A New Real-Time Control Architecture for Advanced Machines

A New Real-Time Control Architecture for Advanced Machines:

Distributed Automation and Digital Machine Vision using IEEE-1394

Edison Hudson, President

MetaControl Technologies Inc.

Introduction

Industrial automation is commonplace in most competitive manufacturing activities today. Advanced automation control systems are central to increasing productivity not only in traditional manufacturing, but also in new fields such as drug discovery, clinical assay, and a diverse range of industries. The movement to higher throughput and extreme miniaturization, (micro and nano scale), makes human control of many processes beyond the dexterity, visual acuity, and attention bandwidth of even the most skilled technician. High performance machine systems executing multi-parameter control are commonplace in automotive, semiconductor, electronics, and increasingly in pharmaceutical production and laboratory environments.

Modern automatic machine systems must be able to control with very high precision a wide range of actuators and sensors simultaneously. Closed loop control of precision motion, force, temperature, flow rate, etc. in complex recipes is already a norm in high performance capital machinery. Increasing competitive pressure insists that these complex control regimes be implemented with cost effective electronics and robust software. Though high end machines for advanced industries are built in relatively small volumes compared to consumer goods, and yet they are under the same competitive pressures for each successive generation to perform at higher levels and lower costs. The industrial adoption of components and methods originally designed high volume consumer applications is a certainty for next generation industrial controls.

IEEE-1394 or FireWire® ( also “1394”), was developed as a consumer technology but shows tremendous potential to tackle both higher performance needs and lower systems costs demanded in advanced machinery. The combination of IEEE-1394 high speed serial bus with fast embedded processors enables a new cost effective, high performance architecture for advanced machine design and other demanding real-time automation tasks.

By enabling the physical distribution of computing power linked by synchronized, deterministic high bandwidth messaging, IEEE-1394 obviates the need for centralized backplane based machine controls. By utilizing the isochronous, peer-to-peer modes and serial data clock of IEEE-1394, the architecture described in this paper shows how distributed embedded processor nodes and intelligent digital cameras can become a preferred alternative to the centralized backplane approach in dominant use today.

Traditional Automation Control Architecture

Most current solutions in high performance machine automation and instrumentation can be characterized as centralized and backplane oriented. It has been a normal assumption in advanced motion, machine vision, and analog process automation that only backplane based controllers could provide the high communication speeds needed to synchronize motion, images, and data acquisition events. The standard implementation of most industrial and laboratory automation controllers is the rack mounted backplane, primarily using bus solutions such as VME, VXI, PXI, or proprietary buses. In recent years, PCI and Compact PCI systems have gained a significant stake in these markets, driven by the advancing capabilities of Windows based PC’s.

In the conventional architecture, all sensors, motors, digital inputs and outputs, and analog signals are cabled from the point of use to converge at the backplane resident cards designed to handle each specialized function. All signals are brought to the physical location of the system controller typically using multi-wire cable bundles. In a typical machine used in semiconductor back-end processing, thousands of individual wires converge from many locations in the machine to the central rack mounted controller. Diagram 1 is a schematic of a typical automation machine with 6 axes of motion control, machine vision, and process control. Examples of systems that use this style of control include semiconductor manufacturing equipment, electronics manufacturing equipment, packaging machines, mechanical assembly, molecular genetic laboratory systems, among others.

The traditional architecture, as depicted by Diagram 1 often contains several specialized backplanes to implement different control functions. Typically, motion control is handled by a specialized controller or board, machine vision by another board or controller, while digital and analog I/O functions add additional subsystems. Bus-to-bus communication between these various subsystems is often through traditional RS-232/422/485 serial communication channels, or in some cases with bus converters. Cabling of these systems is complex and represents a major constraint on complexity. The centralized approach also limits reliability and configurability as hundreds of conductors, many often traversing moving axes, are required to route signals to the central control chassis.

Overall, the traditional approach is cumbersome, physically large, and invariably results in a controls solution per machine that today might typically range from $10,000 to $30,000 depending on performance specifics. The era of the low cost PC begs for solutions that are on of a factor of 10 lower in cost. The software development for traditional architectures such as depicted in Diagram 1 is often expensive and time consuming, due to the fact the subsystems often come from several vendors and are developed with different software approaches and lack standard interfaces..

Distributed Control Systems

As opposed to the centralized, backplane oriented approach illustrated above, a distributed control system ( “DCS”) architecture uses some form of serial or parallel cable to link already digitized information from the point of use. In a DCS, analog signals are quickly digitized, and functions are localized that do not need to be centrally supervised. The conventional wisdom held by most automation designers has been that the distributed approach can only be used for low speed performance or when coordination of devices and events is not highly time critical. Whenever exact time synchronization or computational speed is critical, backplane bus based systems were assumed to always be superior in performance. Additionally in recent years, distributed systems have often been more expensive to implement than low end backplane systems.

However, many advantages of DCS are well accepted by industry including:

Greater signal integrity, (S/N ), can be achieved by reducing the distance that analog signals must travel before digitization occurs, important in applications where signal to noise maximization is demanded;

Cabling can be simplified and functional subsystems can be modularized allowing configuration at a higher level of integration. System configuration is greatly ease by subsystems that can plug into a network, particularly in complex machines such as electronics assembly machines, semiconductor processing, and multi-axes robots;

Large physical scale systems can be controlled more easily with distributed systems, especially large physical plants such as paper processing, chemical refineries, textile processing, and commercial printing systems.

Remote monitoring of signals or control functions over a corporate or public network is generally more straightforward with DCS systems which are naturally data packet driven.

Diagram 1 – Traditional control architecture diagram

There have been many distributed control schemes developed and implemented for industrial applications over the past three decades. The oldest of these distributed schemes are based on Field-Bus and its derivatives, Device Net, CanBus, ProfiBus and others. Most of these schemes are limited in their ability to deal with data rates more than a few megabits / second, far below the capabilities of backplane buses like VME or PCI. In the motion control arena there is a fast serial bus called Sercos, that gained some market share, but in recent years has languished due to high cost and a data rate limitation of 4 megabits / second. For a more comprehensive review of existing distributed control architectures, see the NIST report found at .

IEEE-1394 Distributed Control System

Unlike the aforementioned distributed control standards, IEEE-1394, (“1394”), has most of the advantages and few of their disadvantages of earlier generation DCS buses. Particularly with regards speed of operation, functional signaling modes, and cost, 1394 has the ability to radically change the approach of automation control design. With 1394, the idea that all signals must converge on the bus backplane location is inverted, since 1394 can bring an adequately fast bus to the signals and the point of control.

Though 1394 was originally conceived as a consumer oriented bus, many of its technical features are particularly well suitable to advanced control systems. The following attributes of 1394 are specifically important to distributed control design:

High speed – with 1394-A, 400 megabit/sec is already faster than nearly every industrial DCS serial bus by 3 orders of magnitude. As compared to the widely used industrial distributed buses based on Fieldbus derived technologies, 1394 is nearly 1000 times faster. 1394 B begins to compete in absolute speed with parallel bus backplane solutions (see Chart 1. below). With a roadmap to 400 megabytes/ second (3,200 mbs ), 1394-B on glass fiber offers speed that only a few high performance back planes contemplate, ( future buses like PCI-X, VME-64, and Star Fabric are faster, but multiple times as expensive). In many cases, backplanes systems like VME are more than adequate in speed for most controls and instrument applications. 1394 provides a bandwidth option that meets the majority of machine control and instrumentation needs where messages are typical short in data length, but numerous and frequent

Machine vision is one control technology that demands high bus speed and as a result has not until recently been amenable to distributed control architectures. The high speed of 1394 is needed to allow the digital acquisition of video from multiple cameras, and it is fast enough to replace backplane based framegrabbers.

Isochronousmode– Most serial communication schemes do not have a mode of operation that guarantees the time based delivery of data packets. This time determinism of 1394 is very beneficial in application to closed loop servo control, data acquisition from analog sources, and machine vision using digital video. In addition to the need for guaranteed delivery time, guaranteed delivery order is important for these applications, as each message represents the state of a machine or instrument function at a given point in time and may be part of a closed loop in which the order of the data must be sequential. Networks that do not support ordered and timely data sequences, such as traditional Ethernet, pose impossible conundrums to control designers.

Asynchronous mode – A common requirement of most control systems is to have the ability to respond to instantaneous events. Within the time windows permitted by the clocking scheme of 1394, asynchronous event messages can be generated within every bus clock cycle by any node, thereby allowing a high priority message to propagate with a known latency. The 1394-A asynchronous window of 125 microseconds is adequate for most control applications, with the 1394-B interval dropping to 62.5 microseconds, and ultimately to less than 16 microseconds, accommodating most events on even the most demanding control applications in the future. Another important use of asynchronous mode is in control systems with intelligent nodes, in which asynchronous data packets provide a means to change control parameters on the fly in parallel loop operation. This is very significant for dynamic loop control devices where the initial parameters need to be modified with changing system conditions.

Peer-to-Peer mode - The facility to allow 1394 nodes to communicate directly without sending each message through a host decreases the latency that is associated with host centric networks, such as USB and Ethernet. In many cases, this can be a significant advantage of 1394 when a change in state at one node needs to be passed to another node as quickly as possible and with great determinism. Since host centric networks may suffer from varying delays in message propagation depending on the processing load at the host, the peer-to-peer mode enables control designers to implement schemes in which a message can be sent just to the affected nodes. In low-end applications, where a PC may not be present, peer-to-peer offers the possibility of embedded solutions that do not require a PC.

Broadcast mode – In distributed control, many nodes may need to start or stop a process in synchronization with an event or start signal generated by a coordinating processor. Also safety violation conditions that affect the whole system that all nodes need to be aware of can be implemented using 1394’s broadcast capability that sends a message to all nodes at once. This is a powerful feature for the synchronization and control of multi-axis motion control over a distributed network.

PC compatibility – The proliferation of software standards driven by the PC industry makes integration with mainstream operating systems extremely desirable in a control system. The low level of integration of 1394 using OHCI DMA methods in Microsoft Windows 2000, XP, and Apple OS is very powerful when applied to controls applications. In Windows XP, Microsoft also supports TCPIP over 1394 as a standard to allow PC to PC file transfers transparently and at high speed.With open host controller interface (OHCI) chip sets using direct memory access transfers that OHCI supports, data transfer speed is not dependent on operating system's interrupt latency.

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 All mainstream OS’s are incorporating OHCI type support including Linux and Sun, enhancing its value through industry standards. Despite years of efforts by industrial standards organizations, such as ISO, none of the earlier generation industrial DCS schemes can claim the advantages that arise from being deeply embedded in mainstream OS’s. Ethernet and USB also claim this asset, but do not possess the other technical attractions enumerated above.

Other distributed network schemes for industrial automation include some of the above control features of 1394. 1394 is unique in combining all of these desirable modes in a single architecture, with a promising roadmap of increasing performance that mimics that of backplane buses.

1394-1995 and 1394-A had some limitations and deficiencies that were of concern to control design. In particular, the following issues present design problems for machine control environments:

Galvanic isolation – the earlier versions of 1394 were subject to influence of system level ground fluctuations that could produce unintended “unplugs” and potential data corruption due to the limited electrical isolation. Though these grounding issues could be resolved with system engineering, these earlier versions were less robust than some industrial norms.

Distance between nodes – For most intra-machine applications the 4.3 meter limitation of cabling between 1394 A nodes is not an issue, but limit the scope of application to larger systems or factory level process control where nodes may be tens of meters apart.

RFI interference – In factory and machine environments, the level of radiated electrical noise can be very high when compared to consumer environments where such noise is strictly regulated. In high power applications, such as large motors systems, radiated noise can be intense and potentially poses susceptibility challenges to the earlier 1394 standards.

Each of the above concerns can be mitigated by careful electrical design, and strict attention to system grounding rules. Distributed control systems based on the earlier 1394 A standard have proven to be robust and reliable by adherence to good electrical design, even though usually at a cost of added system components. The distance issues has been overcome in these earlier systems by employing long haul adapters that convert from 1394 twisted pair to plastic or optical signals over greater distances, converting back to 1394 at each end.

Fortunately, the 1394-B standard has eliminated all of the above concerns in its improvements, making the new standard ever more compatible with control system design needs. The simplified electrical isolation scheme adopted by 1394B is also very low cost by comparison to the magnetics approach taken by other buses. By incorporating these enhancements into the consumer standard, 1394-B based systems will be both robust and highly competitive with any specialized industrial DCS network for advanced control.

Comparison to the Predominant Network- Ethernet

Many expect the pervasiveness of Ethernet to be a decisive factor in its selection as the media on which most future distributed control networks will be implemented. This belief is largely based on the wide availability of Ethernet ports on PC and the pre-wired infrastructure support at the corporate enterprise level down to the factory floor. Diagram 2 illustrates the current factory network status and the normal use of Ethernet in the factory.

While some factors favor the incumbency of Ethernet, the installed Ethernet base is less of a factor inside automation machines, or “intra-machine”. Because of the value placed on performance in machine design, technical factors are decidedly against Ethernet as the logical choice for high performance real-time control.

Diagram 2 – Factory Communication Hierarchy

The main control related problem with traditional Ethernet is the Collision Sense, Multiple Access, Collision Detection ( “CSMA/CD”) scheme that is used in the vast majority of installed Ethernets. This issue and the potential solutions are summarized by the following abstract from an industrial journal:

“Traditional Ethernet is not realtime friendly. The CSMA/CD scheme makes access inherently non-deterministic. An Ethernet controller connected to a thin Ethernet (coax - 10BASE-2) or a hub (10BASE or 100BASE) is not able to send a packet as long as the medium is busy sending another packet. The Ethernet controller is free to send its packet as soon as the Ethernet is idle.

The probability for a collision depends on the collision domain, i.e. the range of the Ethernet, and the network load. A traditional CSMA/CD Ethernet with 20% utilization has less than 0.1% collision, while as much as 5% of the packets will experience collisions if the network utilization is above 40%. A CSMA/CD network with 40% utilization is in trouble, and the net data rate will in fact decrease due to collisions if the load is further increased. However, bear in mind, those collisions are not errors. A collision is a normal part of Ethernet networks.