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Ethernet offers simpler distributed motion control

An Inmoco product story

Edited by the Engineeringtalk editorial team Jun 2, 2005

Ethernet based systems are becoming more widely used because the hardware required for faster data handling is now less expensive and readily available, says Gerard Bush of Inmoco.

Today's motion control system architectures usually conform to one of two types, centralised or distributed.

Historically, the choice has been more often central control.

In fact, more than 90% of high-performance motion control systems now use this architecture.

However, this situation is changing rapidly as many new devices are becoming available for easy-to-assemble and more efficient distributed control systems.

A typical centralised control system approach comprises an embedded motion controller plugged into a PC's ISA or PCI bus.

This arrangement enables the host computer to communicate with the motion controller through the bus: a simple and fast process.

This method does have one major drawback, however; it requires that all control signals including motor, encoder and limit signals, be wired to the central controller located in the PC.

The ramifications of this become evident in instances where the various system components are spread over a wide area, necessitating longer wiring, which can increase costs, reduce system reliability and add noise, which may degrade system performance.

Over the last two decades several distributed motion control systems have been developed in an attempt to overcome these problems.

These, including Sercos, Profibus, CANbus (and others), all solved the wiring problem, but proved to have some limitations.

Sercos, for example, controls motion efficiently, but does not allow other devices to operate on the network.

Another limitation is cost: all the proposed system solutions require relatively expensive components.

This fact, combined with the need for every axis to have a separate motion controller, increased the system cost significantly.

This explains why, over two decades, distributed motion systems were not more widely used.

Recently, advances in Internet technology have changed this situation, making Ethernet an attractive candidate for motion-control networks.

This occurred as more users became familiar with the TCP/IP protocol and were able to connect various devices to the network without difficulty.

The result was a reduction in the cost of system components such as Ethernet cards and hubs, leading to further interest in this design approach.

This interest is fully justified, not only from a cost perspective, but also in view of the fact that Ethernet-based motion-control systems address most of the concerns of the designer.

First, they solve the wiring problems.

The controllers can be placed next to the motors and amplifiers, so the wires can be shorter.

Furthermore, when optical fibres are used for communication, noise coupling and grounding problems can be totally eliminated.

Added to these advantages is flexibility; the Ethernet solution lets users place a variety of devices on the network, and due to the common TCP/IP protocol, all devices can communicate.

In addition, the Ethernet-based network also reduces the system cost because of the cheaper hub and the Ethernet card.

Finally, where each controller is a dual-axis card, taking the Ethernet approach reduces the cost per axis.

Two new Ethernet interfaces that are enabling OEMs to leverage the benefits of Ethernet technology for motion control are EtherCAT and SynqNet.

Inmoco offers both of these as plug and play communication cards for its Danaher Servostar range of S300 and S600 drives.

Although EtherCAT technology can be used for distributed control, its primary application is centralised control, where it effectively deploys the benefits of using X86 processors and the Windows operating system.

With the speed of 100BaseT Ethernet, EtherCAT executes motion control and other functionality directly on the CPU and replaces Profibus, ControlNet, and DeviceNet scanner cards, which would otherwise - and normally - plug into the PCI bus, with software.

SynqNet is designed to support high-performance, centralised control systems.

Implementing the technology in a ring topology offers unique, self-correcting fault tolerance that guarantees the system will operate even if there is a cable break, a loose connection, or a complete fault of any node.

Even if two out of five nodes fail, SynqNet can control the remaining nodes and execute alternative motion parameters.

In addition, the use of a closed ring ensures a redundant data path for transmitted data through the entire ring, which SynqNet uses as a secondary data channel.