Out of the rack...and into the fire

The third motion architectural approach, known as a distributed drive, combines the synchronization ability of multi-axis motion cards with the reduced wiring and increased robustness of standalone drives. Such a drive uses a network connection to communicate with 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.

Depending on the application required, two kinds of distributed drives are used. The first can be referred to as a tightly coupled drive, and uses high speed, deterministic networks such as SERCOS, Firewire, EtherCat, or Ethernet/Powerlink.

Standalone driver

Distributed drive

The second can be referred to as a loosely coupled drive, and uses slower speed networks such as CANbus and RS-485, or less deterministic networks such as Ethernet.

One big difference between loosely coupled and tightly coupled distributed drives is that tightly coupled drives require a motion card to synchronize and coordinate the motion of each axis. Loosely coupled drives are controlled directly from the host, by sending commands such as “move the axis to position x using a point-to-point s-curve.” Tightly coupled drives are very different in that each drive receives rapid, synchronized, position and/ or velocity updates, generally several hundred, or even several thousand times per second.

The advantage of distributed drives is reduced wiring and increased reliability. Another big advantage is scalability. Adding one

Integrated motion card

more axis to a distributed drive network is a simple matter of plugging in another drive. In multi-axis motion card architectures, adding another axis can require a whole new card purchase in the situation that (for example) a fifth axis must be added to a fouraxis card. Distributed networks also have the advantage that it is also much easier to mix and match motor types. For example a network could be constructed with some DC Servo motors, some Brushless DC motors, and some step motors. As long as each drive talks the same “language” on the network, the host software need not be aware of motor type.