Switching from high-maintenance encoders to optical measuring sensors
has brought accurate and trouble-free operations to the UK's largest
manufacturer of fine-coated papers
The New Thames Mill in Kemsley, Kent is the UK's largest manufacturing
plant for fine-coated papers. The mill's largest papermaking machine is a
PM6, which produces up to 600 tonnes of product per hour, equivalent to
some 300 'jumbo' reels. These are cut into smaller reels, of more
manageable size, each weighing around three tonnes and transferred to an
intermediate warehouse for storage. Those destined for further processing
are moved automatically into a shunt area to await placement at a
pre-allocated location within the store. The reels are moved by an
overhead gantry crane, which lowers a large suction pad on to them via a
hoist.
Precise control of the gantry crane is essential to ensure correct
placement of the three-tonne reels. Hitherto, position information was
provided by a stainless steel linear encoder, which provided position
pulse signals from 2mm slots cut into the strip. However, this technique
proved incompatible with the warehouse environment and the slots were
subject to frequent clogging, causing disruption to the warehousing
operation. Indeed, it was not uncommon for the crane to be halted two to
three times per eight-hour shift, in order to clean debris from the
encoder slots.
Mill operator, M-Real consulted with its control system software supplier
who recommended an alternative optical measuring system (OMS) from sensor
specialist, Leuze Mayser. The OMS1 system transmits visible red laser
light towards a reflector, fixed to the gantry crane. By measuring the
phase shift between the modulated transmitted light and the reflected
light, the system is able to calculate the crane's position.
A novel solution was also found to control the vacuum grab-hoist
position, by allowing the transmitted signal from a second OMS1 unit to
be deflected through 90o from a mirror mounted on the main gantry to
another reflector mounted on the hoist. The returned light follows the
same path and, again, the phase shift allows the distance to be
calculated. As both sensor units are mounted directly above each other,
automatic subtraction of the main gantry distance from that of the hoist,
shows the precise distance that the hoist has travelled. Both distance
measurements are used to control the exact location of each reel.
The distance measuring sensors supply an absolute value, so no
referencing is required after switching on the system, and will operate
over distances of up to 200m. The method used to calculate distance
guarantees millimetre accuracy with an integration time of less than 2ms
and reproducibility of +/-2mm. Even in this application, the optics
require little maintenance, which can be carried out on a pre-planned
basis, thus avoiding unscheduled stoppages.