MATERIALS ENGINEERING AND TESTING SERVICES
Materials Engineering and Testing Services
Office of Roadway Materials Testing
Instrumentation Services Branch
DATA ACQUISITION SYSTEMS
by Ed Ung
The purpose of data acquisition is to capture relevant data from physical phenomena with sufficient accuracy to permit reliable extraction of required information. A data acquisition system can be generally divided into the following main components:
- Transducer Signal Conditioner
- Recording Device
The transducer converts physical phenomena such as stress, strain, force, pressure, vibration, and temperature into equivalent electrical output signals. The electrical signal is usually an analog signal -- a continuously variable voltage directly proportional to the magnitude of the phenomena. Examples of phenomena and transducers that the Instrumentation Section use include temperature (thermocouples), force and pressure (strain gages, load cells), position or displacement (LVDTs -- Linear Variable Differential Transformer, potentiometers, optical encoders). Some transducers are self-generating -- thermocouples, piezoelectric transducers. Others require external excitation voltages -- strain gages, LVDTs.
10-KIP Load Cell LVDT Installation Strain Gage String Potentiometer
The signal conditioner is an interface device between the transducer output signal and the recording device. Its purpose is to output a high-level linear voltage or current. Signal conditioners perform a variety of functions including amplification of low-level signals, attenuation of high-level signals, isolation, linearization, scaling, and filtering. Filtering is used in data acquisition to eliminate noise, eliminate unwanted frequency components from signals, and eliminate stray pickup of frequencies.
Signal Conditioning Units
The output from the signal conditioner then goes to a recording device such as an X-Y plotter, chart recorder, tape recorder, or a computer. When using a computer to record information, the signal must first be digitized using an analog-to-digital (A/D) converter. A common type of computer to use is a PC with a plug-in data acquisition board to perform the A/D function. This is referred to as a PC-based data acquisition system. Use of computers offer flexibility since various data acquisition parameters are entered through software programs.
Typical Strain Plots on PC DAQ Scanner with Signal Conditioners
Trends in Data AcquisitionHistorically, data acquisition was done using stand-alone dedicated instruments. One example is the oscilloscope. It is dedicated for a specific task. The signal conditioning and recording are contained in a "black box." Beginning in the 1970s, automated test equipment were packaged in rugged enclosures with electronics and cards necessary to capture data across multiple points in parallel. With the economies of advancing electronics, more options were available to users. Data acquisition became less expensive, more programmable, configurable, and accessible by more users for more applications
(1). This trend continued with the introduction of personal computers and plug-in data acquisition boards. Instruments could now be designed by the end user using plug-in boards and peripherals. The controls and displays on stand-alone instruments could be represented "virtually" -- graphical representations of actual controls. The step beyond this is to install the intelligence at the sensor and connect the sensors in a network. This is currently available in industrial environments.
(2).The use of computers in data acquisition increases the dependency on software. Data acquisition parameters such as rate, scaling, and hardware configuration are done through software. There are three basic types. The first is a language add-on type which adds instrument-specific functions to an existing programming language. National Instruments LabWINDOWS is an add-on to the C programming language. The advantage is that it builds upon an existing language and doesn't require the programmer to learn a new language. The second is a complete language type which is a standalone programming language created specifically for computer instrumentation. National Instruments LabVIEW is an example of this type of language. It uses a graphical approach, replacing text-based programming with icons and a data flow-based programming structure. The advantage is a program which runs at comparable speeds to a compiled language program with reduced development time. The third type is menu system software which is a standalone environment in which programs are developed by using menus and filling in parameters.
1. "Data Acquisition: Black Box to Virtual Instrument -- and Beyond," by Rich Lewan, Ed Pastor, Rich Mullen, Keithley MetraByte, Sensors magazine, September 1995.
2. "Integration: The Future of Instrumentation," Automotive Engineering magazine, March 1993.