US2922188A

US2922188A - Control for extrusion apparatus

Info

Publication number: US2922188A
Application number: US641325A
Authority: US
Inventors: David A Bossen
Original assignee: Industrial Nucleonics Corp
Current assignee: Industrial Nucleonics Corp
Priority date: 1957-02-20
Filing date: 1957-02-20
Publication date: 1960-01-26
Anticipated expiration: 1977-01-26
Also published as: DE1148698B

Description

Jan. 26, 1960 D. A. BOSSEN 2,922,188

CONTROL FOR EXTRUSION APPARATUS Filed Feb. 20, 1957 Q nu E INVENTOR DAVID A. BOSSEN United States Patent ()filice 2,922,188 Patented Jan. 26, 1960 CONTROL FOR EXTRUSION APPARATUS David A. Bossen, Columbus, Ohio, assignor to Industrial Nucleonics Corporation Application February 20, 1957, SerialNo. 641,325 8 Claims. (Cl. 18-8) The present invention relates to apparatus for the extrusio-n of synthetic threads and more particularly relates to an apparatus for measuring and automatically controlling the weight per unit length of extruded synthetic fibers.

In conventional systems for the manufacture of yarns from extruded fibers, a liquidous mixture of the synthetic material, such as a mixture of cellulose, acetone and water, is pumped at a constant rate into a header. Constant volume pumps extract the liquid from the header and supply it under force to extrusion heads, producing a plurality of extruded fibers, the number of which is determined by the number of fibers desired per yarn. The extruded fibers from each extrusion head are brought together around a winder bar and then proceed to individual bobbins upon which they are wound, the yarn thus produced being twisted between the winder bar and the individual bobbins. This system is highly flexible and a large number of extrusion heads, pumps and bobbins may be fed from a single header, the pumps being driven by a common pump drive motor and the winder bar and bobbins being driven by a common motor.

The weight per unit length of the fibers is a function of the ratio of the speeds of the pump drive and the winder bar drive. The pump controls the weight per unit time of the individual fibers, since the pump speed determines the quantity of liquidous mixture supplied to the extrusion head per unit time. The winder controls the length per unit time of the extruded fibers, since the velocity of the windup determines the tension on the fibers in passing from the extrusion heads to the bobbins. This of course affects the amount of drawing which the fibers undergo in passing from the heads to the winder bar.

Letting the pump speed equal S the winder bar speed equal S the weight of the fibers equal W, the length of the fibers equal L, the time equal T, and K and K represent constants, the above stated relationships may beexpressed as follows:

Thus, the weight per unit length of the extruded fibers is a function of the ratio of the pump speed to the winder bar speed. In the prior systems the denier or weight per unit length of the fibers was predetermined and the speed ratio of the pump motor and winder bar motor was adjusted and fixed by means of a mechanical or electrical synchronizing apparatus.

It has been found in practice that the weight per unit length of the fibers produced in accordance with such systems varies for a number of reasons, such as a change in the concentration or viscosity of the fluid mixture or as a result of a variation in speed of either or both'of the pump and winder bar drive motors. In a system employing a single header, a number of extrusion heads and a winder bar, a variation in any of these factors alfects the weight of the yarns produced by all of the extrusion heads, since the liquidous supply is common to all heads and a change in speed of either of the two motors is reflected in all of the fibers produced. by the various heads. It follows that accurate measurements of weight per unit length of the fibers produced by a single, selected extrusion head may be taken as representative of the weight of all fibers produced by the machine; hence there is no necessity for a plurality of gauges to measure the output of each extrusion head, nor a plurality of controllers to control each pump individually. Such an expedient would of course be economically unfeasible.

In accordance with the present invention it has now been found that the observed variations in weight per unit length may be substantially eliminated, if the pump and winder bar are separately driven and if the speed ratio of these two elements is adjusted by an automatic controller in accordance with the deviation of the yarns from the desired value of weight per unit length, as detected by a gauge employing a radiation source and detector.

It is accordingly a primary object of the present invention to provide an apparatus for extruding synthetic threads wherein the weight per unit'length of extruded synthetic fibers is maintained substantially constant.

It is another object to provide an effective, reliable and'accurate means for controlling a machine producing synthetic fibers. a

It is a further object of this invention to provide an accurate, continuous and automatic gauging device capable of instantaneously indicating the weight per unit length of travelling synthetic fibers formed in an extrusion apparatus.

It is again an object to provide means for continuously and accurately measuring the, weight per unit length of travelling synthetic fibers formed in an extrusion apparatus, and to provide an automatic control system responsive to the output of the measuring device for maintaining the weight per unit length of the fibers constant at any desired predetermined value.

It is a still further object to provide measuring and control apparatus, capable of fulfilling the aforesaid objects, which may be readily adapted for installation on existing extrusion machines without extensive modification thereof.

It is also an object to provide apparatus in accordance with the above objects which is relatively inexpensive to manufacture, simple to operate, and which requires a minimum of adjustment and maintenance.

These and further objects and advantages of the in vention will become more apparent upon reference to the following specification and appended drawing.

The single figure of the accompanying drawings is a schematic diagram of the physical and electrical elements of a typical apparatus in accordance with the present invention.

Referring specifically to the drawing, a liquidous mixture such as cellulose, acetone and water is pumped into a header 10 at a constant rate by a pumping apparatus,

at 12, 14 and 16 are driven by a single variable speed 4 electric motor 22. Pumps 12 through 16 extract the liquidous mixture from the header through a plurality of

pipes

24, 26 and 28 and deliver the mixture to a plurality of extruding "

heads

34, 36 and 38 respectively through

pipes

44, 46, and. 48. 1

Each of the extrusion heads 34 through 38 is adapted to extrude a pluralityof synthetic fibers 54, which pass from the extrusion heads around a common winder bar 56, where they are brought together to form

individual yarns

58, 60 and 62. The individual yarns 58 through 62 are wound respectively on bobbins 68, 70 and 72, and the yarns are twisted in passing from the winder bar to the individual bobbins to produce a twisted yarn. The bobbins. 68 through 72 and the winder bar56 are driven by means of a common constant speed motor 73 energized by a source of electric power, not shown.

The number of fibers per yarn and the size of the individual fibers are determined by the design of the extrusion head, the various designs required for various yarns being well known in the art. Although only three separate pumps, extrusion heads and bobbins are illustrated in the figure of the accompanying drawings, it is not intended to limit the invention to a particular number of such elements, since it is common in the synthetic yarn industry to employ as many as 100 or more separate pumps, extrusion heads and bobbins.

In accordance with this invention, a radiation source 80, which preferablycomprises a sealed capsule containing a small quantity of a beta ray emitter radioisotope such as strontium-9O or thallium-204, and a radiation detector 82 such as an ionization chamber or Geiger- Mueller tube are preferably disposed on opposite sides of fibers 54 between the extrusion head 34 and the winder bar 56. It is well knownthat the absorption of radiation by a material is a function of the density of the material and the quantity of the material disposed in the path of the emanations. As a result, the emanations intercepted by the detector 82 vary as an inverse function of the weight of material disposed between the source and detector.

Because of the extremely small diameter of individual extruded fibers and because of the possibility of variation from fiber to fiber it has been found appropriate for the sensing unit to detect its information from a number of fibers, thereby to provide an average signal indicative of the total weight per unit length of an entire group of fibers from a selected extrusion head.

The radiation source and detector are preferably mounted adjacent to the winder bar, where the air drying of the extruded fibers is more or less complete, and where the drawing action due to the winding tension has reduced the diameter of the fibers to a value approaching the final diameter attained at the time the yarn is wound upon the bobbins.

The measured unit length of the fibers is determined by the length of the detector parallel to the direction of movement of the fibers and therefore remains fixed throughout the operation of the system. The amount of radiated energy received at the detector is an inverse function of the density of the fibers, the size of the fibers, and the number of the fibers. Since the number of fibers remains fixed in the course of any normal run, the signal generated by the radiation detector is a function of the size and density of the fibers and therefore is indicative of the weight thereof.

Basically, the system of this invention comprises an accurate radiation gauge and an automatic controller.

The gauge includes the source 80, detector 82, resistor 84, a feedback amplifier 90, a calibrating and standardizing network 93 and an indicating device 104.

The controller comprises a comparator network 120-, an optional lead network 124126, an integrating velocity servo system 132-148, and a gear reducer 150 through which the servo motor 146 may regulate the speed adjustment of motor 22.

' Although the accompanying drawing shows one preferred embodiment of the invention in which the winder bar 56 is driven at a constant speed preset by a manual control 154 for the motor 78, and with automatic control applied to vary the speed of the pump motor 22, it is apparent that the arrangement could be reversed to form a fully equivalent system. That is to say, the pump motor 22 could be driven at constant speed, and automatic control applied to vary the speed of the winder bar by regulating the speed of the motor 78.

An alternate arrangement, which is also a preferred embodiment and fully equivalent, may utilize a single motor to drive both the winder bar and the pumps, with one of these elements directly coupled to the motor and the other connected through a continuously variable ratio speed changer. The automatic controller is then used to vary the speed ratio of the speed changer.

The electrical output signal developed by the detector is a minute current which flows through a resistor 84 having a very high impedance. A voltage proportional to current through the detector 82 is thereby developed across resistor 84, and this signal is utilized by the measuring system of the gauge to provide an indication of the weight of the fibers passing between the source 80 and the detector 82.

The measuring system comprises a feedback amplifier 90 with an input on line 86 and ground reference 88;- a calibrating and standardizing network indicated generally at 93, and the weight indicator 104. In operation the signal voltage developed across resistor 84 is compared with a fixed voltage from the network 93, this latter voltage always being subtracted algebraically from the signal voltage so that the amplifier responds to the difference. The output of the amplifier on line 92 is coupled back to the input 86 through the network 93 and resistor 84, so as to maintain the amplifier input at substantially zero or ground potential at all times. Thus the amplifier output voltage between line 92 and ground is automatically maintained equal to the algebraic difference between the voltage developed across resistor 84 and the fixed voltage from the network 93. The amplifier 90 therefore performs an impedance matching function in transforming a high impedance signal into a robust signal for operating the controller and the indicating meter 104, and this is accomplished without appreciable distortion of the signal through the agency of the substantially total inverse feedback arrangement. The indicator 104 is responsive to any output voltage from the amplifier 90, and its pointer will be deflected to either side of its zero center position depending on the polarity of this output.

It can be seen that the calibrating potentiometer 100 provides an adjustment whereby the zero center position of the indicator 104 can be made to correspond to any selected value of fiber weight which it is desired to place at the center of the scale associated with the indicatorb Potentiometer 102 is provided to allow an adjustment of the span of weight deviations on each side of the center value which are readable on the indicator scale. Thus the indicator scale may be calibrated directly in any desired units of weight per unit length such as denier; that is, grams per 9000 meters. Potentiometer 96 and resistor 98 determine the portion of the voltage from the voltage source 94 which is available across potentiometer 100 to provide the opposing voltage in the measuring system. Potentiometer 96 is therefore the means of standardizing the gauge so that the total voltage available across potentiometer 100 may be restored at any time to the exact value of the maximum voltage across resistor 84 when no material is interposed between the source and the detector 82.

The measuring system briefly described above is the subject of a co-pending application Serial No. 628,999, filed December 18, 1956, by Sidney A. Radley, and accordingly the full details thereof'are not included in this specification.

The voltage output of the amplifier 90, which appears on line 92, is indicative of the weight of the fibers passing between the source 80 and the detector 82. This signal, which energizes the weight indicator 104, also provides the input to the automatic controller. The network 120 provides a voltage signal representative of the desired weight of the fibers. This voltage is continuously subtracted from the voltage signal representing the measured weight of the fibers, so that the difference voltage appearing on line 108 at the junction of resistors 106 and 110 is a signal representing the direction and magnitude of the error in the measured weight of the fibers.

The selected weight of the fibers which is desired to be maintained constant by the automatic controller may be preset by means of potentiometer 112 in the network 120, that is, the setting of this potentiometer determines the direction and magnitude of the comparison voltage representing the desired weight. The bridge circuit 120, comprising potentiometer 112 and a pair of

identical precision resistors

118 and 122, is energized by a voltage source represented by the battery 116. The voltage available across the bridge is adjustable by means of potentiometer 114. The potentiometer 112 may therefore be equipped with a graduated dial and calibrated directly in any desired units of weight per unit length to agree with the scale of the weight indicator 104. This provides a direct reading indication of the setting of the control point, independent of the calibration of the weight indicator 104.

The error signal appearing on line 1&8 may be passed through a lead

network comprising resistors

124 and 126 and capacitor 128, Whose function will become more apparent after the servo system has been described. The error signal provides an input voltage to the servo amplifier 142 which energizes the servo motor 146. The motor 146 drives the speed control adjustment 156 for the pump motor 22 through reduction gears 150. The servo motor 146 also drives a tachometer generator 138 which develops a DC. voltage having a polarity in accordance with its direction of rotation and a magnitude proportional to its speed. The tachometer output voltage appears across the voltage divider network of potentiometer 134 and resistor 136. Depending on the setting of potentiometer 134, a portion of the tachometer voltage is fed back through resistor 132 to the input of the servo amplifier 142 in opposition to the error sig-' nal voltage.

The servo amplifier 142 is designed to have an almost infinite forward gain so as to saturate on a very small input signal. If the input voltage representing an error in measured fiber weight has a certain polarity, the servo motor 146 will be driven with full acceleration in one direction. If the error voltage has the opposite polarity, the motor will accelerate in the opposite direction. The motor will continue to accelerate until the voltage derived from the tachometer becomes equal and opposite to the error signal, at which time the input to the servo amplifier on line 130 is reduced to zero. At any greater speed the tachometer output through resistor 132 would become larger than the error signal, so that the input to the servo amplifier 142 would have the opposite polarity, tending to energize the servo motor 142 to drive in the opposite direction. It can be seen that by this means the speed of the servo motor 146 and the rate of readjustment of the pump motor speed control 156 is maintained instantaneously proportional to the magnitude and direction of the error in the measured weight of the fibers. Therefore, over any given period of time, the total amount of adjustment applied to the speed control 156 is proportional to the time integral of the error signal received over the same period of time.

The rate'of correction to the speed control per unit of error in measured fiber weight is referred to as the gain of the control system. The gain of the system is dependent on the ratio selected for the speed reduction gears 150. The gain is variable over a suitable range 6 by adjusting potentiometer 134, which determines the portion of the tachometer voltage which is fed back to cancel the error signal. Potentiometer 134 therefore determines the speed of the servo motor 146 which will be maintained for a given amount of error in fiber weight.

The maximum permissible gain of the control system is definitely limited by transportation lag,. that is, the length of time required for the effect of a step change in pump speed to be observed as a change in weight of the fibers passing between the source and the detector 82. If the gain is set too high, an existing error will be over-corrected before the gauge is able to see that sufficient correction has already been applied. Hence the extrusion process is caused to cycle? or perform forced oscillations around the desired fiber weight. On the other hand, it is desirable to keep the gain as high as possible to secure optimum performance of the controller.

In addition to transportation lag, there are other types ofdelays in the system which can, however, be compensated for' to a great extent. Examples of such delays are friction, mechanical inertia in the pump drive system or other controlled member, and electrical delays such as inductance in the field windings, etc., of a motor which is speed controlled. Such factors will prevent an instantaneous speed change of the controlled element in response to a change in the setting of the speed control represented at 156. The method of compensating for these delays is to provide a momentary overcorrection for any rapid change in the observed error, and a means of accomplishing this result is the provision of the lead

network comprising resistors

124 and 126 and the capacitor 128. Where a constant error signal is present, the signal is divided by

resistors

124 and 126, so that the portion of the signal across resistor 124 is not available as an input to the velocity servo. 'However, if there is a rapid change in the value of the error signal, the potential change is bypassed around resistor 124 through capacitor 128, so that a signal approaching the entire voltage change appears momentarily across resistor 126 alone as an input to the servo system. As a result, it may be said that the controller momentarily observes and attempts to correct for a larger error than actually exists in the measured weight of the fibers. The extra signal voltage may then decay according to the time constant of resistor 124 and capacitor 128, so that the over-adjustment of the speed control effected by the presence of the lead network may be corrected when the error signal approaches a steady state value.

The automatic control device briefly described above is the subject of a co-pending application Serial No. 641,- 414, filed February 20, 1957 by Philip Spergel and Sidney A. Radley, and accordingly the full details thereof are not included in this specification.

As previously indicated, the weight per unit length of fibers 54 is a function of the ratio of the velocities of motor 22 and motor78, the velocity of motor 22 controlling the weight per unit time of the fibers and the velocity of motor 78 controlling the length per unit time of the fibers. Therefore, it is within the scope of the present invention to control the speed of the motor 78 while holding the speed of motor 22 constant, this arrangement or the one illustrated in the accompanying figure being equally applicable to control of the weight per unit length of fibers 54. As a further alternative, the speed of both motors may be controlled.

It is within the scope of this invention to drive both the pumps and the winder bar with a single motor; one of these elements being coupled to the motor through the agency of a fixed ratio speed transmission means, and the other element coupled to the motor through a variable speed ratio transmission means, automaticcontrol being utilized to adjust the speed ratio of the transmission means. a

The'present invention has been described as applied to an apparatus for spinning yarns from a plurality of fibers extruded through an extrusion head. It is within the scope of the present invention to employ the apparatus to control the weight per unit length of any extruded material, such as in the extrusion of synthetic tows, or any other extruded materials wherein it is desired to maintain the weight per unit length of the extruded material within predetermined limits. The system is also applicable to the extrusion of a single fiber or to extrusion of single or multiple fibers extruded from a single or a plurality of extrusion heads. The invention has been illustrated in connection with an apparatus in which a winder bar is utilized as a take-ofi device. It will be understood, however, that the scope of the invention is not restricted to a winder bar take-oil": device, inasmuch as several difierent types of conveyor or transport devices may be utilized with or without the application of tension to the extruded material.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive.

What is claimed is:

1. An apparatus for extruding synthetic yarn comprising at least one extrusion head for extruding at least one synthetic fiber, pump means for forcing liquidous syn thetic material through said extrusion head to produce synthetic fiber, take-01f means for maintaining said fiber under tension, first drive means for driving said pump means, second drive means for driving said take-off means, adjusting means for varying the ratio of the speeds of said first and second drive means, a nuclear radiation source and radiation detector means for producing a signal indicative of the weight per unit length of said fiber, said source being mounted adjacent one side of the path of said fiber to provide a beam of radiation intercepting said path, and said detector being mounted in position to receive radiation from said fiber, and controller means responsive to said signal for actuating said adjusting means to maintain the weight per unit length of said fiber substantially constant.

2. An apparatus for producing extruded material cornprising at least one extrusion head, pump means for forcing liquidous material through said extrusion head to produce a length of said extruded material, take-oil means for maintaining said length of extruded material under tension, first drive means for said pump means, second drive means for said take-off means, adjusting means for varying the ratio of the speeds of said first and second drive means, a nuclear radiation source and radiation detector means for producing a signal indicative of the weight per unit length of said extruded material, said source being mounted adjacent one side of the path of said extruded material to provide a beam of radiation intercepting said path, and said detector being mounted in position to receive radiation from said extruded material, and controller means responsive to said signal for actuating said adjusting means to maintain the weight per unit length of said extruded material substantially constant.

3. An apparatus for producing extruded material comprising a plurality of extrusion heads, a plurality of pump means, each for supplying liquidous material to be extruded to a difierent extrusion head, a common means for driving all of said pump means, take-off means for maintaining said extruded material under tension, means for driving said take-off means, adjusting means for varying the ratio of the speeds of said first and second drive means, a nuclear radiation source and radiation detector means for producing a signal indicative of the weight per unit length of the extruded material produced by one of said extrusion heads, said source being mounted adjacent one side of the path of said extruded material to provide a beam of radiation intercepting said path, and said detector being mounted in position to receive radiation from said extruded material, and control means responsive to said signal for actuating said adjusting means to maintain the weight per unit length of said extruded material substantially constant.

4. An apparatus for producing extruded material comprising at least one extrusion head, pump means for forcing liquidous material through said extrusion head to produce a length of extruded material, take-oil means for maintaining said length of extruded material under tension, first drive means for said pump means, second drive means for said take-off means, adjusting means for varying the ratio of the speeds of said first and second drive means, a nuclear radiation source and radiation detector means for producing a signal indicative of the weight per unit length of said extruded material, said detector means being located between said extrusion head and said takeofi means at a distance from said extrusion head where the stretching of said extruded material is substantially complete, and controller means responsive to said signal for actuating said adjusting means to maintain the weight per unit length of said extruded material substantially constant.

5. An apparatus for producing synthetic yarns, comprising a plurality of extrusion heads each for producing a plurality of synthetic fibers, a plurality of pump means, each for forcing liquidous synthetic material to be extruded through a difierent extrusion head, a winder bar for Winding the fibers produced by each of said extrusion heads into distinct yarns, first drive means for driving said pump means at a determinable velocity, the velocity of said first drive means determining the weight per unit time of said fibers, second drive means for driving said winder bar at a determinable velocity, the velocity of said second drive means determining the length per unit time of said fibers, adjusting means for varying the ratio of the speeds of said first and second drive means, a nuclear radiation source and a radiation detector means for producing a signal indicative of the weight per unit length of the totality of fibers produced by one of said extrusion heads, said source being mounted adjacent one side of the path of said fibers to provide a beam of radiation intercepting said path, and said detector being mounted in position to receive radiation from said fibers, and controller means responsive to said signal for actuating said adjusting means to maintain the weight per unit length of all of said fibers substantially constant.

6. An apparatus for producing extruded material comprising a plurality of extrusion heads, a plurality of pump means each for supplying liquidous material to be extruded to a different extrusion head, a first drive means for driving all of said pump means, take-off means for maintaining said extruded material under tension, a second drive means for driving said take-oil means, a radiation source and detector means for producing a first electrical signal indicative of the weight per unit length of the extruded material produced by at least one of said extrusion heads, said detector means being located between said one extrusion head and said take-off means at a distance from said one extrusion head where the stretching of said extruded material is substantially complete, control means responsive to said signal to vary the ratio of the speeds of said first and second drive means to maintain the weight per unit length of said extruded material substantially constant, said control means including means for producing a second electrical signa indicative of a desired value of weight per unit length of said extruded material, a servo amplifier responsive to the difference between said first and second electrical signals, means for adjusting said ratio of speeds of said first and second drive means, motor means responsive to the output of said servo amplifier for actuating said adjusting means, means for producing a third electrical signal indicative of the rate at which said motor means actuates said adjusting means, and means for applying said third electrical signal to the input of said servo amplifier in opposition to the difference between said first and second electrical signals.

7. An apparatus for extruding synthetic fibers comprising at least one extrusion head for extruding a plurality of fibers, pump means for forcing liquidous material through said extrusion head to form said fibers, takeoff means for tensioning said fibers and conveying the same away from said extrusion head, a first driving means for driving said pump means, a second driving means for driving said take-off means, means for adjusting the speed ratio between said first and second driving means; a radioactive source and a radiation detector disposed at a predetermined distance from said extrusion head, said source being mounted adjacent one side of said fibers formed by said extrusion head and emitting penetrative radiation in the direction of said fibers and said detector being mounted in position to receive radiation from said fibers, a measuring system comprising a feedback amplifier and an electrical network for converting the output of said detector into a first electrical signal indicative of the weight per unit length of said fibers, adjustable means including an electrical network for producing a second electrical signal indicative of a desired value of weight per unit length, circuit means for combining said first and second electrical signals to produce a third electrical signal indicative of the difference therebetween, a servo amplifier having an input and an output, means for connecting said third electrical signal to said servo amplifier input, motor means responsive to sard servo amplifier output for actuating said means for adjusting the speed ratio between said first and second driving means, means for producing a fourth electrical signal indicative of the rate at which said motor means actuates said adjusting means, and adjustable means connecting said fourth electrical signal to said input of said servo amplifier in opposition to said third electrical signal.

8. In combination with an apparatus for producing extruded synthetic material including an extrusion head, pump means for forcing liquidous material through said extrusion head to produce a length of extruded material and driven take-0E means for maintaining said length of extruded material under tension; means for indicating the weight per unit length of said length of extruded material comprising a radioactive source and a radiation detector disposed between said extrusion head and said take-off means at a distance from said extrusion head where the stretching of said extruded material is substantially complete, said source being mounted adjacent one side of said length of extruded material and emitting penetrative radiation in the direction of said material and said detector being mounted in position to receive radiation from said material, a high impedance means for developing a signal voltage proportional to electrical current through said detector, an electrical network including a voltage source for providing an electrical potential opposing said signal voltage, an amplifier responsive to the difference between said signal voltage and said potential opposing said signal voltage, a feedback loop including said high impedance and said electrical network connecting the output of said amplifier to the input thereof so as to maintain said amplifier input at substantially zero potential, potentiometer means for calibrating the output of said electrical network, potentiometer means for standardizing the output of said electrical network, indicator means responsive to any output from said amplifier for registering the weight per unit length of said extruded material, and potentiometer means connected in the circuit of said indicator means for calibrating the sensitivity of said indicator.

References Cited in the file of this patent UNITED STATES PATENTS