Sercos II - Immunity against Interference and for more Communication!
The field of IT and automation technology requires a high degree of technical progression. After all, in this field an increase in safety matters as well as a reduction in time and the resulting costs. When processes in complex installations and systems are to be standardized, there are always efforts to find an internationally recognized standard. However, this also presents problems due to the numerous specifications of individual companies and associations. Often, there is no clearly defined uniformity even after years of cooperation between the various groups. Especially during the 90s, the number of applications that could be found on the market increased. This was based on growth within the industry, and with it the need for industrial communication. After all, machines and their components must be able to act with each other and with other components, so that faults can be recognized and corrected and the process can run effectively and efficiently. One standard broke initially through internationally in 1995 under the designation IEC 61491. In 1998, it also established itself in the European segment as EN 61491. This is SERCOS. This acronym stands for “Serial Real-time Communication System”. This designates a digital drive interface that is suited for use in industrial machines. Since the 90s, Sercos (initially as Sercos I (= Interface), then Sercos II and now Sercos III) advanced to form a communication standard that makes it possible to pick up and transfer data in real-time. Because of this, Sercos II is suited to complex motion control applications with high specification designs. In addition to real-time, performance and interference resistance are among the technological characteristics of this drive interface. Furthermore, there are also a large number of products and a wide range of their suppliers.
Specifications of Sercos II
Sercos II was introduced in 1999 and as a matter of course, it includes the SERCOS real-time protocol. The basic protocol is HDLC (“High-level Data Link Control”) which is a standardized network protocol. The physical medium is the light conductor (LWL). This is a cable which is made using plug-in connectors and light conductors. The lines are used to transmit light. In this process, the light is transferred using fibers made of quartz glass and a plastic which is made up of polymer optical fibers. The topology in which the subscribers of the network are arranged, is in the shape of a ring. Synchronization in this system is performed by the respective hardware. When a data transfer is initialized, its speed is in an interval between 2 Mbps and 16 Mbps. This depends on the length of the cable. With Sercos II the cycle time can be configured, and is at least 62.5 μs. The advantage of this digital drive is its short jitter time. This describes the clock jitter that can occur from time to time with the transferring of digital signals. This causes a small fluctuation in the accuracy of the transfer clock. This inaccuracy here is at just < 1μsec, an amount that makes the suitability of the application clear. Each ring of a topology has only one master, but up to 254 subscriber. However the number of rings and the number of subscribers can be increased.
Communication Opportunities of Sercos II
In addition to creating immunity against digital interference, industrial communication is naturally the focus of the Sercos II functions. As such, it is not surprising that here as well, there are many opportunities for communication. The interface basically instructs three different options to realize an interaction between the CNC (Communication Network Control) and the digital drive controller.
- Transfer of the target position
- Transfer of target speed
- Transfer of target torque
Within these three different communication possibilities, the transfer of target positions has proven to be most effective. Accordingly, it represents the optimal solution when it comes to the quick and accurate interaction of applications. This way, within a light conductor ring, up to six individual axes can run in synchronisation every 0.5 ms. With this, they are all provided with target position values (another name for target positions). This provision is cyclic, which means that it always takes place according to a specified order. Sercos II is thus suited to the displaying and inputting of internal drive data, diagnoses and parameters. However, this requires a SERCOS-compatible CNC.
Configuration with Sercos II
The ability to configure the real-time data makes it possible to handle and control any other operation modes independently. However, the exchange of service data is only performed upon instructions by the master. As such, these data are transferred by means of a handshake procedure in portions of 2, 4, 6, or 8 bytes within the “info” service data field. They are them reassembled during the reception process. With Sercos II, the HDLC protocols provided with an NRZI coding are used for this application.