CAN-bus is one of the main communication buses that is used in industrial mobile machinery, in fact in a wide range of on-highway and off-highway vehicles, to connect control and sensing technologies together so they can communicate; recording and transmitting data, including errors, without the use of a central “host” computer.
‘Industrial mobile machinery’ is a broad term yet, at the same time, CAN-bus is extremely versatile. Industrial mobile machinery for which CAN-bus is suitable ranges from off-highway vehicles such as tractors, lawnmowers, forklifts, cranes, and bulldozers through to static machinery such as industrial robots and automation.
Such machinery may incorporate almost any control and sensing technologies such as servo drives, high-speed analog and digital sensors, fluid power systems and safety-related control systems. CAN-bus enables them to connect and communicate.
CAN (Controller Area Network) is a serial communications bus for bidirectional transmission of control system data. Connections between control units and input/output (I/O) devices are typically made using a two wire, twisted-pair cable that enables data rates of up to 1 Mb/s to be achieved, depending on the bus length, with 8-byte data packets.
CAN FD (CAN with Flexible Data-Rate) is an extension to the CAN bus protocol. It was created for accurate “real-time” data. It enables data transmission up to 8 Mb/s with data packets of 64 bytes.
CAN-bus benefits from guaranteed latency, excellent error detection, cost effective hardware and efficient installation.
CAN-bus is a multi-master protocol, so it lends itself particularly well to decentralised control systems, which are becoming increasingly popular. Furthermore, machines can be electrically noisy environments due to the presence of electric motors and switchgear, but this is not a problem for CAN-bus because it can achieve robust and reliable communications while still delivering high performance and near-real-time (CAN) or real-time (CAN-FD) feedback for deterministic control systems. CAN-bus systems are straightforward to modify or expand, which provides scope for upgrades as well as futureproofing. These technical characteristics, coupled with the low cost of CAN-bus hardware, make the protocol highly attractive for industrial mobile machinery.
For machine builders operating in multiple geographical regions, CAN-bus also offers an advantage over some other communications protocols in that it is truly international.
Before you can implement a CAN-bus project, first you need to know what it is that you are trying to achieve. A functional specification (or functional requirements specification, FRS) defines what the CAN-bus must do and is always important, but even more so if you might use a contractor for all or part of the CAN-bus project. The requirements specification might also be expanded to become more of a CAN-bus project specification that includes information about team members, roles and responsibilities, deadlines, future-proofing, areas that might alter as the project evolves, and other ‘soft’ issues that will help in developing a CAN-bus system that meets all of the needs.
The requirements specification will list ‘things that the system shall do’ as well as standards with which it might comply, standards against which it will be tested, any restrictions on hardware selection, flow diagrams to show how functions are related, external communications, inputs, outputs and key processes (which may be presented as algorithms, formulae, descriptions, etc).
There are four core steps to implementing a CAN-bus system.
The functional specification will greatly assist in preparing the specification for the CAN-bus system itself, which covers many aspects including but not limited to the CAN network, higher-layer protocol(s), test standards, physical limits (e.g. cable lengths), number of nodes (64 with CAN and 110 with CAN-FD), data rates, topology (e.g. linear, star, ring), environmental factors (e.g. ingress protection, shock/vibration levels, operating temperatures, electromagnetic interference), cabling, terminations, connectors and hardware (e.g. controllers).
As the design develops, use can be made of software-based simulators; the digital prototype can be tested and, where necessary, modified to reduce the risk of problems occurring once the hardware is purchased and installed. For some aspects of the system, or later in the project, it may also be prudent to run the simulation with actual controller hardware or I/O devices.
Depending on the nature of the project, the CAN-bus system may be built directly on the machine or a rig could be constructed on a benchtop or an aluminum framework for assembling the controllers and some or all of the inputs and outputs. If a rig is built, thenideally the hardware and cabling should be laid out in a manner that will be representative of the built machine, including correct cable lengths, power cables and electrical equipment that might generate electromagnetic interference (EMI). CAN-bus is a robust and reliable protocol, and CAN-bus systems are often capable of coping with a degree of ‘abuse’ – such as faulty shielding or incorrect terminations – but the greatest long-term reliability will be achieved if adequate precautions are taken when building the network into the final machine.
If a benchtop simulation is being built, care should be taken not to inadvertently avoid faults that might be present on the final machine.
It is essential that connectors are high-quality items that are suitable for the intended purpose and will, therefore, withstand shock, vibration and the ingress of dirt and moisture. Thorough labeling of cable harnesses and connectors will make assembly easier and quicker, and fewer errors will be made. After the machine has been put into operation, the labeling will also simplify maintenance and help to reduce downtime.
To support the implementation, use high-quality cable and connectors, use terminations with the correct resistance, thorough labeling will pay dividends and correct termination is essential.
An amount of testing will have been completed during the development phase by means of software-based simulations and/or hardware-in-the-loop testing.
If a trial CAN-bus system has been assembled on a benchtop, then this should be tested thoroughly to minimise the likelihood of any problems being encountered when the system is installed on a machine for the first time.
A test specification should be prepared first, and this can draw on the contents of the functional specification – for each function, a number of tests can be specified. In addition to testing the functioning of every input, output and process for normal operation, tests can be conducted for proving the system in situations where operating parameters exceed the operating specification – though the necessity and severity of this type of testing will depend on the application.
Testing the functions (e.g. does a particular set of inputs result in the expected set of outputs) is vital but it does not indicate whether, for example, the CAN-bus system is relying heavily on its built-in ability to resend data that is not received correctly the first time. For this level of testing, it can be helpful to have a PC loaded with relevant CAN-bus analysis software or, alternatively, a handheld analyser that typically has a screen, keypad and onboard software. This type of instrument can help in performing tests on resistance, cable impedance, signal level, baud rate, bus load and error frames.
Always refer to the functional specification when preparing the test specification.
Two types of diagnostic data will be of interest once the machine has been built, namely data relating to the CAN-bus system (e.g. baud rate, error rate, etc) and data relating to the physical I/O devices and controllers. The machine may have an HMI (Human Machine Interface) on which diagnostic data can be viewed, but it might also have a port into which a CAN-bus analyser and/or other device can be connected. A machine could also be equipped with Bluetooth for connecting diagnostic equipment wirelessly, or it might benefit from telematics, in which case the owner, supplier, maintenance contractor or other authorised personnel can view diagnostics remotely for faster troubleshooting and reduced downtime.
Data for the CAN-bus system can indicate degradations in overall performance, short circuits, open circuits and I/O devices that are starting to fail. It might also indicate if attempts have been made to carry out unauthorised modifications to the network.
Given the proliferation of sensors on modern industrial mobile machinery, the diagnostic data from these can prove invaluable. The codes give an indication of the I/O devices that are generating the fault, and possibly also the nature of the fault, enabling mechanics to focus on the right area and perform a repair as quickly as possible.
CAN-bus controllers can be configured to store diagnostic data locally or transmit it elsewhere – potentially via WLAN, Bluetooth or a cellular connection. Some controllers have built-in GPS position-sensing capability and/or accelerometers, so this data can be logged simultaneously with the machine diagnostic data to give more insight into causes of failures.
One of the greatest advantages of having diagnostic data available is that a predictive maintenance regime can be implemented, whereby tell-tale signs of wearing or failing components can be identified automatically prior to a catastrophic failure. Replacement parts can be obtained, and repairs scheduled for a convenient time to avoid unplanned downtime as well as the potential for failures that could have a knock-on effect resulting in more extensive damage to the machine.
Having examined the four-steps to implementing a CAN-bus system, it is worth summarising the benefits of CAN-bus for industrial machine builders that maybe seeking cost savings and a competitive technical edge:
Control Technologies UK is a CAN-bus specialist with many years of real-world experience of machine control systems and other applications, helping companies to leverage greater functionality, efficiency and cost savings.