finesse

AllAboutAir Thomas W. Kreher | Applied Pneumatic Controls, Inc. Use Finesse Instead of a Large Hammer A sledgehammer is one device used with great force to turn it on or off abruptly. When the force of a pneumatic actua- speed control finesse include: 1. Flyaway velocity of actuators may cause personal injury or breakage. 2. Acceleration across the actuator to load gap may cause damage on impact. 3. Slowing the actuator to reduce violence reduces production. 4. Loads may spill if accelerated or decelerated abruptly. 5. Energy is wasted with the high force and hold back (meter out) technique. 6. Multiple pressure differentials are desired to vary the velocity or torque. A serious drawback to meter-out speed control for pneumatic cylinders occurs as mentioned previously when the load is blocked or on the first application of pressure. When the pressure is applied with no opposing force, uncontrolled, fly away, speeds are dangerous and potentially destructive. This situation often occurs when a machine has been inactive and cylinders have drifted out of position by gravity or moved out of the home position. Smooth Start Flow 1. To prevent start up cylinder violence, a twostage device called a Smooth Start Valve is often used. The smooth start function may be incorporated into an on/off supply valve including those with high exhaust flow capability for emergency shut down. The smooth start function includes an adjustable orifice that allows the system to fill the existing volume and extend cylinders gradually to a threshold pressure before opening the main valve. A smooth start device may serve the purpose very well. They are restricted to two flow modes, restricted and full flow or creep and race. Variable Pressure Finesse 2. A Pilot Operated Regulator with a “tank circuit” (orifice or needle valve and volume) to build the pilot pressure gradually provides smooth start pressure control. 3. Small solenoid valves plumbed in parallel with multiple preset pressure regulators can provide multiple pressures on demand. 4. With an electronically controlled pilot regulator, the pressure modulation can range from simple to very sophisticated with sensors and feed back loops. Imagine a cylinder that is thrust forward to clamp a part with high flow and low pressure, 30 PSIG. When the cylinder is stopped upon contacting the part, the pressure is increased to 60 PSIG to hold securely. At the end of the cycle, the cylinder is retracted at high pressure just to clear the part and then returned to home position with 30 PSIG. Possibly 40 percent of each cycle could be run at the lower pressure with 40 percent less flow. In this example, the theoretical saving is 16 percent of the compressed air required. I don’t think many will rush out to change all the cylinders using “meter out” force control. If you see someone dragging a large cement block behind their vehicle, please tell them that you were just reading about a similar situation. For More Information: Thomas Kreher started Applied Pneumatic Controls with his wife, Gloria, in 1995. He can be reached at tom@applied-pneumatic.com. tor is too great without being modified, we impede that force with flow controls. Does it seem strange to drive a cylinder forward with 1,000 pounds of force and oppose it with 800 pounds of force to reduce the velocity and violence? This would be similar to pressing the accelerator pedal on your vehicle down with a large cement block chained to the trailer hitch to reduce acceleration. As we strive to conserve energy, the example of “meter out” speed control for pneumatic cylinders (But we’ve always done it that way…) presents a challenge. First, we may ask: Why are we using 1,000 pounds of force if we only require 200 pounds? Meter-out speed controls offer a distinct advantage for loads that vary. If the increased load slows the acceleration of a pneumatic cylinder, more air is exhausted (metered out) and the opposing force is reduced. In the extreme case where the load may be mechanically blocked long enough for all of the opposing volume and pressure to meter out, the results can be dangerous and damaging (fly away) with an unopposed break away. Applications that may benefit from greater 6 |March/April 2007 www.ifps.org | www.fluidpowerjournal.com FPJ07MA07_1-21.indd 6 2/20/07 11:01:28 AM