The robotics net interconnection concept.
Stopper, Markus
Abstract: Based on different enterprise sizes and a certain
economic regulatory environment for the use of industrial automation
solutions, their scalability is looming up large. Particularly in
robotics automation, business solutions are required, which--with a
preceding increase of the number of robots in a factory--do not demand a
paradigm change of established solutions or rather implemented concepts
for the same kind of problem scenarios. The RoboticsNET architecture
solves this circumstance for networked robots by introducing a highly
scalable robot interconnection design based on a fully switched network
layout. An increasing number of robots within the robot network do not
affect the corresponding technical parameters respectively the principal
system behaviour. This paper describes the RoboticsNET fundamental
interconnection concept with the focus on scalability regarding rising
robot counts in the network and having in mind the use of it within a
manufacturing execution system environment.
Key words: open robot interface architecture, industrial software
standards, robot real time interconnection, scalable network concept,
robotics net
1. INTRODUCTION
Networked robots are still a niche in automation solutions. Taking
a closer look to installations already set up, results in finding mainly
proprietary solutions without a uniform continuity. RoboticsNET--certain
aspects of it are described in this paper--is one of the first
approaches to build up a uniform communication infrastructure for
industrial robots.
2. ARCHITECTURE BASE DESIGN
For the analysis and design of an open, extensible and scalable
architecture, which fulfils the up to date requirements of robot
technology on state of current IT technology, the prerequisite of
continuity is mandatory. In other words this condition requires the
patency from connecting the sensor actor level to robot controllers as
far as running up to the office and ERP level which finally leads to the
design of the architecture containing the following main components:
* Central server a host computer in the role of a central server
for database & web. The host collects and saves data from the
various data collectors and enables client web access to robots.
* Data collector server a data collector server is a server
computer which on the one side communicates with the robot controllers
on the network and on the other side with the central server hosting the
database and web services. The introduction of this concept is mandatory
to guarantee the scalability of the system for large installations. For
instance in large installations the data collector server buffers write
demands to the SQL database on the central server during high load
phases (several robot controllers write simultaneously). Events will not
be buffered but passed through directly as well as the messages from the
central server via data collector servers to robot controllers.
* Robot controller one ore more robot controllers with standard
Ethernet network access and the availability of an appropriate
application programming interface (ABB, 2005a).
* Network a network according to the characteristics described in
the following sections.
* Communication protocol a TCP/IP based communication protocol
supporting the robot controller application programming interface.
* Client local or remote PC's which are able to access the
central server respectively the hosted applications by an internet
browser (ABB, 2005b).
On account of the RoboticsNET scalability characteristic described
in this paper, the information flow from or rather to the robot
controllers can be distributed over several data collector servers. The
implementation of the data collector server's interrogator can
either be based on polling or an event driven mechanism. Spontaneous,
asynchronous messages are always event driven. Due to performance
reasons regarding load balancing, the data collector server implements
an in-memory buffer, which also serves short, temporary connection
interrupts. Being offline for a while leads to data loss. For a warm
restart without data loss respectively error recovery the possibility of
using a message queuing server exists to ensure data integrity. However,
this leads to a performance decrease which is in contrast to the
architecture's real time requirements.
3. REAL TIME CONSIDERATIONS
One of the major constraints for the network design of RoboticsNET
was the usage of standard hard- and software from the office world with
all its advantages and disadvantages. Since the aimed application
scenarios the time frames can be significant higher than upper limits
for real time control tasks via network (e.g. movement control of a
milling cutter in a machining centre or control of drive systems) the
determinism of communication becomes more important in comparison to the
representative time frame concerning speed. The developed system design
within the issue of real time Ethernet difficulty can be described as
follows.
The tough requirement for real time properties in wide areas of
automation was always the strong argument against the introduction of
Ethernet technology with the CSMA/CD access method as a base
communication system. Ethernet uses a user shared media realizing the
network functionality. Every participant can send a network package
after checking the media for being unused.
However, simultaneous transmitting and the finite velocity of
propagation may lead to collisions on the media. CSMA/CD takes care of
such collisions and retransmits the packages so that no package will be
lost and finally reaches the target participant. The thereby elapsed
time however, can not be predefined since the MAC access method is a non
deterministic method, which excludes the usage in the area of real time
systems.
The strong development progress in network switching technology
conversely enables the avoidance of the described difficulty if used
properly. Once a package arrives at the switch from a network segment, a
check for the target segment is done. The forwarding just takes place if
the target segment is being unused, which means, that currently no other
package is on that segment. Exists already a package on the segment, the
switch latches the package and transmits it immediately after the target
segment gets free again. Therefore a switch applies a separate channel
between the respective source and target port. But using the switching
technology for the realization of real time operation requires two
additional strong boundary conditions:
* maximum micro segmentation of the switched network (exactly one
participant per port)
* full duplex operation of all used components Considering these
two conditions without exceptions results in avoiding collisions and
blanks the stochastic access method. Further the full bandwidth of the
network is available for every participant. In order that the periods
for package transmission can be appointed with declaration of an upper
limit which results in determinism (non-blocking design with uncritical
total delay). Of course, the backplane of the used switches must have an
adequate bandwidth. For the aspired application scenarios a lowest
switching capacity of 12 GBit/s and a forwarding rate of 9 Mpps are
required (twenty-four 100 MBit/two 1 GBit Ports).
4. SCALABLE INTERCONNECTION CONCEPT
Based on different enterprise sizes and a certain economic
regulatory environment for the use of industrial automation solutions,
their scalability is looming up large. Particularly in robotics
automation business, solutions are required, which--with a preceding
increase of the number of robots in a factory--do not demand a paradigm
change of established solutions or rather implemented concepts for the
same kind of problem scenarios. The fundamental interconnection concept
has therefore be focused on scalability regarding rising robot counts in
the network and having in mind the use of it within a manufacturing
execution system environment (Stopper, 2005).
On the base of a fully switched network RoboticsNET provides a
reasonable delimitation founded on current available standard hard- and
software for the usage in small, medium and large installations. The
empirical tests have yielded in describing three classes of
installations. In small installations up to eight robots, the central
server and the data collector server can be hosted on the same machine
without performance issues. Above that, a separation to different host
computers is required. Up to 24 controllers can then be connected to a
data collector server which corresponds to a medium installation. In
large installations additional robot controllers can be supplied by
adding data collector server's step by step (max. 24 controllers
per data collector). The central server machine has to be designed for
high availability (Stopper, 1999). Figure 1 shows the scalable layout
for e.g. 216 robot controllers.
[FIGURE 1 OMITTED]
Robot controllers are connected with 100 MBit full duplex per port.
The connection of the data collector server as well as the connection up
to the GBit backbone (through which the central server is connected)
takes place via the two GBit uplink ports. For that reason a flat
network structure is the result which concurrently solves the
'many-to-one-traffic' problem too. This difficulty arises by
maximum micro segmentation of a switched network whether a lot of
participants access one micro segment with high frequency (e.g. typical
server operation). That micro segment represents subsequently a
bottleneck within the communication system. By usage of uplinks with a
higher bandwidth the problem is bypassed. If there are no such special
ports on a switch, usually the possibility for bundling of ports
(link-aggregation) to one or more uplinks with higher bandwidth exists.
To reach more performance on higher protocol levels, it is recommended
to use packet oriented protocols (e.g. UDP prior to TCP).
5. CONCLUSION AND FURTHER RESEARCH
For the RoboticsNET architecture an increasing number of robots
within the robot network do not affect the corresponding technical
parameters or rather the principal system behaviour. The real time
behaviour on the level of the network is created by the stringent use of
a fully switched infrastructure with certain additional conditions
avoiding collisions on the Ethernet which results in determinism. For
guaranteeing the real time data accessibility and the scalability of the
system, network micro segments are built with connections to data
collector servers. Data collector servers are common server machines
with an in-memory database, parameterized schedulers and software agents
which are realizing the connection of the various robot controllers to a
central host system with a persistent database. The aimed continuity of
real time requirements within the RoboticsNET architecture considering a
deterministic sense was realized from hardware side up to network level.
Currently the software infrastructure uses legacy protocols and
parameterized standard operating systems to already enable applications
on that platform which lack required real time considerations and
determinism. That case has to be improved within further research. One
promisingly approach is in the direction of service oriented
architectures (SOA) regarding software infrastructures. The point of
central interest in SOA philosophy is platform independent
interoperability. SOA conform implementations just should apply the
process logic of the correlative applications. The manner how data can
be exchanged, whether data transmission is encrypted or not and with
which protocols software components communicate, adequate real time and
quality of service requirements as well as further infrastructural
problems, should be solved just by parameterization and configuration.
6. REFERENCES
ABB (2005a). Robot Application Builder--FlexPendant Software
Development Kit Users Guide, ABB Automation Technologies AB, Robotics,
3HAC 024914-00, RobotWare v5.07, Molndal, Sweden
ABB (2005b). Robot Application Builder--PC SDK Users Guide, ABB
Automation Technologies AB, Robotics, 3HAC 024913-001, RobotWare v5.07,
Molndal, Sweden
Stopper, M. (2005). Virtual Engineering for Industrial Robotic Work
Cells, Final Program and Abstracts of the 4th Asian Conference on
Industrial Automation and Robotics (ACIAR'05), Paper ID-Code: F-35,
ISBN 974-8208-58-3, 2005, Bangkok, Thailand
Stopper, M. (1999). A Concept for Enterprise Automation Networks,
Proceedings of the 10th International DAAAM Symposium, pp. 533-534, ISBN
3-901509-10-0, October 1999, Vienna, Austria
