WHITEPAPER – HEXAPODS IN MECHANICAL ENGINEERING
Dipl.-Phys. Birgit Schulze

Hexapods in Mechanical Engineering
What can be accomplished with parallel kinematics?

Physik Instrumente (PI) GmbH & Co. KG_Auf der Roemerstrasse 1_76228 Karlsruhe, Germany
Tel. +49 721 4846-0, Fax +49 721 4846-1019 Email info@pi.ws, www.pi.ws

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WHITEPAPER – HEXAPODS IN MECHANICAL ENGINEERING
Dipl.-Phys. Birgit Schulze

Mechanical and plant engineering has always benefitted

When position sensors are integrated, they are alsofrom innovations, i.e., from creative ideas and theireach assigned to one drive and only measure the motion

economical implementation. Today this is more true thanin the corresponding positioning axis. Any undesirable

ever, because in many areas of automation technology

motions in the other five degrees of freedom cannot be

precision requirements are on the increase, and an end

detected or compensated, resulting in an accumulation

to this development is nowhere in sight. This is whyof guiding errors.

whenever multiaxis systems are needed, it is worth
having a closer look at parallel kinematics. One exampleof this are Hexapods. The travel ranges of Hexapods gonormally from a few to several hundred millimeters. Evenmajor loads can be positioned accurately to themicrometer using Hexapods. Due to their capacity
acquired recently to communicate directly with PLC or
CNC controls via fieldbus interfaces, they open up
interesting possibilities in mechanical engineering.
Basically, there are two methods of constructing
multiaxis positioning systems: Serial kinematics andparallel kinematics (Fig. 1).

Parallel Kinematics and Parallel
Metrology
In contrast, in a parallel-kinematic multiaxis system, allactuators act directly on the central moving platform.
This is the only way of achieving identical resonancefrequencies and identical dynamic behavior for the axes.
Flight simulators are probably the best-known applicationof six-axis parallel-kinematic systems; here six actuators
act on the same platform, which can move and tilt in all
directions. Industrial Hexapods, suitable for mechanicalengineering,are,however,

much

more

accurate.

Depending on the requirements of the application, they
are driven by high-precision drive screws and precisely
controllable DC motors or directly by linear motors, e. g.
based on piezo actuators, instead of using hydraulicdrives.

Fig. 1 A parallel-kinematic six-axis positioning system ismuch more compact and stiffer than a comparable
serial-kinematic structure of stacked single-axis piezo
stagesSerial kinematics has a simple design, and control of thesingle axes is not very complicated. However, it suffersfrom a series of drawbacks compared with the more
powerful and more elegant parallel kinematic systems.
The reason for this is that in a serial kinematic multiaxis
system every actuator is assigned precisely to one
degree of freedom in direction of motion.

Fig. 2 Hexapods are available in different sizes and withdifferent drive principles

Physik Instrumente (PI) GmbH & Co. KG_Auf der Roemerstrasse 1_76228 Karlsruhe, Germany
Tel. +49 721 4846-0, Fax +49 721 4846-1019 Email info@pi.ws, www.pi.ws

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WHITEPAPER – HEXAPODS IN MECHANICAL ENGINEERING
Dipl.-Phys. Birgit Schulze

Independently of the drive principle, simple integration ofparallel metrology is possible with parallel-kinematicsystems. Here all controlled degrees of freedom can bemonitored simultaneously, thus compensating guiding
errors in real time (Active Trajectory Control). This
results in numerous advantages of the system, such as
much better path accuracy, repeatability and flatness. Inaddition, the moved mass is low enabling a better
dynamic performance, which is the same for all motion
axes. Moreover, cable management is no longer anissue, because cables are not moved, and, last but notleast, the system features a substantially more compact

The latter simplifies in mechanical engineering, forFig. 3 Hexapods have been tested and proven in many
areas of application, including, for example, also
telescopes (Image: ALMA (ESO / NAOJ / NRAO))

example, the safety circuits, because the Hexapod can

Mechanical engineering can benefit from this in many

design.

only move within the easily manageable work space. At
the same time, thanks to their adaptability, Hexapods
offer good integration possibilities in a wide range ofdifferent applications (Fig. 2).

Over the last four decades, PI (Physik Instrumente) fromKarlsruhe, Germany, has become known for highcapacity and high-precision positioning systems with up
to six motion axes.

example,high-precisionelectricaldischarge

machining processes. They are suitable for parts ofclamped ideally. In this case, the electrode is moved by
the Hexapod, while the workpiece remains fixed.
This applies analogously to wire bending work in thehigh-precision range. Wherever optical methods areused for quality assurance, plates or workpieces can bealigned precisely using Hexapods. Further areas ofapplication are, for example, in laser beam control.

PI's product portfolio also includes a large variety of
powerful six-axis parallel-kinematic systems. They aresuitable for different loads, come with different types ofdrives and are designed for all kinds of ambient
Numerousforcomplex contours that, for example in milling, cannot bePositioning with up to Six Degreesof Freedom

conditions.

ways: Applications that require such high accuracy are,applications,

rangingfromhandling systems in electronics production or tool control
in precision machining, to medical technology and opticalsystems, like those found in space telescopes, can becovered (Fig. 3).

However, even if nano- or micrometer precision is not
crucial, parallel-kinematic multiaxis systems provide themechanicalengineerwithinteresting

options,forexample in workpieces that are to be provided withseveral holes. If requirements such as offset or
inclination must be taken into account, Hexapods with
their six degrees of freedom of motion also simplify the
production sequence. This applies analogously tomounting work, because even large and heavy parts canPositioning is in six degrees of freedom: three linear andbe aligned by Hexapods in such a way that they fit

three rotary motion axes. Depending on the geometry ofperfectly, for example in vehicle, aircraft and heavy

the Hexapod, rotations from a few degrees up to 60° andmachinery construction.

translations from a few millimeters to several centimeters
are possible. The repeatability and minimum incremental
motion achieve values of below one micrometer.
Physik Instrumente (PI) GmbH & Co. KG_Auf der Roemerstrasse 1_76228 Karlsruhe, Germany
Tel. +49 721 4846-0, Fax +49 721 4846-1019 Email info@pi.ws, www.pi.ws

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WHITEPAPER – HEXAPODS IN MECHANICAL ENGINEERING
Dipl.-Phys. Birgit Schulze

Synchronous Clock for the EntireAutomation Line

This makes Hexapod systems already compatible with alarge number of fieldbus systems. More fieldbus

Due to a direct connection to the master control,
Hexapod systems can be integrated basically into every

interfaces such as Profibus, CANopen, SERCOS orothers will be available if required.

automated production line. A synchronized clock withother automated components can also easily beachieved, for example, for automated supply systems,etc. (Fig. 4).
In this case, the PLC can communicate with theHexapod system over a standardized fieldbus interface.
The PLC acts as master and defines the target position
in Cartesian coordinates and the trajectories; in return, it
gets the actual positions also over the fieldbus interface.
All other calculations required to command the parallelkinematic six-axis system are done by the Hexapod
controller, i.e. transforming the target positions from
Cartesian coordinates into drive commands for theindividual drives. In this case, the controller acts just likeFig. 4 Standardized fieldbus interfaces guarantee easy
integration: Hexapod in automatic milling

an intelligent drive (Fig. 5). The cycle times for the input

Of course, the Hexapod controller can also act itself asof new positions, signal evaluation and synchronization

master for position control. In this case, however, theare between 1 and 3 ms. Fieldbus interfaces are

PLC only issues a position command. During the run, thecurrently

motion cannot be synchronized with other drive axes,

availableasexchangeable

modulesfor

EtherCAT and Profinet.

measuring systems, etc.

Fig. 5 Block diagram: In this case, the controller acts justlike an intelligent drive. The fieldbus interface can beexchanged to allow communication with numerous typesof PLC or CNC controls

Physik Instrumente (PI) GmbH & Co. KG_Auf der Roemerstrasse 1_76228 Karlsruhe, Germany
Tel. +49 721 4846-0, Fax +49 721 4846-1019 Email info@pi.ws, www.pi.ws

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WHITEPAPER – HEXAPODS IN MECHANICAL ENGINEERING
Dipl.-Phys. Birgit SchulzePrecise Trajectory Control and UserDefined Coordinate Systems

About the PI GroupIf there is no parent PLC or if synchronizing with otherheadquarters in Karlsruhe, Germany, has developed into

system components is not required, the Hexapod

the leading manufacturer of positioning systems withcontroller can now also control the trajectory based on

accuracies in the range of only a few nanometers.

G-code according to DIN 66025/ISO 6983.

With four company sites in Germany and ten sales andThe advantage of G-code is that it can be easily

service offices abroad, the privately managed company

generated by trajectory generation software tools andoperates globally. More than 700 highly qualified

has become very wellestablished in production

employees all over the world enable the PI Group totechnology and mechanical engineering. The G-code

fulfill almost any requirement from the area of innovativeinterpreter now implemented in the Hexapod controller

precision positioning technology.

allows the Hexapod system to move the workpiece orAll key technologies are developed in-house. This allows

tool smoothly and with high precision during machining
so that the mechanical system does not start to vibrate.

Over the last four decades, PI (Physik Instrumente) withthe company to control every step of the process, fromdesign right down to shipment: precision mechanics andTo adapt the trajectory perfectly to the requirements ofelectronics as well as position sensors. The required

the application, it is possible to define various coordinate

piezoceramicsystems by means of the controller, for example forsubsidiary PI Ceramic in Lederhose, Germany, one ofworkpiece and tool, which refer to the position of thethe global leaders for piezo actuator and sensor

workpiece or tool.

products.

Modular Design for Maximum
Flexibility

elements

aremanufactured

byourThe PI miCos GmbH in Eschbach near Freiburg,
Germany, is a specialist for positioning systems forultrahigh vacuum applications and parallel-kinematic

Hexapods can come with many different structures. A

positioning systems with six degrees of freedom andmodular concept allows them to be customized within acustom-made designs.

short time. The Hexapod struts are designed so that theirlength can be easily scaled. They include the requiredAuthor

electronics for reference point switch, limit switch,
position sensor and – depending on the motor –electronic commutation. Their standardized joints allow
to combine them with almost any base plate and top,whatever type of geometry. To find a suitable solution fora certain task will be much easier, in particular because
Hexapods can readily position and move even majorloads of up to several tons with the high precision that is
typical of them.
Automation control and mechanical engineering will
benefit from these advantages in many areas. Look

Dipl.-Phys. Birgit Schulze, Productmanager at Physik

forward to it.

Instrumente (PI) GmbH & Co. KG

Physik Instrumente (PI) GmbH & Co. KG_Auf der Roemerstrasse 1_76228 Karlsruhe, Germany
Tel. +49 721 4846-0, Fax +49 721 4846-1019 Email info@pi.ws, www.pi.ws

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