Filling On The Fly

Feb. 21, 2002
The Allen-Bradley 1394 digital servo controller is a position controller integrated within a modular, digital AC servo drive, providing a single, compact, motion-control package.

The Allen-Bradley 1394 digital servo controller is a position controller integrated within a modular, digital AC servo drive, providing a single, compact, motion-control package.

Machine builders clearly understand that their customers' requests are thinly veiled demands. Either the builder meets the "request," or a competitor will. That's the situation an OEM faced when one of its large customers asked for a machine to fill containers with a heavy, viscous, room-temperature vulcanizing (RTV) silicone at a rate of 150 parts per minute. The OEM knew its current pneumatically driven machines that peaked at 120 per minute would have to change.

"Even though we make innovative pneumatic machinery, we realized the technology wasn't capable of yielding the customer's request of 150 pieces per minute," said the company's inventory and controls manager. "We looked beyond mechanical solutions and researched electronic controls and servo systems to reach the customer's speed, precision, and flexibility requirements."

To migrate to a servo-driven system, the OEM turned to Rockwell Automation and its AllenBradley ControlLogix integrated motion controller. It discovered the servo system not only met its customer's speed requirements, but was more user friendly and flexible than traditional mechanical systems as well.

Pneumatic Nuisances, Servo Solutions
The company's engineers had several challenges with air-powered actuators for filling in high-viscosity applications. For example, pneumatic cylinders provided slow cycle speeds when placed under heavy loads, and filling could only be done at a 1:1 ratio. This meant that for every inch of movement at one end of the cylinder, there was one inch of movement at the opposite end. This inherent limitation required machine designers to rely on adjustable linkages and several sizes of metering cylinders to get the specified degree of movement and filling required for multiple applications. Filling multiple sizes of containers on the same filling line was a daunting task.

Additionally, when used to provide pre-dose volumetric metering of the filling material, the metering cylinders had to be the same size as the containers being filled. That required operators to change the cylinders whenever a production run switched to different-sized containers. These changeovers took about half an hour, lowering daily throughput. Compounding this was the need to stock several cylinder sizes — one for each container size to be filled by the machine.

The Need for Speed
The OEM created its new machine using the ControlLogix integrated motion controller to ensure accuracy and meet the speed requirements unattainable with previous mechanical machines.

Traditional nonintegrated control architectures offer only one function per device, (i.e., a separate controller is required for motion, sequential, and other control functions). Recent developments in control technology integrate motion and sequential control into a single, multi-tasking controller platform, resulting in higher system performance, faster application development, easier maintenance, and lower system costs. Users achieve higher performance because all system-control elements reside on the same hardware chassis and within the multi-tasking control architecture, providing faster communication than possible over a multinode network.

"Even though we've been using servo technology for years, this level of integrated motion control was new to us," the controls manager said. "We took a calculated risk but determined that we could meet our customer's needs using the ControlLogix controller to control five servo motors and drives for five separate motion axes."

The first axis indexes the containers on a conveyor as they move through the machine. Servo indexing moves the conveyor to a fine positioning point quickly and repeatedly. Once the machine finishes indexing, the remaining four servo axes help fill the containers. The machine uses two servos to lift the empty containers from the conveyor to an initial filling point, and the two remaining servos control the volumetric dosing as the cylinders dispense product into the container. The electronic gearing of the equipment ensures that containers drop away from the fill nozzle at an optimum, programmable ratio as they're filled. The result is a fill devoid of air pockets, which could spoil the product, create waste and siphon off profits. This entire process takes roughly two seconds.

Servo controls can reduce setup time too. "On our pneumatically driven machines, our customers have to climb over the machine with a fistful of wrenches to change metering cylinders," he said. "Now, with the ControlLogix system, we can easily adjust the filling parameters to get precise, air-free fills regardless of container size, and machine operators can change the fill volume, speed, and ratio electronically, right at the panel. This saves time and headaches."

Added benefits
Although the switch to servos was driven by a need for faster output, the end user reaps additional electronic motion control benefits, such as space savings, decreased training time and simple programming.

"Both panel and shop-floor space is valuable," he said. "The ControlLogix system eliminates the need to store various dosing cylinders, and because the motion control cards are slotted right into the programmable controller rack, there is no need for a completely separate motion controller in the panel."

Additionally, electronic servo control allows machine operators to change production runs more easily than ever before. Operators can now make adjustments at the touchscreen panel display to program the machine for proper fill rate, speed and volume, greatly reducing setup time. Electronic gearing of the filling station eliminates the need to make mechanical adjustments to prepare the machine for fast, void-free and precise filling applications. This allows machine operators to keep their production lines moving instead of wrestling with their wrenches or waiting for maintenance personnel to make alterations.

The ControlLogix system uses an easy-to-understand ladder logic programming language. Engineers were able to easily write logicbased programs that take full advantage of the new servo systems at the filling station.

"The predefined motion control commands make servo motion programming simple," he said. "Once we programmed the basic movement commands, we just duplicated them for the other axes in the program, dropping them in where appropriate. It is an easy system to program."

In the ever-changing world of machine design, time stands still for no one. The OEM found it could offer customers higher throughput, less downtime and quicker changeovers, all by migrating from pneumatic to electronic servo control.

Based on the experience of designing its newest servo-equipped machine, the company has discovered that servos can greatly enhance machine performance. "Servos have significantly improved our product line. We believe that servo motion control is the wave of our future."

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