The data acquisition system with its computer hardware, software, storage, and peripheral equipment is the starting point for most detailed engineering analysis. Whether the task is a study in a laboratory or a look at a pesky anomaly in a customer installation, it all starts by acquiring data from the equipment being studied.
The typical data acquisition system has certain elements for collecting data and preparing it for computer input. Figure 1 shows the functions of a basic system.
Analog inputs of various types are conditioned so that each has a common voltage range to represent its expected measurement span. This involves gain and offset adjustment and, in many cases, filtering. When a sensor such as a Wheatstone bridge needs excitation voltage, it is supplied by the related conditioner.
Each input is sampled and held for conversion. In the typical case, sampling is sequential. However, some applications require simultaneous sampling of all channels so time relationships are not distorted. One example is the crash test of an automobile, where the user must determine what events happened at each increment of time during a few seconds of intense activity.
The analog-to-digital converter (ADC), the heart of the acquisition system, can be selected from a wide range of types, resolutions, and accuracies. Some of the readily available classes include successive approximation, flash, integration, and delta-sigma conversion.
One or more first-in, first-out (FIFO) buffers are used to hold digital words so none is lost in transfer to the system computer. The size of a FIFO depends on data collection rates and the worst-case accessibility of the computer input path.
All communications with the PC are via bus interface and controller circuitry. This architecture is uniquely defined for each type of bus and the onboard elements that must be serviced.
Many high-performance acquisition systems include one or more analog outputs. These take digital data from the computer to form a continuous analog waveform. A buffer is provided for each channel, so the correct timing relationship between samples can be maintained. Similarly, many systems include a multichannel digital port on which groups of bits can be set as inputs or outputs.
In many systems, a clock controls analog sampling rates and other critical timing functions. The frequency is set by the computer. Some systems have general-purpose counter/timers as well.
The Choice of Equipment
Within the broad boundaries of data acquisition architecture, there are dozens of characteristics that must be considered when selecting a data acquisition system. Start with the basics—resolution, accuracy, and speed. Then come sensitivity, dynamic range, channel counts, and specialized signal conditioning. Obviously, the selection involves a compromise among these characteristics, but there are relative priorities that must be evaluated. You can get a collection rate of 2 MS/s or a resolution of 20 bits, but you can’t have both in the same system. Just how important is each for your application?
Another consideration is noise. “When users find that noise is limiting them,” said Tom Lawson, president of Lawson Labs, “we show them how to solve the problem. We have systems with input impedance as high as 1015 W, true-differential protected inputs with an extra-wide DC common-mode range, very high resolution, noise-rejection converters, and optical isolation to allow ground loops to be broken.”
Finally, maybe you can find a system that improves the data acquisition performance and still allows you to use your existing software tools. Don’t toss out that investment of a few years ago just because the speed and resolution are not sufficient for today’s applications.
Interfaces to a PC
Most data is routed to a PC over some type of communications bus. Presently, low-speed systems can use the universally available RS-232 or IEEE 488. The PC buses and their extensions offer a step up in speed and are very popular.
“When speed is the key,” said Ed Evansen, president of Measurement Computing, “the PCI bus has a clear advantage over other buses. With 130-MB/s bandwidth and typical useful speed of >80 MB/s, it outruns the ISA, PCMCIA card, Universal Serial Bus (USB), Ethernet, and PC/104.”
There are trade-offs with each bus. The key is to find a bus that meets both short-term and long-term goals. For example, when speed is critical, the PCI bus is the way to go. If price is the overriding factor, the ISA bus with its simple interface will save you money. For small size and portability, it’s hard to beat the PCMCIA card.
The simplest hardware/software installation with plug-and-play capability may be PCI, USB, or PCMCIA. For an embedded system, the PC/104 is an excellent choice.
On the horizon is Version 2.0 of the USB. When interface chips to support it become widely available, it may dominate. The jury still is out on Firewire (IEEE 1394), which offers some speed advantages but must gain support from equipment manufacturers to be a viable option.
With all the attention directed toward the newer, high-speed, high-performance buses, it is interesting to see what is happening to the older buses. Yes, many users find the ISA bus or Ethernet suitable for data acquisition, and the 4-20-mA current loop and IEEE 488 bus are alive and well. And this may be a surprise: the RS-232 serial bus still is one of the most widely used interfaces.
Multigigahertz Acquisition
“In most multigigahertz applications,” noted Mike Bayda, product marketing manager at Keithley Instruments, “the goal is not to monitor the data source continuously, but rather to capture a transient of some sort. We can equate this to using a high-speed storage oscilloscope. Consequently, a very important factor is the amount and use of memory.
“It takes just a millisecond to fill a megabyte of storage space at a 1-GS/s rate,” Mr. Bayda continued, “so if you collect data for a long period, the system must be able to store it in an onboard FIFO and shuffle it to a fixed disk at lightning speed. Considering this limitation, it is wise to consider the capture-transfer-analyze sequence, where proper triggering can capture the phenomena of interest in manageable quantities.”
The capability to switch gigahertz signals properly is important in a high-frequency system. A multiplexer should have 50-W input and output impedance, a very low voltage standing wave ratio (VSWR), negligible insertion loss, and excellent channel-to-channel isolation to maintain signal integrity and ensure data accuracy.
Applications
Most data acquisition applications are relatively straightforward. Your data sources are well-defined, your sample rate is fixed, and you need only 12 bits of resolution. However, we never know when a challenge will arise. The following examples describe some unique problems and the solutions that were derived.
Wind Turbine Data Acquisition
Often a user must integrate different types of digitizers into a functional system. One such case involved measuring the spatial relationship of the air turbulance, wind temperature, rotational speed, and wind speed at different points around a wind turbine. Data is collected and time-tagged using inputs from a global positioning system (GPS) receiver. After real-time processing, analog and digital control signals are generated to control other parts of the system.
The signal conditioners for the turbine test are housed in a National Instruments’ SCXI chassis, and an extended PCI bus holds analog input and output modules plus a digital I/O module. LabVIEW software on the system PC provides interactive control of the entire process.
Collecting Data From an Ultrasonic Scanner
How do you store data that is collected at rates that can’t be handled on a PCI bus? When given this challenge, Gage Applied developed a system to collect information from an ultrasonic scanner. Each frame contains 419 MS and must be collected onboard in 1 s.
The CompuScope 8500 captures each record on a memory board, dumps it virtually instantaneously to a large onboard memory, and automatically re-arms itself to capture the next record, all without intervention by the computer. After a total frame of data is stored, the system sends it to the PC.
What’s in That Suspicious Luggage?
A contraband detector used by airport security personnel is made by Ion Track Instruments. The device incorporates a spectrometer and acquires samples from a wand that is swiped over a piece of luggage. A PCMCIA card from Quatech collects spectral data from the spectrometer and prepares it for input to a small computer. The card has eight differential or 16 single-ended inputs, scans at 100 kS/s, and incorporates a 4-kS FIFO buffer.
No More False Failures on Power Supply Test
A switching power supply manufacturer using a successive-approximation analog-to-digital converter (ADC) for test was plagued by false failures because there was very little immunity to the noise on the supply output. By converting to the Keithley Instruments’ 2700 System with an integrating ADC that inherently filters out noise, the problem was solved.
Tiny Bubbles
Are you bothered by cavitation? This is the formation of bubbles or cavities in the flow path of a liquid, typically caused by flowing too quickly through a pump or over a blade. This condition can damage a liquid distribution system.
To collect cavitation data to study the problem, Dynaflow developed the Acoustic Bubble Spectrometer®. This system works with two hydrophones in the liquid. The transmitter emits sine waves at several frequencies, typically 10 kHz to 300 kHz, and the receiver detects the sounds. A PC controls and monitors the process, and a 1.25-MHz module from United Electronic Industries samples the transmitted and received signals simultaneously. Dynaflow software is used to quantify cavitation and evaluate the results of improvements in the flow system.
Particles From Space
An important factor in high-speed systems is channel-to-channel synchronization. Acqiris developed a method called the auto-synchronous bus for ensuring accurate timing. This front-panel interconnection sends trigger and clock signals from card to card over a 1-GHz path, and each card has onboard programmable delay lines. Klaas Vogel, director of North American sales, explained that the system synchronization accuracy is 100 ps after this bus has been installed and delay lines have been tweaked.
The synchronization technique proved useful in a system to detect high-energy particles from space, where several channels are collected in parallel and module-to-module delay is intolerable. Another application is the use of pulsed lasers for measuring fusion-plasma temperature at different spots on a tokamak reactor where time relationships must be preserved.
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July 2001