Visit the San Francisco Giants' Pacific Bell Stadium and the Monterey Bay Aquarium and you'll see impressive structures built with products from RMC Pacific Materials.

RMC, based in Pleasonton, Calif., is a key aggregate supplier in the building materials industry, manufacturing five different types of cement. Aggregate is used in RMC's ready-mix operation and marketed to other ready-mix producers and manufacturers of products such as concrete block, interlocking pavers, and lightweight roofing tiles.

Making sure aggregates are properly mixed is vital to RMC's operation. Mixing begins with mined aggregate, which is washed, crushed, and segregated into various components according to size. These are then recombined according to ASTM specifications to create products used in making concrete, asphalt, and aggregate.

The supplier recently automated its aggregate mixing operations at two of its California plants: RMC Pacific Materials in Pleasanton and Harbor Sand and Gravel in Redwood City. Using Bosch Rexroth electro-pneumatic (E/P) positioners, RMC is now able to monitor and control gate positions and protect its mixing lines from overfeeding in case of power failure or malfunction.

The mixing processes at each plant uses Rexroth E/P positioners to open and close the gates that discharge material directly onto conveyor belts, which transport the material to predefined locations within the plants. Depending on the material being produced, one to four different gates can be opened to blend ingredients.

This type of automation had never been used in aggregate mixing before. Once we tackled the size issue, we needed a 5-inch bore, which was a larger unit than we generally offer. The main challenge was the unit had to be "fail safe" in the extended position.

Maximizing tonnage

At Harbor Sand and Gravel, the E/P positioner opens and closes a gate to control the amount of material released onto a conveyor, to maximize the tonnage being transported without overloading the system. Feedback from a belt scale downstream opens or closes the gate controlled by the E/P positioner to maintain a tons-per-hour flow set by the operator.

Once the E/P positioner is given a set point by a PLC, it moves the gate to the given position and then monitors the position, making adjustments as necessary. Feedback information from the E/P positioner is monitored by the PLC, which alerts the operator if there's a malfunction.

The automated positioners have improved the reliability of the aggregate products made through the load-out system, says Tony Fuentes, of RMC's quality control department. "We've tightened our operating ranges on several products," he says. "One of the biggest benefits is that we can make changes with little or no effect to our product or operating parameters. We can also offer custom blends with more accuracy, which is a huge benefit for special projects."

The E/P positioners replaced existing air cylinders at each mixing location, making installation easier and less expensive since air was already available. A push-button station was already being used to interface with the existing air cylinders, so it was a relatively simple task to integrate the new cylinders into the system while the plant maintained normal day-to-day operations.

"Without the positioner," says Randy Mull, an engineer with Eichleay Engineers, a Concord, Calif., design and services firm, "additional I/O slots from the PLC would have been required to open and close solenoids on the cylinder to maintain the set point."

The size of the cylinders Was big enough to stop the flow of material when closing, move the gates, and control the flow of material. Typically, the actuators default to the retracted position upon signal failure, or stay where last commanded upon total loss of power.

We needed to reverse this operation because the gate closed only when the cylinder was extended. We couldn't risk leaving the chute gate even partially open, as the silo could completely empty on a stalled conveyor and create a messy, costly cleanup operation.

By reversing signal wires and plumbing lines, adjusting command signals accordingly, and adding a normally open the valve, which overrides the pilot pressure to the directional valve that extends the cylinder, the team was able to achieve the desired effect. The system is also equipped with an air reservoir to allow cylinder actuation even after a power failure stops the air compressors.