Customer SolutionsA Computer Controlled Multi-Analysis Titration System
Author(s):Alexander C.. Le Dain, ICON Technologies Pty Ltd
Industry:Basic Materials - Steel/ Lumber/ Construction
Product:Data Acquisition, High-Speed Digital I/O, LabVIEW
The Challenge:To develop a fully-automated titration system that would allow continuous, "round-the-clock" operation of two dual-channel, serially controlled Titrators, with intelligent scheduling of the input sample queue, and the capacity to vary the titration method on a per sample basis.
The Solution:Use National Instruments LabVIEW software to build the application. The multi-threaded LabVIEW environment is ideally suited to building a complex application of this type that requires parallel streams of instrument I/O, with simultaneous data analysis, data distribution, and sample queue management. A range of "off-the-shelf" National Instruments hardware provides the multi-port serial interface, the timer functions to accurately control reagent additions, and the digital I/O for monitoring flow switches and driving switch valve relays.Abstract The alumina refining industry uses caustic solution to extract alumina from bauxite ore. The caustic and alumina concentrations are critical parameters in this large scale, continuous extraction process, and must be monitored around-the-clock at multiple locations in the plant. Manually acquired samples are analysed via a labour-intensive titration process. Each titration takes approximately 15 minutes, with 2-5 minutes clean up between samples. A good analyst may achieve up to three titrations per hour for a single method on one set of equipment, but sustained throughput would typically average 18-24 titrations per analyst in a normal 8-hour working day. ALCOA had previously developed a PC controlled titration system that could queue and schedule samples for submission to a Gilson Autosampler and Mettler Model DL70 Titrator at ca. 5 samples/hour "round-the-clock" sustained throughput. Other requirements for the new system were: 1) that it implement multiple titration methods and allow for the future addition of new methods; 2) that it allow for the substitution of different makes and models of Titrator to accommodate differences in existing equipment between refineries; 3) that it run control samples as required by pre-determined statistical control methods, independent of user input; 4) that it self-diagnose and self-correct errors as far as possible; 5) that it preserve backwards compatibility with a scripting language developed internally by ALCOA for its previous system; and 6) that it allow for future expansion in capacity up to four dual-channel Titrators. The LabVIEW software application includes modules for receiving and scheduling batches of samples, controlling the Gilson Autosampler and Syringe Diluter, controlling the Mettler Titrators, analysing the returned data, archiving data to the local disk and the network LIMS system, handling errors, and running automatic and manual maintenance routines. Samples can be submitted in random order by the operator(s) at any time. The system queues samples for processing based on their priority and method of analysis (some methods require a dedicated beaker available on only one Titrator). The Sample Submission and Run Status screens give extensive visual feedback to the operator about the overall system state and the current queue status of any sample (see Figs. 2 & 3). Each submitted sample has an accompanying sample file that is modified as the sample moves through the preparation, titration, and analysis stages. To maximise flexibility samples are processed according to operator-selected scripts written using ALCOA’s ESL scripting language. The use of ESL scripts means that the amount and order of each reagent addition can be varied, so that existing analysis methods can be "tweaked", and new methods can be added, without rewriting any underlying code. ESL scripts also control all the routine maintenance operations for the system. Scripts are translated into the unique commands required for the Autosampler and Titrator by a low-level driver VI, so the system can easily incorporate alternative Autosamplers and Titrators, provided their gross functionality is similar. All drivers were developed around a standard interface to allow for future extensions to the ESL scripting language. The Mettler Titrator drivers in particular were not trivial to develop. The Mettler DL77 series are very intelligent and can manage a large percentage of the titration process under local control. During this process they alternate between wanting to behave like a server (waiting for the PC to initiate events) and wanting to behave like a client (requesting services from the PC). At all times they must "see" a continuously active communications link from the PC, or they can put themselves irreversibly off-line. In these circumstances reliable messaging and error handling in the presence of major background processes were critical to the success of the system. Under LabVIEW control the new system communicates reliably to multiple Titrators at rates up to 9600 baud, compared with 1200 baud communications to a single Titrator in the previous system. Equally critical to the timely implementation of the application were some of the more advanced LabVIEW features such as accessible ActiveX support, high level implementation of Queues and Semaphores, and simple re-entrant execution of sub-VIs. Each of the four beaker strands is handled by a re-entrant call to a multiple instance of the same sub-VI. The in-built queue functions were used to handle sample submission, system messaging, and error handling. The in-built semaphore functions were used to manage queued use of the single Autosampler and Diluter, key communications between the PC and the Titrators, and access to relay switching of the shared pumps. ActiveX support became critical during the development of the LabVIEW interface to the Mettler DL77. The supplier released, for the first time, an upgraded version of the system level driver as an ActiveX control. The new control was fully integrated into the LabVIEW application, which was at an advanced stage of development at this point, within one working day, and performed flawlessly from day one. At this point we have not had to invoke anything more sophisticated than the default two-thread execution system to achieve the desired performance. It is comforting to know that more sophisticated multithreaded execution options are readily available if required.
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