Comparing drive networks

June 1, 2000
A number of networks are used in motion control applications, however, the right one is not always chosen. Here’s a guide to help you begin your selection process

Most of today’s manufacturing plants use at least one network in each of three control levels of a plant. In what is typically viewed as the first level, one network connects on-off devices, such as photoelectric sensors, limit switches, motor starters, and other similar devices to a control, such as a PLC or PC. In the middle level, another network transmits data between PLCs, PCs, CNCs, and other controls. At what is typically viewed as the highest level, the network is usually Ethernet and it sends information from the control network to mainframes, accounting, and management computers, Figure 1.

Many engineers are finding that within these three levels, they need more than one network. For example, in the middle level, a plant may have Modbus, a proprietary PLC bus, and Profibus. In the device level, a plant may have both DeviceNet and SDS buses, or even a third, AS Interface.

To add to this web of network communications, engineers can add another network, one designed for motion control, and that transmits data between a drive and a control or amplifier. As covered in earlier articles, (see PTD, “Drive networks: separating fact from market position,” 4/97, p. 35, and “Selecting Drivebus architectures,” 5/97, p.49) there are several types of drive networks that engineers can choose from. Such a motioncontrol network falls somewhere between the first and middle levels in a three-level plant hierarchy.

Just as there is more than one network at the other levels, a plant may need more than one drive network for all of its motion-control applications.

Communications web

Many users would like for there to be one network for an entire plant. But technology today cannot meet this want. Each communication need at each level is different. The amount of data that are transmitted ranges from 1 bit to thousands of bytes. These data must be transmitted at speeds ranging from more than 100 Mbits/sec to several bytes per minute. Some devices must connect to inputs and outputs. Other devices must connect to microprocessors or specialized chips.

Some motion applications can be satisfied with a bus such as DeviceNet or a version of Interbus. Others require more particular features, such as those offered by Sercos or Macro buses. “Closing the loop criteria, which is a basic feature of Sercos and Macro, seems to be needed only in specialized applications,” says Bryan Lawson, Baldor Electric Co.

“This is where your definition of motion control becomes important,” said Bryan McGovern, marketing manager, Emerson Electric Corp. “If you’re talking motion control where you have a host controller sending position and current feedbacks — all the things that must be sent down to do a real time system — DeviceNet is not the best choice.”

While DeviceNet can be used in some uncoordinated motion control applications, it was designed to enable communications among devices that traditionally have not been in the communication loop: valves, flow meters, motor starters, limit switches, and photoelectric sensors, Table 1. It works well with smart or positioning drive products, which can execute a program themselves and read I/O.

Interbus-S is another network encountered in the drive-bus world. It provides easy connection to inputs and outputs compared to connection difficulties of proprietary drive buses. But this network functions best in applications that do not require tight synchronization and multiaxis coordinated control. It can handle point-to-point positioning.

The two main open networks for drive communication and high-speed motion control are Sercos and Macro. There is much debate about which is better. Despite the feature differences between them, the only real answer to such a debate is that it depends on the application. Here is a brief summary of the features of these two networks. For more detailed information on each network, see the box, “For more information.”

Sercos is an open-architecture standard interface (IEC 1491) for communications between digital drives, controls, and I/O.

Development began in 1987 by the German Machine Tool Builders Association and German Electrical Standards Association. Many drive and motor manufacturers support it. It also has a large promotional organization, the North American Sercos Promotional Alliance.

Sercos can be applied to high-speed transfer lines; stand-alone milling, drilling, and turning machines; tool grinding machines; cam and crank grinding machines; converting machines; packaging machines; material handling systems; dial machines; robots; assembly and test machines; and woodworking.

It can control multiple prime movers, servomotors, high-horsepower ac induction motors; linear motors; hydraulic cylinders; brush-type dc motors, and I/O. It updates drives at a rate of 65 μsec. It can support up to 254 devices per fiber ring, and multiple rings can be used.

It supports distributed control arrangements, in which the intelligence resides at the drive or amplifier rather than in a massive central control.

According to some users, Sercos can be installed in as few as four hours, which is a considerable reduction from the typical one week installation time.

Sercos also provides access to diagnostic information on the performance of the motor and drive. This is a relatively new feature available because of digital drives, amplifiers, and now communication networks. The information engineers have access to includes current and torque values, conditions of the power block, and heat sink temperature. Engineers can control and set forward and reverse travel limits, torque limits, and determine whether the motor went faster than it was programmed to go.

Features available with Sercos will evolve. Efforts are underway to increase its data transmission speeds and make it easier to connect to I/O.

Macro is a non-proprietary digital interface for connecting multi-axis controls, drives, and I/O. It was developed by Delta Tau Data Systems Inc. Companies that have products compatible with this network include Kollmorgen, Baldor, Rockwell Automation, Galil, Performance Control, and Lutze.

Using fiber optic cable, it updates drives at a 50 μsec rate for a transmission speed of over 100 Mbits/sec. Updates to each amplifier and controller can occur at less than 25 μsec intervals for closing high performance servo loops across the network ring.

It sends data on position feedback, flag status (limits, home flag, registration proximity status), amplifier and machine input status. It can also communicate amplifier enable, amplifier command signals, machine outputs, commands to D/A converters, and it can set up and initialize information for amplifiers and other devices.

Engineers can configure this network for distributed or centralized control. Set up as a ring, it supports up to 256 nodes, and can support up to 16 master substations. For example, in a transfer line with 60 axes, it can be configured to have eight eight-axis controls.

Because of the high data transmission speeds possible with this network, users can acquire data for diagnostics. Users can monitor bus voltage or the current in the amplifier. They can command the drive node to insert these data in the data packet that travels from the drive to the control for tracking purposes.

Additional equipment

Depending on the network, engineers may need to buy additional equipment for proper installation. Engineers should ask the network supplier for details.

Typically, though, engineers will need the specification on the network, cable connection devices, and transceivers such as fiber optic or an RJ45 phone-jack style transceiver.

In addition, for Macro, engineers need:

• A taxi chip, which is a high-speed serial- to-parallel converter. It sends serial data at 125 Mbits/sec. Available from many Ethernet vendors.
• An ASIC chip (which they can design or purchase from Delta Tau) that functions as a bridge between the processor on their drive and the Macro network.

For Sercos, engineers need an ASIC chip that handles the communications interface, and intelligent drives.

For some of these networks, engineers may also purchase developer’s kits to help them get started. These kits often include fiber optic cable and interface boards for communication and testing purposes.

Another part engineers will often need is special interfacing software (software drivers) between the selected network or bus and their motion control equipment. Each network has its own protocol, or way of formatting data for transmission. Each drive or amplifier model expects a signal in a special format that may or may not be compatible with the network. Usually, the two are incompatible. Software drivers function like language translators for the signals that transmit between the network and the motion control equipment.

Most manufacturers will not try to be compatible with all the protocols of all the networks and drives available. This would be cost prohibitive. Companies such as S-S Technologies out of Canada, write such drivers. This code is usually in the form of a microprocessor chip or firmware, and can cost over $3,500 per driver. The drive manufacturer is usually the one to pay this price because it can be spread over a product line.

This is the last of a three part series on communication networks for drives and controls. Part 1 was published in April, 1997, “Drive networks: separating fact from market position,” p. 35. Part 2 was published in May, 1997, “Selecting Drive-bus architectures, p.49.

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