Honeywell Switches and Sensors - Manual (Page 119)

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ULTRASONIC DISTANCE SENSORS
Ultrasonic Distance Sensors
Ultrasonic sensing systems offer no-touch distance measurements to an
accuracy of 1 mm through dust, smoke and vapour, in areas of high noise
level, and with all types of target materials, shapes and colours, with sensing
ranges from 100 mm up to 6000 mm.
High performance no-touch position sensing
Increased reliability, no contamination. Honeywell ultrasonic sensors operate
by exciting an acoustic transducer with voltage pulses, causing the transducer
to vibrate ultrasonically. These oscillations are directed at a target and by
measuring the time for the echo to return to the transducer, the distance may
be calculated. This measurement technique in no way interferes with the
object - it does not contaminate the target, nor does it affect the position. And
being no-touch, there are no mechanical linkages to wear out.
Ultrasonic
Factory noise does not affect operation because the operating frequency is
well above the frequency of ambient sound. And because sound is used, air
pressure, humidity and airborne contamination have little effect on accuracy;
target shape, material and colour are also not critical.
Working method
The sensors work with an ultrasonic transducer used for both transmitting
and receiving. In each cycle, ultrasonic pulses will be transmitted. The pulses
are then reflected back from the target, and received by the sensor. By means
of the temperature compensated measurement of the elapsed time of the
acoustic signal, the target distance is determined, with a high degree of
accuracy. The resulting measurement can be output either as an analogue or a
digital signal.
Figure 1 shows the elapsed time of the acoustic pulse. The diagram shows how the pulse
travels from the transducer to the target, is reflected at time T/2, and reaches the
transducer at time T. Below is a diagram of the voltage at the ultrasonic transducer.
Elapsed time T is directly proportional to object distance a. a = cT/2, where c is the
velocity of sound.
Application criteria
The maximum sensing range depends on a number of factors such as target
shape, surface, inclination to the beam axis, surface composition and
environmental influences. The range values included in this catalogue are
based on a target made of flat, sound-reflecting material at 25°C and still air,
placed vertical to the beam axis.
0 T/2 T
Target object distance
Time
Time
Voltage
Reflective properties
Almost all materials and targets reflect sound, and can therefore be detected.
Only sound-absorbing materials such as cotton wool, or foam rubber are
either difficult or impossible to detect. Certain materials, such as textiles,
weaken the ultrasonic signals, as a result of which the maximum sensing
distance is less than half of the nominal value.
Target shape and surface
All object shapes and surfaces can be measured using ultrasonic sensors, up
to the maximum distance at which a sufficient echo reaches the sensor.
Cylindrical, conical and small objects reduce the measuring range.
Inclination to beam angle
If a smooth, flat target is inclined at more than half of the nominal beam angle
to the normal beam axis (e.g. 5°), the echo is deflected so far that, under
certain conditions, no signal is received by the sensor (see Figure 2 overleaf).
At shorter target distances, the target can be inclined up to the beam (e.g.
10°) from the beam axis. In the case of targets with a rough surface, the
acoustic beam is reflected diffusely. The angle of inclination to the beam may,
under certain circumstances, be up to 50°, but the maximum sensing
distance is reduced.