Machine Design

Digital Networks Link Servos To The Factory Floor

The move to distributed networking for factory automation using Ethernet & TCP/IP is widely accepted, but there is also a move, perhaps not so obvious, to digitally networking servosystems using IEEE-1394.

Allen Presher
Vice President Marketing
Ormec Inc.
Rochester, N.Y.

EDITED BY John R. Gyorki

ServoWire networking based on the IEEE-1394 open standard simplifies wiring, lowers cable costs, and improves reliability by reducing the number of connections required in a total motion-control system. The topology ensures a 200-Mbits/sec serial bus speed with isochronous communications for consistent loop updates, and asynchronous communication for managing command and status.

Industry standard hardware and software, such as the ISA bus, Ethernet adapters, along with TCP/IP and IEEE1394 factory network protocols provide the flexibility and scalability for machine I/O control, operator interfaces, PLCs, and factory network connections needed for high-production throughput.

The ServoWire drive network is more efficient than others. It is all digital and replaces the ±10-Vdc analog interface built into most drives. ServoWire supports servosystems, pacer encoders, high-speed sensors, I/O modules, and programmable limit switches. D/a conversions are eliminated at the controller, as well as a/d conversions at the servodrive which makes sense for working directly into the DSP architecture of the controller.

OEM machinery builders are especially well positioned to benefit from newly emerging open-standard networks for machine and motion-control applications. These networks which include Ethernet, Internet Protocol (IP), and IEEE1394 (FireWire), claim several advantages. Ethernet TCP/IP, for example, extends factory networks to include servodrives and motion-control systems. The ability to remotely connect to these devices using offthe-shelf gear also helps reduce diagnostics, maintenance, and support costs.

Ethernet and IP software standards make it easy to interconnect a wide variety of products such as motion controllers, Ethernet I/O, PLCs, PC-based HMI packages, and vision systems to machine-control networks. Machinery builders can mix and match automation components from a variety of vendors, and select the most appropriate products for a specific application. Application protocols such as Modbus TCP are also gaining broader support from automation vendors as evidenced by the growing number of products supporting this open standard.

A TCP/IP network, the most commonly implemented approach, creates a web of control and diagnostic and production information that can be accessed by a variety of applications. Operator interfaces, data loggers, and Web browsers can simultaneously monitor, gather, share, and report information about a machine or process, in real time. A single operator console can display information from all TCP/IP-enabled devices on the network. Key system information from the motion-control system (speeds, torques, and so forth) or from I/O subsystems can be readily accessed by most PC-based HMIs. The cost per network node on Ethernet is much lower than traditional factory networks, and most plants have the personnel to administer these networks effectively.

Remote Diagnostics and Monitoring
Standard software tools (Windows dial-up networking and applications such as a Web browser or Excel) and modems or network connections, let users access machinery directly for diagnostic and troubleshooting information. Specialized support personnel can work effectively with on-site technicians and machine operators to solve problems quickly and efficiently. Moreover, engineering personnel and software developers can assist without visiting the machine location, saving time and money.

A Web browser is probably the ultimate way to monitor equipment because users don't have to buy special software. For example, a browser can view Web pages associated with a motioncontrol application. These pages may contain detailed data on the motion-control system setup, the status of each axis, software versions, and a history log. Customized Web pages can also display production schedules, job data, and other information.

Digitally Networked Servos
Digitally networked servos are now being interfaced to industrial PC-based motion controllers that run on open standards such as IEEE-1394 (FireWire). The approach effectively extends the reach of networked automation to motion-control systems.

Plus, the technology is reliable, inexpensive to implement, and is already used in millions of PCs and multimedia applications. According to industry analysts, more than 40 million IEEE-1394 interfaces will ship during the year 2000.

One example is Ormec's implementation of IEEE-1394, called the ServoWire Drive Network, which replaces the ±10-V analog interface with an all-digital control network. One PC-based controller can connect to 32 servodrives with servoloop update frequencies from 1 to 5 kHz. The exact value is determined by the number of servodrives connected per module.

A traditional servosystem, in contrast, has 20 to 25 electrical connections per servoaxis. ServoWire, however, lowers cost and raises reliability by eliminating hundreds of interconnections. Furthermore, the standard IEEE-1394 cable reduces wiring costs by 80%.

IEEE-1394 is ideally suited for servodrive networking because it combines highspeed operation (200 Mbits/sec) with guaranteed, real-time network determinism. The 1394 network guarantees real-time updates to 8 kHz by combining two types of network communications. Isochronous transfers are guaranteed to absolutely, positively occur at a predetermined rate and deliver time-critical updates. Asynchronous transfers can use the remaining network bandwidth to dynamically adjust tuning parameters, modify drive setup and monitor system parameters on the fly.

Some IEEE-1394-enabled systems don't need configuration jumpers or programmable DIP switches. Instead, they store drive parameters in a database, and automatically configure each drive during power-up. The control architecture is simpler and fewer spare parts are needed because each servodrive matches up with a variety of servos including ac brushless, linear, dc brushtype, or voice-coil motors.

New devices attached to the network are automatically recognized, and the proper drive firmware is automatically downloaded if not already installed. For OEMs, this means that replacing drives in the field doesn't require a PC-based setup tool or unit configuration.

System Architecture
The system works especially well for multiaxis control and synchronization. It uses a memory-mapped model where all drive setup and motion-control parameters are defined as software variables and communicated in real time over the bus.

Servoloops are managed in real time over the bus, implementing a digital torque control network for as many as 32 axes and eliminating all digital-to-analog conversions. Torque commands to the drives are transmitted digitally as 16-bit variables, eliminating the cost and limitations of traditional 12-bit d/a converters and analog torque signals.

Digital Torque Mode
Digital torque control combines the fundamental advantages of torque-mode control such as greater control flexibility, acceleration feedforward, and torque information with the advantages of digital networking.

In this digital system, a velocity observer replaces the analog tachometer, and software loop parameters replace potentiometers. All loop adjustments are automatically computed when a motor, load inertia, and velocity-loop time constant are selected from a configuration software menu. System parameters such as peak motor-output torque are also set in software.

The drive network provides both high bandwidth and high noise immunity when eliminating analog conversions. Digital servoloops, including 32-bit intermediate results on calculations, provide precise control algorithms for drift-free operation. Moreover, the user's application program can adjust for changing factors, such as load inertia, by changing drive settings or loop parameters while running. An all-software, torque-mode positioning system also provides dynamic monitoring, with easily accessible real-time values for position, velocity, acceleration, and torque.

The 1394 Cable
The cable and connector design are also part of the 1394 open standard and are essential to delivering the high-speed serialbus communications. Standard IEEE-1394 cables contain two power conductors and two twisted pairs (TPA and TPB) for data signaling. Each signal pair and the entire cable are shielded. Cable power is specified at 8 to 40 Vdc at 1.5 A and provides interface power for drives connected to the bus.

Although IEEE-1394 offers many benefits and specific features for real-time control, the most significant benefit may be lower cost.

Servodrive prices have been consistently dropping as equipment makers move to more highly integrated power-block technology and compact electronic design. Industrial servodrives built with surfacemount technology and powerful digital signal processors (DSPs) provide greater value to customers, while IEEE-1394 makes networking these systems easier and less expensive.

How ServoWire uses IEEE-1394
ServoWire implements digital torque-mode algorithms within 1394 for both isochronous and asynchronous data transfers. The isochronous data channel provides guaranteed data transport at a predetermined rate, especially important for just-in-time delivery of data such as timecritical loop updates. The ServoWire protocol allocates network bandwidth for each servo on the network. This guarantees timely transmission of torque commands, position feedback, and high-speed I/O status during each loop update for all drives in a ServoWire network.

Inclusion of high-speed I/O in the isochronous data channel ensures that the ServoWire Axis Module can always drive or respond to high-speed I/O (four programmable limit switches and three sensors per drive) on the next loop update.

Many applications benefit from the precise synchronization of highspeed I/O and motion control. Line-oriented manufacturing systems, many within the packaging and converting industries, frequently demand I/O response far beyond the capabilities of PLCs. An example of this is a rotary knife application on a converting machine, where a knife must cut a web of preprinted boxes to an accuracy of 15 mils at line rates up to 1,200 ft/min.

The second type of data transfer supported in IEEE1394, asynchronous communications, provides a mechanism for managing real-time command and status communications on the network. After all isochronous transfers finish in each loop update, the remaining bandwidth is available for asynchronous transfers. Asynchronous communications can be used to dynamically adjust tuning parameters, modify drive setup, monitor system variables, transfer error messages, and execute other software commands.

Many applications can benefit from the ability to adjust system parameters under software control while the system is operating. One example is adjusting system inertia while motor loads vary in a winding and unwinding operation. The variable Inertia is used by Ormec's MotionBASIC software to let an application program adjust servoloop variables as the roll diameter changes. Dynamically setting servoloop parameters based on roll diameter and tension on-the-fly provides repeatable performance and consistent operation over a wide range of operating conditions. This approach lets high-performance servosystems handle the combination of large roll diameter changes and load-to-motor inertia mismatches that can be as large as 1,000 to 1 in high-speed winding applications.

ServoWire uses plug-n-play technology provided by IEEE-1394 that expedites setup and commissioning of devices on the FireWire network. With a ServoWire system, Ormec's Orion controller searches for servodrives on the network at power-up. Servodrive IDs are displayed on the drives' seven-segment LED, and incorrect or unassigned IDs flash until resolved by pressing a button on the drive to select the correct ID. Drive parameters are then downloaded as defined by the central software.

This approach simplifies replacement of servodrives in the field. New drives are installed as received because offline computer-based setup is not required to properly configure the new unit. Servodrives also configure themselves to match multiple motor models which reduces spare parts and setup time.

IEEE-1394 implements a tree topology, with repeater hardware in each interface rather than a ring, so terminators or hubs are not needed. The tree structure and the IEEE-1394 automatic ID assignment eliminate the need to set physical addresses before attaching a new servodrive to the network. IEEE-1394 cables provide power for the ServoWire interface in the drives which allow the network to operate, even without power to some of the drives in the network.

TAGS: Technologies
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