CAN Newsletter June 2006

Safety at a dizzy height - with CANopen Safety SIL 3

We cannot imagine life today without electronics in mobile machines. Many necessary and convenient functions could not be implemented without electronic systems. In contrast to electronics in ”normal” industrial applications such as packaging machines and conveyors, the requirements for components for mobile use are much higher in order to achieve adequate operational reliability in the requested conditions of use.
Since wiring is to be reduced in many machines, a decentralized machine design is preferred. One or several decentralized input/output modules are connected to a central controller. Via the CAN bus the I/O data are read, processed in the controller and then transferred again to the I/O modules via the bus.
One task, however, could not be implemented by this system until some time ago: the transfer of safety-related data between the network participants. The implementation of CANopen Safety (CiA 304) as an extension of the CANopen communication profile has made it possible now to exchange data safely on the same bus cable in parallel to the PDO communication between the CAN bus participants (e.g. between two control modules).
The prerequisite of this communication possibility is that the bus participants generating or reading these safety-relevant data support the corresponding CAN mechanisms and have hardware according to the requested safety category.
As for the design of a safety controller and the implemented application software the correctness of the data has to be ensured on a safe bus system. If an error occurs during communication, there has to be a reaction within a sufficiently short period of time and the machine has to be passed into a safe state.
At the same time it also has to be ensured that the implemented safety functions do not affect the communication of the normal (not safety-related) bus participants.
The entire safe communication is based on the standard CAN mechanisms and is integrated in the CANopen communication profiles. This ensures that data can be exchanged between non-safe and safe participants simultaneously on one bus cable.

Livestock farming enters the CAN automation world

Quality wants and needs to be verifiable and affordable. Automation and documentation of work processes is normal practice in industry. Industrial methods are however also being increasingly employed in farming. In times of food scandals and animal epidemics, it becomes increasingly important for producers to be able to provide evidence of what goes on in their farms. This leads to the additional use of electronics even in the cowshed. This article explains which objectives one can achieve with such equipment and which demands can be satisfied on the basis of a panel PC with a CAN connection.
The problem of every scandal is that the consequences always hurt the wrong people. While the bad guy is already lying on a sunny beach counting his dishonestly earned cash, the respectable majority in the business must fight the consequences of media reports and resentment against their products.
Farm operations must be documented and verified to regain consumer trust. It no longer satisfies the consumer to be able to inspect the birth certificate and family tree of the cattle; they also want to make sure that the animal has not been pumped full with unnecessary medical drugs. After all, the antibiotics are supposed to heal the next cold, even after a generous portion of one’s favorite steak, roast pork, etc.
However, the producer also has a benefit from controlling and documenting the suitable feed and medication doses. It is a bitter fact that feed and drugs cost money and therefore directly affect profits.
Certification of their health care can also be attached to each cow in the same way as a production report on the creation process of high-quality industrial products. The certificate helps provide clarity if the quality is criticized. The more accurately this work is done, the quicker the issue is cleared from your desktop – that saves time, money and nerves.

Make work routines safer

Humans tend to make errors during routine tasks. Forgetfulness, carelessness and increasingly lax handling of critical materials are the causes of many small and large catastrophes. This militates in favor of delegating such jobs to automated systems. The computer neither forgets Daisy the cow, nor does it mix her pills into her feed according to the slogan ”the more, the better”. It keeps to the dose specified by the vet and it is no problem keeping account of what it has done.
The heart of the system described here is a panel PC. The integration of computer, controller and user interface results in a compact and robust unit. The panel PC communicates with the other plant elements via its CAN interface.

Banding door and table edges via CANopen HMI

An edgebander is used to glue on edges made of veneer, synthetic material or real wood to chipboards, such as tabletops, doors, etc. The edges are glued on with hotmelt adhesive in a through-feed process and are then trimmed. In the following production steps, the board edge is milled, radius-milled and processed by various other units. When the final product leaves the machine, its banded edges have been completely processed and an additional manual refinishing is not required anymore. An edgebander is a processing machine that is continuously fed with work pieces. The work pieces are fed through the machine and the various processing units successively. The edgebander can easily process several work pieces simultaneously. The speed with which the work pieces are fed through the machine depends on the edge material that has to be applied and processed. Typical feed rates are in the range of 8 m/min and 30 m/min. The overall length of an edgebander can vary from about 3 m to up to 12 m, depending on the assembly of units. Every customer has different requirements on the processing of wooden work pieces. In order to be able to meet these requirements more than 100 different processing units have been developed. A modular system allows customers to assemble their special machine in accordance with their requirements. Apart from the chain drive, the essential components of an edgebander are various glue-application units with integrated heating, different edge magazines, a pinch roller, a cabinet scraper, caps, buffing units and milling cutters as well as various additional special components.

Special operation requirements

A special feature of the edgebander is that the basic machine and the individual processing units can be set via a comfortable operating unit. This enables a quick configuration of the machine for different work-piece materials and processing steps. The data is stored in a recipe database and the datasets are clearly arranged with names. These datasets can therefore be quickly accessed again to configure the machine for different requirements.

Enhancing CAN systems with optical fiber links

This article will discuss technical solutions and application examples for CAN-based systems, which combine CAN segments with electrical signaling with communication links based on fiber optics. Among those systems three basic principles to link optical segments with electrical segments are considered.
CAN with the corresponding physical layer standards for electrical signaling is a powerful, robust and cost effective communication network for industrial control applications. So why add optical fiber links to CAN systems? There are a number of ways how the overall behavior of CAN-based systems may be improved by use of optical fiber links as additional system components. Some of the benefits that can be achieved are improved EMC behavior, increased flexibility regarding installation topologies, enhanced communication system speed and throughput with reduced latency. Another reason to use optical fiber links in CAN based systems may simply be to deal with existing infrastructure. Many building automation or transport systems meanwhile include bundles of installed glass fiber, whilst signal lines for electrical CAN data transmission are not in place.

Fiber optic converters

The most straightforward solution to add optical transmission is implemented in most CAN devices in automation systems, however it is rarely applied in the form of a fiber link. It is realized by the optocouplers between CAN controller and CAN transceiver. The same principle - putting an optical link into the transmit and receive line between CAN controller and CAN transceiver - can be applied for fiber systems; functionally the optocoupler is cut into parts, which are connected by fiber segments. This solution is easy to implement and serves tasks like high voltage isolation quite well.
An implementation example of this solution can be found in mining machinery from Voest Alpine Bergtechnik (www.voestalpine.com), a company within the Sandvik group. CAN nodes with fiber optic interface in the bolter miner series of machines are connected to a copper based CAN bus by fiber optic converters. While this is the preferable solution to connect individual nodes to a CAN network and fits the requirements of machinery well, with regard to system design there are limits of this approach. Specifically designed CAN nodes with a matching optical interface are required, off-the-shelf CAN components cannot be used. Furthermore the approach limits the network topology to a single bus structured electrical CAN segment, linking of spatially distributed electrical CAN segments is not possible. If optical fiber links are required to connect bus segments instead of individual CAN nodes, more complex solutions need to be implemented. A characteristic attribute of CAN segments with standard electric signaling is that the direction of signal transmission is not easily available to an external observer. Due to the nature of CAN with bitwise arbitration, acknowledge flag and error frames, the direction of transmission changes even within one CAN frame. Active elements like fiber optic converters however need to know at which time to drive which of the interfaces. So while the direction of transmission is obvious as long as a single CAN controller is connected with its corresponding transceiver (dedicated RX/TX lines), the direction needs to be somehow regenerated to be able to connect bus segments by active elements.