Out of the rack...and into the fire

The third motion architectural approach, and one that has gained in importance in the last several years, is known as a distributed drive. This approach combines the synchronization ability of multi-axis motion cards with the reduced wiring of standalone drives. Such a drive uses a serial network to connect a central host, but still has all the standard drive features of profile generation, amplification, and internal AC or DC power management. This architecture is shown in Figure 3.

Two kinds of distributed drives are used. The first can be referred to as a tightly coupled drive. This type of drive uses high throughput, deterministic networks such as SERCOS, Firewire, EtherCat, or Ethernet/Powerlink. The second can be referred to as a loosely coupled drive, and uses slower or less deterministic networks such as CANBus, and standard Ethernet.

One difference between tightly and loosely coupled approaches is that loosely coupled drives are controlled directly from the host, by sending commands such as “move the axis to position

Standalone driver

Distributed drive

x using a trapezoidal profile.” Tightly coupled drives are different in that each drive receives rapid, synchronized, position and/ or velocity updates. This occurs hundreds or even thousands of times per second.

Just like standalone drives, the advantage of distributed drives is reduced wiring and increased reliability. Another big advantage, particularly compared to a multi-axis motion card approach, is scalability, and motor interchangeability. Adding an axis in a distributed network is simple, since each drive tends to be a single axis module.