FIGURE 5. LRF measurements.
With the LRF design
completed and calibrated, it was
time to take a series of
measurements to determine how
accurate the LRF was and if it was worthy enough to
become a Parallax product. I set up 12 marked distances on
the floor of my lab and took measurements with the LRF
from each location (see Figure 5).
In summary, the LRF has a usable range from about six
inches to four feet ( 48 inches). Within the usable range, the
change in pixels from center/angle is very noticeable,
making the range calculations more reliable. As the distance
increases — although the camera can still “see” the laser
spot — the change in pixels from center/angle is much
harder to determine which causes a reduction in accuracy.
The LRF firmware intentionally limits the maximum distance
to 100 inches. At distances less than six inches, the laser
spot is out of the camera’s field-of-view, so no blob
detection can occur.
Accuracy within the usable range varies from perfect
(no difference between actual distance and the distance
calculated by the LRF) to around ±2% error (approximately
1/4” to 1” difference between actual and calculated
automatic exposure enabled by default (they can be
enabled/disabled with a single command sent to the LRF
module), and will automatically adjust its image to account
for sudden changes in lighting conditions. However, the
module works best in a controlled environment, such as
indoors with minimal changes in brightness across the
frame. The module is also less reliable when the laser is
shining onto a bright object (for example, a white piece of
paper) since the background subtraction done during image
processing could potentially subtract the bright laser from
the already bright frame. Giving the camera time for its
automatic white balance and automatic exposure to settle
helps a bit to make the laser spot stand out. Using a red
filter over the camera will also help the red laser spot
become more visible to the camera in certain situations.
To show how the LRF can be used in real world
The LRF is able to consistently and
reliably detect multiple blobs and make a
determination of which is the primary laser
spot. The range finding math works well
with reasonable accuracy. However, like
any sensor system, it’s not suitable for use
in all conditions and has its limitations:
• Range. As discussed previously, the
design is most accurate within its usable
range of six inches to four feet ( 48 inches).
Longer distances will result in a noticeable
reduction in accuracy, though could still be
used for gross measurements or simple
• Environment. The camera system
has automatic white balance and
FIGURE 6. Demo — BS2.
SERVO 11.2011 45