Video positioning system for a pouring vessel

Abstract: A video positioning system for positioning a pouring vessel 34 over a mold sprue cup 38 comprising one or more light fixtures 18 for illuminating a notch 14 on a pouring mold 12, an image sensor 10 for sensing the position of the notch on the mold by producing a video notch image, a vision interface unit 20 connected to the image sensor for converting the video notch image to digital information, and a processor 22 connected to said vision interface unit for determining the relative position of the notch image compared to a reference position for the notch. Position correction information is sent to a position controller connected to the processor. The position controller causes the position of the pouring vessel to be adjusted by a positioning subsystem comprising a positioning driver for converting the positioning control signals to actuating signals for a positioning actuator, a positioning actuator, and at least one position sensor for sensing the position of the pouring vessel.

Description: Field of the Invention The present invention relates to the accurate positioning of molds in a molten metal pouring operation in foundry installations, and in particular to an apparatus and method which quickly and accurately positions the pouring vessel over the mold to be filled. Background of the Invention In a foundry part casting operation it is extremely important to fill the casting molds quickly and accurately. The relative location of the pouring nozzle in relation to the mold sprue cup is an important factor in achieving this objective. In sand molding operation, the dimensions of a given volume of sand will vary due to differences in molding sand compactability. This means that each mold may have slightly different dimensions, even though the molds may be produced by automated machinery. The dimensional variations lead to changes in the relative location of the mold sprue cup and the pouring nozzle. The resulting misalignment causes splashing of molten metal, sand erosion, and can lengthen the time of pour. Splashing can damage equipment, sand erosion can contaminate the casting, and longer pour times can cause defects in the casting. An earlier method for setting the correct mold position utilized a device (e.g., a roller, a probe) in contact with the side of the mold to physically sense a mark on the mold. Another method used an optical sensor to detect a point of light projected on the mold. Both these devices were placed close to the mold and were often damaged by excessive spillage and splashing of molten metal. They also had a very limited range of operation. The roller had to touch the side of the mold to detect the notch, and the optical sensor operated by detecting a change in distance between a light source and a spot on the surface of the mold. In the event that the reference mark was not present within the effective range of the devices, the positioning system had to hunt for it. This increased the overall time of operation (cycle time) and decreased the production yield. US 4724894 relates to an apparatus for controlling the filling of mold sections with molten metal. The apparatus includes a distance measuring device for locating an illuminated spot on a surface of a mold section and for measuring the distance to the molten metal level in the mold section. DE 4202020 discloses a video system for determining the accuracy of location of a pouring hole of a vessel, by reading the position of three reference points, namely, the pouring hole, a sprue cup and a point on a parting line of a mold. Summary of the Invention According to the present invention there is provided a combination of a mold and a video positioning system as claimed in claim 1. There is also provided a method as claimed in claim 11. Additional features of the present invention are set out in the claims dependent on claim 1 or claim 11. The present invention provides an apparatus and method for positioning a pouring vessel over a mold sprue cup accurately and quickly. The invention utilizes a non-contact method of sensing an indicium in the mold surface by using an image sensor, such as a video camera. The image sensor is connected to a processor that analyses a video image from the image sensor and detects the presence and location of the indicium relative to a predetermined reference point. The processor determines a difference value between the detected indicium image and the reference location and sends positioning information to a positioning controller. The positioning controller activates a positioning subsystem that moves the pouring vessel so that the nozzle of the vessel is positioned in the proper relation to the sprue cup in the mold. The image sensing device is located at a safe distance from the pouring operation. Because the image sensor has a large effective range, there is no necessity to hunt for the indicium. It is located within the field of view of the image sensor. Occasionally a defective mold will emerge from the mold forming equipment. The indicium on a defective mold may be malformed and will not conform to the size, shape and location of the predetermined reference point. The present invention can be arranged to automatically detect bad molds (molds that should not be filled with molten metal) and inhibit the positioning system, preventing a pour into the bad mold. Because the dimensions of sand molds can vary from one mold to the next, it may be necessary for the positioning system to correct the position of the pouring vessel for a succession of molds. In the case where each correction is in the same direction, the pouring vessel may reach the travel limit of the positioning subsystem. The present invention can be arranged to automatically sense the position limit and readjust the location of the pouring vessel when the maximum allowable travel limit has been reached. Brief Description of the Drawings Figure 1 is a functional block diagram of the positioning system of the present invention. Figure 2 is a process flow chart for the method of positioning the pouring vessel using the apparatus of the invention. Description of the Invention The present invention is an apparatus and method for positioning a pouring vessel over a sand mold sprue cup by detecting the position of a video image of an indicium in the sand mold. Hereafter, this indicium will be referred to as a notch (though it may take other forms, such as a cross, an "X", or other visual indicator). The notch may be on the side or top of the mold or it could be the parting line between the halves of the sand mold. In the preferred embodiment of the invention described herein, the notch is formed in the side of the mold. The various components and subsystems of the invention are illustrated in Figure 1. An image sensor 10 is aimed at the side of a line of molds 12. The image sensor 10 may be a video camera or a digital image detector. A notch 14 is impressed on each mold 16 by an automatic mold forming machine (not shown). Light from at least one light source 18 is aimed at the notch 14 at a sufficient angle to cause a shadow to be cast in the interior of the notch 14. The video effect of the shadow is a dark shape representative of the shape of the notch 14 in a brighter field. Where, as in Figure 1, the image sensor is a video camera with an analog output, the image must be converted to digital form in a video interface unit 20. The video interface unit 20 converts an analog video signal into a matrix of pixels, assigning each pixel a numerical value proportional to the intensity of light present in that part of the video image represented by the individual pixel. In this way, a video image may be represented by a two dimensional matrix of numbers corresponding to the number of pixels in a video frame. Each pixel value represents the relative brightness of the small part of the image encompassed by the pixel. In a digital image sensor, of course, the pixel matrix is created within the image sensor, presenting a digital output rather than an analog signal. The digitized image is sent to a central processor 22 where it is analyzed by a location detection algorithm. The algorithm determines the size, shape and density of the image of the notch 14 that has been detected by the image sensor 10. The processor 22 compares the detected location of the notch image with a predetermined notch location stored in memory. The processor 22 calculates a difference value between the position of the detected notch image and the predetermined notch location. The difference value is sent to a position controller 26. The function of the position controller 26 is twofold. It converts the difference value from the processor 22 into driving command signals to be sent to a positioning subsystem. The position controller 26 also monitors the position of the pouring vessel positioning device 32 through a position sensor 36 that sends position data from the positioning device 32 to the position controller 26. The positioning sensor 36 may be any device capable of tracking the movements of the positioning device 32, or its X-Y position, or both, and producing an output signal. The positioning subsystem moves the pouring vessel 34 into the correct position over the mold sprue cup 38 for a proper pour to be effected. The positioning subsystem of the preferred embodiment of the invention comprises a positioning driver 28, a positioning actuator 30, and a positioning device 32. The positioning driver 28 converts signals from the position controller 26 into driving impetus (electrical or mechanical) for the positioning actuator 30. The positioning actuator 30 converts the driving impetus into physical force to move the positioning device 32 and, thus, the pouring vessel 34 into position over the mold sprue cup 38. The positioning device of the preferred embodiment of the invention is an X-Y positioning table, though other apparatus may be employed to move the pouring vessel. The X-Y positioning table is moved by the positioning actuator 30, which may be an electric, pneumatic, or hydraulic actuator or other mechanical means of moving the positioning device 32. Thus, the character of the driving input provided by the positioning driver 28 may be electrical current, pressurized air or fluid, or any other suitable impetus. As the positioning device 32 moves in response to the signals from the position controller 26, the location of the positioning device 32 is monitored by the position sensor 36. In the preferred embodiment of the invention, the X-Y position of the positioning table is sensed and transmitted to the position controller 26. The position controller 26 determines when the notch location correction calculated by the processor 22 has been implemented by the positioning subsystem and stops the movement of the pouring vessel 34. The processor 22 in the system may be any readily available microprocessor for controlling industrial equipment, programmed with the algorithm for analyzing the notch image provided by the image sensor. Pattern recognition algorithms are well known in industrial control applications and are considered to be within the capability of one skilled in the an to select and implement. The position controller may be another processor. The position controller has fewer functions to perform, and need not be as complex as the system processor. The position controller converts difference information from the system processor 22 into signals that are compatible with the positioning subsystem components, be they electrical, pneumatic, hydraulic or mechanical. The position controller 26 should be capable of receiving and processing the signals returned from the position sensor on the positioning device 32. Using the position information returned from the position sensor 36, the position controller 26 can determine when the corrective movement of the pouring vessel required by the processor has been accurately accomplished. The location of the notch 14 on the mold 16 is sensed by the image sensor 10 each time a new mold is indexed into position. The sensed location is compared to a reference notch location by the processor. The reference notch location is established manually by an operator before automated operation begins. The video positioning system comprises a man-machine interface 40 and a video monitor 42 that are used by the operator to establish the reference notch location. The man-machine interface may comprise a keyboard, joystick, mouse, trackball or other input device. The mold line 12 is advanced until a mold sprue cup 38 is positioned under the nozzle of the pouring vessel 34 in the correct position to pour molten metal without spillage, damage or unnecessary delay. The operator commands the processor to store the video representation of the notch 14 on the reference mold in the processor's memory for comparison to the video notch images that will be obtained each time a new mold is indexed into position. The position of the mold 16 when setting the reference notch position becomes the index position for each mold in the mold line 12. As each successive mold 16 is indexed into position during operation, the image of the notch 14 on the mold 16 is processed to detect variation from the reference notch location. Differences in notch position are corrected by the positioning subsystem. The notch 14 on the mold may be any simple shape (complex shapes may be used but would require more demanding analysis by the processor). Notch 14 should have a clearly defined edge and sufficient depth that, when illuminated by a light source 18 at an oblique angle, a shadow is cast within the notch 14 that distinctly defines the notch 14 and distinguishes it from the surrounding mold surface. More than one light source may be employed to clearly define the notch image for the image sensor 10. The notch 14 may be located on the side or the top of the mold 16, and may be on the parting line 44 between two mold segments. The parting line 44 itself may serve as the notch. Since the image sensor 10 and its optics provide a large field of view, the notch 14 is necessarily within the image sensor's field and there is no need for the system to hunt for the notch. The large field of view also allows for accurate positioning of the pouring vessel even when there are large variations in the dimensions of successive sand molds. As noted previously, variations in mold dimensions can result from changes in sand compactibility or from introducing a different sand mold (i.e., a new casting pattern) into the mold line 12. It is possible that successive corrections to the position of the pouring vessel 34 will cause the vessel to be moved by small increments in only one direction. After several such corrections, the pouring vessel 34 may reach the limit of its travel range in that direction. The positioning sensor 36 can detect the limit of the travel range of the vessel 34 and alert the positioning system to that condition. The processor takes corrective action to reposition the pouring vessel over an empty mold. The present invention can also detect defective molds and alert the system to their presence. A defective mold often will be badly formed, resulting in a misshapen notch. The image processor 22 detects the malformed notch when it analyzes the size and shape of the notch image provided by the image sensor 10. A bad mold must not be filled with molten metal. The present invention sends a signal to the operator through the man-machine interface 40 to indicate the presence of the bad mold. The positioning system does not position the pouring vessel over the bad mold, and the pouring operation with respect to that mold is inhibited. The method for positioning a pouring vessel by video detection and positioning is illustrated in Figure 2. For the following discussion, the reference numerals appearing in parentheses correspond to the numerals in Figure 2 denoting a step in the process. After the operator sets the reference notch position (50), the next mold in the mold line is indexed into position for pouring (52). The imaging sensor images the notch on the mold (54). The notch image is digitized (56). The notch image, represented now as a digital matrix of numerical values, is analyzed to determine the notch position (58). Once the notch is imaged and analyzed (60), the image of the notch is checked to determine if the notch conforms to the size and shape that it should (62). If it does not, then the notch is bad and indicates a bad mold that should not be used (62). If the mold is bad, the pouring vessel is not positioned, pouring is inhibited and the processor signals the operator that the mold is bad (64). The next mold is then automatically moved into position by the mold machine. If the notch image is acceptable, the location difference between the notch image and the reference notch position is determined (66). That difference is converted to a position correction value (68). The position correction value is sent to a position controller (70). The pouring vessel is moved relative to the mold sprue cup by an amount sufficient to compensate for the position difference between the notch image and the reference notch position (72). The repositioning continues until the location difference between the notch image and the reference image is zero (74). The operator is signalled when the correct position has been reached (76). On occasion, the ambient light in the pouring area may make it difficult for the imaging system to detect the notch on the mold. The method allows for adjustment of the lighting threshold in the detection algorithm and re-analysis of the digitized notch image (80, 82, 58, 60) until a limit is reached. If the notch cannot be detected after threshold adjustment, too much ambient light is washing out the notch image on the mold and the system must stop and signal an error to the operator (84, 86).

Claims: A combination of a mold and a video positioning system for positioning a pouring vessel over a mold sprue cup; the mold comprising an indicium in its surface; and the system comprising at least one light source for illuminating the indicium at an oblique angle thereto, thereby casting a shadow within the indicium, an image sensor for sensing the position of the indicium on the mold by imaging the indicium shadow, a processor connected to the image sensor for determining the relative position of the sensed indicium compared to a reference position for the indicium, said processor producing a difference signal representing the relative position difference of said indicium from said reference position, and a positioning subsystem for adjusting the position relationship of a pouring vessel to a sprue cup of the mold in response to the difference signal. A combination as claimed in claim 1, wherein the processor is arranged to analyse the size and shape of the image of the indicium shadow for detecting a malformed indicium. A combination as claimed in claim 1 or claim 2, wherein the system is arranged to inhibit filling of a mold having a malformed indicium. A combination as claimed in any preceding claim, wherein the processor is arranged to analyse the density of the image of the indicium shadow. A combination as claimed in any preceding claim, wherein the system is arranged to adjust the lighting threshold. A combination as claimed in claim 5, wherein the system is arranged to signal an error if the indicium shadow cannot be detected after threshold adjustment. A combination as claimed in any preceding claim, wherein the image sensor has a field of view in which the indicium is located, and the field of view has a range such that the indicium is imaged by the image sensor without hunting for the indicium. A combination as claimed in any preceding claim, the system further comprising a position controller connected to the processor for receiving the difference signal from the processor and producing positioning control signals, and the positioning subsystem comprising a position driver, a position actuator and one or more positioning sensors, the position driver receiving the position control signals from the position controller and converting the position control signals to actuating signals, the position actuator receiving the actuating signals and adjusting the location of the pouring vessel relative to the mold sprue cup, and the position sensors sensing the position of the pouring vessel. A combination as claimed in claim 8, wherein the system is arranged to detect the travel limit of the positioning subsystem and to readjust the position relationship between the pouring vessel and the mold sprue cup. A combination as claimed in any preceding claim, the system further comprising a vision interface unit connected between the image sensor and the processor for converting the image of the indicium shadow to digital information. A method for positioning a pouring vessel over a mold sprue cup comprising the steps: providing an indicium in the surface of a mold; illuminating the indicium with at least one light source aimed at an oblique angle to the indicium to cast a shadow within the indicium; imaging the indicium shadow; processing the image of the indicium shadow in digital form using a processor to detect the indicium shadow image position relative to a reference indicium position; determining a difference between the image position and the reference position; producing a difference signal representing the relative position difference of said indicium from said reference position; sending said difference signal from the processor to a positioning controller; controlling a positioning device to correct the position of a pouring vessel relative to a mold sprue cup. A method as claimed in claim 11, further comprising the step of: processing the digital image of the size and shape of the indicium shadow for detecting a malformed indicium. A method as claimed in claim 11 or claim 12, further comprising the step of: processing the digital image of the density of the indicium shadow for adjusting the lighting threshold. A method as claimed in any one of claims 11 to 13, further comprising the step of: detecting the travel limit of the pouring vessel; and readjusting the position of the pouring vessel relative to the mold sprue cup.

Inventors: TABATABAEI EMAD, TABATABAEI, EMAD
Applicants: INDUCTOTHERM CORP, INDUCTOTHERM CORP.
Application year: 1995
Main classification: B22D37
IPC codes: G05B19, G05D3, B22D33, B22D37, B22D11