Page 370 - Mechatronics with Experiments

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However, in practice averaging is used in the position signal processing. In other words,
multiple wave signals are obtained in a row, such as ten samples in a row, and then the
average value of that measurement is presented as one position measurement information.
If averaging is used, the actual bandwidth will be smaller by a proportional amount. In
this case, if five sample averaging is used, the sensor’s maximum bandwidth would be
2.25 kHz∕5 = 450 Hz or if ten sample averaging is used, the sensor’s maximum bandwidth
would be 2.25 kHz∕10 = 225 Hz. The magnetostrictive absolute position sensor is one of
the most rugged position sensing technologies currently available and is widely used in
harsh environment applications, especially with hydraulic cylinders.

6.4.7 Sonic Distance Sensors
Sound is transmitted through the propagation of pressure in the air. The speed of sound in
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the air is nominally 331 m∕sat0 C dry air (343 m∕sat20 C dry air). Two of the important
characteristics of sound waves are frequency and intensity (amplitude). The human ear can
hear the frequencies in the range of 20 Hz to 20 kHz. Frequencies above this range are
called ultrasonic frequencies.
Sonic distance sensors measure the distance of an object by measuring the time
period between the sent ultrasonic pulse and the echoed pulse (Figure 6.27). The sensor
head sends ultrasonic sound pulses at high frequency (i.e., 200 kHz), and measures the time
period between the sent pulses and the echoed pulses. For instance, it may send a short
pulse of 200 kHz frequency and fixed intensity every 10 ms, where the time period of the
sent pulse is only a few milliseconds. Before a new pulse is sent out, the time instant of the
reflected sound pulse is measured. This process continues periodically, every 10 ms in this
example. Knowing the speed of sound, a digital signal processor embedded to the sensor
can calculate the distance of the object,

x = V sound ⋅ Δt (6.75)

where V is known, Δt is measured, x is calculated. The range of the sensor can be up to
sound
20 m with a minimum range of 5 cm. Notice that the sonic distance sensor is not appropriate
for very short distance measurements, that is under 5 cm. The frequency response of the
sensor (distance measurement update rate) varies with the distance measured, in general
it can be around 100 Hz. Clearly, sonic sensors cannot be used in high bandwidth servo
positioning applications which require a 1.0 ms or faster position loop update period.
Typical applications of the sonic distance sensor include the roll diameter measurement,
web loop length measurement, liquid level measurement, and box presence measurement
on conveyors (Figure 6.28).

200 kHz

Amplitude

Sensor Echo pulse
Object Transmitted Transmitted
pulse Time interval pulse
10 ms
FIGURE 6.27: Operating principle of a sonic distance sensor.