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CAN Bus interface:
The standard outdoor controller
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. Units are developed specially for direct sensor-actuator-management
in a compact and robust construction, meeting the needs of bus-networking
in various industries.
The integration
of electronic controls in vehicles and machinery gives savings on
cabling and construction. 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.
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 +85°C. 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.
INOC
- Sensor-Technik
Wiedemann
Email
b114@industrialnetworking.co.uk
CAN in a box
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.
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- Sensor-Technik
Wiedemann
Email
b114@industrialnetworking.co.uk
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