GB2091002A - Annealing metal wires - Google Patents

Abstract

In a continuous annealer the wire (1) passes over a pulley (3) to which is applied a voltage V and over a pulley (4) earthed via a fixed resistor (6). The resultant current in the wire heats it to the annealing temperature. A second fixed resistor (8) is in series with a variable resistor (7) set to produce a reference voltage. If the voltage drop across the two fixed resistors differs, an error signal is developed by a circuit connected to terminals (9 and 10) and used to change the level of power supplied to the annealer and thus stabilize the temperature of the wire. <IMAGE>

Description

SPECIFICATION Annealing metal wires This invention relates to apparatus for annealing metal wires and, in particular to continuous annealers which function by passing an electric current through a section of the wire whilst it is moving, thus causing the wire temperature to rise to a point where annealing takes place. By virtue of the fact that the wire is moving, each part of the wire will be raised to the annealing temperature and a continuous output of annealed wire will result.

Most wire materials used on this type of annealer have increasing electrical resistance with increasing temperature and as the degree of anneal imparted to a wire is a function of temperature it follows that, for specific instances, the resistance of the portion of the wire which is being heated will be an accurate indication of the degree of anneal being imparted to the wire.

According to the invention in its broadest aspect, there is provided a method of controlling a continuous annealer for metal wire in which the resistance value of the wire at a predetermined point is measured as a representation of wire temperature by obtaining a ratio between the current value through the wire and an associated voltage drop, this resistance value is compared with a fixed resistance value and any difference between them is applied as an error signal to change the level of power supplied to the annealer.

Preferably, the voltage is measured across a section of wire by contacts which do not carry the annealing current.

According to a further aspect of the invention there is provided apparatus for controlling a continuous annealer for metal wire comprising means for measuring the resistance value of the wire at a predetermined point as a representation of wire temperature, means for comparing this value with a fixed resistance value, means for deriving an error signal from such comparison and means for using the error signal to change the level of power supplied to the annealer.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings of which: Fig. 1 is a general diagram of the annealer, and Fig. 2 is a schematic diagram of the annealer control circuit.

Referring firstly to Fig. 1 , the wire 1, moving in the direction of the arrowheads, passes from an earthed contact wheel 2 to a power supply contact wheel 3 then, by way of two insulated pulleys 5, to a second earthed contact wheel 4.

The insulated pulleys 5 enable the length of wire between contact wheels 3 and 4 to be greater than that between contact wheels 2 and 3. This has the effect of producing the greatest temperature rise between wheels 2 and 3 and aids the annealing. As the temperature change between wheels 3 and 4 is smaller and the exit point is from wheel 4, it is convenient to measure the current passing to earth via wheel 4 as this gives greater sensitivity.

The aforementioned current passes through a meter shunt 6 and thus generates a small voltage.

A fixed resistor 8 is chosen in value to produce the same voltage as that generated across the shunt 6 when a variable resistor 7 is set to a value proportionate to the supply voltage fed to wheel 3.

Thus when the wire resistance between wheels 3 and 4 corresponds to the resistance setting of the control variable resistance 7, there is no potential difference between terminals 9 and 10 and if the wire resistance is different from the preset value an error signal appears between terminals 9 and 10 which can be used to change the applied voltage to reduce the error signal. This can be done by conventional servo control means but a preferred circuit arrangement is shown in Fig. 2 and will now be described.

The circuit shown in Fig. 2 provides a thyristor firing angle control voltage of O to 5V which is regulated by the miss-match between the actual resistance of the wire in the second annealer leg and the fixed resistor 8. A low actual resistance increases the thyristor firing angle.

Current is measured by the shunt 6 in the earth lead to the second earth contact wheel 4. This gives a control voltage of 2.5V for 50A.

The balancing voltage is derived from the annealer supply via resistors 7 and 8. The ratio of these is such that the required resistance can be set direct, the full range representing 2 ohms.

These two voltages are compared differentially in an operational amplifier 11 which has a gain of 1 00x and is damped to a period of 1 second by a capacitor 12. Zener Diodes 13, 14 and 15 prevent the application of excess voltages.

A variable resistor 1 6 and a diode 1 7 permit the setting of a minimum firing angle, i.e. minimum voltage, because with no supply connected, the circuit will produce no output.

The circuit functions as follows:- When the supply is connected, the minimum firing angle setting causes a low, fixed voltage to be applied to the annealer.

The shunt 6 measures the current delivering a voltage to terminal 18 of amplifier 11 and a proportion of the supply voltage is supplied to terminal 19 of the amplifier.

For the amplifier to produce an output, terminal 18 must be more positive than terminal 1 9. This corresponds to the actual wire resistance being less than the set resistance.

When this occurs, the amplifier advances the thyristor firing angle, thus heating the wire and raising its resistance.

The circuit stabilises when the resistance reaches the set value.

The current and voltage signals are directly obtained and compared on an instantaneous basis and and only the error is averaged by a capacitor 20.

The control will therefor not be affected by changes in wave form.

The ranges:- 0 to 50 Amps O to 2 Ohms will cover 0.5 mm aluminium and 0.5 and 0.4 mm copper for speeds of up to 1,800 metres per minute.

It will be noted that, in accordance with the invention, control of the annealing is effected independently of speed.

In a further embodiment, the voltage input is derived by measuring the voltage drop between the pulleys 5. As no current is fed to the wire via these pulleys, any changes in contact resistance will have a minimal effect on the accuracy of the measured voltage.

Claims (5)

1. A method of controlling a continuous annealer for metal wire in which the resistance value of the wire at a predetermined point is measured as a representation of wire temperature by obtaining a ratio between the current value through the wire and an associated voltage drop, this resistance value is compared with a fixed resistance value and any difference between them is applied as an error signal to change the level of power supplied to the annealer.

2. A method as claimed in claim 1 in which the voltage is measured across a section of wire by contacts which do not carry the annealing current.

3. Apparatus for controlling a continuous annealer for metal wire comprising means for measuring the resistance value of the wire at a predetermined point as a representation of wire temperature, means for comparing this value with a fixed resistance value, means for deriving an error signal from such comparison and means for using the error signal to change the level of power supplied to the annealer.

4. A method of controlling a continuous annealer substantially as described with reference to the accompanying drawings.

5. Apparatus for controlling a continuous annealer substantially as described with reference to the accompanying drawings.