To take pre-cut pieces of tin plate 1m x 0.5m from a stack and pass them through a printing machine, into a pair of lacquer rollers and finally into the wickets of a lacquer curing oven.

The positioning of the sheets are critical throughout the process to ensure that no plates overlap and the spacing is kept constant. At the final transfer point between the "Infeed Conveyor" and The "Oven", the timing is critical as the sheets must leave the "Infeed Conveyor" at just te right time to enter the next pair of wickets without touching the sides.

The four main drives needed to drive the machine, range in power from 4 to 11Kw, with the whole system to operate in a high ambient temperature on  a 24 hour shift system, so that reliability was prerequisite.

How do you control these standard 3 phase motors?

The Solution

Precise speed control of all the motors is required. The "Printer" and The "Coater" loads are cyclic and high inertia. The "Oven" load is low inertia but high level of stiction.

A high performance Flux Vector Hitachi SJ300 Inverter Drive was used to control the speed of each motor Encoder feedback was utilised to further improve the speed holding particularly at low speeds where the cyclic load posed a problem. Dynamic braking using brake resistors ensured no overshoot when accelerating and changing speed.

To meet the safety requirements each motor was fitted with a failsafe brake, controlled by a volt free contact on the inverter and the safety circuit. These brakes were used as holding and Emergency stopping only to wear and maintenance to the minimum.

The precise positional control was achieved using 500ppr encoders on each drive and a digital synchronising card. This card compared the signals from each encoder and produced a correction signal, which was applied to the "Line speed" signal for each inverter.

Initial alignment when first starting the line was achieved using marker pulses on each encoder (1ppr) One revolution of the encoder was equal to one sheet of tin ands each marker pulse represented the leading edge.

After initial alignment of each drive the facility was provided to "Advance" or "Retard" each drive positionally using two push buttons. This was particularly important when transferring the sheets of tin from the "Infeed Conveyor" into the wickets of the "Oven" where 1 degree error could cause a crash.

This system is now operating successfully on 20 lacquer lines on various sites across the UK

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