A beginner's guide to Ethernet Powerlink

Nov 30, 2007 1:40 PM

Industrial communications and automated motion require much data to transmit safely and in real-time. Enter Ethernet Powerlink.

John Mazurkiewicz
Baldor Electric Co.
Fort Smith, Ark.

Ethernet is a high-speed communications-network commonly found in factories. It sends information from PCs to destinations such as other PCs, servers, scanners, and printers. Since the 1970s, Ethernet has been used in offices, as a backbone for the Internet, and is now expanding to industrial automation. To attest to its widespread use, Ethernet is also defined in IEEE 802.3, which covers the physical hardware, voltage levels, and structure of its frame.

Multiple devices transmitting data can collide. Devices 2 and 3 do not detect activity or transmission, causing a collision. ETHERNET Powerlink resolves this conflict by using CSMA/CD, allowing one device to dominate and transmit data. Once Device 2 completes its transmission, Device 3 can send data.

ETHERNET Powerlink devices connect to the network through a switch (router) that directs communication to a specific MAC address. This way, only the destination device processes and acts on the data.

Industrial automation demands that systems react to events within expected time constraints. Otherwise, malfunctions and destructive mechanical collisions can occur, causing downtime or personal injuries. Some factors Ethernet has overcome for factory applications include:

Data size and complexity
Speed
System stability, safety, and security
Environmental influences (electromagnetic, temperature)

Ethernet

Ethernet is a communication medium that transmits digital data. To better illustrate this, suppose several individuals in a room engage in a discussion. Here, Ethernet's physical equivalent is air. If some individuals move to other locations, a physical layer, such as a phone line, must be added to keep all parties in on the conversation. The phone, however, must be equipped with talk and listen signals to prevent people from speaking over each other. Likewise with Ethernet, successful transmissions on one channel require that data not collide or mix. To that end, Ethernet employs simultaneous listening and speaking techniques (transmitting and receiving), which IEEE terms Carrier Sense Multiple Access Collision Detection ( CSMA/ CD). These techniques work best when data (or individuals) listen first, then speak.

Determinism permits network access within specific timeslots. A manager controls data transmission between devices to prevent collisions and ensures precise and timely data exchanges in cycle times down to 200 µsec.

Ethernet relies on a point-topoint configuration where each device connects to the network using a single cable through an infrastructure component, such as a switch. The switch then sends data to a specific Ethernet device with a Medium Access Control or MAC address. All addresses are globally unique; thus, only the destination address processes data.

ETHERNET Powerlink

ETHERNET Powerlink originated at the field-device level (rather than from the top down), which allows smoother data transfers from high-order IT systems down to drives and sensors. This also facilitates data transfers between Internet access and motion control on shop floors for remote control and diagnostics.

Several motion controllers increase cost, complexity, and development time and synchronization between controllers becomes difficult. In addition, field-bus communications often require interfaces on the drive and controller, burdening the CPU.

One important attribute of Powerlink is that it's an open network, or, can be used with any supplier's chips. Another is its determinism (predictability). Specifically, Powerlink can predict the amount of time a device waits to gain network access for data transmissions. A third feature is that Powerlink allows devices with different data rates to mix in the same system and connect to the same switch. As such, 10 Mbit/sec Ethernet devices can be expanded to use faster 100 Mbit/sec products. In other field-bus networks, the slowest device determines the data rate for an entire network.

Automation packages

Industrial automation systems designed around ETHERNET Powerlink employ fewer centralized motion controllers, thereby reducing cost. Decentralized drives can perform velocity, torque, and positioning tasks, such as absolute and relative moves and homing. Powerlink allows any device on the network to act as system manager — critical for establishing a back-up manager. And, fewer motion controllers also reduce cabling and wiring complexity.

Ethernet-compatible controllers deliver benefits to software. For example, motion controllers exercising basic languages provide simplicity and reduce development time. Multitasking permits control of motion, I/O, logic control, HMI and communication tasks. In addition, multitasking simplifies complex applications by breaking them into manageable subtasks.

Here, several machines connect to a corporate network, which decreases the number of motion controllers, cabling, and complexity.

Ethernet at work

A motion application with more than 15 servomotors and six different tools required one setup to automatically move items into place without shutting down or pausing upstream stations. By combining ETHERNET Powerlink and decentralized control, the master position set point broadcast every 400 µsec. It, along with set position points for each axis, were calculated locally via an integrated hub. (The other option, a centralized control approach, would have limited the number of synchronized axes because its broadcast slows the system.)

Using Ethernet with decentralized control improved machine throughput by more than 200% — up to 50 fpm — while also improving tolerances to less than 0.04 in. The new machine produced higherquality parts more economically, reduced setup time and scrap, and increased overall machine flexibility.