Linear Motor "Runaway" – Causes, Risks, and Prevention

What Is Linear Motor Runaway?

Linear motor runaway occurs when the drive outputs electromagnetic thrust in the wrong direction, magnitude, or with incorrect closed-loop feedback, causing the forcer to accelerate uncontrollably beyond its intended travel range.

Unlike rotary servos, which typically drive loads through mechanical transmissions (couplings, ball screws, belts, or gearboxes), linear motors apply thrust directly to the stage. There is no mechanical buffering. If direction, feedback, phasing, or gain is wrong, the stage can hit the end stops almost instantly.

Why Are Linear Motors More Prone to Runaway Than Rotary Servos?

Rotary servo systems follow a typical chain:

Rotary motor → Coupling → Screw/belt/gearbox → Load

The mechanical transmission introduces friction, compliance, and backlash, which inherently dampen speed and limit impact forces.

Linear motors, by contrast, are direct-drive:

Electromagnetic thrust → Load directly

Key characteristics that increase runaway risk:

• No mechanical reduction ratio

• No self-locking mechanism (e.g., screw)

• Low friction

• Fast response

• Direct force application

• Feedback typically via linear encoder (optical or magnetic)

Consequently, any error in control direction, feedback polarity, phase sequence, encoder orientation, or commutation angle does not result in gradual misalignment—it can cause immediate, violent runaway.

This is why leading manufacturers emphasize during first power‑up:
Limit current, limit speed, restrict travel, use low gains, and enable both software and hardware end‑stops.

Primary Causes of Linear Motor Runaway

Runaway can be traced to eight major categories.

1. Incorrect Motor Phase Sequence (U/V/W)

Linear motors typically use three-phase windings. The drive applies U/V/W currents to generate a moving magnetic field that propels the forcer.

If the phase sequence is wrong, the drive may command "forward" thrust, but the motor physically produces "reverse" thrust. In closed‑loop operation:

• The controller commands positive movement.

• The motor moves negative.

• The position error grows.

• The drive increases output to correct the error.

• The error worsens, leading to rapid runaway.

This type of runaway is abrupt and severe.

Diagnosis

During low‑speed jogging, if the actual motion opposes the command, or if the axis jerks immediately upon enable, suspect:

• U/V/W phase miswiring

• Encoder polarity reversal

• Incorrect commutation angle

• Misdefined positive/negative direction

Prevention

During initial commissioning:

• Do not issue large‑stroke motion commands.

• Use the drive's built‑in test run, phase‑detection, or commutation‑search functions.

• Restrict jog increments to a few millimeters.

• Set maximum speed, thrust, and current limits conservatively.

• Verify emergency stop and hardware limit switches are functional.

2. Encoder / Linear Scale Feedback Polarity Reversal

Linear motors commonly use optical linear encoders or magnetic linear encoders (or absolute encoders) for position feedback.

If feedback polarity is reversed, the closed loop becomes positive feedback:

• Command: positive movement

• Motor moves positive

• Feedback position decreases

• Controller interprets larger error

• Drive increases thrust

• Speed escalates out of control

This is a fundamental logic error, not a tuning issue, and is extremely dangerous.

Typical Symptoms

• Axis shoots out immediately upon enable

• Sudden acceleration with minimal command

• Rapidly increasing following error

• Drive alarms: following error, overspeed, encoder fault, or position deviation

• Mechanical end‑stop impact before any alarm triggers

Prevention

Before closed‑loop operation:

1. Manually push the axis in the positive mechanical direction.

2. Monitor the drive's feedback position—it should increase.

3. Verify that a positive jog command produces positive mechanical motion.

4. Ensure command direction, feedback polarity, and thrust direction are mutually consistent.

Golden rule:
Confirm feedback direction first, then thrust direction, and only then run closed‑loop control.

3. Incorrect Commutation Angle / Electrical Angle Alignment

Rotary servos usually come with factory‑matched encoder and rotor alignment. Linear motors, however, are often assembled from third‑party components:

• Motor and magnet track from different suppliers

• Third‑party linear scale

• Third‑party drive

• Separate forcer and stator installation

• No traditional rotary encoder

The drive must know the forcer's position relative to the magnetic field—this is referred to as:

• Commutation alignment

• Initial phase detection

• Electrical angle calibration

• Hall‑based or sensorless commutation search

If the commutation angle is wrong, the drive's current vector will not align with the actual field, causing:

• Abnormal thrust direction

• Low thrust with high current

• Motor vibration or no motion

• Sudden jumping

• Uncontrolled movement and runaway

High‑Risk Scenarios

• First‑time installation

• Scale direction changed

• Motor phase wiring altered

• Drive parameters reset to factory defaults

• Drive replacement

• Scale replacement

• Forcer/magnet track reassembly

• Absolute position lost

• Hall signals miswired

Prevention

Follow the drive manufacturer's prescribed procedure:

• Enter correct motor model parameters

• Set pole pitch, encoder resolution, and feedback units accurately

• Verify Hall signals or encoder feedback are valid

• Execute commutation search / pole‑finding routine

• Apply current and speed limits during search

• Save the alignment result

• Validate direction with low‑speed, short‑stroke moves

• Gradually increase speed and thrust limits

4. Excessive Position / Velocity Loop Gains

Linear motors have high stiffness, low friction, and fast response. If gains are set too high, the system can oscillate.

Unlike rotary servos with mechanical damping from screws or belts, linear motors have no such cushion. Over‑aggressive tuning can produce:

• High‑frequency whine

• Low‑frequency hunting

• End‑point jitter

• Amplified position error

• Velocity oscillation

• Loss of control and impact

Though gain‑induced oscillation is not always classic runaway, in linear motors it can rapidly escalate to mechanical collision.

Typical Indicators

• Humming or whining when enabled

• Axis dithering in place

• Sudden oscillation during slight movement

• Repeated overshoot at target

• Large fluctuations in velocity profile

• Severe current ripple

Prevention

Adopt a conservative tuning approach:

• Start with low stiffness

• Low velocity loop gain

• Low position loop gain

• Reduced acceleration/deceleration

• Disable aggressive feedforward

• Enable notch filters / low‑pass filters

• Increase gains gradually

• Monitor velocity, position error, and current waveforms via oscilloscope

Do not apply high‑gain rotary servo parameters directly to linear motors.

5. Incorrect Electronic Gear Ratio / Unit Conversion

Linear motor control involves frequent unit conversions:

• Pulses per millimeter

• Encoder counts per millimeter

• Micrometers per count

• EtherCAT position units

• Electronic gear ratio

• Scale resolution

• Pole pitch

• Lead equivalent

• Scaling factors

A conversion error can cause a commanded 10 mm movement to be interpreted as 100 mm, 1,000 mm, or more—producing runaway‑like behavior.

Common Mistakes

• Scale resolution set to 0.1 μm instead of 1 μm

• Electronic gear ratio numerator/denominator reversed

• Upper‑controller units (mm) mismatched with drive units (pulses)

• Misinterpreted velocity or acceleration units

• Absolute vs. relative position confusion

• Sign error on target position

• EtherCAT PDO mapping unit misunderstanding

Prevention

Before large movements:

• Manually push the axis 10 mm and observe the feedback count change.

• Verify feedback counts per millimeter.

• Command 1 mm and measure actual travel.

• Command 10 mm and measure again.

• Increase travel distance progressively.

Golden rule:
Never trust software units on the first run—verify with a physical ruler.

6. Failed End‑Stops, Soft Limits, or Safety Logic

The real danger of linear motor runaway is not just loss of control, but the absence of reliable limits to stop it.

Common deficiencies:

• Hardware limits not connected

• Positive/negative limit wiring reversed

• Incorrect NC/NO limit‑switch parameter settings

• Limits wired to PLC only, not to drive

• PLC limit logic not active in program

• EtherCAT communication delays preventing timely limit response

• Soft limits not configured

• Soft limits inactive before homing

• Emergency stop only cuts PLC, not servo enable

• STO (Safe Torque Off) not wired

• Drive alarm output not integrated into safety circuit

Important:
PLC‑based limit logic alone is insufficient—if bus communication, PLC scan cycle, or program logic fails, the PLC may not act in time.

Recommended Three‑Level Protection

Level 1 – Drive‑Internal Protection

• Maximum speed limit

• Maximum thrust / current limit

• Position deviation limit

• Software end‑stops

• Overspeed detection

• Encoder loss detection

• STO (Safe Torque Off)

Level 2 – Hardware Limit Protection

• Positive end‑stop

• Negative end‑stop

• Home switch

• Emergency stop button

• Safety relay

• Mechanical bumpers / shock absorbers

• Hard mechanical stops

Level 3 – PLC / Host Protection

• Motion‑zone validation

• Automatic motion only after homing

• Target position legitimacy check

• Speed / acceleration limits in motion commands

• Axis status monitoring

• Following error monitoring

• Motion timeout alarms

• Immediate stop upon limit trigger

7. Incorrect Load Mass, Thrust Limits, or Parameter Settings

Linear motor performance depends heavily on load mass.

If the actual load is light but the thrust limit is high and gains are aggressive, any control anomaly can cause rapid acceleration and impact.

If the load is heavy but inertia/mass estimation is wrong, problems include:

• Overshoot

• Oscillation

• Slow settling

• Thrust saturation

• Large following error

• Sustained force demand leading to instability

Prevention

Correctly set:

• Forcer mass

• Load mass

• Maximum thrust

• Rated thrust

• Peak thrust

• Acceleration limit

• Friction compensation

• Gravity compensation (for vertical axes)

Crucial tip:
During initial commissioning, set the maximum thrust limit to 10–30% of rated thrust. Increase only after verifying direction and feedback.

8. Vertical‑Axis Issues: Power‑Off Drop, Brake, and Gravity Compensation

For Z‑axis or vertical linear motors, the risk of runaway or free‑fall is significantly higher. Unlike ball screws, linear motors have no self‑locking capability.

If any of the following occur, the load may drop suddenly:

• Servo disabled

• Brake not engaged

• Pneumatic balance失效

• Gravity compensation not configured

• Drive fault disables power

• Emergency stop directly cuts drive power

This may not be "runaway" in the control‑theory sense, but the physical outcome—rapid, uncontrolled descent—is equally dangerous.

Prevention for Vertical Axes

• Mechanical holding brake

• Pneumatic / spring / counterweight balance

• Power‑off drop prevention mechanism

• Correct STO logic and brake sequencing

• Hold torque established before servo enable

• Brake applied before servo disable

• Alarm handling must not release servo abruptly

• Speed and thrust limits active

• Lower end‑stop and mechanical buffers must be robust

Critical reminder:
For Z‑axis linear motors, the greatest fear is not positioning inaccuracy—it is dropping when power is removed.

Systematic Runaway Prevention Workflow

Use the following practical procedure during field commissioning.

Step 1 – Mechanical Safety First (Before Power‑On)

Verify:

• Stage can be moved manually without binding

• No noticeable mechanical obstruction

• Mechanical end‑stops at both travel ends

• Shock absorbers / bumpers in place

• Cable carriers will not pull or snag

• Load is securely mounted

• Vertical axis has anti‑drop provisions

• No personnel have hands near moving area

Key rule:
Never place hands within the stage travel envelope during initial power‑up.

Step 2 – Validate Limits and Emergency Stop

Before any motion:

• E‑stop must cut servo enable

• Positive limit switch must be functional

• Negative limit switch must be functional

• Limit polarity (NC/NO) must be correct

• Drive must be capable of reading limits directly

• PLC must also read limit status

• STO must be verified

• Alarm must stop output

Note: Do not wire limits to the PLC only—wire them to the drive as well. PLC logic is good for supervisory protection, but drive‑level limits are faster and more reliable.

Step 3 – Set Conservative Speed, Thrust, and Travel Limits

Initial settings:

• Speed limit: very low

• Acceleration limit: very low

• Thrust / current limit: 10–30% of rated

• Jog distance: 1 mm to 5 mm

• Soft limits: configure early

• Position deviation alarm: do not set too high

• Overspeed alarm: enable

• Motion commands: avoid large strokes initially

Goal: make the axis "creep" slowly—do not pursue cycle‑time performance on the first run.

Step 4 – Confirm Feedback Polarity

Manually push the axis.

Assuming positive mechanical direction is to the right:

• Push 10 mm to the right → drive position monitor should increase.

• Push 10 mm to the left → drive position monitor should decrease.

If reversed, correct feedback polarity before proceeding.

Do not skip this step.

Step 5 – Confirm Thrust Direction

With low thrust and low speed, perform a jog test:

• Positive jog command → axis moves in the positive mechanical direction.

• Negative jog command → axis moves in the negative mechanical direction.

• Feedback position change must match the direction of travel.

If a positive command moves the axis in the negative direction, do not indiscriminately reverse both motor and encoder directions.

Correct approach:
First ensure feedback polarity matches mechanical coordinates; then adjust motor direction / phasing / drive parameters so thrust direction aligns with feedback.

Step 6 – Perform Commutation Alignment

Strictly follow the drive manufacturer's procedure for:

• Hall‑based commutation

• Sensorless short‑stroke alignment

• Absolute position‑based alignment

• Pole‑search routine

• Phase save / store

• Electrical angle calibration

During alignment:

• Position the forcer near mid‑stroke

• Keep sufficient clearance at both ends

• Limit thrust and speed

• Prevent end‑stop impact

• Save parameters after successful alignment

Step 7 – Low‑Speed, Short‑Stroke Test Run

Recommended test sequence:

1. 1 mm positive

2. 1 mm negative

3. 5 mm positive

4. 5 mm negative

5. 10 mm positive

6. 10 mm negative

7. Measure actual travel distance

8. Verify feedback units

9. Check following error

10. Observe velocity profile

11. Observe current waveform

Do not attempt full‑stroke moves at this stage.

Step 8 – Gradually Increase Speed and Acceleration

After each increment, monitor for:

• Vibration

• Whine / acoustic noise

• Overshoot

• Alarms

• Increased following error

• Current saturation

• Longer settling time

• Velocity profile smoothness

Do not evaluate only final positioning—also check:

• Following error

• Velocity ripple

• Current waveform quality

• Settling time

• Resonant frequencies

Runaway Troubleshooting Sequence

If runaway has occurred, investigate in this order:

1. Feedback polarity – Manual push test.

2. UVW phase sequence – Verify against motor specification.

3. Commutation alignment – Confirm search completed, saved, and electrical angle is plausible; check Hall signals and pole pitch.

4. Encoder / scale parameters – Resolution, direction, A/B/Z signal quality, absolute communication, grounding, shielding, connector integrity, cable condition.

5. Unit conversion – Electronic gear ratio, command/feedback units, velocity/acceleration scaling, PLC axis parameters, EtherCAT object dictionary, host scaling.

6. Gain settings – Position loop gain, velocity loop gain, velocity integral, current loop, feedforward, filters, notch, stiffness, auto‑tuning results.

7. Limit and protection – Soft limits enabled, hard limits connected to drive, limit trigger direction correct, E‑stop cuts servo enable, STO wired, position deviation alarm not overly large, overspeed protection active.

Summary

The most frequent causes of linear motor runaway are:

• Incorrect UVW phase sequence

• Reversed encoder / linear scale feedback polarity

• Wrong commutation angle / electrical angle

• Hall signal miswiring

• Electronic gear ratio / unit conversion errors

• Excessive speed, acceleration, or thrust limits

• Overly high position/velocity loop gains

• Failed limit switches, E‑stop, or STO protection

• Scale signal interference or loss

• Missing brake / anti‑drop mechanism on vertical axes

Core preventive measures:
Low speed, low thrust, short strokes; verify feedback direction first, then thrust direction; complete commutation alignment; enable hardware and software limits; ensure a robust safety chain.

Engineering takeaway:
For conventional rotary servos, the focus is "can it position accurately." For linear motors, the first priority is "can it power up without running away."