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In CNC control engineering, following error refers to the instantaneous difference between the commanded (setpoint) position and the actual position of an axis. This error is inevitable due to system dynamics, inertia, control loop delays, and the physical limits of the drive components. For Siemens controllers such as the SINUMERIK range, the following error is closely monitored and tightly linked to numerous configurable parameters, alarms, and system responses.

It explores the technical dependencies, parameter impacts, and real-world considerations necessary to ensure error remains within acceptable limits during all phases of axis motion.

What is Following Error?

Following error, often labeled as contour error during multi-axis interpolation or simply position deviation for single axis movement, is an essential diagnostic parameter in closed-loop control systems like those used in CNC machines.

Whenever a motion command is issued by the position controller, the servomotor and mechanical load take a finite time to respond due to their combined inertia and dynamics. Hence, at any instant, there is a gap (the following error) between where the axis should be and where it actually is.

Significance of Following Error:

• Small, consistent error means tuned, responsive control.
• Large or fluctuating error signals issues like under-tuned control, excessive load, poor servo performance, or mechanical problems.
• Exceeding the allowable following error can trip alarms or even emergency stops (E-STOPs), halting production and putting the workpiece, tooling, or machine at risk.

Factors Influencing Following Error

1. Position Control Loop Gain

• Parameter: MD32200 $MA_POSCTRL_GAIN
• Explanation: This gain determines the responsiveness and stiffness of the position control loop. A higher gain typically reduces steady-state following error but can also amplify machine vibrations or cause instability if set too high.
• Field Note: Excessively low gain causes sluggish response and bigger errors; excessively high gain introduces oscillations.

2. Maximum Acceleration

• Parameter: MD32300 $MA_MAX_AX_ACCEL
• Explanation: This parameter sets the acceleration ceiling for the axis. The following error tends to peak during acceleration phases as the commanded rate of change of velocity may exceed what the drive system can physically deliver, causing the actual position to lag.
• Field Note: Boosting acceleration without matching drive capability increases following error, especially at high speeds or heavy loads.

3. Maximum Velocity

• Parameter: MD32000 $MA_MAX_AX_VELO
• Explanation: Similar to acceleration, higher maximum velocity puts more demand on the drive and control system, potentially increasing following error at rapid traverse speeds.
• Field Note: Setting this parameter higher than what the axis can reliably achieve worsens position errors.

4. Feedforward Control & Model Parameters

Feedforward control anticipates system requirements rather than merely reacting to position errors. Siemens controllers allow you to fine-tune feedforward contributions using several parameters:

• Velocity Feedforward Gain: MD32610 $MA_VELO_FFW_WEIGHT
  Increases the feedforward component proportional to commanded velocity, directly helping the axis “keep up” during speed changes.
• Feedforward Time Constants:
• MD32800 $MA_EQUIV_CURRCTRL_TIME (Current Control Loop)
• MD32810 $MA_EQUIV_SPEEDCTRL_TIME (Speed Control Loop)
  These parameters model the dynamic behavior of the drive, ensuring the controller’s feedforward compensations match the real system.

Field Note: Properly tuned feedforward can dramatically reduce following error during accelerations and decelerations, but incorrect values may cause overshoot or even instability.

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Typical Following Error Behavior During Motion

When traversing an axis, the following error profile typically looks as follows:

• At Idle/Steady State:
  Little to no following error if the axis is stationary with load in equilibrium.
• During Acceleration:
  Following error increases as the axis strives to catch up to the accelerating command. The magnitude and duration are determined by the mass/inertia of the axis and the tuning of control gains.
• At Constant Velocity:
  Ideally, following error stabilizes at a low, steady value, provided the axis load and control tuning are suitable.
• With Disturbances:
  Fluctuations occur due to mechanical backlash, friction, changing loads, or external shocks.

Following Error Monitoring and Tolerance

To prevent momentary or minor following errors from tripping an alarm (and unnecessarily stopping production), Siemens controllers allow for a tolerance window, defined as:

• Parameter: MD36400 $MA_CONTOUR_TOL
  This sets the allowed following error before contour monitoring alarms are issued.

If the configured limit is exceeded, an alarm such as 25050 “Axis contour monitoring… appears. The alarm is meant to safeguard precision, alert to possible mechanical faults, or indicate incorrect parameterization.

• Alarm-Related Parameter:
• Maximum time for braking ramp and axis E-stop is controlled by MD36610 (Axis emergency stop time).

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Field Diagnosis Workflow: Step-by-Step

Step 1: Evaluate Alarm History and Error Profile

• Analyze axis alarms: review for repeated 25050 alarms or related warnings.
• Use HMI or diagnostic logs to plot following error during typical operation.
• Note timing: when does error spike? Acceleration, deceleration, constant speed?

Step 2: Inspect Mechanical System

• Check for backlash, loose couplings, worn ball screws, excessive friction, or load shocks.
• Verify axis lube and mechanical health.

Step 3: Review and Adjust Parameters

a) Position Loop Gain

• Access MD32200. If following error tracks the load slowly or seems “damped,” carefully increase the gain.
• Monitor for onset of oscillations or audible vibrations.

b) Acceleration and Velocity

• Check MD32300 and MD32000. If these are set higher than realistic for the machine, reduce them.
• Match these values to drive/motor specs and observed load inertia.

c) Feedforward Tuning

• Enable velocity feedforward if not already (MD32610). Observe the resulting error during acceleration and steady-state.
• Fine-tune time constants (MD32800, MD32810) so the controller model matches the real drive response.

d) Adjust Tolerance Window

• Carefully review MD36400 to ensure alarm windows are not set too tight (causing false positives) or too loose (risking undetected loss of precision).

Step 4: Test and Validate

• Command a series of test moves—short, long, slow, and at max speed.
• Plot following error for each run.
• Verify error remains within acceptable and configured limits.

Step 5: Monitor During Production

• After adjustment, monitor axis performance for several cycles or jobs.
• Watch for recurring alarms or abnormal error excursions.
• Document parameter changes and machine observations for future reference.

Practical Tips from the Field

• Parameter Balance: Never maximize gains or feedforward factors without stepwise testing—machine vibrations or unexpected stop alarms can occur.
• Mechanical Issues First: Many following error alarms trace back to loose, worn, or misaligned mechanics rather than control loop issues.
• Documentation: Always log parameter modifications. Revert to saved settings if instability or new alarms appear after changes.
• Tolerance Setting: Set contour tolerance wide enough to avoid nuisance alarms but tight enough to inhibit truly dangerous errors.
• Training: All team members should understand the impact of following error and the safe limits for the specific machine application.

Real-World Example

A customer reports recurring Axis Contour Monitoring (Alarm 25050) when executing rapid G00 moves. Diagnosis reveals:

• Following error peaks occur predominantly during acceleration and at max velocity transitions.
• Position control gain (MD32200) was low—a legacy setting from an earlier, lighter workpiece.
• Maximum acceleration (MD32300) was recently raised to optimize cycle time.
• Feedforward (MD32610) was set to factory default (suboptimal for the current mechanical load).

Diagnosis/Resolution:

• Increment position loop gain while cross-monitoring for oscillations.
• Tune feedforward factor by referencing Siemens tuning documentation.
• Slightly widen MD36400 to account for the higher inertia of the new workpiece.
• Set the Value in MD 36610 ( Maximum Time for the braking ramp during an axis emergency stop)
• Alert maintenance team to periodically check for mechanical looseness and lubrication.

After adjustment, the machine performed rapid traverse moves without tripping alarm 25050, and part quality improved due to tighter following error bounds.

Conclusion

Proper understanding and management of following error is foundational for reliable CNC operation. Troubleshooting involves a mix of parameter expertise, mechanical awareness, and systematic testing.

For Siemens (or any advanced CNC axis control system), the correct balance of control gain, acceleration/velocity limits, feedforward settings, and error monitoring tolerances ensures both performance and safety. Regular review—and responsible adjustment—of these parameters, along with preventative mechanical maintenance, significantly reduces unplanned downtime and supports high-quality production.

• Remember: Set parameters cautiously, interpret alarms intelligently, and always address potential mechanical faults before deep-diving into controller re-tuning.

Disclaimer :The blogs shared on CNC machines are created purely for *educational purposes*. Their intent is to help readers understand CNC controls, alarms, diagnostics, and general troubleshooting methods. We strictly avoid any copyright violations, and all explanations are written only for learning and knowledge-sharing.  

These blogs should not be considered as official repair or service manuals. For detailed instructions, critical repairs, or advanced troubleshooting, it is always necessary to contact and work under the guidance of the respective *machine manufacturer* or *CNC controller support team*.  

The content provided is focused only on *diagnosis and awareness*. We do not take responsibility for any kind of damage, error, or malfunction that may occur if someone directly applies the information shared here without proper technical supervision.#