Automation Fundamentals
Open-Loop Commands Versus Real Feedback
See the difference between a target and a measured machine position.
You must choose between real alternatives covered by “Open-Loop Commands Versus Real Feedback” for a defined machine, circuit or installation.
Distinguish a command from actual motion, form position error from measured feedback, and understand why closed-loop correction still needs stable tuning, limits and independent safety measures.
- Select or check first
- Feedback can be truthful yet incomplete. A motor-mounted encoder can report the motor shaft accurately while a loose coupling, gearbox backlash or flexible structure leaves the load elsewhere. When load position is what matters, either measure it or validate the mechanical relationship across direction, load and time.
- Controlling assumption
- Sensors, controller logic, actuator limits, timing, fault response and environmental conditions are defined together.
- Top risk
- Never use ordinary position feedback as the only protective measure. A failed sensor, controller, drive or coupling can defeat the normal loop. Risk reduction may require hard limits, guards, brakes, safety-rated monitoring or safe torque removal designed and validated for the hazard.
- Proof
- Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Start with the basics Optional foundation before “Open-Loop Commands Versus Real Feedback”
Automation-and-control basics
An automatic system measures a condition, decides what it means and commands an actuator while checking whether the expected result actually happened.
- Sensor
- A sensor converts a physical condition into a signal. Its range, uncertainty, response time, environment and failure behaviour matter as much as its nominal output.
- Controller
- A controller reads inputs, applies logic or a control law and issues outputs. Software cannot correct missing measurements, inadequate hardware authority or unsafe architecture.
- Actuator
- An actuator turns a command into physical action, such as motor torque, valve position or switching. Its real limits depend on load, supply, drive and duty.
- Feedback
- Feedback compares measured behaviour with the desired state so error can be corrected. Open-loop command assumes the action occurred; closed-loop control checks it.
- Fault and safe state
- A fault is an abnormal condition that may invalidate control. The safe response must be defined from the hazard and may require independent hardware, not only software.
How to use this article: Trace one complete chain—physical condition, sensor, signal conditioning, controller, drive, actuator and feedback—then ask how each link can fail.
A controller can issue a perfect command while the machine stops short, overshoots or never moves. Open-loop control assumes that a command produces the expected result. Closed-loop control measures an output, compares it with the target and changes the drive command according to the error. Feedback reduces uncertainty only for what is actually measured, and it does not automatically make the machine accurate, stable or safe.
A target is an intention; feedback is evidence
Diagram description
The diagram is conceptual and does not specify a controller law, sensor or actuator.
Position error
e(t) = r(t) − y_m(t)
The sign convention determines which correction direction is positive. The measured value may differ from true load position because of sensor offset, mounting error, backlash or compliance; closing a loop around the wrong measurement can hide the real output error.
- e(t)
- control error
- r(t)
- reference or desired position
- y_m(t)
- measured output position
| Question | Open loop | Closed loop |
|---|---|---|
| Does the controller measure output? | Not necessarily | Yes, through a defined sensor path |
| Can it notice missed motion? | Only indirectly or through separate logic | It sees persistent measured error |
| Can it reject disturbances? | Only by margin or prediction | Within bandwidth, actuator and tuning limits |
| Does it need calibration? | Commands still need calibration | Command and feedback coordinates both need calibration |
| Can it become unstable? | No feedback-loop instability, though the machine can still behave badly | Yes, delay, gain, resonance and sign errors can destabilize it |
| Does it guarantee safety? | No | No—safety functions require their own risk-based design |
A closed loop links measurement to corrective action
Diagram description
The pictured motor, drive and encoder are generic specimens and do not imply a specific product architecture.
| Block | Job | Limitation to check |
|---|---|---|
| Setpoint | Defines desired motion | Must be feasible and rate-limited |
| Comparator/control | Turns error into a command | Gain, filtering, sampling and windup |
| Power stage | Supplies actuator voltage/current | Saturation, current limit, delay and faults |
| Actuator/mechanics | Produces physical motion | Inertia, friction, backlash, resonance and travel limits |
| Sensor | Measures selected output | Range, resolution, noise, bias, latency and mounting |
| Feedback transport | Returns measurement | Update rate, data age, loss and sign/scaling |
Why more gain is not always better
Higher gain can reduce low-frequency error, but it also amplifies noise and can drive the actuator into saturation. Delay and flexible mechanical modes add phase lag, so aggressive correction can produce overshoot or sustained oscillation. Tuning therefore uses the complete plant, sampling, load range and stability margin—not error magnitude alone.
| Test | Evidence sought |
|---|---|
| Step or point-to-point move | Rise time, overshoot, settling and final error |
| Slow reversal | Backlash, stiction and direction-dependent bias |
| Load change | Disturbance rejection and current/torque margin |
| Sensor disconnect or implausible value | Defined fault detection and safe response |
| Travel-limit approach | Command limiting, stopping distance and independent protection |
| Power interruption and restart | Known position, homing or controlled recovery |
Use Where Automatic Positioning Is Useful to turn a motion idea into measurable requirements. A solar controller example is developed in Why Tracker Controls Should Stay Separate From PV Wiring.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for Open-Loop Commands Versus Real Feedback?
Feedback can be truthful yet incomplete. A motor-mounted encoder can report the motor shaft accurately while a loose coupling, gearbox backlash or flexible structure leaves the load elsewhere. When load position is what matters, either measure it or validate the mechanical relationship across direction, load and time.
Which assumption can change the conclusion?
Sensors, controller logic, actuator limits, timing, fault response and environmental conditions are defined together.
What can go wrong if I copy the example directly?
Never use ordinary position feedback as the only protective measure. A failed sensor, controller, drive or coupling can defeat the normal loop. Risk reduction may require hard limits, guards, brakes, safety-rated monitoring or safe torque removal designed and validated for the hazard.
How do I verify the result in the real system?
Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Sources and further reading
- Servo (Encoder Feedback) Positioning Assembly — Product DataAllen-Bradley / Rockwell Automation · 2026-10-13Manufacturer documentation reviewed for closed-loop positioning, encoder feedback, motion commands, update behavior and drive-disable context.
- S7-1500 Motion Control Function ManualSiemens · 2026-10-13Reviewed for positioning-axis setpoints, feedback, dynamics and homing concepts.