Data Acquisition and Control Handbook

Definition

Although concepts like data acquisition and test and measurement can be surprisingly difficult to define completely, most computer users, engineers, and scientists agree there are several common elements:

1. A personal computer (PC) is used to program test equipment and manipulate or store data. The term “PC” is used in a general sense to include any computer running any operating system and software that supports the desired result. The PC may also be used for supporting functions, such as real-time graphing or report generation. The PC may not necessarily be in constant control of the data acquisition equipment or even remain connected to the data acquisition equipment at all times.
2. Test equipment can consist of data acquisition plug-in boards for PCs, external board chassis, or discrete instruments. External chassis and discrete instruments typically can be connected to a PC using either standard communication ports or a proprietary interface board in the PC
3. The test equipment can perform one or more measurement and control processes using various combinations of analog input, analog output, digital I/O, or other specialized functions.

The difficulty involved in differentiating between terms such as data acquisition, test and measurement, and measurement and control stems from the blurred boundaries that separate the different types of instrumentation in terms of operation, features, and performance. For example, some stand-alone instruments now contain card slots and microprocessors, use operating system software, and operate more like computers than like traditional instruments. Some external instruments now make it possible to construct test systems with high channel counts that gather data and log it to a controlling computer. Plug-in boards can transform computers into multi-range digital multimeters, oscilloscopes, or other instruments, complete with userfriendly, on-screen virtual front panels.

For the sake of simplicity, this handbook uses the term “data acquisition and control” broadly to refer to a variety of hardware and software solutions capable of making measurements and controlling external processes. The term “computer” is also defined rather broadly; however, for most applications, a “computer” means an IBMcompatible PC running Microsoft® Windows® 95 or later, unless otherwise noted.

Data Acquisition and Control Hardware

Data acquisition and control hardware is available in a number of forms, which offer varying levels of functionality, channel count, speed, resolution, accuracy, and cost. This section summarizes the features and benefits generally associated with the various categories, based on a broad cross-section of products. Refer to Appendix A for a comparison of plug-in boards and external instruments.

Plug-in Data Acquisition Boards

Like display adapters, modems, and other types of expansion boards, plug-in data acquisition boards are designed for mounting in board slots on a computer motherboard. Today, most data acquisition boards are designed for the current PCI (Peripheral Component Interconnect) or earlier ISA (Industry Standard Architecture) buses. Data acquisition plug-in boards and interfaces have been developed for other buses (EISA, IBM Micro Channel®, and various Apple buses), but these are no longer considered mainstream products.

As a category, plug-in boards offer a variety of test functions, high channel counts, high speed, and adequate sensitivity to measure moderately low signal levels, at relatively low cost.

External Data Acquisition Systems

The original implementation of an external data acquisition (DAQ) system was a self-powered system that communicated with a computer through a standard or proprietary interface. As a boxed alternative to plug-in boards, this type of system usually offered more I/O channels, a quieter electrical environment, and greater versatility and speed in adapting to different applications.

Today, external data acquisition systems often take the form of a stand-alone test and measurement solution oriented toward industrial applications. The applications for which they are used typically demand more than a system based on a PC with plug-in boards can provide or this type of architecture is simply inappropriate for the application. Modern external data acquisition systems offer:

• High sensitivity to low-level voltage signals, i.e., approximately 1mV or lower.
• Applications involving many types of sensors, high channel counts, or the need for stand-alone operation.
• Applications requiring tight, real-time process control.

Like the plug-in board based system, these external systems require the use of a computer for operation and data storage. However, the computer can be built up on boards, just as the instruments are, and incorporated into the board rack. There are several architectures for external industrial data acquisition systems, including VME, VXI, MXI, Compact PCI, and PXI. These systems use mechanically robust, standardized board racks and plug-in instrument modules that offer a full range of test and measurement functions. Some external system designs include microprocessor modules that support all the standard PC user interface elements, including keyboard, monitor, mouse, and standard communication ports. Frequently, these systems can also run Microsoft Windows and other PC applications. In this case, a conventional PC may only be needed to develop programs or off-load data for manipulation or analysis.

Real-Time Data Acquisition and Control

Critical real-time control is an important issue in data acquisition and control systems. Applications that demand real-time control are typically better suited to external systems than to systems based on PC plug-in boards. Although Microsoft Windows has become the standard operating system for PC applications, it is a non-deterministic operating system that can’t provide predictable response times in critical measurement and control applications. Therefore, the solution is to link the PC to a system that can operate autonomously and provide rapid, predictable responses to external stimuli.

Discrete (Bench/Rack) Instruments

Originally, discrete electronic test instruments consisted mostly of single-channel meters, sources, and related instrumentation intended for general-purpose test applications. Over the years, the addition of communication interfaces and advances in instrument design, manufacturing, and measurement technology have extended the range and functionality of these instruments. New products such as scanners, multiplexers, SourceMeter® instruments, counter/timers, nanovoltmeters, micro-ohmmeters, and other specialized instrumentation have made it possible to create computer-controlled test and measurement systems that offer exceptional sensitivity and resolution. Some systems of this type can service only one channel or just a few channels, so their cost per channel is high. However, the addition of switch matrices and multiplexers can lower the cost per channel by allowing one set of instruments to service many channels while preserving high signal integrity. These instruments can also be combined with computers that contain plug-in data acquisition boards.

Hybrid Data Acquisition Systems

Hybrid systems are a relatively recent development in external data acquisition systems. A typical hybrid system combines a DMM-type user interface with several standard data acquisition functions and expansion capabilities in a compact, instrument-like package. Typical functions include AC and DC voltage and current measurements, temperature and frequency measurements, event counting, timing, triggering, and process control. Keithley’s Integra™ Series, which includes the Model 2700 and Model 2750 and their associated plug-in modules, provides multiple board slots for expanding the system’s measurement capabilities and channel capacity (Figure 1-1).