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News Release from: Status Instruments | Subject: Medacs
Edited by the Engineeringtalk Editorial
Team on 10 September 2002
Modular expandable data acquisition
The success of Status Instruments' Medacs signal conditioning units is bringing them into a growing number of new areas.
The success of Status Instruments' Medacs signal conditioning units is bringing them into a growing number of new areas There is little doubt that the advent of fieldbus in all its various guises has, and will continue to have in the future, a dramatic affect on the design of modern process control systems
This article was originally published on Engineeringtalk on 5 Sep 2001 at 8.00am (UK)
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One criticism levelled at fieldbus systems is that they are only suited to larger installations, especially those on green field sites.
Although fieldbus offers significant advantages and benefits to the user, implementing such systems is often too complex and far too expensive for the majority of smaller applications.
Whether this criticism is fair or not is open to debate, but many companies shy away from fieldbus systems, relying on more traditional PLC and PC based systems using conventional controllers and signal conditioning units.
In addition, integrating or updating older parts of a plant into newer central control systems can be problematic and far from 'seamless'.
With this in mind, Tewkesbury based Status Instruments has developed a revolutionary new family of signal conditioners which offers many of the key advantages of using fieldbus equipment but without the associated high costs involved in embarking on a totally new, and unknown, fieldbus solution.
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The resulting Medacs (modular expandable data acquisition and control systems) series is a radically new concept in signal conditioning and is the culmination of three years R and D, combined with extensive customer liaison.
Medacs is aimed at specifiers of electronic equipment in processing plants, system integrators and specialist panel builders.
The units have the capability of interfacing to a wide range of industrial sensors and can be integrated directly into higher level plant control systems via bus systems, DCSs; often utilising the fast growing OPC software platform.
During the consultation period, one factor became abundantly clear- customers wanted low cost flexibility and connectivity.
So how could Status meet these apparently conflicting challenges and produce a truly 'futureproof'' design with one basic module.
The company was well aware that trying to cover a high percentage of current and future operational requirements using a conventional design approach would result in significant feature redundancy and increase the base cost.
Playing catchup all the time was not something they wanted to do but the alternative would be to produce a whole family of units, each offering a particular function designed to meeting specific requirements.
Status realised that the key to flexibility lies in being able to change the functionality of the signal conditioners in situ, with the minimum of fuss and the answer to the dilemma for them lay, surprisingly, in a form of software called Forth, developed back in the 1960s.
(Forth was developed by Charles H Moore over a period of time starting in the 1960s.
In 1968, he defined a programming language and implemented it on an IBM 1130 computer.
In those days of "third generation" computers, Moore saw the language as a fourth generation language, and named the language Forth (the IBM only allowed five-character names).
Conventional signal conditioners connect to a sensor and measure the sensor parameter, converting this to an industry standard output such as 4-20mA.
However, in order to increase the flexibility of Medacs, Status has pioneered the development of special software programs or routines, based around what it has designated as TFML which can be simply downloaded into the units.
These customised programs are referred to as TFML.
Ultimately, a specific program reads Medacs inputs, applies a transfer function, and produces one or more outputs, hence the acronym TFML - transfer function module library.
In other words, TFML offers the capability of inserting a transfer function between the input and output to provide a special functionality.
This gives customers the ability to configure the units to accommodate unusual or custom applications directly within the signal-conditioning module.
In other words, TFML means that specials are a thing of the past.
Customers have the choice of three options to obtain the necessary TFML function.
They can down load prewritten routines from the Status website to cover a host of standard functions.
They can register for a TFML training seminar, obtain the development tools and learn how to create their own programs.
They can commission Status to develop programs for their specific application.
The main benefits in implementing Forth as the TFML programming language, are its compactness and speed of execution.
In fact, the speed of execution is approaching that of assembly language - something that cannot be said of other higher-level languages like Basic.
This allows the Medacs units to run on an 8bit low-cost microcontroller.
The TFML modules are written in a version of Forth using a PC text editor and saved as a text file (known as a program 'source' file).
Medacs units run an embedded Forth interpreter, so the high level Forth code can be downloaded straight to the units via Modbus RS485 communications.
All the software compilation is done in the Medacs units, so a complex development environment (ie compiler/linker/maker/builder/burner) is not needed.
Units can handle most common sensor inputs including mV, mA, temperature and frequency and, in addition, have two isolated discrete digital inputs.
Optional outputs include an isolated 4-20mA retransmission signal, changeover trip relays and closed-in alarm relays.
Each unit has a RS485 serial communications link, enabling data to be sent to and from a host using Modbus protocol.
An ingenious 'bus jumper' system allows units to be easily bused together, removing the need for complicated and expensive back plane wiring and therefore speeding up the installation process.
Modbus was chosen because it is the most widely used and understood bus system on the market.
Interfacing with other proprietary bus systems such as Ethernet, Profibus, Devicenet and others can be readily achieved via gateway modules.
For high density systems, cost effective dual channel versions are available, while for systems that require more local input, single channel units, featuring a built-in key pad and digital display, allow the user to access critical functions.
Status has achieved full OPC compliance for the Medacs series ensuring international compatibility and connectivity.
We are now fully conversant with the term 'plug and play' in relation to our PCs and expect peripherals such as printers and scanners to work without the need for detailed configuration.
However it wasn't all that long ago when these devices needed special software and interface cards to make them work.
Until the introduction of OPC, plug and play connectivity didn't exist in the process industry.
OPC (object linking and embedding for process control) is a rapidly growing industry standard software platform developed through the collaboration of leading world wide automation and software manufacturers in conjunction with Microsoft - targeted with the simple goal of ensuring full plug and play for equipment.
The organisation that manages this standard is the OPC Foundation which has over 270 members from around the world, including most of the world's major providers of control systems, instrumentation and process control systems.
Based on Microsoft's OLE (now Active X), COM (component object model) and DCOM (distributed component object model) technologies, OPC consists of a standard set of interfaces, properties, and methods for use in process-control and manufacturing-automation applications.
The Active X/COM technologies define how individual software components can interact and share data.
OPC provides a common interface for communicating with diverse process-control devices, regardless of the controlling client software or server devices in the process.
By using the standard OPC client-server model, any OPC compliant device such as the Medacs units will connect easily and seamlessly into overall plant control systems, without the need for lengthy installation procedures and special interfaces.
In the drive for higher efficiencies and reduction in manpower, remote access to equipment is vital.
The Medacs concept has now been expanded to incorporate an embedded web server, bringing almost limitless possibilities for these devices.
The web server gateway module allows access via a user-configurable interactive website.
Remote website access can be achieved via Ethernet, standard PSTN modem, GSM or ISDN Access to the critical parameters allows trouble shooting and system adjustments to be carried out remotely.
In many cases any problems can be resolved without the need for an engineer to visit the site -which may be on the other side of the world.
An Ethernet gateway is also offered.
Status believes, along with many others, that Ethernet, whilst not being suitable for some of the more critical systems, may well leapfrog the current range of fieldbuses as the preferred connectivity system within process control for a great number of applications.
Already proven in the office environment, Ethernet offers simple integration both at factory floor level and also up through the different control levels.
As an example, at remote oyster production plants in the Northern France, Medacs is being used to monitor critical temperature and humidity conditions.
Given the high value of the oysters, it is crucial that they are processed and stored under the right conditions.
The system is designed so that if the parameters go outside preset tolerances, an alarm is texted to one of the service engineers.
Rather than have to rush off to the plant, the engineer can access the system through a laptop computer and check the status.
The system provides three decision levels depending on the severity of the problem.
For instance, at 'level one' the engineer may be able to reset the system and then put the problem right at the next scheduled visit.
At 'level two', the engineer may need to plan a visit within a short period of say two to three days, or at 'level three', it may be necessary to make an immediate visit.
Given the long distances involved, making a calculated decision based on accurate data can save considerably on costs without jeopardising the crop.
A prime example of where modern technology has revolutionised today's process industries can be found in the water and waste treatment industry.
Here modern instrumentation and control systems have had a major influence on the automation and control of plants and remote installations.
Indeed many sites are now fully automated with control and data transferred back to a central location.
However, integrating modern systems with existing equipment is not always that straightforward.
Many installed pieces of equipment, including flow meters, pumps, level sensors and other transducers are not compatible with new fieldbus technologies, yet still have a good ongoing life expectancy.
As a result, engineers have to implement often cumbersome and expensive methods in order to connect these devices into modern control systems.
Even then the amount of flexibility and direct control access is very limited.
The new Medacs signal conditioning units from Status Instruments provide the ideal link between the 'old' and the 'new' in a single module.
At the front end, the units accept traditional voltage, current and frequency inputs from field devices, while at the back Modbus connectivity and a novel bus jumper system allows units to be quickly bussed together as part of a Modbus RS485 loop.
Setpoints within the units can then be accessed via the RS485 link and adjusted as required.
As many of these applications are at remote sites, communications can be unreliable at best.
The Medacs architecture, with the intelligence downloaded to the signal conditioner, is ideal for this type of installation because in the event of a communications failure or interruption, the unit continues to perform its downloaded function.
This enables remote sites to be monitored and parameters changed over a satellite link.
The power and flexibility of TFML is clearly illustrated in actual applications For instance, critical level monitor may require the use of two identical sensors.
A Medacs unit with a dual validation TFML module can read the inputs from two sensors and continually compare their readings.
If one of the sensors fails giving either a high or low reading, then the Medacs will continue to display the sensor with the healthy output and trigger an alarm relay.
The unit can also be set so that if the difference between the two sensors goes outside a set tolerance then again an alarm can be triggered.
Although it will not indicate which of the sensors is faulty it will give an early warning of potential problems.
In another example, a Medacs unit can be used to ensure a constant flow to a process by the regulation of a valve or penstock.
Direct control can cause the valve to 'hunt', whereas a Medacs unit, with a 'three step control' TFML module, compares the actual flow from an upstream sensor with the required target flow and then provides an output signal from the unit to the valve actuator.
This regulates the valve position accordingly and provides a smooth constant flow.
Inputs from two independent sensors can be combined to provide a single output.
For instance, velocity and level inputs can be combined to give water flow readings in an open channel.
Using the TFML has certain advantages over using the capabilities of a PLC.
Typically within a PLC the parameter to be controlled is measured, usually by a multichannel input card, and the digital signal passed to the PLC.
The PLC then operates on that signal and calculates the control algorithm before sending a signal to an appropriate output card.
In contrast, the Medacs approach is to download the appropriate control function from the host to the Medacs unit.
The entire control loop (input/control/algorithm/output) is then done within the Medacs block with full galvanic I/O isolation.
This is truly distributed control.
The host system (which could be a computer or PLC) then monitors the operation of the Medacs module and, for example, downloads any required changes in setpoints or other function.
The Medacs also offers performance levels not offered as standard by many low to medium cost PLCs, including 15bit input resolution, galvanic isolation, bipolar inputs and Modbus connectivity.
As an aside for PLC programmers who are fed up with their work being copied, TFML source code is secure, and cannot be uploaded from preprogrammed MEDACS units.
Through this development, Status Instruments has pioneered the design of low cost flexible distributed control solutions with futureproof communications.
These can be readily tailored to the users' system requirements and Medacs can move forward, adapting to the changing trends in system control without the need for major design changes.
The TFML concept shows that relatively low order instruments can offer a high level of flexibility and connectivity at low cost.
As process control systems become more sophisticated, distributing control and intelligence to the signal conditioning level has significant advantages especially if failures occur higher up the control chain.
The concept of TFML is proving to be highly successful and Status has already introduced this technology into their latest range of panel meters. Request a free brochure from Status Instruments ...
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