The 1000 Vario tractor by Fendt introduced last year not only makes a big impression due to its size and the related performance capability. The energy distribution within the vehicle also implements new ideas generated in collaboration with STW (Sensor-Technik Wiedemann). Both units of the central electrical system provide intelligent energy management, which is revealed in the uncomplicated handling of vehicle versions, simplified wiring, convenient diagnostics and reduced maintenance effort.
The first central electrical system was already developed in 2003 in a collaboration between Fendt and STW and put into series production. Since then, the requirements on these systems, also known as power boards, have continued to increase. Whereas the predecessor models were mainly concerned with the switching of strong currents, an increasing amount of intelligence is today required in the central electrical systems. This may be a CAN connection, via which the switching is conducted and the switching condition can be read out. But also computing and memory capacity in central electrical systems are increasing in importance, so that the function of a decentralized control can also be undertaken. This is also due to the growing complexity of the units being supplied via the central electrical system. Whereas initially only simple consumers such as driving lights or working headlights were connected to the power board, today multi-stage fans or controllable windscreen heating systems are supplied. This, in addition to the connection of digital or analogue consumers, leads to the requirement for the connection of more intelligent subsystems via sensor actuator buses. With the increasingly growing number of consumers, the sum total of currents which a central electrical system has to manage is also increasing. For Dr. Michael P. Schmitt, Managing Director at STW, the increase in intelligence represents a logical consequence of the general trend in mobile machines: “The advance of automation demands decentralized solutions. Therefore the power boards of simple fuse boxes and control cabinets are developing into intelligent devices for the control and monitoring of the distribution and switching of the electrical energy within the vehicle.”
Therefore the task for the new Fendt 1000 Vario power board was to supply, safeguard and control the entire electrical system within the tractor. It should distribute the power circuits and include all necessary connector sockets for the relays and fuses. For the Vario 1000, the principle “divide and command” was used due to the two factors intelligence and number of I/Os. Instead of relying on one single, central electrical system, an approach was selected whereby the system was divided into two. Whereas one unit mainly dealt with the communication with the main control and the assumption of programmable switching procedures, the second unit was primarily responsible for the safeguarding and switching of the currents. An important argument for the separation was also the simplified wiring. With this design, of course, the accessibility of the connections, the ergonomic necessities and the spatial specifications had to be taken into consideration. The construction space was in most cases limited.
Regarding the intelligent unit, known in-house as “ZE” (see Fig. 1), Fendt specified a clear concept. The core of the boards should be an LPC1778 by NXP, an ARM® Cortex™-M3 processor with 512 kByte flash, a 96 kByte SRAM and a 4032 Byte EEPROM memory. One of its two CAN channels which are each guided outwards via a CAN transceiver is connected to the central control bus of the Vario 1000 and can communicate with up to 250 kbit/s. The second CAN bus is provided as a reserve. Three of the five serial UART interfaces are used via appropriate transceivers for the connection to the LIN (Local Interconnect Network), a one-wire fieldbus for sensors and actuators. Different consumers such as the windscreen and side window wipers can be adjusted or configured via the LIN bus. In addition, the processor 165 features any number of utilizable I/O pins, with which the assignment can also provide sufficient choice. As the switching of large currents should also be avoided on the circuit boards for EMC reasons, output pins are used in order to control the appropriate relays on the second unit. The“ZE” receives the commands for switching either via the central command bus or, in some cases, also directly via an input to which a switch is connected. One major advantage of the unit also lies in its diagnostic capability. It permits the condition of important consumers to be read. These include driving and position lights or indicators. If these do not function correctly, operation in road traffic is prohibited. Through the diagnostic capability and the communication via the CAN bus, an error can be shown directly in the cockpit. In addition to the design, STW was responsible for the implementation of the test software and a flash loader. The application software was then written by Fendt. It also includes the possibility of storing different configurations depending on the vehicle version.
The relay board (which explains the internal designation “RB”, see Fig. 2) only deals with the electricity and power distribution via fuses. Here, three possibilities are generally provided. The simplest current course, with which the “RB” only acts as a fuse box, runs via the connector onto the board, through the fuse and back via the connector to the consumer. Simple sensors are generally supplied using this connection type. With the second version, the power supply comes from the “power bolt”, an M8 bolt which is directly connected with the positive terminal of the battery. From here, the current runs via a relay, which is switched via an output of the “ZE”. If the relay is set to “On”, this continues via a fuse and a connector to the consumer. Typically, heating systems or working lamps are connected here. The distribution of the currents from the “power bolt” to the connectors on the circuit boards proved to be one of the major challenges during the layout. In extreme cases, a total current of over 250 A may be flowing over the circuit board at any one time. In a further version for electricity and power distribution which is intended as a reserve, the relay switching is not undertaken by the “ZE”, but rather by an external source. In this way, any voltage /current combinations can be realized. In total, over 100 fuse sockets and 26 relay sockets can be realized on the “RB”. The fuses and relays used are typical products from the automotive sector. These connectors provide space for over 40 inputs and 125 outputs.
Due to the long tractor operation times and the environmental conditions to which they are exposed, special attention was paid to the quality of the products. To do this, tried and tested concepts were used as a basis in the development phase which then proved their worth during the environmental qualification. In addition, a 100% inspection of the central electrical systems was conducted in the in-house production at STW. Special test adapters were developed for the “ZE” and the “RB”. These are used during the production test and the burn-in within the climatic cabinet. Using the test software also developed in-house, all inputs and outputs were activated both at -40°C and also at +85°C, and all functions were tested on the boards.
On arrival at Fendt, the two units are mounted above the rear wheel of the Vario 1000 on an assembly plate under a cover. Although this space stays relatively dry, the boards are cast in polyurethane casting resin in order to protect them against humidity. The connectors and fuses are easily accessible here, which is advantageous when mounting the cable tree. In order to permit easy replacement of the relay, the originally-planned distance between the relay had to be adapted to “thumb width”. As this is a low voltage supply (only 12V or 8.5V connections are available), both units are classified under Protection Class III for electrical devices. They are connected so that even if conductive components are contacted, they do not generate a risk of injury. This also guarantees maintenance-friendly servicing.
For Dr. Schmitt, however, the potential of the central electrical systems or power boards and therefore of the intelligent energy distribution has by no means been exhausted. One possibility for future extensions is the development with telematics functions, whereby a direct remote maintenance access can be created. The question of whether customer-specific versions or standard products will dominate the market is also still open. “We see several approaches available to us here. These extend from real standard products via versions with a customer-specific number of I/Os in combination with standardized control modules, right up to completely customer-specific developments. In the latter case, we specify and develop the central electrical system together with our customers with intelligent control modules, and increasingly use them for networking as a common assembly.” Here additional functions can also be realized using components such as motor bridges or stepper motor drivers. How much intelligence is actually utilized can be defined via the overall concept of the vehicle – and this is where the know-how of a vehicle manufacturer such as Fendt can be used to best effect.