Feetech STS3215
The workhorse of the low-cost arm movement. 4096-step magnetic encoder, TTL half-duplex daisy chain, aluminium gears on the 30 kg·cm variant. This is the servo inside SO-100 / SO-101 and most hobby legged bots.
The role it plays
An actuator turns electrical power into motion. The useful unit is never the motor alone — it is motor + reduction + encoder + driver loop, and most of the engineering is in the last three. The number that decides a design is continuous torque at the joint, not the peak figure on the datasheet. Peaks last milliseconds; gravity lasts all day.
Typical use: 5-DOF printed arms, small quadrupeds, pan-tilt heads.
What the numbers actually mean
The headline figure is 30 kg·cm (2.94 N·m) @ 7.4 V (stall torque), with ~0.222 s / 60° as the next most load-bearing number (no-load speed). At $17 in the actuators tier it sits cheap enough to prototype with and replace without regret, which is the correct way to learn a new subsystem.
How it compares
Same subsystem, closest alternatives. The primary spec column is usually the one that decides it.
| Part | Price | Primary spec | Second spec |
|---|---|---|---|
| Feetech STS3215Serial-bus servo | $17 | 30 kg·cm (2.94 N·m) @ 7.4 V | ~0.222 s / 60° |
| Feetech SCS0009Micro serial servo | $8 | 9 kg·cm (0.88 N·m) @ 6 V | ~0.1 s / 60° |
| Dynamixel XL430-W250-TSmart servo | $50 | 1.5 N·m @ 12 V | 59 rpm @ 12 V |
| Dynamixel XM430-W350-TSmart servo | $260 | 4.1 N·m @ 12 V | 46 rpm @ 12 V |
| Dynamixel XM540-W270-TSmart servo | $500 | 10.6 N·m @ 12 V | 33 rpm @ 12 V |
Where it shows up
Platforms in this atlas that use something in this class: SO-101, Open Duck Mini v2.