EtherCAT in mobile applications

Summary

By Dr. Guido Beckmann, EtherCAT Technology Group

Tractors and their implements, self-propelled harvesters, excavators, cranes, wheel loaders and dump trucks are becoming more and more complex – the use of automation and information technology increases yield rates and energy efficiency while enabling new functionalities. Due to its outstanding performance and flexibility, EtherCAT – as the Ethernet fieldbus – provides valuable opportunities for integrated system architectures and thus is ideal for the use in heavy vehicles and other mobile applications. Challenges of mobile automation Increased market demands in the areas of functionality, productivity and costs in mobile applications lead in turn to higher complexity of electrical system architectures. If, for example, the driving speed for separating grain and chaff in a combine harvester can be controlled or even regulated electrically, new opportunities for optimizing the process arise: Depending on the degree of moisture in the grain, the driving speed of the device, the shape of the grain and chaff or the desired post-processing there are different operating points for the ventilator, which can be adjusted for the process. Software solutions are increasingly replacing existing hardware features, driving hardware costs down. For this purpose, sensor information is processed in the controller (Electronic Control Unit or ECU) to enable flexible adjustments to different working situations. However, the communication systems reach their limits progressively: The required bandwidth can no longer be provided sufficiently for the parallel transmission of real-time data (engine management, process control, etc.), diagnostic information, image data or for the required cycle times of the real-time data, which can lead to requirements for additional sub bus systems.   Therefore, there’s a demand for alternative technologies from similar fields such as the manufacturing industry, computer technology and aviation. The first Ethernet automation applications are already in series production today – namely, in the diagnostic interface for control devices connected via a general Ethernet interface. BMW, for example, uses Ethernet in vehicles for driver assistance systems. Ethernet-based communication systems with industrial protocols that ensure the required deterministic transmission offer the possibility to replace traditional fieldbus systems. EtherCAT is an open, real-time capable communication system which is widely used in manufacturing automation and, due to its properties, is also ideal for mobile applications such as heavy vehicles. Standardized on-board networks Today many on-board networks consist of several coupled, often diversified communication systems. In current vehicles the control units are connected via different system buses (CAN, LIN, MOST, FlexRay). These disparate networks require varied expertise for installation and maintenance and additionally must be coupled via gateways. Looking at the architecture of a modern tractor or combine, those vehicles already have more than 30 electronic modules that communicate among one another.

Current on-board networks reach their limits. Heterogeneous network structures are coupled via gateways.

EtherCAT can simplify the formerly complex electronic architecture into one efficient and future-proof standard. With EtherCAT the bandwidth of the 100 Mbit/s full duplex transmission can be used up to 97%. Extremely short cycle times of <50 µs offer the possibility to close highly dynamic control loops via the bus, too. Since the EtherCAT protocol is optimized for repetitive process data there is no need for protocol stacks such as TCP/IP or UDP/IP.

Devices such as drives that have very tough requirements regarding the cycle time and deterministic data exchange can be operated together with non-real-time critical devices in the same network without affecting each other. Sub-bus systems are avoided and thus system-wide deterministic communication can be implemented: Control joysticks, user panels, process sensors and controls or “infotainment” and image transmission, e.g. for image processing of a rear camera – process data and parameters can be transmitted via the same cable (One Wire Solution). This dramatically simplifies the electrical system architecture and reduces the need for coupling the networks via gateways. EtherCAT supports the synchronization of devices with sub-microsecond level accuracy via the bus. This enables, for example, the usage of a central electrical controller (inverter) in a towing vehicle that provides the energy and the motion control for engines in different implements. The exact amount of power needed for the process is provided with great precision. Additionally, new algorithms to optimize and increase yield can be adjusted flexibly and also retrofitted (reloaded). The additional system information improves the overall control quality as well. The use of hybrid drive systems from internal combustion engines and electric drives / generators provides concepts for energy recovery and lowers fuel consumption. EtherCAT can transmit energy management data in real-time and therefore helps increase efficiency and reduce emissions. Increased life cycle flexibility With EtherCAT the machine structure determines the network topology, not the bus system. Switches and hubs can be used, but are actually not required at all; therefore there’s no limit regarding the cascading, too. With EtherCAT there are virtually no restrictions for the bus topology: line, tree, star and any combination thereof is possible in almost any number of nodes. The network flexibility of EtherCAT enables cost-effective adaption to the various vehicle development stages based on uniform platform architectures (product line). At each stage of development, testing and in series production, the topology can be expanded flexibly and in a nonreactive way.   Automatic network scanning with EtherCAT replaces a manual electrical design of the networked devices. The devices are detected and initialized during startup. Due to automatic link detection, nodes and network segments can be coupled out and back in during operation. This advanced “Hot Connect” functionality can be used for optional or interchangeable implements on tractors or tools for excavators and wheel loaders, for example. To increase system availability the line is extended with an additional cable to form a ring in order to enable line redundancy. Besides the software, only a second Ethernet port is required on the master side. Slave devices support this anyway and remain unchanged; there are no additional protocols or hardware costs to meet the availability requirements in the field of mobile applications. In combination with the Hot Connect functionality the replacement of EtherCAT devices during operation is possible.

 

In addition, master redundancy with “Hot Standby” functions can be realized with EtherCAT. Critical and sensitive machine components can be connected with branch lines so that other parts of the network are not affected in case of interruption. Due to the available bandwidth it is possible to use traditional bus interfaces (CAN, LIN, FlexRay, ISOBUS, …) as a subordinate system in an EtherCAT gateway. This is helpful, for example, when migrating from a traditional bus to EtherCAT. The gradual implementation of a machine to EtherCAT as well as the integration of components that do not (yet) support an EtherCAT interface is therefore possible.

 

Gradual migration via integration of existing networks into the EtherCAT network.

Diagnostics and Condition Monitoring In addition to the possibility to scan and compare the network topology during startup, EtherCAT also natively supports many other diagnostic features.

In each slave the passing telegram is checked for errors by the EtherCAT Slave Controller with a checksum. Faulty telegrams increment a counter and are marked incorrect for subsequent devices in the network. By reading the error counter of the devices the master is able to localize the error very precisely. This is a remarkable advantage over conventional bus systems in which disturbances spread to the shared bus in the system and the source of the error cannot be localized. With EtherCAT, disturbances which only rarely occur can be detected and localized, even if the disturbance does not yet influence the functionality of the machine. The master can synchronously check if all configured devices have processed the data properly. Using status and error information of the devices and, if needed, the Link Status the master can detect the reason for the unexpected behavior. Ethernet network traffic can be recorded with free available software tools. Those tools can be used for EtherCAT as well because the technology uses standard Ethernet telegrams. The well-known tool “Wireshark,” for example, comes with a protocol interpreter for EtherCAT in the setup so that protocol-specific information is displayed in plain text. At the application level the available information of all sensors can be used to implement preventive maintenance. Those sensors measure vibrations on a machine, engine or bearing to detect wear and tear by analyzing the measured values before a production stoppage and thus help to avoid failures and extend maintenance intervals. The performance of EtherCAT is beneficial here especially when a great amount of data from different devices is used for analysis or when the damage frequencies must be measured while dependent on the speed. Convenient condition monitoring systems are only available in hardware and must be coupled with the control system in a complex way; with EtherCAT expensive special hardware can be replaced with cost-effective software modules within the controller. Safety included With Safety over EtherCAT (FSoE), functional safety becomes an integrated part of the network architecture. The FSoE protocol is an international standard (IEC 61784-3) and meets the requirements of ISO 26262 ASIL 3, IEC 61508 SIL 3 and is ISO 13849 PLe ready. The transport medium of the Safety over EtherCAT protocol is considered a “Black Channel” and therefore not included in the safety analysis. The standard communication system of EtherCAT remains single-channel and transmits safe and standard information in parallel. The communication system is arbitrary and not restricted to EtherCAT: Fieldbus systems, Ethernet or similar routes can be used for the transmission on electrical wires, fiber optics or via radio transmission. Connectivity Yield data, maintenance intervals, hour meter and data logging – users expect the ability to access different information about the vehicle at essentially any time and online. EtherCAT offers components for engineering, system testing and data logging all in one uniform interface. Systems and test benches in test laboratories, production facilities and workshops are often equipped with EtherCAT already, so that coupling is simplified by using the same communication system. With EtherCAT a network for all components is possible and using protocols such as CAN application protocol over EtherCAT (CoE) it can be implemented into the networked devices inexpensively. With the protocol Ethernet over EtherCAT (EoE) any Ethernet data traffic can be transported through the EtherCAT system as well. Standard Ethernet devices are connected within an EtherCAT segment via a switchport and the Ethernet Frames are “tunneled” via EoE. This is used for remote maintenance and diagnostics via TCP/IP and web servers. The connection to other vehicles, e.g. for working in parallel at harvest, requires radio communication and has less severe demands related to real-time communication requirements. The EtherCAT Automation Protocol (EAP) meets those demands. It defines interfaces and services for the data exchange between EtherCAT and master devices (master-master-communication) as well as the connection of configuration and diagnostic tools for the machine as well as the device configuration. EAP can be transmitted via any standard Ethernet connection, including radio transmission. Conclusion The increasing number of control devices and sensors, the replacement of mechanical functions with software functions to increase yield rates as well as the flexible adaption of machine functionality are all driving forces behind the real-time requirements for selecting the future communication system for mobile applications. Existing on-board networks increasingly reach their limit regarding bandwidth and system complexity. EtherCAT is the Industrial Ethernet technology which is characterized by outstanding performance, low cost, flexible topology and simple handling. Connectivity between vehicles (machine-to-machine-communication) as well as monitoring and test systems progressively require a radio-based communication with which the EtherCAT Automation Protocol is available to meet every need. Author: Dr. Guido Beckmann, Chairman Technical Committee, EtherCAT Technology Group