US3685271A

US3685271A - Wire twister

Info

Publication number: US3685271A
Application number: US143416A
Authority: US
Inventors: Charles A Wall; F Norman Lutz
Original assignee: Warner and Swasey Co
Current assignee: REED-CHATWOOD Inc
Priority date: 1971-05-14
Filing date: 1971-05-14
Publication date: 1972-08-22
Anticipated expiration: 1989-08-22
Also published as: DE2159646A1; CA944637A; GB1373499A

Abstract

A wire twister having a pre-twister and an infinitely variable positive drive that provides very precise control of the ratio of the velocity of the pre-twister to the velocity of a primary twister to achieve a substantially ''''dead'''' twist of ferrous metallic strands. The drive for the pre-twister uses tachometer generators to provide signals indicative of the pre-twister and primary twister velocities.

Description

United States Patent 151 3,685,27 1 Wall et al. [451 Aug. 22, 1972 [54] WIRE TWISTER 2,526,247 10/1950 Lewis ..57/70 72 Inventors: Charles A w N L tz 2,788,632 4/ 1957 Dewhirst ..57/68 1 both ofNonhfila, g' 3,413,793 12/1968 Stine et a] .......57/68 [73] Assignee: The Warner Swasey Company, I primary E i j h petrakes Cleveland Attorney-Yount and Tarolli [22] Filed: May 14, 1971 57 ABS CT [21] Appl. No.: 143,416 1 A wire twister having a pre-twister and an infinitely variable positive drive that provides very-precise confi S 57/34 57/70 v trol of the ratio of the velocity of the pre-twister to the l 'P i velocity of a primary twister to achieve a substantially 1 o arc q fdead twist of ferrous metallic strands. The drive for the pre-twister uses tachometer generators to provide [56] References Cited signals indicative of the pre-twister and primary UNITED STATES PATENTS t t velocities- 2,484,l79 10/1949 MacCreadie ...57/68 6 Claim, 6 Drawing Figures PATENTEDmczz m2 3.685.271 sum 1 or 2 /VENTUR5 I EHA'RLEE. A. WAL

F. N DR'MAN LLJTZ By M ATTOE/WEY;

3,685,271 sum 20? 2 PATENTEDmczz I972 WIRE TWISTER BACKGROUND OF THE INVENTION This invention relates generally to twisters or stranders, and more particularly to twisters having a pretwister.

.Twisters and stranders have been known and used for many years in the manufacture of rope, stranded wire, and cable. Many stranding operations may be performed on either a strander or a twister, but some operations, for example stranding ferrous wire, have been extremely difficult to perform on a twister. This is because a twister imparts twist to the individual wires whereas a strander does not. However, stranders are restricted in the number of wires they can strand, and may require special reels for holding the wire to be stranded. Twisters, on the other hand, are quite versatile and may be used to twist a large number of different combinations of wire size and number of wires. Therefore, it is greatly desirable to extend the ability of the twister to include twisting ferrous wire strands and thereby eliminate the need for less versatile and more expensive stranders.

Because ferrous metals have a high modulus of elasticity and a high yield point, each wire in a strand twisted on a twister will be torsionally displaced and will tend to spring back to its original shape. This causes the strand to be live and tend to kink and twist upon itself. In order to provide 'a dead strand of ferrous wires with a twister, it is necessary to stress the individual wires beyond the yield point to cause permanent deformation. That is, the strand must be overtwisted and then relaxed so that there are no residual torsional forces in the wires when the twisting torque is completely removed.

It is not enough, however, to merely achieve dead strand. In order to be useful as, for example, tire cord, the strand must be very uniform. This means that, among other things, the lay of the strand, i.e., the length of each full twist measured along the strand, must be very uniform. Therefore, deadness cannot be achieved by simply letting the twisted strand relax until it is dead, because this would change the lay. Instead, it is mandatory that deadness be achieved with a uniform and controllable lay.

In US. Pat. No. 2,526,247 issued Oct. 17, 1950 to Dartrey Lewis for Method and Apparatus for Producing Wire Strandor Rope, there is disclosed the method of overtwisting and then untwisting the strand to produce any desired residual twist in either direction. This method is carried out by a tandem twisting arrangement comprising a first twisting head (pretwister) and a secondtwisting head and flyer (primary twister). The pre-twister is operated at a higher velocity than the primary twister, thus providing overtwist and subsequent untwisting. The Lewis apparatus apparently works well for rope and nonferrous metal wire, but has been unable to twist high strength ferrous wire with the degree of deadness and uniformity required for tire cord.

We have determined that the inability of the Lewis apparatus to provide satisfactory high strength ferrous strands did not lie with the basic method of overtwisting and relaxing or untwisting, but lay primarily with the variable speed drive for the pre-twister. It was determined that mechanical variable speed drives of ble, would not maintain their settings, but varied over time and with load due to slippage. Mechanical drives of the type having gears, teeth, or cogs which positively engage provide good time and load stability, but are incapable of fine enough adjustment to provide the required degree of' deadness. Therefore, we have developed a velocity ratio drive for the pre-twister that has a resolution of one revolution per minute throughout the range of 0-4,000 RPM.

Also because the proper velocity ratio between the pre-twister and the primary twister will vary slightly from reel to reel of wire to be twisted, the required deadness cannot be achieved where more than one pretwister is driven by the same variable/speed drive as suggested by Lewis. This, of course, does not prevent a plurality of primary twisters from being mounted on a common frame and being driven by a common drive. It does, however, require'that the ratio of the velocity of each pre-twister to the velocity of its primary twister be individually controllable.

OBJECTS AND SUMMARY OF THE INVENTION An object of the present invention is to provide an improved apparatus for twisting ferrous metallic wire into a strand having substantially'no residual twist.

It is a further object of the presentinvention to provide an improved velocity ratio control for driving a pre-twister at a desired velocity ratio with respect to a primary twister to produce a twisted strand having the desired deadness.

' It is still another object of the present invention to provide a twisting apparatus wherein a plurality of primary twisters are mounted on a common frame and are commonly driven by a primary drive and wherein an equal number of pre-twisters are mounted on the same frame but each pre-twister is provided with its own adjustable variable speed drive.

The foregoing and other objects of the invention are achieved by generating electrical signals proportional to the velocities of each primary twister and each pretwister, and using these signals to generate control signals for controlling the velocity of the pre-twisters so that each remains in a preselected ratio to the velocity of its associated primary twister.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a generalized perspective view of a fourspindle twister embodying the present invention.

FIG. 2 is a diagrammatic cross-sectional view of one spindle with associated creel, pre-twister, flyer, and take-up reel.

FIG. 3 is a block diagram of the servo control for driving a pre-twister.

FIG. 4 is a graphical illustration of the transfer function of a pre-twister and the motor used to drive it.

FIG. 5 is a schematic diagram of the preferred laglead compensation network used in the servo control of FIG. 3.

FIG. 6 is a graphical illustration of the transfer function of the compensation network of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT 14, and top members 15. Supported by the frame 12 is a housing 21 which houses a conventional spindle drive that rotates and axially reciprocates take-up shafts 22. Each take-up shaft 22 can support a take-up reel 23. Because the illustrated twister of FIG. 1 has four spindles, there are, of course, four take-up shafts 22. Only two of the take-up shafts 22 are visible in FIG. 1, the other two being obscured by a partition 24. I

Mounted on end members 14 are four main spindle and flyer assemblies 25, each of which is coaxial with its associated take-up shaft 22. Also supported by end members 14 are four pre-twister assemblies 27 mounted to the end members 14 by brackets 28. Each of the pre-twister assemblies 27 is coaxial with its associated main spindle and flyer assembly 25 and takeup shaft 22. Each pre-twister assembly is provided with c a protective housing 29.

FIG. 2 shows in greater detail the component parts and relationships between an associated pre-twister assembly 27, main spindle and flyer assembly. 25, and take-up shaft 22. Hereafter, the main spindle and flyer assembly 25 may also be referred to as the primary twister. Wire to be twisted is supplied to the twister from reels 34 on a creel 35. Creel 35 is conventional, having a tensioning device and stop motion, indicated at 36, for each wire to be twisted. As shown in FIG. 2, each wire to be twisted leads from a reel 34 through a tensioning device and stop motion 36 to the pre-twister assembly 27. Between the creel 35 and pre-twister assembly 27 may be located a guide or lay plate 38, which may be supported on frame 12 by appropriate brackets, or may have its own support.

The twisted strand from the pre-twister enters the front end of hollow shaft 55, passes therethrough and The pre-twister assembly 27 comprises a hollow shaft 41 mounted for rotation in

bearings

42 and 43. Shaft 41 is provided with radially opposed and axially displaced

ears

44 and 45 to which are rotatably mounted

sheaves

46 and 47 respectively. Sheaves 46 and 47 through slots in shaft 41. to its hollow interior. The wire to be twisted enters hollow shaft 41, preferably through a closing die 51, passes through the hollow interior to sheave 47, around sheave 47 and across the hollow interior to sheave 46, around sheave 46 and back into the hollow interior, and thence continues through the hollow interior exiting at 52.

The main spindle and flyer assembly, or primary twister, 25 comprises a hollow shaft 55 rotatably mounted in a bearing 56 substantially coaxial with hollow shaft 41 of the pre-twister assembly 27. A flyer 57 is 'a fixed to and rotates with hollow shaft 55. Flyer 57 comprises a base portion 58 and longitudinally extending arm 59 having a proximal end 60 and a distal end 61. The proximal end 60 is afiixed to the base portion 58. Coupled to the base portion 58 and positioned substantially coaxially with hollow shaft 55 is a grooved capstan or roller 61. A second capstan or roller 62 is rotatably mounted to the base portion 58 at a location opposite the longitudinally extending arm 59. The flyer 57 may also be provided with a counter-weight 63 located radially outward from grooved roller 62 that cooperates with grooved roller 62 to counterbalance longitudinally extending arm 59.

over a small pulley 66 to one groove of grooved roller ,62. The twisted strand then makes a plurality of turns around grooved

rollers

61 and 62, travels radially outward to a small pulley 67 mounted near. the proximal end of arm 59, travels along arm 59 to a small pulley 68 mounted at the distal end of arm 59, and fromsmall pulley 68 is wound up on the take-up reel 23. As noted above, take-up reel 23 is mounted on take-up shaft 22 which is rotated and reciprocated by a conventional drive means located in housing 21.

The driving torque for the primary twister is supplied by the twisted strand being pulled around the

grooved rollers

61 and 62 by the take-up reel 23. The number of revolutions of the primary twister per unit length of twisted strand taken up on reel 23 is determined by a selectable gear ratio coupling (not shown) between grooved roller 61 and flyer 57. The pre-twister 27 is driven by a DC motor 71 having a gear 72 mounted on its shaft 73. Gear 72 engages a gear 74 mounted on hollow shaft 41. By controlling DC motor 71 so that the pre-twister 27 is driven at a rotationalvelocity greater than that of the primary twister 25, the wires are first over twisted by the pre-twister 27 and then partially untwisted, or relaxed, between the pre-twister 27 and the primary twister 25.

DC electrical signals proportional to or indicative of the velocities of the primary twister 25 and the pretwister 27 are provided by two

DC tachometer generators

77 and 78. Tachometer generator 77 is coupled to hollow shaft 55 of the primary twister by gears 79 and 80. Tachometer generator 78 is coupled to the pretwister 27 by a gear 81 that engages gear 74. Thus, tachometer generator 77 provides a DC signal proportional to the rotational velocity of the primary twister and tachometer generator 78 provides a DC signal proportional to the rotational velocity of the pre-twister 27 Precise control of DC motor 71 to maintain the velocity of the pre-twister in a desired preselected ratio to the velocity of the primary twister is accomplished by the servodrive illustrated diagrammatically in FIG.

3. Referring now to FIG. 3, there is shown diagrammatically a housing 21 which encloses a main motor control 83, the main motor 84, and a clutch/brake 85. The main motor control 83 is actuated by a start pushbutton 86 and a stop push button 87. The torque transmitted by the clutch/brake 85 is controlled by a clutch control 88. Thus, as shown diagrammatically in FIG. 3, the main motor 84 drives the take-up reels 23 through a clutch/brake 85 that is controlled by clutch control 88. Clutch control 88 regulates the torque transmitted by clutch/brake 85 to effect a smooth controlled start-up of the twister to avoid breaking any of the wires or strands. As each take-up reel 23 rotates and reciprocates it winds up twisted strand from its as sociated primary twister 27. As explained above, the strand passing around grooved

rollers

61 and 62 causes the primary twister to rotate. After startup, the take-up shafts 22 are driven at a constant rotational velocity. Thus, the more strand that is taken up on a reel 23, the faster its primary twister 25 will rotate.

Referring to FIG. 3, the rotational velocity of primary twister 25 is denoted by 6 Tachometer generator 77, driven through gears 79 and generates a voltage E that is proportional to 0,. Thus, letting K represent the combined transfer function of tachometer generator 77 and the gear ratio between gears 79 and 80, we may write E 1 K 6 volts per RPM. The voltage E may be selectively attenuated within a predetermined range by an infinitely variable ratio selector 90, which may comprise a potentiometer having a calibrated control knob to produce a voltage E proportional. to E Letting K represent an infinitely variable attenuation factor or transfer function of ratio selector 90, we may write E K E Tachometer generator 78 produces a voltage E proportional to the rotational velocity of pre-twister 27. Letting the combined transfer function of tachometer generator 78 and the ratio of gears 74 and 81 be represented by K and the rotational velocity of pre-twister 27 be represented by 9 we can write E K 0 Voltages E and B are algebraically combined at a summing point 91 to create an error signal, 2 E E The summing point 91 compares signals (E and E indicative of the velocities of rotation of the primary and neutral strand, voltage E is equal to voltage E and I there is no error signal e.

The error signal e provides the input to compensation network 92, the output of which is designated as E Letting l-l represent the transfer function of compensation network 92, we can write E H e. E is used as the input signal to a DC power amplifier 93, the output of which is designated as V,,,. Letting K 'represent the transfer function of power amplifier 93, we can write, V K E Finally, V,,, is the armature driving voltage for DC motor 71, which taken with primary twister 25 has a transfer function H Thus, we can write 0 H V Combining the above equations, it will be seen that the closed loop transfer function may be written which will be recognized by those skilled in the art as being of the form where A is the open loop gain, B the feedback loop gain, and K the setpoint multiplier.

Because K is fixed by the physical properties of tachometer generator 77 and the ratio of gears 79 and 80, the magnitude of H s), or in other words the steady state ratio of 49 /9 will be a function of the attenuation factor K of the infinitely variable ratio controller 90. Thus, by varying the setting of ratio controller 90, any desired ratio of 0 /0, may be selected.

One must be able to set controller 90 to the required ratio between the velocity of the pre-twister 27 and the velocity of primary twister 25 in order to provide a straight, neutral strand which is dead and which has a desired lay. For given physical characteristics of the wire making up the strand, the primary twister 25 must untwist or relax the strand by a specific amount in order to have a truly dead or neutral strand which will remain straight without any inherent tendency to twist when released. Or, what is the same thing, the pretwister 27- must overtwist the strand by a specific amount in order to achieve a dead strand at the lay determined by the primary twister 25. If the pre-twister imparts too much overtwist to the strand, then the finished strand will tend to twist itself up into a tighter twist. If the pre-twister imparts too little overtwist to the strand, then the finished strand will tend to untwist itself to a looser twist.

The extent to which a particular strand must be overtwisted to provide a straight, neutral or dead" strand will vary depending upon the physical characteristics of the wires making up the strand. Therefore, it

dary twister 27 and primary twister 25, i.e., ti /0 Thus.

if the ratio selector is not adjustable to provide the particular velocity ratio necessary between the primary and

secondary twisters

25 and 27, the strand will not be truly straight and neutral but will have an inherent resilient tendency to twist in one of two directions. To enable the twister assembly to form a strand which is truly dead or neutral when released so that the strand will lay straight and will not tend to twist in one 1 direction or the other, the ratio selector 90 is infinitely variable throughout an operating range of approximately 0.8 to 1.2 to select a desired ratio between the velocity of the pre-twister 27 and primary twister 25. Since the ratio selector 90 is operable to select on of an infinite number of velocity ratios within the operating range, the extent to which the pre-twister 27 overtwists the strand can be carefully adjusted so that the finished strand will be truly dead or neutral.

In addition to providing an infinitely variable speed ratio between the pre-twister 27 and primary twister 25, it is important that the selected speed ratio be maintained with a high degree of accuracy. In one specific embodiment of the invention the servodrive illustrated schematically in FIG. 3 maintained the velocity of the pre-twister 27 to within one revolution per minute of the velocity corresponding to the selected velocity ratio through a range of primary twister speeds of from 0 to 4,000 RPM. Thus, once the infinitely variable ratio selector 90 has been adjusted to provide a desired velocity ratio between the primary twister 25 and pretwister 27, this velocity ratio is accurately maintained during operation of the twister so that the resulting strand is truly dead or neutral. If the velocity ratio between the primary and

secondary twisters

25 and 27 varies by even a relatively small amount from the precise velocity ratio required to produce a neutral strand, the strand will no longer be neutral or dead. Therefore, the formation of a neutral or dead strand requires that the infinitely adjustable ratio selector 90 be set to provide the exact velocity ratio between the primary and

secondary twisters

25 and 27 to overtwist the strand to the extent necessary for it to lay straight and dead when it is released and that this ratio be maintained during operation of the twister assembly.

4 c l 2 A Bode plot of H; is shown in FIG. 4. It will be seen that near zero frequency H has a valueK As will be understood by those skilled in the art, around a frequency of l/T the magnitude of H begins decreasing at a rate of 3 db per octave. At a frequency of l/T the rate increases another 3 db per octave. In order to improve the accuracy of the servodrive of FIG. 3, it isdesirable to increase the loop gain around zero frequency as much as possible without reaching instability. Graphically, increasing forward gain without compensation will move the Bode plot of H vertically upward from its position as shown in FIG. 4. This indicates that the gain in the frequency range of l/T to l/T and above is increased in the same proportion as the gain around zero frequency. Because a small increase in loop gain at the higher frequencies will quickly lead to an oscillatory or unstable condition, the allowable increase in loop gain is severely limited.

In order to overcome the above described limitations, a compensation network 92 was devised. This network is shown in detail in FIG. 5 and comprises an input resistor R connected to the inverting input of an operational amplifier 94 and a feedback impedance comprising the parallel combination of a capacitor C and a resistor R the combination being connected in series with a resistor R between the output of operational amplifier 94 and its inverting input. The transfer function of the compensation network of FIG. 5 may be written as He: 2+ a) It will be seen that this of the general fotm By properly selecting the alues of C, T is madeequal to T Thus, the above equation becomes It will be seen that the effect of the compensation net:

work is to eliminate the break frequency 'at l/T and create a new break frequency at .1/T From zero 8 frequency up to approximately I lT H has a magnitude of (R R3)/R At frequency lfl the magnitude of H begins decreasing at a rate of 3 db per octave until it reaches frequency l/T where the magnitude is R /R,. Further increase in frequency does not change the magnitude of H The inclusion of compensation network 92 in the servo loop permits a very large increase in loop gain around zero frequency without increasing. the gain above frequency 1 /T Thus, the goal of greatly increasing the loop gain around zero frequency without causing instability has been achieved.

In view of the foregoing description, it is apparent that the servodn've illustrated schematically in FIG..3 is adjustableto provide for an infinitely variable velocity ratio between the primary twister 25 and pre-twister 27. The selected velocity ratio between the pre-twister 27 and primary twister 25 ismaintained with a very high degree of accuracy by providing the servodrive with a high loop gain around zero frequency. This high loop gain enables a relatively small error (e) to be corrected so that the velocity ratio is accurately maintained between the primary twister 25 and pre-twister 27. This high degree of accuracy is necessary in order to provide a truly straight and neutral strand.

If the velocity ratio varies from the selected velocity ratio, the strand will have an inherent elastic tendency to twist and will therefore be alive and will be relatively difficult to handle. By providing an infinitely variable velocity ratio selector in combination with the compensation network 92, the velocity ratio between the primary twister 25 and secondary twister 27 can be accurately selected and maintained in order to provide a truly dean or neutral strand. If the ratio selector 90 was not infinitely variable, an operator would be unable to select the precise velocity ratio between the pretwister 27 and primary twister 25 to provide a truly dead orneutral strand. Once the correct velocity ratio has'been selected with a high degree of accuracy, the velocity ratio is accurately maintained during operation of the primary and

secondary twisters

25 and 27 by the compensated servodrive in order to continuously produce a truly dead, or neutral strand.

In the multispindle embodiment of the twister shown in FIG. 1, two control panels 96 are provided, one on each side of frame 12. On each control panel is located a start pushbutton 86, stop pushbutton 87, and a clutch control 88 so that the main drive may be started and stopped from either side of the twister. Also located on each control panel are two ratio selectors 90, one for each spindle on the same side of the twister as the control panel.

It has been our intention to describe our invention in full, clear, concise, and'exact terms that would enable any person skilled in the art to practice it. We have set forth the best mode we have contemplated of carrying out our invention in the foregoing detailed description of the presently preferred embodiment. Changes in the details of construction may be resorted to without departing from the'spirit of the invention. For example, the pre-twister could be driven by a constant speed drive and the primary twister be driven by a servodrive to accomplish the same end.

Having describeda specific preferred embodiment of the invention, the following is claimed:

1. Apparatus for twistirig wires to form a neutral strand, said apparatus comprising means for supplying a plurality of wires, secondary twister means for twisting the wires to form an overtwisted strand which has a resilient tendency to twist upon being released, said secondary twister means including a rotatable'secondary twister assembly for engaging the plurality of wires and secondary drive means for rotating said secondary twister assembly at a first velocity to overtwist the wires engaged by said secondary twister assembly to form the overtwisted strand, primary twister means for partially untwisting the wires of the overtwisted strand to form a neutral strand, said primary twister means including a rotatable primary twister assembly for engaging the overtwisted strand and primary drive means for rotat-v ing said primary twister assembly at a second velocity which is less than said first velocity to untwist the overtwisted strand to an extent which is a function of the difference between said first and second velocities, and control means for maintaining a preselected ratio between said first and second velocities to control the extent to which said primary twister means untwists the overtwisted strand during operation of said apparatus, said control means including means for providing a first signal indicative of said first velocity, means for providing a second signal indicative of said second velocity,

pensator means for improvin g the response of said control means and said one drive means to said error signal I to accurately control the velocity of one of said twister assemblies to substantially maintain said preselected ratio.

2. Apparatus as set forth in claim 1 wherein said compensator means includes amplifier means for amplifying said error to a relatively large extent when the difference between said first and second velocities varies from said predetermined difference by a relatively small amount.

3. Apparatus as set forth in claim 1 wherein said means for providing said first signal includes first signal generator means driven by said secondary drive means at a rate which is proportional to said first velocity, and said means for providing said second signal includes second signal generator means driven by said primary drive means at a rate which is proportional to said second velocity.

4. Apparatus as set forth in claim 1 wherein said selector means includes infinitely variable ratio adjustselector means for preselecting the ratio to be maintained between said first and second velocities to determine the extent to which said primary twister means untwists the overtwisted strand to enable neutral strands to be formed by twisting wires having different characteristics, means for comparing said first and second signals and for providing an error signal to one of said drive means if the ratio between said first and second velocities as indicated by said first and second signals is other than the preselected ratio, and compensator means imena ginr s ment means for infinitely varying the ratio between said first and second velocities tov enable said apparatus to be adjusted to form a neutral strand from wires of different physical characteristics.

5. Apparatus as set forth in claim 1 wherein said secondary drive means includes a motor for driving said secondary twister assembly and wherein said comsignal to vary the rate of operation of said motor to vary the first velocity relative to the second velocity.

6. Apparatus as set forth in claim 1 wherein said compensator means includes lead-lag compensation means that permits increased forward loop gain around zero frequency to increase system accuracy without the lnlnln In An

Claims

1. Apparatus for twisting wires to form a neutral strand, said apparatus comprising means for supplying a plurality of wires, secondary twister means for twisting the wires to form an overtwisted strand which has a resilient tendency to twist upon being released, said secondary twister means including a rotatable secondary twister assembly for engaging the plurality of wires and secondary drive means for rotating said secondary twister assembly at a first velocity to overtwist the wires engaged by said secondary twister assembly to form the overtwisted strand, primary twister means for partially untwisting the wires of the overtwisted strand to form a neutral strand, said primary twister means including a rotatable primary twister assembly for engaging the overtwisted strand and primary drive means for rotating said primary twister assembly at a second velocity which is less than said first velocity to untwist the overtwisted strand to an extent which is a function of the difference between said first and second velocities, and control means for maintaining a preselected ratio between said first and second velocities to control the extent to which said primary twister means untwists the overtwisted strand during operation of said apparatus, said control means including means for providing a first signal indicative of said first velocity, means for providing a second signal indicative of said second velocity, selector means for preselecting the ratio to be maintained between said first and second velocities to determine the extent to which said primary twister means untwists the overtwisted strand to enable neutral strands to be formed by twisting wires having different characteristics, means for comparing said first and second signals and for providing an error signal to one of said drive means if the ratio between said first and second velocities as indicated by said first and second signals is other than the preselected ratio, and compensator means for improving the response of said control means and said one drive means to said error signal to accurately control the velocity of one of said twister assemblies to substantially maintain said preselected ratio.

4. Apparatus as set forth in claim 1 wherein said selector means includes infinitely variable ratio adjustment means for infinitely varying the ratio between said first and second velocities to enable said apparatus to be adjusted to form a neutral strand from wires of different physical characteristics.

5. Apparatus as set forth in claim 1 wherein said secondary drive means includes a motor for driving said secondary twister assembly and wherein said compensator means is operable in response to said error signal to vary the rate of operation of said motor to vary the first velocity relative to the second velocity.

6. Apparatus as set forth in claim 1 wherein said compensator means includes lead-lag compensation means that permits increased forward loop gain around zero frequency to increase system accuracy without the system becoming unstable.