Tensioner

Abstract

 A wire tensioner incorporates a hysteresis brake (1) to apply tension to the wire. The residual torque or cogging effect which is caused when current is switched off when the brake rotor is stationary is reduced or eliminated by applying a decaying alternating current to the stator coil.

Description

[0001] The present invention relates to a tensioner, and in particular to a wire tensioner using a hysteresis brake.

[0002] In tensioners using a hysteresis brake, the wire is fed around a pulley wheel mounted on the permanent magnet alloy cup or ring which rotates in an annular space between circumferentially spaced soft iron poles. A direct current magnetises the poles, and this applies a drag to the rotating cup, thus tensioning the wire. The drag applied is governed by the current.

[0003] Hysteresis brakes have many advantages. For example, torque is proportional to current and a smooth response is obtained, and short and long term torque stability are good

[0004] However, the phenomenon of residual torque, known as cogging, presents a major disadvantage when hysteresis brakes are used to tension fine wire. Cogging occurs when the current is removed from the poles while the rotor is stationary. This phenomenon has been discussed in "Hysteresis Devices Give Precise Control of Tension and Torque", by A.D. Jones, Power Drives, October 1973. If there is residual torque in the brake, this may greatly exceed the maximum desired torque for fine wire, with the result that the wire will break as it is pulled through the tensioner on starting up.

[0005] The present invention aims to overcome the problem of cogging in hysteresis brakes, and to provide a tensioner suitable for fine wire.

[0006] The invention provides a tensioner comprising a hysteresis brake, and means for applying a decaying alternating current to the brake.

[0007] It has been found that by applying a decaying alternating current to the brake, the phenomenon of cogging can be alleviated.

[0008] In particular, whenever the brake stops rotating while a current is applied to the brake, for example due to a wire break, a decaying alternating current is applied.

[0009] We have termed this feature of avoiding cogging in a hysteresis brake the "SOFT-MODE" effect.

[0010] The invention will be further described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows schematically a control circuit for a hysteresis brake used in a tensioner according to the invention,

Figures 2a, 2b, 2c and 2d show waveforms for the normal operation of the hysteresis brake,

Figure 3 shows a waveform for avoiding cogging, in accordance with the invention, and

Figure 4 shows a circuit diagram for the control circuit of Figure 1.

[0011] Referring to Figure 1 or Figure 4, a control voltage V_c is applied to control the hysteresis brake 1 via a comparator 2 and an H-bridge switching circuit 3. The amount of drag applied by the brake is proportional to the current through the brake which, in turn, is proportional to V_c.

[0012] The supply voltage is applied across the brake on the rising edge of a clock pulse (CLK1), and the current flowing through the brake is sensed by a current sensing circuit 5. The voltage output from circuit 5 is filtered by a filter 6 and fed to comparator 2. When the voltage from the filter reaches the voltage V_c, the current to the brake is turned off by circuit 3. The current continues to flow in the brake via the "flywheel" diodes in circuit 3. The current to the brake is restored again by the following rising edge of the clock pulse from clock CLK1. CLK1 is operated at a frequency of 62.5 kHz, for example.

[0013] Figure 2 shows at 2(a) the applied voltage, at 2(b) the current rise time, at 2(c) the clock pulse and at 2(d) the current flow though the brake.

[0014] The tensioner incorporates a feed back mechanism which operates to vary the brake current to maintain a desired tension, for example 15gm. When a wire breaks, the control circuit would apply an increasing current to the brake in an attempt to restore the required tension. In a short time full current would be applied to the brake, and this current might ordinarily provide a tension of, for example, 80gm. The brake will come to rest and when the control circuit sees that there has been no tension present for 1 second a wire break is signalled. At this point a system without the SOFT-MODE system would switch off the current, to allow the tensioner to be re-threaded. Cogging will now be present in the brake, in proportion to the current level at switch off. In this example, cogging may generate a residual tension of 50gm, which greatly exceeds the breaking tension of a wire normally tensioned at 15gm. It is therefore not possible to re-thread the tensioner. The residual torque will remain in the brake for a substantial length of time.

[0015] As indicated by A.D. Jones (above) residual torque can be avoided if the cup rotates during current decay, but this is difficult to achieve when a wire breaks.

[0016] In accordance with the present invention, a decaying alternating current is applied to the brake. This has the effect of decogging the brake, i.e. removing the residual torque.

[0017] Figure 3 shows the current waveform applied across the brake. The waveform alternates at approximately 100Hz. This is achieved by means of a second clock 7 (CLK2) controlling the switch 3 and triggering a reversal of the voltage applied across the brake 1. The voltage Vc is caused to decay to zero, causing the brake current to decay to zero while being reversed by CLK2. Typically, the current/voltage decays to zero over a period of less than one second and preferably about 0.5 or 0.25 seconds. Too fast a decay time leaves residual magnetism in the rotor, while too long a decay time will be unacceptable to the user. The frequency of the clock CLK2 is about 100Hz and is determined according to the inductance of the brake winding.

[0018] The decaying alternating current is applied automatically whenever a wire breakage is detected and whenever the tensioner is switched on, and also when the tensioner is to be re-threaded.

[0019] In one embodiment, the decaying alternating current is applied to stator windings provided on the hysteresis brake.

[0020] In one method of operation, the residual torque is not completely removed. The rotor is partially de-magnetised to produce a residual torque which is greater than zero but which is less than the tension in the wire during normal operation.

[0021] In an alternative method of operation, the residual torque is completely removed.

[0022] Modifications can be made to the preferred embodiment of the invention which has been described above. For example, the frequencies, decay times, voltages and currents are dependent on the brake coil. The figures quoted above are for the HB 106-3294 coil of Magtrol Inc, Buffalo, New York, U.S.A.

Claims

1. A tensioner comprising a hysteresis brake, wherein means is provided to apply a decreasing alternating current to the brake to reduce residual torque.

2. A tensioner as claimed in claim 1 for tensioning wire, wherein means is provided to detect a wire break and for applying the decreasing alternating current automatically on detection of the wire break.

3. A tensioner as claimed in claim 1 or 2, wherein the decreasing alternating current is applied when the tensioner is switched off.

4. A tensioner as claimed in claim 1, 2 or 3, wherein the decreasing alternating current is applied before the tensioner is started-up.

5. A tensioner as claimed in any one of claims 1 to 4, wherein the alternating current is decayed substantially to zero.

6. A tensioner as claimed in any one of claims 1 to 5, wherein the alternating current has a frequency of approximately 100Hz.

7. A method of reducing residual torque in a hysteresis brake, the method comprising applying a decaying alternating current to the brake.

Drawing