The Robot That Moved Fast but Took Forever
A pick-and-place robot was programmed to move at 100% speed. The cycle time was 6 seconds per pick. The customer wanted it faster. We looked at the program. The robot was moving at full speed, but it was accelerating and decelerating at 10% of the rated value — the integrator had set the acceleration low “to be safe.” The robot reached 100% speed only for a fraction of the move; most of the cycle was spent ramping up and ramping down. We raised the acceleration to 80% (the payload was light, the robot was rigid). The cycle dropped from 6 s to 3.5 s. No hardware change — just a parameter. The robot was not slow; it was timid.
Robot cycle time is dominated by acceleration and deceleration, not by the top speed. Here is how to tune it without shaking the robot apart.
Why Acceleration Matters More Than Speed
Most robot moves are short (0.5–1 m). On a short move, the robot never reaches its top speed. It spends the whole move accelerating and decelerating. The cycle time is set by the acceleration rate, not the maximum velocity. A robot that can move at 2 m/s but accelerates at 1 m/s² takes 2 seconds just to reach speed — and on a 0.5 m move, it never gets there. Raising the acceleration to 5 m/s² cuts the cycle dramatically.
The integrator who set 10% acceleration was protecting the robot from vibration. But on a light payload, the robot can handle much more. The 10% setting was over-conservative — it was wasting 40% of the cycle.
The acceleration rule: For short moves, cycle time is set by acceleration, not top speed. Raise the acceleration until the robot vibrates or the payload shifts, then back off 10%. The timid robot was at 10% acceleration; the right value was 80%.
The Payload Limit: Why You Cannot Crank It to 100%
Higher acceleration means higher forces (F = m × a). A heavy payload at high acceleration shakes the robot, loosens bolts, and can cause the payload to slip in the gripper. The acceleration limit is set by the payload mass and the robot’s wrist load rating. The robot’s manual gives a “load diagram” — the maximum acceleration for a given payload. Stay inside it. If the payload is light (a few kg), you can run near 100% acceleration. If it is near the rated payload, run at 30–50%.
Singularities and Axis Limits
Cranking the acceleration can expose a different problem: the robot hits a singularity (a configuration where one axis has to spin infinitely to keep up). At low acceleration, the singularity is hidden (the robot moves slowly through it). At high acceleration, the axis overshoots and the robot faults. If raising acceleration causes a fault at a specific point in the path, you may be near a singularity. Repath the move to avoid it (give the robot a wider arc). Do not just lower the acceleration — fix the path.
Loose Parts: The Hidden Risk
When you raise acceleration, the payload experiences g-forces. A part sitting in a gripper (not clamped) can shift. A cable held by a light clamp can pull loose. A bottle with liquid can slosh. Test the high-acceleration move for 100 cycles — watch for the part shifting, the liquid sloshing, the cable tugging. The 3.5 s cycle was safe only after we confirmed the gripper held the part at 80% acceleration.
A Tuning Procedure
- Start at the current acceleration (note the cycle time).
- Raise acceleration by 10%.
- Run 10 cycles; watch for vibration, payload shift, or faults.
- If clean, raise another 10%.
- Stop when: vibration appears, the payload shifts, or a singularity fault occurs.
- Back off 10% from the limit.
- Record the new acceleration and the cycle time.
- Re-test after any payload change.
A Typical Tuning Result
| Acceleration | Cycle Time (short move) |
|---|---|
| 10% (timid) | 6.0 s |
| 40% | 4.8 s |
| 60% | 4.0 s |
| 80% (safe limit) | 3.5 s |
| 100% (shaking) | 3.3 s (rejected) |
The jump from 10% to 80% cut the cycle by 42%. The last 20% (80→100) bought only 0.2 s but introduced vibration. The sweet spot is just below where vibration starts.
The Bottom Line
The robot that moved fast but took forever was a timid acceleration setting. Short moves are governed by acceleration, not top speed. Raise it until vibration appears, back off 10%, and verify the payload holds. The cycle that dropped from 6 s to 3.5 s was not a faster robot — it was a robot that was finally allowed to move.
