What an actuator does, in one sentence
An actuator is the device that turns a signal into motion. Every example you can name, a Boeing 787 flap motor, a piezoelectric fuel injector, the wrist joint of a humanoid robot, the hydraulic ram on an excavator, the thermal expansion element in a thermostat, fits this definition.
The interesting work is in the conversion. The signal arrives in one form, almost always electrical, and leaves in another, almost always mechanical. The actuator is the bridge.
The three required ingredients
Every actuator needs three things, and removing any of them stops the device from being one.
- A power source. Electrical, hydraulic, pneumatic, chemical, or thermal energy that the device can draw on.
- A converter. The element that translates that power into useful mechanical work.
- A control input. A signal, often small, that commands the converter to produce a specific position, velocity, force, or torque.
Strip out the power source and the device becomes a passive mechanism. Strip out the converter and there is no motion. Strip out the control input and you have an uncontrolled system, a brake without a driver.
The physics, in four equations
The math an engineer reaches for when sizing an electric actuator is small. Four equations cover most of the work.
torque = Kt × current
force = (2π × η / L) × torque
F_net = mass × acceleration
V_back_emf = Ke × angular_velocity
Kt is the torque constant of a brushless DC motor. Ke is the back-EMF constant. η is mechanical efficiency. L is the lead of a ball screw, the linear distance per revolution. The chain reads left to right, from amps in the motor to newtons at the load.
Efficiency in real-world actuators ranges from about 30 percent for cheap pneumatic systems to 95 percent for direct-drive electric machines. The losses live in friction, heat, and parasitic mechanical compliance. Every reduction stage adds another multiplication of less than one.
The eleven families
Practically every actuator in production today belongs to one of eleven families. The first five do the bulk of the world’s work. The other six fill specialty niches that the first five cannot reach.
- Electric rotary. Robot joints, EV traction motors, factory servos, cordless tools.
- Electric linear. Window blinds, hospital beds, satellite solar array deployers.
- Stepper. 3D printers, CNC machines, camera focus rings, where open-loop position is enough.
- Hydraulic. Excavators, aircraft control surfaces, hydraulic presses, where energy density wins.
- Pneumatic. Factory pick-and-place, brake systems, soft robotics, where speed and cleanliness matter more than precision.
- Piezoelectric. Microscope stages, fuel injectors, ultrasonic transducers, sub-micron precision at modest stroke.
- Shape memory alloy. Aerospace deployables, medical stents, micro-grippers.
- Electrostatic. MEMS mirrors, micro-pumps, switches in DLP projectors.
- Magnetostrictive. Sonar, precision tooling, vibration cancellation.
- Dielectric elastomer. Artificial muscles, soft robotics, haptic surfaces.
- Pyrotechnic. Airbag deployment, satellite separation bolts, ejector seats. One-shot devices.
Anatomy of a representative actuator
A typical industrial linear actuator built for robotics looks like this. A brushless DC motor, often a pancake topology for compactness, drives a ball screw through a planetary or harmonic gearbox. The screw rotates inside a non-rotating push tube. As the screw turns, the nut translates, pushing or pulling the tube.
A representative spec sheet:
- Stroke: 200 mm
- Force: 2.5 kN
- Max speed: 50 mm per second
- Lead: 5 mm per revolution
- Repeatability: ±0.05 mm
- Cycle life: 10 million cycles
- Power: 350 W peak
- Mass: 1.8 kg
Total latency, command to motion, falls in the 1 to 5 millisecond range for closed-loop digital servo controllers. That number is what makes actuators viable for haptics, surgical robotics, and balance control on legged robots.
How to choose
Selection is five questions, in order. Most misallocations come from skipping question one.
- How much force or torque does the load require, peak and continuous.
- How fast must it move, and how often.
- How precisely must it stop.
- What is the environment.
- What is the cost target.
Reverse the flow and you back-fit the load to a familiar device, which is how aerospace ends up with hydraulic systems still flying in 2026, and how warehouse robotics ends up with overspec’d harmonic drives that fail under shock loads.
What this reference is for
This guide is the public version of a longer technical brief. For the full chart pack, the worked sizing example, and the supplier landscape, download the artifact pack. Cite NLT143 Research, April 2026.