The High

When choosing a motion network, the key factors to focus on are very simple. Does it deliver the performance you need? Is it reliable and safe? How easy is it to use? Can you afford to pay for it? This article reviews these four areas and the technical background to help facilitate the readers' understanding of digital motion networks.

PERFORMANCE
Ethernet has become the ubiquitous office network platform, and it seems that it will continue its triumphant progress into industrial automation. In the early days of digital networks, the lack of bandwidth, determinism, and high latency led to distributed processing solutions being offered to the market. For motion systems, the result was intelligent servo drive products that are used to interpolate between the irregular and infrequent data points transmitted over the network. For high performance and multi-axis applications, such methods are typically not sufficient. They require a different control model where the processing is performed centrally. The ±10V analog torque interface has, until recently, been the industry standard in centralized motion control, but new networking solutions promise to bring valuable change.

SynqNet is the first commercially available 100BaseT (IEEE802.3) network that offers all the performance advantages of a centralized control model, together with enhanced performance, fault tolerance, reliability and diagnostic features. SynqNet, developed by Danaher Motion, has been adopted by leading drive companies like Yaskawa, Advanced Motion Controls, Panasonic, Glentek, Sanyo Denki, and Trust Automation.

High performance motion control systems depend on a number of key technology components that work together seamlessly in an integrated fashion. A perfect control system must transmit the desired motion profile into the movements of one or more servo axes. Achieving this often requires translation from XYZ ‘space' coordinates to machine or ‘joint' coordinates, using some form of kinematic model.

Kinematic models and compensation techniques are not new concepts, and they rely on a central motion processor to perform fast, accurate matrix computations based on multiple inputs generating multiple outputs. The term MIMO (Multiple In Multiple Out) is often used to describe this class of control systems and the software control models. The exact type of inputs, outputs and matrix computations will vary by application and by the proprietary know-how of the machine builder.

Whatever the final software control model, it is important that the total servo cycle time is minimized. The shorter the cycle time, the tighter the control system, and the higher the performance of the machine mechanism. For fast point-topoint moves, or accurate path motion, the cycle time becomes a significant factor in machine performance.

NETWORK LATENCY & TRANSMISSION RATE DEFINE MACHINE PERFORMANCE
Modern control systems take multiple demand inputs and multiple feedback inputs. For effective high performance control, demand data and feedback data must be transmitted synchronously, with very short cycle times and low latencies. Any transmission delay represents a phase delay in the control system, which limits the achievable gain and the effective response time of a machine.

SYNCHRONOUS NETWORKS OFFER LOW SKEW & JITTER FOR IMPROVED PATH ACCURACY
All networks rely on the sampling of data at some discrete time based on a clock reference. When independent systems with independent clocks are connected together, as is the case with a network, the natural and random variances in clock frequency can present a challenge. Many engineers are familiar with the concept of ‘beating' when two high frequency sources, closely matched but not exact, beat at different frequencies. Digital control systems are no different, and in a collision-free network, the beating ("jitter" in networking parlance) arises primarily from differences between the local clocks at the master and slave nodes. Electromagnetic interference can also contribute to jitter in a real-world network. This jitter is transmitted directly to the path motion; for accurate path motion, it is necessary to have minimal jitter.

Skew is a constant delay of a data transmission between transmitter and receiver, or between network nodes. It's caused by the propagation delay of the cable (approx. 1µs/100m) and delays in internal logic circuitry. For high performance motion control, skew becomes relevant and the network has to be capable of measuring the skew and compensating for it.

For a single axis, jitter can result in erratic control behavior such as variation of velocity or oscillatory final position error. For multi-axis systems, the results are more severe.

Skew introduces a constant phase shift between network nodes. Coordinated axes do not receive a simultaneous set of command values.

SYNQNET OFFERS SUPERIOR SKEW & JITTER REDUCTIONS TO ETHERNET
SynqNet limits the jitter to less than 1µs by using phase locked loop techniques to synchronize the independent clocks of each network slave to the network master. This provides superior performance to other non-synchronized networks such as TCP/IP or IP/UDP based Ethernet networks that can cut jitter down to only 20µs using time stamps.

Ethernet protocols introduce additional overhead burdens that limit typical cycle times and latencies to 1ms or longer. While this level of performance may be adequate for many general automation applications, it is not adequate for high performance motion control systems.

SynqNet limits skew to 20ns using algorithms that measure the system's skew and compensate for it in hardware. Jitter and skew are guaranteed for any number of nodes or network traffic conditions.

PROTOCOL EFFICIENCY IS ESSENTIAL FOR MOTION CONTROL
Ethernet has been designed to transmit long data packages. A data frame, according to the IEEE802.3 specification, consists of 28 controls and at least 46 data bytes. This protocol is oversized for typical industrial motion applications; usually the data needs of a node (device) are small (fewer than 46 bytes). To reduce the cycle time and latency, SynqNet has optimized the data frame on layer 2. Instead of at least 74 bytes, a SynqNet frame consists of at least 24 bytes. This is a key advantage of SynqNet over Ethernet, enabling faster and more predictable performance.

Standard Ethernet relies upon a single pair of wires to transmit and receive data. Access to the wire is managed by a wellestablished mechanism known as multiple access collision detect (MACD). As the name suggests, multiple devices on the network try to access the same piece of wire. If two devices try to talk at the same time, a collision occurs and the device stops transmitting, only to retry later after some random time. Such a mechanism is inherently non-deterministic and as the number of devices on the network increases, the collision time increases, almost exponentially resulting in a rapid degradation of performance. For office networking and general automation, the lack of determinism works just fine; however, for most serious motion applications, alternate solutions are required.

SynqNet was designed to eliminate the MACD mechanism. It uses a synchronous method (hence the name) to transmit data on a regular time-scheduled manner to every device. Independent receive and transmit wire pairs (fullduplex) are used to avoid data collisions and deliver a deterministic date rate of 2 x 100Mbit. The result is cycle times as short as 25 Ms for 4 axes. In addition, SynqNet has a configurable packet structure that allows for cycle times as low as 10Ms.

SAFETY AND RELIABILITY
SynqNet can be configured in either a string or a ring topology. The ring topology offers convenient wiring and tolerance to cable break, loose connection, or complete fault within a SynqNet system.

"Self-Healing" fault tolerance refers to an ability to operate after an actual cable break, loose connection or complete fault of any node or nodes. As an example, if two out of five nodes fail, SynqNet is still able to control the remaining three nodes, flag the application, and then execute alternative motion parameters. A closed ring ensures that there is always a redundant data path for transmitted data through the entire ring. SynqNet uses this redundant path as a secondary data channel.

In the event that a wiring segment fails, SynqNet hardware re-routes the data path within two servo cycles and the network connection remains available. At the same time, the application will be informed about the event and event location, allowing the machine to respond in a manner appropriate to the specific situation.

In addition, each node has its own watchdog timer. Even if the host or whole network fails, each node can react in a predictable and safe way for a smooth and controlled shut down. To predict possible network failures, SynqNet includes transmission error counters at every node. Any abnormal increase in error count can be used to alert the application software and localize the potential problem before it becomes a catastrophic failure.

ELECTRICAL ISOLATION OF SYNQNET (100BASET) DELIVERS SOLID RELIABILITY
Both SynqNet and Firewire (IEEE 1394) networks are designed to accommodate a large number of nodes. When nodes are distributed around a machine or plant, they are often referenced to different ground domains, which introduces ground noise and circulating currents. IEEE 1394 cables provide a DC connection between these grounds and can cause a ground loop. Ground currents will adversely affect IEEE 1394 network reliability. Effects include degradation of data signals and excessive EMI from the cable, which translate to erroneous, possibly dangerous motion performance, or system shut down. If the ground currents are high enough, system components can be damaged, as well as create personal shock hazards.

The IEEE 1394 network is designed to source and/or sink power to/from remote nodes, allowing nodes that do not have their own power to function on the network. This feature, coupled with the highspeed signaling rate required in an IEEE 1394 system, makes DC cable isolation unfeasible.

In contrast, industry-standard network systems like 100BaseT (IEEE802.3) and others employ DC cable isolation that use transformers or optical couplers. Since SynqNet is based on 100BaseT, the EMI problems inherent in 1394 networks are avoided.

EASE OF USE
Networks are conceptually simple, designed to transport data between devices. But the mechanics of transporting data in the real world, reliably, safely, and in a timely and synchronous fashion, demand some very complex underlying technology.

SynqNet was designed with the machine builder in mind. Setup and configuration are simplified using techniques such as auto discovery of network devices, common tuning, and reporting methods.

INTEROPERABILITY DEFINED BY SOFTWARE
The interoperability of networks is often misunderstood and misrepresented. For example, the IEEE 1394 standard defines an interface at the network device driver level. It does not define the software interface to a motion control application and no 1394-automation standard exists to resolve multi-vendor interoperability problems. As a result, 1394 is available from multiple vendors, yet there is no common software API, making multi-vendor interoperability impractical, if not impossible. The machine designer is effectively locked into a specific vendor offering closed 1394 drives and controls.

However, SynqNet offers a common software API for all network devices from multiple vendors. SynqNet products are now commercially available from U.S. and Japanese suppliers offering both standard and custom motion products. The API is available as a set of powerful C/C++ or VisualBasic motion libraries.

POWERFUL NETWORK-READY TOOLS
SynqNet tools are designed to work with networked motion systems that contain components from multiple vendors. Tools for real-time data graphing, network configuration and management, mechanical characterization and optimization are available for windows platforms, and can also be used across any TCP/IP socket connection.

CONFIGURATION CONTROL USING FIRMWARE DOWNLOAD
SynqNet has the ability to verify firmware revisions and to perform firmware downloads to every device on the network. This simplifies the process of configuration management of software, firmware, even FPGA images, and provides an efficient method for implementing machine upgrade packages, or installing spare components of unknown configuration status in the field.

REMOTE DIAGNOSTICS
The availability for real-time node information enables predictive maintenance, remote diagnosis and repair regimes to be supported. For example, if the node is a SynqNet amplifier, parameters such as temperature, fault and warning conditions, configuration, drive motor, and encoder information can all be accessed remotely and in real-time by the user application.

AVAILABILITY & FLEXIBILITY
The key components of a motion system include the motion processor, the drives, and the I/O. SynqNet is supported by a growing number of servo drive vendors, offering standard and custom products, single and multiaxis, ranging from 10W to 10KW. A wide choice of components provides design flexibility and ensures competitive pricing from the world's leading drive and motor vendors.

SUMMARY
Not all networks are created equal. High performance motion networks demand tightly managed timing regimes to ensure synchronous and real-time updates across multiple axes. While Ethernet offers adequate performance for general purpose applications that use distributed control, it is generally too slow for more demanding situations. In these cases, a fast synchronous network is required to connect a centralized motion processor to multiple servo axes.

SynqNet was designed specifically to support high-performance, centralized control systems, and offer additional benefits including fault tolerance, simple discovery-based configuration, and high noise immunity. In addition, SynqNet is supported by multiple drive vendors delivering a wide array of cost competitive products.