industrial wireless book special edition - Networking ...

Wireless avoids cable trouble on
electroplating line automation
Easily damaged and maintenance prone trailing cables on the
suspended monorail of a plastic part electroplating line have been
eliminated through the combined use of an industrial wireless LAN,
Profisafe and an extension of IEEE 802.11 through iPCF. Christof
Kreienmeier explains.
ELECTROPLATING SPECIALIST, BIA Kunststoffund
Galvanotechnik GmbH of Solingen, Germany
has, together with Aucos Elektronische Geräte
GmbH of Aachen, planned, built and commissioned
a new electroplating production line.
Unusually, the suspended monorail tracks above
the electroplating tanks are not powered via
trailing or armoured wire cables, but communicate
with the control system using Profisafe
via wireless LAN (WLAN).
This approach was chosen because cables
collect contamination and dust particles that
end up in the tanks, and because the everincreasing
speeds of the trolleys (typically up
to 120m/min) result in cable breaks. The
resulting production losses can be very
expensive. BIA suggested replacing the trailing
cables with the only viable alternative – a
radio-based industrial solution that had to be
capable of providing cyclic and time-critical
data traffic at predictable transfer rates. This
would ensure reliable emergency stop circuit
functioning.
Industrial Point Coordinated Function – speed equates to safety
In safety technology, the primary issue is transferring safety signals as quickly as possible to the control
to head off critical situations. It is therefore necessary to assign the available bandwidth clearly, and
to afford safety signals the highest priority. Siemens implements this by extending the IEEE 802.11
standard with the Industrial Point Coordinated Function (iPCF) a transmission protocol that – in contrast
to the DCF method – governs access by multiple participants to the wireless network deterministically.
IPCF is a proprietary process that supports roaming in a WLAN infrastructure at times of 50ms. Both
AP and client must support rapid roaming. In automation technology, it is often necessary to transfer
process data and alarms in real time. Existing DCF and PCF roaming procedures don’t allow such realtime
data transfer without interruption.
iPCF is based on the centralised PCF procedure, already described in the first WLAN standard IEEE
802.11 in 1999. With PCF, the AP polls the connected stations and assigns time slices for data
transmission. In contrast, iPCF works with a proprietary modified data frame protocol structure, so that
iPCF clients can log on to an AP on which iPCF is enabled. The AP therefore becomes the ultimate
authority and assigns transmission times to clients as time slices. This produces stable and predictable
response times (typically 16ms with five to eight clients) with intensive use of the available bandwidth
capacity in the radio network. For this reason, iPCF is very suitable for typical production fieldbuses
that achieve their real-time behaviour through short cycle times.
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Controlled wireless field
Siemens supplied the wireless automation
technology, which included rapid-roamingenabled
IP65-protected industrial access points
(APs), plus leaky feeder cables (Fig. 1) to
provide a controlled radio field along the tracks.
Aucos chose a failsafe controller that also
operates the Profinet system connection to
Aucos’ control system. The controller connects
to a managed switch having eight electrical ports
that communicates with four industrial WLAN
rapid roaming APs (IEEE 802.11-compliant)
distributed in the field. These do not set up a
typical omni-directional wireless network in free
space, but use the special leaky feeder cables to
create a uniform field strength along the four
tracks that can be reliably monitored.
Their counterpart in each suspended monorail
trolley is the directional transmitting and
receiving antenna situated alongside the leaky
feeder cable to allow optimal communication.
In the trolley vehicle switch box, each antenna
is connected to a WLAN device used as a
gateway between WLAN and Profibus,
forwarding the relevant Profibus and safety
signals. Failsafe signal modules, motor starters
and frequency converters for the suspended
monorail trolleys completed the Siemens
supplied equipment.
Despite the advanced automation technology,
plastics electroplating still needs much manual
intervention, so there are no barrier fences.
Ensuring operator safety at all times depends
on the reliability of the Profisafe communication
components, as well as a clear prioritisation
of safety signals.
In a fixed installation, the rapid roaming APs
support the free movement of several nodes in
the wireless network and can transfer them
seamlessly from one AP to another. This mobile
installation uses rapid roaming to ensure high
signal transmission speed to and from the
trolleys using the extended iPCF protocol (see
box). It therefore ensures that the safetyrelevant
data is transmitted to the controller
with a cycle time of only 16ms, allowing
predictable response times well below 100ms
– more than sufficient for time-critical
emergency-stop decisions. In addition, process
times can be controlled very precisely.
Fig. 1. With their IP65 enclosures the access points are
installed freely in the field and connected directly to the
RCoax leaky feeder cable (blue).
Interference-free
The leaky feeder cable provides safety in a very
different way. Aucos installed it parallel to the
suspended monorails in strands of around 50m,
ensuring that the field strength along the line
provides reliability. Because the radio field is
directional, it offers optimum reception, even
with significantly reduced field strength. As a
result, electromagnetic exposure is reduced –
and only a few meters away, the radio network
is practically too weak for external users to
receive it. Advanced encryption algorithms
mean that there is effective protection against
unauthorised access.
The decision to use the 5GHz band was made
to avoid interference caused by wireless
sensors, Bluetooth devices and other devices
that operate in the more common 2.4GHz band.
The wireless components use both frequencies
as standard.
BIA is satisfied and says that in future, new
systems will always be equipped with this
WLAN solution, being found to be far superior
to the maintenance-prone trailing cable. It is
quieter, a better system overview is now
possible and the transport trolleys have
virtually unlimited route options. One
unplanned downtime can easily cost several
thousand euros, but this system has eliminated
cable breakages.
Christof Kreienmeier works for Siemens AG’s Industry
Sector – Industry Automation in Düsseldorf, Germany.
First published in the industrial ethernet book July 2011
PHOTO: SIEMENS
Case Study
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