ADVANCED MICROWAVE SENSOR BASED CONTROL CONCEPT FOR CONCRETE PRODUCTION

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1 ADVANCED MICROWAVE SENSOR BASED CONTROL CONCEPT FOR CONCRETE PRODUCTION Jan Kickstein, Johann Held, Uwe Kirchhoff Institute for Applied Systems Technology Bremen GmbH (ATB) The paper describes new concepts for an improved control system for the mamifacturing of concrete products. The control system contains new approaches for the more accurate and reliable water and aggregate dosage, mainly based on a new moisture measurement concept and a Multiple Frequency-Microwave-Sensor-System. A multiple frequency moisture measurement approach and an auto-calibration concept are presented. 1. INTRODUCTION The perfonnance and consistency of quality levels achievable by applying market available sensor and automation technologies for the dosage and mixing process in the concrete industry is dissatisfactory in reference to the increasing quality demands being placed on concrete products by the market. The main problem is the water dosage control. To attain a pre-programmed moisture content for a selected recipe, it is necessary to measure the moisture of the mixture before adding water because the existing water content in the mixture must be subtracted from the theoretically planned quantity of water to achieve the exact recipe. When the aggregates are dosed gravimetrically the water content of the aggregates results in an incorrect material volume that has to be compensated for. Solving these problems, and achieving the required increase in productivity in parallel, is one of the basic requirements w.r.t. the business strategies of companies in this sector, which to a great extent are SME type companies. Even though a wide spectrum of sensor technology is available on the market and has already been implemented in this industrial sector, the moisture measurement's accuracy and reliability still present the key problem, especially in the batch production type of concrete products (tubes, plates, etc.). From an industrial application point of view the required accuracy of the moisture measurement should be in the range of 0.2% with a high reliability under varying industrial production conditions. In addition the minimisation of all calibration efforts is required. All these improvements will provide considerable benefits for the SME's at concrete production sites: material savings due to better process control, cost savings for energy and tools because of reduced mixing duration and reduced wear of the

2 432 Advances in Networked Enterprises mixer blades and bottom, savings in operational costs due to lessened maintenance efforts, remote maintenance and full automatic process control. An increase in productivity based on the higher efficiency of the dosage/mixing process as well as an increase in the quality of concrete products are expected. These improvements will also lead to a reduction in rejected materials, energy savings and by means of this to environmental protection. The RID activities presented have been developed in the scope of the research project MICROSEC "Advanced Micro-wave Sensor Technology for Robust, Remote Maintainable Control system for Concrete Products Manufacturing" funded in the framework of the Brite-EuRam Craft Program. 2. STATE OF THE ART OF SENSOR BASED CONTROL CONCEPTS FOR CONCRETE PRODUCTION 2.1 Sensor Technology Aspects The most successful water dosage systems for concrete production today are based on microwave-sensors [ 1]. The reason for the success of the microwave sensors is justified by the fact that many problems of the other on-line systems that have been developed in the past, using resistive or capacitive techniques, have been solved by the microwave approach. The resistive technique is prone to error because water does not act as a consistent conductor whereas salt water is a very efficient conductor. Dissolved salts (present to a variable extent in all aggregates) therefore have a great influence on the measurement. Capacitive techniques are an improvement, but at the relatively low operating frequencies used are still prone to errors due to dissolved salts. Using the microwave frequency range it becomes possible to separate the water content from the disturbing salt content of the aggregates so that the influence of the salt content on the moisture measurement can now be neglected. Although the use of microwave sensors is a large improvement, there are still problems to be solved. The key problem of the state of the art microwave sensors is the disturbing influence of density (mainly caused by varying fine grain percentages in the aggregates in the scope of aggregate measurements and by varying recipes in the scope of mixer measurements) and temperature on the microwave sensor signal [2] [3]. Both the naturally varying fine grain percentage in sand and gravel and varying recipes can cause large moisture errors up to 1% in the moisture determination. 2.2 Moisture Measurement Location Aspects Actually there exist two methods (locations) to measure the moisture of the fresh concrete using microwave sensors. The first one is based on moisture measurement during the dry-mixing-period with a microwave sensor installed in the mixer to determine the water content of the mixed aggregates. The second is based on moisture measurement of each aggregate during their dosage with microwave sensors installed under the silos or at the aggregate conveyer. These methods in state

3 Advanced Microwave Sensor Based Control Concept 433 of the art control concepts are used exclusively. Both methods have advantages and disadvantages. One disadvantage of the mixer measurement systems is that the microwave sensor signal does not only depend on the moisture but also on the recipe and the temperature of the mixture. The main reason for this recipe dependency is also the general problem of the density dependence of microwave moisture measurement because the density of the mixture depends on the material composition (recipe). A higher portion of sand instead of gravel results in higher density, a higher amount of material causes a higher density etc. This problem in most system solutions is solved by a very work intensive recipe and temperature dependent calibration process. The moisture calculation of new not yet calibrated recipes is then inadequate. For companies that produce a lot of different recipes, this problem makes such solutions inadequate for the daily production. The second disadvantage of the mixer measurement is that when the aggregates are dosed gravimetrically, the water content of the aggregates results in an incorrect material volume that has to be compensated for. The mixer measurement does not permit an additional aggregate dosage which is however necessary for the exact recipe. The advantage of mixer measurements is that once the calibration is made dependant on the recipe, the mixer measurement shows good accuracy under stable production conditions. The main disadvantage of aggregate measurement systems is also the disturbing influence of density and temperature on the microwave sensor signal mentioned in 2.1. The varying density of the aggregates is here mainly caused by the naturally varying fine grain percentages in the aggregates. The presently available system solutions based on aggregate measurement include mostly the temperature but not the fine grain percentage in. their calibration approaches. A further disadvantage of an exclusively aggregate measurement is that in exceptional cases (sensor defects, manual dosage interruption etc.) a restart of the moisture measurement of the current charge is not possible if a sensor is not installed in the mixer. The advantages of an aggregate measurement system are that an additional dosage is possible, the calibration effort is smaller (no problem of recipe dependency) and when done without mixer measurement, the wear of the mixer can be reduced which not only saves energy but also increases the throughput. 3. NEW ADVANCED AUTOMATION-SYSTEM SOLUTION The objective of the MICROSEC project is to enhance the micro-wave (MW) sensor technology and to apply it for the development of an innovative, highly robust and easy to calibrate moisture measurement system for the dosage/mixing process, which is one of the quality critical domains in the manufacture of concrete products. MW based sensor technology for moisture measurement is the unique approach to be clearly favoured for robust, dynamic on-line measurements. However, the accuracy and robustness of the moisture measurements, as well as the reliability and easy calibration of such measurement systems are the most critical issues. To overcome these problems, improvements in MW technology and moisture measurement concepts are needed. Therefore, the specific objectives are: Development of an advanced new Multiple-Frequency Micro-Wave (MFMW) sensor system (sensor components and processing of information) overcoming

4 434 Advances in Networked Enterprises the density and temperature dependency problem of common single frequency MW sensors. The goal to be achieved is an accuracy of moisture measurements of at least ± 0.2%, while robustness (under industrial conditions) should be of at least 99%. Elaboration of enhanced sensor data fusion concepts based on the combination of the advantages of aggregate and mixer location measurements and on the new multiple-frequency microwave sensor system for a reliable and fault tolerant moisture measurement strategy. Development of an auto-calibration of the mixer sensor, based on the redundant aggregate measurement. 3.1 Advanced Architecture for a fully automated water dosage control The new moisture measurement concept is an integral part of the entire production control system for fresh concrete as presented in Figure 1. In the normal operation case the water-dosage calculation is based on aggregate-moisture-measurement. Only in exceptional cases (dosage interruption, sensor defects etc.) is the waterdosage calculation based on a redundant mixer-moisture-measurement. CAN-Bus Concrete Production Process I MWnenso< r- PLC PLC3964RI'Iolocol t Semi inl«face RS232 Moisture Measurement System (PC, WIN-NT 4) ' , :-- Figure 1 - Architecture of the control system Applying this solution results in the following improvements. An additional aggregate dosage by primarily aggregate measurement is possible, higher productivity and less material wear can be achieved by the possibility of reduced mixing times and the system guarantees high reliability due to the sensor redundancy of both the aggregate-and mixer measurement. This concept causes a higher complexity and effort w.r.t. sensor installation, signal data processing (sensor data fusion), wiring, PLC interactions etc. To reduce the resulting higher investment costs and nevertheless guarantee good usability, robustness, maintainability and extendibility, the system solution is based on advanced implementation technologies. So a standard PC with MS-Windows NT4 is chosen for the platform of the control system (MS-Windows NT4 can be used as a "real-time" operation system if the requirements to the response times are not so demanding [4]). The

5 Advanced Microwave Sensor Based Control Concept 435 control system is programmed in C++ to efficiently use the features of a modem object oriented software development. All configuration and production data files (calibration points, charge results etc.) are written in XML-format [5] to apply what is now becoming world standard for the structured storage of data and for the easy development of additional tools for configuring and reporting. To allow for the use of only standardised electrical and logical interfaces, to have an easy wiring and a digital signal data transmission an RS232 interface with protocol 3964R is used for communication with the PLC and the CAN-Bus [6] is used for the sensors. The detailed control and information flow is as follows (see Figure 1 - Architecture of the control system). The sensor signals are pre-processed and digitally transmitted to the control system by CAN-Bus. The PLC (using the 3964R protocol) transmits all production process relevant parameters (charge start with recipe, aggregate dosage start/stop, exact amount of dosed materials, dry/wet-mixing period start/stop etc.) to the moisture measurement system. In normal cases the water dosage value is calculated based on the aggregate measurement at the end of the dosage period and then transmitted to the PLC. In exceptional cases (silo-sensordefect, manual dosage interruption etc.) it is not possible to calculate the exact water dosage value based on the silo-sensors. In this case the water dosage calculation is based on the redundant mixer moisture measurement during the dry mixing period and the value is transmitted to the PLC after the dry mixing period. 3.2 Multiple Frequency Approach for an Improved Aggregate Moisture Measurement The sensor signal of the microwave sensors used for measuring moisture is not only influenced by the moisture F but also by the fine grain percentage K (grain <0.25mm) and the temperature T. Using two microwave sensors with different frequencies and a temperature sensor makes it possible to separate the different influences of moisture, temperature and fme grain percentage. It is presumed that the sensor signals of the two sensors 81 and 82 are linearly dependant on moisture, the fine grain percentage and the temperature (see Equation 1 ). All other influences are neglected. Equation 1: The regression coefficients an1 can be calculated from several calibration measurements with the resulting calibration points (F, K, SJ, 82, 1). When the regression coefficients for the two sensors are known, not only F but also K can be calculated from the on-line measured 81, 82, and T. The fine grain percentage K is a significant parameter for concrete production because it influences the consistency of the fresh concrete. 3.3 Multidimensional Regression for the Moisture Calculation of New Recipes The sensor signal S of the sensor installed in the mixer is not only influenced by the moisture and the temperature of the mixed material, but also by the recipe. So the moisture measurement in the mixer is only possible with a recipe dependent

6 436 Advances in Networked Enterprises calibration. Using a multidimensional regression approach the calculation of calibration curves for new recipes becomes possible by an interpolation between existing recipes. The improvement is that in the presently available mixer measurement systems it is not possible to measure the moisture of new, not yet calibrated recipes. This multidimensional regression is described in the following. The ingredients of a mixture are mathematically characterised by the amount of all the materials that are part of the recipe. Each material is given a fixed index i. If a material with the index i is not part of a recipe the corresponding parameter m; = 0. In the following "recipe" means,m 1 mn" Equation 2: To be able to conclude from the known terms S, T and recipe to moisture, the Equation 4-2 can be solved with respect to F and assuming a linear dependence of moisture for all parameters this leads to Equation 3: Using linear algebra methods, the regression coefficients a; can easily be calculated from the recorded calibration points (F,S,T,m 1. mn) and can also be used for the moisture calculation of new recipes. 3.4 Auto-Calibration Approach for the Microwave-Sensor installed in the Mixer In the new moisture measurement concept the added water is generally regulated by moisture measurement based on the aggregate sensors. The calculation of the water dosage based on moisture measurement in the mixer is only used if an exception during the dosage period occurs that prevents an exact aggregate-moisturemeasurement. After each mixture completed without exceptions, the information is used for the dynamic auto-calibration of the mixer sensor by recording new calibration points (F,S,T,m 1. mn). This results in two reference points for each mixture and its recipe. One reference point results from the sensor mean of the wetmix-period and the moisture which is based on the aggregate measurement and the water already added, the other one from the sensor mean of the dry-mix-period and the aggregate moisture. In this manner the calibration of the mixer is done automatically by the system. 4. FIELD TEST RESULTS OF THE NEW MULTIPLE FREQUENCY MICROWAVE SENSOR SYSTEM The following field tests have been performed to compare the common single frequency approach with the new multiple frequency approach in relation to the required accuracy of 0,2% in the moisture determination. Two sensors with different

7 Advanced Microwave Sensor Based Control Concept 437 frequencies have been used for the measurements and later on the calculation of moisture and fme grain percentage. 4.1 Results of the Single-Frequency Approach The results of the common single frequency approach are as follows. The difference of the calculated moistures (with respect to the calibration measurements) and the test measurement points (real moisture in the following tables) 4f as well as the standard deviation was calculated. Table 1 - Test measurement results Sensor Moisture/Fine grain Moisture/Fine grain Moisture/Fine grain: (Real4,88/90) (%): (Real6,85/80) (%): (Real 5,38/95) (%): Sensor 1 4,84/not measurable 6,57 /not measurable 4, 19/not measurable Sensor2 5,06/not measurable 6,44/not measurable 6,16/not measurable The most characteristic statistical values of the test measurements are: Table 2- Characteristic statistical values of the test measurement results Sensor 14fl<%) (%) Sensor l 0,5 0,6 Sensor2 0,45 0,3 0,2%. The resulting j.1jj is for both sensors distinctly above the required precision of 4.2 Results of the Multiple Frequency Approach The multidimensional calculation of moisture is based on the same measurement points as in 4.1. In addition the fine grain percentage k can be calculated. Table 3 - Test measurement results Sensor Moisture/Fine grain Moisture/Fine grain Moisture/Fine grain: (Real4,88/90) (%): [(Real6,85/80) (%): (Real 5,38/95) (%): Sensor 1 + 4,86 I 87 6,69 I 82 5,16 I 99 Sensor2 And the following characteristic statistical values are: Table 4 - Characteristic statistical values Sensor j.1/j(%) (%) IMI(%) (%) Sensor 1 + Sensor 2 0,13 0,1 3 1

8 438 Advances in Networked Enterprises The multiple frequency approach delivers a significant improvement in the accuracy of the moisture measurement, while IL1fl is reduced to 0,13%. This is an improvement with a factor 3. The initial results indicate that the required accuracy for the moisture of 0,2% can only be achieved with a multiple frequency approach. In addition, it becomes possible to calculate the fme grain percentage with an accuracy of 3% in these measurements. This is a great improvement because the knowledge of fine grain percentage is very interesting for concrete producers since it has an influence on both the water requirement and the quality of the concrete. 5. CONCLUSION In the scope of the research project MICROSEC, a new automation system solution for water and aggregate dosage control for the manufacturing of concrete products was developed. In the new control concept the water dosage control is primarily based on aggregate-moisture-measurement to make possible an additional aggregate measurement. The redundant mixer-moisture measurement that takes place only in exceptional cases (aggregate sensor defects, manual dosage interruptions etc.) makes the system robust. Also, the sensor redundancy makes possible functionality for self diagnosis. Both combined measurement types were improved overcoming the main problem of microwave moisture measurement which is density and temperature dependency. A Multiple-Frequency-Microwave-Sensor-Approach solves the common aggregate measurement problem of wrong moisture determination caused by varying material parameters such as temperature and fme grain percentage. An autocalibration of the mixer sensors based on the aggregates measurement solves the mixer measurement problem of the work intensive recipe dependant calibration. A multidimensional calibration approach for the automatic online calculation of calibration curves for new recipes based on the interpolation between existing recipes solves the problem of a moisture measurement for new recipes. The results achieved are promising to guarantee a significant improvement in the accuracy and robustness of the automated water and aggregate dosage control in concrete production. 6. REFERENCES [1] M.A. Berliner: Feuchtemessung, VEB-Verlag Technik, Berlin (1980) [2] K. Kupfer: Disturbances by the application of microwave moisture measurements, Technisches Messen 60 (1993) [3] A. Kraszewski: Microwave Aquametry, IEEE Press, New York (1996) [4] G. Bittner: Workshop der Automatisierungstechnik, Franzis Verlag, Poing (1999) [5] C. F. Goldfarb: XML Handbuch, Prentice Hall, (1999) [6] R. Buse: Feldbussysteme im Vergleich, Pflaum Verlag, Mllnchen (1996)