GB2242381A - Controlling the pour of molten metal into molds - Google Patents

Abstract

The flow of molten metal from a tundish (30) having a stopper rod (42) into a mold (10) having a sprue cup (12) is controlled by an automatic system having camera means (50). The system observes the actual level of molten metal in the sprue cup over the course of a pour. Based on the flow rate from the tundish and the change of level of molten metal in the sprue cup over time, the system calculates the flow rate of molten metal out of the sprue cup into the mold. By sampling the mold intake behavior over the course of pouring a single mold, the system calculates a number reflecting the mold intake behavior. The mold intake behavior is used to compute a predicted metal level in the cup. This computed metal level is compared to a pour profile representing an ideal pour. The error signal is then combined with a control signal determined directly from the estimated mold intake to produce a control signal applied to the stopper-rod drive. <IMAGE>

Description

j 0 APPARATUS AND 14ETHOD FOR CONTROLLING

THE POUR OF MOLTEN METAL INTO MOIDS Field Of The Invention

This invention relates to automatic control systems for pouring a liquid from a tundish into a mold. More specifically, this invention relates to the pouring of molten metal into a sand mold using real-time vision-based feedback control.

Backcrround Of The Invention In the foundry industry, metal castings are produced by dispensing molten metal into sand molds. The cavities inside the sand molds represent inverse images of the desired part. Figure 1 shows a typical arrangement of sand molds as used in the foundry industry. A succession of molds are filled in sequence through top openings, which are known as sprue cups. The sprue cups communicate with a gating system. which is a series of channels connecting the sprue cup to the cavities which define the shapes of the parts to be nolded.

In order to produce quality castings, the metal flow through the gating system must be quick and continuous. If the flow is uneven, air can be trapped inside the castings, producing a porous part of unacceptable quality. A smooth flow of molten metal through the gating system is achieved by maintaining a full sprue cup for the duration of each pour. However, overfilling of the sprue cup will lead to dangerous spilling and jamming on the production line. Therefore, a need exists for an automatic is control system which can accurately control the level of molten metal in the sprue cup as the metal is being poured.

Prior attempts at automating this process have proven to be unsatisfactory. Typical prior art means for detecting an overflow of molten metal in the sprue cup include providing sensors which detect molten metal rising through vents in the mold, or sensing a change in the intensity of light emitted by the molten metal in the sprue cup. However, common problems with these methods include false signals caused by splashing of molten metal around the sprue cup, and the inability of the control systems to take into account metal which is passing between the pouring orif ice and the sprue cup when the f low is cut off.

Many of these problems are solved by the invention of U.S. Patent No. 4,744,407, assigned to the assignee of the present invention. That patent discloses an optical scanning means for continuously sensing the image of the surface of molten metal in the sprue cup. Means are provided for comparing the image area information to a preselected reference area value and generating a dif ference value representative of the difference between the image area information and the reference area information.

Control means responsive to the difference value generate a control signal to the flow control means for controlling the flow of molten metal to minimize the difference value.

The present invention represents an improvement to the apparatus of that patent, in that the present invention is able to calculate, based on observation of the flow rate from the tundish and the behavior of the liquid level in the sprue cup, the behavior of molten metal flowing from the sprue cup into the gating system. The present invention also takes into account the quantity of molten metal flowing from the tundish into the sprue cup over the duration of the pour. The present invention is able to "learn" the behavior of the sprue cup and gating system and 946-201 GB /slb 1 compute metal f low inside the mold, and adapt to changing casting conditions.

Summary Of The Invention

The invention is a method of and system f or con trolling the flow of a liquid from a tundish having an orifice of variable size into a mold having a sprue cup.

The system takes into account flow control means in the tundish at a given time, and thereby infers the flow rate of liquid from the tundish. By use of a digital camera, the system observes the actual liquid level in the sprue cup over time. Based on the flow rate from the tundish and the change of liquid level in the sprue cup over time, the system calculates the flow rate of liquid out of the sprue cup into the mold, which is known as the Ilmold intake rate". By sampling the mold intake behavior over the course of pouring a single mold, the system calculates the Ilmold intake behaviorllf derived from the history of pre vious pours. The mold intake behavior can be compared to a pour profile of an "ideal" pour, in order to maximize efficiency and the quality of the casting. The system also predicts, based on the flow rate from the tundish and the change in liquid level in the sprue cup, when the f low passing through the orifice at a given time will reach the sprue cup, thus enabling the system to take into account the quantity of liquid "in transit" when the flow is cut off. The system then uses the mold intake behavior to generate a control signal that will control the flow control means in response to the mold intake behavior and the quantity of liquid "in transit", thereby controlling the liquid level in the sprue cup.

Brief Description of the Drawings

For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this inven tion is not limited to the precise arrangements and instru mentalities shown.

946-201 GB /slb 1 1 Figure 1 is an isometric view of a number of sand molds, as they are arranged in a typical automated casting system.

Figure 2 illustrates the interrelationship of the tundish, stopper rod, servo motor, sprue cup, and camera.

Figure 3 is a schematic diagram of the tundish, sand mold and control system.

Figure 4 is a functional diagram illustrating the calculations performed by the control system.

Figures 5a and 5b illustrate the technique by which the level of molten metal in the sprue cup is observed. Detailed Description of the DrawingLs

Figure 1 shows a series of sand molds 10. At the top of each sand mold 10 is a sprue cup 12. Sprue cup 12 is has an opening in its bottom which communicates via a series of channels, known as a gating system 14, with any desired number of cavities 16. Each of the cavities 16 represents an inverse image of a part to be cast. To cast parts, molten metal is poured into sprue cup 12, from which it flows through the gating system 14 to the cavities 16. In the embodiment shown in Figure 1, each sand mold 10 has opposite sides of the mold facing outward so that each side of each sand mold 10 interacts with its opposite on the previous or subsequent sand mold moving down track means 18. In this way the spaces between each sand mold 10 become the two halves of the shape f ormed by the sand molds. It should be noted that each sprue cup 12, formed by two sand molds 10, has a predetermined volume of its own, and that the openings from each sprue cup 12 to gating system 14 have a cross-sectional area significantly less than that of the bottom surface of each sprue cup 12. Thus, molten metal poured into each sprue cup 12 accumulates therein for a short period of time if, as is preferred, molten metal is poured into the sprue cup 12 at a faster rate than it can flow out to the gating system 14.

946-201 GB /slb i Of course, as those skilled in the art will ap preciate, any type of mold may be employed without depart ing from the scope of the invention, and the particular type of mold should not be regarded as essential to the invention.

For reasons mentioned above, in order to produce quality castings, the flow of molten metal into gating system 14 and cavity 16 must be continuous, and the most effective way of ensuring a continuous flow is to maintain a constant level of molten metal in the sprue cup 12 f or the duration of the pour. Improper or erratic pouring of molten metal will result in unsatisfactory castings having pockets of trapped air.

Figure 2 illustrates a system for maintaining a substantially full sprue cup through the duration of a pour, using real-time vision-based feedback control.

Molten metal 20 (identified as 20a when in the sprue cup 12, 20b when in transit between the tundish and the sprue cup 12, and 20c when in the tundish 30) is poured into sprue cup 12 of sand mold 10 from a tundish 30. The flow of molten metal 20 from tundish 30 to mold 10 is controlled by a stopper means 40.

Tundish 30 holds a quantity of molten metal 20c.

Tundish 30 typically comprises an outer shell 32 with a refractory lining 34. At the bottom surface of tundish 30 is a nozzle 36 with an orifice 35 through which molten metal passes by gravity. Tundish 30 is preferably, al though not necessarily, of a design, such as the funnel design shown, which allows for the head pressure of the molten metal 20c to remain relatively constant as tundish empties.

Stopper means 40 comprises a stopper red 42 and a servo means generally labelled 44. In the preferred embodiment, stopper rod 42, typically of a cylindrical shape, passes through opening 38 in the top of tundish 30, and has a free end adapted to engage nozzle 36 and open and 946-201 GB /slb 1 close orif ice 35. Generally, the higher the f ree end of stopper rod 42 in relation to nozzle 36, the greater the flow rate of molten metal 20b from the tundish 30. Of course, the flow can be stopped completely by moving stopper rod 42 downward so that its free end engages nozzle 36 and forms a seal around orifice 35. Servo means 44 enables the position of stopper rod 42 to be precisely controlled. In the preferred embodiment, servo means 44 comprises an electric motor which drives stopper means 40 through a lead screw 45.

Also shown in Figure 2 is a camera 50, which is arranged to view the top opening of sprue cup 12, the molten metal 20a in sprue cup 12, and the molten metal 20b emerging from the orifice 35. The function and operation of camera 50 will be discussed in more detail below.

Figure 3 shows a general overall view of the apparatus of the present invention in the context of a casting line in a foundry. Tundish 30 is preferably supported over sand mold 10 by means of a movable carriage 60. Preferably, carriage 60 enables the position of tundish 30, along with stopper means 40, to be adjusted in two horizontal dimensions so that orifice 35 is aligned over the sprue cup 12 of any sand mold 10 no matter what the configuration of sand mold 10 may be. (Generally, in the art of sand mold casting, the height of the mold is standardized, so vertical adjustment of the tundish is not necessary.) To facilitate mass production, a series of sand molds 10 are produced in known manner by a mold machine and may be arranged, as shown in Figure 1, on a track means 18, on which a number of sand molds 10 can be filled in relatively rapid succession by being indexed under the orifice 35. Depending on the size of the -sand mold 10, track 18 may be mounted on a series of conveyor bases 62.

Camera 50 is connected to a control computer and servo drive control which may be housed in a cabinet 70.

946-201 GB /slb i i 1 For monitoring and inspection, the picture f rom camera 50 may also be displayed on monitor 52. The Control System What follows is a description of the control system, describing how the visual signal from camera 50 is processed and used to control the flow of molten metal 20 via stopper means 40.

The control loop begins by taking account of the vertical position X(t) of stopper rod 42 relative to the nozzle 36. Generally, within the range of positions of stopper rod 42, it can be assumed that there is a linear relationship between the position X(t) of stopper rod 42 and the rate of f low Q (t) of molten metal through the nozzle 36. (The 11(t)11 in all variables indicates that all is of these values vary over time.) Q(t) and X(t) are related by a constant K1. thus:

Q (t) = K1 X (t) (1) The molten metal 20b that flows from nozzle 36 will therefore enter the sprue cup 12 at the rate Q (t). once the molten metal is in the sprue cup 12, it will f low out of the sprue cup 12 into gating system 14 at a rate F (t), which is primarily a function of the design of the gating system. Preferably, X(t) is initially chosen so that F(t) is less than Q(t). The difference between incoming flow Q(t) and outgoing flow F(t) will therefore cause accumulation of molten metal in sprue cup 12, providing a varying quantity of molten metal 20a in sprue cup 12. Because the orifice 35 of tundish 30 is spaced a measurable distance above sprue cup 12, it takes a measurable amount of time T before the molten metal 20b exiting orifice 35 reaches the sprue cup. This "transit delay" r will remain constant regardless of the flow rate Q(t) from orifice 35. Q(t), F(t) and r all are related to the molten metal volume V(t) in the sprue cup 12 by the following equation:

946-201 GB /slb V (t) = f (Q (t-.r) - F (t)) dt (2) By substituting equation (1) into equation (2), a relationship between the volume V (t) in the sprue cup 12, the position X(t) of the stopper rod 42, and the rate of flow of molten metal into the gating system F(t) can be obtained:

V (t) = f (K, X (t-r) - F (t)) dt (3) While the molten metal 20a is accumulating in sprue cup 12, the level of molten metal in sprue cup 12 is being observed by camera 50, which will be described in greater detail below. The sprue cups of most sand molds are configured so that there is a proportional, linear relation between the observed level A(t) of molten metal in the sprue cup and the volume V(t) of molten metal. This linear relationship can be written as:

A (t) = K2 V (t) where K2 is a constant describing the linear relationship and is determined by the design of the mold. By substituting equation (3) into (4), a relationship between the observed level A(t) of molten metal in the sprue cup and the f low rate F (t) into the gating system 14 can be obtained:

A (t) = f (K, K2 X (t-,r) - K2 F (t)) dt (5) Taking the derivative of both sides of equation (5).yields:

Jk(t) = K1K2X(t-r) - K2F(t) 946-201 GB /slb I i i 1 1 which can be rewritten to obtain a relationship between the flow rate of metal into gating system 14, the position X(t) of the stopper rod 42, and the observed changes A(t) of the level of molten metal 20a in sprue cup 12:

F' (t) = K1 X (t-.r) - A (t) /K2 (6) The f low rate into gating system 14 is designated Fl(t) and represents the estimated mold intake for the current pour cycle, that is, the estimated rate at which molten metal 20a in sprue cup 12 enters the mold through gating system 14. The rate of change A(t) of the molten metal level A(t) is measured in real time by sampling the observed level A (t) over a series of frames produced by camera 50. In the preferred embodiment, camera 50 sends one video frame into the computer every 33 milliseconds; that is, camera 50 operates at a rate of 30 frames per second.

The estimated mold intake Fl(t) is used in two ways. First, the value of Fl(t) is used for learning the mold intake behavior Fi(t) of a given pour. The mold intake behavior Fi(t) for a given pour is a weighted average of values of the estimated mold intake F' (t) over a number of previous pours and is expressed as:

F 1 (t) a + (1-a) Fi-1 (t) (7) where a is a weight coefficient for combining the value of the estimated mold intake Fl(t) with an accumulated average of the values for F(t) from previous pours. In equation 7, the estimated flow rate of the current pour is given as F' (t), the caret indicating that, in practice, the signal representing the running value of the estimated flow rate Fl(t) in the course of a single pour is usually run through a low-pass filter to eliminate noise. In equation 7, the 946-201 GB /slb subscript 'fill represents the currently-computed pour, and represents the previous pour.

In equation 7, the greater the value of a, the more the value Fl(t) will reflect the characteristics of the most recent pour. Conversely, a small value of a will bias the value of Fi(t) to more heavily reflect the long-term trends of the behavior o,f the system over the course of many pours. The value of a is usually unchanged in the course of a series of pours. The value a is used to reduce the noise created by the control signal by averaging the mold intake over a series of pours.

Fi(t) is used for direct control of the stopper rod 42, producing a "feed forward" signal D (t) = Fi/K1 - G which accounts for typical mold behavior.

The estimated mold intake F' (t) is also used for the prediction of the level of molten metal A(t) at time t + r. This value is important to know because the volume of the metal 20b in the stream that flows from orifice 35 of tundish 30 will not have reached the sprue cup 12 after the frame sample is taken and is processed by the computer. This molten metal "in transit" must be taken into account when pouring a precise quantity of molten metal into a mold. The time delay for the molten metal 20b to fall from the opening of orifice 35 to the sprue cup 12 is given by In order to take into account molten metal 20b, the system of the invention must be able to predict the molten metal level at time t + r, or A(t+r). This can be determined by the following equation:

A(t+r) = A(t) + K1K,tft+TX(t-r)dt - Ktft+rFi(t)dt (8) 946-201 GB /slb t S - 1 1 Equation (8) is derived f rom equations (2), (3), and (4), assuming that after interval r the mold intake is equal to:

F, (t) 1 t+.r t In the preferred embodiment, the predicted level of molten metal A(t+r) is compared with a desired level L(t+7), which has been pre-programmed into the control computer based on previous experience with the tundish and type of mold. The pre-programmed level L(t+r) may, but need not, be a predetermined profile of the level of molten metal 20a as a function of time over the course of an "ideal" pour, a profile which provides both the most constant rate of f low into the gating system 14 and the is minimum of waste. Alternatively, the reference profile may be obtained by having a skilled operator pour the f irst molds of a job manually, as described below. From comparison with the desired level L(t+.r), an error signal E(t) may be obtained by:

E(t) = L(t+r) - A(t+r) (9) This error signal is, in turn, amplified by gain G to provide a correction signal:

C(t) = G.E(t) (10) The combination of the direct "feed-forward" signal and correction signal X(t) = D(t) + C(t) is sent to servo motor 44 which controls stopper rod 42. In this equation, D(t) represents the feed-forward signal and C(t) a correc tion signal to control the position of the stopper rod 42.

This new value of X(t) is used as the starting position for stopper rod 42 for the next pour. The new value of X(t) causes a change in the flow rate Q(t) from the tundish, 946-201 GB /slb thus af f ecting the liquid level in the sprue cup, and the cycle begins again. After pouring a number of molds, the pour profile A(t) over the course of pouring an individual mold converges to and closely follows the preselected input profile L(t + f).

Measurement Technigue An important element of the above-described auto matic control system is accurate monitoring of the level of molten metal 20a in sprue cup 12. In the preferred embodi ment, monitoring is accomplished by camera 50, which is preferably but not necessarily a digital camera. Figure 5a shows a stylized view of the raster display 54 of camera when camera 50 is focused on the top of the sprue cup 12. Typically, digital camera 50 will have a resolution of 256 x 256 pixels. For the purposes of the invention, it is preferred that the image formed on raster 54 be adjusted in intensity, by conventional means of optical filters and electronic thresholding, so that the image of molten metal, which is customarily luminous, appears as white on raster 54, and the surrounding area appears as dark.

Figure 5b shows the relationship between the behavior of the molten metal stream and the image formed on raster 54. On raster 54, what is of importance to the control computer is the number of horizontal lines 56 which are completely occupied by the image of molten metal 20a, which is shown in Figure 5a as (20a). By counting the number of completely white horizontal rows 56 of pixels on raster 54, the control computer can determine a height el of molten metal in the sprue cup 12, which in turn may be normalized to the vertical height d' of the total area of interest upon which digital camera 50 is focused. Figure 5b shows how digital camera 50 can be arranged so that the image of the molten metal on raster 54 can correspond to the entire range of levels of molten metal 20a within sprue cup 12. In Figure 5b, vertical dimension d represents the entire height of sprue cup 12, while vertical dimension e 946-201 GB /slb R 1 1 represents the height of molten metal 20a at a given time. If the image (20b) of the stream of molten metal 20b is ignored by the control computer, a completely empty sprue cup 12 will be represented by a raster which is completely blank (no white lines) as far as the control computer is concerned. Conversely, a completely full sprue cup 12 will appear on raster 54 as a white signal from every horizontal line. Of coursef the digital camera 50 and the control computer can be adapted for other requirements, such as having camera 50 focus on the area on the top of the sand mold 10 around sprue cup 12, so as to detect spilling or splashing of molten metal as it is poured.

When a "run" of a series of molds of a given part (for example, a crankshaft) is begun to be poured, the particular type of mold will have associated with it an "ideal" pour profile. A pour profile is the relationship between liquid levels and time over the course of pouring one mold. For a mold of a part having a complicated shape, different size voids within the mold will fill at varying rates and will result in each type of mold having a unique pour profile associated with it. An "ideal" profile is one that either minimizes wasted metal left in the sprue cup after the mold is full or represents an optimization between waste and speed of filling the mold, depending on the needs of a particular run.

Equation 9 above describes the use of a desired level or pour profile L(t + T) in the control system of the present invention. This desired level L(t + r) is compared to the actual level of molten metal observed in the sprue cup, A(t - r), to produce an error signal E(t). The desired level L(t + r) can be obtained in a variety of ways. one way is to have an experienced operator manually pour a succession of molds, thus arriving at the ideal pour profile by trial and error. When a mold is being poured manually, even the most experienced operator will require five or more attempts before sufficiently learning the 946-201 GB /slb behavior of the f low rate to perf orm. a neat, continuous pour. With the large manufacturing runs common in the mold industry, the down-time for manual learning of the ideal pour profile ' is usually not economically significant.

During this manual pouring, the control system will "learn" the ideal pour profile from the manual pouring by monitoring the behavior of X (t), the position of the stopper rod over time, and comparing to it the simultaneous values of F (t) over time. In this way, the control system of the present invention will acquire the expertise of an experienced manual operator for each new mold.

Alternatively, if an experienced manual operator is not available, a simple "box" input pour profile can be entered as an ideal profile, and the control system will automatically determine the ideal profile through its own trial and error. A "box" input involves simply holding the value of F (t) at a certain constant value f or a period of time sufficient to fill the mold:

F(t)1const = VO/T where VO is the total volume of the mold and T is the total time of the pour.

The splashing and discontinuities of a "box" profile will be taken into account by the control system, particularly through equations 6, 7 and 8, and over the course of several pours, the ideal profile will gradually be defined.

Another consideration that must be taken into account is the precise position of stopper rod 42 in relationship to nozzle 36. By monitoring this position, the computer can infer the rate at whicb molten metal is flowing from the tundish 30. When the lower end of stopper rod 42 is in contact with nozzle 36, there will of course be no flow from tundish 30. When the end of stopper rod 42 is near but not touching nozzle 36, the rate of molten 946-201 GB /slb i 1 i metal flowing out of tundish 30 will vary with the distance of stopper rod 42 from nozzle 36. However, once the lower end of stopper rod 42 is removed far enough from nozzle 36, the position of the stopper rod will cease to have a direct ef fect on the rate of molten metal passing through the nozzle 36, and the linear relationship between the position of stopper rod 42 and the flow of molten metal will no longer hold. This linear relationship is important to the algorithms by which the computer of the present invention 10 functions.

When the automatic control system is first used to pour a particular set of molds 10, it is important that the value K,, which is the slope of the relationship between flow Q(t) from the tundish 30 and position X(t) of the is stopper rod 42, be established. Assuming a constant metal level in the tundish 30, and constant dimensions of nozzle 36 and stopper rod 42, K1 may be measured by lifting the stopper rod to a distance XO and completely filling the mold with the stopper rod 42 held at X0. It is assumed that position X. would be within the range wherein the linear relationship between flow rate and stopper rod position would still hold. The volume of the mold VO may be determined beforehand. The time T that the mold takes to fill when the stopper is at position XO can be measured.

In mathematical terms, VO = 0 f TQO (t) dt = QO T = K, XO T, QO = K, X0 (12) where QO is the inferred flow rate from tundish 30 when the stopper rod 42 is at position XO. From equation 12 the initial value of constant K1 can be calculated as:

K, (t=O) = VO/XOT 946-201 GB /S1b -is- (13) t This value of K. is valid only at the beginning of the pouring process. However, with the buildup of slag in the nozzle, the diameter of orif ice 35 changes and therefore K1 should be adjusted to reflect this change. The value of K1 is recalculated after each pour. Integrating the mold intake, from equation (6) above, over the period of pour, yields the equation:

V() = 0 f TF (t) dt = K, 0 f TX (t) dt - A (T) K2 (14) where A(T) represents the level of molten metal left in the sprue cup 12 at the end of the pour.

Equation 14 can be rewritten, solving for K, VO + A (T) /K2 (15) K, = 15. f TX (t) dt Besides the obvious advantage of eliminating manual operation, the closed loop vision control system of the invention may be applied to generate a mold intake chart. This information is extremely valuable to mold designers and foundry engineers. By analyzing the mold intake, mold designers can optimize the gating system to allow quicker filling of the mold, thereby increasing productivity. Foundry engineers can determine the correlation between metal level in the sprue cup and the mold intake. With this knowledge, an optimization can be made between the amount of metal used versus line productivity, that is, the optimal balance between conservation of material (by keeping a low metal level in the sprue cup towards the end of each pour) and f illing each mold at a high speed (by maintaining a full sprue cup during a pour). Summary of Operatio

Figure 4 is a diagrammatic f low chart showing the interrelationship among the various parts of the control 946-201 GB /slb i i system and the signal f low. The diagram illustrates the processes described in detail above.

Box 100 shows the tundish 30 with stopper rod 42.

The flow rate Q(t) from the tundish 30 is related to the position X(t) of the stopper rod 42 by a constant K..

Molten metal from orifice 35 passes into the molds, which are shown in box 102. The volume of metal in the sprue cup V(t) at any given time is given by the integral of the flow rate Q (t) into the cup minus the f low rate F(t) into the gating system. The control system also takes into account the delay time r f or the molten metal to pass f rom the orifice of the tundish to the sprue cup.

Box 104 shows camera. 50 observing the behavior of molten metal in the sprue cup. The volume V(t) of molten metal in the sprue cup is related by a constant K2 to the actual level of molten metal A(t) viewed by the camera. Camera 50 observes both the actual levei of metal A(t) at any given time and monitors the instantaneous---changein A(t) over the course of several video frames, so the control system can determine the derivative A(t) in differentiator unit 105.

The values of A(t) and A(t) are applied to two different equations shown in box 106, an estimator equation (equation 6 above) and a predictor equation (equation 8). The estimator equation establishes the relationship between the flow rate from the tundish and the position of the stopper rod at any given time. The predictor equation takes into account the fact that there is no way to control the molten metal in transit between the orifice and sprue cup. The predictor equation predicts the level of molten metal at the point in the future when the metal leaving the orifice at time t reaches the sprue cup at time t + r.

The estimated flow rate from the estimator equation is then used for learning the mold intake behavior, the equation for which (equation 7) is shown in box 108. At the same time, the predicted valu- of A(t + v) is compared 946-n0J- GB 1 slb to a preselected pour profile, shown in box 110, which has been preprogrammed into the control system for a particular type mold. This comparison yields a difference signal E (t), which is sent to a controller, box 112, to be con s verted into a control signal C(t). The control signal C(t), which represents the difference between the current pour and the ideal profile, is combined with Fi(t), the mold intake behavior, to make fine adjustments to the position of the stopper rod, given as X(t), through the servo drive unit, box 114. The value of X(t) will thus have a direct ef fect on the flow rate Q (t) from the tundish, thus completing the control cycle and closing the control loop. The newly-adjusted value of Q(t) is detected by the camera as a new actual metal level A (t), and the cycle begins again.

It will be appreciated that there are two control loops in the system: a "fast" loop which regulates the level of molten metal in the sprue cup in the course of each pour, to prevent splashing and discontinuities, and a "slow" loop which adjusts to pour profile over a succession of pours so that the pours converge to an ideal profile. This slow loop is shown in hatched lines 120 in Figure 4.

The system of Figure 4 may be embodied in any number of ways known in the art. The equations that are used in the control system may be embodied as an arrangement of analog computing elements, or alternatively the equations may be easily programmed into a microprocessor.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.

946-201 GB /slb 1 J 1

Claims (13)

1. Method of controlling the pour of a liquid f rom a vessel into individual molds, each mold having a sprue cup, comprising the steps of: monitoring the flow rate of the liquid from the vessel to the sprue cup of the mold; determining the rate at which the liquid flows into the mold from the sprue cup and generating therefrom at least one parameter representative of pour behavior; calculating, based on the flow rate and the at least one parameter representative of pour behavior, a predicted liquid level in the sprue cup for a future point in time; comparing the 'calculated predicted liquid level with a reference predicted liquid level and generating a difference signal representative of the comparison; periodically updating the at least one generated pour behavior parameter; generating a control signal derived from the difference signal and the updated pour behavior parameter; and using the control signal to controllably vary the flow rate of the liquid from the vessel.

2. Method as in claim 1, further comprising the step of updating the at least one generated pour behavior parameter based on updated pour behavior parameters from previous pours.

3. A method of filling a mold having a sprue cup and a gating system with liquid from a vessel having a flow control means, comprising the steps of: pouring liquid at a preselected flow rate from the vessel into the sprue cup; sensing actual liquid level in the sprue cup over time; 946-201 GB /slb 1 calculating the derivative liquid level with respect to time; of the actual periodically determining from the preselected flow rate and-the derivative of the actual liquid level an instantaneous estimated mold intake rate at which the liquid passes out of the sprue cup into the mold gating system; determining from each instantaneous estimated mold intake rate a periodically updated mold intake characteristic by taking a weighted average of previously determined instantaneous estimated mold intake rates over a preselected interval; determining a predicted liquid level from the actual liquid level, preselected flow rate, mold intake characteristic and the time required for liquid to travel from the vessel to the sprue cup; comparing the predicted liquid level with a preselected reference level and generating therefrom a difference signal representative of the difference between the predicted liquid level and the reference level; generating from the difference signal a control signal; combining the control signal with the mold intake characteristic and generating therefrom an updated preselected flow rate; and repeating said steps until the mold is full.

4. A method as in claim 3, wherein the step of determining the mold intake characteristic comprises including in said weighted average a value representative of a mold intake characteristic from a previous mold.

5. A method as in claim 3, further comprising the step of recalculating the predicted liquid level after each pour by measuring the time required to fill the mold in the previous pour.

6. A method of controlling the flow of molten metal from a tundish having a reciprocally-movable stopper 946-201 GB /S1b i i 1 f 1 rod f low control mechanism to a mold having a sprue cup, comprising the steps of: determining the f low rate from the tundish to the mold as a function of stopper rod position; sensing the level and rate of change of the level of molten metal in the sprue cup over time; periodically calculating from the flow rate, the sensed level and rate of change of the level an instantaneous mold intake rate representative of the flow rate of molten metal out of the sprue cup into the mold; determining a mold intake characteristic from a weighted average of instantaneous mold intake rates over a preselected interval; calculating from the flow rate, rate of change of level, mold intake rate and distance travelled by the molten metal from the tundish to the sprue cup a predicted level of molten metal in the sprue cup after a delay interval; and controlling the position of the stopper rod in response to the mold intake characteristic and the predicted level of molten metal to thereby control the flow of molten metal from the tundish.

7. A method as in claim 6, wherein the delay interval is approximately equal to the time required for metal leaving the tundish to reach the sprue cup.

8. A method as in claim 6, further comprising the steps of: comparing the predicted level of molten metal with a preselected reference level and generating therefrom a difference signal representative of the comparison and generating from the difference signal a control signal for controlling the position of the stopperrod.

9. A method as in claim 7, further comprising the steps of:

946-201 GB /slb f combining the control signal with the mold intake characteristic and generating therefrom an updated flow rate.

10. Apparatus for controlling the flow of liquid from a vessel to a series of molds, comprising: means for monitoring and controlling the flow of liquid into the molds during filling of individual molds in accordance with preselected pour parameters; means for predicting the value of at least one pour parameter at a time in the future corresponding to the interval between the passage of liquid from the vessel to the mold; and means operatively associated with the monitoring and controlling means for updating the values of the pour parameters from mold to mold during filling of a plurality of molds in sequence.

11. Apparatus for controlling the pour of a liquid from a vessel into at least one mold, comprising: means for controlling and monitoring the flow rate of the liquid from the vessel; means for sensing the flow rate of liquid into the mold; means for generating from the flow rates of liquid from the vessel and into the mold at least one parameter representative of pour behavior; means for predicting the at least one generated pour behavior parameter at a time in the future equal to the interval between the flow of liquid from the vessel and the flow of liquid into the mold; comparison means for comparing the at least one generated pour behavior parameter to a preselected reference pour behavior parameter and generating therefrom a difference signal representative of the comparison; means for periodically updating the at least one generated pour behavior parameter; 946-201 GB /slb 1 I 1 1 1 i 1 1 1 1 1 1 i k means for deriving from the pour behavior parameter and the difference signal a control signal; and means for applying the control signal to the means for controlling the flow rate of liquid from the vessel.

12. A method of controlling the pour of liquid into a mold substantially as hefeinbefore described with reference to the accompanying drawings.

13. Apparatus for controlling the pour of a liquid into a mold substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

946-201 GB /slb Published 1991 at The Patent Offize. Concept House. Cardiff Road. Newport. Gwent NP9 1 RH. Further copies may. be obtained from Sales Branch. Unit 6. Nine Mile Point. Cw-mfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques ltd. St Mary Crav. Kent.