Banishing those wire knitter jitters
Knitting wire with a diameter of just 0.003in calls for some rather
special control techniques. Anything less, and you'll be forever
re-threading the machine. In this example, we find out how a low-power ac
drive has been used to control the process
Knitting with wire is not the easiest of machine processes. When you are
dealing with metal threads as thin as 0.003in, the slightest snatch on
acceleration or delay on soft stopping will result in a breakage, and
that means taking the machine down while it is laboriously re-threaded.
Leicestershire based Tritex Design & Development builds machines that
knit fine wire into circular tubes measuring anything from 0.2 to 30
inches in diameter. These knitted tubes end up in a variety of products,
including reusable filters, exhaust seals and chemical de-mister pads -
even domestic pot scourers!.
The machine must stop without snapping the materials, explains Tritex
director, Kevin Roberts. If there is a breakage, the machine must stop
rapidly to prevent the loose end entering the main part of the web; and
should a hole be created in the knitted mesh, it's a matter of
re-threading and restarting.
The smoothness of machine operation is only as good as the drives that
control the motors, and as far as Tritex is concerned, Yaskawa's VS Mini
J7 single phase inverter meets all their expectations. On set-up, the
J7's jog function is used to increment the machine slowly for threading
at 5Hz. The knitting head speeds vary according to the diameter of the
knitted tube, from almost zero speed up to 2,000rpm. The product line
rate also affects the knitting head speed, as does the type of material
being knitted, which, for Tritex, can be anything from stainless steel to
platinum.
The 0.4kW Mini J7 inverter chosen for this application has a very small
footprint, yet it is able to deliver 150% torque at 3Hz or less. A
high-speed current limiting function reduces the incidence of
over-current trips as well as providing inrush current protection.