If the cylinder response is not sufficiently linear, the modeling option just described will not work, and one must use one of the first two methods of active damping. If none of the active damping choices are an option, then one must rely on passive damping or make the system stiffer. With a fluid power system this involves using a larger cylinder, valve, and pump. The extra cost of these components may make one reconsider active damping. Tradeoffs then exist between cylinder sizing, valve sizing, and the use of an active-damping-capable motion controller. The less expensive long term route is usually to buy the linear valve and use a controller capable of active damping, rather than making the cylinder diameter bigger, because the bigger diameter cylinder requires more hydraulic fluid and a bigger valve.

An active damping example
Because of the compressability of air, damping is important for closedloop pneumatic control. Figure 3 contains a motion plot showing active damping of a pneumatic system using pressure transducers to measure the differential force on the piston. The vertical axis shows the magnitude of various system parameters and the horizontal axis represents time.

In this example, the goal is to move the cylinder from 1 to 11 in. at 10 in./ sec. The actual position (red line) is shown as a function of time, increasing from the 1-in. position in the lower left toward the target position (cyan line) at the top of the screen. The control output (green line) initially makes a small increase but then hesitates because the differential force (gray line) is increasing very fast.

A motion controller without the differential force feedback would just keep increasing the control output until it was too high. Initially, the differential force is increasing due to air being added that has nowhere to go because the cylinder isn’t moving. When the differential force is greater than the frictional forces the cylinder starts to move and the differential force drops. This in turn allows control output to be increased. The speed (blue line) builds to a relatively constant 10 in./sec.

Finally, the actual position gets close to the target position, so the control output starts to decrease. As the control output decreases, the differential force decreases, because the speed of the cylinder is increasing the volume of the cylinder on the push end faster than air is being added. Eventually the differential force drops below the amount needed to overcome friction and the cylinder slows down and stops. The key point is that this controller used the differential force feedback to keep from adding too much energy (pressurized air) and thus eliminated any overshoot or oscillation.

Active damping is a much more energy efficient and more flexible option than friction damping or special cases of passive damping such as braking. In systems that are prone to oscillations, smooth and accurate motion requires the ability to control using position and its derivatives. While some controllers only have the ability to detect errors in the first derivative, others can detect errors in second and third derivatives, too. This extra capability provides for better control when accelerating or decelerating rapidly which is often necessary to shorten production cycle times. And this capability is available today for both hydraulic and pneumatic applications, using a high-performance electronic motion controller.

For more information, email the author at peter@deltamotion.com.