Air smooths motion transitions
Most motion bases, whether used in flight simulators or theme park attractions, use stiff, servo-controlled hydraulics for precise control of motion. But the Cyber Air Base, designed by Brad Engstrand, chairman of Virtogo Inc., Chicago, uses a set of discrete air valves to produce, in effect, proportional motion control
In the Cyber Air Base, compressed air flows to the cap end of each cylinder through four directional control valves piped in parallel. This is a six-axis motion base, so there are six cylinders and 24 valves. These are not proportional valves; instead, they are on/off type poppet valves. Response is typically under 50 msec. The valves go from fully closed to fully open in less than 5 msec, so most of the response time involves waiting for enough air to flow into the cylinders to build adequate pressure. When a valve opens, the compliance of air acts as a cushion to accelerate the load, instead of producing the jerky motion of a hydraulic on/off valve. This type of motion is similar to that produced by a soft-shift hydraulic valve.
More importantly, each of the four valves is a different size. So to accelerate rapidly, all four valves shift to connect the cap end of the cylinder to pressure. When lower acceleration is needed, any combination of one, two, or three valves open. Flow is regulated by controlling which valves are open, which is determined by control software. For sake of discussion, say valves A, B, C, and D can pass 1, 2, 3, and 6 scfm, respectively. Therefore, any flow rate can be achieved in increments of one anywhere between 0 and 12 scfm. Opening valves A, C, and D produces flow of 10 scfm; opening B and D produces 8 scfm. Compliance is important because it creates transitions that smooth out motion as valves shift.
Brad Engstrand was chairman of Virtogo Inc., Chicago, which manufactured the Cyber Air Base, when this article was opriginally published. He is now president of Motion Controls LLC, Hartford, Wis., a manufacturer of pneumatic cylinders and the InSight positioning system using QVLA sensors.