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Ageing control system? Deciding when it’s time to upgrade

02 January 2014

Most new machines already include the latest technologies in their control systems. This is expected from an OEM keen to retain a hard-won reputation and market leadership. End users, however, have different criteria to those of OEMs.

Legacy systems are driven hard to earn a decent return on investment, but at some stage the end user will need to decide whether or not the time has come to embark on a machine control system upgrade. When and to what extent a migration from an ageing to a fully up-to-date control system is undertaken will depend on a range of factors.

In most cases, the single factor driving the decision to upgrade is that the existing control system’s reliability has diminished to the point where it begins to impact significantly on a business’s commercial performance (productivity being the issue here), or its vulnerability to failure and the consequences of failure are considered too risky. In such cases, a control system replacement may be an imperative.

Upgrading the ageing control system of any machine involves the full or partial replacement of the existing control system with new control equipment, re-designed to embrace the latest control techniques and supporting technologies. So, how does a company justify the need, and subsequently prepare, for a migration from an ageing to a fully up-to-date system?

In business most questions must satisfy a commercial rationale. Any monetary investment needs to have justification with a confidently predicted return on that investment. So what improvements can be reasonably expected from the decision to migrate? Let’s consider a few of the more frequently undertaken migration projects:

Moving from analogue to digital control
The widespread introduction of low cost microprocessor technology revolutionised not just industrial equipment but domestic appliances, too. Those readers who can remember tuning a radio via a rotating dial are probably also familiar with the need to continually adjust the tuning dial for an optimum signal quality. With digital systems all this is a thing of the past. We might compare this with the speed holding performance of a variable speed motor.

Variability of analogue control systems - due to temperature, for example - means that the best speed holding one can hope for is about plus or minus 0.1 percent of maximum speed (that is, with a high quality analogue tachometer feedback device and a good quality motor controller). Digital technologies offer at least a ten-fold improvement on this. The impact on both product quality and rate of production is obvious.

PLCs – new for old
PLC systems have now been commonplace since the 1970s, but in forty years they have moved on apace in terms of their capabilities and performance. With technological developments come new product ranges; and when a manufacturer releases a new range, the previous one has two limited support lifetimes.

One, the official one, is provided directly by the manufacturer; the second one - by independent means which has no defined time span. The latter generally outlasts the former, particularly for main brand equipment where second-hand units find their way onto the general spares market.

Some obsolete PLC equipment is seen more often than others as a direct result of its original popularity and evident reliability. Siemens S5 equipment, for instance, is still in widespread use despite the fact that it ceased to be manufactured as long ago as the early 1990s. The spares and support requirements for this equipment has dwindled to dangerously low levels with increasingly difficult support strategies needed to maintain it. An irreparable failure is likely to cause significant disruption, so a planned PLC migration may prove a wise course of action.

There are several benefits to be gained by employing the latest PLC technology. Recently introduced models have greater memory capacity and take up less panel space. Depending on a machine’s specifications, the PLC size can be anything from ‘nano’ to ‘micro’ to large. A single modern PLC may run several machine sections concurrently, and as most PLC manufacturers offer modular systems that provide only those functions needed for the job in hand, machines can be successfully controlled at lower cost.

Motor speed control
Until the advent of vector ac motor control, the most common options available for industrial variable speed drives were mechanical (positive infinitely variable gearboxes, for example) or electronic variable frequency inverters controlling motor input voltage and frequency (V/Hz).

Motor torque varies as a function of the stator and rotor fields and is at its peak when the two fields are at right angles (orthogonal) to each other. In V/Hz based control the angle between these two fields varies considerably. However, vector control seeks to recreate the orthogonal relationship in the ac motor, and control the torque producing current separately from the magnetic flux producing current so as to achieve the responsiveness and torque performance of a dc motor.

Vector control requires knowledge of the rotor flux position and can be calculated using advanced algorithms from knowledge of the terminal current and a dynamic model of the ac induction motor. Vector control is thus highly dependent on computational resources and it only became viable as an advanced motor control technique with the widespread availability of cheap but powerful microprocessors.

Vector and other sophisticated methods of controlling motor torque are now commonly implemented in modern affordable inverter drives, along with other control features, including PLC, PID and application specific functionality.

Fieldbus and the art of copper reduction
While remote I/O hardware comes at a higher price than the centralised version, the cost of installation (from the point of view of cabling and manpower resources) is reduced dramatically and in most cases outweighs the increased hardware costs.

Add to this the much-improved reliability that comes with lower numbers of wire ends and the vast amount of real time diagnostic data available from the technology, and the argument for fieldbus adoption becomes a compelling one.

Summarising, control system migration should only be seriously considered when some or all of the gains outlined above can be made. Specifically, if your process is difficult and time-consuming to setup, product quality drifts and the control system is suspected, downtime is becoming an issue, production systems are not flexible enough or you simply can’t find spares anymore, then it may be time to consider migration.

Article submitted by Optima Control Solutions

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