Electricity and Control February 2020

SENSORS + SWITCHES

Photoelectric sensors for precision edges

One of the numerous processing steps in the manufacture of furniture is smoothing and levelling of veneer surfaces. In this case study, ifm reports how high-performance diffuse reflection sensors are used for contour detection in the processing of veneer top surfaces to ensure the highest quality in the sanding process.The sanding machine in use here is from Heesemann and is used for large furniture panels such as table tops for conference tables.

O ne of the challenges presented in the sanding process was that the edges of both the outer ends and the cut-outs – where they occur – of the high-quality veneer should not be rounded by sanding. There should be no pressure applied by the sanding shoe segments where there is no veneer underneath. For this reason the contour and cut-outs (if any) of the workpiece are detected during each cycle and transferred to the controller. Sensors replace mechanics The scanning of table tops used to present a problem for this longitudinal sanding machine because the scanning was effected by mechanical switches. The table top was detected via levers with rollers and evaluated by an old PLC. The PLC inputs required only a low current of about 1 mA and there were problems with the switching contacts of the mechanical switches which, because they were worn away over time, did not always switch reliably.

Another problem was the detection of the table tops by the rollers. The sanding dust took its toll on the rollers’ bearings and this caused the rollers to fail more and more often. As a result table tops were spoiled and could no longer be used. A solution using non-contact detection without mechanical switches was therefore sought. The company consulted one of its long-standing suppliers, ifm electronic, and found the correct technical solution. ifm provided its diffuse reflection sensors, O6 design, free of charge, for testing in the sanding process. After the tests which specifically assessed the sensor’s reaction to dust and to the different colours of the veneers, it was found to meet all the criteria and selected for the process. In this case the PLC was also completely updated and the sensing of the diffuse reflection sensors was visualised on a 19" display so that any incorrect detection could be seen at an early stage.

28 Electricity + Control

FEBRUARY 2020