Motion
Jerk
Jerk is the rate of change of acceleration over time, and limiting it is what makes a robot start and stop smoothly instead of snapping into motion.
What it is
Jerk is the third derivative of position: velocity is how fast the tool moves, acceleration is how fast that speed changes, and jerk is how fast the acceleration itself changes. When you feel a car lurch as the driver stabs the brake, that lurch is jerk, not the deceleration itself.
In robot motion planning, jerk is the quantity that decides whether a move ramps in gently or slams from zero torque to full torque. A jerk-limited profile caps how abruptly acceleration can build, which is why the same point-to-point move can feel either glassy or violent depending on how the controller shapes it.
How it actually works
Think of a simple move as a stack of profiles. A trapezoidal velocity profile switches acceleration on and off in steps: the instant the move starts, acceleration jumps from zero to its full value. That step is theoretically infinite jerk, and the arm answers it by shaking. An S-curve profile instead ramps acceleration up and back down along smooth shoulders, so jerk stays finite. The velocity trace picks up its S shape from exactly that softened acceleration.
The payload is what feels it. Every arm has structural compliance and resonant modes, and a sharp acceleration step is broadband energy that excites those modes. The tool wobbles, a mounted camera blurs, liquid in an end-of-arm gripper sloshes. Dialing jerk down trades a little cycle time for a settled tool: the move takes marginally longer, but the arm stops ringing and the payload arrives still. On a loaded arm the effect is bigger, because more mass on the faceplate lowers the resonant frequency the jerk step is exciting.
How it differs
- Acceleration · Acceleration is how quickly speed changes; jerk is how quickly that acceleration changes. Two moves can hit the same peak acceleration yet feel completely different, because the one with lower jerk eases into that acceleration instead of stepping to it. Capping acceleration limits force; capping jerk limits the shock of applying it.
- Cycle time · Aggressive jerk shortens a move on paper by getting to peak acceleration sooner, but the vibration it leaves behind adds settling time at the end. Lower jerk can make the net motion, including settle, actually faster and far more repeatable.
Where you meet it in the field
- FANUC M-20iD/25 · A mid-payload arm where jerk limiting earns its keep: with a heavy tool on the faceplate, smoothing the starts and stops keeps the load from ringing at the ends of moves.
- Arc welding · On continuous welds, path smoothness and a steady torch matter more than raw speed. Lower jerk keeps the tool from twitching at direction changes, which shows up directly in bead quality.
Common questions
- My part placement drifts under high speed but is fine when slow. Is that jerk?
- Often, yes. If the endpoint scatter grows only as you push speed and acceleration up, the arm is likely still ringing when the position latches. Softening jerk lets the tool settle before the controller calls the move done, which tightens placement without dropping peak speed.
- I already lowered acceleration and it still shakes. What else?
- Lowering acceleration caps peak force but does nothing about how abruptly that force arrives. If the profile still steps acceleration on, jerk stays high and keeps exciting resonance. Look for an S-curve or jerk-limit setting rather than dropping acceleration further.
- Does adding a heavier tool change how much jerk I can run?
- Yes. More mass on the faceplate lowers the arm's resonant frequency, so the same jerk step excites bigger, slower ringing. A profile that behaved fine bare-flanged can shake with a full end-of-arm tool, so retune jerk after a payload change.