How to Measure Length and Diameter of an Object Travelling on a Conveyor

An application which requires an object to be measured for length and diameter as it travels along a conveyor belt may be solved using two types of sensors and a PLC.

Position a set of photoelectric sensors in a through-beam configuration so that the infrared beam crosses the conveyor. The object to be detected should break the beam as it travels. The photoelectric sensors will be used to measure the length of the object and also to trigger the measurement of the diameter by the ultrasonic sensor.

The ultrasonic sensor should be positioned above the conveyor facing downward and it should be well out of striking distance. The analog output from the ultrasonic sensor will be sent to the PLC, which should be programmed to calculate the diameter of the post based upon the sensor’s output.

The output from the photoelectric sensor actually serves two purposes. First, it will send a signal to the PLC indicating that the object is present. The PLC will then acknowledge the output from the ultrasonic sensor and begin its calculation. At the same time, the photoelectric sensors will help the PLC determine the length of the object using a calculation based upon the speed of the conveyor and the amount of time that the photoelectric sensors are blocked.

Though it sounds complicated, the hardest part of the application is the programming of the PLC, which should only be done by a trained professional. The benefits of using this equipment to solve the application are that the sensors are easy to install and they perform efficiently and reliably. Both measurements are done without physically contacting the object, so there are no moving parts to wear down.

Related products and information
Infrared photoelectric sensors for conveyor applications
Infrared photoelectric multiplexer for mutiple sets of photoelectric sensors

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