How to choose a CANbus industrial controller
Initial price for an industrial PC is higher than a consumer equivalent, but long-term cost is lower for industrial control applications. Here's why.
Ulrich Luetke Entrup, Janz Automation Systems AG, and Alan Lowne, Saelig Co. Inc.
Industrial PCs (IPCs) with an integrated CANbus industrial network can help today’s machine designers incorporate needed hardware and software, with minimum development time, greater reliability and flexibility, and easier maintenance and upgrades. While, product managers insist on low manufacturing costs, the up-front purchase price of a controller certainly is not the best arbiter of a wise decision.
Increasingly complex applications demand such features as high-speed control, complex motion handling, special analog I/O modules, fast human-machine interface (HMI) development, and other features. Industrial PCs typically cost significantly more than consumer PCs, but using a cheap product today can translate into expensive problems later. Field servicing is always many times more costly than producing robust, reliable devices that seem more expensive initially. Important technical considerations can make cheaper off-the-shelf solutions an unwise choice.
Total cost of ownership
Many equipment purchasers (and their supervisors) fail to understand the concept of total cost of ownership (TCO) versus purchase price. TCO helps managers evaluate direct and indirect costs related to the design-in decision, so the magnitude of an initial price can be offset by the ensuing maintenance cost savings realized over the life of the equipment. This almost always justifies a higher initial outlay for more reliable equipment.
Most engineers are familiar with the selection process for a home PC (speed, memory, I/O devices, etc.) Industrial PC designs also should consider communications, remote access, operator interface (custom keyboards and operator panels or touch-panel display), provisions for future expansion, such as extra PCI slots, and other considerations. Network connections, in addition to CANbus, can include RS-232/-422/-485, IrDA, Bluetooth, USB2.0 (soon USB3.0), WiFi or other wireless connections, in addition to various display options, such as multiple displays or rugged touch-screens.
One of the first decisions is selecting the optimum operating system software. Is Linux appropriate, or must the system handle various Microsoft Windows operating systems? Some applications require instant-on with no boot-up time. CoDeSys Soft PLC Runtime Environment, a device-manufacturer-independent IEC 61131-3 programming system from 3S-Smart Software Solutions GmbH, can simplify external peripheral access and display. The IPC manufacturer should be well-versed in such packages.
Using an operating system like Microsoft Windows CE or Linux offers notably fewer possibilities for malicious attacks from viruses or trojans. Local operating systems can be reduced to “bare bones,” which also reduces boot-up time. PCs can sometimes be switched off inadvertently, or damaged by power failures. A LAN connection helps system administration—even operating system kernels can be changed via LAN in a running system.
In selecting an industrial PC, consider:
Pre-load of required application software to simplify product assembly;
100% test and burn-in;
Infrequent component changes. When changes occur, make available full, traceable historical information, guaranteeing transparency, without effect on product operation or specification. (Commercial PC component and design changes are frequent, and product life is measured in months, not years.)
Electromagnetic compatibility/electromagnetic interference/electrostatic discharge (EMC/EMI/ESD) requirements. The IPC must not interfere with other electronics on the machine. It should be immune to any radiated interference from the other electrical controls and environments. It should also function and survive severe static charges. Exact standards requirements depend where the equipment is to be sold. U.S. marketed equipment must at least meet the emissions standards of FCC 47 CFR Part 15-2001 (Class A).
Robustness, by eliminating moving parts like fans and hard-drives, low-power design techniques, solid state memory. These can reduce or eliminate service costs for the controller and increase vibration tolerance. Low-power designs allow reliable passive cooling.
Keyboard availability, customized to suit the alpha/numeric standards worldwide, with features like emergency stop switches;
Enclosures appropriate for the environment. For IPCs mounted to the outside of a machine, an IP65-rated enclosure may be necessary—even for keyboards. The machine may have to run in extreme hot and cold conditions—typical industrial controllers will operate at ambient temperatures from 0 to 50 °C, with humidity up to 90% and at altitudes of up to 3,000 m above sea level. Mounting options might require DIN-rail accommodation, remote, inaccessible positioning, or weatherproof installations.
When selecting a IPC supplier, consider:
Longevity (including long-term availability) and reliability;
Reputation. An IPC manufacturer with a respected reputation should participate in the product design cycle, customizing to meet application needs, including the “trade dress” of the product;
Design knowledge to accommodate a wide range of input voltages and other hardware and software specifications, anticipating situations the customer may not have considered; and
Service and support with “one-stop” advice.
Also read from Control Engineering - How to choose a controller - cover story.
Ulrich Luetke Entrup serves the south/west Germany sales regions for Janz Automation Systems AG.
Alan Lowne is president of Saelig Co. Inc., supplier of industrial and embedded PCs from Janz, Amplicon, and Comfile.
CAN networking provides crucial data integrity, error detection
CAN protocols are becoming standard for under-the-hood connectivity in cars, trucks, and off-road vehicles, as well as medical instruments, and production lines. CANbus runs real-time critical functions with high transmission reliability. Multi-master hierarchy, sophisticated error detection, and retransmission techniques also provide a high level of data integrity, which is crucial to many applications.
CANbus often can help to facilitate diagnostic checks using a laptop computer—even remotely over a dial-up phone line or Internet connection, simplifying service dramatically. Advantages of CANbus compared to other network solutions are based on robustness and the price/performance ratio. Highest speed currently supported is 1Mbit/sec, and modern CAN interfaces have hardware filtering and buffering features that can ignore unwanted communication and buffer back-to-back messages.
Suppliers familiar with high-level protocols, such as CANopen, can help with system integration. While CANbus adapters or USB dongles can decrease cost, they reduce CANbus system performance. Designs with higher complexity can lower reliability, shortening mean time between failure (MTBF).
Ideally, a CANbus PC should have the CANbus ports built-in using intelligent CANbus ICs like the Philips SJA 1000, which handles the CANbus protocol without burdening the CPU. Integrated opto-isolation of the CANbus port also is worth specifying to eliminate ground loops.
CANbus trade organization, CANbus in Automation, helps equipment manufacturers and end users apply CAN-based industrial communications networks.