The actuation of cams, detents, and levers has long been the forte of pneumatic or hydraulic systems in machines. These systems are generally open loop. The only verification that the system has executed a command is the absence of an alarm signal as generated by some sensor, usually a mechanical or Hall-effect switch located near the mechanical stop. The speed at which events happen under open-loop control is dictated by physical factors such as mechanical friction and the amount offeree the actuator generates. An adjustable mechanical stop usually sets the final position of an air cylinder. Open-loop pneumatics are increasingly giving way to closed-loop servosystems. Closed-loop servos continuously monitor feedback from speed or position sensors as a means of controlling not just the distance actuators move, but also their speed and acceleration while they travel to their final destination.
展开▼
