Technical Information

Dynamic Precision – Machining Dynamically and withHigh Accuracy
Increased productivity and cost reduction are preeminent goals of modern manufacturing enterprises. Demands for reduced scrap, fastermachining times and avoidance of additional work steps or rework are closely related to these goals. In the conflict of interests between
machining time, surface quality and workpiece accuracy, machine tool controls must therefore be capable of making an approach
optimized for the milling machine and the manufacturing process.
Under the concept of Dynamic Precision, HEIDENHAIN describes a group of functions for TNC controls that dramatically improve thecontouring accuracy of machine tools even at high feed rates and in complex contouring moves. The dynamic accuracy of a machine
tools is determined by the feed-axis acceleration required in order to produce precise movement between the workpiece and tool.
When feed axes are accelerated, machine components can be deformed by inertia forces or even begin to vibrate. With Dynamic Precision,
the dynamic errors at the Tool Center Point (TCP) that arise during machining are significantly reduced so that NC programs are run with
better component accuracy and surface quality, and even noticeably faster.

Through a significant reduction of error at
the tool center point during the highly
dynamic execution of NC programs,
Dynamic Precision makes a valuable
contribution to improving the performanceof machine tools.
Users save time and costs for unnecessary
scrap because the higher dynamic accuracyof machine tools with Dynamic Precision
manifests itself in shorter machining times,
improved workpiece accuracy and greater
surface quality.

September 2013

Dynamic Precision
Shorter machining times, higher accuracy, better surfaces

Thermal errors of machine tools can become
apparent on the workpiece in periods offrom a few minutes to several hours. With
the software option KinematicsOpt, usersof five-axis machines can effectively
compensate the effects of thermal errors
on the position of rotary table in short time
periods.
The dynamic errors of a machine tool
include brief deviations or vibrations at thetool center point. Dynamic errors have thefollowing causes:• Feed forces and feed torque as well as
machining forces cause position errors
and angular errors at the tool center
point.
• Following errors between the nominal
position and actual position of the feed
axes cannot be completely compensatedby the drive control.

Velocity

The class of thermally induced errors
describe the effects of temperaturefluctuations in the machine frame or in theworkpiece on accuracy at the tool center
point (TCP). Temperature fluctuations in theframe are caused by:• Cold or warm air streams in the machinehall• Exposure to sunlight• Heat generation from components anddrives in a machine tool
• Cold or warm cooling lubricant movements in the machine’s working space

The applies particularly to the machining of
free-form surfaces with the highest
possible surface quality together with highaccuracy. Many free-form surface
operations combine corners with slightly
curved surfaces. Frequent changes inmilling direction are the result. Axes haveto accelerate or decelerate with eachdirectional change. The measure for theduration of changing acceleration is jerk.
High jerk makes for a fast increase inacceleration and therefore saves time.
However, high jerk also can excite machine
vibrations and result in inaccuracy anderrors on the workpiece surface. To keep
the deviations from dynamic error as small
as possible, traverse must be slow.

Time

Acceleration

The effect of errors in the real machine
kinematics are described by the kinematic
model in the control in the form of kinematicor static errors. In practice, the following
factors have effects on the accuracy ofmachine kinematics.
• Production and assembly accuracy of themachine components• Weight-induced sag or the associateddeformations of the machine frame
On high-value machine tools, the kinematic
errors usually change only slightly and canbe mapped and compensated using the
TNC software options KinematicsComp
and KinematicsOpt.

The dynamic errors generally increase
when the speed of NC program execution
increases. Therefore, efforts to reducemachining time can have negative effects
on accuracy and surface quality. The flipside is that more exact contours have to bebought with longer machining times.

Time

Jerk

The defects visible on the workpiece surface
and the measurable geometrical errors canbe attributed to three classes of error.

Time

Y
X

2

Machining a corner: velocity, acceleration andjerk in the two axes X and Y

Slightly curved or lightly inclined surfaces
often cause problems through shading on
the surface. This can be the result ofmachine vibrations, but also come fromexternal disturbances.
With 5-axis machining, linear axes are
subjected to highly dynamic motions (highfeed rate, high acceleration) due tocompensatory movements. This can result
again in large deviations due to vibrations
or other dynamic errors.
Because the dynamic accuracy of amachine can change with increasing age
and depending on the load, additional
deviations can occur. These effects
become evident mainly in the acceleration
phases.

The concept of Dynamic Precisioncomprised optional functions for
HEIDENHAIN controls that effectively
reduce the dynamic errors of machine
tools. They improve the machine’s dynamic
performance, attain higher stiffness at the
TCP and therefore permit milling at thelimit of the technologically possible
depending on the machine’s age, load andthe machining position.
Benefits of Dynamic Precision for theend user
It’s no longer necessary to machine slowly
in order to produce accurate workpieces
with high surface quality. Machine toolsthat operate with Dynamic Precision canmachine quickly and precisely at thesame time.
High precision together with fastmachining also means an increase inproductivity. Unit costs are reduced
without compromises in accuracy andsurface quality.

Dynamic Precision also ensures that accu­racy is retained regardless of operatingtime and weight. It isn’t necessary to
reduce feed rates due to age or load.
What is Dynamic Precision?
The functions of Dynamic Precision areavailable as options for controls from
HEIDENHAIN. They can be applied
individually as well as in combination.
• CTC – Compensation of accelerationdependent position errors at the tool
center point, thereby increasing accuracy
in acceleration phases• AVD – Active vibration damping for
better surfaces• PAC – position-dependent adaptation ofcontroller parameters• LAC – Load-dependent adaptation ofcontrol parameters enhances accuracy
regardless of load and age• MAC – Motion-dependent adaptation ofcontrol parameters
The following is a detailed description ofthese functions.

How does Dynamic Precision work?
The functions of Dynamic Precision are
adapted at high clock rates in the controller
unit—a component of HEIDENHAIN
controls—to the movements and loads ofthe machine tool.
Because Dynamic Precision consists ofsoftware functions, it requires nointervention in the mechanics of themachine or in its power train. However, themachine manufacturer has to enable theindividual functions, enter their parameters
and adapt them to the machine.

3

CTC
Compensation of acceleration-dependent position errors at thetool center point
Dynamic acceleration processes cause
forces that can briefly deform parts of themachine. This can result in deviations at thetool center point (TCP). Besides deformationin axis direction, the dynamic accelerationof an axis due to mechanical coupling canalso cause deformation of axes that areperpendicular to the direction of acceleration.
This applies in particular if the feed forces
are not at an axis’s center of gravity. The
mass and inertia can therefore cause
pitching movements during the braking andacceleration phases (see Figure 1).
The resulting position errors in the directionof the accelerated axis and perpendicularaxes are proportional to the acceleration of
the moving feed axes (Figure 2). The errors
also depend on the stiffness of theguideways, the distance between the feed
force application point and the center ofmass as well as the distance between thecenter of mass and the tool center point.

Example 1: Milling circles
The stud (R32 F10000) is machined too large
due to a dynamic elasticity in the X and Y
axes. This is illustrated when the squarebeneath the stud is machined. Because thelength of its side exactly equals the diameterof the stud, the square and the circular studideally form a tangent (Figure 3). However,
because the stud is too large due to elasticity,
a part of the stud is milled away when thesquare beneath it is machined. A flat area
appears on the circle (Figure 4).

S

F

Figure 1: Pitch error in an axis

These deviations are not recognized by the
position encoders. The feed axis servo
control therefore cannot react to them.
Effects on the workpiece
The effect becomes most visible when thesame point on a workpiece is approachedfirst with high acceleration and then withlow acceleration.

Figure 3: Stud on a square, milled with CTC.
No visible flat spot.

Figure 4: Effect of elastic deflection (the overlarge
stud is flattened when the square is milled)

Errors (2 µm/div)

Nominal contour of stud
Y axis [mm]Acceleration (1 m/s2/div)

3

Position (100 mm/div)
2
1

Error of stud (40 µm/div)

40
30
20

0
10
-1
0
-2
0

0.5

1

1.5

2

2.5

Time [s]Figure 2: Error due to acceleration during position changes

-10
-20
-30
-40
-40

-20

0

20

40

X axis [mm]Figure 5: Accuracy of the circular movement. The deviation from thenominal contour is shown enlarged 500-fold

4

Figure 6 shows the error without
enlargement. It clearly illustrates that theelasticity causes a radius enlargement in
the X direction of approx. 5 µm, which
results in a 12.5 mm wide flat area on thecircle when the surface beneath it is milled.
This also equals the error measured on theactual workpiece.

Nominal contour of stud
Y axis [mm]This effect of elasticity is illustrated in
Figure 5. The acceleration-induced deviations
known from measurements are added tothe actual movement. Due to the various
conditions in the X and Y directions(masses, machine geometry, etc.), theresulting path forms a slight ellipse.

Contour of stud

0.8

Contour of square

0.6
0.4
0.2
0
-0.2
-0.4

Example 2: Pocket milling, elasticity
perpendicular to the direction ofacceleration
When pockets are milled, accelerationinduced deformations of the axes
perpendicular to the direction of motion
can cause surface blemishes (Figure 7).
The acceleration process results in apitching movement. The cutter is briefly
pressed into the material. CTC removes
this faulty movement (Figure 8).

-0.6
-0.8
−32.03−32.02−32.01−32−31.99−31.98−31.97

X axis [mm]Figure 6: This detail section shows the damage to the stud from milling thesquareCompensation through CTC
With its CTC function (Cross Talk
Compensation), HEIDENHAIN offers acontrol option for compensation ofacceleration-induced position error at thetool center point. The quantities andparameters involved are either known inthe control (acceleration) or can be
determined in a measuring process(machine stiffness).
It makes it possible to manufacture moreaccurate parts without changing themachine mechanically or increasing themachining time. In addition, the accuracy
attainable with CTC does not depend on
the traversed acceleration movements.

Figure 7: Machining without CTC

Effects in practiceWith the aid of CTC, it proved possible toreduce mean error by up to 80 % asmeasured by a grid encoder. This makes it
possible to increase jerk (measure for theduration of changing acceleration) andtherefore also significantly reducemachining time.
An increase in jerk by the factor of 2 madeit possible to reduce contouring times by
up to 15 %. Thanks to CTC, the mean error
was nevertheless only 50 % of thatattained without CTC.

Figure 8: Machining with CTC

5AVD
Active vibration damping

Inclined or curved surfaces often havefinish problems in the form of visible
shadows or fluctuations in contrast. They
are especially disturbing because at typicalviewing distances of 30 or 60 cm thehuman eye is very sensitive to contrast
fluctuations with a position period of
0.5 mm to 5 mm. The shadows can be
attributed to fundamentally different
causes:• Mechanical vibrations resulting fromelasticity in the power train or themachine setup• Position errors within one signal period(interpolation error) due to the encoder.
(See the Technical Information document
Perfect Surfaces through HEIDENHAIN
Encoders)
This Technical Information describes thesurface errors caused by mechanicalvibrations.

Periodic shading is usually caused under
low-frequency vibration up to 100 Hz. At
the usual finishing feed rates of 3000 to
6000 mm/min, such vibrations become
visible exactly in the area of these position
periods. Depending on the incidence oflight, contour deviations as small as 1 µm
and smaller can be visible.
There are two frequent causes for surface
quality problems:• Elasticity in the drive train
Elastic deformations of the ball screw or
elasticity in the drive belt, for example,
can case vibration in the power train
between the drive side (motor) and thefriction side (slides).
• Vibrations from machine setup
Vibrations from machine setup are
inevitable. Machine setup vibrations are
typically in the frequency range of 10 Hz
to 30 Hz.

Nominal contour

Errors without AVD (10 µm/div)

60

Errors with AVD (10 µm/div)

80
60

40

40

20

20

0

0

-20

-20

-40

-40

-60

-60

-80

-80
-50

0

50

X axis [mm]Figures 2 and 3: Errors due to vibrations

6

Y axis [mm]Y axis [mm]Nominal contour
80

Figure 1: Vibrations from machine setup

-50

0

50

X axis [mm]

Conventional countermeasures
Vibrations can be excited by acceleration
processes in the machine or cross-couplingthrough the floor, by the cutter engagement
as well as by torque ripple in the motor.
While excitations through acceleration
processes can be reduce by lowering thejerk, this causes longer machining times.
Compensation through AVD
The AVD feature (Active Vibration Damping)
suppresses dominant low-frequency
vibration (machine setup vibrations orelasticity in the power train).
AVD makes milling operations fast andvibration-free. By suppressing the disturb­
ances resulting from acceleration
processes, high jerk values and therefore
higher acceleration can be realized. This
reduces machining times without impairing
the surface quality of the workpiece.

Effects in practiceIn this example, two squares are arranged
at different angles to each other. The
acceleration processes at the corners excite
vibrations in the X and Y axes (Figures 2
and 3). The vibration components
perpendicular to the workpiece surface
are visible in the shading (Figure 4). The
period length of 2 mm at a feed rate of2000 mm/min results from the measured
setup vibration of 16.5 Hz. With AVD, it waspossible to nearly eliminate the vibration
amplitude (Figure 5).
To attain comparable surfaces without AVD
it would be necessary to reduce the jerk
values by a factor of 3.
Conclusion
AVD increases the productivity of amachine tool and/or improves the surface
quality of the workpieces.

Figure 4: Vibrations can significantly impair surface quality.

Figure 5: With AVD, visibly superior surface quality is achieved

7

PAC
Position-dependent adaptationof control parameters
Depending on the positions of the axes ina working space, the kinematic conditionsof a machine give it a variable dynamic
response that can adversely affect thestability or quality of the servo-control.
Changes in axis positions also change themass ratios in a machine (see Figure 1).
Stiffness values can also change depending
on position, as for example on ball-screw
drives. The changed mass ratios andstiffness values cause a shift of natural
frequencies in the drive train. This results ina varying control behavior depending on
position.

Adjustment to the weakest position leaves
unused potential for improving the dynamic
accuracy.

Control can be optimally adapted to themachine through position-dependent filter
settings and control factors in order toachieve the best results at any position
within the working space. In addition, it canincrease dynamic accuracy at the positions
that are relevant for the largest share ofmachining.
The higher the control factors are set, thebetter the suppression of interference (e.g.
gear transmission error, torque ripple in the
motor) and the smaller the following error
becomes. And this, in turn, results in better
contour accuracy.

Benefits of PAC
The PAC option (Position Adaptive Control)from HEIDENHAIN can change machineparameters depending on the axis positions
and in this way better exploit the machine’s
dynamic capabilities.

Following error [mm]The following error can serve as a measureof control quality. It is an an indicator of
how well the control traces a nominal
contour.

Conventional measures
The control loops of the axes must always
be adjusted so that they can stay stable
and robust at any possible position. It istherefore always necessary to adjust for
the weakest position. These positions are
frequently located at the edge of thetraverse range (limit of the working space,tool changer, loading position of the table,etc.). In the center of the working space,
where most of the most precise machining
is required, the controller dynamics and the
dynamic accuracy resulting from it could
be significantly increased.

Following error
Nominal feed rate (for information)

Time [s]Following error [mm]Servo control optimized for Z = 0, following error within the tolerance band (± 1 µm)

Following error without PAC
Following error with PAC
Nominal feed rate (for information)

Time [s]Servo control at Z = -500
• Without PAC: Clearly visible oscillations and following error outside of the tolerance band (± 3 µm)• With active PAC: Following error stays within the tolerance band (± 1 µm)

8LAC
Load-dependent adaptationof control parameters
The dynamic response of machines withmoving tables can vary depending on themass (linear axis) or mass moment ofinertia (rotary axis) of the fixed workpiece.
The values for the friction and acceleration
feedforward control of an axis apply only
for the mass or the mass moment ofinertia that existed during the adjustment
process. Under other load conditions, thefeedforward values no longer apply to theactual situation. This expresses itself as anenlarged following error during theacceleration phases, which can then result
in contour deviations.
Conventional measures
Machine tool builders can prepare parameter
sets for various load situations that can be
activated through a cycle call. This does
reduce the following error, but residual
errors always remain depending on theload conditions.

The situation becomes more critical with arotary table. Here the inertia, not the mass,is relevant for the feedforward parametervalues. Suboptimal workpiece setup caneasily multiply the mass moment of inertia
from the same mass.
Compensation through LAC
The LAC option (Load Adaptive Control)from HEIDENHAIN enables the control toautomatically ascertain the current mass
with linear axes and the mass moment ofinertia with rotary axes as well as thefriction forces. In order to optimize changed
control behavior at differing loads, adaptive
feedforward controls can exploit data on
acceleration, holding torque, static friction
and friction at high shaft speeds. During
workpiece machining, the control can also
continuously adjust the parameters of theadaptive feedforward control to the current
mass or mass moment of inertia of theFollowing error [0.001°]

A typical example is two parameter sets
for the load conditions 0 kg to 150 kg(adjusted for 75 kg) and 150 kg to 500 kg(adjusted for 325 kg). In the worst case,the actual mass differs from the adjusted
load situation by up to 175 kg.

workpiece. The adaptation velocity ispreset by parameter. Because the machine
operator cannot enter the load situation on
his own, this rules out operator error.
Further benefits of LAC
The aging of machine components, suchas guideways or ball screws, can greatlychange frictional forces over the service lifeof a machine tool. An optimal feedforward
adjustment for a machine in shipping
condition no longer applies after a few
years. The LAC option ensures that the axis
is always optimally adjusted.
However, quickly changing friction
conditions that result, for example, fromlubrication pulses on sliding guides, canalso be optimally compensated by LAC.

Following error
Nominal feed rate (for information)

Time [s]Following error [0.001°]

Optimal feedforward control for rotary tables without additional load and with following error within the tolerance band (± 0.001°)
Following error without LAC
Following error with LAC
Nominal feed rate (for information)

Time [s]Additional load changed• Without LAC: When the feedforward control is unchanged, the following error is outside of the tolerance band (± 0.008°)• With LAC: When LAC in active in the feedforward control, the following error is within the tolerance band (± 0.001°)

9

MAC
Motion-dependent adaptationof control parameters
Machine behavior changes depend not only
on the position of the axes in the working
space, but also on their velocity. Amongother things, this can also be attributed tothe influence of velocity on friction in theguideways. Changed frictional conditions
can affect the vibration behavior of amachine tool. Optimal controller settings
that are typically conducted for standstill,
can lead to strong vibrations at rapid
traverse.
In addition, MAC makes it possible toeasily change the bearing preload of a rackand-pinion drive with two independent
feed motors depending on the velocity.

Benefits of MAC
Example 1
The MAC option (Motion Adaptive Control)
makes it possible to change machineparameters depending on other input
quantities such as velocity, following error
or the acceleration of a drive. Through thismotion-dependent adaptation of thecontrol parameters it is possible, forexample, to realize a velocity-dependent
adaptation of the control loop gain on
motors whose stability changes throughthe various traversing velocities. In thisway, the optimal controller settings can beapplied for any machining situation. This
provides optimal interference suppression
and improves the dynamic performance ofthe machine.

Without MAC
M

Mmax

Mmax

MV1

MV1




a
a1max

10

In addition, a reduction of the tensioning
torque at standstill significantly decreases
heat conduction into the machine, which
also reduces thermally induced
deformation or drift.

With MAC

M

MV2

Example 2
A further application is the accelerationdependent change of the tensioning torquebetween master and slave for master-slave
torque control. With the MAC option, thisarrangement makes it possible to attain asignificantly higher maximum acceleration
at rapid traverse, for example throughparameterized reduction of the tensioning
torque with increasing acceleration.

aa1max a2max

a2max > a1max

MV2

Dynamic Precision
Functions are perfectly matched

The functions comprised by Dynamic
Precision complement each other perfectly.
A simple example contour—shown black in
the respective graphs—illustrates this for
CTC and AVD. The errors at the TCP wererecorded with a grid encoder at a feed rateof 10 000 m/s. The deviations from thecontour are shown in the graphic enlarged
500 times.
Figure 1 shows the deviations from thenominal contour as a red line. Particularly inthe acceleration phases at the corners, theerror values are quite high due to the highvelocity.

High jerks more strongly excite themachine. The resulting vibrations canbecome visible as shadows on theworkpiece. These strong vibrations arethen reduced by the AVD option.
Figure 2 compares the initial condition (redline: without Dynamic Precision at 100 %jerk) with machining with Dynamic Precisionat 200 % jerk (Figure 2, green line). This
clearly illustrates that Dynamic Precision,even with high dynamics, improves error
by a factor of 2. Doubling the jerk made it
possible to reduce the traverse time at thiscontour by 12 %.

Time

2.6 µm1.04 sCTC on
AVD onR100%

0.4 µm1.04 sCTC on
AVD onR200%

0.6 µm

0.91 sConclusion
With Dynamic Precision your work becomessignificantly more accurate or—if jerk isincreased—both faster and more accurate.

Nominal contour

Nominal contour

Without CTC/AVD, 100% jerk

Without CTC/AVD, 100% jerk

With CTC/AVD, 100% jerk
15
10

Z axis [mm]Z axis [mm]With Dynamic Precision, here by switchingon the CTC and AVD options, these errors
are compensated (green line in Figure 1).
The part is machined at the same feed ratewith a much more accurate contour.
Thanks to the reduction in error, the jerk
can be increased in order to elicit more
dynamic performance from the machine.

Mean error
CTC off
AVD off
R100%

With CTC/AVD, 200% jerk
15
10

5

5

0

0

-5

-5

-10

-10

-15

-15

0

-15

-10

-5

0

5

10

15

Y axis [mm]Figure 1: Errors with and without Dynamic Precision at 100 % jerk

0

-15

-10

-5

0

5

10

15

Y axis [mm]Figure 1: Errors with and without Dynamic Precision at 100 % jerk and with
Dynamic Precision at 200 % jerk

11

HEIDENHAIN controls

Contouring controls for milling, milling/turning, drilling, boring machines andmachining centers
The TNC controls from HEIDENHAIN cover
the whole range of applications: From thesimple, compact TNC 128 three-axis
straight cut control to the TNC 530 (up to
18 axes plus spindle)—there's a TNC
control for nearly every application. The
TNC 640 is a control for milling machinesthat are also capable of turning operations.
HEIDENHAIN TNC controls are versatile:
They are workshop oriented and feature
both shop-floor and offline programming,and are therefore ideal for automatedproduction. They handle simple milling
tasks just as reliably as the TNC 640 and
iTNC 530, for example, can handle highspeed cutting—with especially jerkreduced path control—or 5-axis machining
with swivel head and rotary table.
HEIDENHAIN combines innovative control
functions for efficient high-precision
machining under the hypernyms Dynamic
Efficiency and Dynamic Precision.
Dynamic Efficiency helps the user tomake heavy machining and roughing more
efficient while also enhancing its processreliability. Dynamic Efficiency is available
on the TNC 640 and iTNC 530 controls.
Dynamic Precision makes workpieces
more exact, with clean surfaces and highspeed machining, thereby providing highprecision and higher productivity. You can
use the Dynamic Precision software
options with the TNC 640, iTNC 530 and
TNC 620.

TNC 640iTNC 530

TNC 620

TNC 640iTNC 530

TNC 620x

x

x

CTC – Compensation of position errors Option
through axis coupling

OptionOption

AVD – Active vibration damping

OptionOptionOption

PAC – Position-dependent adaptationof controller parameters

OptionOptionOptionLAC – Load-dependent adaptation ofcontrol parameters

OptionOptionOptionMAC – Motion-dependent adaptationof control parameters

OptionOptionOptionxx–

ACC – Active suppression of chatter

OptionOptionOptionAFC – Adaptive feed control

OptionOption–Trochoidal milling

•

•

•

Dynamic Precision

Dynamic Efficiency

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1081196-20 · 10 · 4/20114 · F&W · Printed in Germany

For more information:• Catalog: TNC 640• Catalog: iTNC 530• Catalog: TNC 620• Technical Information:
Dynamic Efﬁciency