ServoWire, a drive network protocol version of the IEEE-1394 specification, integrates digital servo drives with a PC-based controller through special axis modules (ISA adapters). Up to eight servo drives and servo motors can be controlled by one axis module with position loop update rates of 2 kHz. For four axes, the update rate is 4 kHz.

Few would deny that networking is the way to go in motion systems today. By networking servo drives, you gain greater flexibility in system design, 50% or more reduction in wiring and installation time, and more software control and diagnostic information. You can also solve many nuisance noise problems.

Despite these benefits, less than 10% of drive systems shipped in North America during 1997 included networks, says a recent survey by Automation Research Corp., Dedham, Mass. What's the holdup? So far, everything out there either can't do the job or costs too much. Complexity has been a stumbling block as well.

For example, networks such as Universal Serial Bus (USB), Ethernet, and DeviceNet can link to servo drives, but with restrictions. USB is limited to transmission rates of 12 Mbps (megabits per second). It's unlikely this specification will change for two reasons. USB's architectured design does not include provisions for such modification and there are faster buses already available.

One of them is Ethernet, which provides data rates up to 100 Mbps. However, because of the way Ethernet manages multiple message access requests, it is not a good choice for multimedia, audio, or servo drives. DeviceNet, on the other hand, is too slow at 500 kbps, plus it was not designed as a drive interface and is highly limited for such use.

Out of the PC and into motion

One network that does offer the speed, determinism, and low cost desired in servo applications is IEEE- 1394, also known as FireWire. It was designed to cope with the increasingly stringent multimedia data needs of computer and consumer electronics. As it turns out, such requirements mirror those found in servo drive applications as well.

Fast transmission speed, for example, is a must for both multimedia and servo operation. Video data must flow at 30 frames/sec to prevent gaps in the film. This requires a transmission rate in excess of 200 Mbps. FireWire can move data at up to 400 Mbps, which is plenty fast for real-time servo loop closure.

FireWire came out of the computer industry, driving the "plug-n-play" era, so connecting a servo drive to a control is a matter of just plugging in each end. The cabling contains two power conductors and two twisted pairs for data signaling. Each signal pair, as well as the cable, is shielded. Cable power can be from 8 to 40 Vdc at up to 1.5 A and is used to provide interface power for drives connected to the bus.

The next version of FireWire will be even faster, offering rates of 1.6 and 3.2 gigabits per second (Gbps). At a recent developer's conference, engineers with Texas Instruments demonstrated a 1.6 Gbps version using standard copper cables. The main limitation is that cabling must be kept to lengths of less than 5 m.

As for determinism, IEEE-1394 allocates network bandwidth and time slots for specific communication tasks. Such tasks in servo applications might include command synchronization and multiple-axis motion. Allocation is the latest networking technique for guaranteeing the timed delivery of message packets.

Networks for servo applications also need to respond to asynchronous events, such as high-speed inputs and outputs, changes in software parameters, error and diagnostic messages, and on-the-fly system monitoring requests. Allocation of resources in 1394 are all that's needed to meet these and other requirements, opening a communication window for servo and pacer encoders, sensors, I/O, and programmable limit switches.

The servo interface

Creating a servo interface based on FireWire requires modifying the memory map to define drive setup and motion control parameters as software variables. By taking advantage of the digital architecture, a servo interface can eliminate digital to analog (d/a) conversions in servo loop operations, such as torque control.

Torque commands to the drives can then be sent as 12-bit variables, taking out the cost and limitations associated with conventional analog signal transmission and d/a converters. Eliminating analog conversions also reduces the number of data packets that move through the servo network, leaving it open for critical data.

What's more, in digital torque mode, a velocity sensor eliminates the analog tachometer and software loop parameters do away with potentiometers.

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