Servos

The six joints use Feetech STS bus servos with the standard STS register map. The full register dump is in the register map.

Identity

Joints	Model number (registers 3–4)	Firmware	Supply (measured)
J1–J3	0x0809 (2057)	3.9	~7.6 V
J4	0x0709 (1801)	3.9	~6.5 V
J5–J6	0x0209 (521)	3.9	~6.4 V

The common STS3215 reports 0x0309 and the STS3250 0x0B09. These three model numbers are not in any public table found so far. They can be custom versions made for Elephant Robotics.

Replacement servos

The servos are Feetech STS-series bus servos: they use Feetech’s protocol and register map, and register 3 is 9 as on other STS models. There are three types. Their settings are also different:

Type	Joints	Model	Max voltage (14)	Min voltage (15)	Phase (18)	Protection current (28–29)
A	J1, J2, J3	0x0809	13.0 V (J1, J2), 24.0 V (J3)	6.0 V	4	300
B	J4	0x0709	24.0 V	4.0 V	100	1000
C	J5, J6	0x0209	24.0 V	4.0 V	100	1000

The Feetech product name of each type is not known yet. To find it:

1. Read the label on the servo. This needs the joint covers off.
2. Connect Feetech’s FD software (Windows) through the FT232R at 1 Mbaud. It reads the model number and shows the name.
3. Ask Elephant Robotics for a replacement servo for that joint and give the model number. Elephant sells parts for the arm (not checked for these servos).

A replacement must report the same model number. Before you use it, set its ID, the PID gains (21–23) and the other settings in the register map for that joint, and calibrate its zero (stock command SET_SERVO_CALIBRATION, 0x54, or the offset in 31–32).

Units and encodings

• Multi-byte values are little-endian.
• Position: 12-bit, 4096 steps per turn (0.088° per step).
• Speed: steps/s, sign-magnitude with bit 15 as the sign. The present speed is quantised to 50 steps/s (4.4°/s).
• Load: 0.1 % units, bit 10 as the sign. It behaves like PWM duty, not torque: on J1 it is ~0.53 % per °/s plus ~3 % static friction.
• Voltage: 0.1 V. Temperature: °C.
• Calibration offset (registers 31–32): sign-magnitude, bit 11 as the sign.

Modes

Register 33 sets the mode.

Mode	Command register	Status
0 — position (default)	Goal position, 42–43	Used for all the results.
1 — velocity	Goal speed, 46–47	Tested on J1: 102–103 steps/s for 100 commanded, with ~0.25 s of dead time at that speed. No joint limits.
2 — PWM (open loop)	Goal time / PWM, 44–45	Not tested. The closest mode to torque control.
3 — step	relative position	Not used.

Multi-turn (J6)

The STS servos can read and move past one turn (tested on J6 on 2026-10-06, through the ATOM’s REG_READ and REG_WRITE, with J6 turned by hand and then moved):

Setting	Present position (56–57)	Goal position (42–43)
As shipped: phase 100, angle limits 0/4095	One turn: wraps at 0/4095 (±180°)	Clamped to 0–4095: J6 stopped at −179.3° for a −190° goal
Phase bit 4 set (100 → 116), angle limits 0/4095	Past one turn: read −231.8° (4685)	Clamped as above
Phase bit 4 set, angle limits 0/0	Past one turn	Past one turn: J6 moved to −209.3° and back across ±180° the correct way

• Values past one turn are sign-magnitude: bit 15 is the sign.
• The EEPROM lock (55) stays 1, so these settings last until the servo’s next power-off. Our firmware (4.5+) writes them at each power-up for the joints marked multi_turn in servos.yaml (J6).
• After a power-up the servo counts from its one-turn reading. The firmware finds the turn from the J6 limits: they span one turn (−225° to +135°), so a reading above +135° is one turn lower. It keeps a turn_offset for the joint. Within 1.5° of ±135° the turn is not known: J6 goes limp and the ATOM stays in the error state until a HOLD finds the turn (the status log says TURN UNKNOWN).
• The positions in STATE, STREAM and plans go past 0–4095 on J6: −225° is step 4608.
• This also explains an older problem: after many turns by hand, J6 turned away from its goal when motion was enabled (see Known problems). The servo counts turns inside, even when it reports one turn only.

How goals and torque behave

These rules come from measurements. They are also in Known problems and rules.

1. Goal position uses the same units as present position. To hold a joint, write goal = present.
2. Goal speed 0 means “do not move” in position mode. A nonzero goal speed is a speed cap.
3. Goal position and goal speed read 0 after power-up. Set every goal to the present position before you set a nonzero speed.
4. Writing a goal position turns torque on. Writing the mode (register 33) turns it off.
5. In velocity mode, the present position is raw (without the calibration offset). It looks like a jump of the offset size when the mode changes. The joint does not move.
6. Register 55 (EEPROM lock) is 1. Changes to the EEPROM area (for example the mode) are probably lost at power-off. This is not verified.

Response in position mode

The servos follow a stream of goals with a repeatable lag and smoothing. Fitted on the sine recordings (see Servo response):

Joint	Delay	Time constant	Pure-delay fit	PID (21/22/23)
J1	40 ms	80 ms	120 ms	32 / 8 / 0
J2	88 ms	25 ms	114 ms	32 / 8 / 0
J3	44 ms	75 ms	118 ms	10 / 0 / 1
J4	4 ms	50 ms	54 ms	10 / 0 / 1
J5	0 ms	40 ms	40 ms	10 / 0 / 1
J6	4 ms	25 ms	28 ms	10 / 0 / 1

The joints also stick for 0.2–0.5 s after a reversal (static friction). The dead zone (registers 26/27) is 3 steps on every joint.

Register names

Elephant’s documentation calls register 22 “I” and register 23 “D”. This is wrong: 22 is D and 23 is I, as in Feetech’s table. See Stock protocol commands.

The PID column was read on 2026-10-03 with the stock ATOM firmware. See the next section for the gains in use now.

Position-loop gains

Registers 21, 22, 23 are P, D, I of the servo’s position loop (Elephant’s documentation names 22 and 23 the other way round; see Stock protocol commands).

Joint	Ours (MyCobot.GAINS, written at power-up by the controller firmware v3+)	Stored in the servos	Stock ATOM firmware writes
J1	32 / 4 / 16	32 / 8 / 0	32 / 8 / 0
J2	32 / 4 / 16	32 / 8 / 0	32 / 8 / 0
J3	32 / 4 / 16	32 / 8 / 0	10 / 0 / 1
J4	32 / 8 / 0	32 / 8 / 0	10 / 0 / 1
J5	32 / 8 / 0	32 / 8 / 0	10 / 0 / 1
J6	32 / 8 / 0	32 / 8 / 0	10 / 0 / 1

• Ours adds integral action on J1–J3. On the circle it halves the flange error (5.3 → 2.6 mm) with almost the same end-effector vibration. See Servo dynamics.
• Stored: what the servos use if nothing writes the gains (read on 2026-10-04 with the controller firmware v2, which did not write them).
• Stock: read on 2026-10-03 with Elephant’s ATOM firmware, which writes these at power-up.
• With the EEPROM lock (register 55) at 1, written gains last until the next power cycle. The controller firmware writes ours again at each power-up (gains_ok in PING).
• Change them for a test with scripts/set_gains.jl ours|stored|stock --atom=IP.