WHITEPAPER – Industrial Safety Device for Hexapods-- Dr. Christian Sander, Jens Matitschka --Industrial Safety Device for Hexapods
How to Connect a Safety Light Barrier to a Hexapod System

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Page 1 of 8

WHITEPAPER – Industrial Safety Device for Hexapods-- Dr. Christian Sander, Jens Matitschka --1 Motivation
Strict regulations apply for the safety of personnel inmanufacturing environments. When fast movements arecarried out and large forces act, it is necessary to take special
safety measures. Typically barriers, e.g. fences that spatiallyseparate people from the machines, are common andrelatively easy-to-integrate solutions. However, if mechanicalsystems cannot be installed or if the work process is
influenced by them, contact-free safety concepts such as alight grid or a light curtain can be used. A light curtain forms aclose-meshed protective field and, therefore, secures theaccess to the danger zone.

system as well.
In stacked systems, however, the lower drives not only haveto move the mass of the payload, but also the mass of thedrives that follow, along with their cables. This reduces the
dynamic properties. Guiding errors that accumulate on theindividual axes also impair accuracy and repeatability.

Hexapod robots are multi-axis positioning systems with alimited workspace that can often be safely integrated into
industrial setups. However, if people can move into theworkspace of the hexapod or of the setup on its motion
platform then a safety measure must be set in place.
This current white paper explains:





1.1

when it is useful and necessary to use a safety device;
how a safety light barrier (or light curtain) works;
how the risk assessment for hexapods is carried out;
how the safety device can quickly be integrated in thecontroller environment.

Parallel-Kinematic Hexapods Compared
to Serial-Kinematic Systems

Serial-kinematic systems consist of individual axes or
actuators that build on one another, which means that they
are mechanically connected in series. For example, aplatform-bearing Z axis can be mounted on a Y axis, which inturn can be mounted on an X axis (Fig. 1) and so on.
In hexapods, all six actuators act directly on a single platform.
This allows a considerably more compact setup than is thecase with serially stacked multi-axis systems.
Since only a single platform is moved, which is also often
equipped with a large aperture, the mass to be moved isconsiderably smaller. This results in a considerably improved
dynamics.
In addition, hexapods exhibit higher accuracy, since theyusually do not contain guides with corresponding guide errors
and the errors from the individual drives do not compound.
Since the cables are not moved with the platform, theprecision is not influenced by corresponding friction or
torques. In addition, the parallel structure increases thestiffness and thus the natural frequencies of the overall

Fig. 1 Functional scheme of parallel-kinematic hexapods in comparison
with serially stacked axes

1.2

Fields of Application for Hexapods in
Automation

Manufacturing and quality assurance processes in electronics
production, mechanical engineering, and the automotive
industry increasingly require compact multi-axis positioning
systems. At the same time, the requirements for accuracy arealso increasing.
Depending on the model, parallel-kinematic hexapod robots
can move and position tools, workpieces, and complex
components with masses ranging from a few grams to several
hundred kilograms or even several tons in any spatial
orientation with high precision – regardless of the assemblyorientation.
The PI hexapod controller with integrated EtherCAT®
interface makes it easy for users to integrate hexapods into
automation systems without having to do challenging
kinematic transformations.
An essential feature of the hexapods are the freely definablereference coordinate systems for the tool and workpiece
position (Work, Tool). This allows the movement of thehexapod platform to be specifically adapted to the respective
application and integrated into the overall process.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Page 2 of 8

WHITEPAPER – Industrial Safety Device for Hexapods-- Dr. Christian Sander, Jens Matitschka --1.3

Safety-Relevant Applications

Generally, the risk of injury due to hexapods, which are used
for precision positioning in six degrees of freedom, is low
because of the low speed of movement and the limited rangeof movement.

Fig. 3 Dynamic motion hexapods are used for motion simulation purposes,
for example

2 Function of the Light Curtain
Fig. 2 The workspace of a hexapod is limited

The situation is different for dynamic motion hexapods (seeFig. 3) because of their high speed and acceleration which canbecome a hazard for people who work in their immediate
workspace. Mainly, this is due to the limited human reaction
time to quickly remove endangered body parts from a given
danger. When a collision occurs high impulse forces due tomass inertia and crushing of limbs are possible. A safetysystem can protect people and minimize this risk of injury.
Depending on the version, PI hexapod controllers feature a
motion stop input (e.g. C-887.522). The input is used forconnecting external hardware (e.g. push buttons or switches)
and it deactivates or activates the power supply of thehexapod drives. However, the motion stop socket does notoffer any direct safety function in accordance with applicablestandards (e.g. IEC 60204-1, IEC 61508, or IEC 62061). [2]Hereafter, the integration of a light barrier (or safety curtain)in the control of the hexapod is described. The light barrier
itself is compliant and approved in accordance with EN/IEC
61508 and EN/IEC 61496-1/-2.

Safety light curtains are made of an emitting (transmitting)and a receiving unit. In the emitting unit, light sources thatradiate toward the receiving unit are arranged at an evendistance from each other (e.g. LEDs). The receiving unit isequipped with detectors in the same arrangement (e.g.
photodiodes), each being responsible for the detection of its
opposite light source. The resolution, i.e. the distance
between the light sources, determines the level of protection
provided (finger, hand, or body protection).
If one of the light beams is interrupted, the light barrier
outputs the information via two so called OSSD (Output
Switching Signal Device) outputs, which nowadays are
standard for barrier-free safety devices. In the standard state,
the outputs are at 24 V and fall to 0 V briefly andasynchronously from each other for the purpose of selfmonitoring. To be able to interpret the output signals, a
suitable and certified safety relay is required that switches off
or stops the machine movement in the event of a failure.

3 Safety Requirements and Risk
Assessment
Different things need to be considered when selecting anddimensioning the safety device.
The relevant EN ISO 13849-1 standard specifies thesafety levels for the components used.
1.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Page 3 of 8

WHITEPAPER – Industrial Safety Device for Hexapods-- Dr. Christian Sander, Jens Matitschka --With the EN ISO 13855 standard the minimum distance
between the safety device and the danger zone is calculated.
2.

Depending on the degree of protection, the minimum
distances need to be increased accordingly.
3.

The safety level is defined as described in the EN ISO 13849-1
standard. With this standard, a tree diagram can be used todetermine the safety level that a system must meet. In Fig. 4
the risk diagram according to the EN ISO 13849-1 standard is
depicted. To reach a defined safety level, all safety
components must meet the requirements of that level.[1]beams is large. Therefore, the safety distance has to be
increased by 850 mm (length of the arm). This is also the casewhen the safety device is not high enough and a person could
reach above or below it [1]. For our test implementation, aclose-meshed light barrier with a resolution of 30 mm waschosen and, therefore, the safety distance did not need to be
increased.
In the test setup, it was assumed that at a speed of 100 mm/sof the hexapod, the required stopping distance was 3 mm.
Using the following formula (applies to light barriers with aresolution of < 40 mm):𝑆 = 𝑣(𝑡1 + 𝑡2 ) + 8(𝑑 − 14)
Where the safety distance is S, v is the approaching speed, t1
and t2 are the triggering time of the electronics and braking
time of the machine, d is the resolution of the light barrier.
For our test setup, this resulted in a safety distance of
316 mm from the hexapod.

Severity of the injury
Minor injury (normally reversible)
Serious injury, including death (normallyirreversible)

S
S1
S2

Frequency and/or duration of the exposure to
the hazard
Seldom to more often and/or short duration
More frequent to continuous and/or long
durationF

Possibilities for avoiding the hazard
Possible under certain conditions
Hardly possible

4 Implementation in the
Laboratory

F1
F2
P
P1
P2

Fig. 4 Risk diagram and evaluation criteria according to EN ISO 13849-1.

The minimum distance between the safety device and thedanger zone is calculated with the EN ISO 13855 standard.
This dimension is important to be able to define themaximum range the light barrier must monitor. It should be
noted that different speeds of approach apply to the different
degrees of protection (person, hand, or finger). Depending on
the degree of protection, the required distance may have to
be increased as necessary.
The safety distance for optoelectrical safety devices for handand finger protection is calculated according to EN ISO 13855.
For this purpose, an approaching speed of 2000 mm/s istaken into account. Depending on the degree of protection
the safety distance must be increased accordingly. For
example, when protecting a whole person, it is possible tograb through the light barrier since the distance between theFig. 5 The hexapod is surrounded by the safety light barrier on four sides.
1: Hexapod. 2: C-887 hexapod controller. 3: Safety light curtain. 3a:
Emitter. 3b: Receiver. 3c: Mirror. 4: Safety relay, relays, power
supply. 5: Emergency-stop switch, reset button

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Page 4 of 8

WHITEPAPER – Industrial Safety Device for Hexapods-- Dr. Christian Sander, Jens Matitschka --4.1

Mechanical Setup

Thanks to the three deflecting mirrors, the mechanicalalignment of the components of the safety light barrier canbe efficiently achieved with only one laser that emits light inthe visible range. At the beginning, all three columns need tobe adjusted vertically to the surface using setscrews.
Following that, the laser can be mounted to the emitting andreceiving unit and then one column after the other can becentered to each other.

monitoring ensures that resetting is only possible when thesafety relay switched before the failure. The coils of the relays
must, therefore, be connected to the safety contacts of thesafety relay.[3]The information has to be passed on from safety relay tocontroller so that the program stop can uniquely traced backto the safety device, the light curtain. In the test setup, an
auxiliary output, which can also be used to display the status,
is connected to the I/O connection of the controller via afurther relay. This I/O connection has digital inputs as well as+5 V and GND outputs. These can be connected with achangeover contact on the relay in such a way that thecontroller has a High or Low signal at the digital input, which
can then be queried in the program.
The wiring is depicted in Fig. 7.

Fig. 6 Schematic alignment

The schematic alignment can be seen in Fig. 6 in the planview. The laser beam can be seen when a paper is placed in
between the mirrors. This is then centered on the mirrors andfinally aligned with the receiver. When the emitter and thereceiver are connected electronically, the LEDs on thereceiver show that the alignment was successful.

4.2

Integration with the Electronics

The supply line between controller and hexapod is
disconnected and then switched via the relays of a safety
relay. Thereby, Stop category 0, per IEC 60204-1 isimplemented, and the circuit offers the safety characteristics
described in the EN ISO 13849-1 standard. The outputs of thelight barrier are connected to the input contacts of the safety
relay and, if required, an emergency stop switch can beconnected in series. This is unproblematic from a safety pointof view since series connection does not endanger safety.
Various options exist for resetting the safety relay after it was
triggered. The easiest solution is an automatic start by
applying 24 V to the reset/start contact. For a manual start, abutton can be installed between the 24 V source and thecontact. An additional feedback loop monitoring can be
implemented via two relays with NC contact, which are eachconnected in series to the button. A feedback loop

Fig. 7 Wiring diagram for connecting the C-887 hexapod controller to theemitter and receiver of the described light curtain

4.3

Implementing the Safety Function in the
GCS Software from PI

In this chapter, an example is shown of how the safetyfunction can be implemented when written in the Python
programming language for the PI Python library. It is always
possible to choose a different software solution or to use a
PLC to connect the hexapod, for example, via EtherCAT. The
PI Python library facilitates the use of the General CommandSet (GCS) from PI. GCS offers a standardized set of commands
that is independent of the connected controller or the drive
principle used.
When using Python, each controller error throws an
exception in the running program as soon as the next GCS

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Page 5 of 8

WHITEPAPER – Industrial Safety Device for Hexapods-- Dr. Christian Sander, Jens Matitschka --comman is executed. A controller error occurs, for example,
when the motor voltage is disconnected by the safety circuit.
This is why, in Python, try-except blocks can be used toensure a controlled continuation of work after the activationof the safety function.

Table 1: Parts list for safety light barrier (Pilz GmbH)[3]

In the following example, a Python program is shown whichmoves the hexapod in an infinite loop in the Z axis. After thesafety relay is triggered, the program tries to switch thehexapod's servo back on. The safety relay can be reset to its
normal operation by pushing the reset switch. The hexapod
controller keeps retrying to switch on servo. This only
succeeds when the safety relay returns to normal operation
while no new controller error has occurred.
from pipython import pitools
from pipython import GCSDevice
import timewith GCSDevice() as pidevice:
pidevice.ConnectTCPIP('172.16.244.54')
print('connected:{}'.format(pidevice.qIDN().strip()))
position = 0#----Start the monitored movement------------------while True:
print("Start the monitored movement")#----Switch servo on and start reference move-----if pidevice.qSVO()['Z'] == False:
pidevice.SVO('Z', 1)
if pidevice.qFRF()['Z'] == False:
pidevice.FRF('Z')
pitools.waitontarget(pidevice, 'Z')#----Loop-------------------------------try:
print('Loop started')
while True:
if position == 0:
pidevice.MOV('Z', 0.5)
pitools.waitontarget(pidevice,'Z')
position += 1
if position == 1:
pidevice.MOV('Z', 0)
pitools.waitontarget(pidevice,'Z')
position += 1
position = 0#----#----Motion stop exception--------------------------------------------------------except:
print('Try to switch on servo')
if pidevice.qDIO(1)[1] == False:#Search for state of the safety relay
while True:try:#Try to switch on servo
pidevice.SVO('Z', 1)
break#If successful, leave servo switch on loop
except:#If not successful, wait for a short period of time
time.sleep(2)

4.4

Item

Order no.

Quantity

PNOZ s3 24VDC 2 n/o

750103

1

PSEN op2H-s-30-075/1

630724

1

PSEN op Protective Column-090/1

630951

2

PSEN opII mirror column-090 Set

632008

3

PSEN op cable axial M12 4-p. shield.
3m

630303

1

PSEN op cable axial M12 5-pole 3m

630310

1

PSEN op Protective Bracket-4/1

630956

2

Table 2: Other electronics

Item

Quantity

24V power adapter

1

Relays with changeover contact

1

Relay with NC contact

2

Emergency stop switch and reset button

1

Control line

Table 3: PI mechanics and electronics[4]ItemQuantity

H-840.D2 hexapod mechanics

1

C-887.522 hexapod motion controller

1

C-887.MSB motion stop box

1

Parts for Test Setup

In Table 1, 2, and 3 all required parts are listed for the safety
light barrier described in this paper.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Page 6 of 8

WHITEPAPER – Industrial Safety Device for Hexapods-- Dr. Christian Sander, Jens Matitschka --5 Summary

7 Authors

The risk assessment for the hexapod in the test setup was
carried out according to the EN ISO 13849-1 standard. As asuitable safety device with the highest degree of protection(finger), a safety light barrier (or light curtain), was set up
around a hexapod and integrated into the controller circuit.
As an example, programming with the help of the PIPython
library enabled a fast and reliable startup. The Stop category0, per IEC 60204-1 safety function was, therefore, successfully
implemented.

6 Sources
Valid Standards

Dr. Christian Sander, Senior Expert for Hexapod Development
at PI (Physik Instrumente).

EN ISO 13849-1 „Safety of machinery - Safety-related parts ofcontrol systems – Part 1"
EN ISO 13855 „Safety of machinery - Approach speed of partsof the body for the positioning of safety devices"[1]

Neudörfer, Alfred: Konstruieren sicherheitsgerechter
Produkte. Berlin, Heidelberg : Springer Berlin
Heidelberg, 2016 — ISBN 978-3-662-49818-7[2]

Siemens AG: Einführung und Begriffe zur
funktionalen Sicherheit von Maschinen und Anlagen( Nr. E86060-T1813-A101–A5). Nürnberg, 2010

[3]

Pilz GmbH & Co. KG: Das Sicherheitskompendium, 5.
Auflage ( Nr. 775 740 8-8-1-0–100). Ostfildern, 2017[4]

Physik Instrumente (PI) GmbH & Co. KG: PI
Hexapoden. URL
https://www.physikinstrumente.de/de/produkte/he
xapoden-parallelkinematiken/. - abgerufen am 201907-22

Jens Matitschka, Design Engineer for Hexapod Development
at PI (Physik Instrumente).

EtherCAT® is a registered trademark and patented technology
licensed from Beckhoff Automation GmbH, Germany.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

Page 7 of 8

WHITEPAPER – Industrial Safety Device for Hexapods-- Dr. Christian Sander, Jens Matitschka --8 PI in Brief
For many years, PI (Physik Instrumente), founded in 1970, hasbeen a market and technology leader for high-precision
positioning technology and piezo applications in thesemiconductor industry, life sciences; photonics, and inindustrial automation. In close cooperation with customers
from all over the world and for 50 years now, PI's specialists(approx. 1,300) have been pushing, again and again, theboundaries of what is technically possible and developing
customized solutions from scratch. Technologies from PI
achieve reproducible accuracies in the millionth of amillimeter range. More than 350 granted and registeredpatents underline the company's claim to innovation.
PI develops, manufactures, and qualifies all core technologies
in-house, thereby constantly setting new standards for
precision positioning: Piezoceramic patch transducers andactuators, electromagnetic drives, and sensors working in thenanometer range. As the majority owner of ACS Motion
Control, PI is also a leading global manufacturer of modular
motion control systems for multi-axis drive systems anddevelops customized complete systems for industrial
applications with the highest precision and dynamics.

Physik Instrumente (PI) GmbH & Co. KG, Auf der Roemerstrasse 1, 76228 Karlsruhe, Germany
Phone +49 721 4846-0, Fax +49 721 4846-1019, Email info@pi.ws, www.pi.ws

WP4025E Industrial Safety Device for Hexapods 01/2020 0 Subject to changes. © Physik Instrumente (PI) GmbH & Co. KG 2020

With six manufacturing sites and 15 sales and service offices
in Europe, North America, and Asia, PI is represented
wherever high-tech solutions are developed andmanufactured.

Page 8 of 8