Most hobby robotic arms use rotary servos at every joint, but there are real situations where linear motion is a better fit: a gripper that needs long jaw travel, a Z-axis on a SCARA or delta arm, or any joint where the geometry wants a push-pull actuator rather than a rotating link. Here’s how the options compare and what works at the DIY scale.
When linear beats rotary
A rotary servo is optimal when a joint sweeps an arc — shoulder, elbow, wrist. A linear actuator suits three situations:
- Long straight travel. A jaw gripper that opens 80 mm travels farther than any standard servo can swing within a compact package.
- High force in a small footprint. A lead-screw actuator can produce tens of kilograms of push force from a small NEMA 17, far more than a hobby servo of the same size.
- Non-back-drivable locking. A lead screw won’t back-drive under load — the joint stays where you left it with no holding power from the motor. Useful for a gripper that must clamp without burning power.
The four main types
1 — Electric (lead screw or ball screw)
A DC or stepper motor turns a threaded rod; a nut converts rotation to linear translation. This is the practical choice for almost every DIY application:
- Lead screw + NEMA 17 — direct 3D-printer DNA; cheap, precise, slow
- Hobby linear servo — a standard servo with an internal lead screw; tiny, limited force, drop-in replacement for a servo in a servo-based arm
- Electric linear actuator module — a 12 V unit with an integrated DC motor, gearbox and lead screw; rated in force (kg) and stroke (mm); driven with a simple H-bridge
Control an H-bridge-driven actuator the same way you control a DC motor: PWM for speed, direction pins for extend/retract. Pair it with a limit switch at each end of travel to avoid overrun.
2 — Pneumatic
Compressed air pushes a piston. Industrial delta and SCARA arms often use pneumatic grippers because they’re fast (millisecond response), light (the compressor stays off-arm), and reliably strong. For DIY: the compressor, solenoid valves and fittings add cost and complexity. Worth it for a purpose-built pick-and-place machine; overkill for a learning project. The hydraulic arm guide covers the manual syringe version of the same principle.
3 — Hydraulic
High-force, high-pressure fluid drives a piston — the mechanism used in excavators, heavy industrial arms and the classroom syringe build. Not practical at the hobby bench (hydraulic fluid, high-pressure lines, pumps) except the low-pressure syringe build, which is excellent for teaching Pascal’s principle but has no electronics.
4 — Shape-memory alloy / soft actuators
Nichrome and SMA (Nitinol) wires contract when heated — they’ve been used in research grippers and soft-robotics hands. Interesting for small, lightweight tasks but slow, energy-hungry, and hard to control precisely. Mostly a research/experimental area for hobbyists right now.
DIY electric actuator options
NEMA 17 + lead screw (best precision)
Exactly the Z-axis design from every Cartesian 3D printer. A NEMA 17 stepper, a 300 mm T8 lead screw and nut, a coupler and a linear rail give you a precise, strong linear axis. Control with an A4988/TMC2209 driver and an Arduino Mega. Travel resolution: with a 2 mm-pitch lead screw and 200 steps/rev, each step moves 0.01 mm.
Travel per step = lead_screw_pitch / (steps_per_rev × microstepping)
= 2 mm / (200 × 16) = 0.000625 mm per microstepHobby linear servo (simplest)
A standard-sized servo body with an internal mechanism that pushes a rod out 0–30 mm. Controls like any servo (PWM 1000–2000 µs), drops into any servo mount, and needs no extra driver. Force is limited (under 5 kg typically) but for a lightweight gripper or a tool-change actuator it’s the simplest solution.
12 V electric actuator module (most force)
Widely available in 50–400 mm stroke lengths, 5–50 kg force ratings, powered from a 12 V supply and driven by an H-bridge (L298N or IBT-2 for higher current). Add a potentiometer feedback version for closed-loop position control — the pot wiper voltage maps linearly to rod extension, readable on an Arduino analog pin.
Wiring a linear actuator to Arduino
A 12 V DC linear actuator (no feedback) needs an H-bridge:
const int IN1 = 7, IN2 = 8, EN = 9;
void setup() {
pinMode(IN1, OUTPUT); pinMode(IN2, OUTPUT);
pinMode(EN, OUTPUT);
digitalWrite(EN, HIGH);
}
void extend() { digitalWrite(IN1, HIGH); digitalWrite(IN2, LOW); }
void retract() { digitalWrite(IN1, LOW); digitalWrite(IN2, HIGH); }
void stop() { digitalWrite(IN1, LOW); digitalWrite(IN2, LOW); }For a feedback actuator, read the pot:
int pos = analogRead(A0); // 0–1023 maps to fully retracted–extended
int targetPos = 700; // target extension
if (pos < targetPos - 5) extend();
else if (pos > targetPos + 5) retract();
else stop();Combining linear and rotary
Most capable arms mix both: rotary servos handle the arm joints (shoulder, elbow, wrist) while a linear actuator or lead screw drives the gripper jaws or the Z axis. The wiring diagram guide covers mixing servo PWM signals with H-bridge control on the same controller, and the powering guide explains running 12 V actuators from the same bench supply as 5–6 V servos with a buck converter.
Frequently asked questions
What is a linear actuator in a robotic arm?
A linear actuator converts energy into straight-line motion — it pushes or pulls a rod rather than rotating a shaft. In a robotic arm, linear actuators are used when a joint or axis needs to extend and retract rather than rotate, or when a servo's limited arc range (0–270°) doesn't suit the geometry. The Z axis of a SCARA arm and the gripper travel of many industrial arms use linear actuators.
What is the difference between a servo and a linear actuator?
A servo motor produces rotary motion (shaft turns) and is geared to hold a specific angle. A linear actuator produces straight-line motion — the output shaft extends and retracts. Many linear actuators are actually electric motors with an internal lead screw that converts rotation to linear travel. Servos are lighter and cheaper for rotary joints; linear actuators suit extending/retracting axes and grippers that need long travel.
What types of linear actuator are used in robots?
The main types: electric (motor + lead screw or ball screw — most common in DIY), pneumatic (compressed air drives a piston — fast, used in industry), hydraulic (fluid pressure — very strong, used in heavy industrial arms and the hydraulic STEM arm build). For hobby robots, electric lead-screw actuators are the practical choice: 12 V, controllable with an H-bridge driver, widely available.
Can I use a stepper motor as a linear actuator?
Yes — a NEMA 17 stepper motor paired with a lead screw and nut is one of the most common DIY linear actuators, directly borrowed from 3D printer Z-axis design. The lead screw pitch sets the travel per step, giving very precise positioning. The downside is speed: lead screws are slower than belt drives and pneumatics.