Product category:
Pressure sensors
News Release from: Sensor-Technik UK | Subject: CAN-Bus interface controllers
Edited by the Engineeringtalk Editorial
Team on 03 July 2001
CAN Bus interface: the standard outdoor
controller
Controllers with CAN-Bus interfaces have proven themselves in outdoor applications such as controlling hydraulics on farm vehicles, and are now finding uses in the wider mobile machinery market
Two universal controllers with CAN-Bus interfaces have proven themselves in arduous outdoor applications such as controlling the hydraulics on farm vehicles, and are now finding applications in the wider mobile commercial machinery market Initially developed for farming machinery, sensor manufacturer Sensor-Technik Wiedemann (STW) now also offers its universal electronic controller for other industries
This article was originally published on Engineeringtalk on 26 Jun 2008 at 8.00am (UK)
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There are two versions of the controller model: ESX with one CAN-Bus interface and the EST, which has two CAN-Bus connections.
Both real-time systems can work as individual units in a bigger process system.
The ESX can be customer specified with the expansion boards using two standard internal interfaces and various modules to provide up to twelve additional input or output ports.
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The ESX works independently as a measuring, driving or controlling device for sensor-actuator management.
It is capable of executing a number of separate or related tasks in real time.
This means that proportional valves can be driven without expensive amplifier or controller cards: either a PWM (pulse width modulated) output with internal current measurement or a special output for proportional valves with integrated circuitry may be used.
CAN specification Data exchange with other intelligent units is made possible with the RS232 or the CAN-Bus interfaces: the device complies with CAN specification 2.0B with standard and extended formats supported.
Thus, a number of ESX controllers may work in tandem and/or be integrated into a CAN network.
Also, modules with further CAN interfaces may be built into the ESX, allowing it to take on a bridge function, and allowing multiple independent C1: The ESX controller has one CAN-Bus interface, while the EST has two.
Both real-time systems can work as individual units in a bigger system buses to communicate freely.
The model ESX processor system is a Siemens C167, having 16 digital and eight analogue inputs.
It can control up to twelve outputs (proportional valves up to 4A), including isolated supply to any sensor.
Programming can be effected in the high level language C or by using the IEC 1131-3 graphical interface, using a range of languages: Function Block Diagram (FBD), Ladder Diagram (LD), Instruction List (IL), Structured Text (ST) and Sequential Function Chart (SFC).
The Philips PCA82C251 transceiver chip, which acts as the physical connection (physical layer) to the bus wires, is ISO 11898-24V compliant, giving short circuit protection in 24V systems.
It can attain transfer rates up to 1Mbit/s.
The ESX electronic controller may be expanded using two standard internal interfaces and various modules to provide up to twelve additional input or output ports.
The type of port is not restricted.
These modules may also include further design features, such as a real-time clock, adding flexibility.
Software tools Characteristics, calibration data and critical parameters for sensor-actuator management, as well as controller configuration, can all be stored in a non-volatile EEPROM.
Using editor software, this data may be accessed on either the CAN-Bus or the RS232 interface.
A line of software tools is available for reading the error buffer system diagnostics, visualisation and maintenance or repair services.
Again, communication is established using the standard CAN-Bus or the RS232 interface.
The freely programmable ESX controller was developed for use in harsh conditions and will function in vehicles and machines over the temperature range -40 to +85øC.
All inputs and outputs are protected against short circuit to ground or to voltage overload and have a built-in diagnostic capability.
A robust aluminium housing (protection grade IP65, optionally IP67) with a Gore-Tex breathing filter offers high immunity to electromagnetic disturbances and protects against mechanical shock.
Applications in mobile commercial machines include: Controlling complete hydraulic machinery Steering systems (steer- by-wire) Drive controllers Anti-lock braking (ABS) systems Anti-slip regulation (ASR) Engine management systems Gear shift controllers Platform inclinations and level control systems Fluid level controllers.
For safety-critical applications, including waste disposal vehicles and hoists, the ESX has been developed to conform to the following standards: Anforderunsklasse 4 of the DIN V VDE 0801 and DIN V 19250; to Category 3 of EN 954-1; and Safety Integrity Level 2 of IEC 61508.
All input and output channels of the control systems are diagnosable, meaning that short circuits or open circuits can be detected by the software.
In addition, a continuous software diagnosis of the internal hardware is performed.
A safety relay provides a second means of switching off the digital and PWM outputs.
The function library with ready-to-use function components provides the possibility to quick and easy programming application software and may even be used without previous knowledge of any high level languages.
The Controller Area Network (CAN), originally developed by Bosch for use in the automotive industry, has established itself as the standard bus system for mobile applications (international norm ISO 11898).
Owing to their widespread use, components for CAN-based systems are available in large quantities at very reasonable prices.
CAN-Bus systems exhibit high and reliable data transmission rates (CAN low-speed up to 125kbit/s, CAN high-speed to 1Mbit/s).
A number of different capabilities (CRC, frame checking, acknowledgement, bit monitoring and bit stuffing) enable the CAN protocol to recognise errors in transmitted data caused, for example, by electromagnetic disturbances, and to correct them (transmission stop with error flag and automatic repetition of the message).
Since the length of the data packages is limited to eight bytes per message, correction takes place with very little loss of time.
A pair of wires suffices as the transmission medium.
The length of the network can be up to 40m with transfer rates of 1Mbit/s, or networks without repeaters up to 1000m in length are practical with rates of 80kbit/s or less.
The number of participants per network (in theory unlimited) depends on the types of IC chip used (transceiver, physical layer) in the equipment.
With commonly used chips, 32, 64 or up to 110 (with restrictions up to 128) are possible.
Further extensions require 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 (ie temperature, rotational or linear speeds).
All participants check out the identifier being transmitted and decide it the type of message is relevant to them (known as acceptance filtering).
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.
In case a number of participants try to access the bus simultaneously, the higher priority message is guaranteed to gain bus access.
For these reasons it is important to incorporate functional procedures and safety requirements into the process of defining identifiers.
Standard format (11-bit identifier) and extended format (29-bit identifier) are two different message formats which can coexist on the same physical CAN-Bus.
The specification CAN 2.0B supports both formats, while CAN 2.0A only allows frames with 11-bit identifiers.
Through content-oriented identifiers in a message, the system achieves a high degree of configuration flexibility and allows a simple extension of the network to include further devices.
The various semiconductor manufacturers offer CAN controllers with differing functionalities.
One common type has one data buffer for transmitting and one for receiving - here the receiver buffer is followed by a shadow buffer and the message filtering function utilises the associated microprocessor (Basic CAN).
Another type, called Full-CAN, has a number of buffers for managing and filtering multiple messages simultaneously - this reduces CPU workloads.
In addition, there are so-called SLIO (serial-linked I/O) devices, which require no further microprocessor, but function only as CAN slave modules for I/O extension. Request a free brochure from Sensor-Technik UK ...
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