18/12/00 - Article: AS-Interface Specialities
AS-Interface Specialities
Special slaves to interface standard field devices to AS-Interface
The AS-Interface has
established itself as fieldbus system for the lowest field level. All leading
suppliers offer AS-Interface components because of its ease of handling and the
high cabling flexibility and especially the low costs for the user.
AS-Interface differs from other fieldbus systems because of its focus on the
lowest field level, especially on the installation of the binary sensors and
actuators.
The new specification 2.1 with its improvements as well as
AS-Interface Safety at Work offer new possibilities to use AS-Interface. Often
AS-Interface can not be used because some small but important funtionalities
can not be solved with AS-Interface. Exactly for that case Bihl+Wiedemann has
developed special AS-Interface components which enable the user to interface
standard field devices very simple to AS-Interface.
Field devices can be
connected to the AS-Interface either via market known I/O modules or the
AS-Interface slave interface can be directly integrated in the sensor or
actuator. In many applications is the connection via standard I/O modules for
different reasons not possible e.g. (too less space etc.).
On the other
side it is not useful to develop different AS-Interface slaves for each
product. Because the quantities are not so big. The development costs including
all test for CE sign, certification cost for the AS-Interface certificate could
be an insurmountable obstacle for a direct integration of the AS-Interface
slave inside the device. The special AS-Interface slaves by Bihl+Wiedemann may
offer the solution.
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| Figure 1: AS-Interface 4I/4O-Module |
The AS-Interface
4I/4O module is the PCB based solution of an AS-I slave. Because of its small
size it can be optimal used for integration cutomized devices and applications.
An example is the use of this board in push button boxes. The module can also
be mounted on moving machine parts like robot arms or gripper because of ist
small inertia.
Another posibility is their use in mounting and handling
systems. E.g. the company EPS has integrated the AS-Interface 4I/4O module
inside their Pick-and-Place-System. The handling systems can be equipped with
valves, suckers, grippers or turn modules according to customer requests.
Regarding the respective needs 2 or 3 AS-Interface modules are mounted inside
the handling system.
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| Figure 2: 4I/4O modules in Pick-and-Place-Systems by EPS |
The board is completely powered by AS-Interface. The inputs and outputs are short circuit and overload protected. If bus communication is interrupted, the outputs are switched to their currentless switching state by the watchdog. The designer can choose between connecting sensors and actuators via screw terminals to the module or embedding the board into existing electronics as a piggy back via wiring pins.
The Cylindrical
AS-Interface Actuator is the pendant to the AS-Interface 4I/4O Module for
devices with high protection. This AS-Interface module with one digital input
and 3 digital outputs in a M18x1 stainlless steel sleeve with 55mm length
offers the possibility to interface standard filed devices with high protection
in a simple way to AS-Interface.
This can be demonstrated very good with
motors with integrated frequency inverters. In transportation applications
multi speed motors are replaced more and more by the combination motor and
integrated frequency inverter.
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| Figure 3: Example application |
The above figure shows a production line with 12 conveyor belts. Each conveyor belt is variable speed driven, with the frequency inverter motors communicating via AS-Interface. For those simple drive problems the AS-Interface is the best possible bus system to transmit the control data. Only a few commands "start/stop, forwards, backwards" have to be transmitted to the frequency inverter motors. This is the optimal case for the use of a sensor-actuator-bus.
With the help of the Cylindrical AS-Interface Actuator it is possible to interface standard motors with integrated frequency inverters to AS-Interface.
This slave offers the possibility to design decentralized plant structures at an optimal price-performance ratio with the drives communicating directly via AS-Interface. In that way the motor directly has got the AS-Interface interface.
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| Figure 4: Frequency inverter moter with AS-Interface slave interface |
Via respective
reduction adapters (e.g. PG-16/M18x1) the Cylindrical AS-Interface Actuator can
be mounted in the terminal box of the frequency inverter motor. The slave
should be mounted that the two diagnosis LEDs are good visable.
The
inputs/outputs of the Cylindrical AS-Interface Actuator have to be directly
connected to the digital clamps of the frequency inverter. The external voltage
is normally supplied by the inverter. AS-Interface is connected via M12x1 round
connectors which are commonly available for flat cable and round
cable.
Beside the use with drives the Cylindrical AS-Interface Actuator can
also be used in combination with other devices e.g. valve actuators etc. Other
I/O configurations than 1I/3O are available on request.
The transmission of
analog values via AS-Interface has always been a tricky issue. In many
application there was a need to process analog values like temperature or
pressure. With the new AS-Interface specification it is possible to transmit
analog values via AS-Interface as simple as binary signals. For these reasons
the new AS-Interface slave profile 7.3 for the transmission of analog values
has been defined. According to the profil 7.3 the AS-Interface master puts the
analog slaves into operation in the same way as the digital slaves and startes
the data exchange automatically. The host system (PC, PLC, Fieldbus) can read
the 16-bit-value directly out of the AS-Interface master. The analog value
transmission between AS-Interface master and AS-Interface slave is done
invisibly for the user.
The user's advantages are abvious. He does not have
to pay attention to the handling of the data transmission. This means in the
majority of applications that the transmission time of analog values via
AS-Interface will be reduced because the transmission time depends on the
AS-Interface cycle time and not on the cycle time of the PLC program as it was
before.
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| Figure 5: AS-Interface Analog Module |
Bihl+Wiedemann has
developed AS-Interface analog modules for safe and direct connection of sensors
and actuators according to the new standardized Profile 7.3 (see figure
5).
Analog data such as pressure and room temperature is transmitted by the
module in interference-free digital signal form. The modules are simply clamped
onto the AS-Interface cable via AS-Interface penetration technology and then
directly supported by the AS-Interface master: Simple Plug and Play! There is
no need to program a function block to transmit the AS-Interface analog value
via AS-Interface. The range consists of 5 AS-Interface analog modules: 2 analog
inputs 0 -10V, 2 analog inputs 0/4 - 20 mA, 4 analog inputs PT 100, 2 analog
outputs 0 - 10 V, 2 analog outputs 0/4 - 20 mA. The modules with IP65
protection can be directly installed in the field.
The special AS-Interface slaves - the 4I/4O Module as board based solution, the Cylindrical AS-Interface Actuator inside the stainless steel sleeve or the AS-Interface Analog Modules - offer possibilities to connect also particular field devices simply to the AS-Interface and to solve in that way automation problems with AS-Interface.
Author
Dipl.-Wirtsch.-Ing. Lutz
Bieberstein
Bihl+Wiedemann GmbH
Floßwörthstr. 41, 68199
Mannheim
Fon: 0621/33996-0, Fax: 0621/3392239
eMail:
lbieberstein@bihl-wiedemann.de
Mr Bieberstein is Head of sales and marketing
at Bihl+Wiedemann GmbH