Electro-Pneumatic Positioner 4-20mA Input

Electro-pneumatic positioners are used with rotary air actuators to accurately position control valves used in throttling applications. These valve positioners convert a 4-20mA input control signal to a proportional pneumatic output. This output is fed to an air actuator which in turn controls the valve position and flow (combined with mechanical feedback). Air actuated valves with positioners have a history of rugged and reliable service in tough applications. Valworx positioners offer fast and accurate control of ball valves, butterfly valves and other type valves. These positioners direct mount to rotary type air actuators with the standard VDI / VDE- Namur style top shaft mounting. Universal Namur mounting bracket kit is included with each positioner. Suitable for use with Valworx double acting or spring return air actuated valves, as well as most other brands utilizing the standard Namur mount. Rugged aluminum die cast housing has superior anti-corrosion surface treatment and an easy read visual valve position indicator. Easy to adjust zero and span. Positioner is PRE-INSTALLED when ordered with a Valworx air actuated valve package.

Note: The cycle time of assemblies using a positioner could be slower than the actuator cycle time by upwards of 10 seconds.

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Input Signal4-20mA DC Impedance250 +/- 15 ohms Rotation0-90 degree (clockwise or counter-clockwise) Supply Pressure20 to 100 PSI (filtered, dry, oil free air) Flow Capacity2.8 SCFM Air Consumption0.10 SCFM Ambient Temperature-4 to 158 degree F Linearity+/- 1.5% F.S. Hysteresis+/- 1.5% F.S. Sensitivity+/- 0.4% F.S. Repeatability+/- 0.5% F.S. Air Connections1/4" NPT Gauge Connections1/8" NPT EnclosureAluminum die casting Enclosure RatingGeneral purpose, IP66 weatherproof Electrical Connection1/2" NPT female conduit Mounting BracketFits Namur 80x30 and 130x30 mounting holes with 20 or 30mm shaft height

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In addition to electropneumatic valves, EPPS circuits typically include standard cylinders, Hall-effect or other position sensors, and a position controller.

The electropneumatic positioning system (EPPS) uses two proportional pressure regulators mounted on the pneumatic cylinder, one on either side of the cylinder piston. One side maintains a constant pressure using the first proportional pressure regulator (E/P valve), while the second E/P valve controls pressure on the opposite side.

The unit&#;s electropneumatic positioning controller (EPPC) receives data from and sends analog pressure command signals to the proportional pressure regulators. The controller compares the position command to piston feedback. A proportional-integral-derivative (PID) loop provides an analog output to the second proportional pressure regulator to adjust the pressure as needed. Greater pressure on the second side means the piston will move forward, and vice versa.

Air&#;s Advantages

While electromechanical and flow-based electropneumatic systems offer effective positioning capabilities, direct proportional pneumatic control systems such as the EPPS hold a number of inherent advantages.

The main advantage of this method is higher accuracy. Proportional pressure regulators constantly and precisely control the pressure, eliminating effects of dithering. The direct-acting pneumatic pressure regulators can control pressure within 0.10 psi, and the EPPS system can use any of the company&#;s pneumatic actuators. That gives system designers a lot of flexibility.

Note that every actuator has different physical characteristics, so it is difficult to make a broad statement regarding positional accuracy. However, with a properly selected actuator and optimized tuning of the controller, ±1 mm positioning accuracy is certainly possible. This is better than traditional flow controlled pneumatic positioning.

The EPPS is designed to economically deliver the benefits of pneumatic positioning to a broad range of automation applications. The system uses standard, off-the-shelf components which are easy to integrate and can be quickly replaced when needed. The EPPS uses an AVENTICS SM6-AL (Hall Effect) sensor, and does not require the cylinder piston rod to be gundrilled as in traditional pneumatic systems. Eliminating gundrilling lets standard cylinders be used, thus reducing cost and delivery time.

Although it relies on electronic controls, the EPPS does not require complex programming: A user simply inputs an analog position command, saving time and resources. Standard analog command for stroke set-point is 0 to 10 Vdc or 4 to 20 mA.

In general, electropneumatic systems are more durable than electromechanical systems because they do not require electric motors or drives at the actuator. Thus, they can withstand harsher conditions such as temperature extremes, dusty conditions, and wet and dirty environments.

Finally, the EPPS improves positioning accuracy compared to conventional pneumatic positioning, and it eliminates drawbacks (such as dithering) associated with flow-based pneumatic positioners. For many automation applications requiring proven performance, high throughput, and ultraprecise positioning accuracy and control, the EPPS offers a viable alternative to electromechanical and flow-based positioning systems.

Electropneumatics at Work

For example, a mechanical-components OEM turned to the EPPS to automate a press that&#;s used in an assembly process for installing bushings in housings. The press is designed to handle two different sizes of housings. The smaller housing requires two pneumatic cylinders to press two sintered bronze bushings into place, one on each side of the housing. The machine simultaneously installs both bushings in a single operation.

A larger housing requires three bushings to be pressed into place. As with the smaller part, a bushing is pressed into each side of the housing. However, for the larger unit, a second operation is needed to press the third bushing in place inside the housing&#;a cumbersome and tricky task when performed manually.

To speed the process and improve quality, the manufacturer chose the EPPS system to precisely and quickly press the bushings into the proper positions. The operator selects the required housing size via an HMI; a PLC then sends a command signal to the EPPS controller (EPPC) and on to the electropneumatic control valves, ultimately controlling actuator position during the pressing operation.

The system uses two ED02 electropneumatic pressure-control valves and two PRA pneumatic ISO piston-rod cylinders for pressing, plus a third cylinder for clamping parts in place. In addition, an RTC rodless cylinder opens and closes an operator safety shield.

The OEM automated this process to ensure accurate bushing placement, reduce the cycle time for the operations, and improve operator ergonomics. Results show the bushings are precisely mounted with less effort and fatigue, and cycle times decreased about 30%.

One final note: When designing the automated press, the company&#;s manufacturing engineers considered an alternative system using an electric servodrive. The EPPS system won out, based on an estimated cost savings of approximately 40%.

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