Optical sensors are used in a variety of industrial applications. One such application is the production of paper. In the production of paper, several measurements are required during the manufacturing process to ensure consistent quality of the final product. Two such measurements are consistency and brightness.
During the manufacture of paper, wood fibers are separated from bulk wood by either mechanical or chemical means, or a combination. Water is then mixed with the wood fibers to form a wood pulp slurry. In order to achieve some measure of quality control during the process, it is essential to know the ratio of wood fibers to total mass (consistency) at every step in the process.
Some sensors used in the industry to measure consistency are mechanical in nature. One early method requires that a calibrated tapered rod about six inches long be dropped from a vertical position a standard distance above the stock. A reading is then taken of the depth the rod sinks in the stock. Other mechanical sensors, by one means or another, measure the force which moving pulp slurry produces on a mechanical arm, plate, or the like. Some limitations of these sensors are distortion due to velocity of the slurry, different wood species and drainage. Also such mechanical system cannot be readily installed in a tower or chamber through which pulp slurry is moving slowly or in which pulp slurry is contained. Finally, accuracy of mechanical sensors is limited for certain pulp consistencies.
The quality of paper is also dependent on the brightness of the pulp. Traditionally, brightness measurements are normally performed on an off-line basis. In this type of instrument, a sample is periodically taken from the pulp washer, dried, and its brightness determined from a reflectance meter. This determination can take 20 to 30 minutes.
Most modern methods of determining consistency and brightness of the pulp employ some means which emit radiant energy in the direction of the pulp. The magnitude of the energy which either passes through the pulp or is reflected back from the pulp is indicative of its brightness and consistency. This reflected energy can either be measured or compared to the magnitude of the energy emitted to determine the consistency and brightness of the pulp. The magnitude of the reflective energy can be separated into components for both consistency and brightness.
In an example of an on-line system, two sensors are positioned in the pulp to measure brightness of the pulp entering and leaving a bleaching stage. The two sensors measure the intensity of back scattered light and then compare results to control bleaching chemicals in order to optimize brightness of the pulp.
One drawback of the radiant energy methods is that as the sensor ages or certain conditions within the measurement chamber change, the sensor may output erroneous readings. There are mechanical systems which provide data normalization due to condition changes. In most cases, these mechanical systems extend out into the pulp flow and are susceptible to wear and premature failure from a variety of sources including vibration.
Therefore, providing data normalization which is less susceptible to vibration and does not require exposure to pulp has significant advantages.