Active microwave moisture sensors advanced solutions for the pulp and paper industry AMS an integrated system of sensors installed in various phases of the pulp and paper manufacturing process to measure moisture levels in real time and make data available online. By Eng. Claudio Salvador, President, Advanced Microwave Engineering S.r.l. Figure 1. Active Microwave Sensors Use in paper manufacturing. INTRODUCTION Modern paper manufacturing machines are evolving towards ever greater automation and control in order to obtain ever better performance in terms of productivity, flexibility and cost reduction, as well as controlling quality. Furthermore, to face the challenges presented by today s demanding and difficult market, the most advanced paper mills are focusing on other increasingly important factors: these include reducing the environmental impact of operations and optimising the use of energy in this highly energy intensive process. To optimise these aspects, paper mills are becoming increasingly complex and the number of parameters that have to be constantly controlled and compared is growing exponentially to the point where operator experience is no longer sufficient to manage the process to the degree required. In addition to these manufacturing considerations, there is one other aspect that is gaining ever greater significance: employee safety. Regulations on employee safety require access to hazardous zones to be increasingly restricted. Put another way, hazardous zones, including those there is one aspect that is gaining ever greater significance: employee safety. close to machinery in movement, are being closed off from operators in order to avoid risk. By definition, these regulations make it impossible to take manual measurements of parameters during operations a practice which used to be commonplace and entrusted to specialised operators. Regulations are pushing paper mills towards automated sensors, integrated with machinery which then require no manual intervention whilst the plant is in operation. In conclusion, the new control requirements, together with ever
stringent targets for quality, efficiency in production and energy use (not to mention environmental compatibility, combined with strict safety regulations) are very clearly moving in one direction: the development of innovative technologies, specifically, integrated sensors that, when installed on a machine, can measure automatically and in real-time, correlate these measurements with the main process parameters and make data available on-line. Naturally, in the paper manufacturing process, where controlling moisture is crucial, the technological development of specific on-line moisture sensors takes on a fundamental role and it is precisely for this scenario that the integrated AMS system was created. INTEGRATED SYSTEM OF SENSORS FOR THE ENTIRE PAPER MANUFACTURING PROCESS For over 20 years, Advanced Microwave Engineering (A.M.E.) has designed and built sensors to measure moisture and permeability on-line specifically for use in the pulp and paper industry. conditions (mechanical configuration, temperature, etc. of the various parts of the machine) in the three main zones are very different. These considerations led A.M.E. to develop a family of sensors, each one unique and designed for a specific task, each one optimised for use in the various measurement positions required. The differences between the sensors in the AMS family are in the technology used (radiative, resonant), the frequencies used (millimetre waves, low microwaves), the way in which the sensor interacts with the material (contact, contactless), the mechanical characteristics and the materials used to make the sensor in order to withstand the various environmental conditions. The range of sensors that A.M.E. provides (the AMS range) covers every phase of paper manufacturing (see Figure 1) starting from the forming board (AMS_P1), going to the press section (AMS_P2) up to the drying section (AMS_R1 and AMS_R2). The sensors continuously measure and correlate the quantity of water in the forming board zone, the press zone and directly on the paper sheet, both in the drying section as well as in other zones deemed particularly crucial to the process, such as before the size press, allowing these parameters to be controlled well before the finished product is output. All this, however, is just one piece of information that the AMS system can provide the production control system. The complexity of the required control systems does not stop at the need to know the moisture content at certain points in the process. Control systems have to be able to aggregate and transfer information, in an integrated manner, to the machine s control system so that the information can be used in strict correlation with other information from other locations and, above all, so that the information can be used in conjunction with the multitude of other parameters that are measured such as temperature, Each of the main phases in the paper manufacturing process, which can be associated with the three main parts of the machine the forming section, the press section and the drying section has significantly different measuring contexts. In each of the three phases, the moisture content in the formed paper sheet varies continuously, dropping from 98% as a near-liquid mixture at the start of the process to approximately 5% in the finished product as an almost completely dry sheet. Even the supports carrying the material and the environmental measuring Each of the main phases in the paper manufacturing process has significantly different measuring contexts Figure 2.R1 Sensor.
pressure and so on. Aggregating and correlating information allows a better understanding of how the various parts of the machine interact and how this interaction is reflected in the characteristics of the finished product and, therefore, how checks can be managed to optimise production, quality, energy consumption, the environmental impact, and so forth. The intelligent, integrated use of this information allows the person managing the machine to perform timely, automated checks to improve production efficiency and energy usage, runnability and the uniformity of the finished product quality with regards to the requirements set by clients. To achieve this, the AMS system was designed so that every sensor in the system uses the same digital protocol and is orchestrated by one device: the AMS_HUB. This single device handles the true integration of the entire system: it receives and aggregates the data coming from the sensors, guaranteeing temporal correlation and integrity before sending the data to the process control system (the DCS/QCS) in the most appropriate format. Correlating and integrating the data obtained in this way allows every phase of the various parts of the machine s operation to be viewed in real time, allowing the best possible management and control of: the quality of the product; energy usage and emissions; the use of chemical products; unscheduled machine downtime; scheduled machine downtime for ordinary maintenance. One extremely important element concerning the use of these AMS sensors (apart from being able to use correlated measurements, delivered continuously and in real time during the machine s normal operation), is simultaneously avoiding having to make these measurements manually, thereby removing the need to expose operators to a potentially serious risk. Collecting data manually, as well as being hazardous, is also relatively significant in terms of the process. It is a spot activity, frequently done on more than a weekly basis, and correlating manual measurements is impossible, both in terms of timing and in terms of metrological reference (calibration). Moreover, given how hazardous it can be to take these measurements manually, this task is being prohibited in more countries around the world. Thus, the AMS system also radically improves employee safety by removing the need for such hazardous activity whilst the plant is in operation. R1 CONTACTLESS MOISTURE METER FOR PAPER SHEETS OR PULP AMS_R1 represents real innovation in microwave sensors in terms of measurement technology and, above all, because it does not need to be in contact with the material to be measured. Being able to use the sensor in the most disparate types of production from tissue paper to pulp two of the most stringent requirements were: this information allows timely, automated checks to improve production efficiency and energy usage, runnability and the uniformity of the finished product firstly, to have a wide active range of measurement (0-1500 gr/m 2 ); and secondly, to have extremely high resolution and accuracy (< 0.1 g/m 2 ). Today, AMS_R1 is an incredibly advanced moisture sensor. It uses innovative contactless technology, derived from sophisticated microwave radar technologies, to take continuous and extremely accurate measurements on sheet like materials such as paper sheets or pulp or felts. The measurements are provided in real time during plant operation, without ever coming into contact with the sheet itself (or with any machinery in movement) and the sensor is able to meet, with extreme reliability, the requirements of the measurements requested. The ability to take measurements without contact is absolutely critical to this application given the fragility of the sheet. Simultaneously, it allows the reliability index of the equipment itself to be considerably improved during operation, as it is not subject to mechanical stress; furthermore, neither the machine, nor the finished products, are in any way interfered with. HOW IT WORKS The AMS_R1 sensor has both an active and a purely passive component. At the radio level, the active component generates a microwave signal that is transmitted via passive reflector towards the interposed material (the paper sheet) that runs between the two components. This signal is reflected back to and received by the same active component. The moisture measurement is obtained by accurately measuring the energy dissipated on this path which passes through
the material (the paper sheet). The sensor s active component converts the analogue measurement into high resolution digital data (corresponding to a resolution better than 0.1 g/m 2 ) and is connected to a suitable digital data acquisition unit (the AMS_HUB). The measurement provided by the sensor is converted into a moisture value by the AMS_HUB where a special inversion curve has been implemented. This inversion curve was developed by the laboratories at A.M.E. to obtain extremely accurate values of the water contained in a material (g/ m 2 ). Through the AMS_HUB, it is also possible to store and process the measured data, in real time, making it available on-line through remote connections as well as being able to transfer the data collected - transparently and in real time - directly to the DCS/QCS via standard analogue data lines (4/20 ma). The paper or pulp material being measured will then run between the sensor and the corresponding reflector (they are set 10-30 cm apart) without any direct contact. Measuring residual moisture allows the dryness percentage to be calculated at the points relevant for process control The data generated is managed by a dedicated CPU (integrated in the AMS_ HUB) which runs software that allows both the acquisition and the storage of the data, its display and its processing (FFT analysis), even independently of the DCS. The AMS_HUB also converts digital data to an analogue 4-20 ma format. The sensor provides its data continuously with a sampling frequency of 50 Hz (one reading approximately every 24 ms). The amount of data acquired is therefore relevant when considering the continuous operation of the sensor. It can highlight the effective dynamics of the process, given that the sampling frequency is sufficiently high when compared to the frequencies involved in this type of application to the point of being able to identify typical periodic phenomena such as vibrations, pulsations and oscillations connected to the mechanics of the entire system through FFT analysis. Since paper manufacturing is a periodic mechanical structure (cylinders, felts, sheets but also pump engines, etc...), periodicity analysis, that is found traced onto the finished product, can be extremely useful when fully understanding the issues tied to the structural elements and how to best control them. AMS_R1 is, in short, the solution that transforms tactile and visual sensations, acquired over decades of experience, into something objective. This sensor provides the most complete, reliable, quantitative, very accurate, high resolution vision of the process, making the information available to an advanced, computerised process control system. APPLICATIONS The AMS_R1 sensor was developed to continuously measure residual moisture on the paper sheet during the manufacturing process in such a way that measurements are reliable, highly accurate and do not require contact with the said sheet. Measuring residual moisture allows the dryness percentage to be calculated (one of the fundamental parameters used to control production) at the points relevant for process control, such as leaving the press section, in various areas inside the drying section, Figure 3. Moisture measured at the forming board vs. moisture measured leaving the press vs. basis weight produced.
Figure 4. Consistency measured at the forming board vs. consistency measured leaving the press vs. basis weight produced. Figure 5. CMD scan. entering the size press or even whilst leaving the machine altogether. With this information, it is possible to control and to optimise the operation of the press section; maintenance condition of the wet felts; operation and optimisation of the steam plant, the consequent maintenance of repeatable production standards and the optimal use of the steam itself; elimination of operator risk during the phases when samples are taken for measurement off-line; elimination of the risk of breaking the sheet when taking a sample; immediate correlation to operational status and adjustments; speed of intervention in the event of problems with the subsequent reduction in both waste and machine downtime. The AMS_R1 sensor can be used both in a fixed configuration to make MD measurements (i.e. in the direction in which the sheet is moving), or it can be mounted on a bridge for CMD scans to measure the effective moisture profile of the paper sheet. In the first case, the result allows changes of moisture to be highlighted over time at a fixed point on the forming board. Figure 3 shows an example of the change in basis weight produced compared to the data measured by the AMS_P1 on the forming board. This graph shows the effect of the press section on moisture changes in the two points (the red curve is before the press whereas the blue curve is after the press). Knowing the basis weight, it is possible to calculate the consistency percentage as a variable of time (see In the case of a transversal measurement, however, it is possible to highlight the moisture profile over the entire format of the finished product
Figure 4 with reference to the results in Figure 3). In this example, note how the consistency tends to remain substantially constant. In the case of a transversal measurement, however, it is possible to highlight the moisture profile over the entire format that is reflected in the uniformity of the finished product (Figure 5). In this example, note how the transversal moisture profile is anything but uniform. Here, downstream from the measurements, adjustments can be made to the machine in order to equalise the profile to obtain a more repeatable and more uniform product over the entire format of the sheet. SAVINGS AND QUALITY The AMS_R1 system has been installed in numerous plants and initial feedback emphasises the extreme ease of installation and the immediacy of the integration of the data measured by the sensor and sent to the AMS_ HUB and then onto the DCS/QCS, given the availability of a standard 4-20 ma analogue line. Equally simple is installation on a scanning beam where the passive component can be on a single frame and so just the active component is movable along a generic scanning beam. Correlating data to a position is possible with the beam s PLC or by implementing a special function on the AMS_HUB. The information acquired by the sensor allows many of the machine s main adjustments to be optimised in real time. the Wire/Suction Cylinder motors for which it is possible to optimise their percentage usage whilst maintaining the same dryness value in the press. The vacuum on the suction boxes can also be considerably optimised. The ability, therefore, to continuously and accurately control dryness at the press exit has allowed the energy consumption of some components that are fundamental to the process to be calibrated thereby improving the quality parameters and introducing benefits to every level in the manufacturing process. CONCLUSIONS The AMS integrated moisture measurement system is made up of two parts: firstly, the different types of sensor (with each one optimised to operate in various critical zones of the process); secondly, the data acquisition and management device (the AMS_HUB). The system provides the process control system directly, immediately, continuously and in real time with the moisture levels in the most important sections of the manufacturing process. The technology employed is stateof-the-art for this type of application, having been developed to best meet, not only the metrological requirements, but also to operate reliably and with minimum interference, adapting itself in the best way possible to each type of production layout. Finally, the ability to provide correlated data offers further added value to the system which, thanks to this element, allows estimates to be made on parametric sensibility to adjustments that are impossible to obtain with uncorrelated measurements. The results of applying this technology are now tangible in many of the most important and technologically advanced paper mills in Europe where the system has been operating on a daily basis for years. CASE STUDY Holmen Group Iggesund paper mill - KM2 In terms of the shear number of both sensors installed and control points implemented, the installation at the Iggesund paper mill (part of the Holmen Group) in Sweden is perhaps the most relevant example. The system in question was installed as part of a project to optimise the KM2 machine at the Iggesund paper mill. The project was part of a framework that included other The information acquired by the sensor allows many of the machine s main adjustments to be optimised in real time. These optimised adjustments have been used to calculate an average payback period of approximately 6 months (across the various installations) focusing in particular on the energy usage of Figure 6. AMS_P1 measurement points.
Figure 7. AMS_P1 sensor innovation and process improvement projects that have resulted in this plant becoming one of the most advanced in the world, distinguishing itself not only for its superior efficiency but, most importantly, for the incredibly high quality of its products. The Iggesund project required the installation and integration of 5 AMS_P1 measurement points and a scanning beam carrying an AMS_R1 (see Figures 6 & 7). The scanning beam required some minor modifications to the machine in order to obtain sufficient space for the mechanical components (Figure 8). The results of the AMS_R1 sensor scans were obtained through the integration with the beam s positioning system (Figure 9) The installation of the 5 AMS_P1 sensors allows control over the values on the various forming wires and on the final coupling (Figure 10). Figure 9. Result Figure 8. AMS_P1 sensor Figure 10. Result