Pouring Systems: Vision vs. Laser Sensing

June 22, 2005
Each has advantages, but a combination of the two may be your best bet.

Closed-loop automated pouring systems allowed casters to shift from time-based pouring to sensor-based control of the stopper rod. Stopper rods were opened to allow a specific amount of metal to flow for a pre-determined time. This predicted the next pour would behave precisely as the previous pour. But wear of the nozzle, slagging, and changes in the level pouring vessel result in flow-rate changes, making time-based control difficult. To maintain proper pour control, the operator would have to constantly adjust the stopper-rod opening or the pour time — hardly an automated process.

That opened the door for closed-loop automated pouring, according to Goran Lowback, with Koins Corp.. The key: sensors that provide real-time feedback of the metal level in the pour cup, making it possible to control the opening of the stopper rod to maintain a certain level in the cup throughout the pour. Such systems match the flow rate from the nozzle with the mold’s intake rate, using the metal level in the cup to provide feedback.If the sensor detects that the level in the pour cup is increasing, the system closes the stopper rod slightly. If the level decreases, the stopper rod is opened slightly. A steady level indicates that the flows are perfectly matched.

Today, these systems may employ one of three sensing technologies: vision systems that employ CCD-based cameras; traditional point laser systems; and newly introduced line laser systems that combine laser and vision measurement.

“Line laser systems are designed for high-speed production with small pour cups,” explains Lowback, noting that Koins offers all three sensing technologies in its PourTech automated pouring system, plus enhancements. “For example, a variation on our line laser, what we call the 3D sensor, is used with a Disamatic-type vertical-mold line. It consists of a line-laser generator that will put a line across the pour cup, and as the metal fills the mold, that line will project on the metal. The sensing system located on the opposing side of the pouring unit will see the line and measure variation of the metal in the cup.”

Larger Pouring cups with vision sensing — This is best suited for flask lines that use large pour cups and don’t require constant repositioning of the pouring vessel,. and costs less than laser-sensing technologies. But, a much slower sampling rate, combined with difficulty in “seeing” the pour cup and recognizing details of the process, limits its capabilities.

According to Lowback, vision systems look at the areas around the iron stream, reading the number of pixels illuminated on the sensor and trying to interpret the level in the cup. If the iron stream starts to move back and forth, and the stream starts to fan out, very little metal surface area remains in the pour cup, so the vision system can’t see the metal level.

“We only recommend vision-based systems for pouring with very large pour cups — about four times the diameter of the pouring nozzle,” he said. “I recently visited a foundry that poured with a 2.5-in. nozzle, and for us to feel confident in the effectiveness of the vision system, we recommended a 10-in.-diam. pour cup. Such a large cup ensures an adequate metal surface for the camera no matter the actions of the iron stream. Line laser sensoring could do that job using a cup only about half that size.”

High data-sampling rates with lasers — Line laser sensoring exhibits data-sampling rates that are much higher than with vision sensing. The rates depend somewhat on cup size: the smaller the cup, the greater the data range.

“This allows the line laser technology to perform many more data calculations than a vision system,” he says. Of course, more calculations mean more refined and accurate sensing.

Line laser sensoring combines a line laser with a special C-MOS-based vision camera, consisting of a laser line generator and a line sensor, both housed in water-cooled, air-purged jackets. Each device mounts on opposing sides of the pouring vessel, allowing triangulation measurements across multiple points in the pour cup. By projecting a line across the pour cup, the sensor becomes less sensitive to deteriorating streams, making it much more effective than vision-based sensing in many applications. It differs from point laser sensing in that it does not require a laser tail. Point laser sensing employs a triangulation laser aimed into the pour cup, and takes 16,000 direct level measurements per second.

Often, a pour cup with this type of sensor is outfitted with a small tab on its side to separate the laser point from the pouring stream as much as possible. It reduces the turbulence in the cup, slowing down the waviness, permitting a stable signal in the laser tail.

The tail provides a well-defined surface for the laser to find during the positioning phase and helps separate measurement points from the iron stream during the pour-control phase. The result: a gain in sensing capability that allows use of smaller pouring cups than with vision sensing.

Line laser sensing requires only a portion of the line to be read with the sensor, negating the need for the well-defined surface created by the laser tail. As long as the sensor sees a portion of the line, a level measurement can be made. No need for modification of pouring cups.

Access affects laser selection — Because it requires mounting of equipment on opposite sides of the pouring vessel, access difficulty may preclude its use.

“That includes applications where it is difficult to get both the line laser generator and the sensor to see the pouring cup,” Lowback explains. “Laser line sensing was designed for pouring furnaces with access from both sides.”

Sensors determine stopper-rod action — “With PourTech, the operator tells the controller what levels must be maintained in the pour cup throughout the pour,” Lowback explains.

“The control system monitors what the laser measures in the cup, then positions the stopper rod accordingly. If the actual level is above the ideal, the control will close the stopper until the level drops to the ideal level or below. If the actual level drops below ideal, the control will reopen the stopper. That describes the closed-loop nature of the system.”

“The goal is to match the amount of metal coming out from the nozzle with the intake rate of the mold. If the amount is exactly at the same amount that the mold is accepting, the level in the cup will stay perfectly still because everything we deliver is consumed by the mold.”

But, maintaining an ideal level is an elusive proposition.

“With constant sensing and adjustment of the stopper rod, we approach that ideal cup level,” said Lowback. “It is essentially impossible to measure the flow, so the sensors measure the cup level. That will tell us if we are pouring more or less than what the mold is accepting at that moment.”

“As the mold nears full, less metal will be accepted,” continues Lowback. “The stopper rod slowly closes, delivering less metal. When the mold is full, the stopper rod closes completely. No more metal is delivered, signaling completion of the pour. Now, the mold line indexes and we attack the next mold.”

Finding the best means to attack that next mold means taking advantage of automated pouring and sensing options .