TechArchive

Original article date: September 1999

Transparent containers, such as PET bottles, have great attractions for the packaging industry but, unfortunately, such containers are difficult to detect reliably with conventional transparent object detectors. Tony Malin, of Telemecanique has details of an innovative new sensor which pruvides a complete solution to this problem.

Most packaging equipment relies heavily on retro-reflective photosensors. They’re easy to install and adjust, they’re reliable, and they offer excellent sensing ranges. Ordinary retro-reflective sensors are, however, not well suited for use with transparent targets, such as PVC waapping film, glass jars and PET bottles.

The problem is not hard to understand. Since the target absorbs very litte light, the amount of light reflected back to the sensor when a the target is present is almost the same as when there is no target in place. Some photosensors are undoubtedly sufficiently sensitive to detect the small change in light level, but that’s not the end of the story.

First, adjusrment for the best operating point with such a tiny ,change in light level is very critical. Second, it is perfectly possible that the operating point of conventional sensors may drift slightly -due, pernaps, to changes in temperature or component ageing. This drift can easily be enough to upset the critical adjustment.

The third, and most important, problem is lens soiling. It is inevitable that, even in the best maintained plants, dust and other contanants will build up on the lenses of photoelectric sensors over a period of time. The result is a reduction in beam strength, which conventional sensors will interpret as the permanent presence of a transparent target.

With these problems in mind, the Telemecanique brand of Schneider Electric has developed the innovative new XUKT retro-reflective sensor which is specifically designed for use with trannsparent targets. XUKT sensors have a range of 1.5 metres, and are supplied in the rectangular housings which are the most popular choice for packaging and mechanical handling equipment.

The ciruuitry of the sensors is optimised to detect the small changes in light level produced by transparent targets, and to provide excellent stability under all operating conditions. In addition, the difficulty of setting precisely the correct operating point is overcome by an automatic adjustment feature.

After installing and aligning the sensor, the user simply presses a “teach” button on the sensor body while there is a target in the beam, then presses it again with no target in place. With no further effort on the part of the user, the sensor then sets itself to the best possible operating point.

That’s not all, however. During their normal operation, XUKT sensors constantly monitor the strength of the beam with and without the target present. As soiling builds up on the lenses, and the beam strength falls, the sensors compensate without the need for any adjustment by the user. In this way, reliable sensing is achieved over long periods, ensuring maximium productivity, and minirnising maintenance costs.

The self-monitoring feature also compensates automatically for changes in the operating environment, such as the presence of steam or other vapours, and for small amounts of misalignment between the sensor and the reflector. Should any combination of these effects reduce the light beam strength to a level where reliable operation can no longer be guaranteed, the sensor provides an alarm output which can be used to signal the need for corrective action.

To ensure simple interfacing with modern PLC-based control systems, Telemecanique’s XUKT transparent target detectors are designed for operation from DC supplies in the range 10-30V. They incorporate both PNP and NPN outputs rated at l00mA for the main output, and 50rnA for the alarm output. To maximise their versatility, the sensors feature an integral delay-on/delay-off timer, which is adjustable with a potentiometer over the range 0 to 15 seconds. They also have programmable light/dark switching via DIP switches, located under a lockable perspex cover.

XUKT sensors are also provided with two LED indicators which show the output status, and provide instant visual cofirmation that the light beam is of sufficient strength to allow dependable detection. The sensors are available in pre-cable versions, or versions with M12 connectors, both types have an IP66 protection rating. They are housed in industry standard 50mm x 50mm housings

With the introduction of XUKT photosensors from Telemecanique, detecting transparent targets reliably is simple. The automatic set-up feature guarantees that the best operating point is selected, and self-monitoring means that the sensors will operate for long periods without adjusment or maintenance. In fact, when its clear objects you need to detect, XUKT sensors offer the solution that’s clearly superior!

• Telemecanique,
  Schneider Electric Ltd,
  University of Warwick Science Park,
  Sir William Lyons Road,
  Coventry, CV4 7EZ.
  
• Tel: 02476 416255
  Fax: 02476 690209

September 1999