Streaming Servo Control

A control mode where the host streams target joint angles or poses at a fixed rate and the controller just tracks them, without planning.

Robot arms typically accept two kinds of motion commands. MoveJ/MoveL only specify an endpoint, and the controller plans the entire trajectory itself. Streaming servo control — also called servo mode or pass-through mode — instead has the upper-level computer continuously send a dense stream of target points at a fixed cycle, and the controller does essentially no trajectory planning of its own, just tracking whatever it receives. Chinese vendors often distinguish ‘joint-angle pass-through’ (sending joint angles) from ‘pose pass-through’ (sending an end-effector pose, with the controller solving inverse kinematics). It suits scenarios where the target changes online — visual servoing, teleoperation, and real-time action output from a VLA or reinforcement-learning policy. Because the controller no longer smooths anything for you, trajectory continuity and the stability of the send cycle become the upper computer's responsibility: cycle jitter or a large jump between adjacent points can cause vibration or even trigger a safety stop, so policy output is usually interpolated into a high-frequency, smooth trajectory before being streamed. Universal Robots' servoj is the most commonly cited interface of this kind.

ExampleUR's servoj(q, a, v, t, lookahead_time, gain) must be called once every timestep: the e-Series uses t = 0.002 s (500 Hz), the CB3 uses 0.008 s; lookahead_time is set to 0.03–0.2 s for smoothing and to reduce overshoot; gain is set to 100–2000, with higher values responding faster but vibrating more easily. RealMan's rm_movej_canfd interface requires a streaming cycle no longer than 10 ms in its high-follow mode.

Also called

Pass-through Mode, Servo Mode, servoJ, Streaming Servo Mode

Related

MoveJ / MoveL · RTDE · Control Frequency · Trajectory Interpolation · Real-Time Control · Visual Servoing

Sources

Universal Robots 支持文章：servoj command (Chinese)
睿尔曼 RM_API2 Python 接口（rm_movej_canfd / rm_movep_canfd 角度、位姿透传说明） (Chinese)

As of

2026-09