A new generation of electropneumatic valves offers the flow and response of large valves in a spacesaving package.
Bosch Rexroth Pneumatics
Electropneumatic-proportional (E/P) valves are typically used where applications demand precise control of pressure or force. The combination of a pneumatic valve and electronics accurately maintains preprogrammed pressures along with the ability to quickly react to changes in flow, pressure, or other external influences on a system.
Designers typically evaluate E/P valves in terms of linearity, hysteresis, nominal flow, repetitive precision, dynamic response, size, and cost as they relate to a specific application. As with most engineering decisions, specifying the correct valve usually means balancing performance and cost.
A new generation of compact E/P valves can eliminate many of the trade-offs and ease the decision-making process. They offer flow capacity, dynamic response, and precision typically associated with larger, more-expensive valves, giving designers new possibilities for precise pressure regulation in space-saving packages.
E/P valves modulate pressure in proportion to an analog electrical-input signal, typically 0 to 20 mA or 0 to 10 V. A piezoresistive pressure sensor measures output pressure, and integrated electronics compares it to the nominal electrical input. If command and actual pressures do not match, the controller sends an output-positioning signal to the solenoids, which adjust the valve to produce the required output pressure.
Two direct-acting, 2/2-way valves independently control inlet flow and exhaust. With direct-acting valves, a proportional magnet generates force and moves the valve seat avoiding the inertia and hysteresis of mechanical transfer elements.
Proportional control also eliminates the stepping behavior of on/off solenoids. Precision basically depends only on the quality of the pressure sensor and the controlling current sent to the solenoids, letting the valve offer exacting control even under constantly changing process conditions.
Compact valves not only take advantage of advances in electronic controls and proportional solenoids, the latest designs pack more flow capacity into smaller packages. For example, Rexroth ED02 compact E/P valves measure only 76 X 60 X 30 mm yet generate nominal flow of 100 lpm (3.5 scfm) over a standard pressure range of 0 to 10 bar (145 psi). Traditionally, such flows required much larger valve bodies.
Even with the small size, the valves provide excellent control. The ED02, for example, offers a repetitive precision of 30 mbar (0.44 psi), meaning the valve's output deviates only slightly from a given setpoint. Similarly, hysteresis is only 50 mbar (0.73 psi) and linearity varies <1% over the full control range. Together, these three characteristics give highly precise and repeatable operation.
Another area of significant improvement lies in valve dynamics the speed with which a valve responds to external changes. Two-valve configurations quickly, precisely, and independently control inlet and exhaust, resulting in higher dynamics than single-valve versions. Flow rate also affects dynamics, so compact valves' ability to handle relatively high flows improves response. Finally, intelligent onboard electronics reacts to inputs in milliseconds and permits sophisticated control strategies.
VALVES AT WORK
The compact valves readily adapt to closed-loop operations, where feedback based on torque, temperature, position, force, speed, and other variables permits continual adjustment to ensure precision under varying conditions. This makes them suited for industries from automotive to semiconductor. Typical applications include:
Painting. E/P pressure-control valves maintain constant pressure in containers for consistent, even paint application.
Filling. Electropneumatically controlled metering valves let operators quickly, precisely, and repeatedly fill containers to the ounce.
Flow control. Airflow through a nozzle can be adjusted using controlled pressure. Installing a flowrate sensor and an overriding rate control ensures precision.
Contact pressure. In clamping operations, keeping tooling pressure constant provides uniform results for surface processing.
Speed control. Maintaining constant air pressure provides an even and easily adjustable flow and speed control for pneumatic motors and turbines. Adding a rotary-speed sensor lets the units provide extremely precise control. In linear actuators, controlling pressure in the cylinder chamber lets users define piston movement with various speed profiles.
Compensating rollers. Pressure-controlled tensioning devices prevent lengths of fabric from tearing or tangling and optimize material flow.
Welding. Electropneumatic pressure control in welding tips makes it possible to quickly and gently close the tips and apply precisely the same welding force every time.
Because most applications use more than one E/P valve, the ease with which they combine is often an important design consideration. Most devices mount on a baseplate, although newer valves such as the ED02 can link together without one. Supply air can feed in from both sides, while a single connector handles electric power, analog setpoints, and feedback readings. A pneumatic control module coordinates the valves and connects to the control system via standard fieldbuses or Ethernet.
Bosch Rexroth Corp., boschrexroth-us.com