Getting on board the right fieldbus

Oct. 20, 2005
Larger and more complex machines, using centralized controllers such as those with multiple axes, need substantially more wire and cable.

More wiring increases the potential for errors. The sheer size or volume of the wiring can make cabling too big to flex as axes move and articulate.

A distributed-architecture strategy can shorten wires by locating the intelligence on the machine near sensors and actuators. Also, a fieldbus sends information via a single wire. Still, much of the distributed hardware and centralized software available today was developed for process-control applications, which don't need the high determinism necessary for motion control.

Transmitting bidirectional communication with the node at a higher-level approach allows lower frequency communications and determinism. It is practical to turn a motion axis into a node on the I/O fieldbus in some applications, particularly if the motion is not coordinated or precise. But some fieldbus-based axis drives only handle multiple tasks through use of numerous setup parameters such as setpoints, speeds, accelerations, multiple gain terms, and safety limits. Even when most parameters are constant, it is much more complicated than setting variable parameters such as end points and speed with a centralized controller. In essence, hardwire complexity is exchanged for software complexity.

Most I/O fieldbuses lack the necessary synchronization mechanisms to assure precise coordination in applications requiring tight coordination between axes. There's a potential for problems if multiple axes move simultaneously, whether or not they are tightly coordinated. For example, consider the case where an axis faults out, usually caused by slow feedback update rates. The controller might not receive information in time to prevent damage to the machine, product, or both. Even with sequential moves, the distributed approach may be unmanageable. For example, a distributed system and typical I/O fieldbus may experience delays confirming a properly executed move. The delays can add 30% to overall cycle time when compared to a highly centralized system.

There is a way to get the hardware benefits of a distributed system while maintaining the software benefits and performance of a centralized system. The approach uses distributed-interface electronics with centralized control software computing most of the calculations on a single processor.

A fundamental enabling technology for this linkage is 125-Mbps Ethernet. Its software protocols are not deterministic, but deterministic systems can use its underlying hardware. Use of high-bandwidth fiber-optic transmission eliminates electromagnetic interference. And fiber optics provides automatic isolation between nodes, eliminating potential ground-loop problems.

To control the stages and rigging for the airborne acrobatics as well as various other moving prop elements, K¡ uses over 300 axes of motion. An enormous amount of rigging equipment is hidden in the rafters.

K¡ requires 1,500 ft between nodes on the computer network ring because of the size of the stages and their moving components. The Macro fiber-optic-fieldbus can support nodes as far apart as 10,000 ft. Control of all the axes is via closed loop using the Macro fiber-optic communication system between nodes.

A Delta Tau UMAC controller maintains system variances between the four vertical cylinders of less than 0.125 in. The Macro loop controls remote systems and its fiber-optic ring eliminates noise or electromagnetic interference. A 100° tilt is controlled via an additional four cylinders paired with two hydraulic servovalves. The Macro loop also controls two safety nets each having nine hydraulic winches with a dead band of ±4 V. Controllers from Delta Tau handle the 18 identical winches used in the performance.

125 Mbps (Mbits/sec)
40 KHz (16 nodes)
Fiber optic & RJ-45 twisted pair
12 Mbps (Mbits/sec)
RJ-485 twisted pair
16 Mbps (Mbits/sec)
16 KHz
*(Data current as of completion of this paper.)

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