22/12/99 - Article: AS-Interface Slaves for Frequency Inverters
The AS-Interface has become the standard bus system to network binary sensors and actuators at the lowest field level. In industrial plants the problems of electrical drive technology have been solved conventionally. The following article is about how to combine frequency inverters and the AS-Interface in an effective way and what advantages the user gets using this combination.
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.
The product range starts with different
types of masters and gateways, and expands to include integrated AS-Interface
sensors, I/O modules for cabinet mounting (IP20) and field mounting (IP67) as
well as intelligent actuators. The multifunctional variable speed drives of
different suppliers are used for machinery building as well as for industrial
plants. The frequency inverters’ possibilities to optimize processes and
to save energy can is particularly effective in new innovative machines and
plants when used in combination with communication interfaces, which integrate
the drives in the best possible way into the automation environment.
This environment is also influenced by sensors. For this reason
Bihl+Wiedemann has developed AS-Interface slaves together with the various
drives suppliers to solve these Automation problems which do not need the use
of powerful fieldbus systems thus saving costs for the user.
The example of a production line demonstates the benefits which result
out of the use of frequency inverters in combination with the AS-Interface.
Picture 1 shows a production line with 12 conveyor belts. Each
conveyor belt is variable speed driven. The power cabling between frequency
inverter and and the decentrally located motors is conventionally installed.
The parallel wiring starts at the central cabinet and ends at the particular
motor. This parallel wiring means a big cabinet with all PLC I/O slots as well
as all frequency inverters inside.
Picture 2 shows a cabling
alternative for the same production line, with frequency inverters or frequency
inverter motors decentrally located and communicating via AS-Interfacenterface.
In opposite to the parallel power wiring only two cables go through the plant -
the AS-Interface to transmit the control signals and the the power bus.
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| Picture 1: Traditional concept with parallel wiring |
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| Picture 2: Concept with decentral frequnecy inverter motors communicating via AS-Interface |
As you can see from this example there are possibilities to minimize the cabinet and the power cabling if AS-Interface is used in combination with decentrally used frequency inverters. Further advantages are the possibility to design modular plant structures, to minimize the installation time and test time. This all leads to an earlier start of production which means less „dead“ money.
To connect frequency inverters and frequency inverter motors to the AS-Interface Bihl+Wiedemann offers slave modules in protection class IP20 for frequency inverters and in protection class IP65 for frequency inverter motors (see picture 3). The main operation fields of these AS-Interface slaves are transport systems, filling and packaging machines as well as handling machines. Up to now AS-Interface slaves have mostly been used in protection class IP20. These slaves are mounted together with a frequency inverter in a decentral clamp case near to the motor.
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| Picture 3: AS-Interface slaves for frequency inverters with IP20 and IP65 protection class |
But these slaves are also used in machines’cabinets e.g. easy packaging machines, with the PLC only having an AS-Interface master to connect the decentralized I/O modules. The slaves in protection class IP65 are used in combination with frequency inverter motors. In particular integrated frequency inverters are suitable for easy decentralized plants with the nodes communicating via AS-Interface.
The AS-Interface slaves for frequency inverters and frequency inverter
motors consists of a 4I/4O module as a AS-Interface bus interface as well as a
serial interface to communicate with the frequency inverter. The respective
inverter protocol is implemented on the slave. The serial communication between
the AS-Interface slave and the frequency inverter is invisable to the user.
This means the slave can be used as a normal 4I/4O module resp. analog module.
On the other side the access via the serial interface of the inverter offers
the best possible connection to use the large functionality of the frequency
inverter. The use of the serial interface also reduces the cabling between
AS-Interface slave and frequency inverter. Similar to the As-Interface, the
frequency inverter is connected using a two wire cable. The AS-Interface slaves
with a high protection class can be used in an identical way to devices for
cabinet mounting. This allows frequency inverters from different suppliers can
be used in the same way.
Which functionality is implemented on the
AS-Interface slaves? This question can best be answered with application
samples. Often multi-speed motors have been replaced by a combination of
frequency inverter and motor. For these applications the AS-Interface is
particularly suited. The user has just to set a few bits in his application
program and various speeds can be adjusted. In „normal operation“ the
AS-Interface Slave devices work as 4I/4O modules for AS-Interface. With the use
of the 4 AS-Interface output data bits per AS-Interface Slave up to 7 preset
speeds as well as the direction (forward/ reverse direction) can be controlled.
The frequency inverter sends bit status information to the PLC via the 4
AS-Interface input data. This offers the possibility to evaluate diagnostic
messages from the inverter in the PLC.
A communication error between
Frequency inverter and AS-Interface slave will of course be detected and
displayed via the AS-Interface input data. In transportation plants there is
often a need to have 1 or 2 binary sensors near the driving motor e.g. to stop
the conveyor belt in case of an tail back or to increase the speed in case of
shortage of products on the belt. The AS-Interface slaves for frequency
inverters can be parameterized so that 2 digital inputs of the inverter are
read from the AS-Interface slave via the serial interface and transmitted to
the PLC via AS-Interface. So switches, sensors etc. can be directly connected
to the inverter and the status information will be transmitted to PLC. In that
way an additional I/O module can be saved.
Further, it is very important to
detect a component’s failure quickly and replace the component if
neccessary. The monitoring of the frequency inverter’s status information
has already been described . But with the AS-Interface slaves for frequency
inverters it is furthermore possible to store the inverter’s parameter set
inside the slave. This offers the possibilty for a very quick inverter
exchange. The damaged inverter has just to be replaced by a new one and the
parameter set will be downloaded from the AS-Interface slave to the
inverter.
Setpoint values and ramps can also be transmitted to the inverter via AS-Interface. This can be demonstrated with picture 4 which shows a production line at a glass works at PLM in Bad Muender. After the actual bottle manufacturing the bottles are tranported on conveyor belts through various stations till at the end the bottles will be pallatized and packed. Here you can see a station of the production line where bottles are coming in rows with four bottles and at the end are transported one bottle after another. The 4 conveyor belts are driven with 4 different but co-ordinated speeds. With each glass production job, various types and volumes of bottles will be transported through the same production line. To achieve this requires a very flexible controlled application.
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| Picture 4: Production line at PLM glass works Münder |
In that case the PLC’s software is written such that only the
diameter and the number of bottles per hour has to be given from the user to
the controller. Resultant from these 2 parameters the different belt speeds are
calculated within the PLC and are transmitted via the AS-Interface analogue
protocol to the AS-Interface slaves and so to the frequency inverters. Picture
5 shows an integrated frequency inverter in combination with an AS-Interface
slave with IP65 protection.
The whole plant could be operated in this way
especially with the use of the AS-Interface slaves for frequency inverter
motors. Through the use of these devices it was possible to fulfill all control
tasks with the AS-Interface. The alterative was to operate the whole
application with a more powerful bus system. The disadvantages in such a case
would be the higher component costs and that the binary I/O signals have to be
connected via I/O boxes with at least 8 I/Os. This would mean a step back to
the parallel wiring of binary sensors and actuators. The second alternative was
to network the frequency inverter motors via a more powerful fieldbus system
and to scan the binary I/O data with a gateway between the superior fieldbus
system and the AS-Interface. Again, the costs would be higher compared with a
pure AS-Interface solution.
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| Picture 5: AS-Interface slave in combinations with a frequecy inverter motor |
In summary, the reasons for the use of the AS-Interface in combination with frequency inverters and frequnecy inverter motors are:
The AS-Interface - originally designed to network binary sensors and actuators - offers in combination with the AS-Interface slaves for frequency inverters and decentralizes frequency inverters motors a possibility to create new plant structures together with drives. The AS-Interface’s advantages (low costs, high flexibility, easy handling) can be combined with the functionality of frequency inverters, especially frequency inverter motors in an efficient way. This concept provides the user a real alternative to create new solutions for machines and plant which were formerly operated with more powerful fieldbus systems.
Literature
[1] Zeyer, G.:
Aktuator-Sensor-Interface-Systeme. Franzis Verlag, Feldkirchen 1997
[2]
AS-International: Online Brochure, 12.05.1997, http://www.as-interface.com
[3] Lachmann, K.: Motorstarter in AS-Interface-Netzen, Vortragsfolien vom
AS-Interface Anwendersymposium 1997
[4] Gutsche, A.: Neue Aktuatoren
für AS-Interface, Vortragsfolien vom AS-Interface Anwendersysmposium
1997
Author
Dipl.-Wirtsch.-Ing. Lutz
Bieberstein
Bihl+Wiedemann GmbH
Floßwörthstr. 41, D - 68199
Mannheim
Fon: (+49) 621/33996-0, Fax: (+49) 621/3392239
eMail:
lbieberstein@bihl-wiedemann.de
Mr Bieberstein is head
of sales and marketing at Bihl+Wiedemann GmbH
This article has been published also in following periodicals:
•
A&D Kompendium 2000
• MSR Magazin 11/99
• Computer &
Automation 5/99 (November 1999)