Sercos I - The Digital Drive Interface of Past and Future
1987 was the year in which SERCOS’ first digital drive for industrial machines was introduced. This took place under the name Sercos I (the Sercos Interface). The entire thing was developed for the purpose of securing against uncontrolled drive movements and speeds which are not classified as allowable. In order to be able to realize this process, a light conductor is integrated as a physical medium. By means of this light conductor, it is possible to satisfy the requirements for the safety in industrial machines. This is because the desired high immunity against jitter (as periods of electromagnetic interference are called) is achieved. After all, with the use of Sercos I, this value is just < 1μsec. Further, in this area of business, the secure exclusion of HF interference is of key importance. This is done by controlling the power which takes place through pulse width modulation. Furthermore, this is where the locally separated components of the network and the earth connections on the control units and drives play a role. The foundations of Sercos I include the SERCOS real-time protocol and the HDLC basis protocol. The topology of the network is that of a ring, and the synchronization takes place by means of hardware. Each ring has only one master, but up to 254 subscribers. This can be increased through the coupling to another ring. A rate of 2 Mbps or 4 Mbps is achieved with the transmission of real-time data. With this, the cycle time remains configurable and at least 62.5 μs.
Since the introduction of Sercos I at the end of the 80s and its increasing popularity in the 90s, this digital drive interface served in numerous applications to create higher safety as well as optimized communication within a system. As such, Sercos I can be found in nearly all processing technologies that have CNC. However, with this achievement combining IT and industrial communication, the range of possible applications is much wider. As such, the following areas belong to the application possibilities of the interface:
- Turning and finishing work
- Machining centers including HSC
- Milling work including HSC
- Crankshaft shape grinding
- Tool grinding machines
- Transfer machining lines
- Cog wheel fine machining
- Rotary transfer machines
- Assembly robots
- Assembly lines
- Cam form grinding
The Monopoly Position of Sercos I
Sercos I has now become so successful, that a third version has been able to conquer the market. This is because only this digital drive interface satisfies all the requirements for such an application. Among other things, these requirements include:
- top precision,
- high speed,
- minimized upgrade costs,
- openness, standardization and
Only Sercos I and its successors offer these preconditions. As such, the monopoly position of the technology is not surprising. This interface has appealed even to car manufacturers. Essentially, it is found in all fields of automation technology. Additionally, there is also use in modern machine concepts, e.g. those that cover the fields of packaging, textiles, and printing.
The Light Conductor
In this digital drive interface, the light conductors (LWL) can vary in length. In the case of a light conductor made of plastic, a transfer section has a length of 50 m. When classical glass fibers are used, this value is increased fivefold. The exact number (max.) in a drive (the max. value is 254), depends on the required cycle time that is necessary for one communication to pass through. Furthermore, the selected scope of operation data and the data rate itself are decisive. Through the use of several of these types of light conductor rings, the number of drives which are to be found in a control unit can be expanded as desired.
With this digital drive interface, initialization is performed in part by means of a number system, which serves the task of identification. As such, the real data to be transferred in the process are specified in advance. These may be numerical data, including the nominal values and actual values, but also bit lists with I/O instructions.
Advantages of using the Sercos Interface
The internal drive functions of the digital drive interface enable self-monitoring, which by itself already works perfectly. This is done by monitoring the target position values and the actual position values, as well as the drive parameters, which are connected to a forced shut-down which also works in the case of a fault or the failure of the drive processor. Through the logical monitoring of the target values received by the control unit within this drive processor, Sercos I is also capable of completely excluding inadmissible speeds of the axis or run-away caused by faulty or wrongly-transferred target position values. This way, safety redundancy within the control unit is achieved by the monitoring component which acquires the actual values that are reported back. The requirement of immunity against faults is helped by the ring technology. After all, this exhibits the lowest number of optical components and as such, it does not require any complex T-branches. This also offers the opportunity of a forced shut-down.