-------------------------Monitor------------------------ The Newsletter for PC-Based Data Acquisition and Control Issue 130 www.windmill.co.uk May 2009 --------------------ISSN 1472-0221---------------------- Welcome to Monitor, the data acquisition newsletter. I hope you find it useful, but should you wish to remove yourself from our mailing list please go to http://www.windmill.co.uk/newsletter.html CONTENTS ======== * Choosing how Often to Sample an Analogue Signal * DAQ News Roundup ________________________________________________________ ________________________________________________________ Choosing how Often to Sample an Analogue Signal ________________________________________________________ When monitoring analogue signals, the data acquisition equipment digitises the signal before transferring the reading to the computer. It does this by "sampling" the signal at regular intervals. How do you choose the sampling rate to get an accurate picture of an analogue waveform? The first thing to decide is how fast the signal is changing. For faster signals this is determined from the maximum frequency component in the signal. For slowly changing signals, such as those from industrial plants, you can use the maximum expected rate of change. For frequency components the Nyquist theorem demands that the signal be sampled at least twice in each cycle, otherwise the amplitude of this frequency component will distort the signal at lower frequencies (see the picture at http://www.microlink.co.uk/sampling.html). So, sample at least twice as fast as the highest significant component. This rate would not mimic the waveform very closely though. To get an accurate picture of the waveform you will need a sampling rate of 10 to 20 times the highest frequency. For slowly changing, essentially DC, signals, then all that is necessary is to consider the minimum time for a significant change in the signal. Related topics: Monitor Issue 26, Multiplexing ________________________________________________________ ________________________________________________________ DAQ News Roundup ________________________________________________________ Welcome to our roundup of the data acquisition and control news. If you would like to receive more timely DAQ news updates then grab our RSS newsfeed at https://www.windmillsoft.com/monitor.xml. Read http://www.windmill.co.uk/newsfeed.php for notes on how to display the news on your own web site, read it via e-mail, mobile phone or in your browser. Ageing satellites to hit GPS accuracy GPS devices calculate their position by comparing time signals from at least four satellites. To keep that many within range at all times requires a fleet of at least 24. For now there are 31 operating, but 13 of them are more than four years past their design lifetime. Bad news for position-referenced data acquistion. Source: New Scientist http://www.newscientist.com/ New Way to Test Water Quality Researchers at RMIT University have developed a low-cost, portable way to test water quality and help authorities deal with pollution. The prototype sensor uses selectively adsorbing polymers designed to quantitatively detect specific water-borne contaminants. Source: RMIT University http://www.rmit.edu.au/ New acoustic monitoring system for sewers Engineers from from Bradford University are developing an acoustic monitoring system that will allow engineers to rapidly identify blockages and damage in sewer pipes. The key elements of the acoustic system are a small multi-sensor array and an advanced, real-time signal processing algorithm which overcomes the effects of ambient noise and reverberation in the sewer environment. Source: The Engineer Online https://www.theengineer.co.uk/ Industrial Controls Shipments plunge US association NEMA's Primary Industrial Controls Index experienced its largest quarter-to-quarter decline on record, contracting more than 23 percent in the first quarter of 2009 versus the fourth quarter of 2008. Source: NEMA http://www.nema.org/ ________________________________________________________ ________________________________________________________ * Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see http://www.windmill.co.uk We are happy for you to copy and distribute this newsletter, and use extracts from it on your own web site or publication, providing the above notice is included and a link back to our website is in place. An archive of previous issues is at http://www.windmill.co.uk/newsletter.html and an index of articles at http://www.windmill.co.uk/newsletter.html Windmill Software Ltd, PO Box 58, North District Office, Manchester, M8 8QR, UK Telephone: +44 (0)161 833 2782 Facsimile: +44 (0)161 833 2190 E-mail: [email protected] http://www.windmill.co.uk/ https://www.windmillsoft.com/
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