by;  E. Tabatabaei, Inductotherm Corporation; Rancocas, New Jersey

ABSTRACT
A closed-loop automatic molten metal dispensing system based on vision technology is presented.  Vision technology and its effects on industrial automation and reasons for selecting vision sensor as opposed to other sensing devices such as X-ray and laser is discussed.

The control is self-compensating and utilizes automatic learning schemes to repeatedly and accurately fill the molds.

The level in the sprue cup is sensed by a vision camera and compared to a predetermined level profile.  The difference is then used to drive a servo-driven actuator.  The control calculates the flow of molten metal into the mold to learn the gating system characteristics.  This information is used to improve the future pours, which may be affected by the change in the orifice diameter and the level of molten metal in the tundish.

In case of a flaskless mold line, the pour tundish is automatically positioned over the sprue cup.

The system has been successfully tested both in the laboratory and on the production lilne.

The rising cost of material and labor has forced today's foundries to automate the various aspects of their operation to increase efficiency and productivity.  The extent of this automation has also reached the casting process.  The modern delivery systems and high speed molding machines have necessitated the need for an automatic pouring system to produce high-quality castings as a high throughput, while eliminating costly overpours.

This paper will discuss a closed-loop automatic pouring system based on Vision Technology for non-contact measurement of the molten metal level in the sprue cup.

After a brief discussion of the vision sensing, the control scheme will be introduced.

VISION SENSING
A typical vision system is comprised of a camera, lens, and image-processing electronics.  The object is viewed by the camera and a live picture is sent to the image-processing electronics at a rate of 30 frames or samples per second.  Each point of the picture is converted to an element called the pixel with a value representing the brightness of the original picture.  The collection of the pixels form an image, which can be processed by a computer for feature extraction (Fig. 1).  

Level Measurement
To measure the level of the molten metal in the sprue cup, it is essential to distinguish between the molten metal and the sprue cup.  This is accomplished by optical filters and electronic thresholding of the image.

The brightness of the molten metal provides an excellent contract between the sand mold and the metal in the sprue cup, and, therefore, a crisp image of the molten metal represented by white pixels can be obtained without utilizing any special lighting.  Since the camera looks at a large area and not only one point of the sprue cup, it is necessary to reach the area of the sprue cup or the region of interest (Fig. 2a).  The region of interest has two functions, it is used in metal level calculation (Fig. 2b) and in the elimination of the stream contribution in the total image area.  By looking directly at the sprue cup and electronically blocking the stream, the need for special sprue cup design is eliminated (Figs. 2c and 2d).

Fig. 2.  Sprue cup area (region of interest).

The molten metal level is calculated by counting complete lines of white pixel, filtering the stream out, and normalizing the total count to the cup area.

Why Vision?
The technological advancement of the past 10 years in the areas of computer and sensor technology has made a tremendous impact on automating industrial process control.  The increased productivity, reliability, and safety is what this automation has offered.

Vision systems, being a part of this advancement, are being widely used to automate industrial quality control, process control, and gauging, to name a few.

The advantages of a vision system in the automatic pouring process are: 

• Availability and relatively low cost of the components
• Safety
• Visual feedback

The availability and low cost of the vision components, plus the fact that -- unlike other measuring devices such as X-ray and laser -- there is no need for any sort of radiation, makes this system attractive.  Another feature is that, with the vision system, the process of positioning and pouring can be viewed live on a video monitor.  The camera, in conjunction with a VCR, can provide a valuable tool to analyze the pouring process.

VISIPOUR AUTOMATICE CONTROL SYSTEM
The Visipour System consists of a power enclosure that houses the servo amplifier, power supplies, and line conditioner; a control station that contains the computer, vision electronics, servo controller, the input-output interface, control software, and a video monitor; two air-cooled camera housings; and a servo-driven stopper rod mechanism (Fig. 3).

Figure 3.  Vision system equipment.

The operator interfaces with the system via a set of user-friendly menus through a front panel membrane keypad.  

The stopper rod mechanism is a servo-driven computer-controlled actuator, which provides fast response and accurate positioning of the stopper rod for quality pouring of molten metal.  The quick response of a servo actuator, as opposed to a pneumatic one, which is slow in response for today's fast sampling sensors, is vital to automatic control of the stopper rod during opening, closing, and throttling.

The servo closed-loop control system offers accurate positioning of the stopper rod and provides the means to automatically adjust the seating force of the rod into the nozzle.

The vision system performs two functions:  it automatically positions the tundish over the sprue cup, and it fills the mold according to its filling requirement.

Tundish Tracking
In the flaskless molding operation -- such as Disamatic line -- because of the change in the sand compactibility, the thickness of a given volume of green sand will vary from mold to mold.  This variation in the mold thickness offsets the sprue cup in relation to the pouring nozzle, and, therefore, the pour stream will not fall in the center of the sprue cup.  This causes splashing and sand erosion, and may increase the pour time.

Another reason for the tracing is that the vision system requires the sprue cup (region of interest) to always be in the same position.

The tracking is done by aligning the tundish over a mold and acquiring a picture of a notch formed on the side of the sand mold (Fig. 4).  The ideal notch location to be used as a position reference is calculated by the vision system.

Fig. 4.  A typical flaskless mold line.

The automatic positioning program will use this location to position the tundish by acquiring an image of the new mold's notch and comparing it to the ideal one.  The system is moved according to the difference between the present notch location and the desired one.

Automatic Pouring Control
To produce quality castings, it is essential to fill the sand mold quickly and continuously, without interruption in the flow of the molten metal in the gating system.  To assure this continuity, the sprue cup should be kept full during the pour to eliminate air entry into the gating system, which will result in unacceptable casting quality.

The vision system is a closed-loop, computer-controlled, molten-metal-dispensing system designed to fill sand molds with varying metal intake characteristics at a high throughput.

Due to the variation in the gating system, which determines the flow characteristic in the mold, the level of molten metal in the metal holding container, and the diameter of the orifice (which changes over time), the control system should be self-compensating.  To assure repeatability and accuracy of the pours over time, a powerful adaptive scheme is incorporated, which learns the characteristic of each pour and uses the acquired information to provide the controls with prior knowledge for the succeeding pours.

The control system (Fig. 5) requires a tracking profile to use the guidance in controlling the level of molten metal in the sprue cup.  This profile is based on the total time of pour and the desired levels of the molten metal in the sprue cup, during filling and at the end of the pour (Fig. 6).

Figure 5.  Vision system control loop.

Figure 6.  Level profile.

The vision camera measures the level of molten metal in the sprue cup, which is directly proportional to the difference between the flow out of the orifice and the flow into the mold (Eq.1).  This information is then used to predict the future level in the sprue cup (Eq. 2) due to metal in transition and also to calculate the mold intake (Eq. 3).

(1) (2) (3)

It takes a certain amount of time for the molten metal released from the orifice to reach the sprue cup (metal in transition).  This time delay is directly related to the distance between the orifice and the sprue cup and must be accounted for in the control scheme.

The comparison of predicted level to desired level, in conjunction with the estimated mold intake (Eq. 4), produces a process error (Eq. 5).  The error is then fed to a proportional control (Eq. 6) to drive the servo actuator to correct the level of the molten metal in the sprue cup and eliminate the error.

(4)	(t) = g [F(t) ]
(5)	E(t) = L(t + t) - A(t + t) + (t)
(6)	C(t) = G E (t)

where:  

Kc	= calculated gain
Ka	= a measured gain
G	= the control gain
t	= the measured time for metal in transition
L(t)	= the desired level of molten metal in the sprue cup
Q(t)	= the flow out of the orifice

The learning process takes place after a pour is finished.  While the next mold is being indexed into position, the calculated mold intake (Eq. 3) is analyzed, and the estimated mold intake (Eq. 4) is updated based on the result of the analysis (Fig. 7).  

Figure 7.  Mold intake profile.

The learning process is essential to the proper operation of the control system in order to keep the pour time in the acceptable time window without short pouring, due to gradual changes in the level of molten metal in the tundish and diameter of the orifice.

CONCLUSION
The vision system as a closed-loop, self-compensating automatic pouring system increases the productivity and reliability of the casting process by eliminating the costly over pours and short pours.  The utilization of a vision camera, which directly views the sprue cup, eliminates the need for oversized sprue cup design, which may hold more molten metal than needed.

The control system adapts itself to the changes in the pour process caused by variation in the molten metal level in the tundish and diameter of the orifice.

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