Two engineers can agree that a motor trace looks good and still choose different controllers. One values quick arrival; the other worries about voltage peaks. A written specification makes that disagreement visible before tuning starts.
Describe the task and its conditions
Name the controlled output and its unit, the target or target range, the actuator and its bound, controller sample period, initial condition, load test and observation window. A settling-time value without its tolerance band and a peak voltage without saying requested or applied are ambiguous.
Begin with a baseline run before changing the controller. If one candidate sees a different load or starting speed, a better-looking plot may be due to the changed task rather than the controller.
A small requirement sheet
Fill the criterion column before seeing the candidate runs. The numbers below are example placeholders, not guaranteed attainable goals for the current Lab preset. Set them from your use case and available actuator authority.
| Question | Measure and condition | Example criterion |
|---|---|---|
| How fast? | Settling time, stated ±5% band, fixed target step | ≤3 s, illustrative |
| How much overshoot? | Peak relative to nonzero target | ≤10%, illustrative |
| Does it reject load? | Maximum speed drop and recovery after one stated pulse | Choose before testing |
| What does it cost? | Applied voltage peak and saturation duration | Respect ±12 V; limit saturation |
| What if the model changes? | Repeat at stated inertia/friction scales | Choose tested range |
Use the Lab as a comparison, not a beauty contest
Try the Lead, Lag or PI guided experiment. Pin the baseline and change only controller type while holding target, load, seed and duration fixed. Read output and applied voltage together. Mark each requirement as met, unmet or not measurable in the available window; never invent a settling time for a run that has not settled.
If no controller satisfies every condition, show the tradeoff instead of silently relaxing a requirement. You may need a different actuator, a revised target or a design beyond the controllers currently offered.
Write a one-paragraph decision
A useful conclusion sounds like this: 'Under target A and load B, controller C met the ±5% settling requirement but briefly saturated. Controller D recovered the load more slowly but used less voltage. We prefer D because voltage headroom matters more in this task.' State what remains untested.
Decide what success means before looking at curves. A reproducible comparison connects a controller choice to measured requirements and known limits.
CHECK YOUR UNDERSTANDING: Why specify actuator effort and disturbance conditions before tuning a fast response?
A faster trace may rely on unavailable voltage or force, or only work without the required disturbance. Feasibility and test conditions belong in the specification.