Customer SolutionsNVSI Uses NI LabVIEW Real-Time and FieldPoint to Update 50 Ton Load Cell Testing System
Author(s):Christopher Relf, Neo Vista System Integrators Pty Ltd; Liam Granger, Neo Vista System Integrators Pty Ltd; Laurent Raynal, Neo Vista System Integrators Pty Ltd
Industry:Industrial Controls/ Devices/ Systems
Product:Compact FieldPoint, FieldPoint, LabVIEW, LabVIEW Real-Time, Programmable Automation Controllers (PACs)
The Challenge:Replacing an antiquated load cell testing system with a networked, database-driven, automated system with higher accuracy, expanded capability, and lower test times.
The Solution:Using a National Instruments LabVIEW Real-Time and NI FieldPoint programmable automation controller (PAC) to control the system and achieve highly accurate force application and measurement, while controlling the UUT steady-state temperature and humidity set points.
Improving Load Cell and Environmental Control Parameters The National Standards Commission (NSC) performs pattern approval type testing on measuring instruments used for trade purposes in The NSC requested a general improvement to the load cell and environmental control parameters, with database control and comprehensive data logging features, which proved impossible with off-the-shelf solutions that were currently available. The load cell consists of an inner stack containing ten 500 kg weights and an outer stack containing ten 150 kg weights. Test loads of 50 tons are possible as a result of the balance arm, which amplifies the load using a 10:1 ratio when test objects are placed inside the large test chamber. We developed the solution using two LabVIEW Real-Time executables residing in two separate FieldPoint controllers (one controls the temperature environment of the chambers and the other controls the motors used to apply the load). We used the LabVIEW Real-Time PID algorithms running in both FieldPoint systems. The NI LabVIEW Database Connectivity Toolkit provided us with the ability to communicate to the NSC SQL server, which stored test profiles and recorded test results. We used the following FieldPoint modules:
Two Allen Bradley motors control the loading and unloading of the weights on the inner and outer stacks, and we used the quadrature encoder module to determine the position of the stacks. Setpoints stored in the SQL database indicate the position required for the addition of each of the weights. A further motor lowers and raises the balance arm to ensure that it is horizontal at all times. A Schick laser displacement sensor provides the input parameter passed to an embedded LabVIEW PID application running on the FieldPoint controller, which controls the motor speed profile as the balance arm is adjusted. A test typically consists of exercises (the maximum load of the test is loaded during a short time, as defined by the OIML R60 recommendation), steps (the maximum load of the test is gradually applied in a series of steps, a profile of which is contained in the SQL database) and/or creeps (the maximum load of the test is loaded during 30 minutes). A standard test can last anywhere between a few hours and several days, containing numerous cycles, and operating without user intervention, which requires the ruggedness and reliability of the FieldPoint controllers running a real-time operating system. We used two NI Compact FieldPoint systems (one for load application and another for environmental monitoring and control) and a PC for full-system control and load monitoring. We linked these systems using the LAN. We also uploaded test sequences and part identifiers from a network SQL database. UUT Environmental Control The system records temperature, load, and pressure then stores it in the SQL database during the test, at intervals defined by the test profile. Reusing code that had been developed on the NSC environmental chamber monitoring system, we attain temperature setpoints to an accuracy of ±0.1°C. This code is modular and easily extendable to incorporate the monitoring and control of other physical phenomena, including humidity. We execute a test at different temperatures controlled by the environmental portion of the software. A typical test may require the load tests to be applied when stability has been achieved at +20°C, +40°C, -10°C, +10°C, and again at +20°C.
The system is easily configurable using a database front end and can be quickly altered using the LabVIEW Real-Time development system and FieldPoint hardware. The system can perform user-customizable tests on load cells up to 50 tons, with strict environmental control and monitoring of the load cell itself, satisfying regulatory recommendations and constraints. We developed the environmental control systems used in this solution using an existing NVSI user solution, currently used by the NSC. Using the existing modular code and documentation, we delivered the overall solution at a significant time and fiscal saving, as opposed to creating the solution from scratch. Similarly, we are currently porting the code produced for this system to a smaller 500 kg Load cell Test Rig at the NSC site in For further information, contact: Christopher Relf National Manager - Neo Vista System Integrators Pty Ltd Tel: +61 2 9809 7899 Fax: +61 2 9809 7499 E-mail: christopher@nvsi.com.au Web: www.nvsi.com.au
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