Electro-magnetic needle selection is now available on many types of knitting machines; this was first commercially used on circular rib jacquard machines (Fig. 11.11). The electronic impulse that energises an electromagnet is usually assisted by the field of a permanent magnet, and the minute selection movement is then magnified by mechanical means.
If all the needles, or a block of needles, were to be simultaneously selected, each would require its own actuator. It is much cheaper to select the needles at a single selection position in serial formation, using between one and six actuators, although the time interval between each selection impulse is shorter.
Many of the modern electronic selection units are now mono-system, i.e. the selection butt position for each needle is at the same height, so the time interval between each selection impulse is the time between one needle and the next passing the selection position. The selection speed can be as fast as 6000 needles per second. These selection units are very compact and can now be fitted into the dials of
large-diameter circular machines for dial needle selection in addition to cylinder needle selection .
The Moratronic was one of the earliest machines and was first exhibited in 1963 (Fig. 11.12). For each feeder, a photo transistor scans its own track of an endless 35-mm film, giving a selection for each jack control spring as it passes the control position of the feeder. If the position on the film has a transparent spot, light is transmitted to generate an impulse. If the position on the film is opaque, no impulse is generated for that control spring. The impulse is magnified to energise a coil and thus neutralise its permanent magnet at the control position at the precise moment when the jack control spring is guided onto it. The spring is thus not held by the magnet and stands vertically to pass on the far side of a wedge-shaped control cam.
As the cam presses onto the spring, it depresses the jack into a deep recess of the trick so that the jack butt is pushed away from the cylinder raising cam and the needle supported by the jack is not lifted to knit. If no impulse is generated, the control magnet can hold the spring so that it passes in a bent position on the near side of the control cam and is held away from its jack, which stays out of its recess with its butt remaining on the raising cam to lift the needle above to knit.
The film is driven in phase with the needle cylinder to make a selection in
0.5 milli-seconds. Twelve million selections are possible - enough for a full-width selection 1564 pattern rows of three-colour design deep.
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