Machine Design

Tanks a lot — PLC is a key to talkative tank filling system

Inside the vibrationproof cabinet of this Sur-Fill single gas system (right) are sample cylinders on scales. A Mitsubishi A-500 frequency drive controller in a NEMA enclosure to the rear of the system (left) reads the weight measurements for turning gas and vacuum on and off, and for operating pump motors. Operators interact with Mitsubishi A-900 a touch screen incorporating 3D graphics. The touch screen is on the control cabinet front panel.

Operators don't have to worry about keeping an eye on new Sur-Fill systems, used to fill gas cylinders. The Q Processor platform from Mitsubishi Electric Automation lets the system issue prompts in a human voice. It can even remind forgetful operators and warn them of impending events.

A prerecorded voice chimes in with messages like "Time for your PS 1000 leak check," or, "Approaching your pressure" as these events grow near. "This way the operator doesn't have to stand there and watch the filling process," says Gary Schueman, principal of Computer Integrated Automation Inc. CIA Inc. (Carol Stream, Ill.) working with Weldcoa, a maker of cylinder handling and filling gear in Northlake, Ill, devised the Sur-Fill system. "We've had customers that were able to reduce their labor force just because of the prompting feature," says Schueman.

One reason for the use of the Mitsubishi PLC was "I had worked with many different types of automation controls over the years and knew I wanted to avoid a PC-based platform," says Weldcoa senior R&D developer Maynard Klotz. A Mitsubishi Q Processor platform got the nod for its dual high-speed Risc processors and 0.15 sec/step execution speeds. Says Schueman, "The Q processor option is probably the fastest high-end PLC on the market right now. Its floating-point abilities and ease of programming with high-level commands are what makes the gas system possible."

Sur-Fill operators first weigh spent tanks that have been returned. The measurement gets checked against a database to find out what the particular tank should weigh when completely empty. Any foreign matter in the tank raises a flag. The system evacuates it before refilling to prevent any contamination.

The system can fill cylinders with gas to a proscribed weight, or with specific mixtures of gases based on their individual pressures. The latter process is complicated by the increases in gas temperature that accompany rising pressures. To that end, gas physicists at CIA Inc. devised filling algorithms that factor in heat of compression. They let the Mitsubishi controller compute shut-off pressures for each gas going into the mixture so that the final cylinder pressure is correct.

Under control of the Mitsubishi controller, the system can fill as many as four manifolds simultaneously each holding 16 300-ft 3 tanks. Mitsubishi A-500 variable-frequency drives power the pump motors. Alarms signal the detection of failure in system components such as thermocouples or pressure transducers.

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