METHOD AND MEANS FOR MONITORING OF CONTINUOUS CASTING

Abstract: Means and equipment for monitoring of the operation of a continuous casting equipment comprising a mould (1), a drive mechanism (3, 4), adapted to generate an oscillatory movement, and a mechanism to transmit the oscillatory movement from the drive mechanism to the mould. There are generated, by means of at least one motion sensor (2), which operates in dependence of a parameter of the actual movement of the mould, a first signal (A1...AN) corresponding to the said parameter. A pulse emitter (3), operating in dependence of a parameter, corresponding to the movement of a part of the mecanism (4, 5), is utilized to generate pulses (S1...SN) corresponding to the said parameter each time the mould passes a predetermined, arbitrarily chosen phase position during its oscillatory movement. There are formed, by means of pulses from said pulse emitter (3), a number (N) of pulses (S2) which are evenly spaced in time between two (S1)-pulses. The sensor signals (A1...AN) are sampled, at each (S2)-pulse, and are accumulated in successive registers (R1...RN), while restarting the accumulation in the first register (R1) for each time a pulse (S1) occurs.

Description: Method and means for monitoring of continuous casting. The present invention relates to a method and means for monitoring of continuous casting. In SE-C-8004494-4 there is disclosed a method for monitoring of the operation of a continuous casting equip¬ ment comprising a mould, a drive mechanism adapted to generate an oscillatory movement, and a mechanism to transmit the oscillatory movement from the drive mechanism to the mould. The method as disclosed therein comprises (a) measuring a parameter, depending on the actual movement of the mould, viz. the displacement, speed or acceleration thereof, generating a first signal, corres¬ ponding to that parameter, chosing a first frequency band, and determining the component of the first signal, lying within that frequency band, termed "useful signal U", (b) measuring a parameter corresponding to the move¬ ment of a part of the mechanism, preferably as close to the drive mechanism as possible, viz. the displacement, speed or acceleration of that part, generating a signal corresponding to that parameter, chosing a second fre¬ quency band, and determining the component of the second signal, lying within that frequency band, termed "refe¬ rence signal R", (c) continuously or intermittently comparing the useful signal U with the reference signal R, particularly by calculating the difference U-R or the relation U/R, (d) and determining therefrom the deviation of the actual mould movement from the movement of the said part of the mechanism. Also US-A-4 758 943 relates to a method of sampling and utilizing signals from an oscillating system. The invention as disclosed in that patent has for its object to automatically adjust the setting of a regulator. A relay feed back is introduced to replace the regulator, and the self-oscillation generated thereby is used to calculate a new setting of the regulator. The fundamental tone of the generated oscillation forms the base for the choice of sampling of the measured signal. A number of samples are taken, the same number for each half period, and in any case at least two. Then, the number of samples for each half period may be successively increased in order to gradually make the calculations more exact. In the patent it is also mentioned that a number of periods of the signal may be utilized to calculate a mean curve, following the stabilization of the signal. Processing signals from sensors, arranged on an os¬ cillating continuous casting mould is known per se from DE-A1-3 022 963. According to that patent the movement is measured, in principle, at two points within the mould machinery. One point is at the mould itself, and the other one at the drive machinery, as close to the drive motor as possible. In principle, the two movements are measured in the same manner at both points. Signals, processed in principally similar manner, are derived from signals from both of the measuring points. These processed signals are then compared with each other in one of a plurality of defined ways. From that compari¬ son between the signals there is formed a measuring value which is monitored during the casting. On processing of the signals from the sensors it is also possible to chose the part which is situated within a predetermined frequency range, and the range between 0 and 5 Hz is mentioned as an example. By measuring at the point which is closest to the motor it is also mentioned that this advantageously may be disposed at a location where an absolute freedom from play has been determined. In this context also the invention according to US- A-4 532 975 should be noted, which comprises measuring the motion at two locations, at least, at the mould, and then using the peak-to-peak value of the comparison signal from two sensors at these two locations to indicate that there exists a difference in the oscillatory motion at these locations. The invention involves monitoring the oscillatory motions by means of load sensors, so called load cells, or other sensors, which emit signals that represent the os- ciallatory forces, and forming at least one comparison signal is formed, and detecting the peak-to-peak value of that signal. A plurality of sensors of these types may be used, and the signals therefrom may be combined to form a signal which corresponds to the sum of the forces on the mould. This signal is then registered, when the machine is free- running. The difference between the peak-to-peak values of this registered signal and of the one which is detected during casting operation is utilized as a measure of the frictional load on the mould. Further, the invention according to EP-A1-0 044 291 should be noted in this context. This resembles the just above described invention in that it utilizes a plurality of load cells to sense the loads between the mould and the drive machinery. From the total of these sensed loads are subtracted on one hand the static own weight of the mould, and on the other hand the so called dynamic load, sensed by means of an accelerometer on the mould table. The momentary value and/or the peak value of the resulting signal is detected and shown. The methods and equipments as described above have been found to operate comparatively satisfactorily, but it has been noted, however, that they suffer from certain limitations and drawbacks, for which reason considerable improvements and refinements may be achieved. To the just mentioned end the method according to the present invention is characterized, in principle, in utilizing a parameter, corresponding to the movement of a part of the mechanism, to generate pulses by means of a pulse emitter each time the mould passes a predeter¬ mined, arbitrarily chosen phase position during its oscil¬ latory movement, forming, by means of said pulse from said pulse emit¬ ter, a number of pulses which are evenly spaced in time between two pulses from said pulse emitter, and sampling said sensor signals at each pulse and accu¬ mulating them in successive registers, while restarting the accumulation in the first register for each time a puls occurs. Corresponding to what has been mentioned above, an equipment to carry the just defined method into effect according to the present invention is characterized, in principle, in that it comprises a pulse emitter, adapted to generate, in dependence of parameters of the movement of a part of the mechanism, pulses corresponding to the said parameters each time the mould passes a predetermined, arbitrarily chosen phase position during its oscillatory movement, a device to form, by means of said pulses from said pulse emitter, a number of pulses which are evenly spaced in time between two of said pulses, and a device to sample said sensor signals at each (S2)- pulse and accumulating them in successive registers, while restarting the accumulation in the first register for each time a pulse occurs. The invention will be disclosed in more details below with reference had to the accompanying drawing wherein an equipment according to the invention, picked as example, is illustrated very diagramatically. In the drawing a mould with a mould table has been generally denoted 1. The mould and the mould table are provided with a plurality of movement sensors (accelerome- ters) 2, which are adapted to emit a plurality of signals Al-AN. In connection with the mould there is arranged a motor driven shaft 4, which is provided with one or more excenter devices 5. The motor driven shaft 4 has a member which is adapted to cooperate with a pulse emitter 3, which emits pulses SI in dependence of the rotation of the shaf 4. The equipment operates in such a manner that the movement sensors 2, which are located directly on the mould, or a part rigidly secured thereto, convert the acceleration of the mould at the respective sensor locat ion to an electric signal Al, A2 .... and so on. The pulse emitter 3, located in connection with th mould drive machinery, emits a pulse (SI) each time th mould, during its oscillatory movement, passes a prede termined but arbitrarily chosen phase location. By means of the pulse from the pulse emitter ther are formed a number (N) of pulses (S2) , preferably an eve power of 2, which are evenly spaced in the interval bet¬ ween two (SI)-pulses. The sensor signals Al, A2 ... are measured (sampled) by each (S2)-pulse and are accumulated in successive re¬ gisters Rl, R2, ... RN. Each time a (SI)-pulse occurs the accumulation restarts in the register Rl. Because of this accumulation method there is ensured a reduction of con¬ tribution to the signal from frequency components that cannot be expressed as an integer multiple of the oscilla¬ tion frequency. By a methematical process the contents of the accumu¬ lator registers Rl ... RN is converted to a discrete fre¬ quency spectrum of the mould speed, or - more correctly- an estimation thereof. From this spectrum the final mea¬ sured value (D) is calculated as follows: D = s q rt (.-SHΓT-. K * «n) ) * 100 where a± is the amplitude of the component having the frequency i*f f is the oscillation frequency n has the values 3, 4 ... , N m has the values 1, 3, 4 ....,N The variation of the D-values as a function of the time is presented as an indication on variations of the pattern of movement of the mould, which in turn are depen¬ dent on variations of the friction loads and the bearing loads. By use of the method and the equipment according to the present invention it is contemplated to be possible to achieve the following effects slightly better than in the past, viz. - optimizing of the casting parameters for different steel qualities, - detection of breakouts by skin sticking, - detection of mechanical faults in the mould machi nery, - prediction of the blank quality. While using the method according to the invention by testing one embodiment of an equipment according to the invention the following has been noted: When the output signal D has been used as a measure of the friction steel-mould, it has been found that a high l2θ 3 -content of the slag (up to 20% has been noted by start up of the casting as compared to normally 7-10%) , gives a bad lubrication and a high friction. Blanks pro¬ duced close to start up and end of the casting operation exhibit most of the faults. The friction (distorsion signal) increases by start up and when the casting is stopped, e.g. by shift of mould box. By shift to a different steel quality during casting (lowering of the level in the mould and stop of the cast¬ ing bar) an obvious deflection with higher friction is shown. A sweeping of the pouring spout results in a friction increase for 10 - 20 minutes following the sweeping. An adjustment of the width towards a narrower casting bar results in an increase of the pressure against the casting bar, which leads to a notable higher friction level during the adjustment. The casting speed has an influence on the friction. A replacement of the end walls leads first to a slightly higher and irregular friction which then decreas¬ es. Qualities having a higher content of carbon (>0.10%) exhibit a higher friction, from which may be concluded that the casting powder which is presently used for all steel types, is not the optimal one for higher contents of carbon. Wider mould forms give higher friction levels. A varying level within the mould gives higher fric¬ tion due to disturbance of the lubrication. The effect of the difference between automatic and manual control of the level can be evidently observed. By casting an abnormal friction can be linked with: - return steel with additional lance scavenging, Al-addi¬ tion beyond the standards, etc., i.e. a "bad" charge, - the period immediately prior to a mechanical failure of the link system, - an additional hole in the pouring spout (below the bath level) , that disturbed the melting of the casting powder (a lower consumption could be observed) , with attendant lubrication problems, - a crack in the pouring spout above the bath level (noted subsequently) . The high friction could be reduced, during the cast¬ ing operation, by decreasing the casting speed and shift of the casting powder. Irregular and coarse oscillation marks could be ob¬ served on the actual blanks. From the above it may be concluded that the method and the equipment according to the invention form good aids to detect disturbances of the mould lubrication, and that the system is useful in operation as a monitoring instrument that gives an early warning that something in the production chain is not correct. The invention may also be expressed in the following way: A dynamic model is made to describe the interrelation between forces, which act on the mould, and the movement thereof. The model may be, for instance: Drive forces = Frictional forces + dynamic force, where the driving for¬ ces are described with one degree of freedom, viz. the difference between driving motion and speed, or the mould movement and speed, respectively. Here, the drive machi¬ nery is assumed to possess a certain resiliency, kl, and a certain damping, dl. The dynamic force is described as the product of the mass of the mould and the acceleration thereof according to Newton's 2 nd law. In the next moment, the fixed model parameters comp¬ rised in the above disclosed model are determined, i.e. kl, dl and the effective mass of the mould, mO, and, furthermore, the relationship between the signals from the pulse emitter, and the movement (position) and speed, respectively, of the drive mechanism, as well as the amp¬ litude the oscillation of the mould attains without influ¬ ence from frictional forces from the steel. These parame¬ ters are determined either through calculations from known design values or by operation of the machinery without steel, and measuring on the same, or by a combination of these two methods. Within the scope of the general inventive idea it may also be contemplated to proceed in the following manner: A model is formed that describes how the frictional force is combined from the influence of different physical phe- nomenons. For instance, this model may be designed in the following way, where the frictional force is composed from the following portions: 1. A solid body friction, comprising a constant for¬ ce, acting against the relative motion mould-steel, which is modfified in such a manner that this constant value is attained at first as a predetermined lowermost mould-steel speed difference has been attained. This friction is as¬ sumed to be zero when the speed difference is zero, to then increase linearly, unto the constant value has been attained. 2. A viscous frictional force, which is proportional to the steel-mould speed difference. 3. A viscous frictional force, which is proportional to the steel-mould speed difference, and which, furthermo¬ re, depends on the thickness of the slit between solid steel and the mould, which is filled by a viscous material (molten steel and/or molten casting powder) . As the mould oscillates, this slit will vary in thickness, due to the fact that the mould may be slightly tapering and because the steel casting cools, and thus shrinks. Further, the frictional force is maximized by the force which will be able to lift the portion of the cast¬ ing bar that depends below the mould. During measuring the total frictional force is cal¬ culated from the relationships that follow from the dyna¬ mic model. Then, the total frictional force is described as a combination of the various portions of the model of frictional forces. The combination is chosen that gives the best description, where, as a criterion of which de¬ scription is the best one, e.g. that criterion may be chosen which gives the smallest deviation, measured as the sum of the square deviation at each sampled point, between the combined force and the total frictional force, deter¬ mined as set forth above. Finally, the frictional course at the actual moment of measuring is described as the set of parameters within the frictional model that has given the best description. Then, the model of the frictional force may be suc¬ cessively improved in such a manner that it, with a cer¬ tain predetermined number of parameters, gives descrip¬ tions which, seen over a longer period of time, result in the smallest deviation, as an average, between the model and the actual measuring. As compared to the prior art, as manifest by the equipments according to the above mentioned patents, the method and the equipment according to the present inven¬ tion imply decisive differences. As contrasted to the invention according to US-A- 4 758 943, the sampling frequency according to the present invention is not determined directly by the measured sig¬ nal. This signal may be so distorted that determination of passes through zero and number of periods may be rendered difficult. Instead, the determination is performed by means of a separate puls emitter on the drive machinery, which affords a more distinct definition of the frequency of the fundamental tune of the motion. This also enables a measuring of momentary phase displacements between the motions of the machinery and the mould, which is of great interest for estimation of the magnitude of the frictional forces. Due to the fact that, according to the present invention, the number of samples per period is a power of 2, e.g. 32, 64, 128 and so on, great advantages are achieved by use of a Fast Fourier Transform-technique at determination of the power of the harmonics, which is imperative for the performance of the invention. According to the present invention it is defined that a separate device forms pulses which are evenly spaced over the period time of the machine oscillation. These pulses form the sampling occasions. The values obtained in this manner are used to form a mean curve. This means that the requirement for a stabilization of the measured signal is strongly reduced. The measuring may be perform also when the number of periods varies strongly, which is very well possible by continuous casting operations. Comparing with the invention according to DE-A1 3 022 963 it should be noted that, according to the present in¬ vention, only one measuring of the motion at the mould is performed. As a reference only the puls emitter is util¬ ized to give a phase reference from the drive machinery. This means a great simplification. According to the pre¬ sent invention a measured value (D) may be obtained by processing only the signals from an emitter at the mould, where the pulse emitter is utilized solely to sample the signal from the "motion" emitter. In this manner the problem of finding a location with absolute freedom from play is avoided, which is important with the equipment according to the just mentioned German patent. According to the present invention also most of other concievable ways are avoided, in which disturbances of the motion of the second sensor location - or of the measuring of that motion, which for various reasons are not transmitted to the mould - could otherwise affect the calculation. The present invention differs fundamentally from the equipment according to the German patent also in another respect, viz. in that therein the whole signal within a certain frequency interval is utilized, whereas in the present case, in principle, only those discrete frequency components are utilized which are harmonics to the funda¬ mental frequency of the machine. Further, the frequency components which are predomi¬ nantly used according to the present invention are rather disposed entirely outside the interval (0 - 5 Hz) that has been mentioned as an example in the German specification. It has been verified by tests that one, by omitting the lowermost portion of the frequency range, may achieve a more reproducable signal, which at checked test could more easily be referred to known deviations of the casting conditions. Now, to compare with the invention according to US-A- 4 532 975, it will be noted that this invention requres measuring of internal loads or forces within the machine¬ ry. This in turn requires that certain parts of the machi¬ nery are replaced against parts having integrating force sensors, or that the parts are machined in order to be provided with some type of force sensors. Experience has shown that calculations of the load on the mould, with some types of machinery, requires measuring of internal forces at many locations. This means practical difficul¬ ties by the installation of the sensors, which may be avoided according to the present invention in that accele- rometers may be utilized, which are easier to install. It has been possible to prove, by measuring on acce- lerometer signals, that a dominating proportion of the disturbance of the acceleration, and thus of the force according to Newton's 2 nd law, may occur during those periods of the oscillation cycle when the frictional for¬ ces change direction. These periods do not occur during maximum or minimum acceleration, for which reason the phenomenons do not directly affect the peak-to-peak values of the force. According to the present invention the vari¬ ation of the acceleration, and thus of the frictional force, is detected with the same sensivity, irrespective of when it occurs during the oscillatory cycle. Measurements have also shown that the amplitude (the half of the peak-to-peak value) is clearly less good, when it comes to detection of most of the normal casting faults (cracks etc.), than is e.g. the phase displacement between force (acceleration) and feeding motion, steel level in the mould or the first harmonic of the force. As the pre¬ sent invention is directed towards measuring of harmonics, it is more sensitive when it comes to detection of dis¬ turbances of the casting conditions. With respect to the use of force sensors the same comments may be made regarding the invention according to EP-A1- 0 044 291 as has been made with regard to the just discussed invention according to US-A- 4 532 975. However, there exists a fundamental problem with the method as applied according to both EP-A1- 0 044 291 and US-A- 4 532 975. This problem resides in that fact that the measuring value sought for, the variation of the fric¬ tional force, is formed by subtracting a plurality, in this case three, different types of signals. Further, one of these signals is formed by summing, in the disclosed case, four load cells. Disturbances and measuring errors with these individual signals will strongly affect the difference between them, and will thus aggravate the mea¬ suring method. Another fundamental problem that the invention ac¬ cording to US-A- 4 532 975 suffers from is that the static own weight, to which it is referred, in practise will vary during the casting operation, and during the individual oscillatory period, because the casting bar within the mould will move to a greater or lesser extent together with the mould. This means that the moving mass varies, which also affects the dynamic force. Therefore, the cal¬ culated signal does not exhibit the simple relation to the real frictional force that the inventor assumes. By making oneself dependent on a plurality of load cells and an accelerometer as well as a static mass calcu¬ lation unnecceεary sources of errors have been introduced into the method, and at the same time a measuring equip¬ ment has been obtained which is complicated and elaborate to mount.

Claims: CLAIMS 1. A method for monitoring of the operation of a continuous casting equipment comprising a mould (1) , a drive mechanism (3,4), adapted to generate an oscillatory movement, and a mechanism to transmit the oscillatory movement from the drive mechanism to the mould, comprising generating, by means of at least one motion sensor (2) , which operates in dependence of a parameter of the actual movement of the mould, a first signal (Al ...AN) corres¬ ponding to the said parameter, characterized in utilizing a parameter, corresponding to the movement of a part of the mechanism (4,5), to generate pulses (S1...SN) by means of a pulse emitter (3) each time the mould passes a predetermined, arbitrarily chosen phase position during its oscillatory movement, forming, by means of said pulse from said pulse emit¬ ter (3), a number (N) of pulses (S2) which are evenly spaced in time between two (SI)-pulses, and sampling said sensor signals (Al...AN) at each (S2)- pulse and accumulating them in successive registers (R1...RN), while restarting the accumulation in the first register (Rl) for each time a puls (SI) occurs. 2. An equipment for monitoring of the operation of a continuous casting machine, comprising a mould (1) , a drive mechanism (3,4) adapted to bring about oscillatory movements, and a mechanism to transmit the oscillatory movement from the drive mechanism to the mould, wherein there is generated, by means of at least one motion sensor (2), which operates in dependence of a parameter of the actual movement of the mould, a first signal (Al ...AN) corresponding to the parameter, characterized by a pulse emitter (3) , adapted to generate, in depen¬ dence parameters of the movement of a part of the mecha¬ nism (4,5) , pulses (S1...SN) corresponding to the said parameters each time the mould passes a predetermined, arbitrarily chosen phase position during its oscillatory movement, a device to form, by means of said pulses from said pulse emitter (3) , a number (N) of pulses (S2) which are evenly spaced in time between two (Si)-pulses, and a device to sample said sensor signals (A1...AN) at each (S2)-pulse and accumulating them in successive regis¬ ters (R1...RN), while restarting the accumulation in the first register (Rl) for each time a pulse (SI) occurs. oooOooo

Inventors: FRIDH CONNY, FRIDH, CONNY
Applicants: CONTECH HB
Application year: 1990
Main classification: B22D11
IPC codes: B22D11, G01H1, G05B21