Page 371 - Mechatronics with Experiments

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JWST499-c06
JWST499-Cetinkunt
SENSORS 357 254mm×178mm

FIGURE 6.28: Applications of of a sonic distance sensor in various manufacturing processes:
(a) liquid level sensing, (b) two-dimension sensing (i.e., box height and width), (c)
one-dimension sensing (i.e., box height), (d) roll diameter sensing, (e) web slack sensing.

6.4.8 Photoelectic Distance and Presence Sensors
Sensors which measure the distance or presence of an object from a reference point using
“light” as the transduction mechanism are collectively called “photoelectric sensors” or
“light sensors.” The light sensors which provide only two discrete outputs (ON/OFF) are
called photoelectric presence sensors (Figure 6.29). The sensor emitter sends a light beam.
The receiver (phototransistor) turns its output either ON or OFF depending on the received
light. The light threshold that separates the ON and OFF states is adjustable by the op-amps
in the electronic circuit of the sensor. The light sensors are immune from the ambient light
variations due to the modulated nature of the emitted light. The light emitting diodes (LED)
are tuned to emit light at high frequency. Figure 6.30 shows the frequency spectrum of light
waves. The phototransistors (receivers) are also tuned to respond to that light frequency
and reject the other frequencies, much like tuning a radio channel. As a result, the sensor
output is very robust against ambient light conditions. If the receiver is across from the
emitter, it is called a through light sensor (or through beam photoelectric sensor). If the
receiver and emitter are on the same sensor head, and the receiver turns ON/OFF its output
based on the amount of reflected light, it is called a reflective light sensor (or reflective
beam photoelectric sensor).
A category of light-based distance and presence sensors uses reflected light intensity
for measurement. The presence sensors have a two state output: ON or OFF, representing
the presence or absence of an object at the target location. Such reflected light based
presence sensors (sensors with ON or OFF output) work on the principle of reflected light
strength. An emitter sends a focused light. The object reflects the light. The reflected light

Regulated
voltage Power source
Modulated LED Phototransistor supply

Regulated
Power Amp Demodulator Timing Load
source voltage Oscillator logic
supply
Emitter Receiver
FIGURE 6.29: Components and operating principle of a through-beam light presence sensor.
Reflective and diffusive versions of this type of sensor can also be used for distance sensing. (a)
Liquid level sensing, (b) part dimension sensing (height and width), (c) part height sensing, (d)
roll diameter sensing, (e) web slack level sensing.