Actuators are the muscles of machines, from cuckoo clocks to robot arms
Every machine that moves needs something to turn a weak command into real force. That something is an actuator: a camshaft opening engine valves, oil pushing a piston, a bimetal strip bending inside an old thermostat. Engineers now build soft versions from polymers that respond to heat or light much as muscles do.
An actuator takes a low-energy control signal, such as a voltage, a current, fluid pressure or even a hand on a lever, plus a power source, and produces force, torque or movement. Output is usually straight-line or rotary. Rotation suits small machines making big movements, and a lead screw or ball screw can convert it into linear travel. Some actuators are purely mechanical, driven by other moving parts: the camshaft in a car engine, or the mechanism striking the hour in a grandfather clock.
Hydraulic actuators use nearly incompressible liquid, usually oil, to shove a piston along a cylinder, so they can deliver huge forces and hold position without drawing much energy, though their acceleration is limited. In a single-acting design fluid pushes one way and a spring or gravity returns the piston; a double-acting one pumps fluid to both sides. Pneumatic versions use air instead and skip the return plumbing, but still need compressors, tanks and filters. Steam locomotives ran on the same principle, turning piston strokes into wheel rotation through a crank.
Electric designs have multiplied since 1960. Electromechanical actuators turn an ordinary motor's spin into a push through belts or screws, reaching forces around 100 kN with little upkeep, and appear in medical devices and factory automation. A linear motor is effectively a rotary motor sliced open and unrolled; with low friction it can outlast a hundred million cycles, and for loads up to about 30 kilograms it offers top speed and precision, which is why watchmakers and chip and drug manufacturers favour it. On valves, a brake is fitted so fluid pressure cannot force the valve open and set the motor oscillating until it breaks.
Other actuators exploit physics directly. Thermal types rely on expansion, like the two-metal strip in a mechanical thermostat, or on shape-memory alloys. Magnetic ones include reed switches used as door sensors in alarm systems. Soft actuators made of flexible polymers suit robots working near people, and 3D printing now lets them be made in one step.
Source: Actuator
