Image Credit: Baumer Ltd. | Allied Electronics, Inc. | Newark / element 14
Ultrasonic proximity sensors use reflected or transmitted ultrasonic waves to detect the presence or absence of a target component. The output is Boolean, that is, the sensor merely detects whether the target is or is not within the detection range.
Ultrasonic Senor, Operating Principle
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Ultrasonic proximity sensors emit and receive sound waves. The carrier signal is a high frequency, inaudible sound wave. They detect the presence of the target object in one of two configurations.
Diffuse or Reflective sensors have the transmitter and receiver packaged in the same housing. When a target enters the sensing range of the device, the ultrasonic waves are reflected back to the sensor.
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Opposed or Thru-Beam sensors have the transmitter and receiver packaged separately. The receiver is mounted facing the transmitter and when an object enters the sensing range of an opposed sensor, it blocks the transmitted signal. Rather than activating the trigger when the frequency is received, the trigger is activated when the signal is broken.
Image Credit: Peppurl+Fuchs
Advantages / Disadvantages
Ultrasonic proximity sensors can detect a variety of objects regardless of its material or surface properties. They are useful for object detection over intermediate distances, on the order of several feet. At close proximity they possess a blind spot where objects are not detected. They can also operate in a wide variety of operating conditions.
|Detects a Variety of Materials / Surfaces||Blind Zone at Close Proximity|
|Detection in Adverse Climates||Noise Interference|
When searching for ultrasonic proximity sensors, sensing performance and carrier signal are the most important parameters to consider. Other considerations include switch performance, body type, electrical connections, and various features.
Specifications for sensing performance include operating distance and repeatability.
- Rated operating distance is the critical distance at which switching takes place. It is important to choose a sensor that will operate in the required sensing range. This could be dictated by process requirements and mounting option.
- Repeatability is the distance within which the sensor repeatably switches. It is a measure of precision. Depending on the application, precision could be an important design criterion when selecting a sensor.
Specifications describing a sensor's carrier signal include blind zone and transmitter frequency.
- The blind zone is the distance extending from the face of the sensor where no objects can be detected. The blind zone is equal to half the wavelength of the ultrasonic wave.
The transmitter frequency is the range of broad-casted and received signals, measured in hertz or waves per second.
Electrical Switch Performance
Depending on the power options available, power requirements may be a key specification when selecting a sensor.
The device can be powered by either an AC or DC power source.
Load configurations are important parameters to consider. Ultrasonic proximity sensors may switch an AC load or a DC load. DC load configurations can be NPN or PNP.
NPN is a transistor output that switches the common or negative voltage to the load; load connected between sensor output and positive voltage supply.
PNP is a transistor output that switches the positive voltage to the load; load connected between sensor output and voltage supply common or negative.
Wire configurations are 2-wire, 3-wire NPN, 3-wire PNP, 4-wire NPN and 4-wire PNP.
Image Credit: Fargo Controls, Inc.
2-Wire DC circuitry typically includes a diode bridge in series with the sensor, allowing functionality either as NPN or PNP in a DC circuit.
Acceptable values of leakage current must be evaluated in this type of circuit
Switch types can be normally open (NO) or normally closed (NC).
Ultrasonic proximity sensors are typically housed in either a barrel or rectangular housing.
Threaded barrel and rectangular housing. Image Credit: Baumer Ltd. | Banner Engineering Corp.
A barrel body style is cylindrical in shape. The barrel of the sensor is typically threaded so that with the help of two lock-nuts the sensor can easily be adjusted closer or further from the target as necessary.
A rectangular or block body style is a one piece rectangular or block shaped sensor.
Electrical connections for ultrasonic proximity sensors can be fixed cable, connector(s), and terminals.
A fixed cable is an integral part of the sensor and often includes "bare" stripped leads.
A sensor with connectors has an integral connector for attaching into an existing system.
A sensor with terminals has the ability to screw or clamp down.
There are a number of features that may be important to consider when selecting ultrasonic proximity sensors.
- Field adjustable sensors are required when adjustments need to be made while the proximity sensor is in use. Potentiometers are commonly used to control the detection range.
Self-teaching proximity sensors can be auto-calibrated to trigger the switch at a specific target location.
Materials of construction and enclosure ratings must be considered when the sensor will be located outdoors or exposed to extreme temperatures, or under the influence of wet, humid, dusty, dirty or corrosive process conditions.
Enclosure ratings are specified by "National Electrical Manufacturers Association (NEMA)" or "Ingress Protection (IP)" ratings.
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