Siemens SINUMERIK 840D Following Error, Contour Deviation and the Axis Service Screen

Introduction

On Siemens SINUMERIK 840D, 840Di and 810D controls, the Axis/Spindle Service screen is one of the main service displays used for diagnosing axis and spindle faults.

This screen can display:

• Position setpoint
• Actual position
• Following error
• Control deviation
• Contour deviation
• Servo gain
• Speed setpoint
• Actual speed
• Active measuring system
• Compensation values

The purpose of the screen is not only to show errors; it is to evaluate the setpoint chain, the actual value chain, the encoder feedback, the servo behavior and the axis monitoring functions together.

What Is Following Error?

The following error is the difference between the position setpoint and the actual position reported by the active measuring system.

Following Error = Position Setpoint - Actual Position

The unit can be mm, inch or degrees.

The following error depends on the dynamic behavior of the axis. At constant speed, the following error is related to the servo gain and the speed.

According to the example in the Siemens documentation, with KV = 1 and a constant feedrate of 1 m/min, a following error of approximately 1 mm is expected. This example only illustrates the control principle and is not an acceptance limit for every machine.

How to Interpret the Following Error

When evaluating the following error, keep these conditions identical: axis, direction of travel, feedrate, acceleration, feed override, workpiece load, feedforward status and measuring system.

Comparing the same movement under different conditions can lead to wrong conclusions.

Following error rising at constant speed

Possible causes:

• Insufficient servo gain
• Mechanical friction
• Ball screw or guideway problem
• Insufficient motor torque
• Drive current limit
• Increased axis load

Following error rising during acceleration

Possible causes:

• Feedforward setting
• Incorrectly defined motor and load inertia
• Acceleration set too high for the axis dynamics
• Speed control loop settings
• Filter and jerk settings

Following error rising at direction reversal

Possible causes:

• Backlash
• Friction
• Mechanical play
• Quadrant error
• Incorrect backlash compensation

Control Deviation and Contour Deviation

The control deviation is the difference between the setpoint at the position controller input and the actual value of the active measuring system.

The contour deviation is the difference between the position calculated from the control model and the real measured value. It is used to evaluate following error fluctuations caused by the effect of load changes on the speed control loop.

Contour deviation is particularly important during circular interpolation, high-speed contour machining, die and mold machining, radius transitions and simultaneous axis movements.

The Servo Gain Value

The servo gain value shown on the Service screen is calculated by the NC using the following relationship:

Servo Gain = Velocity Setpoint / Following Error

According to the documentation, the displayed servo gain value may not always match the value set in the machine data exactly.

If feedforward is active, the calculated servo gain may appear higher. If jerk filters or dynamic matching filters are active, the calculated value may appear lower.

For this reason, MD 32200 POSCTRL_GAIN must not be changed based only on the servo gain value shown on the screen.

Feedforward Control

Feedforward control is used to reduce the following error at high speeds and accelerations.

On SINUMERIK systems, the basic mode selection is:

MD 32620 FFW_MODE

Typical options:

0 = Feedforward off
1 = Velocity feedforward
2 = Torque feedforward

Torque feedforward can only be used with the appropriate hardware and option conditions.

Switching on and off from the part program:

FFWON
FFWOF

For these commands to work, the following must be set:

MD 32630 FFW_ACTIVATION_MODE = 1

Feedforward should not be switched on or off while an axis or spindle is moving. Siemens recommends performing the operation at standstill to avoid jerk.

Following Error with Feedforward Active

If feedforward is set correctly, the following error becomes smaller, particularly at constant speed and during acceleration.

For velocity feedforward, the values MD 32610 VELO_FFW_WEIGHT and MD 32810 EQUIV_SPEEDCTRL_TIME are important.

If the equivalent time constant of the speed control loop is defined correctly, the system deviation approaches zero when the feedforward factor is approximately 1.

For torque feedforward, the values MD 32650 AX_INERTIA and MD 32800 EQUIV_CURRCTRL_TIME are used.

When the inertia and the current loop time constant are defined correctly, the following error during acceleration can approach zero.

Alarms Related to the Following Error

Alarm 25040 – Zero Speed Monitoring

Condition:

Following Error > MD 36030 STANDSTILL_POS_TOL

This alarm occurs when the permitted position window is exceeded while the axis is at standstill.

What to check:

• Vertical axis brake
• External load
• Mechanical drift
• Encoder instability
• Servo oscillation
• Standstill tolerance

Before increasing the tolerance value, investigate the possibility of physical drift.

Alarm 25050 – Contour Monitoring

Condition:

Contour Deviation > MD 36400 CONTOUR_TOL

Possible causes:

• Excessive feedrate or acceleration
• Axis synchronization problem
• Servo saturation
• Mechanical binding
• Incorrect feedforward setting
• Speed control problem

Alarm 25060 – Speed Setpoint Limitation

Condition:

Speed Setpoint > MD 36210 CTRLOUT_LIMIT

This alarm is related to the axis control output exceeding the defined limit.

What to check:

• Motor and drive scaling
• Speed setpoint limit
• Servo gain
• Axis load
• Feedforward output
• Programmed acceleration

Alarm 25080 – Positioning Monitoring

Condition:

Following Error > MD 36010 STOP_LIMIT_FINE

The axis has failed to reach the exact stop fine window.

Possible causes:

• Servo oscillation
• Mechanical play
• Friction
• Insufficient position gain
• Brake or clamp problem
• Encoder instability

Alarm 26000 – Clamping Monitor

Condition:

Following Error > MD 36050 CLAMP_POS_TOL

The alarm occurs when the axis moves outside the defined position tolerance while the Clamping Active signal is present.

What to check:

• Clamp mechanism
• Hydraulic or pneumatic pressure
• Clamp signals
• Axis movement while clamped
• Mechanical clamping play
• Encoder movement

Backlash Compensation

Mechanical play causes a position difference between the measuring system and the moving machine element when the axis reverses direction.

On SINUMERIK systems, the basic backlash value is defined with:

MD 32450 BACKLASH

Backlash compensation becomes active in all operating modes after the reference point has been approached.

An incorrect backlash value can cause dimensional errors at direction reversals, quadrant marks on circles, contour distortion, and axis over- or under-travel.

The backlash value must only be determined with a dial indicator, a laser interferometer or another suitable measuring method.

Temperature Compensation

Temperature compensation corrects position errors caused by thermal expansion of the machine body, the ball screw and the axis elements.

Basic settings:

SD 43900 TEMP_COMP_ABS_VALUE
SD 43910 TEMP_COMP_SLOPE
SD 43920 TEMP_COMP_REF_POSITION

These values must be determined from measured temperature–error curves.

The compensation type is selected with:

MD 32750 TEMP_COMP_TYPE

For the compensation to work, the option must be active, the compensation type must be selected, the setting values must be defined and the axis must be referenced.

These values must not be changed based on a single deviation measured on a workpiece.

Sag Compensation

Sag compensation is used to correct geometric deviations caused by a bridge, a column, a table or a heavy workpiece.

The weighting factor:

SD 41310 CEC_TABLE_WEIGHT

multiplies the values in the compensation table.

If different tool or workpiece weights only change the amplitude of the error curve, the factor can be used. If the shape of the curve changes, different compensation tables must be used.

When changing the factor, the maximum total compensation limit must not be exceeded.

Circularity Test

The circularity test records the actual positions of two axes during a circular movement and displays the deviation from the programmed radius as a graph.

The test is used especially for evaluating quadrant error, friction, backlash, axis synchronization, QEC tuning and contour accuracy.

The SINUMERIK documentation states that the test records the real axis positions and shows radius deviations, particularly at the quadrant transitions.

Basic test sequence

1. Prepare an NC program that performs a circular movement.
2. Open the Circularity Test screen in the Diagnosis area.
3. Select the axes.
4. Enter the radius and the actual feedrate values.
5. Start the program in AUTO or MDA mode.
6. Start the measurement with the Start softkey.
7. Evaluate the resulting graph.

The control does not check whether the selected axes match the axes in the NC program, or whether the feedrate entered on the screen matches the real feedrate.

PRESET and Software Limits

The SINUMERIK 840D documentation states that software limits are active after referencing and are deactivated after a PRESET operation.

PRESET may also require the absolute encoder reference status to be re-established.

For this reason, after a PRESET operation, check the encoder reference, the software limits, the machine coordinates and the status of the absolute measuring system.

Diagnostic Sequence for Service Technicians

For an axis or contour alarm, the following sequence can be applied:

1. Record the alarm and axis information.
2. Check the reference status of the axis.
3. Compare the position setpoint and the actual position.
4. Measure the following error without load and under load.
5. Compare the speed setpoint with the actual speed.
6. Test in both the positive and negative directions.
7. Check the feedforward status.
8. Check the motor current and the drive limits.
9. Measure the backlash and the mechanical friction.
10. Perform a circularity test if necessary.
11. Back up the existing data before changing any parameter.
12. Repeat the measurement under the same test conditions after any change.

Disclaimer

This content was prepared to explain the service and fault-diagnosis logic of SINUMERIK control systems. Machine data, servo, drive, encoder and compensation settings must only be changed by authorized and experienced technical personnel. The same parameters and functions may behave differently on different SINUMERIK versions and on machines from different builders; the machine builder’s manual always takes priority.