Basis weight regulation
Basis weight control is the most important control on a paper machine because the basis weight is the main criterion for production and subsequent marketing.
The basis weight is regulated on a modern paper machine with the help of a process computer, which is a further development of a meshed regulation based on individual regulators. The variable to be controlled is measured directly or indirectly and the measured "actual value" is compared in the controller with a specification, the setpoint . Depending on the result of this comparison, the manipulated variable Y is changed on the actuator . This change in the manipulated variable is intended to adjust the controlled variable , the actual value X, to the setpoint value W.
The measuring device
For the measurements on a paper machine, a low-vibration and dimensionally stable support construction is used, which extends over the entire width of the paper web to be measured. This support structure is firmly connected to the subsurface on both sides. The support structure is optically measured very precisely during and after assembly and, if necessary, mechanically corrected so that there are as few fluctuations as possible in the distance to the paper web for later measuring operations. The carrier construction is the track for the traversing scanner, which is divided into two parts above and below the paper web and covers the entire width and indirectly measures the weight per unit area.
The measuring head
The multiple measuring head is installed in a water-cooled plastic housing filled with nitrogen at a slight overpressure. The respective sensor sources are located in the lower head and the receiver devices are located in the upper head. The measuring head needs exactly 30 seconds for a level crossing, whereby the exact alignment of the two head parts on top of each other is guaranteed at all times by a coordinated toothed belt drive. A deviation of more than 5 mm is not tolerated and leads to the measurement being switched off. The sensors in the measuring head measure the weight per unit area, air humidity and air gap temperature and continuously transmit these to the process computer. The individual sensors are explained in the next sections.
The humidity sensor
Infrared light with two defined wavelengths is used to measure the humidity, one wavelength is sensitive to water, so the second wavelength is not absorbed by water. The intensity of the two wavelengths is measured during the passage through the train and transmitted to the process computer. The process computer determines the moisture in percent from the ratio and the weight per unit area measured at the same time. A moisture measurement is carried out at defined time intervals using a filter that simulates a known water content and compared with a calibration curve. The comparison with the calibration curve provides a correction factor that is taken into account by the process computer when determining the percentage moisture.
The basis weight sensor
The basis weight is measured indirectly via the beta particle absorption capacity of the paper. In the lower part of the measuring head there is a beta radiation source encapsulated in a tungsten housing. The beta particles are partially absorbed by the paper web and the remaining part of the radiation in the top part of the head is picked up by the radiation sensor. An automatic shutter ensures that the radiation source is only open during the railway passage and during calibration measurements.
- Encapsulation material: tungsten
- Radiation source: Krypton 85
- Radiant energy : 0.5 curie
- Half-life: 10 years
- Radiation exit opening: 12 mm
- Weight range: 10-750 g / m²
- Measurement accuracy: 0.25%
- Reproducibility: 0.1%
The absorption ratio follows a so-called e-function
- where R = transmission ratio
- e = Euler's number
- i = absorption coefficient
- z = weight
An absorption curve is stored in the process computer, which was built up on the basis of known and specified weight samples; this serves as a reference curve. Every 30 traverses, a standardization measurement is carried out outside the paper web. During this measurement, any dirt deposits on the sensor are determined and taken into account by the process computer when calculating the basis weight, in this way the source of error dirt deposits is eliminated. A second source of error, which affects the basis weight measurement, is caused by the air gap temperature, since the absorption capacity for beta particles changes with temperature. For this reason, the air gap temperature is recorded over the width of the web, evaluated and dynamically included in the basis weight calculation.
The influence of air temperature
As mentioned in the previous section, the air temperature influences the measurement of the basis weight. The reason for this lies in the different density of warm and cold air, this different density has the consequence that different numbers of beta particles are absorbed by the air between the radiation source and the radiation sensor. The influence is negligible, but is taken into account in the basis weight calculation in order to meet the high quality requirements. The air gap temperature is measured separately for the top of the head and the bottom of the head, in that a fan guides the air at high speed along a thermistor in the respective head section. The measured air gap temperature is sent to the process computer at the same time as the data on humidity and weight per unit area.
The actuator is used to influence the manipulated variable, which changes the controlled variable "basis weight". The manipulated variable is the amount of material that is fed to the paper machine. High demands are placed on the actuator: low wear, no bearing play, if possible, good positioning and low responsiveness. In most basis weight control loops, a ball sector valve with stepper motor control is used. A ball sector valve is characterized by its excellent characteristic curve, which is logarithmic. The spherical structure prevents deposits and other unwanted changes in cross-section. A sector in the inlet part of the ball ensures an equal percentage change in flow rate in relation to a change in stroke and gives this valve its name.
The consistency measured by the consistency transmitter is compared by the process computer every second with the previous consistency value. The actual change in consistency is calculated from this comparison. Since the stock consistency transmitter is located in front of the stock valve, the process computer waits until the actual change in stock consistency has arrived at the stock valve and only then changes the stock flow rate. The necessary change in the material flow rate determined by the process computer represents the actual material density compensation and results in a more uniform amount of material to the headbox, which significantly reduces the short-term scatter of the basis weight and also the moisture.
- Otto Föllinger, Regeltechnik 5th improved edition, Huthig Verlag, Heidelberg 1985
- Theo Gruber, measurement and control technology, manual for the paper and pulp industry, paper from Austria, Steyrermühl
- Bert Rackmunisz, information sheets on control engineering, VDI Verlag, Düsseldorf 1988
- Chris Krifed, Datasheets for Radiometry, TG Teubner, Stuttgart 1988