-------------------------Monitor------------------------
The Newsletter for PC-Based Data Acquisition and Control
Issue 169 www.windmill.co.uk August 2012
--------------------ISSN 1472-0221----------------------
All measurements are contaminated by noise - today we
discuss how you can minimise its effects on your signals.
We also pleased to announce that you can now buy the
new Microlink thermocouple data logger from our
web site.
I hope find the newsletter useful, but should you wish
to remove yourself from our mailing please go to
Monitor Newsletter
CONTENTS
========
* Windmill News: Thermocouple Data Logger also Measures
Voltage, Flow, Level etc
* How to Remove Noise from your Measurements
* DAQ News Round-up
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Thermocouple Data Logger also Measures Voltage,
Flow, Level, etc
________________________________________________________
The new Microlink themocouple
data logger not only logs
temperature data but
can also measure voltage,
flow, level etc. Additionally,
it provides digital I/O and
event and frequency counting.
Read on for more details or see
https://www.windmillsoft.com/daqshop/thermocouple-data-logger.html.
The Thermocouple Data Logger:
- Monitors 16 thermocouples or voltage signals
- Has built-in linearisation for B-, E-, J-, K-, N-, R-, S-
and T-type thermocouples
- Automatically detects broken thermocouple leads
- With extra hardware also measure strain, current and pH
- Reads data over an Ethernet network or over the Internet
- Reduces electrical noise with an integrating analogue-to-
digital converter
- Lets you select the resolution of the A-D converter from
12- to 18-bits: choose high resolution or high speed
- Automatic recalibrates itself
- Offers two alarms on each input
- Lets you choose real-time data acquisition on your PC or
stand-alone data logging
Themocouple Measurement
=======================
The system comprises a Microlink 851 measurement unit, an
isothermal box, Windmill data acquisition and control software
and technical support for life.
You connect the thermocouple wires to screw terminals in the
Microlink isothermal box. This keeps the temperature of the
thermocouple junctions constant, measured by a cold junction
sensor in the box. Plug the isothermal box into the
Microlink 851 using its ribbon cable. The isothermal box will
also detect broken thermocouple leads for you. For more
information on monitoring thermocouples, see our tutorial on
computerised themocouple measurement
https://www.windmill.co.uk/temperature.html.
Further Reading
===============
More information on the 851-TC package:
https://www.windmillsoft.com/daqshop/thermocouple-data-logger.html
How to make temperature measurements - pitfalls of
monitoring thermocouples and RTDs:
https://www.windmill.co.uk/temperature.html
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How to Remove Noise from your Measurements
________________________________________________________
All measurements are contaminated by noise. It may be
generated within the electrical components of the
input amplifier: called internal noise. It may also be
added to the signal as it travels down the wires to
the amplifier: called external noise. Noise is more
of a problem for millivolt signals, from sensors
like thermocouples, than it is for larger, volt-level,
signals.
How do you minimise the effect of noise on your signal?
Reducing Noise Generated by the Amplifier
=========================================
Internal noise arises from thermal effects in the
amplifier and although it can be minimised it cannot
be removed completely. (The amplifier in data acquisition
equipment increases the voltage signal to a level
suitable for the analogue-to-digital converter.)
Typically amplifiers will generate a few microvolts of
internal noise which limits the resolution of the
signal to this level. The amount of noise added to the
signal depends on the bandwidth of the input amplifier,
from the lowest frequency allowed into the amplifier
to the highest. Amplifiers with a bandwidth matching
that of the input signal will be less noisy than, say,
feeding a 100 Hz signal into an amplifier that covers
0 to 10 MHz.
Integrating A-D converters reduce noise by
integrating the signal over a period, which is
effectively reducing the bandwidth.
Reducing Environmental Noise
============================
External noise reduction is much more of an art form.
Noise is added because the signal leads act as aerials
picking up environmental electrical activity. Much of
this is common to both signal wires, and a differential
amplifier will remove a lot of this common mode voltage.
Differences between the signal wires (for example if
they are separated rather than twisted together) will
lead to residual voltages being added to the signal,
increasing noise. Keeping the signal wires as short
as possible, twisted together and as far away from
electrical machinery as possible, will help. You can
also shield your signal wires.
Filtering Noise
===============
Filtering can reduces noise errors in the signal.
For most applications a low-pass filter is used. This
allows through the lower frequency components but
attenuates the higher frequencies. The cut-off
frequency must be compatible with the frequencies
present in the actual signal - as opposed to
possible contamination by noise - and the sampling
rate used for the A-D conversion.
A low-pass filter that's used to prevent higher
frequencies, in either the signal or noise, from
introducing distortion into the digitised signal
is known as an anti-aliasing filter. These generally
have a sharper cut-off than the normal low-pass
filter used to condition a signal. Anti-aliasing
filters are specified according to the sampling
rate of the system and there must be one filter
per input signal.
You can read more about filtering in Issue 8
of Monitor.
Summary of Reducing Noise
=========================
- Keep the signal wires short
- Keep the wires away from electrical machinery
- Use twisted pair wires
- Use differential inputs to remove noise common the both wires
- Use an integrating A-D converter to reduce mains frequency interference
- Filter the signal
- Consider shielding the wires
Further Reading
===============
Monitor Issue 8: Filtering - Removing Interference from your Signal,
https://www.windmill.co.uk/filter.html
Monitor Issue 122: Debugging Noisy Measuring Systems,
https://www.windmill.co.uk/monitor122.html
Monitor Issue 155: Differential or Single-Ended Inputs?,
https://www.windmill.co.uk/monitor155.html
Comments
========
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DAQ News Round-up
________________________________________________________
Welcome to our round-up of the data acquisition and
control news. If you would like to receive more
timely DAQ news updates then either follow us on Twitter at
http://twitter.com/#!/DataAcquisition
or grab our RSS newsfeed at
https://www.windmillsoft.com/monitor.xml
NIST Releases Test Framework for Upgrading Smart Electrical Meters
Next-generation 'smart' electrical meters for residential
and commercial buildings will have computerised operating
systems just as laptops or mobile devices do.
Source: NIST
https://www.nist.gov/
Industrial PC Market Driven by Need for Data
According to the Arc Advisory Group, the Industrial PC
(IPC) market continues to grow faster than most other
automation hardware markets as IPC-based automation
solutions replace PLCs in traditional machine control
applications. Growth is also driven by the preference
for IPCs in comparably new and growing areas such as
wind energy.
Source: Arc Advisory Group
http://www.arcweb.com/
ISA Charters New Standards Committee on Intelligent Device Management
The International Society of Automation has
established a new standards committee, ISA108,
Intelligent Device Management. The committee will
define standard templates of best practices and work
processes for design, development, installation and
use of diagnostic and other information provided by
intelligent field devices in the process industries.
Source: International Society of Automation
http://tinyurl.com/c6kwzhx
Hairy sensors to give robots sensitive skin
A coating of hairy, electronic skin could soon help
robots feel the slightest breath of air.
Source: New Scientist
http://www.newscientist.com/
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* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see https://www.windmill.co.uk
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An archive of previous issues is at
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and an index of articles at
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Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 834 6688
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E-mail: monitor@windmillsoft.com
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