Control For Roll Gap Of A Rolling Mill

Abstract

A feedback control system for a gap between rolls in a mill includes a variable inductance with a core on one roll and an armature on the other roll. This inductance, as well as a reference inductance and two auxiliary inductances, are interconnected to establish AC biased bridge. The relative phase of the bridge diagonal voltage is used to control the gap width. The pick up inductance and one of the auxiliary inductance are mounted in physical proximity to each other.

Description

The present invention relates to an arrangement and apparatus for controlling the gap or clearance of and between the rolls of a pair of rolls in a rolling mill, to operate by means of automatic control equipment and using particularly inductance means for measuring the width of the gap. The inductance means is to include a core and an armature, and the effective inductivity of that inductance is variable through variation of the air gap between core and armature.

It is customary in rolling mills to adjust the desired thickness of the product to be rolled by adjusting the pressure exerted upon and by the operating rolls. Such a more or less indirect control does not permit compensation for all kinds of disturbances as they may arise. Accordingly, the rolled stock has accurate thickness only within a fairly large range of tolerances. In order to narrow the tolerances it is necessary to provide direct control which includes rather exact measuring of the roll gap. Such a requirement for obtaining sufficiently accurate control is best carried out by means of electronic control and regulating equipment.

For purposes of gap control through a feedback system it is, of course, necessary to measure the gap between the rolls and to provide representation thereof as an electrical quantity. The roll gap width of a pair of rolls has been detected previously directly and electronically by means of optical scanners or by means of feeler and lever systems coupled to a potentiometer or to an adjustable inductance. It was found, however, that particularly the latter kind of electromechanical equipment is rather complicated, and adjustment thereof is rather difficult to obtain. Cumbersome adjustment procedure is particularly disadvantageous for cold rolling because for reasons of surface finish it is required here to change the operating rolls about every 2 to 3 hours.

Another known method for measuring the roll gap is ascertained by contactless gauging. This method utilizes a stationary magnetic field for measuring the roll gap. The strength of the magnetic field is approximately inversely proportional to the roll gap operating as an air gap within the magnetic field path. The dimensions of this air gap are measured by placing Hall generators or indium antimony probes (field plates) into the magnetic field path. The electrical output derivable from these pick up and field sensing element includes indeed representation of the roll gap as an electrical signal.

It was found that the aforementioned method requires rather strong magnetic fields across this roll air gap because field plates and Hall generators are rather insensitive to weak magnetic fields. In case a strong magnetic field is employed, inaccuracies are introduced in the measuring result particularly if the rolled stock is ferromagnetic material. In this case shavings and other particles may be attracted by the rolls, and it is difficult to protect the measuring instrument against the resulting disturbances on the probing field. Furthermore, the production of a stationary and constant magnetic field requires that the field generating current has to be maintained constant rather accurately, as fluctuations in the exciter current deteriorate the accuracy of measuring.

Another point to be considered is that production of a sufficiently strong magnetic field is accompanied by considerable heating of the instrument. This fact, in turn, requires additional provisions for temperature compensation. It can be seen that employment of a strong, constant magnetic field requires considerable expenditure and has many inherent disadvantages; it is, therefore, not surprising that this method has not gained acceptance.

The problem, therefore, still exists to provide a method for measuring the gap width between the rolls of a pair of rolls in a rolling mill, and seemingly utilization of capacitive or inductive phenomena may be suited best. However, the gap detector cannot be protected against cooling liquid during rolling. Thus, capacitive methods must be excluded for all practical purposes because the coolants have relatively large dielectric constants. This narrows the suitable apparatus to employment and utilization of inductive phenomena. Considering this point, the invention relies on particular utilization of inductive phenomena as was outlined in the introduction but obviating the prior art deficiencies.

In accordance with the preferred embodiment of the present invention, it is suggested that the gap width be represented by variation of an air gap between a core of a pick up inductance and an armature thereof. Armature and core are disposed in two separate carriers which respectively receive journals of the rolls, so that the distance between core and armature represents the roll gap as air gap in the inductance. The core of this pick up inductance as well as an auxiliary inductance are mounted together in one of the two carriers. These two inductances are included in a bridge as two serial branches connected across a bridge biasing source. The bridge includes additionally an adjustable inductance to provide a desired or reference value, and another auxiliary inductance completes the bridge. A control signal is derived from one of the diagonals of the bridge. In case the actual value of the roll gap deviates from the desired value, that control signal is used to operate upon a controller for adjusting the roll gap and completing a feedback loop.

The inventive arrangement is characterized particularly by a rather simple construction, by insensitivity against mechanical disturbances, and particularly by ready adaption to large and small gaps. Moreover, the resolution of the roll gap measuring method and accuracy thereof are particularly high. Installation of the equipment does not pose difficulties particularly because a reference value of zero for the gap width to be measured can be established simply by moving the armature into engagement with the core corresponding to a gap width zero, and the resulting inductivity defines the base line or zero reference for gap measurement.

The combination of pick up inductance and of one of the auxiliary inductances of the bridge provides a pick up device from which, within certain limits, influences of temperature have been eliminated, because both inductances as well as their loss resistances are subjected to the same thermal conditions. Another advantage results from a rather simple layout of the entire electronic circuitry, because voltage and current stabilization is no longer required. Moreover, it is no longer necessary to provide particular linearization as far as relationship of desired and actual value for the gap is concerned. Furthermore, the dynamics of the control loop is independent from the adjusted air gap of the pick up inductance.

The arrangement in accordance with the invention is also useful in rolling mills used for rolling ferromagnetic material because the low AC induction in the gap area is insufficient to attract any shavings or cuttings. Finally, the inductive phenomena utilized is not sensitive to engagement with a coolant.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention, and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings, in which:

FIG. 1 illustrates somewhat schematically the basic arrangement of a roll gap control apparatus improved in accordance with the present invention;

FIG. 2 illustrates the measuring and pick-up instrumentality employed in the arrangement of FIG. 1; and

FIG. 3 illustrates somewhat schematically a circuit diagram of the entire roll gap control loop.

Proceeding now to the detailed description of the drawings, FIG. 1 illustrates, as stated, somewhat schematically the basic arrangement for roll gap control. A journal 11 extends axially from a lower roll 10 and carries a support ring 13 for a pick-up means carrier 12. The rolling mill includes an upper roll 14, and the material 16 to be rolled, i.e., the stock is disposed in the gap 19 between rolls 10 and 14. The upper roll 14 has likewise a journal 15 carrying a ring 18 upon which is placed a second carrier 17 constituting measuring plate.

The measuring instrumentation and pick-up means are included in carrier 12 as will be described more fully below. Suffice it to say presently that an output cable means 12a provides electrical signal which represents the width of roll gap 19. A control circuit 20 receives in addition a reference signal representing the desired value for the gap and being developed by an appropriate signal source 21. The control circuit 20, of course, includes amplifier means and provides an output used to control an electrohydraulical servo valve 22. Valve 22 governs an hydraulic actuation 23 which determines and adjusts the pressure exerted upon rolls 10 and 14 for adjustment of the roll gap.

The pick-up device 12 is shown in greater details in FIG. 2 and particularly its cooperation with the measuring plate 17 is shown with particularity. The pick-up means 12 includes a pair of coils 30 connected in series and being wound upon the two legs 33 of a U-shaped core 32 to establish a pick-up inductance. The two legs 33 respectively have front faces that are oriented coplanar to each other and transverse to the plane of the U of core 32, whereby particularly these two front faces of legs 33 define a datum plane 34. The second carrier 17 of the pick-up system is positioned adjacent to the datum plane 34 and defines particularly a second measuring plane 36 that is parallel to plane 34. Carrier 17 is provided particularly for the support and positioning of an armature 35 extending lengthwise to plane 36, and the one side of armature 35 facing the legs 33 of core 32 is in that plane 36. Plane 36 may also define the lower surface of measuring plate 17 as a whole. The two planes 34 and 36 are spaced apart by a gap 37. The entire equipment is adjusted so that distance between planes 34 and 36 corresponds to the distance or gap width of the rolls across gap 19. Therefore, the roll gap is represented by the distance between the core 32 and armature 35, establishing a variable air gap for the pick up inductance.

The pick-up means includes further an auxiliary inductance 31 which includes a pair of coils connected in series and wound upon the legs of a U-shaped core 38. The auxiliary inductance includes likewise an armature 39, and the two legs of core 38 have front faces facing armature 39. There is an air gap between core and armature which is adjusted to obtain a fixed spatial relationship.

The auxiliary inductance 31 is completely embedded in shielding 40 made of material of a high magnetic permeability. Likewise, core 32 is embedded in a shield 42 of similar material but being open in plane 34. Armature 35 is lined by shielding 41 of material of high magnetic permeability but the shield is open in plane 36. Thus, the air gap between armature 35 and core 32 is not lined with shielding.

As is shown also schematically in FIG. 2, the inductance as established by two serially interconnected coils 30 on legs 33 is connected in series with the auxiliary inductance 31. The common junction provides and is connected to a measuring outlet line 26. The respective two other ends of inductances 30 and 31 lead to two additional connecting lines 25 and 27, provided for applying voltage across the two serially connected inductance systems. Accordingly, there are three connecting lines for this particular pick-up arrangement.

FIG. 3 illustrates a circuit in which the several coils appear as inductances 30 and 31 and wherein particularly inductance 30 represents the principal source for measuring input signal. The series circuit connection of inductances 30 and 31 is likewise depicted in FIG. 3, and it can be seen that the two inductances form two branches of a bridge circuit. Two inductances 50 and 51 are connected in series to each other, and they are additionally connected in parallel to inductances 30 and 31, for completing the bridge. Inductance 50 is variable to provide and to establish reference signal in representation of a desired value for the roll gap. Inductance 51 is the second auxiliary induction in the system.

While not essential in principle it is convenient to think of inductances 30 and 50 as being similarly constructed. Thus, inductance 50 may include a core with an armature, and the armature is adjustably positioned to vary the air gap in that inductance. For example, the air gap is adjusted by means of a micrometer screw to assume the same value that is desired for the roll gap. Inductances 31 and 51 may also be similar. This similarity is appropriate to obtain complete electric symmetry in the bridge circuit.

Inductances 50 and 51 can be regarded as representing the reference means, summarily denoted by numeral 21 in FIG. 1. The reference inductance 50 and the auxiliary inductance 51 are particularly connected to inductances 30 and 31 in that the one end of auxiliary inductance 51 connects to one end of auxiliary inductance 31 to form a bridge terminal or junction A. The one end of inductance 50 connects to one end of inductance 30 to form another bridge terminal or junction B. The two terminal A and B are connected to a biasing source, AC generator 55. The voltage drop across inductance 30 can be regarded as signal representing the actual value for the gap, while the voltage drop across inductance 50 constitutes the reference signal.

The junction between inductances 50 and 51 constitutes a third bridge terminal D and is grounded. The bridge output signal can, therefor, be taken from the fourth bridge terminal C, with reference to ground. Terminal C is, of course, the connecting junction of inductances 30 and 31.

Resistances RCU are included in the bridge circuit, merely to represent ohmic losses of the respective inductances. However, during operation the bridge is presumed to remain balanced as to ohmic losses. Trimming resistors may be included in the circuit if necessary to obtain balance between ohmic losses and inductance drops.

A phase detector or phase sensitive rectifier 56 has one input of a first pair of AC input terminal connected to ground (i.e. to bridge terminal D), while the second AC input of the first pair is connected to bridge terminal C which is actually line 26 as leading from the junction of inductances 30 and 31. The phase dependent rectifier 56 has a second pair of AC input terminal connected to generator 55. The output of rectifier 56 represents the phase difference between the bridge bias voltage and the bridge voltage as derived from across diagonal C-D, and this phase difference, in turn, is a representation of the differences in effective inductance of the inductances 30 and 50.

The DC output voltage of phase dependent rectifier 56 is passed through a filter 57 to eliminate AC components and is fed to a control amplifier 59. Amplifier 59 controls the servo valve 22 which, in turn, and as was outlined above, controls hydraulic means 23 for adjusting pressure upon the lower roll 10. As a consequence, the gap between rolls 10 and 14 is changed which, in turn, reflects upon the inductance 30 for closing the control loop. The dashed line between elements 22 and 30 shows the actuation path of the feedback loop.

Control amplifier 59 is additionally connected to a potentiometer 58 having its tap connected to the micrometer spindle by means of which inductance 50 is adjusted. The adjustable resistor 58 is included in the amplifier circuit to determine the gain of the amplifier. Thus, the amplifier gain is made dependent upon the adjusted reference value for the gap.

All four inductances of the system in the bridge circuit are rated, and are particularly constructed, so that they have similar inductivity for similar air gaps. As stated above, the measuring inductance 30 and the auxiliary inductance 31 are combined to form a measuring pick-up means, and the reference inductance 50 may be disposed in relation to auxiliary inductance 51 to form a structurally combined reference means. Due to physical association of inductances that connect serially across the supply source 55, the bridge is rendered independent from temperature variations that affect serially connected inductances similarily. Temperature variations affect particularly loss resistances R.sub.cu in the respective inductances. Even if the bridge is detuned to some extent because of imbalance among the loss resistances, errors are not introduced into the measuring output, because unbalanced signal components across the loss resistances exhibit a phase shift by 90.degree. in relation to the control information proper included in the bridge voltage across terminals C and D. Therefor, these residual errors appear as AC signals without DC components in the output of phase dependent rectifier 56 and are eliminated by filter 57 accordingly.

If the measuring equipment is employed, for example, in a hot rolling mill, temperature variations that the equipment may have to experience can be expected to be quite large. Severe thermal imbalance can be compensated by embedding resistors with negative temperature coefficients in the coils of the several inductances. As these resistors are also included in the bridge circuit, temperature compensation is obtained, that is effective particularly for large scale temperature variations, requiring very little additional equipment. As the bridge circuit is symmetrically constructed and connected even significant temperature variations hardly affect accuracy of measurement.

Generator 55 supplies the bridge with AC having a frequency selected so that WL/RCU for any of the individual inductances is quite large. On the other hand, the frequency should not be so high that eddy current losses in the cores of the several inductances or changes in capacity of the connecting lines exert any significant influence upon the accuracy of measurement.

As can be seen, the circuit in FIG. 3 is connected and designed to control amplifier 59 receives information from the bridge circuit via phase dependent phase rectifier or phase detector 56 as so-called zero information, i.e., in case the reference value for the roll gap agrees with the actual value thereof. To obtain this static condition, additional off-set means may be included in the input circuit for amplifier 59. Amplifier 59 receives zero input. This mode of developing control signals for the regulation eliminates the requirement for the usual constant current or constant voltage sources which, if needed in the present case, would require considerable expenditure due to requirement of high signal-to-noise ratio. The high permeability shields of the several inductances eliminate external stay field and improve the signal-to-noise ratio further.

The control circuit includes amplifying circuitry 59 that is operated in accordance with the principles of operational amplifiers, in which the desired response is obtained through adjustment by means of an external network. Additional input signals for control and regulator amplifier circuit 59 may be combined with the output from filter 57, for example, in accordance with the principle of error feed forward operation.

As stated above, potentiometer 58 is ganged with the micrometer spindle for adjusting the reference value in inductance 50, and this potentiometer 58 is included in the amplifier circuit. This way the loop gain of the control and regulating loop is maintained constant and, therefore, optimizes transient response behavior of the system to be independent from the adjusted reference value for the roll gap. This compensation of loop gain is necessary as the measuring inductance varies hyperbolically with width of gap 37, and the voltage gradient across the bridge decreases with increasing in the air gap.

The arrangement in accordance with the embodiment of the present invention as aforedescribed, may be modified in that the carrier 12 for the pick-up means is by itself constructed of ferromagnetic material to provide magnetic shielding directly. Analogously, the measuring plate 17 can be constructed from ferromagnetic material obviating in this case the particular shield 41.

The invention is not limited to the embodiments described above, but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

Claims

I claim:

1. Apparatus for control of the gap between a pair of rolls in a mill, the rolls of the pair having journals, comprising:

a first carrier on one of said journals;

a second carrier on the other one of the journals, the first and second carriers spaced apart;

first means defining a pick-up inductance in the first carrier and including a U-shaped core having legs establishing a datum plane, and coil means on the core;

second means defining an auxiliary inductance in the first carrier and disposed in vicinity of the pick-up inductance to be subjected to similar temperature;

armature means in the second carrier and magnetically cooperating with the core of the first carrier but spaced from the datum plane, so that the distance between the first and second carrier determines an air gap in the inductance of the first means corresponding to the gap width between the rolls;

third means, including first and second reference inductance means, at least one thereof being adjustable, both of them being connected to the inductance of the first and second means to define a bridge circuit;

fourth means including a source of AC potential connected to electrically energizing the bridge circuit;

fifth means connected to the bridge circuit to derive therefrom a control signal; and

control means connected to be operated in response to the control signal to adjust the gap of the rolls so as to complete a control loop.

2. Apparatus as in claim 1, the inductances of the first, second and third means each having an air gap, the air gap of the auxiliary inductance and of the first and second reference inductance means being selected and adjusted to remain constant, the air gap of at least one of the reference inductance means adjustable to obtain reference value adjustment, there being means for adjusting the air gap of the one reference inductance means, the inductances having equal inductivity for similarly adjusted air gaps.

3. Apparatus as in claim 2, wherein at least one of the reference inductance means is adjustable by micrometer means.

4. Apparatus as in claim 1, the inductances of the first and second means connected in series to each other and parallel to the fourth means, the reference inductances of the third means connected in series to each other and in parallel to the inductances of the first and second means as connected to the fourth means, the control voltage being derived from between the junction of the inductances of the first and second means and the junction of the references inductances of the third means.

5. Apparatus as in claim 4, the AC voltage having frequency so that the inductivity vs ohmic loss ratio of either inductance is relatively larger at still low eddy current losses in the respective cores and relatively low capacitive losses.

6. Apparatus as in claim 4, the fifths means including a phase sensitive rectifier connected to receive the AC voltage as provided by the fourth means and further connected to receive a bridge voltage as derived from the junctions to provide the control voltage, the control voltage being zero when the roll gap agrees with the reference gap, the fifths means including a filter to eliminate the a.c. component from the control voltage.

7. Apparatus as in claim 6, the control means including an operational amplifier, the one reference inductance having adjusting means coupled to the operational amplifier to vary the gain thereof to maintain the loop gain of the control loop.

8. Apparatus as in claim 6, the operational amplifier having means to adjust its response.

9. Apparatus as in claim 1, the first and second carriers made of non-magnetic material, the inductances in the first carrier and the armature in the second carrier being shielded by means of material having high magnetic permeability.

10. Apparatus as in claim 1, the first and second carriers made of ferromagnetic material to serve as magnetic shields.