1979 was a highly significant year - Margaret Thatcher was first elected as Prime Minister, Manchester was bombed, Chrysler asked the US government for $1bn to avoid bankruptcy (sounds familiar?), I got married, and drives were custom built in cubicles. Variable speed drives were dc, with ac only used in specific applications such as textiles where many motors (usually reluctance) were fed from a single inverter. Those inverters employed 'fast thyristors' and commutation circuits of such complexity they were instruments of torture for undergraduate students.
At the second IEE international conference on electrical variable speed drives, that year, Professor Leonhard presented a paper "Microprocessor control of induction motors employing field co-ordinates" to academic acclaim. At the same conference an attendee was ridiculed for suggesting that the potential sales volumes for drives could justify the economic use of custom integrated circuits. It was a time of great innovation but many of the leading drives companies of the time failed to embrace the change and have become marginalised or disappeared completely. It was not a time where energy efficiency or carbon footprints were critical issues!
Today, industrial drives represent a £8bn business, and the modern product is unrecognisable against its distant relative of 1979. So what facilitated such radical innovation? A critical factor was power semiconductors, not simply the device type or switching performance, but the packaging. 1979 saw the introduction of isolated thyristor packages, which meant that heatsinks could be monolithic and earthed. Drives designers were not slow to recognise the implications. Drives shrank dramatically, firstly to smaller sizes of 19in rack for cubicle mounting and then to products recognisable as the forbears of today's offerings.
In 1983 bipolar transistor technology arrived and, despite initially poor reliability, the ac drives market boomed. Control was simple: constant Volts to Hertz with low frequency boost. Field oriented control was known but used only on large drives. It was not until 1989, with the launch of the Control Techniques Vector drive, that such control, offering superior performance to a dc drive, was available to the mass market.
In the mid 1980s advances in microprocessors facilitated cost effective digital drives. Drives were introduced containing custom integrated circuits and new plastic materials were introduced giving structural strength, weight, size, assembly and cost advantage. By the late 1980s IGBT (insulated gate bipolar transistor) technology arrived, heralding the era of relatively quiet, efficient and ultimately reliable ac drives. Drives continued to get smaller, easier to use and volumes grew.
The birth of 'intelligence'
The mid 1990s saw the introduction of the first truly Universal drive, which met without compromise, by parameter selection only, the diverse requirements of an open loop (vector) drive, a closed loop drive, a servo drive, and a sinusoidal supply converter. This was also the birth of what has become known as the intelligent drive with user programmable functionality and fieldbus connectivity. Integrated ac motor drives were introduced at the end of the 1990s. These products were, for the most part, open-loop, inverter-driven induction motors, initially targeted at replacing mechanical variable speed drives. Integrated servomotors followed.
Multi-axis requirements inspired a range of developments: multiple drives in a single package; drives with the position and speed loop embedded in the encoder housing on the motor itself. This later concept gave advantage by processing position information close to the source avoiding noise problems, which allowed dramatic improvements in control resolution, stiffness and reduced wire count between drive and motor.
In the early years of the new millennium, rapid change continued. Users looking at drives as components in a large control system, demanded ever greater connectivity. The ieldbus 'wars' were raging with passionate claims for many systems (I counted over 200 in a 12 month period), most of which have since disappeared. This has now morphed into the Ethernet wars, with advocates of the different protocols all predicting dominance.
Modern communications systems open up new opportunities for the highest levels of system performance through dynamic distributed control. Synchronising all control loops (with <2µs jitter) within and between drives brings a new level of determinism. Indeed, PLC functionality is now available for users to programme very complex and demanding system applications, including motion control with drives - some having multi-axis interpolation capability.
Customer driven
Ongoing development is driven by customer needs, component technology, design techniques and the vision of the industry. Power devices drive efficiency, whilst microprocessor performance yields not only improved motor shaft performance, but enhanced functionality. Ease of use remains a key focus. Whilst there remain specific motor types and controls best suited to certain applications, users should not have to deal with a plethora of different user interfaces and modes of operation. The Universal drive has been a major step forward but more will come. Similarly, automatic tuning routines in drives, matching the drive to both motor and mechanical load, ease the burden on the commissioning engineer; but further advances are needed. Robustness of control and stability even within non-rigid structures, will be important future areas. Significant advances occur when users and drive designers get together and consider system solutions holistically.
Motor technology is changing too. The brushless permanent magnet motor, once only used in high performance applications, is now being considered where efficiency or size is critical. Linear motors have made an appearance - though mainly at trade shows. For the most demanding applications, the quality of speed or position measurement is critical. Sine/cosine encoders have facilitated very high-resolution position feedback, while all digital methods such as EnDat, point the way forward.
Amazingly, some drives are sold to customers who have no motor to control It has been purchased purely to use the comprehensive auxiliary functions. Will manufacturers continue to pour more and more functionality into drive products? Perhaps not as drives are, despite LCD displays with menus, macros and wizards, already bewildering for many users! The drive may become more like a PC with a basic operating system and a limited number of core functions and then have functionality loaded by the user.
The market for industrial drives will continue to grow apace, despite the hiccup in the present global economy. Automation drives the development of the industrial world and drives are at the heart of automation. Energy efficiency will, at last, be a substantial driver of growth, through both regulation and common sense. Alternative energy sources rely upon efficient power electronic energy conversion and drives companies are well placed to play a significant role in this market. Spin-offs and leverage will result.
The last 30 years have been fantastic, for which I thank my wife. It's been pretty exciting in the drives world as well, and in very many ways the story has only just begun!
Professor Bill Drury is executive vice-president, technology, Control Techniques