Electricity + Control September 2019

SENSORS, SWITCHES + TRANSDUCERS

A stilling tube is commonly used to obtain accurate liquid level measurements in streams, irrigation channels, wet wells and other open water areas with turbidity, foam or moving obstacles. The

multiple times before it dampens out enough to become non-detectable as a first-time echo. (Figure 5.) This can be addressed by setting the measurement rate to allow dissipation of the signal to occur naturally before a subsequent pulse is sent. For example, if it is determined that a signal will reverberate four times before dissipating below the detection threshold, the measurement can be set at a rate slower than four times the travel time for a single measurement. Moving sensors/moving objects In applications where either the sensor is moving or the object is moving, the rate of the movement may be too fast for the sensor to detect the object. Smart sensors enable users to calculate and then configure the required measurement rate, tak- ing into consideration the beam width at the meas- urement distance, size of the object, the maximum speed of either the object or the sensor, and the speed of sound in the environment. (Figure 6.) Synchronising sensors Two or more sensors installed in close proximity may interfere with one another. Multiple smart sensors can be networked and programmed so that only one sensor is measuring at a time. Each sensor may require only a few milliseconds to take its measurement. A group of 20 or even 30 sensors can complete a synchronised measurement in less than a second. Sensors in a network can also be programmed to take measurements at the same time to get a level or distance measurement for a wide area at a single point in time. In this scenario, the first signal returned to each sensor is recorded and other, subsequent signals are ignored. Detecting object dimensions When the dimensions of an object cannot be detected using a single sensor or multiple standalone sensors, three smart ultrasonic sensors can be configured to collect data that identifies the height and width of an object, such as a carton moving on a conveyor line. (Figure 7.) One sensor measures the difference in distance between the top of the object and the conveyor or floor to determine the height. Two sensors can be used to measure the object from two sides to determine the width. If one side of the object is always in the same position, such as against a wall or another reference guide, the width can be determined using one sensor.

Figure 3.

Figure 5.

ultrasonic sensor is secured at the top of a tube, which is extended into the liquid being measured. (Figure 3.) The surface of the liquid within the stilling tube remains calm so the measurement is not affected by turbulence or the movement of objects beyond the tube. This technique is used in various outdoor liquid level measurement applications. In irrigation systems, for example, smart ultrasonic sensors and stilling tubes are used to measure water levels above and/or below control gates where water may be flowing and turbulent. Temperature variations The speed of sound changes by about one percent for every 10°F (approximately -12°C) change in temperature. Temperature variations therefore have the potential to distort measurements. Smart sensors incorporate temperature compensation circuitry that can adjust distance measurement to compensate for changes in the speed of sound. Detection of irregular objects In applications where users need an ultrasonic sensor to detect the presence or absence of an object, the echo signal may go astray and not be detected if the sound pulse strikes the object at an oblique angle. One common application, for example, involves using ultrasonic sensors to de- tect the presence or absence of a car in a parking space. (Figure 4.) This challenge can be addressed by programming the sensor to detect the ground instead of the object. To avoid false triggers caused by animals, debris or other unintended objects, smart sensors can be configured to ignore objects that are smaller or closer to the

Figure 6.

ground than the intended target. Smart sensors also can be configured to register only an object that remains for a minimum amount of time.

Figure 4.

Sound reverberation in tanks In a closed tank, sound energy may dissipate slowly, causing an echo signal to reverberate

Figure 7.

Electricity + Control

SEPTEMBER 2019

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