IEPE-Link
™
-LXRS
®
Wireless Sensor Node
User Manual
System Operation Overview
18
3.5.1
Sensor Conversion Values
Because the wireless sensor system is digital, the analog voltage readings from the sensor
are converted into a digital equivalent value based on the volt-to-bit scale of the internal
analog to digital voltage converter (A/D converter). A/D value can be read directly or further
converted to engineering units by applying conversion values and a conversion formula.
Sensor readings can be displayed and recorded in A/D value (bits) or engineering units.
For the IEPE-Link
™
-LXRS
®
the engineering unit is
g
-force. The conversion value is
obtained from the sensor manufacturer and is typically referred to as sensitivity in units of
mV/
g
(milli-volts of sensor output per g-force it experiences). For example, a sensor with an
operating range of +/-50
g
may have a sensitivity of 99.8mV/
g
.
NOTE
In order to report accurate readings, many sensors require calibration.
Calibration coefficients normalize the sensor output to a known reference
device and are often expressed in the conversion values. Calibration accounts
for the variations between sensor elements.
Conversion Formula:
The default formula assumes a linear relationship between the A/D
value (bits) and engineering units and is expressed mathematically as
y=mx+b
, where
y
is
the engineering units at a given point (measurement),
m
is the slope of the line that
represents the linear ratio,
x
is the A/D value at a given point, and
b
is the fixed zero load
offset of the sensor.
Slope:
The slope is the ratio of A/D value (bits) to engineering units (EU) that is used to
convert the sensor measurements, or bits per EU. The slope conversion value will vary
depending on the engineering units desired. For example, if the units are a measurement of
acceleration in units of g-force, the desired slope conversion would describe how many bits
equal unit of
g
-force (bits/
g
). Mathematically, the slope is
m
in the formula
y = mx +b
.
Offset:
The offset value is the starting output value of the sensor with no load applied.
Mathematically, the offset is
b
in the formula
y = mx +b
. Because IEPE accelerometers are
AC coupled there is no offset value.
