For some automatic control engineering applications, primarily within motion control, the performance and functionality offered in the basic Profibus DP-V0 protocol is not enough. A new application profile, PROFIdrive, has therefore been developed so that Profibus can also be used in fast servo-steering and synchronised processes. PROFIdrive uses functionality from all three of the basic DP protocols. From DP-V0 comes the cyclic data exchange and diagnostics; from DP-V1 the acyclic data exchange for parameterising slaves. To achieve additional function levels the DP-V2 functionality such as slave-to-slave communication and isosynchronous transmission is used.
Functionality from DP-V1
DP-V1 expanded DP-V0 functionality primarily for use in process automation. A special functionality demanded in such applications was an acyclic data exchange between master and slaves. The acyclic data exchange was therefore added as a complement to the cyclic. An online connection between the master and slaves can thus be achieved, allowing parameters to be changed at will during operation.
DP-V2 functionality
Despite the improvements with the addition of the acyclic data exchange, it was still not enough to be able to use Profibus in time-critical processes. Expansion with the aim of making use in drive technology possible was therefore necessary. As a result, DP-V2 functionality such as slave-to-slave communication and isosynchronous transmission of process data was added.
Slave-to-slave communication means, as the name implies, that slaves can exchange information with each other via broadcast messages without communication being initiated by the master. This type of communication is therefore very fast and reduces response time on the bus by up to 90%. Two concepts are added with the advent of slave-to-slave communication, 'publisher' and 'subscriber'. The significance of this is that slaves in the Profibus network may be publishers that can send their process data over the bus in a broadcast message. The message can then be read and dealt with by all subscribers on the bus. Slaves may also be subscribers.
An additional functionality necessary for Profibus (to be able to be used in drive technology applications with multiple shafts) was the ability of the master to reach several slaves simultaneously with set points, for example, as well as the complimentary ability to read off actual positional values. This was solved with the isosynchronous transmission technology found in the DP-V2 protocol. The isosynchronous mode means that a time-synchronous communication is established between the master and slaves by the master sending out a special broadcast telegram called 'Global Control'. This telegram, together with parameter settings, control when the slave should set the set points and read the actual value. With the isosynchronous mode a system can be set up where all slaves set the reference value and read the actual value at set times - and with high accuracy. This in turn means that reading of absolute position from Leine & Linde encoders may be synchronised within 1 ms.
Leine & Linde's adaptation of PROFIdrive
During the last years Leine & Linde has taken part in the preparation of a new encoder profile that states how an absolute angle encoder shall behave in a PROFIdrive network. Parallel to this the products have also been further developed to implement the new functions the PROFIdrive profile lays down. In the future we will thus be able to offer absolute encoders that support both traditional Profibus DP-V0 communication and PROFIdrive. Both solutions will be presented with the same hardware, the only difference being that different GSD files are used for configuration. Just as today both single turn and multiturn absolute encoders will be manufactured, both in hollow-shaft and shaft models. For installations in tough working environments the reliable gateway concept with support for PROFIdrive can be delivered.
For the interested reader the entire PROFIdrive profile may be ordered from PNO (Profibus Nutzer Organisation), order number 3.172.
Profibus and angle encoders
Profibus stands out among other fieldbuses due to its flexibility, compatibility and user-friendliness. One of the reasons for this is that Profibus developers have developed special profiles that solve basic needs for users in many different areas of industry. That they have also succeeded in standardising the most important aspects from the various profiles in an open fieldbus, EN50170, has further improved conditions for creating a widespread interface to many different components in hundreds of applications.
The angle encoder has long been a component with its own 'encoder profile' which describes its functionality in a Profibus network. This profile, developed by Leine & Linde, among others, is based upon communication over a DP protocol. The DP protocol is the standardised fieldbus protocol developed for fast data exchange at field level and is used in all Profibus networks. In the Profibus network the central programmable control system (PLCs) communicate with the distributed field units (I/Os, drive equipment or encoders) via a serial interface, RS485. Today, there are three versions of the DP protocol: DP-V0, DP-V1 and DP-V2. The basic communication functions such as cyclic data exchange and other functions (such as diagnostics) are described in the basic DP-V0 communications protocol, the same protocol that Leine & Linde encoders have used up till now. Since different applications make different demands on performance and function of the components and communications used in the Profibus network, the DP protocol functionality is continually being extended.
For more information contact Wim Annandale, ATI Systems, +27 (0) 16 986 2267, [email protected]
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