Visualising temperature

01 April 2007

Industrial thermography is growing beyond its predictive maintenance roots to become a primary sensor technology. Process industry applications include the manufacture of glass and plastic packaging film.

Just as thermal imaging is becoming white hot for predictive equipment maintenance, industrial engineers are becoming increasingly aware that it makes an excellent primary sensor to keep temperature control systems on track.

Thermal imaging, or thermography, maps the surface temperature of an object by means of the infrared (IR)
radiation it emits. A thermographic scanner codes different temperatures as different colours, making it possible to visualise very narrow temperature ranges. All thermal imagers use ‘false colours’ to display these images, since the true colours are all infrared, and therefore invisible to the human eye. More importantly, the equipment is also capable of analysing the images to extract precise control information.

We are now at the stage where process engineers are starting to use thermography to help control manufacturing processes. Armed with detailed knowledge of how critical temperatures vary in time and space, engineers can adjust automated control setpoints and parameters to improve quality and maximise yield.

Tempering glass
Tempering glass requires bringing an entire glass sheet up to a uniform temperature above its softening point, then quenching it rapidly enough to introduce just the right amount of internal strain. This strain creates glass four to five times stronger than ordinary glass, which shatters into tiny glass cubes instead of large, sharp shards.

Ordinary glass can be likened to an extremely viscous liquid. It is so viscous that its flow rate under typical forces is imperceptible. As the temperature goes up, however, the glass viscosity goes down. At annealing temperature (290- 340ºC) glass holds its shape well if supported, but the atoms are able to move around to relax any internal strains.

‘After [the glass sheet is] heated in the furnace, it goes into the air quench,’ explains Clifford Matukonis, process engineer at Tamglass Tempering. For best temperature control, he moves the glass from the furnace to the quench on a conveyor moving at 2,500-3,800 metres per minute. The scanner, a Raytek EC100 line scan thermograph, looks down onto the glass sheet with the scan line perpendicular to the
conveyor motion and scans parallel stripes across the sheet.

The air quench is a flat plate perforated by holes through which cool air blows onto the sheet. That cool air
rapidly ‘freezes’ the glass, locking in a uniform strain field that depends on the annealing temperature and quenching rate. This uniform strain field gives tempered glass its useful properties.

‘We typically put the scanner between the furnace and air quench. At that point, we find out what’s the
temperature gradient of the entire sheet of glass,’ says Mr. Matukonis.

Tamglass controls its process through a number of radiant heating elements mounted over a conveyor belt in the furnace, as the photo shows. Each heating element has a proportional controller using a thermocouple as sensor. Overall process control is via a PCbased system communicating with the
proportional controllers over Profibus.

Heating element setpoints vary from about 315-370ºC. Modifying the relative temperatures of heating elements can adjust the sheet’s temperature uniformity, while the amount of time it spends in the oven (controlled by the oven conveyor speed) controls the ultimate temperature. Slowing the conveyor leaves the plate in the oven longer, so the glass reaches a higher ultimate temperature.

Thermography allows Mr. Matukonis to monitor the actual product temperature at the point where it counts most: between the furnace and air quench. At present, only thermography can capture spatially resolved temperature data in the time available. Gaining insight into both ultimate sheet temperature and temperature gradient enables him to adjust heater setpoints to tune the control system on the fly.

From supporting actor to star Now that control engineers are seeing thermography’s ability to help optimise
control loop setpoints, the next step surely will be to take the human out of the loop. They will develop algorithms to tune the process controls automatically, adding a new level of control to make automated
manufacturing systems more robust.

In industries now experimenting with thermography as a primary sensor for quantitative information to help
engineers tune process controls, future systems will have thermographs backed up by controllers that calculate appropriate setpoints for PID control loops for individual process controls. While these thermographs will be collecting spatially and temporally resolved temperature data, the controllers will analyse that data to more directly measure process parameters, such as gradients, fill levels, and control system stability. Such systems will optimise the entire manufacturing process for maximum quality and yield.

(The full article appears on Control Engineering Europe’s web site, www.controlengeurope.com)


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