Monday 31 May 2010

Geotechnical mwasurements

Simplified Linear Measuring System Uses Displacement Transducers
The ability to measure linear displacement over an extended range means access to new applications and industries for Applied Measurements. With the existing AML-SGD products starting as low as 0 - 5mm and measuring up to 50mm over four models, the latest introduction measuring 0-100mm is the result of increased demand from customers preferring to use a strain gauge based measuring system.

Based on a four-arm Wheatstone-Bridge design with a nominal impedance of 350 Ohms, the design ensures excellent non-linearity, low power consumption and excellent temperature stability. Additionally, the characteristics of strain gauge based sensors provide higher accuracy linear sensing than that of less accurate and lower resolution LVDT sensors.

Because these sensors operate at similar voltage levels and produce output signals compatible with other commonly used strain gauge sensors such as load cells and pressure transducers, they can easily form part of the same measurement chain in one central instrumentation system.

These displacement sensors operate on low cost DC strain indicators or digital voltmeters, whereas standard LVDT transducers require conversion of the signals to achieve the same level of compatibility. The simplicity of this type of measurement requiring no further processing of the basic signal, can make this type of test or measurement significantly more attractive to a user.

Additionally, strain gauge based displacement transducers exhibit typical non-linearity errors of a quarter of that of typical LVDT’s. As with other strain gauge devices, the resolution is infinite and strain sensitivity is constant with a stabilised input voltage between 1.5Vdc and 15 Vdc.

These advantages are realised by Fugro Offshore Geotechnics, who use these sensors within their accredited soil testing laboratories. It is within these high-tech establishments where sub-sea soil samples are analysed and the sensors are used (together with submersible load cells, also from Applied Measurements) to monitor compression of the sample relative to the force applied.

As Fugro Senior Lab Manager Phil Robinson says, "We test soil samples from destinations all over the world and much time and considerable effort is spent retrieving them from sub-sea locations. By the time we receive them, they are worth a great deal of money and cannot be replaced. The tests can run for only a few hours or for several weeks and as we can only ever run a test once, we need confidence in our testing equipment and that the results can be relied upon."

The range of AML-SGD displacement sensors are precision assemblies contained within a rugged stainless steel outer casing. The internal construction of the AML-SGD transducer ensures a smooth, virtually frictionless movement of the inner core, resulting in high overall accuracy of <+/- 0.1%.

The return force of the spindle is maintained at a minimum level for effective operation when installed upside-down, without having to attach the spindle to the subject. This makes the sensor less intrusive to the application when measuring on soft or pliable materials. The sensors’ are designed for long life and excellent mechanical reliability, hence the 3-year manufacturing warranty offered. This is evidenced by existing applications where these transducers have passed 14 million cycles in normal operation. And unlike some LVDT transducers, the AML-SGD product is completely serviceable by the manufacturer in the event of damage in use.

Any transducer device would be quite useless without means of providing appropriate supply voltage and output signal and this is often a significant part of any data acquisition programme. The AML/SGD range of displacement sensors ensure these costs remain at a minimum, by utilising strain gauge signal inputs of existing data acquisition equipment. In this way, these displacement sensors limit expenditure to the cost of the sensor only, thus eliminating further capital outlay on dedicated instrumentation.

However these advantages would be of no use if this combination of user benefits meant extended lead times. As such it is the policy of Applied Measurements to keep most standard ranges in stock, meaning that the payback period is shorter and the benefits can be realised sooner.

Compact, rugged and easy-to-install, AML/SGD displacement sensors are used by industries such as R&D, machine builders, automotive, marine and geotechnical. The total cost of ownership of instrumentation is now more relevant than ever for these industries and improvements in the versatility of sensors that allow their use with simple DC signal conditioning is but one method of spreading the life costs.

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