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Theatre Application for New Non-Contact Tension Measurement Device
This development was prompted by a customer asking for a non-contact method of determining the tension in reinforcing cables and wire for pre-stressed concrete beams. Peter Lewis, the managing director of Applied Measurements investigated the dynamics of the measurement and discovered that a vibrating steel cable or wire would have on an externally applied transverse magnetic field.
After significant development the 'Digiforce' was born and the patented device has now been supplied to many concrete manufacturers' including RMC, Concrete, Tarmac, Spiroll Precast, Coltman Precast, Milbank in the UK, Burns in the USA and Impact in Australia - and other blue chip construction companies all who share the need to make rapid and accurate tension measurements on-site.
Following the success of the Digiforce in pre-stressed concrete, other applications are now being realised. One such application is within the theatre industry, where the product is used to monitor the tension on various cables and wired elements used to support stage scenery.
Some of these cables support very large loads and the stage engineers need confidence that the safe working load of cables is not being exceded. Prior to this the process of load measurement on these cables was very time-consuming and was not easy to log many readings with any great accuracy or confidence, or compare to earlier measurements taken.
Within the concrete industry especially, the equipment used for the tensioning process must be tested on a regular basis and accuracy of the measurement is in part dependent on the skill of the operator. The Digiforce has enabled users to achieve rapid, repeatable, non-contact tension measurement using cutting-edge technology.
Integral to the durability and structural performance of these concrete products is the correct tensioning of the pre-stressing steel tendons. There are approximately 30 tendons in the average 1200mm-wide hollow-core floor slab, which may be up to 160m in length. By striking the wire or cable under tension and observing their fundamental natural frequency of vibration, it has proved possible to develop a battery powered hand-held instrument capable of accurately determining the tensile forces applied to them.
Although simple in theory, the low frequencies - Hz to a few tens of Hz and complex sound spectra have up to now, prevented their accurate analysis on-site. The Digiforce addresses this issue via the integral mathematical software, to process only those frequencies of interest that contributes to the measurement. This means that the user no longer has to refer to look-up charts or return to the office to run complex calculations pertinent to that piece of equipment or installation.
The instrument is first programmed with the length of the wire or cable whose tension is to be measured. Also inputted is its weight per unit length. The magnetic sensor at the end of the instrument is then placed adjacent to the part to be measured. The measurement process is triggered and the part is struck lightly with the aluminium striker supplied. After a few moments the tension is displayed on the LCD display of the instrument.
Tensions in stainless steel or other non-ferromagnetic wires can be measured by attaching clips (supplied), made of ferromagnetic materials, provided that their masses are insignificant compared to that of the cable to be measured.
The vibrations being analysed are complex and the fundamental frequencies that the method uses are in the range 3Hz to 70Hz and not the metallic note heard by the ear. The tension is determined from the formula, T = 4 x length2 x Hz2 x weight per unit length. Because of the very low frequencies, tuned circuit filter methods are not an option. Instead, the device makes an initial 20 measurements of each oscillation and discards these. The next five measurements from 10 oscillations are then used to establish a tolerance band to establish which measurements are reasonable to contribute to the calculation and which should be rejected as spurious. Subsequent measurements are then made and those that fall within the tolerance band are used to calculate the final measurement value.
Because the principal business of Applied Measurements is in design and manufacture of precision load cells and force sensors, the measurements are traceable back to NPL calibration standards.
The Digiforce boasts an on-board data-logging facility for collecting results and an RS232 port / USB converter for later downloads to a host PC, driver software, aluminium striker, rugged carrying case and full instructions. Additionally the device is supplied with a three-year warranty.
18th March 2010
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