Tradition often holds back evolution. Today, the needs of the manufacturing industry are changing so rapidly and are getting so sophisticated that tradition has no option but to synchronise itself with realtime. One of the last bastions of tradition in process control is the support for conventional hardware such as industrial, ruggedised PCs designed to withstand the rigours of the manufacturing environment as opposed to new approaches such as thin client HMIs.
There is a different form of industrial revolution taking place in the 21st century. It is unusual because it is taking industry forward by going backward - by applying new technologies in the fashion of another era. Today's industrial revolution is applying advanced thin client computing on the plant floor in a manner reminiscent of the decades old, centralised approach of mainframe computing.
The latest revolution is quite different however, because it is a return to the basics of mainframe computing as the world knew it decades ago - when computing was centralised and people accessed the data and application programmes they needed to do their work via dumb terminals. It is not the thick client environment, in which each plant-floor workstation had its own complete copy of application programmes that were run locally by operators that emerged with the advent of the personal computer (PC) in the 1980s.
The new revolution is taking industry back to the era of the 1950s and 1960s by applying advanced thin client computing which replicates a mainframe environment by running applications on networked server computers and accessing them from dumb 'client' devices, which can include anything from PCs to wireless handheld devices. The benefits are numerous and this article will examine the application benefits and reduced total cost of ownership provided by thin client computing.
A bit of history
In the late 1980s, PLC and other automation equipment manufacturers began to offer panel-mounted operator interfaces to replace pushbuttons, switches, thumb-wheels and seven-segment digital displays. For the most part, these interfaces were monochrome cathode ray tube units based on Intel 8086 or Motorola 6800 microprocessors. They were generally programmed in variants of Basic and offered only character-based displays. By the late 1990s, these units had been upgraded to vector-based colour graphics, with configuration software running on Microsoft Windows platforms. These units were appropriate for general operator interface tasks such as placing systems into manual/auto modes, starting/stopping motors and loading simple setpoints. They were low-cost as well, ranging up to about R10 000.
For more demanding applications, users bought industrialised PCs (or office PCs mounted in conditioned enclosures) running Microsoft Windows and PC-based HMI software. In doing so, however, they then faced a tradeoff between the lower functionality of typical embedded HMIs and the higher cost of Windows-based HMIs.
The arrival of Microsoft's Windows 2000 operating system and thin client terminal services technology has brought to the plant floor a dramatic improvement in HMI capability and reliability - with a corresponding dramatic reduction in the total cost of ownership for these HMI systems. Today's thin client technology brings a multitude of benefits to the plant floor, including:
* Complete Windows experience on all clients.
* Highly reliable, solid-state client hardware.
* Centralised maintenance, development and deployment of applications across the plant floor or remotely via dialup or Internet access.
* Simplified system installation and deployment, with reduced costs for training and maintenance.
* High scalability for serving large numbers of remote clients.
* Client support for a multitude of operating systems and versions, such as Windows CE, Embedded NT, WFW 3.11, Windows 95/98, Windows NT 3.51/4.0, Windows 2000, Linux and Unix.
* Resistance to hardware obsolescence and re-use of existing hardware.
* High levels of operator and system security.
These changes do not mean the demise of the embedded panel-mounted HMI, only that there is an evolution occurring in the use of these embedded devices. Thin client technology allows users the operational benefits of Windows-based HMI, with the lower cost structures and high reliability of embedded HMI by enabling the use of solid-state thin client devices on the plant floor.
PLC communications and networking
PLC communication technology has actually changed very little in the last 20 years. For the most part, the communications protocols used today were designed in the late 1970s and early 1980s. Modbus, Modbus+ and Modbus Ethernet are good examples. Consider the task known as Function Code 3- Read Output Register. It is the same in all three protocols and is limited to 128 contiguous words of data in a fetch. The only differences from the data acquisition of yesterday and today are the speed of the protocols and the number of devices that can request the data from a particular PLC. This is like having freeway speeds on the highway and congested rush hour speeds at the on and off ramps.
The universal problem is that every PLC manufacturer's product family has a limit on the amount of data that can be requested from a particular PLC in a timely fashion. This data bottleneck is exacerbated when the PLC programmer fragments HMI data throughout the PLC memory map. This may require the HMI to issue dozens of data requests for relatively small amounts of data. Another limit to PLC communications is the number of data requestors (HMI I/O servers and drivers or other PLCs) that can be serviced by a single PLC. In the drive to reduce PLC costs, most PLC manufacturers now utilise a single CPU that must execute logic and communicate to the outside world. This means that users are now required to tune PLC scans to balance the tradeoffs of fast scans versus fast communications.
These communications issues can be worsened by improperly applying embedded HMI systems, Windows-based HMI systems or even thin client technologies, which can result in slow data updates and frustrated operators. It is critical for users to understand the communications limitations of a particular PLC system so as to avoid these potential problems, no matter what HMI technology is deployed.
Application space of traditional embedded HMI
An embedded, panel-mounted HMI can be an obvious and cost-effective choice for original equipment manufacturers (OEMs) of plant floor devices such as palletisers, case packers and rewinders as well as for skid-mounted equipment that may be driven by a single PLC. The primary drivers for using an embedded HMI for these types of applications are their limited HMI requirements and the cost structure of the OEM environment.
In some higher-end OEM systems, multiple embedded HMIs may be needed for proper operation of the system. A multimaster or peer-to-peer PLC network is often required as a prerequisite for this mode of operation. The drawback of this configuration, however, is that having too many HMIs requesting data from the same PLC can adversely affect performance. This occurs because each embedded HMI is independent from every other embedded HMI and, therefore, each requests its own data from the PLC. Perhaps the only real benefit offered by such an architecture is that it is inherently redundant.
One of the frustrating issues that users of embedded HMIs have had to deal with is hardware obsolescence. No sooner does a user select and start using a particular embedded HMI, than the vendor releases a new, and often incompatible, HMI with different configuration software. While it might be appropriate in some cases to berate the embedded HMI vendor for discontinuing a particular embedded HMI line, in many cases the manufacturer has had no choice. Embedded HMIs are dependent on the integrated circuits, CPUs and low-level software used by these devices. There is usually a very tight relationship between the software and the hardware used in them and integrated circuit (IC) development and manufacture are exceptionally dynamic. When an IC manufacturer discontinues a particular circuit, the embedded HMI vendor sometimes has to completely redesign both the HMI hardware and software. The history of embedded HMIs is rife with discontinued hardware that is incompatible for user applications from one version to the next.
Concluding part will be included in a future issue.
For more information: Mike le Plastrier, Futuristix, 011 723 9900, [email protected], www.futuristix.co.za
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