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| Fig.1: The certificate for a complete AS-Interface circuit: active
contribution to preventive maintenance |
AS-Interface is and remains the
simplest networking in automation. Moreover, it has been designed such that
self-configuration, inter-operability and compatibility allow simple, safe and
rapid setup and commissioning of a network for continuous operation. The master
monitors the operation and corrects possible individual telegram errors
automatically, reports lost slaves automatically reaccepting them or a
substitute slave.
This is the
theory and it is true. An AS-Interface circuit is usually
characterised by a very high degree of functional security (Fig. 1). There are,
however, cases in practice where possible sources of errors need to be
protected even beyond this standard. Usually this concerns machines or major
plants where occasional downtime would result in high costs or difficult
restart. Or one would like to have more detailed information for instance
concerning a series prototype to avoid expensive service operations.
For such
cases Bihl+Wiedemann now offers the certification of complete AS-Interface
circuits. In addition to the certificate (as a token of functional security)
the principal will receive a test report describing in detail the condition of
the circuit or circuits.
Benefit:
The objective of the
certification is not so much the discovery of acute errors (which are rather
rare anyway) but safeguarding high availability. This is quite comparable with
the TÜV-test of a car or a building. The principal thereby gains the
following:
- documentation of the plant
condition with identification of the slaves,
- the security
concerning the correct setup and operation of an AS-Interface
network,
- the security
of high future availability,
- if
applicable, indications of weak points which should be rectified immediately or
during subsequent maintenance,
- information
as to how and on which components improvements could be performed.
Reasons for
the certification of networks:
Repetition and error?
With AS-Interface it is important
to clearly distinguish between "Repetition" and "Error". If the receiver in
master or slave has difficulty recognising a telegram through impedance caused
deformation of the signals or external interferences this does NOT mean that
incorrect data could reach the control — AS-Interface protection is too
good to allow this.
What happens are repetitions of individual telegrams
(each of which extend the individual cycle by a mere 150
µs). Such repetitions on a small scale are quite normal and
harmless. If several repetitions do not help the affected slave is removed from
the configuration until the master is able to resume communication with that
slave. In this case a compulsory error message is issued by the master followed
by a plant stop — depending on the machine concept — availability
drops. |
Again: in
many cases certification of the AS-Interface circuits is not necessary. It is
not a "must" but an additional often cost-effective measure to safeguard plant
availability.
However,
there are a number of reasons for somewhat diluting the "plug and play concept"
of AS-Interface in this regard:
- Some errors
in the setup of a system cannot generally be detected by the network management
of the master or the individual master is unable to detect: unfavourable
routing of the cables, excessiveline length, loose contacts on connections,
excessive voltage drop in the network, component ageing, defective installation
of the functional earth, earth faults or double
addressing1.
- (Unfortunately) components are
often used in practice that have not been certified2 by the user
organisation. Experience shows, however, that these often deviate from the
AS-Interface specification to a greater or lesser degree. Interoperability is
threatened in this case. Even if a network appears to be operating smoothly
there could be faults in the background that would change the availability of a
plant or render it more susceptible to faults.
- Individual
slaves can be defective caused by manufacturing or installation
defects.
- The
insensitivity of AS-Interface to EMC faults is based inter alia on the fact
that defective telegrams can be safely detected and repeated several times with
little loss of time. Telegram repetitions are therefore allowed to a certain
extent and hardly have an effect on the cycle time. The user is not generally
aware of this. The flipside of the coin: if the background of repetitions is
excessive a minor additional fault may cause the permitted limit to be
exceeded, resulting in plant downtime. Network certification recognises
unnoticed faults, locates their causes, and increases the safety reserves
especially with regard to EMC faults, in this way clearly increasing the
availability of a system.
- More complex
slaves are increasingly employed which require data from several slaves (slave
addresses) for the transmission of a value of several AS-Interface cycles.
Their protection against possible data errors is always as good as that for
simple slaves. But the probability of incurring extended transmission times
through otherwise harmless repetitions increases with such slaves. There is
therefore also an interest in ensuring particularly stable network
operation.
- The same
applies to the transmission of safety-oriented signals (Safety at Work). Here
the safety of a system is always guaranteed through the concept and the
components. However, in order to allow quick disconnection in a dangerous
situation the number of permitted repetitions with Safety at Work was
deliberately reduced: a network is already stopped after two instead of three
failed AS-Interface cycles. This is where network certification comes in: it
discovers hidden telegram repetitions. The availability of the system
increases
- With the
increasing security in handling AS-Interface some basic rules of AS-Interface
were generally and justifiably diluted somewhat: bus termination and tuner
allow longer networks than 100 m; even slaves which are close to
be according to the specification can be partly compensated3.
According to recent considerations this can indeed be accepted but has to be
verified4. The certification of such AS-Interface circuits supplies
this verification.
- Some
manufacturers will only accept responsibility for a network if no foreign
equipment is involved. This is absolutely contrary to the spirit of
AS-Interface, which demands the interoperability of all devices from the widest
range of manufacturers. In such cases, network certification very easily serves
to confirm proper operation.
Implementation:
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| Fig. 2: Individual tests |
The
certification measurements are conducted by Bihl+Wiedemann staff using their
own equipment (Fig 2). The test is performed independent of the manufacturer of
the equipment employed. If the results of all individual tests are as expected
the certificate will be issued. It confirms that the examined circuit operated
free of problems in the given environment at the time of the measurement and
that it is likely to continue operating smoothly also in future.
The
AS-Interface circuit need not be disconnected for the tests — to be exact:
it must not be stopped5 at all so that the test is conducted under
operating conditions. This is often not only important for the system operator
so as not to interfere with the operation. Furthermore at the same time it
ensures realistic information and the high level of requirements Bihl+Wiedemann
has in a certificate. A certificate will always be issued only if the test can
be conducted under these conditions.
AS-Interface circuits still under
construction will not be covered by a certificate. But certainly the same
methods can be applied to these circuits also. Troubleshooting and fault
rectification are usually the main aspects here. Since the data of all
equipment and all telegrams in the circuit can be rendered visible (Fig. 3) it
is possible in this case to not only check the actual AS-Interface circuit but
the entire periphery including sensors and actuators. For testing a new system
this means that periphery and AS-Interface circuits can be checked separately
from a primary system and the application program — a considerable help
for the system engineer. If the application is already running on the system
optimisation potentials for the machine can also be located together with the
principal if required.
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| Fig. 3: In the event of error every single telegram can be
displayed. |
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