Encoder Profile for the sercos III Automation Bus
sercos is one of the world's leading serial interfaces for communication between controls, drives and decentral peripheral devices and has been used, for example, in machine engineering for over 20 years. sercos III provides an Ethernet-based automation bus that supports a speed of 100 Mbit/s at full duplex and allows for the use of copper and fiber optics cables . sercos III real-time telegrams are transmitted as collective telegrams in a broadcasting procedure and processed by all connected devices during a cycle. The cycle times are set from 31.25 µs to 65 ms and can be adjusted according to the application requirements. The sercos transmission method allows for the exchange of real-time data between any devices within one communication cycle. The data is transmitted via direct cross communication) as quickly as possible and is also available synchronously, i.e. in an identical cycle. The synchronization procedure used by sercos is derived directly from the cyclically transmitted telegrams. Runtimes in the network are compensated so that a synchronization accuracy and simultaneity of well below 1 µs is reached in the sercos III network. Any additional Ethernet protocols can be transmitted in a configurable, separate time slot without affecting the real-time performance of the network. This channel is used to operate EtherNet/IP devices in a common network infrastructure, for example. Furthermore, the safety aspects: the "CIP Safety on sercos" concept is a protocol extension compatible with sercos III to enable the use of the technologies in safety applications up to SIL3 in accordance with IEC 61508, even with minimal cycle times.
Uniform profiles for interoperability
Profilstruktur von sercos-III-GerätenHowever, standardization must unify not only communication (physics and protocol, but also the functions of the different types of automation devices so that devices from various manufacturers can be combined in the network without much engineering effort. For this reason, various profiles were specified by the sercos user organization. The communication functions are determined in the sercos communication profile (SCP). General device functions are summarized in a basic profile, the generic device profile (GDP). The device-specific functions are unified via function-specific profiles (FSP). The FSP drive defines the commands and parameters for electrical servo drives, frequency converters, as well as hydraulic drives. The FSP I/O defines the functionality for compact and modular I/O assemblies. The underlying device module was designed so that, in addition to pure devices, hybrid devices that combine various applications in one device can be realized.
The encoder profile for sercos III
The FSP encoder profile recently developed by a work group at sercos international (SI) ensures that encoder functions are available via clearly defined interfaces, uniformly and network-wide, regardless of the manufacturer. Already specified parameters for motion control encoders that were a component of the sercos drive profile were considered while determining the profile. The requirements of various encoder manufacturers were also integrated, especially with regard to their experience with existing, comparable device profiles of other bus systems. The encoder profile defines which encoder functions are supported and how these functions can be used by other devices, e.g. controls. The interoperability of devices from different manufactures is ensured thanks to the scaling of the position information defined in the profile. Users can select the encoder best suited for the respective application without having to fear compatibility problems. The encoder profile can be used in diverse applications. In addition to a stand-alone encoder, the profile can also be instantiated multiple times and combined with other function profiles in a hybrid device (e.g. servo regulators with two connected encoders).
Schnittstellen des sercos-III-Encoder-Profils
Thanks to the sercos III transmission method (producer-consumer principle in combination with direct cross communication), the data provided by an encoder ("producer") can be processed directly by one or multiple connected sercos III devices ("consumer"). Moreover, it does not matter whether the consumer is a master or a slave.
Implementing encoder solutions based on sercos
Various types of hardware solutions are available for the implementation of a sercos III encoder. ASIC and processor solutions are supported in addition to FPGA modules of renowned manufacturers (such as Altera, Lattice, Xilinx). Some examples are the Hilscher netX family and Sitara AMA 335 x microprocessors by Texas Instruments. Frequently, device manufacturers can adopt an already existing hardware design that was implemented for another Ethernet protocol. Only the software has to be adjusted in order to connect the encoder logic to the sercos III profile and the sercos III protocol. The first implementations of the encoder profile were introduced at the SPS/IPC/Drives in Nuremberg. TR Electronic showed an absolute encoder with integrated sercos III interface; CANNON-Automata presented an SSI gateway that can be used to integrate SSI encoders of any manufacturer into sercos III networks.
Information:
The encoder profile specification is available to interested companies as a PDF file. Registration on the SI specification server is sufficient (see link below). For SI members, a version of the specification can be found online in sercos Wiki. Initial training is simplified considerably thanks to search functions and links to relevant parts of other specifications. Furthermore, new specification drafts can be accessed.
Link: www.sercos.org/accredit