Ultrasonic proximity sensors use a transducer to send and receive high frequency sound signals. When a target enters the beam the sound is reflected back to the switch, causing it to energize or deenergize the output circuit.
A piezoelectric ceramic disk is mounted in the sensor surface. It can transmit and receive high-frequency pulses. A highfrequency voltage is applied to the disk, causing it to vibrate at the same frequency. The vibrating disk produces high-frequency sound waves. When transmitted pulses strike a sound-reflecting object, echoes are produced. The duration of the reflected pulse is evaluated at the transducer. When the target enters the preset operating range, the output of the switch changes state.
When the target leaves the preset operating range, the output returns to its original state.
A blind zone exists directly in front of the sensor. Depending on the sensor the blind zone is from 6 to 80 cm. An object placed in the blind zone will produce an unstable output.
The time interval between the transmitted signal and the echo is directly proportional to the distance between the object and sensor. The operating range can be adjusted in terms of its width and position within the sensing range. The upper limit can be adjusted on all sensors. The lower limit can be adjusted only with certain versions. Objects beyond the upper limit do not produce a change at the output of the sensor. This is known as “blanking out the background” . On some sensors, a blocking range also exists. This is between the lower limit and the blind zone. An object in the blocking range prevents identification of a target in the operating range. There is a signal output assigned to both the operating range and the output range.
The radiation pattern of an ultrasonic sensor consists of a main cone and several neighboring cones. The approximate angle of the main cone is 5°.
Free zones must be maintained around the sensor to allow for neighboring cones. The following examples show the free area required for different situations.
OPERATING MODES
Sonar sensors can be setup to operate in several different modes: diffuse, reflex, and thru-beam.
DIFFUSE MODE
This is the standard mode of operation. Objects, traveling in anydirection into the operating range of the sound cone, will cause the sensor output to switch states. This mode of operation is similar to a proximity sensor.
The reflex mode uses a reflector located in the preset operating range. The operating range is adjusted for the reflector. The pulses are bounced off the reflector and the echo pulses are returned to the sensor. When a target blocks the echo pulses the output is activated. Typically used in applications where the target is not a good sound absorber.
Thru-beam sensors consist of a transmitter, which emits ultrasonic pulses, and a receiver. If the beam between the transmitter and the receiver is interrupted the output of the
receiver switches state.
Sound travel time can be affected by physical properties of the air. This, in turn, can affect the preset operating distance of the sensor.
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