Actuators Are the Muscles of a Mechatronic System
An actuator converts an input energy source, electrical, pneumatic, or hydraulic, into mechanical motion or force. Choosing and sizing the right actuator is one of the first concrete design decisions in any mechatronic system, since it constrains the mechanical design (how much space and structural support the actuator needs), the electrical/control design (what drive electronics and feedback are required), and the overall system's power budget and weight. This guide covers the three major actuator families, the core sizing methodology that applies across all of them, and a practical decision framework for choosing between them.
Electric Actuators
Electric actuators convert electrical energy directly into mechanical motion, most commonly through one of three motor types. DC motors (brushed or brushless) produce continuous rotary motion with speed roughly proportional to applied voltage and torque roughly proportional to current — simple to control with a basic PWM driver, and the default choice for continuous-rotation applications like wheels, fans, and pumps. Stepper motors move in discrete, fixed-angle steps in response to electrical pulses, giving inherently good open-loop position control (the controller knows exactly how far the motor has moved just by counting pulses, without needing a position sensor) at the cost of lower top speed and a real risk of "losing steps" (silently missing commanded steps) if the load torque briefly exceeds the motor's capability. Servo motors pair a motor (DC or brushless) with a built-in or closed-loop position/speed feedback sensor and a dedicated controller, giving precise, actively-corrected position or speed control that automatically compensates for load variation, at higher cost and complexity than an open-loop stepper.
Electric actuators dominate modern robotics, CNC machines, and consumer mechatronic devices because they integrate cleanly with digital control electronics, run cleanly and quietly, and don't require a separate compressed-air or hydraulic power infrastructure. Their main limitation is a comparatively lower force/torque-to-size ratio than hydraulics for very high-force applications, though modern high-torque-density motors and gear reduction largely close this gap for most mechatronic (as opposed to heavy industrial) applications.
Pneumatic Actuators
Pneumatic actuators use compressed air, typically supplied at 80-100 psi (5.5-7 bar) from a shop compressor or dedicated compressed-air system, to drive a piston in a cylinder, producing linear (or, with a rotary pneumatic actuator, rotational) motion. Pneumatic cylinders are simple, robust, inexpensive, and inherently compliant — because air is compressible, a pneumatic actuator that encounters an unexpected obstruction yields somewhat rather than stalling with the full, potentially damaging force a stiff electric or hydraulic actuator would apply, a genuine safety advantage in applications where the actuator might contact a person or an unexpected object.
The tradeoff is control precision: standard pneumatic cylinders are naturally suited to simple two-position (fully extended / fully retracted) motion controlled by a solenoid valve, and achieving fine position control partway through the stroke requires more sophisticated (and expensive) proportional valves and position feedback, at which point much of pneumatics' cost advantage over electric actuation disappears. Pneumatics remain the standard choice for fast, simple, repetitive motions in industrial automation, pick-and-place mechanisms, and clamping/gripping, wherever a compressed-air supply is already available or easily justified.
Hydraulic Actuators
Hydraulic actuators use pressurized hydraulic fluid, typically at 1,500-3,000+ psi (roughly 20-40× typical pneumatic pressure), to drive a piston, delivering far more force per unit of actuator size than electric or pneumatic actuation can achieve. This makes hydraulics the standard choice wherever very high force or torque is needed in a compact package: construction and earthmoving equipment, industrial presses, aircraft flight control surfaces, and heavy-duty industrial robots.
The cost of that force density is system complexity: a hydraulic actuator needs a hydraulic power unit (a pump, typically motor-driven, a fluid reservoir, filtration, and control valves) supplying pressurized fluid, along with hoses or rigid lines routing that fluid to the actuator — infrastructure that adds significant weight, cost, and maintenance burden (fluid leaks and contamination are the most common hydraulic system failure modes) compared to an electric motor that just needs a power and signal wire. Hydraulics are rarely the right choice for small mechatronic devices; they earn their keep specifically where the force requirement genuinely exceeds what electric actuation can practically deliver.
Core Sizing Methodology
Regardless of actuator type, a sound sizing process follows the same core steps.
1. Determine the required force or torque at the point of application, including static load (weight, friction, any process force like a clamping or cutting force) and dynamic load (the additional force needed to accelerate the moving mass at the required rate — remember that starting/accelerating a load typically demands meaningfully more force than sustaining it at constant velocity).
2. Account for any mechanical transmission between the actuator and the load — a gearbox, belt drive, or lead screw changes the torque/force and speed seen at the actuator versus at the load by its transmission ratio (see the companion article on gear trains and mechanical power transmission on this site for the full relationship). Reflect the load back through this transmission to determine what the actuator itself must actually produce.
3. Add a safety margin, commonly 25-50% above the calculated steady-state requirement, to cover starting/breakaway torque (often several times the running torque, especially for loads with static friction to overcome), unmodeled friction and efficiency losses in the transmission, and supply voltage or pressure sag under load.
4. Check the actuator's continuous (not just peak) rating against the actual required duty cycle. A motor or cylinder that can briefly deliver the required force may not be able to sustain it — electric motors in particular are thermally limited, and a motor sized only against its peak torque rating for an application that actually runs continuously will overheat in service even though it performed fine on the bench.
5. Verify required speed and the resulting power (power = force × velocity, or torque × angular velocity) fall within the actuator's rated operating range at the actual supply voltage or pressure, not just its no-load or maximum rating.
Decision Framework
| Requirement | Best-Fit Actuator Type |
|---|---|
| Precise position/speed control, digital integration | Electric (servo for closed-loop precision, stepper for simple open-loop indexing) |
| Fast, simple two-position motion, compliant/safe on contact | Pneumatic |
| Very high force/torque in compact package | Hydraulic |
| Weight-constrained mobile/wearable application | Electric (best practical power-to-weight once full system is weighed) |
| No existing compressed-air or hydraulic infrastructure, small-scale project | Electric (lowest infrastructure overhead) |
| Continuous, quiet, precise operation near people | Electric |
Worked Example: Sizing a DC Gearmotor for a Linear Lift
Consider a small lift mechanism raising a 5 kg load via a pulley and cable, requiring a lift speed of 0.1 m/s, driven through a gearbox and a 40 mm diameter drum.
Required force: F = mg = 5 × 9.81 ≈ 49 N (ignoring friction for this estimate; a real design would add 10-20% for cable/pulley friction, say 55 N).
Required torque at the drum: T = F × r = 55 N × 0.02 m = 1.1 N·m.
Required drum speed: ω = v / r = 0.1 / 0.02 = 5 rad/s (≈ 47.7 RPM).
Required power: P = T × ω = 1.1 × 5 = 5.5 W.
Applying a safety margin: multiplying by 1.4 (40% margin for starting torque and losses) gives a target continuous torque of ≈ 1.54 N·m at ≈ 48 RPM — a spec directly comparable against a gearmotor's continuous torque-speed curve at its rated voltage, which is exactly the number to check against a candidate motor+gearbox datasheet before committing to a part.