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Operation
of industrial checkweighers
Dynamic checkweighing systems have been
used in industry for over 20 years. Technological advancement in
that time has focused on accuracy, speed of operation, user friendliness
and data logging and control. Today's checkweighers offer easier
programming, greater control and integration into data networks
and easy to read colour screen options.
With a host of extra's like variable speed, servo control, twin
infeed, scroll indexing, network capability and smart card it's easy to feel
bewildered. Here we try to offer a
simple overview of what you can and should expect from dynamic
weighing systems. Typical industries covered are:
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Food industry |
Medical industry |
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Chemical industry |
Cosmetics industry |
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Printing industries |
Pharmaceutical industry |
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Checkweigher
technology
LVDT Systems
The original
technology that provided the weight measurement at the heart
of checkweighers.
When an AC excitation signal
is applied to the Primary Coil (P), voltages are induced in
the two Secondary Coils (S). The MAGNETIC CORE inside the
COIL WINDING ASSEMBLY provides the magnetic flux path
linking the Primary and secondary Coils. Since the two
voltages are of opposite polarity, the Secondary Coils are
connected series opposing in the centre, or Null Position.
The output voltages are equal and opposite in polarity and,
therefore, the output voltage is zero. (Thanks to
Collins Technologies the leading LVDT
manufacturer).
There are many of
systems still in reliable operation today, the main limiting
factor is the low accuracy and relatively slow speeds less
than 80 packs per minute.
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Load cell-Strain gauge systems
Each strain
gauge consists of one or more fine wires cemented to the
surface of a beam, ring, or column (the strain element)
within a load cell. As the surface to which the gauge is
attached becomes strained, the wires stretch or compress
changing their resistance proportional to the applied load.
One or more strain gauges are used in the making of a load
cell.
Strain gauges
have replaced LVDT weigh cells in most new equipment due to
higher reliability and accuracy. |
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MFR- Magnetic Force Restoration systems
Electro-magnetic force, or
force restoration, is the name given to the system in which
a delicate mechanical structure is maintained in equilibrium
by an electromagnetic field acting against the gravitational
force applied by the mass to be weighed. The current
required to maintain this equilibrium provides a very fine
measure of the mass itself. This is a far more precise form
of measurement and is the system most commonly used in
quality dynamic weighing machines with much higher speeds
and accuracy than conventional systems.
The latest
technology employed offering much higher pack rates coupled
with high sensitivity. Pack speeds in excess of 400 packs
per minute with an accuracy of less than a gram are possible
with this technology.
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Common
terminology:
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Auto zero |
The checkweigher performs an automatic zero to offset any
debris that may be accumulated on the platform. Auto zero
uses trend or single mode of operation. Care should taken
that checkweighers are equipped with this function and that
it is operational. |
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Average weights legislation |
Also known as European weights legislation, the 1 pack in 40
rule, items with the "e" mark have been weighed to this
standard. |
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Calibrate |
Some checkweighers operate as comparator systems which
compare the actual weight of the inspected pack to a known
calibration pack. Normally a checkweigher would require the
same known weight pack passing 5 times to perform a
calibrate. MFR checkweighers do not need to be re-calibrated
once commissioned. |
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Combo unit |
A
combined metal detector and checkweigher operated through
one integrated control panel, has the advantage of reduced
line length requirements and integrated reporting. |
| Fault
monitoring |
Checks the
internal voltages to ensure correct operation. |
| Low air
detect |
Monitors the
air line to ensure there is sufficient pressure to remove
the non conforming product. |
| LVDT |
The LVDT
(Linear Variable Differential Transformer) is a device that
produces an electrical voltage proportional to the
displacement of a movable Magnetic Core. Now approaching
obsolescence having been superseded by strain gauge and
later by MFR technology. |
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MFR |
Magnetic Force Restoration - latest loadcell technology |
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Pack Rate |
PPM - packs per minute - shows the maximum weight
measurement per minute - it is directly proportional to the
weight and length of the product measured. |
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Pack Pitch |
The space in between each pack presented to the checkweigher
only one pack can be on the PWT at any one time. It should
be noted that instantaneous pack rate can be exceeded when
presenting products with close pack pitch. |
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PWT |
Powered Weigh Table - the moving section of the checkweigher
containing the weighing device. |
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Reject |
The method of removal of the rejected product from the
product flow includes: side pneumatic pusher, pneumatic air
blast, drop flap and pneumatic plough. |
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We have many
years experience of practical metal detecting systems
experience. Should you have any specific questions relating to
metal detection please do not hesitate to contact us by your
most convenient medium. |
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TEL: 01274 663311
FAX: 01274 665511 |
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