Sugarcane Mechanisation: CANbus Networks Explained

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1 Sugarcane Mechanisation: CANbus Networks Explained B. P. Weston Booker Tate Ltd. Masters Court, Church Road, Thame, Oxfordshire, OX9 3FA. Abstract This paper sets out to provide a basic overview of the CANbus network technology used in equipment utilised in sugarcane production. It establishes that CANbus network technology enables other systems such as GPS auto-steer and machine control to work. The Paper highlights the importance the Agricultural Industry Electronics Foundation (AEF) has had in establishing industry standards required to obtain compatibility between independent equipment manufacturers and their CANbus networks, underlining the benefits being realised through data collaboration. The availability of new technologies utilising CANbus networks allows the industry to adopt new farming systems increasing cane production on the same area and drive down the cost of production. Future developments that utilise the CANbus network technology will without doubt reduce the level of human involvement in the production of sugarcane but will demand a higher level of technical understanding to maximise the technologies potential. Keywords: CANbus, CAN, bus, binary, network, Nodes, data, standards, sensors, telemetry, AEF, ISOBUS, ISO-11783, computer. Introduction. Machinery utilised in sugarcane farming continues to embrace new technologies and farming systems continue to evolve utilising available data transmission and associated machinery control. Data sources can originate from any of the following; onboard telemetry, GPS systems, remote computers, satellite data links, cellular networks and radio waves. Multiple data sources can be utilised, the choice being determined by output objectives and design criteria. Some systems are unidirectional i.e. from source to machine, and other s bidirectional transmitting information from onboard equipment to remote systems for the purpose of monitoring, processing and/or control. This paper will look at examples of integrated data management/control systems found on equipment operating in field environments and best describe their purpose and functionality. The acronym CANbus provides a specific label for onboard systems that process data received from external sources as well as data generated onboard equipment. CANbus stands for Controlled Area Network with the bus element of the acronym describing packages of data travelling across the controlled area network via multiplex wiring. Data packages leaving the CANbus will be referred to but remain outside the scope of the paper. Mutiplex. Is the physical wiring that connects the various components making up the network. There are two wires known as the Twisted Pair that are used to carry data packages over the network, it s important to understand that these wires are not used to power the system, their purpose is to carry the data.

Controlled area network CAN. In the late 1970 s Electronic Control Units, ECU s were commonly incorporated into onboard equipment control systems.

Frequently ECU s were connected together to share information with the objective of improving vehicle performance, see Figure 1.

2 Controlled area network CAN. In the late 1970 s Electronic Control Units, ECU s were commonly incorporated into onboard equipment control systems. An ECU can be considered a small computer designed to control the operation of an individual or group of components. Frequently ECU s were connected together to share information with the objective of improving vehicle performance, see Figure 1. For example the ECU controlling the brakes could be linked to the ECU controlling the engine. In this example the engine would not accelerate while the brake pedal was being depressed, increasing all round braking performance and vehicle safety. Figure 1 Comparison of ECU s connected via conventional wiring harnesses and connected by CAN. As the number of ECU s utilised on equipment increased so did the quantity of wiring connecting the ECU s, this increased production costs, complexity and decreased system reliability. In 1980 Dr Robert Bosch and Intel worked together developing the Controlled Area Network, CAN system, which is now found on nearly all agricultural, automotive and industrial equipment. The CAN connects all ECU s together via the twisted pair (Multiplex) allowing data packages (bus) to be transmitted over the CAN, see figure 1. The control of a bus over the network being achieved by each ECU. An ECU in a CANbus system is referred to as a Node. Each Node generates its own specific identity code, see Figure 2. Nodes are pre-programmed to recognise a bus (data package) containing data for their specific requirements, via an address code, ignoring ones that do not match their imbedded address code. This simple concept greatly minimised the amount of wiring used, reducing production cost and complexity. Other benefits from this new concept provided an improvement in fault finding and the ability to increase network connectivity. Figure 2 Concept of Nodes communicating over the CAN. Source:

3 Other important design features of the CAN are; CAN is referred to as a Multi-Master network meaning each Node can send a message across the network and has its own independent power supply. Nodes are connected to the twisted pair; one side to the wire referred to as CAN-H (High) and the other referred to as CAN-L (Low). The Twisted Pair do not supply any power to the network. The CAN-H circuit is used to transmit data of high importance e.g. auto-steering or ABS braking information, while the CAN-L side of the CAN carries data of lower priority e.g. turning lights on or controlling the climate temperature in the operators environment. CAN-H and CAN-L wires are twisted to eliminate any electromagnetic interference, caused by inductance etc. Any external influence on the signal voltage is equally replicated on the CAN-H and CAN-L circuits and therefore its effect is nulled, see Figure 3. Figure 3 External influence on voltage replicated on CAN H and CAN L circuits Nodes supply two voltage values. To the CAN-H wire it is either 2.5V or 3.5V and two voltage values to CAN-L wire either 2.5V or 1.5V, see Figure 4. Figure 4 Range of CAN-H and CAN-L. The CAN voltage is equal to CAN-H (3.5 V) minus CAN-L (1.5 V) or CAN neutral (2.5V), so when subtracted from each other the difference will always equal 2V or 0V, see Figure 5.

Figure 5- Variance between CAN-H and CAN-L voltages A CAN value 2V is represented by the Binary code value logic 0, and a CAN value 0V is represented by the Binary code value of logic 1 A bus

4 Figure 5- Variance between CAN-H and CAN-L voltages A CAN value 2V is represented by the Binary code value logic 0, and a CAN value 0V is represented by the Binary code value of logic 1 A bus (message/data package/frame) on the CAN is made up of a string of 1 and 0 which in binary code are called bits, see Figure 6. The data field of a bus might look like this Figure 6 CAN voltages represented as binary code Transmission rates across the CAN are typically sent around bits per second (125 kbits/s) A 120 Ohm resistor is installed at each end of the twisted pair (Wires). Resistors absorb signal energy and stop any signal reflections (noise), see Figure 7 Figure 7 Twisted pair terminal resistors. A bus is a message (data package/frame) and all Nodes are pre-programmed to construct their own buses. Each Node also reads every bus that is sent out over the network only responding to the bus if the bus is addressed with an identifier code that the Node has been pre-programmed to recognise

5 The binary output of the Node is achieved by using an integrated circuit IC (A Chip). IC s are made up of many wafer thin slices of silicon (a semiconductor) onto which many thousands of minute transistors, capacitors, and/or resistors are fabricated/etched. The IC can be designed to function as an amplifier, timer, oscillator, counter, computer memory or a microprocessor. In a CANbus Node the IC is designed and programmed to operate as a digital microprocessor. Digital microprocessor have fundamental building blocks known as Logic Gates, specifically arranged components on the silicon wafers which output voltages at three specific values, 1.5, 2.5 and 3.5V. The Logic Gate outputs are influenced by data received from the various sensors on the equipment. As explained earlier the output voltages determine the binary values i.e. logic 0 and logic 1. Buses are constructed in a format referred to as a Frame. There are basically four frame types, Data frames used to send data from one Node to many, Remote frames used by Nodes to request data from one another, Error frames Nodes use this to send an error condition, incorporating it in Data or Remote frames; and, Overload frames Nodes can identify an overload condition and request delays in further Frame transmissions. A typical Data frame format is shown in Figure 8. Figure 8 A typical Data Frame format. SOF Start of Frame always commences with a dominant 0. Arbitration ID - This identifies the message priority and the message itself, the example shows that an Arbitration extension format is available as a programming option. Remote transmission request, RTR -. Differentiates Data and Remote frames. Data length code, DLC Identifies the length of the code i.e. how many bytes there are. Data field Contains between 0 to 8 bytes of data. Cyclic redundancy check, CRC Used for error detection Acknowledgment slot, ACK The transmitting Node monitors the CAN checking that the addressed Node/s return an acknowledgement code indicating they have received the Frame.

6 End of frame, EOF A code that identifies to all Nodes the end of the frame. Data field As previously identified the data field contained within the Data Frame can be made up of 8 numbers. In Binary terms this means that each number can have a maximum value of 255 (8 bytes), see table 1. Therefore it is possible for the individual numbers to represent data from 8 different data sources i.e. engine temperature, alternator output (volts or amps), oil pressure etc. Table 1 Binary value of 255. Binary 8 bit (Base 2) Converted to Base 10 = = 255 Arbitration The purpose of Arbitration is to set a level of priority/importance of sequencing and dispatching buses (Frames) and denotes the CAN H and CAN L buses. As already identified all bits of a message are 0 s or 1 s, 0 s are dominant and 1 s are recessive. In the programming of the Node dominant bits always have priority to overwrite recessive bits, therefore the lower value has the higher priority. Telemetry. Telemetry is responsible for feeding the CANbus network the vital data required for processing. It is possible for data to be imported from external sources outside the equipment and used to assist controlling an action onboard the equipment e.g. turn off a function. Telemeters or sensors supply data to the Nodes which in turn through internal programming, use that data to construct the data fields in the Frames. The data can be supplied in analogue or digital format the Node will convert all data to Binary code. A Node can be designed to receive data from more than one sensor and include up to 8 sensor readings in the data field. The power (+) to the sensors as well as the ground (-) can be supplied by the Node being specific to individual design. Examples of sensors Figure 9 provides some examples of telemetry sensors that can be found on modern agricultural tractors. The Central Processor represents one of the Nodes in the CANbus network. Wheel encoder This sensor provides data relating to the angle of the wheels and its general steering direction. The inertial measurement unit (IMU) Data from this unit is incorporated with received GPS data to determine the equipment s geo position and direction of travel. Refreshed data is used to actuate the Electro-Hydraulic (EH) valve and adjust the steering direction. The result of this action changes the steering angle and starts the whole auto-steer cycle off again. Speed radar This measures the true forward speed of the equipment, the data output can be used to accurately calibrate equipment on the move.

7 Figure 9 Examples of a selection of sensors used on an agricultural tractor. The development of standards. The steady uptake of the CANbus system into the agricultural machinery industry exposed issues relating to integration between manufactures which were unique to the industry. Other industries were not so exposed to the diversification of additional auxiliary equipment and/or the variety of operations found in the agricultural industry. Original Equipment Manufacturers (OEM s) required the development of industry standards. The idea being that these standards would then be reflected in the design of bespoked CANbus networks facilitating meaningful communications with other OEM s networks using the same standards. The Agricultural Industry Electronics Foundation (AEF) was originally formed in October 2008 by seven international agricultural manufacturing companies and two associations. The AEF remains an independent international organisation and is a resource for industry to use in the continual development of electronic systems used in farming. The standard ISO-11783, referred to in the agricultural equipment manufacturing industry as ISOBUS, has the objective of standardizing communication protocols between mobile equipment and implements while ensuring protocols are maintained with data transfer between mobile systems and office based software. For a visual representation of ISOBUS refer to Figure 10. ISO identifies all areas making up a CANbus network and its functionality covering components making up the Physical Layers as well as going on to include higher levels such as Application Layers and Universal Terminal, Task Control functionalities. For more information refer to the web address supplied above.

8 Figure 10 Visual interpretation of ISOBUS network. Source, S. Peets, Harper Adams University. In Figure 10 a tractor is being used to illustrate the prime mover, however, this could equally be the outline of a modern day chopper harvester, self-propelled sprayer or lime spreader etc. The concept remains the same with possibilities being endlessly variable. In Figure 10 two CANbus systems are illustrated. The blue line represents the equipment manufactures own system and the red line shows what could be an aftermarket ISOBUS system or an ISOBUS system installed by the OEM to facilitate aftermarket equipment. In the operational environment there are frequent issues relating to the accessibility of OEM data through the interface between the OEM s CANbus system and the ISOBUS system. The AEF have developed three levels of integration between the two systems, these are TECU Class 1 Allows the transfer of basic tractor internal measurements e.g. true ground speed rear PTO speed etc TECU Class 2 Used with complex auxiliary equipment allowing better operational control and data integrity. TECU Class 3. Allows direct communication between CANbus Nodes but the tractor may not produce a response to the external data source request. The AEF provides a database of OEM equipment and what TECU class of ISOBUS has been built into the equipment. This helps other OEM s and purchasers of equipment to understand the level of integration possible between the OEM and third party CANbus networks. Photo 1 shows a tracked prime mover with a sugarcane billet planter and a chemical doser. It is quite possible that each unit will have individual CANbus networks and each will have the need to communicate with each other.

Photo 1 Tracked prime mover with sugarcane billet planter and chemical dosing unit The ability for individual CANbus networks to collaborate allows the unit to establish the optimum workrate under

9 Photo 1 Tracked prime mover with sugarcane billet planter and chemical dosing unit The ability for individual CANbus networks to collaborate allows the unit to establish the optimum workrate under varying conditions across the work area. For example the desired seedcane planting rate may be 8 tc/ha, if the planter knows at all times what the forward speed of the tractor is then the planter, via the planter machine controls, can ensure the planting rate will always remain at 8 tc/ha irrespective of the tractor forward speed. At the same time the chemical doser will respond to ensure the correct amount of chemical is applied to the seedcane as the volume of seedcane being planted increases or decreases with variations in forward speed. Without this collaboration units would have to be calibrated at fixed values and the forward speed of the tractor would have to be constant. Obviously as the unit starts and stops its forward speed will fall outside the planting calibration requirements resulting in not enough or too much seedcane and chemical being planted/applied. The same goes for any deviation in speed across the field. In this scenario resources would be wasted, there would be a negative impact on the future crop and cost of production would be higher, becoming significant over larger areas. At the same time as the unit is planting the onboard performance data could be transmitted back to a central computer along with geo references in real time, see Figure 10. This data could be used to generate reports identifying daily work rates, area planted, tonnes seedcane planted, tractor hours worked, fuel consumed, fuel consumption per hour and cost, engine temperature, litres of chemical used, the list being limited by what is being measured. While separate CANbus networks now have the ability to collaborate by sharing varying levels of information. Frequently individual CANbus networks are required to display the network information separately, see Photo 2. Obviously there is a way to go with some aspects of CANbus integration.

10 Photo 2 Individual CANbus network displays. Conclusion. New and evolving systems and technologies are making use of the standards developed by the AEF and the adoption of ISO by an increasing number of OEM s means that it is here to stay. The collaboration of CANbus networks on individual pieces of equipment in the operational environment, now mean greater efficiency being designed into agricultural equipment and office based networks. These improve the effectiveness of farming sugarcane, resulting in improved crop production, better use of limited resources and a real opportunity to drive down the cost of production. The industry will become less dependent on labour but will demand higher levels of technical understanding to maximise the technologies potential. References. Duncan. T. Electronics for Today and Tomorrow, (2 nd Edition). Hachette UK, London, Gibson, J. R. Electronic Logic Circuits, (3 rd Edition). Hodder Headline Group, London, Watson, J. Mastering Electronics (4 th Edition).Macmillan Press Ltd, Basingstoke, England, Ross, D. et al.electronics for Dummies, John Wiley & Sons Ltd, Chichester, England Analog Devices AN-1123 Application Note Controller Area Network (CAN) Implementation Guide, Dr. Conal Watterson. Application Report SLOA101 August 2002 Introduction to the Controller Area Network (CAN) Steve Corrigan Sensor Architecture and Task Classification for Agricultural Vehicles and Environments Francisco Rovira-Mas. Peets, Dr. S. Controller Area Network (CAN). E7009 Precision Farming Technology. Harper Adams University. February

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