-----------------------------Monitor----------------------------- The Newsletter for PC-Based Data Acquisition and Control Issue 26 www.windmill.co.uk September 2000 -------------------------ISSN 1472-0221-------------------------- CONTENTS ======== * Windmill News: Windmill Help Files Available from Web * Multiplexing Multiple Measurements _________________________________________________________________ Windmill News: Windmill Help Files Available from Web The Windmill data acquisition software comes with comprehensive on-line Help. We hope you'll be able to find answers there to any questions you have. However, we operate a policy of continually upgrading the Help: adding more information depending on the questions you ask us. All updated versions are available from our web site at http://www.windmill.co.uk/help.html Download the zip files and copy *.hlp and *.cnt to your Windmill folder. Previously we only listed updated Help files for Version 4.3 of Windmill, but now we've added those for Windmill 6. To keep improving the Help we need your comments, especially your criticisms. Please fill in the form on the Help page. http://www.windmill.co.uk/help.html _________________________________________________________________ Multiple Measurements: Multiplexing or Simultaneous Sampling? When data acquisition equipment receives an analogue signal, it digitises it - converting it to number that the PC can understand. This is done by an analogue-to-digital converter. However, what happens when monitoring multiple signals? Clearly, one A-D converter cannot digitise all the signals at once. There are two answers to this: provide an A-D converter for each analogue input, or switch (multiplex) the signals in turn to one A-D converter. Simultaneous Sampling ===================== In true simultaneous sampling the data acquisition hardware provides an A-D converter for each analogue input signal. Providing sampling is initiated from a single clock source, this both permits simultaneous sampling and ensures that there is no reduction in sampling rate as the number of inputs increases. However, this method is very expensive. Multiplexing ============ The second, and more common, approach is to use a single analogue-to-digital converter and switch each signal in turn to the converter. This is a much cheaper solution. However it does not provide simultaneous sampling across the inputs and as more inputs are added the maximum sampling rate is reduced per input. For example, if your system can read 1 input channel 100 times a second, when reading 10 input channels it is limited to 10 readings per second per channel. Settling Time and Cross Talk ============================ As the multiplexer switches one input after another to the A-D converter, you must allow time for each reading to settle to its new value. This settling time defines the minimum interval between reading each input: the inter-channel interval. If you try to go faster than this there will be increasing cross-talk between channels, leading to inaccurate results. Mixing Small and Large Signals ============================== Low-level signals, like thermocouple voltages, take longer than higher voltages to settle to their true value. It's always good practice to group the signal sizes. Some data acquisition systems let you set different inter-channel intervals for different groups of channels. This is especially valuable when streaming data directly to disk at high speeds. For slower applications, software like Windmill lets you set different settling times for different channels. Another consideration when monitoring different sized signals, is being able to choose the range of each input channel individually. The range refers to the maximum and minimum voltage that will be digitised by the A-D converter. It's always best to choose the smallest range that encompasses the signal, as this optimises resolution. For low speed applications (Hz) some software will automatically select the best range for you, but this should be turned off for high speed applications (kHz). Independent ranges lets you to mix, say, thermocouples, 2 V instrumentation outputs and 4-20 mA process signals in one multiplexed scan. Burst Scanning ============== When collecting data fairly slowly it is possible for the scan rate (the total time to read all the channels once) to be much slower than the minimum inter-channel interval suggests. In this case it may make sense to sample the channels fairly quickly once a scan starts, giving readings that are taken closer together in time and so more easily compared. This relatively long break between scans is known as burst scanning. Mixing Fast and Slow Changing Signals: Split-Rate Scanning ========================================================== Suppose you are monitoring an engine. You are recording pressure signals from the cylinders as well as coolant temperatures. It is sensible to record the pressure signal much more often than the coolant temperature. This leads to the idea of split-rate scanning where a subset of channels is recorded every scan, while all channels are recorded every nth scan. Channel Sub-Sets ================ For test applications, you may be monitoring some inputs in one test run and a completely different set of inputs in another. The ability to select a sub-set to scan is important to avoid having to rewire your connections each time you want to record some waveform data. Software ======== Windmill software supports low-speed multiplexed systems with independent input ranges and channel sub-sets. Streamer software supports higher-speed multiplexed systems with independent input ranges, channel sub-sets, inter-channel intervals, burst scanning and split-rate scanning. (Assuming in both cases that the hardware also supports these features.) For more details of Windmill or Streamer send for a free leaflet at our web site. http://www.windmill.co.uk/contact.html Further Reading =============== Monitor Issue 3 - A-D Converter Specifications Monitor Issue 4 - Analogue Signals and Types of A-D Converter _________________________________________________________________ * 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/ Comments: Email:
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