*As Featured on NI.com
Original Authors: Chris Hudson, ProMetric Systems, Inc.
Edited by Cyth Systems
Developing an accurate, cost-effective, multicell automated test system that continuously controls and analyzes industrial HVAC units in real-time by acquiring data from more than 1,500 sensors, simulating field, and engineering conditions, and logging all data during tests that last for months.
Using NI hardware and software to create a five-celled automated test system that incorporates two infrastructure support systems with LabVIEW shared variables distributed across more than 1,500 physical I/O points to control, monitor, and report all aspects of HVAC unit testing accurately and automatically.
ProMetric Systems is a national systems integration firm that provides engineering, software development, program management, and production services. One of our clients, a leading manufacturer of industrial HVAC systems, wanted to increase its market share while lowering its test and development costs for its rooftop line of air- and water-cooled chiller products. The company needed to automate its test facility and approached us with a system concept that incorporated five test cells and two infrastructure support systems. These systems required significant levels of automation, high-accuracy control, and high-channel-count data acquisition capabilities.
System Requirements and Challenges
Our client needed a system that could acquire data from more than 800 resistance temperature detectors (RTD) and 500 thermocouple sensors as well as various pressure, flow, and voltage sensors. The system also had to monitor I/O, log data, and provide robust, reliable functionality on a near-perpetual basis.
Controlling the common support systems that service each of the five test cells was technically challenging. These systems were responsible for providing the necessary heat load and cooling mechanisms to the unit under test (UUT). Control changes or disturbances in one cell could significantly impact an adjacent cell if our solution failed to address these interdependencies. Other technical challenges involved distributed data generation, alarming at the local and system levels (including safety shutdowns), data storage, and requirements to maintain a common-core software architecture across multiple real-time cells without creating any Microsoft Windows dependencies.
Left: Omega Cable Clamp Miniature Thermocouples, Right: NI PXIe-1095
Choosing NI Tools
To meet our client’s needs in a cost-effective manner while addressing the demanding performance requirements, we developed and deployed a custom automated test system that simulates field and engineering conditions to accurately study the performance of the UUTs. Because our client approached us with the requirement to automate its facility, we specifically chose to base the system’s hardware on NI CompactRIO and PXI real-time hardware. These real-time platforms are ideal for making complex automated test system designs more efficient. We chose to control the system using LabVIEW graphical system design software because of its inherent capabilities for high channel counts and parallel operations. We also designed the system within the LabVIEW environment because of the software’s graphical dataflow programming, which is especially well-suited to designing large, complex systems.
Left: HVAC system’s programmable logic controller. Right: LabVIEW user interface visualizing all I/O and workflow functions.
The HVAC test system we developed consists of 12 subsystems that interface NI signal conditioning hardware through an NI DAQ module. Seven of these systems direct the control mechanisms and the others direct data acquisition and test sequencing.
We implemented a unique solution to the problems created by the need for two infrastructure support systems and five parallel test cells. Instead of implementing custom TCP/IP communication structures, we deployed LabVIEW network-published shared variable libraries at each CompactRIO and PXI real-time controller and interfaced them to the terminal-client PC through bound controls. This provides reliable, real-time communication for numerous I/O points while maintaining Microsoft Windows independence. Each real-time controller runs one or more Timed Loops, including subprocesses for acquisition, proportional integral derivative (PID) control, and shared variable updates.
The test sequencers communicate from the test controller to the control subsystem executing on the support real-time controller by providing updated setpoints through a shared variable. Because of the flexible capabilities of LabVIEW, we were able to construct an automated test solution that incorporates more than 1,700 shared variables distributed throughout the system – across more than 1,500 physical I/O points.
Using LabVIEW, our engineers also developed 40 PID loops, which control temperatures, flows, pressures, and humidity. We implemented thermodynamic calculations such as isentropic efficiency and superheat using a dynamic link library (*.dll) from NIST that integrates equations of state (EOSs) for a variety of refrigerants. The support real-time controllers communicate to system chillers and multiple variable frequency drives (VFDs) through Modbus I/O servers. In addition, each test controller deploys multiple external power analyzers through a Virtual Instrument Software Architecture (VISA) TCP/IP (VXI-11) connection.
Energy, Cost, and Time Savings
Our configuration saves energy because it provides heat integration between the condenser and evaporator loops. This minimizes the need for an external heat load on the UUT (except for transitory step changes) by controlling the flow of fluids through the loop interface heat exchanger with PID modules written in LabVIEW. Because it saves energy, it will also save our client significant costs because the company runs long-duration tests on a continual basis.
The system also saves significant time in the test setup process because it incorporates not only fully automated functionality but also an easy-to-use graphical programming interface. This eliminates the need for operators with programming experience and contributes to increased supportability and reliability. The system includes seven operator workstations (OWSs) that can remotely communicate to any of the 12 real-time controllers. These workstations provide a window into the test, control, and data systems while helping the operator easily start, stop, or configure tests. Once an operator starts a test, the OWS is no longer required unless the operator wants to stop or monitor a particular test or system. The system’s automated data-logging functionality uses the transition minimized differential signaling (TMDS) file format, coupled with an FTP transfer mechanism, to store test information on a multiterabyte server.
Using NI products, we successfully designed and developed a highly accurate, reliable system that helped our client efficiently study the behavior of its HVAC units while saving energy, time, and capital.
Additionally, using NI products helped save our company significant development time in designing and deploying the system because the network-published shared variable eliminated the need for custom TCP/IP data transport logic. Because of the tight hardware-software integration of LabVIEW, CompactRIO, and other NI tools, we developed this test system within just eight months, a process that would have taken well over a year using traditional solutions.
Chris Hudson, ProMetric Systems, Inc.
Edited by Cyth Systems