advantage
of this change in capacitance and converts it to a proportional output voltage for the microprocessor. The
microprocessor uses calibration data along with a transfer function to convert the input voltage to an equivalent
acceleration value in G, where 1G = 9.8m/s².

© Analog Devices

16920-A

HOBO Pendant G White Paper

Logger Orientation and Frame of Reference

The logger label uses an X, Y , Z axis of orientation where each of three axes is orthogonal. When the logger is
oriented as shown below, the X axis points down, the Y axis points right, and the Z axis points away (into the page).
To help visualize the Z axis, this can also be depicted using a 3 dimensional cube.

Axes of Acceleration Sensitivity

The accelerometer output varies depending on the direction of acceleration. To illustrate this, the motion of
acceleration along each axis is depicted below with a black arrow. The increase and decrease in axis output is
shown in green and red respectively. For the Z axis, arrow directions into the page are depicted with an X symbol
and out of the page with a colored dot.

1-800-LOGGERS 2 www.onsetcomp.com

HOBO Pendant G White Paper

Accelerometer Location

Locating the accelerometer on the printed circuit board may be useful for experiments dealing with centripetal
motion where the radius distance from the center of rotation to the accelerometer must be known. Dimensions
are from the edge of the logger and bottom of the logger to the center of the accelerometer and shown below in
inches (5/8" and 31/32").

Logging Modes

There are two logging modes: Normal and Fast. The mode is determined by the Logging Interval selected in the
Logger Launch window in HOBOware. If the Logging Interval is set to 1 second or greater, the logger will operate in
normal mode. If the Logging Interval is set to “Fast,” the logger will operate in Fast Mode (you must also select the
Hz in this mode).

Normal mode is ideally suited for tilt applications due to the low frequency of events. When in normal mode, the
logger takes one instantaneous sample every logging interval and stores it to internal memory. It does not do any
averaging or peak measurement over the logging interval.

Fast mode is better suited for dynamic acceleration events where movement, vibration, and shock need to be
captured. See the section Dynamic Acceleration: Fast Mode on page 11 for more details.

Memory Fill Time

The logger has 64K bytes of memory for data and logger header storage. The time required to fill this memory is a
function of how many channels are being logged and the logging interval/mode. The following formula can be used
to determine the “Fill Time.”

Fill_Time(s) = (Memory_size-Header_size)/(# samples/sec * # enabled channels)

For example, a logger with a header size of 500 bytes, 1 sample/s, 3 enabled channels would be:

Fill_Time(s) = (65535-500)/(1 * 3) = 21678s (~6hrs)

The results of this calculation, for fast mode, are shown in Plot A.

1000

100

1 Channel
2 Channel

10

Fill Time (min)

3 Channel

1

1 112131415161718191

Samples/sec (Hz)

Plot A: Logger Memory Fill

1-800-LOGGERS 3 www.onsetcomp.com

HOBO Pendant G White Paper

The logger header size will vary based on the logging parameters entered during launch. The fill time or “Logging
Duration” calculation is determined automatically by HOBOware and can be found in the Launch Logger window.

Tilt Sensing

The ability of the logger to perform tilt sensing relates to the type of accelerometer used and its construction.
Gravity acting on the mass of the beam inside the accelerometer creates a constant force that displaces the beam,
allowing it to measure the static acceleration of gravity (9.8m/s² or 1G). This is true even when the logger is sitting
motionless. As the logger orientation changes in space, gravity acts on each of the three axes, allowing it to be
used in applications to determine tilt, inclination, leveling, relative position, or orientation.

Tilt Angle Calculation

Tilt angle is not measured directly by the logger, but is processed in HOBOware when the data is offloaded. The
HOBOware Status window also displays the angle graphically and uses the following formula.

Tilt Angle = 180° – ArcCos (Acceleration in G’s)

Although the logger’s full scale range is from -3G to +3G, HOBOware performs a post-processing function
truncating values outside the -1G to +1G range for tilt angle reporting and leaving the original acceleration data
intact.

This formula assumes acceleration varies between -1G and +1G and generates corresponding angles between 0°
and 180° with the following orientation.

0°

-X

Z

-Y

-Z

X

90°

Y

-Z

180°

The frame of reference for tilt angle can be either a vertical or horizontal plane and is somewhat arbitrary. We
have chosen a vertical plane since it corresponds to the vector associated with gravity.

0°

Y

-Y

180°

0°

X

Z

Y

90°

X

-Z

-X

180°

Z

90°

-Y

When any of the three logger axes is pointing toward earth, for the sake of discussion using X, the measured
acceleration will be 1G with a corresponding calculated tilt angle of 180°. When in a horizontal position, a 0G
measurement corresponds to 90°. When pointing up (away from earth), a -1G measurement gives 0°.

1-800-LOGGERS 4 www.onsetcomp.com

HOBO Pendant G White Paper

Tilt Orientation: Examples of HOBOware Status Window

These are examples of different logger orientations and corresponding HOBOware Status window screens.

1-800-LOGGERS 5 www.onsetcomp.com