Pneumatics mixes things up

Electropneumatic positioners help automate aggregate-mixing operations.

Aggregate, a combination of sand and gravel, is a primary ingredient in concrete. Mixing aggregate begins with washing, crushing, and segregating mined material into various components according to size. These components are then recombined according to ASTM specifications to create products used in making concrete, asphalt, and aggregate.

RMC Pacific Materials, headquartered in Pleasanton, Calif., uses aggregate in its ready-mix operation and markets it to other ready-mix concrete manufacturers of products, such as concrete block, interlocking pavers, and lightweight roofing tiles. These products have been used in such high-profile projects as the San Francisco Giants' SBC Park and the Monterey Bay Aquarium.

With the help of Randy Mull, P.E. with Eichleay Engineers Inc., distributor Motion Industries, and Bob Atchison of the Pneumatics unit of Bosch Rexroth Corp., RMC automated aggregate mixing operations at two of its California plants: RMC Pacific Materials in Pleasanton and Harbor Sand and Gravel in Redwood City. Thanks to Rexroth electropneumatic positioners, comprised of a cylinder with an integral feedback potentiometer, RMC can monitor and control gate positions and protect mixing lines from overfeeding in the event of power failures or malfunctions.

Automation in the mix

The mixing processes at each of the RMC locations use the electropneumatic positioners to open and close the gates that discharge material directly onto conveyor belts, which transport the material to different locations within the plants. Depending on the material being produced, one to four different gates can be opened to blend ingredients.

"This is the first time this type of automation has been used in aggregate mixing," says Atchison. "Once we tackled the size issue - we needed a 5-in. bore, which was a larger unit than we generally offer - the main challenge was making the unit fail safe in the extended position."

Failure is not an option

At the Harbor Sand and Gravel plant, the electropneumatic positioner opens and closes a gate to control the amount of material released onto a conveyor. This maximizes the tonnage being transported without overloading the system. Feedback from a belt scale downstream opens or closes the gate to maintain a tons-per-hour flow set by the operator. Once the electropneumatic positioner is given a setpoint by a PLC, it moves the gate to the given position and monitors it, making adjustments as necessary. The PLC monitors feedback information and alerts the operator of malfunctions.

"Quality-control samples are taken daily, and the results are used to fine-tune positioner settings," says Richard "Butch" Kelly, project and resource manager for RMC's Aggregate Div. "These are periodically updated to make a more consistent blend, with fewer operator adjustments. Also, less out-of-spec material means less rework and less repurchasing of unsatisfactory material."

According to Tony Fuentes of RMC's quality-control department, "The new system takes a lot of the guesswork out of the gate positions. One of the biggest benefits is that we can make changes with little or no effect on our product or operating parameters. We can also offer custom blends with more accuracy, which is a huge benefit for special projects."

The electropneumatic positioners replaced existing air cylinders at each aggregate mixing location, making installation easy and economical because air was already available. A pushbutton station was already being used to interface with the existing air cylinders, so it was relatively simple to integrate the new positioners, while the plant maintained day-to-day operations.

"Without the positioner, additional I/O slots from the PLC would have been needed to open and close solenoids on the cylinder to maintain the setpoint," says Mull.

Typically, the actuators default to the retracted position upon signal failure, or stay where last commanded upon total loss of power.

Power failures occur an average of three to four times per year at the plant. In the past, each time the power failed, the cylinders remained in their last position because the solenoids were electrically actuated. This caused the contents of the bunker to spill onto the stopped conveyor belt, burying the belt and requiring several hours of cleanup before the belt could be restarted.

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

The answer was found in reversing signal wires and plumbing lines, adjusting command signals, and adding a normally open 3/2 valve, which overrides the pilot pressure to the directional valve that extends the cylinder. The system is also equipped with an air reservoir to allow cylinder actuation even after a power failure stops the air compressors.

The cylinders now fully extend in the event of power failure," says Mull. "When power fails, any residual air pressure in the lines is used to extend open cylinders to their fully extended position, thus closing open gates and eliminating spills and cleanup."

Setting the electropneumatic positioner apart from others is its ability to provide feedback on its position. After operators send a signal to the positioner telling it where to go, they can look to verify the gate has moved and is in the correct position.

The Pleasanton plant introduced the positioners in an aggregate blending application where units are used to divert the effluent from a rock crusher onto two conveyors. In the two years since, it has been bombarded with gravel, sand, dirt, water, and harsh air, yet the unit has functioned daily with only minimal maintenance required.

"The cylinder and valves in this system are designed to withstand these elements. The control card is the only part of the system that must be enclosed, but it doesn't necessarily need to be located at the cylinder; it can be several feet away and still function as well," says Atchison.

More on the machine

Bosch's electropneumatic positioner provides strokes from 1 to 10 in., or in 2-in. increments between 10 and 18 in. Strokes up to 60 in. can be special ordered. The positioner's accuracy is ±0.05 in. or 1% full stroke, whichever is greater; and its repeatability is ±0.05 in. Stroking speeds range from 0.5 to 2 ips, with operating temperatures of 41° to 122° F.

The unit's feedback device is an internally mounted linear potentiometer for applications in which infinite positioning requirements allow electrical analog control signals. The positioner interfaces with a computer, PLC, or potentiometer and consists of a cylinder with an integral sensor, optimized valving, and electronic controller. The basic concept involves a cylinder with an integral feedback potentiometer in conjunction with a controller and matched solenoid valves.