Daktronics C44 - Manual (Page 55)

Infrared Photocells A-3
the sensing position, decrease the sensitivity adjustment (CCW rotation) until the
receiver indicator LED goes off, plus two more full turns. Remove the object
and confirm that the LED indicator comes on and is pulsing more than two beats
per second.
A.3 Installation and Alignment Retro-reflective
Photocells
Retro-reflective mode photoelectric sensing is ideal for off-start-line applications where
opposed mode sensing would be the first choice, but where sensing is from only one side.
Retro is the most popular sensing mode for applications where objects are large and the
environment is relatively clean. The effective range for retro-reflective photocells is
approximately 30 feet.
Retro-reflective sensors work with special target materials that reflect the emitted light beam
back to the sensor. The efficiency of these targets (and, therefore, the sensing range) depends
upon the size and the reflective nature of the target. Size is important because, at ranges
beyond a few feet, the retro target may not intercept the complete beam. At an extended
range, a 3" diameter target will intercept nine times as much light as a 1" diameter target (the
area ratio is the square of the diameter ratio). The 1" target will, therefore, require nine times
the excess gain required for the 3" target. Reflectivity is a function of target construction.
Most plastic targets are made up of small, highly efficient corner-cube reflectors.
Successful retro-reflective mode sensing depends upon adequate optical contrast between the
dark (beam broken) state and the light (beam unbroken) state. Retro-reflective sensing,
therefore, works best with objects of low reflectivity. Highly reflective objects such as glass,
polished metal, mirrors, etc. may not be sensed because they can reflect as much or nearly as
much light back to the sensor as does the retro-reflective target. This effect is known as
proxing. At the other extreme, transparent objects are difficult to sensor retro-reflectively
because they may not sufficiently interrupt the sensor's light beam.
Proper operation of retro-reflective mode sensors requires that they be mounted securely and
aligned properly. Excessive movement or vibration can result in intermittent or false
operation caused by loss of alignment to the retro-reflective target.
Use the LED on the photocell in the following alignment procedure.
1. Begin with the sensor at the desired distance from the retro-reflective target and at
the approximate position where it will be mounted. Retro-reflective targets are rather
forgiving to beam angle in that they do not begin to lose effectiveness until they are
more than 15 degrees off of perpendicular to the beam axis. An object at the sensing
position should pass through the core of the sensor's light beam.
2. Apply power and perform one of the following steps (see notes below). If in either
case the LED appears to be on steadily, it is actually pulsing at a rate too fast to be
seen. Slow the pulsing to a countable rate by reducing the sensitivity (counter-
clockwise rotation of the adjustment). Being able to detect a change in the pulse rate
when the position of the sensor or reflector is changed will allow accurate alignment.
a. If the target position is fixed, tilt the sensor up/down and rotate right/left to
obtain the fastest indicator LED pulse rate (no object at the sensing position).
Secure the sensor in position.
b. If the sensor position is fixed, move the target up/down and right/left to obtain
the fastest indicator LED pulse rate (no object at the sensing position). Secure
the target in position.