Windmill Software Ltd
Data Acquisition Intelligence

Measuring Strain
December 1999

Issue 17: How to use a PC to Measure Strain

CONTENTS

Windmill News | Measuring Strain | Strain Gauges | Computerising Strain Measurements | Strain Measurement in Action


WINDMILL NEWS: MORE SUPPORT FOR SARTORIUS BALANCE USERS

You can now link Sartorius equipment to the Windmill logging, charting and control software. For Sartorius users who have previously downloaded Windmill, and don't want to do so again, we've provided a small .dll file to upgrade your system. Click labimdll.zip to download, then unzip the file - using PKZip, WinZip or similar - into your Windmill folder.

To further aid Sartorius balance users, we've added their configuration files to our library. With these you can be instantly up and running, with no need to fill in any dialogue boxes. Visit the library and download the set-up files.

As well as Sartorius balances, Windmill works with a vast range of other hardware. If you would like a free copy of Windmill subscribe to the Monitor newsletter.


HOW TO USE A PC TO MEASURE STRAIN

In 1856, Lord Kelvin's experiments revealed that exposing a metal conductor to a mechanical force affects its resistance. Around 80 years later, this discovery was put into practice in the development of strain gauges. Today, we still employ strain gauges, and using computers can measure faster, more accurately and in previously unattainable situations.

For example, a manufacturer of earth moving equipment wants to test that its mechanical digger can withstand being driven at high speeds over rocks and holes in the terrain. Strain gauges can be mounted on the loaded digger arm and continuously monitored by computerised equipment in the driver's cab. More about this later, but first a little about the strain gauges themselves.

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How do the Strain Gauges Work?

When a force is applied to a structure, the length of the structure changes. Strain measures this change of length. The strain gauge is glued to the structure being tested, so any distortion of the structure will also cause a distortion of the strain gauge. As the strain gauge contains conducting material - often a pattern of metal foil - the distortion results in a change in the resistance of the gauge. By measuring the change in resistance we can measure the strain.

Easily said - but how is resistance to be measured? Generally through a "Wheatstone bridge" arrangement. This has 4 arms, arranged in a square. Each arm generally contains either a resistor of known resistance, or a strain gauge - strain gauges will occupy 1, 2 or 4 of the arms.

Taking the case of 1 strain gauge in the Wheatstone bridge - a quarter bridge arrangement. Power lines are connected to opposite corners of the bridge, lets call them A and C, providing an excitation voltage. The measurement is made across the other corners - B and D - of the bridge. If the resistance of the strain gauge changes, the current across the bridge will change (according to Ohms law, V=IR). If we measure this change we gain a measure of the strain. With computerised systems the change in voltage across the bridge is usually measured, rather than the current.

This of course assumes nothing else beside a dimensional change is causing the resistance to alter (temperature for example) and that the excitation voltage remains constant. Luckily such potential error sources can be compensated for.

In some applications, the sensitivity of the system can be increased by using "half" or "full" bridges. For example, imagine a tube sticking out of a wall. A downward force is applied to the end of the tube, compressing its underside and stretching (tensioning) its upperside. The compressive and tensile strain measurements are equal. Using a strain gauge on either side (a half bridge) increases the sensitivity of the measurement, and automatically compensates for changes due to temperature. Similarly, mounting two gauges in compression and two in tension makes a full bridge which again increases the sensitivity.

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Computerising Strain Measurements

You don't need a computer to measure strain, but you can speed up measurements, measure continuously, increase accuracy, eliminate transcription errors, automate testing, generate health and safety records, minimise human error and so on.

A computerised system measures the "bridge output voltage" before the strain is applied, then at regular intervals or whenever certain conditions apply.

8 Points to Look for in a Computerised System

  1. As you need to measure the voltage across the bridge, you need two leads for each measurement. Your data acquisition interface should therefore accept differential inputs.
  2. You need to apply an excitation voltage to the bridge. Some hardware provides this excitation, but is limited in the number of strain gauges it can handle. Other hardware needs an external power supply, but will distribute the excitation voltage to many more strain gauges.
  3. We're assuming that the measured change in voltage is only due to the change in strain. However, other factors might also affect the reading, such as a change in the excitation voltage. This is of especial concern in long-term measurements where component values may drift with time and temperature. However, with suitable hardware and software like Windmill, a computerised system can measure the excitation at any time and then adjust the strain results for changes in the excitation voltage.
  4. The changes in voltage due to strain are tiny: measured in microvolts. Monitoring such small signal changes can often produce jitter in the readings from noise. For slow sampling you can counteract this with an integrating analogue-to-digital converter. By averaging the signal over 50 or 60 Hz the noise is rejected. (For a discussion of integrating A-D converters see Issue 4 of Monitor.)
  5. The initial unstrained bridge measurement is often much larger than the change in signal due to strain. If this initial value is not accounted for, it restricts the resolution you can obtain - i.e. the smallest signal you can measure. A computerised system can zero or balance the bridge, nulling the offset of the initial voltage. So instead of measuring over a wide range, 0 to 0.5 V for example, you can choose a narrow range such as 0 to 0.01 V. (Each range is divided into a fixed number of steps. The smaller the range, the more precise the reading. For more information on resolution and ranges see Issue 3 of Monitor.)
  6. If your system is not capable of zeroing gauges, ensure that your A-D converter has high enough resolution to give the dynamic signal range required. Even if you can zero the gauges, in some applications a high resolution converter might still be needed (16- or 18-bit for example).
  7. For high speed sampling of strain, it's useful if your data acquisition hardware has RAM storage of gain (range) and balance voltage for each bridge.
  8. When using quarter bridges (1 strain gauge) or half bridges (2 strain gauges) your hardware will typically provide high quality termination resistors. This makes it easy to permanently wire your signals to the hardware.

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Computerised Strain Measurement in Action

Earlier we gave the example of testing strain on an earth-moving digger. A computerised system is essential in this case - but the test environment causes several problems which need to be overcome. To withstand being shaken in the cab, the data acquisition hardware is mounted on shock-absorbing legs in a vibration-proof housing. The controlling computer is safe in an office 100 metres away, connected to the equipment by an Ethernet RF link. The digger drives at high speed over a test area, with bumps, steps and holes. 64 stain gauges are mounted on the digger's arm and measurements taken 50 000 times per second. The computer operator starts and stops the scans of strain gauges by pressing keys on the keyboard, with a pre-defined maximum number of scans. The software controls the zeroing of all bridges simultaneously, before data collection. In this case our Windmill Streamer software was used. To download a free demo of Streamer visit http://www.windmill.co.uk/streamer.html.

For more information on computerised strain measurement please contact Windmill Software.

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Further Reading


Our apologies for the lack of a Visual Basic corner this month. It returns next year with a discussion of how to use controls on a web page to send events to your Visual Basic application. Meanwhile, a very prosperous new year to you all and many thanks for subscribing to Monitor.

Do you have a question, comment or suggestion on this newsletter? E-mail the editor - Jill - at [email protected]

Copyright 1999 Windmill Software Ltd. All rights reserved. You may freely forward this newsletter in its entirety, but please don't reproduce individual sections without the prior written agreement of Windmill Software Ltd.


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