Improving the dynamic behavior of pneumatic actuators

“Air spring” shortens travel time and reduces compressed air consumption

In a development project sponsored by the German Federation of Industrial Research Associations, the Steinbeis Research Institute Processing Machines and Systems has been working to significantly improve the dynamic behavior of working cylinders and reduce travel time. This can be achieved by using the energy of the discharged air via a reserve, to dampen the cylinder in its final position.

Conventionally, pneumatic actuators are operated according to the principle of exhaust throttling, and using open-loop control of the final position. With this method, after a short acceleration phase, a constant speed is established at which the actuator reaches its final position. The residual kinetic energy is dampened using standard methods such as hydraulic dampers, pneumatic final position dampers and elastomer cushions. This final damping often results in the formation of a large dynamic load on the system, which can destroy it. The problem can be counteracted by reducing the actuator speed, although this leads to a huge increase in travel times.

The disadvantages of insufficient dynamics are particularly evident at large stroke lengths and high effective loads. In such instances, a sufficient cycle time cannot be reached, despite high dynamic loads. If the final position brake and the external throttle check valve are ideally set, the cylinder reaches its final position after approximately 1.72 seconds. The speed of travel reaches a maximum value of approximately 0.25 m/s and remains roughly constant over twothirds of the stroke.

In the “air spring” system, developed by the Chemnitz-based Steinbeis Research Institute together with Dr.-Ing. Eberhard Zipplies from the Chemnitz University of Technology for a medium-sized firm in Saxony, the discharged air space of the working cylinder is connected to a pre-stressed pressure reserve. The air mass expelled by the piston flows into the reserve and increases the reserve pressure in proportion to the displacement. The resultant increase in the pressure of the discharged air acts in opposition to the actuator motion and noticeably brakes the actuator before it has reached its final position. Compared to the exhaust throttling method, the actuator dynamics can therefore be significantly increased for both di- rections of travel. However, the “air spring” system can also be operated so that the pneumatic energy of the compressed air in the reserve is used to perform an exhaust-throttled return stroke. This also leads to a significant reduction in compressed air consumption.

Compared to conventional actuators, the cylinder reaches its final position much sooner in the new system, and its maximum speed of travel increases to approximately 1 m/s. This transmission method significantly improves actuator dynamics and reduces cycle times. This development means pneumatic actuators can be used in more situations, as well as increasing the allowable effective dynamic load. As a side effect, compressed air consumption can be lowered by up to 50 percent for unilateral transmission.

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