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01 November 2004

Finding interoperability

Through PXI, hardware, software team to create hybrid systems.

By Garth Black

When hybrid gas-electric vehicles first hit the street, the goal was to promote an environmentally friendly vehicle with improved gas mileage and reduced air pollution emissions—all without sacrificing overall performance.

Hybrid control, monitoring, and test systems have a similar goal of increasing total system performance, preserving legacy equipment still adding value to the system, and reducing total system cost—all without compromising the overall functionality of the test system.

Today's control engineers face the daunting task of balancing beleaguered budgets and building high-performance systems for process control, supervisory control, and industrial monitoring applications. These systems may require interoperation between multiple hardware architectures for unique functionality and lower cost solutions. In addition, engineers may need to upgrade outdated components or instruments on these systems while preserving other legacy devices still serving valuable roles in the overall system.

One architecture that can fill that role is PXI, a modular instrumentation platform designed specifically for measurement and automation applications. With PXI, you can select the modules you need to integrate into a PXI system from multiple vendors. Communication between the modules uses PC-based technologies such as the 132-megabyte-per-second peripheral component interconnect (PCI) bus, allowing high-performance communication that leverages widely available software. PXI also integrates timing and synchronization into the system, so you can pass signals between instruments for high performance and accuracy, without additional cabling.

PXI, whose system alliance has more than 60 members, is a hardware architecture that offers more than 1,000 instruments for control, monitoring, and test systems. The design of PXI hardware and software architectures calls for it to fully interoperate with alternative hardware platforms such as distributed I/O (including programmable automation controllers [PACs] and programmable logic controllers [PLCs]), PCs, and vendor-defined instruments (General Purpose Interface Bus [GPIB]). This is one way users can preserve an existing system investment with flexibility for the future.

Case in point

The need for hybrid test systems incorporating multiple hardware architectures is evident in control and monitoring applications today. Soliton Automation in India had to develop a supervisory control and monitoring system for a highly accelerated stress test system for X-ray tubes. The system required custom communication to an X-ray generator, a serial interface to the measurement device for fault and data monitoring, a temperature monitoring and supervisory control system, and a high-speed 100-megahertz data acquisition system for the X-ray tube stress measurements. In addition, the system needed a flexible software environment to interface with and control each hardware platform and to provide means for data analysis, management, and storage.

Soliton chose a PXI-based architecture for the supervisory control and monitoring system. A PXI controller area network (CAN) module was able to communicate with and send measurement parameters to the X-ray generator. With PXI serial modules, Soliton achieved measurement data monitoring and system fault monitoring. A distributed I/O system monitored temperature and supervisory control of safety interlocks. The company used PXI-based 100-megahertz digitizers for the high-speed waveform measurements from the stress testing on the X-ray tubes. A standard desktop PC running Microsoft Windows networked into the distributed I/O system via Ethernet and connected to the PXI system with a transparent PCI-PCI bus extension device called multisystem extension interface (MXI). The PC networked via Ethernet to a second PC for data storage and data broadcasting over the Web, which allowed viewers to monitor data in real time without interrupting the supervisory control, monitoring, and test events.

With the hardware architecture in place, a flexible software program had to interface and control each hardware device, as well as provide software tools for fault analysis, signal analysis, database management, and data broadcasting over the Internet. Soliton also required automated e-mail notification of system events, including system shutdown or failure. Because of its array of device drivers for a variety of hardware platforms, including CAN, serial, distributed I/O, and data acquisition devices, Soliton selected National Instruments' LabView as the software development and control environment. The software also provided tools for data analysis, structured query language (SQL) database management, Web broadcasting, and automated e-mail notification.

By implementing the system based on PXI, distributed I/O, and networked PCs, Soliton's cost analysis showed it saved more than $30,000 in hardware costs compared to a system with vendor-defined, stand-alone instrumentation. Because of PXI's smaller form factor (compared to traditional supervisory control and monitoring systems), Soliton preserved valuable space on the manufacturing and test floor. In addition, the company said it saved one year of development efforts compared to previous attempts to build similar systems with alternative, vendor-defined hardware and software architectures.

PXI platform

The PXI Systems Alliance (www.pxisa.org) governs the PXI hardware and software specifications (revision 2.1). PXI works in environments with extended temperature, shock, and vibration requirements. These requirements make PXI viable for rugged environments that are unsuitable for other types of instruments (including PCs and desktop instruments).

The PXI hardware architecture calls for interoperability with an array of hardware devices. The PXI system controller is the heart of the PXI system and is the primary means for interoperation with other hardware platforms. PXI-embedded controllers are available with processors up to 2.2 gigahertz, integrated hard drives, PS/2 connectors for keyboard and mouse, VGA video interfaces, universal serial bus (USB) 2.0, and parallel ports. Many PXI-embedded controllers also include integrated Ethernet (10BaseT, 100BaseTX), serial (RS-232), and GPIB interfaces. These integrated interfaces provide for hybrid interoperation with alternative hardware platforms such as distributed I/O (PACs, PLCs), serial, GPIB, and CAN. PXI systems can also connect to PCs, auxiliary PXI systems, or VME extensions for instrumentation (VXI) systems with transparent PCI-PCI bus extension devices, such as MXI. With bus extension devices like MXI, fragile PCs or desktop instruments located in a protected environment can still maintain communication between platforms such as PXI and distributed I/O, which are more suitable for harsh environments.

Some PXI devices include serial modules, CAN, DeviceNet, high-speed data acquisition, motion control (up to eight-axis), image acquisition, and high-speed (gigabit) Ethernet. Per the PXI specification, all peripheral PXI devices must include a manufacturer-produced software device driver. Because of this regulation, the end user does not have to deal with the burden of creating complex device drivers.

Software a key

The PXI software architecture is just as important to multidevice interoperability as the hardware architecture. The PXI software specification requires complete functionality with industry standard technologies such as the Windows operating systems, and it supports all commercial application development environments. Many of these application development environments support industry standard communication methods such as OPC for distributed I/O systems, Foundation fieldbus, and Virtual Instrument Software Architecture (VISA) for GPIB, serial, and VXI systems.

For example, a PXI-embedded controller running Windows XP and LabView can control distributed I/O systems via device drivers for hardware such as PXI CAN and serial modules, interface via OPC with devices such as Allen Bradley or Rockwell PLCs, control GPIB-based instruments with VISA, and stream data to an isolated PC with an Internet or database connectivity tool kit—all in one networked system.

The requirement for multiple hardware architectures with unique functionality is necessary in today's control, monitoring, and test systems. Interoperability between these architectures to form hybrid systems is paramount for the success of the system, allowing engineers to make the most of their legacy systems, reducing system cost, and adding flexibility and increased productivity to future systems.

Behind the byline

Garth Black is a PXI product manager for National Instruments. His current projects include product management of PXI chassis and PXI industrial applications. He has a B.S. in chemical engineering from Brigham Young University.