Much is changing in the world of motion control networks, but one is emerging as the favorite choice.
In communications, what are the features that matter most? Based on the growing appeal of Ethernet, they are wide compatibility, installation ease, and relatively low cost. Almost all other capabilities and enhancements available with the latest technology are seen more as a challenge than an aid to communication.
Even as we speak, Ethernet is pervading the enterprise - from executive offices to the plant floor. The list of motion device manufacturers offering an interface to this protocol grows daily, so every device, including motors, actuators, drives, and even bearings, will connect to it at some point.
But Ethernet is not, and may never become, the sole solution for every application. It easily handles byte-size data transmission from mid to high-level control devices. For bit-size data, additional networks or buses are often needed. The choices for this level of connectivity are increasingly narrowing to DeviceNet.
For the other side of drive communications, the signals between drives and motors, the choices are less clearcut. Sercos, Macro, and ±10-V analog still hold sway. However, FireWire is gaining adherents, and USB is mounting a challenge to all of them.
The appeal of standardizing on one network is strong as companies anticipate cost savings in implementation, maintenance, and training. Ethernet’s promise of greater access to operations information, eventually down to individual I/O points from anywhere in the world through the Internet or an intranet, is the engine behind the implementation of this protocol.
However, even though the choice seems settled, obstacles to seamless communication still remain. Ethernet comes in multiple flavors, many of which are incompatible with each other (see the March 2000 issue of PT Design, page 44). And an additional version was introduced in March.
This latest version, EtherNet/IP, is sponsored by the Open DeviceNet Vendors Association (ODVA). It consists of off-the-shelf physical media and components and an “open” application layer. Open in this case means that the protocol will have no trouble communicating with DeviceNet and ControlNet networks. There will also be a routable application layer for connection to FL-Net and FireWire.
Most applications use Ethernet to conduct program maintenance, send data to and from upper management information systems and manufacturing control systems, and log events and alarms. Some use it for limited control purposes. And a few use it for everyday device as well as controllevel communication.
The main obstacle to more device and control-level applications is the lack of compatibility between the various Ethernet versions. Almost every Ethernetbased protocol uses a different top layer, the application layer, which prevents a seamless exchange of data.
EtherNet/IP was developed to solve this problem. The IP in this case stands for Industrial Protocol, not Internet Protocol. Borrowing some of the features of TCP/IP, Ether- Net/IP uses encapsulation to help create a common application layer.
In a typical TCP/IP frame, the data field can contain, or encapsulate, instructions as well as protocol information. Thus, it’s possible to pass the parameters needed to establish compatibility among otherwise incompatible communication systems. Ether-Net/IP uses this feature to allow a DeviceNet node to encase a message along with any DeviceNet protocol specifications, objects, and device profiles, easing the message’s move to the datalink layer. This message format is typically used for device configuration and diagnostics and tends to vary in size and frequency.
For real-time messages, EtherNet/IP relies on User Datagram Protocol/Internet Protocol (UDP/IP). These messages contain only data in the data field. Thus, they have low overhead because there’s no protocol information. In addition, receiving nodes know what type of data to expect in this format, so processing time is further reduced, enabling real-time message.
Continue on page 2
For those applications already using DeviceNet or ControlNet, this protocol will make it easy to add in an Ethernet link.
USB speeds through
Version 2.0 of USB is out and claims an impressive data transmission rate of 480 Mbps. That’s 40 times faster than the first version, and 80 Mbps faster than FireWire.
Such speed enables faster broadband Internet connections and lets USB handle other tasks more quickly. For example, backing up a gigabyte of data on a PC hard drive will take less than one minute, compared to about half an hour with Version 1.1.
Developed by a consortium of companies led by Intel, the new specification was able to increase speed through a change in architecture. The new version of USB is divided into physical and link layers. A parallel interface between them helps move digital signals from the link layer to the physical layer and the analog transceiver.
Fully backward compatible, Version 2.0 operates with current USB cables and connectors. It also allows several highspeed peripherals to share the same port.
USB may not be a typical choice for drive communications. As far as speed, though, even the 12 Mbps version holds its own with other drive networks.
Those engineers who’ve used it say that it’s easier to install and program than other communication systems. The biggest benefit cited is that device drivers are easier to write because of a software language called OPC, an object oriented language frequently used in automation programming.
Plus, the 480 Mbps version can also be used with linear motors, not just rotary. Previously, if engineers wanted to use a PC-based network with their linear motor systems, their primary choice was FireWire. Linear motors have high bandwidth needs because of the amount of data they transmit. Until recently, only FireWire had sufficient throughput.
As if on queue, the Sercos Alliance has recently released a higher speed version of the protocol. With two pins, engineers can now select 2, 4, 8, or 16-Mbps transmission speeds for moving data. The 16 Mbps speed is a 400% increase over the initial Sercos release. According to studies, the effective data transfer rate of the highest speed setting is comparable to that of 100 Mbps Ethernet.
For softCNC and soft motion-control applications, there’s a software only version of the Sercos driver - SoftSercans. It will handle master network functions, as well as initialize and manage the communication loop, letting engineers treat it as a black box during system design. A passive Sercos card is required.
SoftSercans interfaces to Ventur-Com’s Windows NT extension for realtime operation, RTX. The main benefit is that it enables real-time Windows programs to address digital drives.
During installation, (it’s available from the web), it checks and dynamically adapts to a microprocessor’s capabilities. Its operating performance advances with hardware improvements.
These advances in motion communications are only the beginning. The speed issue is well on its way to being conquered. Connecting to more motion components is the next step. Now it seems that the biggest obstacle may be the speed at which design engineers take advantage of these advances to bring motion on-line.