What does Powerlink mean?
Powerlink is a high-speed (up to 100m) communication protocol based on Ethernet.
In view of the vigorous development of Ethernet and the wide application foundation of CANopen in the field of automation, Ethernet Powerlink combines the advantages and disadvantages of these two technologies, that is, it has the high-speed and open interface of Ethernet, and CANopen has good SDO and PDO data definition in the industrial field. In a sense, Powerlink is CANopen on Ethernet The data link layer uses Ethernet medium, while the application layer retains the original SDO and PDO object dictionary structure
-Powerlink can be realized without many changes;
-Protect the interests of the original investment;
Ethernet Powerlink version 1 was developed by bernecker lenner industrial electronics, an Austrian control manufacturer, in November 2001 and provided to other companies. EPSG was established in November 2002. In November 2003, the specification adopted the real-time industrial Ethernet protocol powerlinkv2, which includes V1, the most important extended application layer: Based on the mechanism defined in CANopen, standardized application interface.
In order to avoid conflicts and maximize the utilization of bandwidth, data exchange between devices is based on time slots. A mobile device in the Powerlink network needs a “management node” (MN), which controls the communication function, defines the synchronous clock pulse of all nodes, and assigns the transmission right to each device“ The manager required when the control node (CN) sends. The Powerlink cycle is divided into four time periods (Figure 1)
Start time: the manager here sends a “start” loop frame (SOC) broadcast message to all controllers. Synchronization on the SOC of all devices in the Powerlink network.
Cycle period: the cycle synchronization data exchange takes place in this period of time. According to the preset schedule (configurable), the manager sends a “poll request frame” (preQ) for each controller compared to the previous quarter. The “head response” frame (PRES) of the addressing controller response. Interested in receiving these data from all nodes, a real producer / consumer communication station can be implemented between stations similar to can.
Asynchronous period: this time interval is asynchronous, non time critical data exchange. For example, a controller gets the right to send the manager, and then it can send the IP frame.
Idle time: unused until the beginning of a new Powerlink cycle.
Any topology can be implemented by using a hub. Due to the fact that there is only one device sending time and no collision, the number of hubs is no longer limited because it is two fast Ethernet. From the design point of view, this is an obvious advantage. If the Powerlink device has an integrated 2-port hub line structure, it can easily realize the Ethernet of application program interface. Powerlinkv2 is based on can defined mechanism of CANopen communication protocol ds301 in automation (en50325-4). This opens up a wide range of ready-made, used device and application configuration files, and realizes the continuity of communication services between Powerlink, CANopen and Powerlink systems, which is conducive to the migration from CANopen to Ethernet Powerlink on the software level.
Application of Powerlink redundant network topology in process industry
In the process industry, high availability (effectiveness) is very important. The safety and reliability of key applications must be ensured. A way is needed to prevent the functional interruption caused by hardware damage or cable damage. This is realized through the redundancy of the central control unit. If the IPC is down, The second unit needs to immediately notify the fault and take over the task of the failed unit to prevent the delay of the task in operation. Another necessary condition for achieving high availability (effectiveness) is cable and control fault location.
Alstom is one of the members of EPSG (Powerlink Standards Organization), which is active in the field of global energy and transportation. The company has developed a set of redundant system based on Powerlink, which has the characteristics of short response time, high real-time synchronization, high transmission rate and easy diagnosis.
Alstom only used “factory execution protocol (FIP)” before, unless it is necessary to use another fieldbus solution. In our latest process sequence, the amount of data now exceeds the bandwidth of FIP. “Therefore, we need a bus alternative that can handle more real-time data and adapt to redundant topology.
Boteher explained that Alstom’s criteria are not only important for technical performance such as bandwidth, but also for system standardization. After all, choosing a new technology always brings a lot of investment protection problems.
History of Powerlink
Powerlink was developed in 2001 by B & R, an Austrian manufacturer of control technology. The goal of becalais is to achieve real-time data transmission in microseconds by extending the standard Ethernet protocol of IEEE802.3.
Powerlink is open to other developers. In 2002, it was led by a democratic and open EPSG organization. In 2003, EPSG voted to adopt the Powerlink V2 technical specification. Its most important extension is to standardize the application interface based on the mechanism defined by CANopen standard.
The key feature of Powerlink is the clock management of network devices. Unlike the standard Ethernet system, where the transmission speed depends on the amount of data, the expansion of Powerlink adopts the method of time slot, which can ensure the synchronization of data exchange. In order to synchronize the data transmission, the whole bandwidth is applied and the data conflict is prevented, A device in a Powerlink bus is assigned a management node (MN) function, while all other devices act as a controlled node (CN). Mn defines the synchronous clock pulse of all devices and controls data communication by sending a request (poll request frame, preQ), including a response frame (PRES), to all controlled nodes in a clock cycle. Because Ethernet is a broadcast system, all network nodes can receive data. A Powerlink cycle consists of four cycles. In the “start” cycle, Mn sends cycle frame start (SOC) to all CN to synchronize the devices.
The synchronous data exchange of the cycle takes place in the second cycle (the cycle cycle), and the following “asynchronous cycle” makes the non real time data transmission possible. A “idle” cycle completes the whole cycle. Because the time slot method prevents different devices from sending data at the same time, the number of hubs in the network segment is unlimited, which allows users to implement any topology. In any network segment, each Mn can be assigned up to 240 controlled nodes, More advantages of Powerlink are based on the software implementation and existing standardization. A loop is composed of real-time pulse of real-time poll request and poll response and non real-time pulse for transmitting non real-time data.
Through the work of other organizations, we have provided many advantages in this field. We have accumulated rich experience in redundancy technology. We can position our company’s Development Foresight and make great contribution to high availability standardization.
Alstom redundancy mode is designed as follows: two or more redundant management nodes (rmns) are at the top of the network hierarchy, only one is the active management node amn, and the others are in the stand by (SMN) state and act as a controlled node. The only difference between SMN and cn is that SMN continuously monitors all network and CN functions. In case of emergency, this enables SMN to have AMB function. As an Rmn in the network, each Rmn must be able to configure all network devices, and other rmns are no exception. This redundancy pattern allows a wide range of topologies. Rmn can access a simple network layer through hub. A feasible design is to put Rmn and CN in a ring to close the network structure. The third advantage for Alstom includes redundant data lines, which can protect the bus not only in case of Mn failure, but also in case of bus cable failure and fault.
Network management status
Alstom developers have to make many changes to the network state to enable the system to integrate multiple redundant management stations. Since Powerlink only allows the operation of one Mn at first, NMT is a monitoring and control service based on CANopen. Each network device has an NMT state machine, which can provide request and operation state information from the NMT master station of Mn. Four states are defined, namely, initialization, pre operation, operable and stop. In the initialization phase, the device is not allowed to start and activate the network. In the pre operation phase, the device synchronizes the network parameters, but it cannot start to transmit data, The operable indication unit is fully operable. Stop means that the device is disconnected from the network and only accepts one NMT instruction. For redundant operation, Mn can assume the control function at any time. The programmer adds a standby to the NMT state machine. In the pre operation stage, Rmn checks the bus state. If no relevant signal is detected, Rmn checks the bus state, It switches to activation mode and plays an amn function. If Rmn detects network activity, it will switch to waiting mode in operation mode, but it will enter the system with a standard CN.
In addition to stack replacement, Alstom experts have developed connection selectors, an innovation that enables the network to operate on redundant data lines. Similar to hub, it connects the data line input and the controlled node, plays the role of an associated operator, and connects the signal transmission line with CN. If one line is interrupted, the switch switches to another. A broken cable can also be located by sending a message through the connection selector.
After the completion of all modes and technology development, Alstom enters a simulation stage. In the industrial installation of Alstom, whether the system can adapt to the special process must be verified by computer-based mode. Most importantly, the simulation confirms our preliminary estimation, which shows that 100Mbps Ethernet is enough to meet the requirements of data load. As a result, we have created a wealth of special processes and tested the system functions in these cases. Potier describes this long simulation phase. We can’t put into operation before confirmation unless it works as we wish.
Alstom is encouraged by the results. We now have a field network that can meet all our requirements. The system has a very clear level. The other advantages are that it supports hot swap. There is no need to make any effort in configuration to connect additional devices to the system. In addition, we can easily integrate network configuration into engineering design tools, Since Powerlink is a network based on CANopen, the most important thing is that this system belongs to us. We know the development process. We have all the resources available for further development when necessary. New functions will be implemented in EPSG standard and open to anyone.
Alstom provides turnkey integrated power plant solutions, equipment and related services for hydropower, gas, coal and other power plants using various energy resources.
The openness and convenience of Powerlink technology has given Alstom experts the idea of the next step. The first step to implement Powerlink has been successful. Now we start to integrate Powerlink into all our product ranges. In the next step, we will adopt Powerlink safety technology, which can make sil3 applied to machine and factory protection. Safety is a new development direction of EPSG, It specifies the network emergency shutdown method in case of control failure. Moreover, we are considering integrating the connection controller in the controller. Another potential project is to develop the network timestamp to synchronize with the GPS clock. In the future research and development process, there will be more possibilities.