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Synchronizing Multiple Ultrasonic Sensors

Posted by Sam Horn on Mon, Apr 10, 2017

Ultrasonic sensors are non-contact devices used for object detection or level measurement. They operate on the principle of sound traveling through a medium, where a transducer emits a sound wave at a specific frequency. Based on the time it takes the reflected sound pulse to reach the transducer, the sensor’s internal logic determines if the object is within the nominal sensing range and generates an output signal.

Ultrasonic sensors are non-contact devicesThe sensor’s emitted sound wave follows a cone-shaped pattern. If multiple sensors of the same transducer frequency operate next to each other, the emitted pulse of one sensor can be detected as a reflected pulse by the other. Depending on the application, this scenario may not be ideal. Two transducers emitting ultrasonic waves of the same frequency simultaneously can generate false outputs.

An ulrtasonic sensor’s emitted sound wave follows a cone-shaped pattern

To prevent this interference, or cross-talk, a simple wiring connection can synchronize the emit/receive timing pattern. For example, the V15 (5-pin male) connector on a UC2000-L2-I-V15 ultrasonic sensor has one wire dedicated strictly for the purpose of synchronizing multiple sensors. To manually sync multiple UC2000-L2-I-V15 sensors, simply interconnect the synchronization input wire of each sensor.

UC2000-L2-I-V15 ultrasonic sensor

Because the timing is automatically set when the synchronization wires are connected, an ultrasonic sound wave emitted by one transducer cannot be mistaken as a received wave by the other. Even though multiple sensors can be interconnected via the synchronization wire, each sensor tied together will still have distinct outputs. The sole purpose of using the synchronization wire is to alternate the timing at which each sensor pulses.

Note: When installing ultrasonic sensors, it may not be possible to adhere to the minimum separation distances. Pepperl+Fuchs provides models with synchronization inputs. This prevents sensor cross-talk and allows the minimum separation distance to be reduced.

A drawback of using the synchronization wire is increased response time to target an object’s movement. Because multiple ultrasonic sensors alternate pulsing, each sensor pulses in a series pattern. As more sensors are tied together, it will take longer for an output to generate because each sensor in the group must emit and receive on its own before the next sensor can follow suit.

For some applications, a wiring connection and sequential strobing pattern is not an option. In collision avoidance for automated guided vehicles (AGVs), for example, sensors simply need to detect if an object is present or absent in front of the robot. All ultrasonic sensors of the same frequency can be spaced far apart such that they do not cross talk, but an emitted pulse of one sensor can be detected as a reflected pulse by another to reliably detect a present object in front of an AGV. 

An interface can be used to program these parameters on certain ultrasonic sensors; for example, some software allows the timing of alternating pulses to be adjusted. If all sensors in one array are mounted too closely together and pulse at the same time, cross-talk can occur. The response time is minimal, but false outputs can be generated. Programming all sensors in a sequential pattern greatly increases the response time, so programming via an interface can allow groups of sensors on an array to emit/receive as desired. 

For example, six sensors ordered 1-2-3-4-5-6 on an array can pulse in groups of 1-3-5 and 2-4-6 separately to drastically reduce the response time compared to pulsing in a sequence of 1-2-3-4-5-6.

Pepperl+Fuchs offers ultrasonic sensors with synchronization inputs

Some ultrasonic sensor applications work better when the sound cone width is narrower or the sensors rely on each other’s output to regulate timing. Parameter adjustment is specific depending on the application, and timing is key to optimize the response time.

Despite having two or more ultrasonic sensors synchronized via a wiring connection or interface parameter programming, outputs of each sensor in the configuration will still be distinct from one another. Both synchronization methods have their benefits and downsides, but the key takeaway in utilizing multiple ultrasonic sensors simultaneously is preventing interference between sensors of the same transducer frequency. 

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Topics: Ultrasonic Sensors

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