21/02/00 - Article: The Power-Extender for Field Installation
For several years, AS-Interface has been established as the simplest automation networking solution1, used both as an independent system and as a subsystem for higher field buses in decentralized applications.
 |
Figure 1: The Power Extender of Bihl and Wiedemann. It is designed for an operating current of 2.8 A and is connected to the AS-Interface network by means of standardized coupling elements for flat and round cables. In this combination, the device complies with IP65. |
Decentralization often means use in the field with a high degree of protection. For this purpose, AS-Interface offers a great number of components provided with IP65 and higher, e.g. for masters, gateways, slaves and modules. Until recently, however, the mains power supply represented an exception. Due to its higher amount of power dissipation, it was usually put in an IP20 case and often had to be housed in the central service cabinet or a local control box. If more than one network was instal-led, several power supplies had to be mounted in these cabinets or boxes and were lead to the pertaining networks by means of separate leads. These mounting restrictions (control box, separate supplies, "unnecessary" line lengths within the 100-m limit) are not really desirable for such a simple system and were regarded as limitations for some purposes.
A new AS-Interface device by Bihl + Wiedemann, the "Power Extender" (figure 1), now eliminates these restrictions. It is basically an independent data decoupler in an IP65 case (figure 2) and can be used in many different kinds of configurations (fig. 3 - 7). It eliminates the need for additional cable lengths, additional power supplies and additional control boxes. Used together with gateways with a high degree of protection, there is no need for a local control box in the field even in big applications with two or more AS-Interface networks, and several networks can be fed by one single power supply.
 |
|
Figure 2: Circuit
diagram of the Power Extender (PEX) in an IP67 case. The "real" Power Extender
is additionally provided with a self-resetting fuse and 3 LEDs for monitoring
of the operating voltage in the network. |
To understand how a Power Extender works one only needs to remember how AS-Interface works: AS-Interface uses a two-wire cable for data and energy which supplies all users with the regulated d.c.-voltage. Data are exchanged between the users by means of pulses which are modulated onto this voltage. A standard power supply would interpret these pulses as momentary interference and either compensate or short-circuit them via its output capacitor. An AS-Interface power supply, however, is provided with an inductance in the output which works as a low-pass filter. The control unit of the power supply cannot "see" the pulses; the "data decoupler" (figure 2) separates network and power supply from one another2.
While AS-Interface power supplies usually contains a combination of voltage source and data decoupler, the Power Extender separates these: it contains only the data decoupler. Thus, it is no problem to put a Power Extender into an IP65 case, because its power dissipation is low. And it can separate the data of two or more networks, because it is not transparent.
Separating the data decoupler from the voltage source and housing the Power Extender in an IP65 case has the consequence that the user has to install two devices instead of one. In a surprising number of cases this, however, increases flexibility when more complex applications are installed. And, in addition to this, it saves costs. In the following paragraphs, five examples will be described. They can be combined. For standard applications, of course, where the separation does not have an advantage, the user will continue to use the well-established AS-Interface power supplies.
 |
|
Figure 3a: An AS-Interface network with a great distance between control unit and the first slave. The figure shows the usual installation with a data "extender". |
 |
|
Figure 3b: An AS-Interface network with a great distance between control unit and first slave. As an alternative to the configuration shown in figure a, this configuration does not require a local control box for the power supply, as it is provided with a Power Extender. |
In plant applications, a great distance must often be spanned between the service cabinet on the one hand containing the control unit (the host) and an IP20 power supply and the network itself on the other hand. Either due to the maximum admissible cable length of 100 m or to the maximum admissible d.c. voltage drop of 3 V, this distance can limit the application of AS-Interface. As a solution for this problem, the (data-) "extender" has been developed long ago. It moves the generation (or refreshing) of the signal into the field and spans the great distance between the service cabinet and the first slave. In this case, however, it is still necessary to find a local solution for the protection of the power supply in a IP20 case (figure 3a). By means of the Power Extender, however, the power supply can now be integrated in the remote control cabinet. Data and Power Extender can, without additional protection, be mounted locally as IP65 components (figure 3b). The analogy to the "Data Extender" in this particular case has lead to the expression "Power Extender".
The Power Extender separates the signals of two or more networks from each other. Thus, several networks can be operated with one single power supply and two or more Power Extenders. There is no need for additional power supplies. This is shown in figure 4 for two masters.
 |
|
Figure 4: Two (or more) AS-Interface networks can be supplied by one single power supply, if they are separated from each other by one Power Extender per network. |
AS-Interface is often used as a subsystem below higher field buses, e.g. in order to process tasks in a decentralized way and at particularly low costs. The connection to the field bus is established by a gateways which serve as master for the individual AS-Interface networks. Until now, each gateway needed a separate power supply for its network and, therefore, also a control box. This can now be solved by one single power supply, an IP65 gateway and a Power Extender (figure 5).
 |
|
Figure 5: Three AS-Interface networks are used as subsystems for a higher bus and are supplied by a central power supply. There is no more need for local control boxes for these three networks. |
This configuration saves the mounting of several local control boxes. Gateway and Power Extender can be mounted almost anywhere. The use of AS-Interface is thus an even more economic investment, particularly for very large systems, in which the components are mounted far apart3.
If a network extends for more than 100 m, AS-Interface requires a repeater. Until now, the two parts separated by the repeater had to be supplied by two separate power supplies. If two Power Extenders are installed, only one power supply is needed now (figure 6).
 |
|
Bild 6: Repeater mit zwei Power Extendern. Die beiden Netzbereiche werden durch ein gemeinsames Netzgerät versorgt. |
Normally, AS-Interface transmits data and energy via one single two-wire cable. It is one reason why it is so widely accepted. This way of transmission, however, reaches its limits when in a network some devices with a high power demand are mounted far from the point where the direct voltage is fed. If the voltage drops by more than 3 V in the AS-Interface cable, there may not be enough voltage left to operate the actuators4. The standard solution for such a case is the installation of the AS-Interface power supply near the devices with the highest demand. If a high degree of protection is required, this can either be reached by mounting the power supply in a local IP65 control box or - more easily - by using a Power Extender (figure 7) which is connected to the power supply in the service cabinet by means of a line with a large cross-section and, thus, a low voltage drop.
 |
|
Bild 7: Minimierung des Spannungsfalles auf der Leitung: Die Einspeisung der Energie erfolgt an einem fernen Punkt des Netzes nahe dem größten Stromverbraucher. |
In principle, the Power Extender can be combined with a standard power supply, because it is an independent, separate data decoupler. In practice, however, this does not seem to be recommendable, because the specification of the system imposes several prerequisites on the power supply concerning start-up reserves, electromagnetic compatibility and voltage tolerance, which are not always fulfilled by standard power supplies. Therefore, it is recommended to use the Power Extender together with an AS-Interface power supply. Its dimensions are such that the doubling of the data decoupling has no influence on the system.
In situations like the ones described here, the Power Extender for AS-Interface - an independent data decoupler in an IP67 case - makes the configuration of networks surprisingly easy and flexible, and, additionally, it helps reducing costs. This makes it an interesting complement for the product range of the suppliers; not for standard applications, but for many applications which are more complex and require, for example, energy feeding in the field, multiple networks or networks with repeaters.
Author:
Dr. Otto W.
Madelung, Technical Management Consulting Dr. Madelung; 1991-1999 operations
manager of the AS-International Association; Odenthal, tel.: +49 -2174-40756;
fax: +49-2174-41571; email: otto.w.madelung@gmx.net
Annotations:
1 Kriesel, W.R., Madelung, O.W. (editors.): AS-Interface - The
Actuator-Sensor-Interface for Automation; 2nd edition, Carl Hanser Verlag 1999,
ISBN 3-446-21065-2
2 Strictly speaking,
the decoupling serves two more purposes:
· It balances the entire
network, which is the basis for the high interference immunity of
AS-Interface.
· As a consequence of the inductivity of the data
decoupler, the current pulses of the transmitters in slaves and master are
converted into voltage pulses on the lead.
3 see (1), page 190 or: Madelung, O.W.: Hierarchisches Netz in der Prozeßindustrie (Hierarchical networks in process industry); mpa 35, 10, p.12-16 (10/99)
4 Numerical example: the voltage in a standard cable with a cross-section of 1.5 mm² drops by 5 V approximately, if 2.5 A are not absorbed before a line length of 80 m is covered. In a 2.5 mm² cable the voltage drops by only 1.9 V approximately.