AR4000 User’s Manual
17
LLL004001 – Rev 2.7
8.1 Detector Thermal Noise
Range measurement accuracy at high sample rates is limited by thermal noise in
the sensors’ detector. Typically, a range measurement will be made by timing a
number of cycles of the output. The greater the number of cycles timed, the
better the averaging or filtering of this noise will be. Without going into the
theory of noise power and noise bandwidth, the effect is that the standard
deviation of the measurement error increases proportionally with the square root
of the signal bandwidth, or in this case the sample rate. The noise in this sensor
is 0.0005 in/(Hz
1/2
). Multiplying this value by the square root of sample rate will
give the rms. noise value (approximately the same as the standard deviation) for
the measurement. Thus a sample rate of 10,000 Hz gives readings with a
standard deviation of about .05 inches.
8.2 Laser Diode Noise
There is another source of measurement error that needs to be considered when
taking high accuracy measurements, caused by noise in the laser diode. This
noise is characterized by random changes in the range reading that tend to
increase as the time over which the readings are taken increases, when the sensor
and target are stationary. This becomes noticeable over times of about 0.3
seconds or more, and increases up to times of several hours. The standard
deviation of this drift is about 0.01 in. at 1 second, and .05 in. at 10 hours for the
IR version and up to .1” in 10 hours for the visible model. Much of this noise
shifts to higher frequencies (up to several kilohertz) if the target is moving or
vibrating, even slowly. This fact can be used to filter out this noise if low
frequency sampling is being done on a moving target. The motion effectively
dithers the range reading, and an average value can be obtained that is more
accurate than is possible if the target is stationary.
8.3 Maximum Range Specification
One of the configuration options for the AccuRange 4000 is the maximum range
expected. This is to allow the sensor to obtain readings with the best possible
resolution and accuracy. Internally, the time required to take a single sample
depends on the distance being measured and the resolution used to take the
measurement, If the ranges are known to be short, better resolution and accuracy
at high sample rates may be obtained by reducing the maximum range. For most
applications the default of 650 inches should be adequate. If you are measuring
ranges greater than this, or ranges much shorter in situations where maximum
resolution and high sample rates are required, your maximum should be specified
using the Set Maximum Range command.
8.4 Sampling Resolution
The diagonal line shown in Figure 2 is an accuracy limit due to sampling
resolution, assuming that the ranges to be measured are 30 feet or less. This
becomes the limiting constraint above 15,000 samples/second. For ranges up to