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.
