CAN Newsletter March 2005
| Business | Fairs, congresses and expositions - The K-fair, plastic-processing with CAN - CANopen market survey 2004 |
|---|---|
| Device | Motion control drives and motors - Sensor survey - Driving four step motors with a CANopen module - USB-to-CAN gateway survey - HMIs with CAN, CANopen and DeviceNet - Industrial PC, CANopen controller - PLC, iPC, vehicle controller |
| Application | CANopen controls envelope stuffing machine - Driving concept reduces setup times - CANopen pressure sensors in practice - CANopen listens to the grass growing - CANopen in medical testing laboratory - CAN in case packing machinery |
| Tool | CANopen protocol decoding - Tool extensions, updates and new tools - EDS-to-XML converter |
| Semiconductor | Micro-controllers and transceivers |
| Specification | The CANopen layer Setting Services (LSS) - CiA DSP 309-3 implementation |
CANopen in medical testing laboratory
Medical testing laboratories must automate their test procedures in order to stay competitive and keep costs at bay. Drug and biotech companies do the same to speed up their development cycles. Automated material handling and storge systems for medical laboratories have become available to do just that. They contain trays for storing chemicals and test materials in temperature-controlled environments and handling systems for moving the trays to test areas where materials can be mixed automatically. The resulting mix is placed in test tubes or plates and is then either stored or transferred.
Necessary for such a system are two main parts: a database that provides information on where the materials are stored and that keeps a history of each individual test tube or plate, and a computer that controls all activities and movements.
TekCel, a process management provider for drug researchers, used to build their servo systems in-house. The increasing complexity of keeping systems up-to-date made it very time-consuming to dedicate enough manpower to servo systems. The company decided to buy rather than build. The systems are used by pharmaceutical companies to put disease cells in thousands of test tubes and the add chemicals that might serve as antidotes. When the test chemicals show positive results they are taken out for further research. Test are run on as many as 250,000 test tubes, and it is not unusual for researchers to take out as many as 2,000 vials for further research. This means that the system has to cover a large area and move racks and transfer tubes at fairly high speeds with the utmost accuracy. The incentive to find a commercial rather than a self-built solution grew as staff spent much of their time away from what was their core business. The challenges to the system were manifold:
- the servos had to work in a -20 °C frigid environment,
- the space was very limited; the drive had to be placed close to the motor,
- the company wanted a distributed system architecture,
- the system had to provide power for linear and rotary motors,
- and was to be controlled by self-contained programs (position mode)
CANopen in envelope stuffing machine
Mailing is an indispensable process for any company, and one in which considerable savings can often be made. This is particularly the case with companies with highly structured business processes that rely on effective document management in the mail room. This includes the cutting, folding, adding of inserts, stuffing of envelopes, addressing and franking. The envelope machine has a key function here because the speed of operations depends on its flexibility and capability.
With a speed of 15,000 cycles per hour, the Challenge mail processor from Grützmacher System is a very powerful machine; it can process envelope formats from DIN C6 to C4. Up to 13 supplement feeder stations can be integrated as modules and can also be added at a later time. The machine’s ability to adapt to the type of paper feed required provides the flexibility it offers. The inserts can be fed in from the top or bottom with pushing or manual feeders, as well as with suction. The modular design of the stations even enables customers to re-arrange the order of the insert feeder stations in order to meet their own requirements. Grützmacher System is based in Germany, and is one of the leading suppliers worldwide of envelope and mail processing machines. The company set the requirements for a new control concept that would offer increased flexibility and processing speed: A fast PLC cycle time for optimizing machine performance; a rugged decentralized operator station with a scratchproof color touch pad; open software and hardware standards; remote diagnostics and tele-service via modem or web server; Ethernet connectivity to other third-party systems within the document management system of the end customer. Grützmacher System chose a PLC concept: The XC200 PLC with the MV4 and MV4-CFG-1 configurator (Galileo) were used. The XC200 comes with CANopen integrated on board, and with a cycle time of 0.05 ms/1000 instructions. Ethernet, RS-232 and USB interfaces are provided as default features as well as an OPC and web server. These provide the basis for remote diagnostics, remote programming and tele-service facilities; they also allow the vertical integration of customers’ third-party systems within the document management. I/O modules for up to 32 channels enable installation, and decentralized I/Os can be connected via CANopen using the company’s XI/ON system. Frequency inverters, such as the DF5 series used at Grützmacher, can also be networked via CANopen. The concept also includes the integration of CANopen-compatible systems of other supplier such as rotary encoders.
The CANopen Layer Setting Services (LSS)
CANopen devices, which do not have a hardware interface (e.g. DIP-switches, RS-232) to set up the CAN bit-rate and the node address use the LSS protocol for that purpose: Two identifiers are used for communication between the LSS master (CAN-ID 7E5h) and the LSS slave (CAN-ID 7E4h). In order to identify a certain CANopen node, the LSS address of that node is used. The LSS address consists of four 32-bit keys (vendor-, product-, revision- and serial-number), so that every CANopen device has an unique 128-bit key for identification. By means of the “configure bit timing parameters” service the LSS master sets the new bit timing on a LSS slave. For CANopen devices, the following bit timings are defined:
Configuring a wrong bit-rate for a CANopen device is a critical issue. Depending on the data stream stuff-bit, CRC, form or acknowledgement error occurs and the CANopen device will signal this by transmitting an error frame. The process of error detection and signaling stops, when the device with the incorrect bit-rate goes into bus-off state.
A misconfigured CANopen device that supports bit-rate setup only via LSS is in a deadlock situation now: it cannot be reached via the network. The misconfigured device must be removed from the network and the stored bit-rate must be evaluated by “try-and-error”, using a point-to-point connection between a LSS master and the LSS slave.
In order to avoid the described deadlock situation two solutions seem reasonable:
- Usage of a default fall-back bit-rate
- Usage of automatic bit-rate detection
