Hands-on sensing01 August 2006

A novel vibration dose meter is being developed that can be worn on the hand, in order to measure exposure to damaging hand-arm vibrations from machinery. In its present form, it can advise of excessive risk levels and store information for download at the same time that it is being recharged.

Because it is small, compact, durable and inexpensive, there is already significant interest in developing it further for use in wireless systems for monitoring process machinery, equipment in telephone exchanges and buildings.

The HAVSence White Finger Dosimeter is the combined brainchild of the HAVS Co team consultant Daniel Parman; David Mawdsley, managing director of Laplace Instruments, Cromer; and Dr Mark Harper and Dr Martin Thompson of data logging experts Mecon, Cambridge.

Recent progress has been made on the HAVTEC database run by OPERC - as reported in our last issue (p4) - and this has advanced the research into hand arm vibration.

This work apart, until now both employers and HSE inspectors have had to rely on guesses and estimates deduced from measurements made under laboratory conditions.
The EU directive presently states that the ideal would be to measure the vibration entering the fingers, but suggests that, as this is not possible with currently available commercial equipment, the measurements should take place on the machine as close as possible to the hand.

Data so obtained is very unsatisfactory, especially since tests are usually conducted on new or recently maintained pieces of equipment, while that encountered in the field may be in need of maintenance and running 'rough', with rotating loads out of balance and other problems.

The new system has two accelerometers, which allows it to measure in X, Y and Z axes, a logic array, a rechargeable battery and various other surface mount components. It is small enough that, in its encapsulated enclosure, it is normally worn between the second and third fingers of the hand, held by a tee piece that sits below the fingers in contact with the load and with a miniature electronic package above the fingers. As it is in direct contact with the hand and vibration source, it provides an accurate measure of human exposure and is expected to assist greatly in ensuring conformance to the new European Union regulations described in Plant Engineer in the November-December 2005 issue. An LED flashes once per second to indicate that the device is working. After half the allowed daily vibration dose has been accumulated, it double-flashes every second. When the full allowance has been reached, the LED stays on permanently and all work should cease for the remainder of that day.

The docking station, developed by Laplace Instruments, can accommodate five dosimeters units to download data and recharge them through contacts on the outside, so there are no connectors to become dirty or damaged. It connects to a PC via a USB port.

The devices are in the prototype demonstrator phase and there are seven complete systems of dosimeters and docking stations out in the field under test. Industrial design is being undertaken in consultation with Zeller Design in Kettering in order to optimise the ergonomics, and final shape and configuration. It might eventually be possible to reduce the size of the device considerably, but the battery, which occupies half the board space in the present device, sets a limit. A single battery charge is sufficient to power a dosimeter for 16 hours.

The system is expected to come to market as a commercial product towards the end of 2006. The final price will depend on manufacturing and marketing arrangements, but it is expected that a complete system of base station and five dosimeters will sell for around 10,000 euros.

Daniel Parman of HAVS Co describes the results of measurements taken so far as "surprising". The measured levels closely confirm those deduced from laboratory experiments. Tests have been conducted on a hammer drill, a grinder and on five different machines, including lawnmowers, a lawn strimmer and hedge trimmers, in conjunction with a county council team in Barking. Tests have also been conducted on a cyclist riding on a bumpy, cobbled road.

However, when the dosimeters are used on equipment away from laboratory conditions, they show, says Parman, that "vibration levels found to be experienced by workers in the field are much higher than expected" and that some equipment needs to be serviced far more often than it usually is.

Although the system has been developed by a small company and is intended to be used in a very specific way, its low price and small size has already attracted interest from a major telecommunications company among others.

They are interested in using the technology to monitor vibration in buildings and structures, because it is seen to be much cheaper than competing industrial systems. The telecoms company, and other interested parties, are interested in developing it to communicate data by wireless in real time, without having to wait for it to be attached to a docking station. There is also a great deal of interest in using it to monitor process machinery.

The extra power demands of wireless communication could be met by techniques already established in remote data logging, which include sleep periods at very low currents, unless significant events are detected.

SOE

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