CANbus control: how does it work?

Morten Moller of Sensor-Technik UK explains the background to CANbus control, from where it's used to how it operates.

CANbus control units and HMIs (human machine interfaces) are used in a wide variety of functions in mobile and stationary applications.

Via the various configurable inputs and outputs it is possible to connect all types of sensors and actuators directly to the control unit; the user-programmed controller monitors the system inputs and allows either closed loop or open loop control of the outputs.

This way, components such as proportional valves can be controlled directly.

Together with extended diagnostics functions and the CAN interface, the system offers unmatched in-service facilities.

In case of failure (such as broken cables or short circuits) it allows users to restore the vehicle to safe conditions and provides service staff with the right tools for quick and easy diagnostics.

With the additional possibility of remote (wireless) maintained diagnostics, the CANbus is capable of making a significant contribution to increased vehicle availability and productivity.

More and more vehicle manufacturers and automotive component suppliers are realising the benefits of the CANbus system.

This technology has now reached a stage where internationally agreed standards exist, so that different manufacturers' electronics can communicate together.

A typical modern truck might have separate electronic modules controlling engine, automatic gearbox, ABS brakes and onboard hydraulic control.

The engine module has a sensor reading rotational speed.

All the other modules, such as the gearbox, need to know the engine speed to operate correctly.

The engine module will broadcast the engine speed message on the CANbus, and a module connected to the bus can receive the message and use that information.

This means that the engine speed sensor need only be connected to one of the modules.

The versatility of CANbus modules is evident from the wide range of applications in which they are used.

They include specialised civil engineering, mobile cranes, agricultural/harvesting machinery, forklift trucks, refuse collection vehicles, steamrollers, aircraft towing vehicles, road sweepers, excavators, fire engines, military equipment, boats, shipyard cranes and diggers.

But what's the background to this technology?.

The Controller Area Network (CAN) was originally developed by Bosch for use in the automotive industry, but has established itself as the standard bus system for mobile applications, and is defined under ISO11898.

CANbus systems exhibit high transfer rates up to 1Mbit/s and high data transmission reliability.

A number of different capabilities (CRC, frame checking, acknowledgement, bit monitoring and bit stuffing) enable the CAN protocol to recognise errors in the transmitted data (caused for example by electromagnetic disturbances) and to correct them.

This correction takes place with very little loss of time, making CANbus extremely reliable in demanding applications.

A pair of wires forms the transmission medium.

The length of the network can be up to 40m for the maximum 1Mbit/s transmission speed, but networks can be up to 1000m in length without the need for repeaters and are practical for transmission rates of 80Kbit/s or less.

The number of nodes on the network is unlimited in theory, but in practice depends on the type of chip used.

With commonly used chips, 32, 64 or up to 110 nodes per network are possible (or 128 with restrictions), whilst further nodes can be accommodated by using repeaters or bridges.

CAN is a "multimaster system" with line topology and real-time capability.

Unique "identifiers" contain information not directly related to the address of a participant, but to the contents of a message (such as temperature, rotational speed or linear speed).

All participants check out the identifier being transmitted and decide if the type of message is relevant to themselves.

In this way, all messages can be received from many or all of the participants simultaneously.

The unique identifier also determines the priority of the message relating to bus access.

Should a number of participants try to access the bus simultaneously, the higher priority message is guaranteed to gain bus access.

Standard format (11bit identifier) and extended format (29bit identifier) are two different message formats that can exist on the same physical CANbus.

The CAN 2.0 B specification supports both formats, whereas CAN 2.0 A only allows frames with 11bit identifiers.

Through content-oriented identifiers in the message, the system achieves a high degree of configuration flexibility and allows a simple extension of the network to include further devices.

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