Ultrasonic Sensors Knowledge (Part 6) – Synchronizing Ultrasonic Sensors

If a synchronization input remains open, the ultrasonic sensor operates in normal mode. By applying a defined potential (L+/L-), the sensor can be locked and enabled again using an external trigger signal. As long as the sensor is locked, no ultrasonic pulses are emitted. The outputs (analog and switching outputs) are frozen while in this state. As soon as the sensor is enabled for at least one measurement cycle with the synchronization input, the outputs are updated. This feature can be used for external synchronization or multiplex operation.

In external synchronization mode, the synchronization inputs for all sensors are linked and controlled by an external clock-pulse source (each ultrasonic sensor is triggered separately in multiplex mode).

The required signal level, cycle time, and the maximum possible number of sensors are found in the data sheet of the respective sensor. In internal synchronization mode, the synchronization inputs for all sensors are linked and controlled by the sensors themselves. There is no need for an external clock-pulse source.

In common mode, the ultrasonic sensors work in parallel. This means they all emit a burst of ultrasound at the same time and wait for the reflected echo from an object within the sensing range. To do this, the synchronization inputs of all sensors must be connected. Depending on the sensor type/family and configuration, synchronization mode either runs automatically (internal synchronization) or requires an external trigger signal (external synchronization).

Application:

Several sensors are installed in an ultrasonic array to monitor a wide area. Where space is limited, synchronizing sensors can be useful, as a lower minimum interval is required between the sensors. The intervals given in the table below must also be observed for opposite mounting.

Pros:

• Lower wiring costs, connection of enabled inputs for each proximity switch
• Fast response times, as every proximity switch is continuously active

Cons:

• The object cannot be assigned to a specific proximity switch

Multiplexing ultrasonic sensors allows ultrasonic bursts to be emitted at staggered intervals. This prevents crosstalk between sensors, and there is no need to maintain a minimum distance between adjacent sensors. However, as the sensors are activated in sequence, the response/reaction time increases with each sensor added to the multiplex array.

A distinction is made between internal and external multiplex mode. When using internal multiplex mode, the synchronization inputs of all sensors must be connected. Depending on the sensor type/family and configuration, multiplex mode either runs automatically, or the sensor must be assigned an address using the configuration tool. In external multiplex mode, an external trigger signal is required and the timed sequence must be coordinated by an external controller.

Application:

The sensors are used in equipment and machines with very confined installation spaces, or sensors of the same type in immediate proximity, or to prevent crosstalk in various measurement tasks. Minimum intervals do not need to be adhered to, even if the sensors are located opposite of one another.

Pros:

• Reliably prevents crosstalk
• An object can be assigned to an ultrasonic sensor

Cons:

• Additional costs for clock generator when using external multiplexing
• Longer response/reaction time than synchronization mode because the sensors work in sequence