With servohydraulic motion control systems, the huge amount and complexity of data that must be generated, transmitted, converted, received, and ultimately executed is astounding. You wonder why they work at all, until you realize how much you’ve spent on all the high-precision components and how complicated it was to get them to work in concert. This is especially important with six-axis motion bases, because when you extend one cylinder, it affects the positions of the other five, some to a greater or lesser degree than others. With a servohydraulic system, you can’t just command a cylinder to extend without commanding the other five to extend or retract accordingly. This means motion commands must be synergistic. Otherwise, motion of one cylinder that is not consistent with that of the other five will either overload that cylinder or tend to distort the motion base platform shared by all six cylinders.

Because of their stiffness, electric servohydraulic systems are able to produce sharper motion than can be readily achieved with pneumatics. For example, if your virtual spaceship is struck by an asteroid, an electric servohydraulic system can produce a convincingly sharp impact or make the whole vehicle shudder. However, where is it written that a sensation of an impact must be generated by the motion base itself? Maybe the Cyber Air Base can’t reproduce a believable impact with its pneumatic system, but by positioning an auxiliary cylinder on the underside of the motion platform, and having the rod strike it right under the seat, I can produce the sensation of a sharp impact. This solution produces the same result an as elaborate electic servohydraulic system, but without the complexity and steep price tag.

A reality that all motion-control designers must deal with is that there are no ideal motion systems, so we must deal with compliance, deadband, backlash, inertia, and other challenges. Trying to control an ideal motion base would be complicated enough. But trying to make a motion base seem ideal from a control standpoint by factoring in algorithms for compliance, backlash, deadband, and other variables adds tremendous complexity and cost to the system. Moreover, the degree of success varies because the parameters vary — especially over the life of the application.

Instead of factoring in algorithms for compliance, so we can tackle the motion-control problem, we can rely on the compliance of pneumatics to provide the necessary forgiveness that compensates for error that may exist between the control program and the physical limitations of the motion base. To put it another way, instead of treating compliance as an obstacle to be overcome through complicated algorithms that only add complexity to the control program, we can use compliance to compensate for errors introduced by real-world conditions.

For example, one of the most elusive goals of motion control for mechanical and hydraulic systems is achieving optimum acceleration and deceleration. Getting from point A to point B in a certain amount of time isn’t as difficult as executing smooth or steep acceleration and deceleration ramps at the beginning and end of a motion profile. Pneumatics, by virtue of its compliance, has the potential for acceleration and deceleration built in. So compliance can be an asset, not a liability. Granted, the Cyber Air Base may not be able to match the ultra-sophisticated acceleration-deceleration ramps or a wide variety of motion-control profiles of a servohydraulic system, but it certainly produces believable motion — and without servovalves or even proportional valves.

Compliance and hardware

In the Cyber Air Base, compressed air flows to the cap end of each cylinder through four directional control valves piped in parallel, Figure 2. This is a six-axis motion base, so there are six cylinders and 24 valves. These are not proportional valves; basically, 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 the software. For sake of discussion, say valves A, B, C, and Dcan pass 1, 2, 3, and 6 scfm, respectively. Therefore, we can achieve any flow rate in  increments of one anywhere between 0 and 12 scfm. Opening valves A, C, and Dproduces flow of 10 scfm; opening Band Dproduces 8 scfm. Compliance is important because it creates transitions that smooth out motion as valves shift.

Another unusual aspect of the Cyber Air Base is that it uses double-acting cylinders, but compressed air is pumped only into the cap end of the cylinders. The rod ends of all cylinders are piped together in series, Figure 3. This greatly simplifies motion control because as one cylinder extends, air from it flows into a cylinder that is retracting. This trading off of fluid (air in this case) is inherent to motion bases, and the combined volume of air in the rod ends of all cylinders remains relatively constant, because whenever the motion platform pitches, rolls, or yaws, air exiting one cylinder is always needed by another. It also eliminates the problem of having to account for differential piston area between the opposing sides of each cylinder.

This rod-end air is held at a constant pressure in the Cyber Air Base. If this shared volume was simply open to atmosphere, gravity would be the only means of retracting cylinders. Pressurizing this volume not only assists gravity in retracting the cylinders, but adds some stiffness to the system. The magnitude of pressure is determined by mass of the load: the lighter the load (person), the higher the preload pressure — typically 35 to 80 psig. The only time this pressure requires intervention is when the motion platform lurches vertically upward or downward. This occurs when the software commands all cylinders to extend or retract, respectively. When the platform lurches upward, rod-end pressure exceeds the initial setting. This causes a pressure transducer in the circuit to open a relief valve to vent air to atmosphere. When the motion platform lurches downward, pressure drops below the setpoint, so the transducer opens a supply valve to route regulated pressure into the circuit.