Variable speed drives have brought precise and intelligent motion control to a range of industrial applications. The manufacturing sector alone depends upon machines that rotate and convey materials, pump liquids, cool or heat air with fans, pack and stack finished products – and do so as part of a spread of interlinked operations that plays out for the most part automatically.

In order to function as they do in this environment, drives depend completely on the controlling, co-ordinating power of the PLC (programmable logic controller). Given, though, that it is by no means a new technology – the first models were made almost exactly fifty years ago – the question inevitably arises: what is it about the PLC that makes it so suited to the job?

PLCs were developed to substitute the power of the computer for the hardwired banks of power relays formerly used to control factory machinery. The difficulties of maintaining and troubleshooting these old electromechanical nerve centres were legion: gigantic enclosures presented the technician with walls of relays, timers, counters, fuses and terminals amid swathes of crisscrossed, point-to-point wiring. Replacing a failed coil or worn out contact was challenging enough; modifying the purpose of the system itself might entail a wholesale rebuild.

It was inevitable that the arrival of the microchip would sweep these cabinets of curiosity into the annals of engineering history. Both Odo Struger (1931-1998), a research engineer at Allen-Bradley in the 1960s, and Dick Morley (1932-2017), who answered a call for ideas issued by General Motors in 1968, have been called Fathers of the PLC. They both saw that the sequence of events carried out by relay systems in order to control machinery could be translated – and miniaturised – into the form of a computer program.

A computer it is, then. But the PLC is a very specific type of computer; it was conceived as such and to this day remains so. But in what ways?

Why PLC rather than, say, PC?

Perhaps the most obvious answer – to look at it – is that a PLC is physically tough; the thing is ruggedised. This means that all aspects of its design (from a choice of component materials to features such as temperature control and style of casing) are meant to protect the device from challenging levels of dust, humidity, vibration, temperature and so on.

A PLC’s distinctive design must also accommodate significant input/output arrangements – far more than the odd memory stick or printer. The PLC’s list of signals coming in (from switches, sensors, circuit breakers, etc.) combined with outgoing commands (to motors, lights, valves and the like) is as long as the operations it controls are complex.

But the most fundamental difference between PLCs and personal computers is their programming language. Ladder logic (or Ladder Diagram) encodes operational instructions sequentially, in a manner that is directly modelled on the flow of work through a scheme of electrical relays. This makes it extremely engineer-friendly. And alongside a small number of other simple languages, notably Function Block Diagram, it remains the standard programming method.

PLCs communicate with variable speed drives either through direct control signals or through a digital communications protocol (Modbus has long been the most popular) or through both in combination. The full range of device commands can thus be executed: from instructing the drive simply to run the motor, and in which direction of rotation, to the all-important real-time adjustment of acceleration and deceleration parameters.

The potential of drives to operate motors at optimal speed can only be fully realised when they are in real-time, two-way communication with PLCs. It is the PLC that monitors the drive’s performance, continuously checking status and fault codes derived from, for example, the comparison of the target with real output current. The way in which this monitoring of output influences the nature of the drive commands is crucial to the intelligence of the system.

PLCs impact on the automation industry

PLCs have had a revolutionary effect on the automation industry, enabling insight into and control over complex mechanical systems a far cry from the days when a single unidentified ‘fault’ might cause the best part of a manufacturing plant to grind to a halt. And their enduring success over the years has been in large part due to their essential simplicity – for all their processing power.

No technology, however, remains untouched by progress indefinitely. And PLCs are in the end as bound as anything else to reflect key developments in the way in which machines and devices are built. 

Continuous development

Miniaturisation in particular – that very force that saw early electronic activity transposed from the relay wall to the circuit board – continues to make processors, component parts and circuit boards themselves ever more compact. PLCs are in consequence becoming more powerful (faster and with dramatically improved memory capacity) even as they decrease in size. A single PLC can now easily do the work of several of its predecessors. Such progress may be seen in their ability to accommodate multiple communication protocols simultaneously or the fact that their software developers may mix and match different programming languages.

The irony here, of course, is that this order of capability is not in fact necessary for the control of many devices, drives included. Where simple efficiency is the priority, complex capability may be at best an irrelevance and at worst a liability (for instance in terms of cybersecurity). For this reason, a new generation of machine controllers – compact PLC-like devices – has evolved to take over some of the work which high-end PLCs have outgrown.

More limited than a PLC in terms of memory and input/output capacity, a controller of this type for a variable speed drive, supplied onboard, bespoke-programmed and with an intuitive graphic interface, is relatively inexpensive, time-saving and easy to use (as well as to integrate with the larger network or system).

PLCs and drives, the perfect match?

The traditional relationship between drives and PLCs is, therefore, going through a time of change. It’s an upheaval which perhaps only old-style system architecture might not survive. The fundamental principle – of drives made smart through programming power – is as fully charged as ever.