EUREKA FEBRUARY 2004 COVER FEATURE STORY
Flying food takes off for speed
Novel handling techniques allow the very rapid handling
of food products and other delicate items: Tom Shelley reports
Biscuits need not just be conveyed mechanically, they can be
flown to different destinations on cushions of air.
Fast moving cucumber and tomato slices can be sorted by conveyors
getting out of their way, instead of directing them, and sticky
cherries picked and placed at speed, a task difficult for humans,
let alone machines.
It is not just food items that can be moved around by such means.
The techniques are applicable to anything delicate, and the
research group with the expertise in food handling, can also
handle and monitor animal embryos.
John Reed, who is leader of Automated Handling at the Silsoe
Research Institute, explained to Eureka that while it is possible
to float balls on passive air jets, it is not possible to do the
same for biscuits. The momentum of an air jet is maintained out
to a substantial distance, even though it spreads out. Hence a
biscuit will either fail to be lifted, or if the air pressure is
increased, "Will rise up and hit the ceiling." It is,
however, possible to support biscuits on an air cushion generated
above a layer of porous material. The problem then comes at
conveyor junctions. Left to themselves, the biscuits will
normally continue in their direction of travel. Redirection is
found to be best effected by using two opposing sets of air jets,
one below the junction for lifting, and the other above, aimed
downwards. With just the lower set activated, the biscuit
continues on its original path, but switching on the upper set
blows the biscuit down onto a transverse conveyor. Using this
technology, it has been found possible to convey biscuits at 1m/s
and sort them at 8 biscuits/second. Reed is convinced that by
using faster responding valves, it should be possible to
substantially increase both speeds, without risking damage to the
biscuits.
Handling sliced cucumbers and tomatoes for sandwich making has
been studied in an Advanced Food Machinery LINK project. One of
the problems with these items is that their mechanical behaviour
is governed by water film surface tension. Effective friction
coefficients sliding down a metal chute can vary over a ratio
range of 13:1. Rippled steel can be used to break the surface
tension. Air jets can be used to reject imperfect slices, but the
blown off items are liable to "Go in any direction."
The best method of handling in this instance has been found to be
synchronised conveyors. These can be stopped, started, or moved
aside to allow unwanted items to drop, which is easier than
trying to make them suddenly change direction.
Experimental developments have led to trials of a prototype
system by CKF Systems of Gloucester, which includes a vision
system to identify items, and the ability to pick them up, sort
them, pattern them and place them on the sandwiches. The system
handles 120 slices per minute. Development was undertaken using
stepper motor drives, but the prototype is a six axis servo
system for extra speed. Reed says that he uses steppers for
development work, "Because they are easier to work
with", but recommends servos for high speed commercial
systems.
His proudest and most recent achievement, however, is a system
for picking and placing sticky items. In this words, "The
Bakewell tart cherry has been the demise of many an automation
engineer. The problem is not picking them up, but letting them
go."
The patented final gripper design has a strip of 50 micron
thick food grade polyester wrapped round each finger. For
release, the grasped item remains stationary while the tape
progressively peels away as the fingers are withdrawn. The free
ends of the tape are reeled in by sprung spools, although they
could also be wound in by motors. Either configuration has
potential for handling stacks of pancakes, working with
horizontal fingers. It then works in the same way as whipping a
tablecloth from underneath objects placed on it. A rapid
withdrawal leaves the pancakes where they were, provided the pile
is not too high. The stacks can thus be placed or dropped
vertically onto surfaces or into packs.
Being able to release products without opening the gripper allows
objects to be packed into confined recesses or in close proximity
to each other. The gripper also has a self cleaning capability
that makes it ideal for use in processes that are subject to
strict hygiene constraints.
The same kinds of expertise, although not the same methods, have
been applied to handling pig and shrimp embryos. In this case the
techniques are described as Micro Chamber Technology or MCT.
Manipulation of the embryos, which in the case of the pigs are
0.1mm across, and 0.2mm across in the case of the shrimps is
achieved using pipettes. Funding is by Sygen in conjunction with
a BBSRC research grant.
Design of mechanical handling systems is undertaken with the help
of Cosmos Motion in conjunction with SolidWorks. The team is
currently evaluating MotionWorks. Reed describes the simulations
as being, "On the limit of what can be modelled." Much
of the modelling is funded by a BBSRC Competitive Strategic
Grant.
The gripper development rig is based on a Rexroth robot, using an
SMC cylinder in the gripper itself, with a movement amplifying
mechanism.
Industrial products that are as delicate as biscuits include
'green' powder compacts prior to firing. More than a few
processes involve handling items that are at some point sticky.
Compact packing is a requirement of all industries, allowing the
reduction of shipping costs. And the growth of nanotechnology
means that more and more engineers can expect to asked to find
ways of handling delicate items that are very small.
Silsoe Research Institute
email John Reed at the
Silsoe Research Institute
email Simon Miles at the
Silsoe Research Institute
"The bakewell tart cherry has been the demise of
many an automation engineer. The problem is not picking them up,
but letting them go."
Eureka says: Improved high speed handling of
delicate items, particularly food items is the goal of many
automation engineers, especially as it reduces both damage and
cost incurred by using low wage human labour.
Pointers
* Biscuits can be flown on air cushions at 1m/s and sorted at 8
biscuits/second
* Cucumber and tomato slices can be handled by techniques that
take account of variable water film stickiness
* Sticky items can be reliably picked up and released by using
film that is peeled back from the item being grasped
For more technical
developments see www.eurekamagazine.co.uk
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