Emerald Group Publishing Limited
Copyright © 2008, Emerald Group Publishing Limited
Displacement sensors guarantee quality of paper
Article Type: Mini features From: Sensor Review, Volume 28, Issue 2.
For paper manufacturers, any unforeseen breakdowns in production machinery will result in costly downtime. A number of critical parameters such as paper thickness, lip gap and guide roller wear can seriously affect the quality of the end product. Therefore, these parameters need to be measured and monitored, which can be achieved using a number of different sensor technologies, including eddy current and laser optical displacement sensors. In fact, a number of leading European paper manufacturers are already using Micro-Epsilon sensors to help guarantee the quality of their paper.
Lip gap measurement
Take lip gap measurement. In paper manufacture, the lip gap at the head box determines the thickness of the paper. Paper making machines usually have a lip width of between 4 and 6m and the measuring task involves constantly monitoring the variable gap between the upper and lower lip. To solve this, paper manufacturers are using Micro- Epsilon's “eddyNCDT EU22” sensor, a non-contact eddy current displacement sensor, which is fitted to one or both sides of the lip. A steel or aluminium target plate is also fitted opposite the sensor to serve as the measuring target (Figure 1). The “EU22” sensor is ideal for harsh environments where water and other aggressive liquids are present.
Figure 1 Schematic diagram (left) and in situ photograph (right) of Micro-Epsilon's eddyNCDT EU22 sensor
Micro-Epsilon's range of non- contact, wear-free, eddy current displacement sensors measure distance, displacement or the position of any electrically conductive target. Because of their immunity to dust, oil, dirt, moisture and interference fields, the sensors are ideally suited to harsh industrial environments such as paper manufacture. The sensors also have active temperature compensation and field calibration capabilities. The range includes more than 400 different types of eddy current sensor, including miniature versions, special ceramic, screened housing types and pressure- resistant sensors.
Automatic paper web control
At the so-called “wet end” of paper manufacture, the prepared paper pulp flows onto a conveyor belt of fine wire mesh. Here, most of the water is removed using a vacuum. The paper web is now generated and transported to a second station, the wet press rollers, which determine the paper web speed by means of a second drive unit.
In order to prevent the paper web tearing, a laser-based position measurement sensor is used to measure the position of the paper web between the two stations (Figure 2). Micro-Epsilon's “optoNCDT” laser optical displacement sensors are ideal for these types of applications because they are reliable, accurate (±1 percent FSO), non-contact and have a measuring range (to the paper) of up to 950mm.
Figure 2 Laser-based sensors measure the position of the paper web at the “wet-end” of the manufacturing process
The analogue output signal of the sensor is fed into a closed-loop control circuit, which influences the speed of the drive motor at the first station, therefore ensuring that a specified set point distance value is maintained at all times and the paper web does not tear (Figure 3).
Figure 3 Schematic diagram showing the set up to measure and maintain the specified set point distance
Rebound behaviour of the felt
In paper production, the paper web is transported over a felt from the screen station to the pressing station. This felt removes the residual moisture from the paper. For water separation the felt passes through a defined gap between the central roller and the suction roller.
The wear and the rebound behaviour of the felt determine the hygroscopic qualities of the paper and are therefore critical to paper quality.
Laser optical position sensors from Micro-Epsilon are being used to monitor the wear at the guide roller. Measuring with two sensors compensates for the ovality/stroke of the guide roller (Figure 4). The result of the measurement is the effective thickness of the felt. If the measured value falls below the specified wear limits, the production can be stopped and the felt can be replaced. Because of the difficult environmental conditions, Micro-Epsilon's “optoNCDT” sensors are installed in a protective housing.
Figure 4 Two sensors compensate for the ovality and stroke of the guide roller
Acquisition of coil diameters
Even after the paper has been manufactured, Micro-Epsilon displacement sensors can then be used to measure the quantity (diameter) of paper wound on or off the large storage coils or rolls in the warehouse.
Micro-Epsilon's “optoNCDT ILR” is used here and is an optoelectronic sensor that operates according to the “time of flight” principle. A laser diode in the sensor produces a short laser pulse, which is projected onto the object being measured. The light reflected from the measurement object is detected by the sensor element. The propagation time of the light pulses to the target and back, determines the measurement distance. The integrated electronics inside the sensor calculate the distance from the propagation time and conditions the signal for analogue or digital output (Figure 5).
Figure 5 Sensors are used to measure the paper quantity (diameter) after manufacture
The sensor comes in two main types: laser gauging versions are suitable for direct measurement against an object at distances of up to 10m. If extremely high accuracy at large distances is required (up to 250m) then the distance sensors are used by measuring against a special reflector mounted onto the measurement object.
The sensors offer excellent repeatability, fast response times, a variety of interfacing options and a good price-performance ratio.
For further information please visit the web site: www.micro-epsilon.co.uk