GB2174516A

GB2174516A - System for controlling a puller assembly

Info

Publication number: GB2174516A
Application number: GB08509704A
Authority: GB
Inventors: Friedrich Wilhelm Elhaus
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-02-10
Filing date: 1985-04-16
Publication date: 1986-11-05
Also published as: GB2174516B; GB8509704D0

Abstract

In a system for controlling a puller assembly 3 for withdrawing an extruded section as it exits from an extruder, in which the puller assembly 3 is moved in a pulling direction by means of a drive mechanism 9, the actual speed with which the extruded section 1 is withdrawn is sensed by means of a tachometer 34 the output signal from which is delivered to a speed controller 32 for controlling the speed-of the drive mechanism 9 for the puller assembly. Optimum pulling force may be adjusted by providing follower means 11, 12 between the puller assembly 3 and the drive mechanism 9, said follower means being acted on by an actuator 15 for permitting relative movement between a drive member 8 and the puller head 5. The displacement of the follower 11 which corresponds to the relative movement between the puller head 5 and the drive member 8 is sensed by a displacement detector 30 which provides a signal to the speed controller 32 which modifies its output signal in such a manner as to tend to maintain the displacement of the follower 11 intermediate limiting positions thereof. <IMAGE>

Description

SPECIFICATION System for controlling a puller assembly This invention relates to a system for controlling a puller assembly for withdrawing an extruded section as it exits from an extruder, in which the puller assembly is moved in the pulling direction, ie the direction of withdrawal, by means of a drive mechanism.

It has been known to control a puller assembly to provide a constant pulling force and to this end to measure the pulling force (DE-OS 2,933,262).

This is an effective solution for extrusion operations in which every fresh billet is initially pressed from a standstill condition, in other words during discontinuous operation. By virtue of the pull control it is ensured that the same withdrawing force or pull will always be exerted on the extruded section.

Two puller assemblies are required for continuous operation of an extruder. The speed of the respective pulling means to be moved in the operative position first has to be synchronized to the extrusion speed of the extruded section. Subsequently, the extruded section is taken over, i.e. a load change-over between the first and the second puller assembly takes place.

It is an object of the present invention to provide a system of the type described above, which permits synchronization of the puller assembly to the moving extruded section and take-over of the moving section by a similarly moving puller assembly.

In order to solve this object it is provided in a system of the type described above that the actual speed at which the extruded section is moved is sensed by a speed sensing means and is delivered as set-point or reference input value to a speed control means for controlling the speed of a drive mechanism for driving said puller assembly.

By virtue of the invention it is possible to achieve speed control of the puller assembly, in which the set-point value is formed by the actual speed of the extruded section and the actual value is formed by the respective speed, e.g. rotational speed, of the drive mechanism of the puller assembly. The actual speed of the extruded section, i.e. the set-point value, may for instance be sensed by a roller urged into frictional engagement with the extruded section. When at least two puller assemblies are employed, the set-point value may also be provided by the actual speed of the puller assembly which is still in engagement with the extruded section.

When an extruded section is withdrawn by a puller it is desirable to maintain an optimum pulling force so that the section will remain as straight as possible and dimensionally stable. Even slight differences between the speed of the extruded section and that of the puller assembly will, however, result in large variations of the pulling force so that the extruded section may be over-strained, the cross-sectional dimensions may be reduced and breaking may be caused.To prevent this, in preferred embodiments of the invention follower means influenced by the exerted pulling force is disposed between the puller and its drive mechanism, and an actuator actuates said follower means so that it permits a relative movement between a drive member of said drive mechanism and the puller assembly, and a displacement detector is provided through which said relative movement is delivered as disturbance factor to the speed control means.

The follower means acts like a so-called jockey roller for deflecting a rope or a sheet material or the like which is to be wound, wherein said jockey roller may have a force applied thereto that varies with the operating conditions to produce an initial tension in the rope or the like.

According to a preferred embodiment, the follower means comprises a follower which is movably guided in the withdrawing direction on a truck of the puller assembly and which is influenced in the direction of pull by said actuator, which may be a fluid cylinder.

In order to enable operation of the puller assembly with a selected pull, the actual pulling force is preferably sensed by means of a pulling force sensor and a corresponding signal is then delivered to a pulling force control means which produces a control signal for the actuator on the basis of an adjustable pulling force reference input value.

The invention may be employed in many cases where a travelling section which e.g. exits from an extruder outlet at the extrusion speed, is to be taken up by a puller assembly during operation, i.e. without stopping.

The invention may be advantageously employed particularly in continuous extrusion processes providing two puller assemblies which alternately act on the extruded section. Embodiments of the invention are described in detail below by way of example with reference to the accompanying schematic drawings, in which: Figure 1 illustrates a first embodiment of a puller assembly with a control system according to the invention including the associated control circuit, Figure 2 illustrates a variant puller assembly; and Figure 3 illustrates a system for continuously withdrawing an extruded section, in which two puller assemblies are employed.

Referring to Figure 1, an extruded section generally referenced 1, exits from the outlet of a continuously operating extruder 2 (not shown in Figure 1) and is pulled from the extruder in the direction of extrusion by a puller assembly generally indicated at 3. The puller assembly 3 comprises a truck 4 and a puller head 5. The truck 4 is movable via rollers 6 on guide rails indicated schematically at 7, by means of an endless chain 8, which is adapted to be driven via a sprocket (not shown) by a variable-speed d.c. motor 9. On the truck 4 there is mounted a guide means 12 oriented in the direction of puller movement i.e. parallel with the rails 7, for guiding a follower 11 which is movable along said guide means between one limiting position a and another limiting position b through a central position m.In the embodiment of Figure 1, the puller head is fixed to the truck 4 and the chain 8 is secured to the follower 11. The follower 11 includes an arm 13 which extends transversely to the direction of movement of the follower and has the piston rod 14 of a double-acting hydraulic cylinder 15 pivotally mounted thereon. Pressure is applied to the pressure chambers 16, 17 of the hydraulic cylinder 15, from a hydraulic pressure circuit. Since the effective area of one side of the piston in the cylinder 15 is reduced, as compared with the opposite side by the cross-sectional area of the piston rod, although the pressure is the same on opposite sides of the piston, the piston exerts, via the piston rod 14, a net force on the follower 11 which corresponds to the product of piston rod area and pressure.The hydraulic pressure circuit includes a reservoir 18 from which hydraulic fluid is pumped, by a pump 19, through a check valve 21 (and/or depending on delivery rate etc, through a pressure relief valve connected between the reservoir 18 and a point intermediate the pump 19 and check valve 21), through a flow regulator 22 to a conduit which is connected with the cylinder 15, and with a hydraulic accumulator 23.This conduit is also connected with a pressure sensor 24 and with a valve 27 through which, in certain portions of valve 27, fluid can flow from said conduit, through a flow regulator 28.The pressure sensor 24 provides a signal corresponding to the actual value of the pulling force, (assuming follower 11 to be intermediate its limiting positions a and b) to a pulling force regulator 25, in which a comparison is performed between the actual value and a set-point value for the pressure and thus the pulling force, said set-point value being adjustable and being provided by an adjustable set-point device 26. The regulator 25 provides an output signal dependent on the sensed deviation between said actual value, and said set-point value. The thus provided control deviation acts on the motor 20 driving the pump 19, in such a way that the motor speed is kept at a value providing a pressure applied to cylinder 15, corresponding to the desired pulling force.The regulator 25 may also act to cause the valve 27 to relieve pressure from the conduit which is connected with cylinder 15 in the event that the sensed actual value of the pressure exceeds or exceeds by a predetermined amount, the set value.

Thus, the control signal delivered by the pulling force regulator 25 is also delivered to valve 27 which cooperates with flow regulator 28 for controlling the return flow of hydraulic fluid from the cylinder 15 to the reservoir 18. This permits the load change-over between two puller assemblies alternately acting on the extruded section 1 after speed synchronization of the respective puller assembly that takes over the extruded section 1, as explained in more detail below.

The speed of motor 9, and thus of the puller assembly 3 is controlled as follows. The output signal from a tachogenerator 33 is delivered as a setpoint value signal to a speed controller 32, the tachogenerator being coupled to a roller 34 held in frictional engagement with the extruded section 1 for sensing the actual speed of the extruded section. The speed of the d.c. motor 9 for driving the chain 8 is likewise measured by a tachogenerator 35 and the signal from tachogenerator 35 is supplied as an actual value signal to the speed controller 32. The speed controller 32 provides an output signal which (neglecting any disturbance factor signal as referred to below) corresponds to the deviation of the actual speed of the motor 9, as sensed by tachogenerator 35, from the set or desired speed, as determined by tachogenerator 35.

This output signal of the speed controller is supplied via a power converter 36 to the d.c. motor 9 for controlling the rotary speed thereof, in such a way that the speed of motor 9, is corrected in a sense to eliminate any deviation sensed.

The position of the follower 11, on the guide means 12, relative to the truck 4 is sensed by a displacement sensor 30 having a measuring amplification 31 connected to its output, whereby the amplifier 31 delivers to speed controller 32 a signal significant of the deviation of the follower 11 from its central position m. The arrangement is such that the signal supplied to controller 32, from sensor 30 via amplifier 31 tends to cause controller 32 to act on motor 9 in such a sense as to return the follower 11 to its central position m.Thus, even if the extrusion speed, as sensed by tachogenerator 33 is equal to the pulier assembly speed as determined by tachogenerator 35, if the follower 11 is displaced from its central position m towards its limiting portion a, the controller 32 will provide an output signal such as to slow the motor 9 until the follower 11 returns to is central position m whereas if the follower 11 is displaced from its central position towards position m, the controller 32 will provide an output signal such as to speed up the motor 9 until the follower 11 returns to its central position m. Otherwise expressed, the movement of the follower 11 is feed forward-coupled as disturbance factor to a speed controller 32 via displacement detector 30.

Figure 2 shows a variant of the puller assembly (comprising the truck 4 and puller head 5, and associated cylinder 15) of Figure 1, which may be used in a system otherwise identical with that of Figure 1. Except in the respects noted below the assembly of Figure 2 is identical with that of Figure 1.

In Figure 2 the same reference numerals as in Figure 1 have been used to denote corresponding parts.

Whereas in the embodiment illustrated in Figure 1 the chain 8 engages the follower 11, the chain engages the guide means 12, i.e. the truck 4, in the embodiment illustrated in Figure 2, whilst the puller head is mounted for movement parallel with rails 7 on guide means 12. Furthermore in the embodiment of Figure 2, the cylinder 15 is mounted to act in opposition to its operative direction illustrated in Figure 1. In other words, in the embodiment shown in Figure 1 the pressure applied to the pressure chamber 16 acts in opposition to the withdrawing direction indicated by the arrow F, i.e.

the follower is displaced to the left relative to the truck, whereas in Figure 2 the pressure in the pres sure chamber 16 acts on the follower in pulling direction F. In both cases, of course, the fluid pressure in the cylinder 1 tends to move the puiler head downstream, i.e. in the pulling direction F, relative to the chain 8. The embodiment according to Figure 1 is advantageous when there are large speed variations of the drive mechanism 9. In this case the low-mass follower 11 may rapidly follow such speed variations by virtue of its being directly coupled to the drive mechanism via the chain 8.If it is desired, on the other hand, to compensate for fluctuations of the travelling speed of the extruded section, it will be more advantageous to couple the chain 8 to the high-mass part, viz. the truck 4 instead of to the follower 11, as illustrated in Figure 2.

Figure 3 shows the use of the system according to Figure 1 or Figure 2 in an arrangement comprising two puller assemblies movable on either side of a run-out path and permitting continuous withdrawal of an extruded section 1 from the outlet 2 of an aluminium extruder. Suppose that the extruded sections 1 are initially being withdrawn by the puller assembly 3 illustrated on the right-hand side in Figure 3 in the direction of the arrow F. By the time that this puller assembly 3 releases the extruded section 1, after which its head 5 is pivoted out of the run-out path 100 in the manner illustrated, the puller assembly 3' shown on the lefthand side in Figure 3 will have taken over pulling of the extruded sections. To this end speed synchronization of the left-hand puller assembly 3' with the extruded section 1 will be necessary.Such speed synchronization is performed by means of the control circuit of the kind described with reference to Figure 1.

In the described system, at the commencement of the synchronizing phase the puller assembly 3 acts with a set pulling force on the extruded sections 1. The feedforward-coupling of the disturbance factor represented by the follower movement is cut off, and the puller assembly 3', which in the meantime had been returned (to the right in Figure 3) to its ready-position "empty", commences the synchronizing movement to the left at "zero" pulling force. Due to hydraulic fluid discharge from the hydraulic accumulator 23 of the puller assembly 3', the minimum pressure Pmin is provided there at, which by virtue of the acceleration and frictional forces causes either the follower 11 (embodiment of Figure 1) or the head 5 (embodiment of Figure 2) to move to the one end position.During this synchronizing phase the speed of the puller assembly 3' is controlled exclusively in accordance with that of the other, still engaged puller assembly 3. When the puller assembly 3' has controlledly reached the speed of the puller assembly 3, load change-over will take place. By proper setting of the flow regulators 22, 28 it is possible to dimension the decreasing force of the puller assembly 3 and the increasing force of the puller assembly 3' such that the sum of the forces will always remain constant. Only when the force of the puller assembly 3' has reached the set pulling force value and the force of the puller assembly 3 has reached "zero", will the centre deviation of the follower 11 be fed forward-coupled to the control circuit as disturbance factor.During the withdrawing operation the command variable "speed of extruded section" will always be effective with respect to the engaged puller assembly.

In one arrangement utilizing two puller assemblies in the manner described above each puller assembly has a respective control system such as shown Figure 1, including pressure setting system and speed control system for the respective motor 9, and overall control means, not shown, is provided which, in addition to controlling the operation of the jaws of the puller heads, in a manner not shown, controlling the application of the respective disturbance factor signals from the respective sensors 30 to the respective controllers 32, and controlling whether the speed of the extrustions or the speed of the other puller assembly, is to be used as the set value for the speed controller 32 of either puller assembly, is capable of controlling the set point devices 26, or of controlling the connections of the regulators 25 is of the two assemblies to set point devices providing respectively "zero" pulling force and "desired" pulling force set values, in such a way that when the action of the puller head which has nearly completed its pull is about to be taken over by the other puller head, now synchronized with the puller head which has nearly completed its pull, the set value presented to the regulator 25 for the puller head about to release the extrusion can be changed from the "desired pulling force" value to the "zero pulling force" value, at the same time that the set value presented to the regulator 25 for the puller head about to take over is changed from the "zero pulling force" value to the "desired pulling force" value. The "zero pulling force" value may correspond with Pm,n.

Claims

1. A system for controlling a puller assembly for withdrawing an extrude section exiting from an extruder, wherein the puller assembly is moved in a pulling direction by means of a drive mechanism and the actual speed at which the extruded section is moved is sensed by means of speed sensing means and a corresponding signal is delivered as a reference input value to a speed control means for controlling the speed of a drive mechanism for driving said puller assembly.

2. A system as claimed in Claim 1, in which follower means influenced by the exerted pulling force is provided intermediate the puller assembly and the drive mechanism, and in which an actuator acts on said follower means to permit relative movement between a drive member of said drive mechanism and the puller assembly, and a displacement detector is provided through which said relative movement is feedforward-coupled to the speed control means as a disturbance factor.

3. A system as claimed in Claim 2, in which the follower means comprises a follower movably guided on a truck of the puller assembly and adapted to be influenced by said actuator, in the direction of pull.

4. A system as claimed in Claim 3, in which the drive member engages the follower, and the actuator provides a force, acting on said follower in a direction opposite to the direction of pull.

5. A system as claimed in Claim 2, in which the actual pulling force is sensed by means of a pulling force sensor and is delivered to a pulling force control means which produces a control signal for the actuator on the basis of an adjustable pullingforce reference input value.

6. A system as claimed in Claim 6, in which the pulling force sensor comprises a pressure sensor and is connected in a fluid circuit for energizing said actuator, and in which the fluid circuit comprises an inlet to and an outlet from an accumulator and wherein the fluid flow through said inlet and said outlet is controllable, at least in part, by means of respective regulators.

7. A continuous extrusion arrangement in which two puller assemblies are movable independently of each other along both sides of a runout path for the sections to be extruded and wherein a system according to any of Claims 1 to 7 is provided for controlling said puller assemblies whereby said puller assemblies may alternately be synchronized by said system to the extruded section to be withdrawn.

8. A system according to Claim 1 and substantially as hereinbefore described with reference to, and as shown in Figure 1 of the accompanying drawings.

9. A system according to Claim 1 and substantially as hereinbefore described with reference to, and as shown in Figure 2 of the accompanying drawings.

10. A continuous extrusion arrangement according to Claim 8 and substantially as hereinbefore described with reference to, and as shown in Figure 3 of the accompanying drawings.

11. Any novel feature or combination of features described herein.