-----------------------------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
https://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.
https://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.
https://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 https://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
https://www.windmill.co.uk/newsletter.html
and an index of articles at
https://www.windmill.co.uk/newsletter.html
Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 834 6688
Facsimile: +44 (0)161 833 2190
E-mail: monitor@windmillsoft.com
https://www.windmill.co.uk/
https://www.windmillsoft.com/
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