Actuation & Mechanics
Actuators: How a Robot Moves
The three actuator families (electric, hydraulic and pneumatic) and the handful of properties that decide which one a robot should use.
beginner · 13 min read
Lesson content
Sensors let a robot notice the world. Actuators let it do something about it.
An actuator is the part that turns stored energy into motion. Every wheel that rolls, every joint that bends and every gripper that closes has one behind it. Without actuators, a robot is just a very expensive sensor on a stand.
Nearly every actuator you will meet belongs to one of three families. This lesson is about telling them apart and choosing between them.
Definition
Actuator
A device that turns energy (electricity, pressurised oil or compressed air) into movement whenever a control signal tells it to.
Formal definition
A transducer that converts a low-power command into a controlled flow of power from a supply, delivering force or torque to a mechanical load.
The motor or cylinder is only one link in a longer chain. Power has to be stored, switched and delivered before it becomes useful motion. Every stage along the way loses a little of it.
The actuating chain
Power supply
A battery, a mains supply, a hydraulic pump or an air compressor. This is where the raw energy comes from.
Power amplifier
The controller only sends a tiny signal. The amplifier uses that signal to switch a much bigger flow of power: an H-bridge chopping current for a DC motor, or a servo valve metering oil for a hydraulic cylinder.
Motor or cylinder
Turns that power into torque or force. This is the actuator itself.
Transmission
Gears, belts or screws that trade speed for force and carry the motion to the joint. Some robots skip this step and drive the joint directly.
| Electric | Hydraulic | Pneumatic | |
|---|---|---|---|
| Runs on | Electric current | Oil at 100–350 bar | Air at 6–10 bar |
| Power for its weight | Moderate | Highest by far | Lowest |
| Stiffness | Moderate (gearboxes flex a little) | Very high, because oil will not compress | Very low, because air compresses |
| Control | Excellent | Good, with servo valves | Poor between the ends of its travel, fine for simple on and off |
| Usually needs a gearbox? | Yes | No | No |
| Support equipment | A driver board | Pump, tank, filters, hoses and a cooler | Compressor, filter and regulator |
| Cleanliness | Clean | Can leak oil | Clean but noisy |
| Efficiency | High | Moderate | Low |
| When the power fails | Drops unless it has a brake | Holds if the valves stay shut | Sags, because trapped air is springy and slowly leaks |
| Typical jobs | Almost every robot | Excavators, presses and very large robots | Grippers, clamps and simple on/off moves |
If hydraulics are the strongest for their size, why is nearly every robot electric?
Because strength for size is only one property, and electric actuators win almost all the others.
Electric motors are easy to control precisely. They are efficient, quiet and clean. All they need is a driver board, and they talk directly to a computer.
Now count what a hydraulic system needs: a pump, a tank, filters, hoses and a cooler. All of that weighs something. Once you add it up, the advantage of that small, strong cylinder shrinks a lot.
Hydraulics can also leak, which keeps them out of clean rooms and food factories. Cold oil also turns thick and sluggish.
So where do hydraulics still win? Anywhere you need enormous force from a small package: excavators, presses and large robots that have to survive hard landings.
Raw force is only part of the story. A few other properties decide what a robot actually feels like to work with.
- Stiffness and compliance
How much the actuator gives when the load pushes back.
A stiff actuator holds its position accurately, but it also hits hard if something is in the way. A compliant (springy) one is less accurate and naturally gentler. Hydraulics are stiff, pneumatics are springy and electric actuators sit in between.
- Backdrivability
Whether you can push the output by hand and turn the motor backwards through its gears.
A backdrivable joint is safe to bump into and easy to guide by hand. High gear ratios make joints hard to backdrive. Robots that work near people either sense your push and move out of the way, or use low-ratio gearing that gives way on its own.
- Duty cycle
How long the actuator can work at full effort before it needs a rest.
A motor's continuous rating is often only a third of its peak rating. Choose a motor for its peak and then run it that hard all day, and it will overheat.
- Holding without power
What the actuator does when the supply is cut.
An electric joint falls unless it has a brake. A hydraulic joint with its valves shut stays put. A pneumatic one slowly sags. This decides what a failure looks like and whether it is dangerous.
Example
One joint, three ways
Imagine an arm joint that has to lift 20 kg.
Electric: a brushless motor driving a 100:1 harmonic gearbox. Precise and clean, but it needs a brake to hold the arm when the power is off.
Hydraulic: a compact rotary actuator with a servo valve. Much smaller for the same torque, and it holds still with the valve shut. Somewhere, though, there has to be a pump.
Pneumatic: a poor choice. The trapped air would act like a spring, so the arm would bounce under the load and could never be positioned accurately.
Now change the job to opening and closing a gripper. Suddenly pneumatics win. There are only two positions, accuracy does not matter and air is cheap and fast. The springiness even helps, because it will not crush the part.
Two smaller families are worth a mention, even though they rarely drive a whole joint.
Shape-memory alloy wires shorten when they are heated, usually by passing a current through them. They power tiny mechanisms such as steerable medical catheters.
Piezoelectric actuators change size by a tiny amount when a voltage is applied, very quickly and very precisely. They drive microscope stages and ultrasonic motors.
Both make the same bargain. They give up range of motion in return for smallness, speed or precision.
Sizing an actuator, step by step
1 · Find the peak torque or force
Work out the load at its worst pose, then add what it takes to accelerate it. Use the peak, not the average, because the peak is what stalls a motor.
2 · Find the speed you need
It comes from how quickly the task must be done. Torque multiplied by speed is power, and power is what you are really buying.
3 · Choose the gear ratio
The gearbox turns a motor's natural fast, weak spin into the joint's slow, strong turn. It also makes the load feel n² times lighter to the motor.
4 · Check the continuous rating
Compare it with how hard the actuator will work over a whole shift, at the temperature it will really run at.
5 · Decide what happens when the power is lost
A brake, a counterweight or a controlled drop. This is a safety decision, so make it early.
Sizing the actuator only for holding the load still.
Getting the load moving usually takes two to four times the holding torque. A motor that holds the arm fine can still stall when asked to lift it quickly.
Using pneumatics to stop in the middle of the stroke.
Air compresses, so the rod's position changes with the load. Pneumatic cylinders are excellent at moving between two hard stops and poor at everything in between.
Forgetting what a power cut does.
An electric arm without a brake falls under gravity. Choose a brake, a counterweight or an accepted drop on purpose, and check that the choice matches your safety assessment.
Key takeaway
- An actuator turns stored energy into motion, at the end of a chain of supply, amplifier, motor and transmission.
- Electric, hydraulic and pneumatic actuators differ most in strength, stiffness and how well they can be controlled.
- Electric actuators dominate because they are precise, clean and efficient, even though hydraulics are stronger for their size.
- Stiffness, backdrivability, duty cycle and behaviour without power decide how a robot really behaves.
- Size for the peak load, check the continuous rating and decide early what a power failure should look like.
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