What DAQ System Should We Use?

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...Factory Information Technologies offers a product called FIN (Factory Intelligence Network), that provides real time and historical trending, long-term data storage, easy exporting of data to Excel, powerful report features, and true client-server operation, and comes completely configured so it works out of the box. The FIN data collection package can collect data from existing PLCs or Ethernet I/O modules.

Ron Iannacone, President

Factory Information Technologies, www.factoryintelligence.com

Consider Technical Requirements First

There is a wide variety of methodologies available for data acquisition today, and there are is an equally wide range of applications that need to be satisfied. Choosing the most suitable system is, to some extent, predicated by the type and quantity of data to be recorded, but there are far more aspects that should influence the choice. This article is a very brief, broad-brush overview considering just three technologies: instrumentation recorders, programmable digital voltmeters, and PCs with DAQ cards.

...There are many applications where all three of the above technologies will provide successful data acquisition, and in such cases, the choice will probably come down to cost and personal preference. In applications where this is not so, the first step is to establish a concise specification about the data to be acquired, including details of where that data is physically being obtained from, where the acquired data is intended to be finally located and what analysis or other post-acquisition operations must be performed before the end product is supplied to users. Choosing the most appropriate data acquisition device requires consideration of the whole system, from obtaining data from sensors through providing processed results to users in their requested format.

...There are many parameters that will affect choice, the most important being technical capability. The highest bandwidth analog signal and the highest bit rate digital input need to be noted. Also the aggregate bandwidth (i.e. the sum of all the input signal bandwidths and bit rates) should be calculated. The mission-minimum duration must be defined, with this in turn yielding the size of storage media required for the mission [aggregate data rate multiplied by mission duration]. The operating environment should be defined, including issues such as portability, mobility, and extremes of temperature, humidity, pressure, vibration, shock, and EMC/RFI.

...Programmable voltmeters operate at very low bandwidths and can only be used for slowly changing signals. Typically, they will sample the input channels sequentially. In some circumstances, this will create problems with the time alignment of data from different channels, although this is less likely to be a concern for low bandwidth data. The signal type is more or less only one type - voltage. There are a wide variety of methods for transferring data from programmable voltmeters to analysis platforms; potential users should ensure that the method is appropriate to their needs and is accompanied by desirable analysis and display tools. Methods for distribution of the data to several users need to be considered. These devices can be portable and suitable for mobile use, though environmental capability may be limited.

...PC DAQs can acquire data over a range of rates, from a few Hertz to megahertz, and can handle from one to many channels. Some DAQs share a single analog-to-digital converter [ADC] between several channels; this will create time misalignment between channel data. If phase relationships are an issue, ensure that the channel interfaces contain one ADC per channel and the sampling clock is synchronized across all ADCs.

...A wide variety of DAQs interface to a variety of signal types like analog voltage, bridges, gauges, logic levels, and others. PC-based systems are generally not suitable for use in non-benign environments outside of typical laboratory use, although certain manufacturers provide rugged and/or portable systems. PC-based DAQs are frequently supplied with and closely coupled to comprehensive analysis and visualization tools. PC-based systems rely on the robustness of the PC operating system for reliability of acquisition, this sometimes being problematic if the acquisition application is run concurrently with other applications. The acquired data will typically end up on a hard drive, and the user will have to make arrangements to copy data to tape or use network file transfers if the data is to be distributed among several users/customers.

...Instrumentation recorders like those manufactured by Heim Data Systems are purpose-built platforms for data acquisition and, as such, provide the widest selection of signal input types and the widest range of bandwidth (up to many megahertz), typica lly have the most economical solution for high aggregate bandwidth applications, and have standard solutions for non-benign environments. The signal sampling systems are designed to ensure accurate timing of data both between channels and in absolute time. They appear in laboratory, portable, mobile, and airborne packages. Their purpose-designed operating systems are optimized for robustness, speed, and user operation.

...Recorders can use many different types of media: tape is a low-cost media that can be removed from the data acquisition system for easy transfer from acquisition site to analysis platform and later distribution. Hard drives provide an economical solution for high aggregate data rate, long-duration applications. Solid-state Flash memory drives provide the most robust solution for non-benign environment operation. Although the price of flash memory continues to come down, it is presently much more expensive than tape and hard drive solutions.

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