Q&A

Q: What kind of accuracy can I expect from the pneumatic servo?

A: The final accuracy of any closed loop system is based on a variety of factors, including friction and inertia. However, for a properly sized system with modest levels of friction (5-15 lbf.), an accuracy of 0.020” can be expected. Repeatability, moreover, may be as high as 0.001”.

Q: Doesn’t this kind of accuracy in a pneumatic system come at the expense of cycle time?

A: Like any closed loop system, allocating additional time will reduce the error between the actuator’s state and its target. However, the pneumatic servo has a high enough bandwidth that the cycle rates are competitive with electric servo systems. Refer to the following question for a numerical example.

Q: I’ve tried other pneumatic positioning products before, and they worked fine so long as the system inertia was low. How well does the pneumatic servo handle heavy inertial loads – say 20 pounds on up?

A: Maximum acceleration will of course diminish with increasing load, but a properly sized pneumatic servo will be responsive and controllable even in systems with high inertia. For example, a 2” bore actuator with 80 psi factory supply can be expected to drive a 100 lbfm load over a 10 inch travel in 400 milliseconds, without overshoot, and within 0.020” of target.

Q: This seems to defy conventional wisdom. Doesn’t the compressibility of air make closed loop pneumatic systems sluggish and spongy?

A: All fluids are compressible to some extent, gasses much more so than liquids. But then, magnetic fields are compressible as well. In fact, the arrangement between magnets and coils in a linear motor yields a magnetic field that’s almost infinitely compressible. Yet linear motors are renowned for their speed and bandwidth. Air IS very compressible in comparison to hydraulic fluid. But only when this physical reality is ignored does the performance of a closed loop pneumatic actuator fail to achieve its potential.

Q: How well does the pneumatic servo perform when changes in load occur?

A: The pneumatic servo does not have a “gear box” through which the load can be isolated. Consequently, changes in load will alter the system dynamics and have a subsequent effect on performance. Nevertheless, a properly sized servo will perform well over a broad range of inertial loads. For example, a servo tuned for 50 lbfm will have minimal overshoot if the load drops to 25 lbfm, or increases to 75 lbfm. As a general rule, keep the lightest inertial load within 40% of the heaviest.

Q: Can acceleration and velocity be controlled?

A: Yes. Acceleration and velocity limits are factory set, and can be easily reprogrammed through the Sunstream teaching pendant, or by PC. For each point-to-point positional movement, the servo’s electronics will compute a positional trajectory based on a trapezoidal acceleration profile. Since the bandwidth of the pneumatic servo is 10 to 100 times higher than that of conventional closed-loop pneumatic actuators, it is able to smoothly track the positional trajectory and maintain acceleration and velocity limits.