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.
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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Also, drift can be eliminated through control algorithms that use 32-bit resolution. And, application programs can be set up to dynamically adjust drive settings or loop parameters for changes in such factors as load inertia.
Tapping into other features of IEEE-1394, a servo-specific implementation can set aside an amount of bandwidth for each servo, accommodating torque commands, position feedback, and high-speed I/O status during loop updates. This allocation is based on how 1394 uses isochronous and asynchronous data transfers. The isochronous data channel guarantees data transport at a pre-determined rate, which is especially important for timely delivery of data such as loop updates.
The high-speed I/O in the isochronous data channel ensures that any axis module can drive or respond to programmable limit switches and sensors in the following loop update. This feature is particularly beneficial in line-oriented manufacturing systems, such as those found in packaging and converting. In the case of a rotary knife, for example, suppose a machine has to cut a web of pre-printed boxes to an accuracy of 15 mils at line rates up to 1,200 ft/min. Here, PLCs may not be able to respond fast enough to the inputs of the rapidly changing I/O.
Once each loop completes isochronous transfers, the remaining bandwidth is available for asynchronous transfers. Asynchronous communication manages real-time command and status information on the network. It provides a way to dynamically adjust tuning parameters, and modify drive setup.
In winding-unwinding operations, the ability to adjust parameters through software lets engineers adjust system inertia as motor loads vary, for example, as roll diameter changes. In such cases, load-to-motor inertia mismatches can be as large as 1,000 to 1.
Plug it in
Like many new networks, FireWire is designed for "plug-n-play." This, obviously, can expedite set up of devices on the servo network. At powerup, a controller will search for any servo drives connected to the network. To each drive, it downloads drive parameters according to instructions in the control software.
This greatly simplifies servo drive replacement in the field. In addition, new drives do not need off-line setup for proper configuration, and so can be installed as received. The drives can also automatically configure themselves to match various motor models.
In FireWire, the cables link together in a tree topology rather than a ring. This eliminates terminators and hub devices typically found on other networks. What's more, when combined with automatic ID assignment, it eliminates the need to set physical addresses before attaching a new servo drive to the network.
Gordon Presher is President and CEO of Ormec Systems Corp., Rochester, N.Y.
Evolution of the servo drive interface
The first drive interface, the ±10 V analog standard, handled motor speed data between electronic drives and early numeric controls. In the mid-1980s, another standard based on ±10 V arrived for motor torque data and commands. By the late-1980s, the interface took on phase-quadrature position information from resolver or encoder feedback.
About this time, most large machine tool control suppliers in the U.S. were developing proprietary digital drive interfaces. In Germany, though, machine-tool builders joined to create an open drive interface standard, Sercos.
By 1990, however, many suppliers adopted the ±10V analog torque-mode control interface and used digital signal processors for position and velocity loops. This improved performance and noise immunity, and reduced the installed cost of servos.
Digitally networked drives, led by Sercos, continued to gain members. But there has been an ongoing question regarding Sercos' ability to meet the technical demands of many applications. In some applications, particularly those using small servos, it can add upwards of 40% additional cost to the system.
First developed under the name FireWire by Apple Computer in 1985, the new network came under the direction of an IEEE Working Group in 1989.
IEEE-1394 is a serial bus that offers a high-bandwidth, digital-todigital interface. It has been adopted by a broad cross section of consumer electronics manufacturers, including Sony, Mitsubishi, Toshiba, and Philips; and computer companies such as Texas Instruments, Intel, Microsoft, IBM, Sun, and Apple Computer.