Sensor hones in on sharp operators.
Risk is being driven out of the use of heavy duty diamond
cutting tools thanks to a Euro 800,000 EU-backed research
programme at Greenwich University in London, to determine
the true operating characteristics of such equipment.
At the heart of this vital work is a high tech torque sensor from Sensor
Technology in Banbury, which proved to be the only effective way of
measuring the true toque in the rotating saw blades and drill bits.
Previous research has simply estimated the torque by calculation from the
drive motors’ current requirements and consumption. This was known to be
highly inaccurate, because every motor has its own unique signature of
characteristics, which can have a significant effect on actual performance
in highly demanding applications, such as drilling 1000mm bore holes
through reinforced concrete, or sawing through the barnacle encrusted
steel of sub-sea structures.
In order to accurately profile the performance of each tool, it is necessary
to measure the torque of the blade or cutting tip as it rotates; and for
this a non-contact sensor is required. The Greenwich team considered
several options around which to build their test rig, and the clear winner
was the Torqsense unit from Sensor Technology.
This is a transducer that requires no physical contact between the sensor
mounted on the high speed tool and its adjacent static pick-up, instead
using Rayleigh Waves or surface acoustic waves (SAWs).
Such waves were originally identified by Lord Rayleigh as being the
destructive force in front of earthquakes, but now Sensor Technology is
able to harness their power to measure torque.
To do this they first create waves by passing an alternating voltage across
the terminals of two interleaved comb-shaped arrays, laid onto one end of a
piezoelectric substrate. A receiving array at the other end of the
transducer converts the wave into an electric signal, the frequency of which
is dependant upon the spacing of the teeth. Tension in the transducer
reduces the operating frequency while compression increases it.
Thus to measure torque, two sensors are bonded to a shaft at 45deg to the
axis of rotation. When the shaft is subjected to torque, a signal is
produced which is transmitted to a stationary pick up via a capacitive
couple comprising two discs, one of which rotates with the shaft, the other
being static.
For this project the sensor had to be mounted so that it could move linearly
to accommodate the cutting progress of the tool. This proved quite difficult
at first and lead to some inaccurate data, but careful tuning and damping of
the electronic noise along with an accurate mechanism to allow the sensor to
shadow the progression of the transducer was developed and the research
could continue.
Until about 10 years ago, the existing research into heavy duty tools was
adequate for the needs of the day. But in recent times more demanding
operating standards will become legal requirements, while simultaneously the
number of poorer quality tools imported from outside the European Union has
risen sharply. (Often these masquerade under the livery of brands recognised
for their quality, but are in fact made by unscrupulous ‘copycat’
manufacturers.)
In order to advance the state of the art it was necessary to build a
database of the torque, power and other characteristics by the actual tools
themselves under a great variety of operating conditions. This could then be
used to profile the tools’ performance and thus allow accurate forecasting
of their performance in the field. In order to log the performance data as
accurately as possible it was fundamentally important to measure the true
torque at the tools’ shaft rather than to estimate it from a more convenient
measurement at the motor. The SAW techniques offered through Torqsense has
proven to be the only effective way to do this, and in fact have proven to
be remarkably simple and straightforward to implement.
“The level of danger to operators was becoming untenable as the number of
‘rip off’ tools grew,” says Dr Alec D Coutroubis, who is leading the
Greenwich project and who has been awarded his doctorate for the fundamental
research involved. “Also the European tool manufacturers were seeing their
business severely undermined by the imports, which otherwise look absolutely
identical to quality tools.”
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For more information, please contact :-
Sensor Technology Ltd.
Tel: +44(0)1295 730746 Fax: +44(0)1295 738966
Email: info@sensors.co.uk
Website: www.sensors.co.uk
October 2003
