TechArchive

Original article date: January 1998

An alternative approach to industrial control means a flexible way to control a range of processes from a central point. Jumo’s RUDI RABER compares the options available.

Perhaps the world’s first commercially available peer-to-peer industrial process control system, Jumo’s new MTRON (standing for modular TRON) offers the same functionality as conventional PLC systems, yet is estimated by its manufacturer to cost 30% less to control and maintain. In addition, because the central control point can be completely isolated, the safety, security and integrity of the processes are assured.

Currently, there are two types of PLC: compact PLCs, which contain all the functional components needed to solve small and medium-sized control tasks, and modular PLCs. The modular PLC consists of individual modules which are mounted on a rack with a system bus. Special modules are available for tasks which straddle the interface between process and control, such as positioning, pulse processing, communications, diagnostics and visualisation. They enhance the original PLC concept, which was directed towards switching logic. But they place an added burden on the CPU.

Concentrating all processing functions in a central CPU can be seen as a main disadvantage of PLCs, since it demands increasingly short program cycles to accommodate the additional functionality. This, in turn, demands a progressive increase in the data transfer rate on the system bus and a consequent reduction in the size of network that can be supported.

So as a central system becomes more complex and more overloaded, its operation and design become less and less transparent. A further disadvantage is that separate connecting lines are needed between every module and each of the sensors with which it communicates. This arrangement is expensive and inflexible, and takes a considerable amount of time to install. Furthermore, it cannot be changed easily at a subsequent stage.

Recognising these disadvantages, some PLC manufacturers have produced decentralised systems, of which the units are linked by a fieldbus system. This technology is based on master-slave communication, in which central intelligence is linked by fieldbus to remote peripherals which also have their own intelligence.

Compact controller

The compact controller is a single instrument which combines a power supply with analogue and digital process inputs and outputs, a microprocessor with program and data store, a digital display and keys for setting the parameters and for configuring and operating the unit. An internal digital interface card allows the compact controller to operate on a data network, performing functions such as process visualisation, system operation and reports, with the bus system designed on master-slave principles.

Compact controllers are mainly used in measurement and control and also as the hardware interfaces to sensors and actuators. They can be connected to a wide range of devices, such as thermocouples, resistance thermometers and transducers. They can operate as single or double set-point controllers, as proportional or modulating controllers, or as proportional controllers with subordinate driver functions.

Modern compact controllers contain complex functions, such as limit comparators, window discriminators, maths and logic functions, fuzzy control elements and even special start-up circuits, such as a ramp function for set-point control.

Compact controllers can be extremely powerful instruments, but they also have their disadvantages, especially in processes with complex automation structures. They are really best suited to applications involving only one or two process variables. This is because their small size means they cannot accommodate large amounts of wiring. In addition, it is often inconvenient to run large amounts of wiring for power supplies, sensors and actuators through cabinets to their doors, which is where controllers are most often located.

Peer-to-peer alternative

The MTRON system resolves these problems by separating the individual controller functions into individual modules. It is effectively an intelligent control network, sometimes hundreds of metres long, based on low cost, easily installed communications technology. It can be used in virtually any conditions, and the intelligent sense and control devices that plug into it are programmed remotely from a PC located, for example, in an office and/or via a display module on the shop floor. The system is therefore ideal for operation on the shop floor or in other dirty, hazardous environments.

The communications network at the heart of the MTRON system is Echelon Lonworks’ local operating network (LON). This is a reliable control network that operates as both sensor bus and fieldbus. It is based on an isolated, screened, single twisted pair cable loop, to which spurs can be connected. This is significantly less expensive than point-to-point wiring, and much easier to install and maintain.

The real advantage of a peer-to-peer system is that any device linked to it can communicate directly in real time. The process control and monitoring functions of the MTRON system are performed by MTRON units. These are individual modules which plug directly into the LON. At the heart of each MTRON is a neuron microprocessor, which provides the unit with its own, embedded intelligence. This allows it to be programmed remotely across the network and to communicate its identity to the other devices on the network. It also optimises, switches and controls the process conditions and system parameters/set-points for the module so that it can perform its designated functions. New MTRONs can be added to the network at any time to meet changing requirements.

The first MTRON units include display, controller, universal relay, analogue input and output, communications and logic modules. Development is currently under way on others.

The combination of the LON system and MTRON results in a system that can be assembled, configured and adapted for each application, yet which requires far less custom development of hardware and software than alternative process control environments. This contrasts with PLC systems, which require expensive, time-consuming programming, and often suffer from incompatibility problems. System start-up is simpler, and it is also far quicker and easier to increase the capabilities of the initial network simply by adding extra modules.

Maintenance is also easy. Modules can either be switched out for repair or upgrade, or diagnostics and reprogramming can be done long distance by telephone. Because each unit on the network has its own integral intelligence, system vulnerability is spread over a wide area instead of being confined to a central point.

The MTRON system is aimed particularly at the builders of physically large systems, production lines and chemical plants, and applications such as tunnel ovens which require distributed processing. Because it is so easy to install and expand, the system is also well suited to use with complex or multiple control loops where communications are required.

The MTRON lineup

Display module This is particularly useful in local network control, both for showing alarm texts and process conditions, and as one of several means of modifying the programs and set-ups of other MTRON units.

Controller module This offers all normal controller functions, including ramp starts and steps, mathematical control, limit comparator and alarms. It contains one binary and two analogue inputs, as well as two relay outputs and a constant output.

Universal relay module This offers a sophisticated level of control, but without a display (since this is normally provided locally by the display module or remotely on a PC or other central display system). Featuring four internal set-points, it acts as a limit comparator, pulse modulator and pulse frequency modulator. Its eight outputs can be used to adjust the processes controlled by other units on the network.

Analogue input module This is used for external communications, for example configuring the signals received from all the MTRONs on the network and transferring them to a central control point such as a PC. Features include two analogue inputs, two internal maths functions, linearisation and two filter limit comparators.

Analogue output module This features two analogue outputs for communicating distributed data. It can operate with 0-10V, 0-20mA and a 4-20mA signals.

Communications module This permits full communications between the LON network and remote printers, modems, faxes, telephone systems and other external devices. It enables the LON network to raise an alarm and to send it to the central control station, a remote security post or to another appropriate destination. It could, for example, generate a print-out on a remote printer, send a fax or even trigger a telephone call to a duty officer. The module can also translate signals for transfer between the LON and other systems such as Modbus.

Logic module This is effectively the software equivalent of a PLC. With eight binary inputs and six binary outputs, it is capable of controlling arithmetical, logic or “structured text” functions. This is powerful facility predefines and controls all communications links across the LON network. It contains a library of functions for a wide variety of control operations.

MTRON-ITOOL software This is the software used to configure the MTRON system, to program the individual units, and to define the relationships between them.

Why operate a peer-to-peer network?

Intelligent control networks have until now been costly or limited in applicability. Communications protocol are difficult to design and implement and tools for developing and testing networked applications have been lacking.

The networks which Lonworks is designed to build, install and maintain, may comprise quite simple individual sense and control devices which are quite simple. But they can be interconnected with large numbers of others to create powerful and sophisticated systems, perhaps containing anywhere between two and tens of thousands of units.

In most industries, there is a trend away from proprietary control schemes and centralised systems. Manufacturers prefer to use open, off-the-shelf chips, operating systems and parts.

In centralised control systems, remote sensors provide feedback to a central controller, which in turn controls sensors and actuators. Each system typically has its own unique I/O and processing architecture. Such systems are expensive to develop, costly to install and difficult to expand:

• Designers of such systems find that most of their engineering effort is spent implementing and testing wiring harnesses and communication systems, rather than developing control features and applications.
• Systems integrators spend time trying to connect systems that were not designed to intercommunicate.
• End users pay a premium for maintenance and expansion.

In a Lonworks network, intelligent control devices, called nodes, communicate using a common protocol. Each node in the network contains embedded intelligence that implements the protocol and performs control functions. In addition, each node includes a physical interface that couples the node’s microcontrollers with the communications medium.

With Lonworks, there is much less need for custom development of hardware and software. Costly point-to-point wiring is replaced by low-cost shared wiring or even by power line and RF signalling techniques that require no new wiring.

System start-up is easier and additions and changes are easily accommodated. The network can be grown incrementally, so there is no danger that a manufacturer will outgrow the platform. And because each point in the network has local intelligence, systems can be designed with no central point of vulnerability.

• Jumo Instruments
• Rudi Raber
• Tel: 01279 635533
• Fax: 01279 635262

January 1998