We have made 89c52 based custom plc to control wire cutting m/c. The
machine cuts, strips the pvc insulated copper wires (electrical wires).

wire dia. = 0.5 - 4.0 sq.mm.
wire length = 100 - 9000 mm

The machine:

The machine uses 3 stepper motors for feeding, cutting & pulling. { The
feeding and pulling motions are rotary motions. The rpm is typically 1500
at shaft end, and 2500 at motor end.The cutting motion is oscillatory
(clock-anticlock). It is then mechanically converted through rack-pinion to
a linear up-down motion}.

The wire passes through 2 feed rollers, 1 guide tube, 2 cutting blades & 2
pull rollers.

All these parts are made from mild steel, they are hard-chrome plated for
rust prevention. Blades are alloy steel.The machine body is properly earthed
using 2.5 sq.mm wire.
The earthing wiring is tree structure.

The PLC:

The PLC consists of 1 control unit (4 X 89c52). Man-machine-interface(MMI)
connected to PLC using a 2.5 mtr. 6-core shielded cable. The power supply
for the PLC(24V), the MMI(24V) & all I/O's external to PLC(24V) are
galvanically isolated (optically isolated).

The MMI controller is AT89C55WD.

The drives:

The 60V stepper drives are galvanically isolated from PLC. The 0V(GND) is
earthed.

Problem:

1. The machine runs troublefree, if runs without wires.
2. The 1.0 sq.mm wire is loaded & cutting length > 500mm (typically).

now the problem starts,

the machine hangs after cutting few wires. The stepper motors remain
enabled. All the switches are disabled. We have to switch off & on the power to the PLC to start again.

Observations:

1. If the second end of the wire is earthed, the problem of hanging
reduces.
2. If the PLC to MMI cable length is < 1mtr., the problem of hanging
reduces.
3. In dry & cold air the problem is more.
4. The problem depends on the type, diameter & set length of the wire.
(bigger dia, more length and certain insulations give more problems).

Please suggest a solution.


The input power is 240 V ac, 50 hz. Mostly through constant voltage
transformer or servo stabiliser.

It is then stepped down to 60V, 12V and 16V for the drives through
isolation transformers. The transformers are shielded and earthed.

The power supply to PLC, MMI and I/Os is through : ELcom make EMI/RFI
suppressor and then through step down transformer and then through D2D
convertor.