Author(s):
Robb Wallen -
University of Colorado at Boulder
The 400 g-ton geotechnical centrifuge at the University of Colorado at Boulder is one of the nation’s most powerful centrifuges capable of spinning two tons of material at 200 times the force of gravity. Using industrial NI PXI chassis and LabVIEW Real-Time controllers, researchers replaced the existing 200 I/O-point PLC controller – along with 2,000 rungs of antiquated PLC ladder logic – to create a more efficient, rugged, and easily upgradeable system that reduces system setup time and increases reliability. The centrifuge requires two independent systems for control and data acquisition. One system provides centrifuge operation control and monitoring provisions and the other provides flexible experiment control and data acquisition for the centrifuge payload (experiment) package.
The previous centrifuge control system was based on proprietary hardware and software. It was extremely complicated, with more than 300 discrete digital I/O lines, most of which were dedicated to operating a hard-wired control panel. The fixed control panel layout and the 2,000+ lines of vintage PLC code made it impossible to add functionality to the system. As the system aged, pieces of it began to fail. Repairs required time-consuming troubleshooting, and downtime was excessive. We decided that we needed a new modern control system based on PC technology. We also needed a system that we could maintain and easily update.
PAC Flexibility and System Reuse
With NI PACs, we replaced an aging, unreliable PLC control system with the next generation of PLC – a PAC system. With the flexibility of LabVIEW, we replaced both the centrifuge control system and the in-flight experiment control and data acquisition system with the same PXI hardware platform using a single development environment to program the entire application.
With a limited budget, we had to implement the new system in four months while the centrifuge was down for a major cooling system update. Using PACs, we delivered a more reliable and capable control and data acquisition system within this short time period and under budget.
Centrifuge Control System
The new control system uses an NI PXI-8176 real-time controller running LabVIEW Real-Time software. We bolted this PXI system into the cabinet that held the original PLC system, consuming about 1/20 of the cabinet volume. We used an NI PXI-6526 and NI PXI-6031E respectively, to interface with the digital and analog I/O through the existing wiring. The I/O compatibility between the old PLC and the new PXI system saved time and money because we reused the original wiring while capitalizing on the measurement density of the PXI platform. We replaced the static, hard-wired control panel with a standard Windows-based PC running a graphical user interface built with LabVIEW. The CAT5 standard Ethernet cable replaced hundreds of individual conductors connecting the PLC to the old control panel.
Additional NI PAC products complemented the PXI platform to fulfill control system requirements. In addition to controlling the 900 horsepower DC drive system, brake systems, and cooling systems, the centrifuge controller must interface with imbalance-sensing equipment mounted on the centrifuge arm. The original system handled this task by routing many analog signals through an electrical slip-ring assembly and up two building floors to the PLC. We replaced this noisy and failure-prone system with NI FieldPoint hardware that digitizes the analog signals before sending them to the PAC over a serial interface. This technique reduced the system wiring complexity while improving the centrifuge balancing system resolution.
We exploited the new processing power of the PXI embedded controller to perform centrifuge vibration measurements using the NI PXI-4472 dynamic signal analyzer, LabVIEW, the LabVIEW Sound and Vibration Toolkit, and the LabVIEW Order Analysis Toolkit. This hardware and software processes vibration data measured at key mechanical equipment points in real-time, extracting quantitative data points that we track historically. Comparing the historical vibration data against real-time data helps identify potential centrifuge motor, gearbox, and main bearing system problems.
Experiment Control and Data Acquisition System
The old centrifuge data acquisition system, a key component required for research projects, consisted of a loosely integrated collection of equipment. These digitizers, signal conditioners, servo systems, and control computers spread out over three centrifuge-building floors and linked together with miles of cabling, suffered from limited functionality and over-complexity. With this system, failures were common and resulted in wasted time and test specimens. Because centrifuge experiments require precise coordination between different hardware and software systems, tight integration between components is a critical concern.
Lessons learned from the old system, and limitations placed on routing signals to and from the spinning centrifuge, led us to choose a PXI-based PAC as our experiment control and data acquisition platform. The PXI system spins on the centrifuge mounted near the center of rotation, and is a solid industrial rugged platform for the centrifuge. The industrial form factor helps the PXI PAC withstand the mechanical stresses placed on it by the machine’s centripetal acceleration. The wide range of hardware available for the platform including machine vision systems, multifunction I/O, and motion controllers replaced the old system with a tightly integrated, robust package. Centralizing the equipment on the centrifuge arm dramatically simplifies the experiment setup.
The data acquisition system runs a Windows OS, and is programmed using LabVIEW. The PXI system includes a multifunction data acquisition device that is connected to an SCXI chassis with several SCXI modules for signal conditioning. The SCXI modules include an NI SCXI-1520, NI SCXI-1102, NI SCXI-1540, NI SCXI-1125, and NI SCXI-1124. In addition to strain, temperature, and accelerometer measurements, there is a PXI-4472 in the PXI system for high-speed accelerometer measurements, and an NI PXI-7344 motion controller for remote-test specimen manipulation equipment control.
The operator accesses data acquisition system via an off-the-shelf wireless network using a Windows integrated remote desktop client and LabVIEW datasocket connections. With network access, the operator can use the system from inside the centrifuge chamber, the control room, or from anywhere on the Internet. Analog transducers interface to the PAC using standard RJ-45-style patch panels and CAT 5 Ethernet cable spanning 18 ft between the centrifuge center and the payload platform. The RJ-45 patch panels are well-suited for handling the variety of analog sensors used in geotechnical research, and the CAT 5 cabling has preserved analog signal integrity. The RJ-45 panel and CAT 5 wiring installation provided an order-of-magnitude cost and labor savings over a traditionally wired system.
A Simple, Comprehensive Solution
Using NI products, we created a comprehensive solution for our control and data acquisition needs in a minimal amount of time, at a near-order-of-magnitude cost savings over a new PLC control system. The majority of these savings resulted from using LabVIEW to program both systems, and using in-house expertise to translate the deficiencies of the old systems into requirements for the new systems, eliminating the need for a third-party system developer. The high level of integration and modularity provided by the PXI platform increased the reliability and productivity of our centrifuge lab dramatically and resulted in much simpler and more powerful systems.
Author Information:
Robb Wallen
University of Colorado at Boulder
n/a
n/a, CO n/a
United States
Tel: n/a
Fax: n/a
wallenr@colorado.edu
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