Glossary term
Jerk
The time rate of change of acceleration, used to describe abruptness in motion profiles, vibration, and ride comfort.
Definition
quantityJerk is the time derivative of acceleration, indicating how abruptly acceleration changes.
Jerk appears in motion control, vehicle dynamics, robotics, elevators, rail systems, machine tools, vibration analysis, and human comfort assessment. Even when velocity and acceleration remain within limits, high jerk can excite structural vibration, create impact-like loads, reduce ride comfort, increase tracking error, and stress mechanical components. Motion profiles often limit jerk to make acceleration changes smoother.
Jerk is the rate at which acceleration changes:
For rotational motion, angular jerk is the rate of change of angular acceleration. Jerk does not describe how fast something is moving, or even how strongly it is accelerating. It describes how abruptly that acceleration is being changed.
Why it matters
A system can satisfy velocity and acceleration limits while still feeling harsh or exciting vibration if jerk is high. In elevators, rail vehicles, and passenger cars, jerk affects ride comfort because the human body responds strongly to sudden acceleration changes. In robots, pick-and-place machines, CNC axes, and camera stages, high jerk can excite flexible modes, produce overshoot, reduce accuracy, and increase wear.
Mechanical loads also change. A step change in acceleration implies a theoretically infinite jerk, which no real actuator or structure can follow without bandwidth limits, compliance, vibration, or impact-like force transmission. Smooth motion profiles reduce these effects by ramping acceleration gradually.
Motion profiles
Trapezoidal velocity profiles have constant acceleration segments and abrupt transitions between acceleration levels. These transitions create high jerk. S-curve profiles limit jerk by smoothing the acceleration transitions. This can reduce vibration and improve tracking, although it may increase move time or require careful tuning.
In servo systems, jerk limits interact with motor torque, reflected inertia, controller bandwidth, encoder resolution, structural resonance, and load flexibility. A low jerk limit can make motion smoother, but if set too low it can reduce throughput or make the machine sluggish.
Common mistakes
A common mistake is ignoring jerk until commissioning, when vibration or comfort problems appear. Another is specifying a jerk limit without defining whether it applies to commanded motion, measured motion, translational axes, rotational axes, or combined path motion. Good motion specifications state velocity, acceleration, jerk, payload, axis orientation, control bandwidth, and allowable tracking error.