Servos

A servo is a motor, a gearbox, a position sensor and a little controller, all packed into one small box. You send it a pulse, and it drives itself to the matching angle and holds there. That makes servos perfect for claws, wrists, flippers and latches — and easy to break if you ask too much of them.

How a servo is controlled

A servo has one 3-wire cable: ground, power and signal. The hub sends a pulse on the signal wire many times a second, and the width of that pulse sets the position. The Control Hub can send pulses from 500 to 2500 microseconds, with 1500 µs as the centre.

For example, the REV Smart Robot Servo (SRS) maps 500–2500 µs onto its full 270° range. In code you set a position from 0.0 to 1.0, and the SDK turns that into a pulse width.

Standard or continuous rotation

Standard (positional) servo

The pulse sets a position. Set 0.5 and it goes to the middle and holds there. Use these for claws, wrists, latches and anything with a defined end point.

Continuous-rotation servo

The pulse sets a speed and direction, and there is no position control. Centre means stop; either side spins one way or the other. Use these for small rollers, intakes and winches that just need to turn.

Many FTC servos are dual mode: the same servo can be either, depending on how it is programmed. The REV Smart Robot Servo and goBILDA's 2000 Series Dual Mode Servo both work this way.

Servo programmers

You change a dual-mode servo's settings with a small handheld servo programmer, not with your robot code.

• The REV SRS Programmer runs on 4 AA batteries. It can switch an SRS to continuous rotation, set custom left and right angle limits, and reset it to factory defaults. It also works as a tester for any standard servo.
• goBILDA sells a Servo Programmer for its 2000 Series Dual Mode Servo that switches between standard and continuous mode, and also works as a servo tester.

The hub's servo power limits

The Control Hub and Expansion Hub supply 5V to their servo ports, and that supply has limits. From REV's Control Hub specifications:

Limit	Value
Servo port pairs	0–1, 2–3 and 4–5 each share a supply
Current per pair	2A total
All servo ports plus the two +5V ports	5A total

The current that matters is stall current — what a servo draws when it pushes but cannot move, like a claw squeezing a game piece or an arm held against gravity. For example, REV lists the SRS V2 (Balanced) at 2.1A stall at 6V.

Worked example — why port choice matters. Put two of those servos on ports 0 and 1 and have both clamp hard on a game piece at the same time. Together they can ask for around 4A — about double the 2A that the 0–1 pair can supply. Move them to ports 0 and 2 and each one has a pair to itself.

That is why FIRST's Robot Best Practices page recommends using only the even ports or only the odd ports on each hub — at most three servos per hub — so no two servos share a pair.

When you need 6V servo power

High-torque servos often need more current than the hub's servo ports can supply, and some are designed for more than 5V. The Competition Manual (rule R502 in 2026–27) notes that the REV hubs give servos 5V, while these approved devices give 6V:

Device	Input	Output	Notes
goBILDA 6V Servo Power Injector	8–15V via XT30	6V, 24A total	Signal passes straight through from the hub; ships with servo extensions that have the red wire removed, so only signal and ground come from the hub
REV Servo Power Module	12V via screw terminals	6V, 15A total, about 3A per port	Six channels, over-current shutdown
REV Servo Hub	7–15V via its power terminals	6V, up to 15A total, about 3A per channel	Controls six servos over a single RS485 or USB link
Studica Servo Power Block	Robot battery, via its JST VH power connector	6V	See Studica's documentation for current limits

Game Manual 0 reports that the REV Servo Power Module has a history of failing under load, sometimes damaging the hub it is plugged into, and recommends the newer REV Servo Hub or the goBILDA Servo Power Injector instead.

The rules about how these are powered matter for inspection:

• They must be powered only from the robot's main battery, through the connection the manufacturer intends (for example, the SPM only through its screw terminals).
• Their 6V output may only power servos — not sensors, LEDs or anything else.
• Power from one port only powers the device on that port. Do not cross-wire or combine port power.

These are rules R608 and R613 in the 2026–27 manual. Check the current Competition Manual — servo limits and the approved device list change between seasons. The manual also limits how powerful a legal servo can be and points to a list of pre-approved servos in its Inspection Quick Reference.

Keeping servos alive

Most servo failures are mechanical. Game Manual 0's top tips:

• Do not back-drive a powered servo by forcing the arm by hand — it strips the internal gears.
• Support side loads. Servo output shafts break when something hits the arm sideways. A servo block or bearing-supported shaft takes that load off the servo.
• Watch connector orientation. Most servo plugs are not keyed and will fit backwards. Match the colours: black or brown is ground.
• Clip or tape extension joints. Servo extensions pull apart easily; use locking connectors or retaining clips.

For the code side, REV's Programming Servo Basics walks through setPosition() in Java.

Check yourself

You have three high-torque servos to plug into one Control Hub. Which ports spread the load best?

• A0, 1 and 2
• B0, 2 and 4
• C3, 4 and 5
• DIt does not matter — all six ports share one supply equally

Check yourself

You command a claw servo to position 0.3 and it spins non-stop instead of moving there and stopping. What is the most likely cause?

• AThe servo is set to continuous-rotation mode
• BThe servo needs 6V instead of 5V
• CThe servo is plugged in backwards
• DThe hub's servo pair is over its current limit

Check yourself

Your goBILDA Servo Power Injector has spare 6V output. A teammate wants to use it to power an LED strip and a sensor. Is that OK?

• AYes — it is the same robot battery underneath
• BNo — 6V from a servo power device may only power servos
• COnly the sensor, because sensors draw very little current
• DOnly if you add a fuse

Next

That completes the actuators. The next unit is about how the robot senses the world: switches, distance and colour sensors, the IMU, odometry and cameras.

Sources and further reading

• REV Control Hub specifications — servo port voltage, pairs and current limits
• FIRST Robot Best Practices — servo voltage, current and the odd-or-even port recommendation
• REV Smart Robot Servo, SRS V2 specifications and SRS Programmer
• goBILDA Servo Programmer for the 2000 Series Dual Mode Servo
• goBILDA 6V Servo Power Injector
• REV Servo Power Module and REV Servo Hub
• Game Manual 0: Servo Guide, Choosing a Servo and Servo Usage Tips
• FTC Competition Manual — section 12.5 (Motors & Actuators) and 12.6 (Power Distribution)