Much is written about high performance factory automation, and indeed, about linking entire factory Management Information Systems by high performance SERCOS, Firewire, and Ethernet fiber and cable buses.
The CANopen protocol, which was pioneered for automobile electronic control, has a definite and under-appreciated role for industrial systems -- specifically, for multi-axis automation. Designers of assembly, test, and packaging machines can exploit CANopen networking for cost, performance, and simplicity benefits on several fronts.
Eliminate Motion Controller & Cabling Complexity
CANopen motion control works with motor drivers (servo amplifiers) that close the motor-amplifier feedback loop locally. (Example: Copley''s CANopen compatible Accelnet™ servo amplifier family.) This simple statement fails to do justice to the tremendous benefits that immediately accrue. Traditional analog systems must send motor/amplifier feedback data to a distant motion controller for computation of the ±10 V position control signals. CANopen operation, which works with servo amplifiers that close the feedback loop locally, means that no separate motion controller is needed.
In a typical ±10 V servo amplifier system, up to 12 wires are involved in communicating with the motion controller. For a multi-axis system with 6 motors, the rat''s nest of wires per drive stage expands sixfold, implying significant noise, crosstalk, RFI and wiring errors. The result: serious system setup and commissioning angst.
CANopen systems not only eliminate the motion controller. Gone too are the daunting wire bundles, replaced by a rugged and low-cost two-wire bus. Under the aegis of the CANopen protocol, all servo amplifiers, limit switches, and other I/Os, are connected via a low-cost two-wire bus, which merely requires a simple modem for connection to the control computer. The control computer sends simple positioning instructions over the bus to individual drive stages.
Lightened Load Permits Embedded Processor
Distributed motion control intelligence, stemming from each servo amplifier''s ability to close its feedback loop locally, lightens the control computer''s computation burden. In fact, instead of requiring a computer capable of heavy lifting, the entire system can be operated by a low-cost embedded microprocessor.
In summary: the motion controller goes away, elaborate wiring (along with noise and erroneous connections) is eliminated, and a simple embedded computer rather than a high-end PC controls the entire system or machine.
Further important CANopen benefits are software borne. Copley Controls'' CME2™ motion explorer software provides a powerful algorithm that completes amplifier-motor current loop tuning at the click of a mouse. CME2™''s auto phasing algorithm, also invoked by keyboard commands and mouse clicks, eliminates the arduous rewire and try of system commissioning. Comprehensive software libraries, plus a graphical user interface for setting up motion profiles, speed the completion and implementation of motion system designs.
The transition from ±10 V analog control to networked digital control introduces a potent new data capture capability. The operating parameters of a new multi-axis motion system can be captured during initial system setup. Data collected during subsequent years of operation can then be compared with the initial baseline values, for performance analysis, maintenance scheduling, etc.
Portal Connection
Wiring simplification and motion controller elimination are the hard cash benefits of CANopen control. Similarly, software libraries and CME2™ system setup software provide indirect but vital expense reductions, while speeding new equipment designs to market. But CANopen process control offers more.
Every candy wrapping machine, for instance, is linked to the main MIS bus for reporting the day''s throughput and to receive update instructions. CANopen now plays a cost-cutting and equipment-simplifying role in the interface (portal) between the MIS bus and equipment under CANopen control.
Remember that CANopen control of multi-axis automation "unloads" the control computer''s computation burden. Individual drive amplifiers make their own loop closing decisions in response to updates sent over the two-wire CAN bus. Computation demands on the control computer are therefore quite modest. Turning the candy wrapping system into a node on the MIS bus involves but a minor increment in processing horsepower. This simplification also reduces the MIS engineer''s setup and commissioning time.
CANopen Background
CANopen technology has proven itself in roughly two decades of use in automobile actuation. Roughly two-thirds of all modern automobiles depend on CAN bus automation. A two-wire CAN bus links door locks, window motors, air conditioner, and wipers, includes lights, radio, and other appurtenances, to reduce the wire bundle that would otherwise extend from driver to these individual destinations. A further CAN bus interconnects ignition control, fuel pump, shaft speed monitor, automatic transmission for engine control and fuel economy.
This proven CANopen technology has been migrated to the industrial automation arena, and enhanced to meet the needs of motion control. Standard motion commands are now incorporated into the CANopen plan, enabling multi-axis system designers to invoke "prefabricated" functions, instead of writing the software from scratch.