Putting PC power to work in industrial automation applications
The most significant technological shift in industrial automation over the last few years has been the emergence of the PC on the plant floor. The graphical and networking abilities of the PC have already established it as the most popular platform for HMI/SCADA operations. However, thanks to increasing performance, decreasing prices, faster data buses, and more stable operating system (Windows NT), PCs are also being used more and more for direct I/O and control operations.
There are several ways to add I/O capabilities to a PC. Most methods can be classified into one of two general approaches (Fig. 1):
– Plugging a data acquisition (DAQ) board or PC card directly into the PC
– Connecting a networked (or distributed) I/O system to the PC.
Although each way has its advantages for certain applications, both approaches allow the power of the PC platform to be applied to a variety of industrial automation applications.
Plug-in data acquisition boards
DAQ boards that plug directly into the PC internal bus offer a versatile solution for industrial measurement and control applications. Compared to most PLC and industrial I/O systems, plug-in DAQ boards offer high-quality, high-speed analog signal measurement capabilities at very low cost, a combination that lets the user put together sophisticated and specialized systems (see table).
A wide variety of ISA and PCI plug-in boards are available (Fig. 2). Although ISA-based boards have been used for years, PCI boards are quickly becoming the standard for plug-in DAQ boards. With a maximum throughput of 132 Mbytes/ sec, PCI is able to handle large amounts of data at high rates.
PCI has also been implemented in a more industrial format using CompactPCI and PXI specifications (Fig. 3). CompactPCI is a multivendor standard for modular, Eurocard-style computers that uses PCI as the backplane bus. The PXI specification builds on the CompactPCI standard, adding specifications for power, cooling, timing and triggering pins, and software. The PXI extension adds a functionality that makes the platform especially useful for measurement and instrumentation applications.
PC cards for portable DAQ
The growing popularity and increased performance of notebook PCs have fueled the development of a wide range of PCMCIA cards (or PC cards) with high-performance data acquisition and I/O capabilities. Using a notebook computer equipped with a PC card with I/O and instrumentation capabilities frees portable and mobile applications from the limitations of hand-held instruments and dataloggers. This configuration puts the complete resources of a PC at the user’s disposal in the field, providing new capabilities for datalogging, storage, onsite analysis, and diagnostics. PC card/DAQ interfaces are available with general purpose analog and digital I/O as well as with more specialized functions such as a digital multimeter (DMM) or oscilloscope.
Although the PCMCIA bus supports a theoretical throughput of 20 Mbytes/sec and performs well in many high-speed data acquisition applications, it lacks direct memory access (DMA). However, PC card performance will soon benefit from a new PCMCIA standard called CardBus. Based on PCI technology, CardBus includes 32-bit operation, 132 Mbyes/sec throughput, and DMA.
Adding signal conditioning
Whether the addition of I/O to the PC is made using a plug-in DAQ board, CompactPCI/PXI DAQ module, or PC card, front-end signal conditioning will probably be required for most industrial applications. External signal conditioning subsystems precondition field input and output signals to improve performance, accuracy, safety, and reliability.
Depending on the application, signal conditioners add some combination of electrical isolation and protection, noise filtering, signal amplification for improved sensitivity and accuracy, and specific conditioning for different sensor types. For example, most signal conditioning systems provide a direct interface to thermocouples, RTDs, and strain gauges.
Typically, the external signal conditioning subsystem conditions the field signals, then passes the conditioned, high-level signals directly to the DAQ board that is plugged into the computer. This modular approach lets users match up the needed DAQ device type with the specific signal conditioning functions required for the application (see Fig. 1). Some signal conditioning systems also implement signal multiplexing, letting the user condition and acquire very large numbers of signals with a single, plug-in DAQ board.
New PC buses
Two new external buses (see table) appearing on PCs recently offer new connectivity options for PC-based DAQ. As Microsoft and Intel call for the removal of ISA, parallel, and serial ports from PCs over the next few years, it is expected that the universal serial bus (USB) and FireWire, along with PCI, will become the new standards for adding peripherals to a PC. Both USB and FireWire are officially supported in the Windows 98 operating system.
USB offers improved performance over the serial and parallel ports and ease of use with plug-and-play configuration, multidrop configurations, and standard connections. Al- though the theoretical throughput of USB is 1.5 Mbytes/ sec, its frame-oriented protocol and lack of DMA limit continuous acquisition throughput to less than 100 Kbytes/ sec.
FireWire (IEEE 1394) is a very high performance serial bus designed for such devices as video cameras, audio devices, printers, and disk drives. The bus currently features speeds up to 400 Mbits/ sec; future versions should reach 1.2 Gbits/ sec. Inexpensive cabling and connectors, versatile device topology (up to 63 devices), and built-in power distribution should make FireWire a popular alternative for high-performance I/O.
An important caveat of the bus, however, is that the cable drop lengths for the two buses are limited to 5 meters and 4.5 meters, respectively. These relatively short cable lengths limit their use in industrial I/O applications that require a more distributed or networked topology.
Plugging a DAQ or I/O board directly into a PC backplane has several advantages. However, many industrial applications involve sensors or devices located far from the PC. The traditional solution would require the installation of long wiring, usually carrying 4-20 mA signals from each sensor to the I/O system.
The emergence and acceptance of open, digital, industrial networks has facilitated the distribution of I/O and DAQ functionality out of the PC and into the field closer to the sensors. Because all I/O devices can be connected with an inexpensive network cable, wiring costs are dramatically lower. In addition, the use of high-speed digital networks with more intelligent I/O and DAQ devices results in access to more information from distributed locations, improved diagnostics, and reliable, autonomous operation (Fig. 4).
The ability to add networked I/O to a PC depends on the particular network. Specialized industrial buses and networks require an interface adapter, usually in a plug-in board format. For example, ISA, PCI, and even PC card interfaces are available for RS-485, DeviceNet, Profibus, and Foundation Fieldbus networks. Some networked I/O systems are able to connect directly to the standard RS-232 COM port on the PC.
One network gaining momentum today in industrial automation and data acquisition systems is Ethernet. Available with 10 Mbit/sec and 100 Mbit/sec speeds, Ethernet provides an economical, high-performance communications bus. More importantly, it has a large installed base, is well-integrated into the PC, and is familiar to most users. Although once considered appropriate only at the information systems level and used solely to connect PCs and servers, the network is rapidly proving itself a capable and attractive alternative for connecting I/O data acquisition and control devices.
Software ties it together
Today’s software tools make the PC a flexible engine for a variety of instrumentation and I/O applications. By combining the right I/O and software with the capabilities of the PC, users can build custom measurement and control tools and apply them in such areas as machine control, process monitoring and control, motion control, datalogging and analysis, and predictive maintenance.
Whether the application-level software being used is an HMI, data acquisition, control, analysis, or datalogger software package, a specific software component called a driver or server is needed to communicate with the I/O hardware. For example, most plug-in DAQ boards include a Windows DLL driver to handle all the low-level hardware communications and memory management required for data acquisition.
Compatibility and availability of I/O drivers and servers for different application-level software packages have traditionally been a problem with PC-based I/O. However, a new industrial standard is making it easier to connect these software pieces together.
OLE for process control (OPC) is a standard based on Microsoft COM and DCOM technology. It defines a common interface for communications between software packages and I/O servers so that users no longer need to worry whether their favorite software package has a driver for their preferred I/O system. As long as both items are supplied with an OPC interface, the two will work well together (Fig. 5).
Making the choice
Which approach should you use? The answer, of course, depends on the nature of the application. In general, plug-in DAQ boards provide versatile, high-speed data acquisition and control applications. For example, plug-in DAQ boards are ideal for capturing and analyzing multiple channels of high-speed waveforms or for applications that require advanced timing and triggering.
Plug-in DAQ boards make it easy to integrate I/O and data acquisition with other functions provided by plug-in boards such as motion control and image acquisition. Adding external signal conditioning modules lets plug-in DAQ boards also handle the demands of industrial applications.
If an application involves distributed sensors, processes, or devices, an external, networked I/O system is probably more appropriate. Although they tend to be less versatile and flexible than plug-in boards, networked I/O systems are excellent, economical solutions for most industrial monitoring and control applications. As intelligent networked I/O devices evolve, they can provide very reliable, autonomous operations that are less dependent on the host PC. — Edited by Jeanine Katzel, Senior Editor, 630-320-7142 , firstname.lastname@example.org
I/O capabilities can be added by plugging a DAQ board or PC card directly into a PC or by connecting a networked or distributed I/O system to the unit.
Plug-in DAQ boards offer high-quality, high-speed analog signal measurement capabilities at very low cost.
Applications involving distributed sensors, processes, or devices are generally more suited for an external, networked I/O system.
The author is available to answer technical questions about this article. He may be reached at 512-794-0100.
Guide to acronyms
The computer and controls industries are fraught with acronyms that are often complex and confusing. The following list is designed to promote an understanding of some of the terminology commonly encountered in these fields.
CardBus: New 32-bit revision of the PCMCIA standard that brings PCI performance to portable PCs, including 132 Mb/sec throughput and DMA.
CompactPCI: A Eurocard configuration of the PCI bus for industrial applications.
DAQ (Data Acquisition): (1) Collecting and measuring electrical signals from sensors, transducers, and test probes or fixtures and inputting them to a computer for processing; (2) Collecting and measuring the same kinds of electrical signals with A/D and/or DIO boards plugged into a PC, and possibly generating control signals with D/A and/or CIO boards in the same PC.
DCOM (Distributed Component Object Model): A set of Microsoft concepts and program interfaces in which client program objects can request services from server program objects over a network.
DMA (Direct Memory Access): A method by which data can be transferred to/from computer memory from/to a device or memory on the bus while the processor does something else. DMA is the fastest method of transferring data to/from computer memory.
Fieldbus: An all-digital communication network used to connect process instrumentation and control systems. It will ultimately replace the existing 4-20 mA analog standard.
Fieldbus Foundation: The organization developing a standard digital communication network (fieldbus) for process control applications. The network developed by the Foundation is referred to as the Foundation Fieldbus.
FireWire: See IEEE 1394.
HMI (Human-Machine Interface): The means by which an operator interacts with an industrial automation system, often a graphical user interface.
IEEE 1394: The shortened notation for IEEE standard 1394-1995, which defines a high-speed serial bus growing in popularity for connecting computers to peripheral video, memory, and instrumentation devices. The maximum transfer rate, currently at 400 kb/sec, is scheduled to increase in the future. (Also known as FireWire.)
Industrial Device Networks: Standardized digital communications networks used in industrial automation applications. They often replace vendor-proprietary networks so that devices from different vendors can communicate in control systems.
I/O (Input/Output): The interface, or the products that interface, to raw, real-world signals and transducers, including both analog and discrete, or digital, inputs and outputs. I/O products include the signal conditioning and acquisition functionality to connect directly to a variety of sensors and voltage and current sources.
MMI (Man-Machine Interface): see HMI.
Noise: An undesirable electrical signal. Noise comes from external sources such as the A/D power line, motors, generators, transformers, fluorescent lights, soldering irons, CRT displays, computers, thunderstorms, arc welders, radio transmitters, and internal sources such as semiconductors, resistors, and capacitors.
OLE (Object Linking and Embedding): A set of system services that provides a means for applications to interact and interoperate. Based on the underlying Component Object Model, OLE is object-enabling system software. Through OLE automation an application can dynamically identify and use the services of other applications to build powerful solutions using packaged software. With OLE it is also possible to create compound documents consisting of multiple sources of information from different applications.
OPC (OLE for Process Control): An open industry-standard device interface that provides interoperability between disparate field devices, automation/control systems, and business systems. OPC is based on ActiveX, OLE, Component Object Model (COM), and Distributed COM (DCOM) technologies.
PC Card: A credit-card-sized expansion card that fits in a PCMCIA slot, often referred to as a PCMCIA card.
PCI (Peripheral Component Interconnect): A high-performance expansion bus architecture originally developed by Intel to replace ISA and EISA. It has achieved widespread acceptance as a standard for PCs and workstations and offers a theoretical maximum transfer rate of 132Mbytes/sec.
PCMCIA: An expansion bus architecture that has found widespread acceptance as a de facto standard in notebook-size computers. It originated as a specification for add-on memory cards written by the Personal Computer Memory Card International Association.
PLC (Programmable Logic Controller): A highly reliable special-purpose computer used in industrial monitoring and control applications. PLCs typically have proprietary programming and networking protocols, and special-purpose digital and analog I/O ports.
Plug and Play ISA: A specification prepared by Microsoft, Intel, and other PC-related companies that will result in PCs with plug-in boards that can be fully configured in software, without jumpers or switches on the boards.
PXI (PCI eXtensions for Instrumentation): A rugged, open system for modular instrumentation based on CompactPCI, with special mechanical, electrical, and software features.
RTD (Resistance Temperature Detector): A metallic probe that measures temperature based upon its coefficient of resistivity.
SCADA (Supervisory Control and Data Acquisition): A common PC function in process control applications where programmable logic controllers (PLCs) perform control functions but are monitored and supervised by a PC.
Thermocouple: A temperature sensor created by joining two dissimilar metals. The junction produces a small voltage as a function of the temperature.
USB (Universal Serial Bus): A serial bus gradually replacing RS-232 on PCs because of its higher speed.