Motor Control and Timing
Why Steppers Lose Position
Compare commanded motion, observed motion and feedback loops.
A real system shows a symptom related to “Why Steppers Lose Position”, and you need to find the cause before replacing parts or changing settings.
Learn why a stepper can miss motion while command counts continue, how to diagnose lost position, and what feedback can detect, correct or safely contain.
- Select or check first
- Holding torque is not running torque. Holding torque is measured at standstill under stated excitation. During a move, acceleration, speed, current dynamics, resonance and temperature change available torque. Size from the relevant torque-speed information and test the real motion profile with margin.
- Controlling assumption
- Sensors, controller logic, actuator limits, timing, fault response and environmental conditions are defined together.
- Top risk
- Position feedback is not automatically a safety function. A standard encoder and ordinary controller may fail together or report the motor while the load moves differently. Where unintended travel can injure someone, use risk-assessed guards, stops, brakes and safety-rated functions as required.
- 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 “Why Steppers Lose Position”
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.
On this page
An open-loop stepper system usually knows how many electrical step commands it issued, not how far the load moved. If required torque exceeds available torque, the rotor can fall behind or lose synchronism while the command counter advances normally. The error may persist until a reference is found again. Good design therefore combines operating margin with a deliberate answer to a simple question: what evidence proves the load is where the machine believes it is?
Command history is not position evidence
Diagram description
The graph is conceptual. Feedback does not automatically prevent a stall or reconstruct the correct load position after a mechanical slip.
Following error
e_p(t) = p_cmd(t) − p_meas(t)
Both positions must use the same coordinate, scale and sign. A motor-shaft encoder can miss backlash, a loose coupling or a slipping transmission downstream; a load-side sensor is closer to the controlled result but may add cost and different dynamics.
- e_p(t)
- position following error
- p_cmd(t)
- commanded position
- p_meas(t)
- measured position at the selected sensor
| Cause | Mechanism | Evidence to collect |
|---|---|---|
| Excess process load | Required torque crosses the motor-driver envelope | Position error and current near the load event |
| Acceleration too high | Inertia demands more torque than available | Failure versus ramp and payload |
| Resonance | Oscillation prevents the rotor from settling into sequence | Repeatable failure in a speed band |
| Current not reached | Supply, inductance, wiring or driver setting limits phase current | Bus voltage and phase-current waveform |
| Mechanical bind or impact | Friction spikes or travel ends abruptly | Manual inspection, force trend and stop timing |
| Thermal protection | Driver reduces or interrupts current | Temperature and diagnostic status |
| Electrical interference | False or missing STEP, direction or enable signals | Logic trace at the driver pins |
| Transmission slip | Motor moves but coupling, belt or screw does not transmit it | Compare motor-side and load-side position |
Trace energy forward and feedback back
Diagram description
This is a functional map, not converter-output breaker-placement guidance. Actual protection topology follows the drive, motor and governing requirements.
| Approach | What it can establish | Boundary |
|---|---|---|
| Open loop with margin | Reliable operation within a validated envelope | Cannot directly prove each move |
| Home or index switch | Known reference at a defined event | Position between references is still inferred |
| Motor encoder monitoring | Motor-shaft following error | Cannot see downstream slip or backlash |
| Load-side position sensor | Actual output coordinate | Needs installation, calibration and loop design |
| Driver sensorless stall indicator | A model-dependent load or stall clue | Validity depends on speed, tuning and device conditions |
| Closed-loop stepper | Uses measured error to adjust drive behavior | Still limited by motor, drive, sensor and mechanics |
| Redundant protective limit | Independent evidence near a hazardous boundary | Must be designed and validated for its protective role |
What should happen after excessive following error?
- Debounce or filter measurement only enough to reject known noise without hiding real error.
- Compare error with a limit that accounts for expected transient lag and sensor accuracy.
- If the limit persists, stop issuing normal motion and place outputs in a defined fault state.
- Prevent an automatic restart when the machine coordinate is no longer trustworthy.
- Remove or control the cause, inspect the mechanics and verify the sensor path.
- Recover position through an approved homing or absolute-reference procedure.
- Record command, feedback, current, voltage, temperature and fault timing for diagnosis.
Test loss and recovery before the machine depends on them
Verification should include maximum payload, warm windings, minimum supply, reversal, speed transitions and credible friction changes. Introduce controlled disturbances only where a risk assessment and test method permit it. Confirm not just that a fault flag appears, but that motion stops as intended, hazardous energy is controlled, the displayed diagnosis is useful and restart cannot occur with an unknown coordinate.
The control-loop distinction is developed in Open-Loop Commands Versus Real Feedback. For mechanical verification and stopping boundaries, continue with Lifetime and Safety Checks for a Motion Axis.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for Why Steppers Lose Position?
Holding torque is not running torque. Holding torque is measured at standstill under stated excitation. During a move, acceleration, speed, current dynamics, resonance and temperature change available torque. Size from the relevant torque-speed information and test the real motion profile with margin.
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?
Position feedback is not automatically a safety function. A standard encoder and ordinary controller may fail together or report the motor while the load moves differently. Where unintended travel can injure someone, use risk-assessed guards, stops, brakes and safety-rated functions as required.
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
- Stepper Motor BasicsOriental Motor · 2026-10-13Motor-manufacturer guide reviewed for open-loop command sequencing, lack of inherent position verification, encoder feedback and lost-synchronism context.
- AN-032: TMC4361A Closed-Loop Motor Control for Stepper Motor DriversAnalog Devices · 2026-10-13Semiconductor-manufacturer application note reviewed for encoder position, closed-loop stepper control and commanded-versus-measured position handling.
- S7-1500 Motion Control Function ManualSiemens · 2026-10-13Controller-manufacturer manual reviewed for positioning-axis feedback, following error, dynamics, homing and fault/restart concepts.