IT'S SMART TO UNDERSTAND DIGITAL IN ALL ITS FORMS
26/09/2019 Sprint Electric Ltd
Modern manufacturing and processing relies on digital control for its efficiency, reliability, consistency and flexibility. But, as Neill Drennan of British drives manufacturer Sprint Electric explains, the word 'digital' can mean different things in different situations.We are living in an increasingly digital world and it is very easy to become complacent and assume that any digital variable speed drive will be able to do any job. However, in reality there is more to digital drives than meets the eye, and it is a wise user that takes the time to understand the basics - and seeks expert advice before installing or re-setting a critically important drive.
The first thing to realise is that there are two types of digital drive. The first is the basic fixed block diagram drive with very limited flexibility; the second is the fully programmable drive.
The programmable drive gives you the flexibility to set the drive functionality to match the application without the need for a PLC to perform the process logic. Such drives are sometimes described as having an on-board PLC, and while a control engineering purist would scoff at such terminology, it is in fact fine for most situations if enough I/O is available.
Of course, you can use a block diagram drive with a PLC. Typically you may do this when the application has a high I/O count and you need the capabilities of an expandable PLC. Alternatively in a simple price-sensitive application it may be cost effective to marry a programmable drive with a small low cost PLC.
It is perhaps easiest to understand today's technology by bearing in mind a bit of historical perspective. When the first modern drives came along they were analogue, which in real terms meant you did not have the availability of any type of chip set. Thus process logic such as multipliers, dividers, PIDs and all signal processing and amplification was done using discrete components like resistors, diodes and capacitors. From today's perspective this appears to be limited, but the drives companies quickly developed the ability to conceive advanced process circuits and were ingenious in producing rack mounted systems to meet the customers' needs. Even in those early days PID, winder and other types of control could be remarkably sophisticated (although they did usually need the support of expert engineers to set them up).
As technology advanced and the microchip became available to industry the change from analogue circuitry to digital based programming started to take place. In the early days the chips were very slow and did not match the existing analogue performance, but as Gordon Moore predicted in "Moore's Law" the number of transistors in integrated circuits doubled every couple of years and so with it did the speed of the processing.
It is worth noting that the early digital drives required dual processors, one for logic and one for control. This soon moved to single processors but there was still some effort required to understand how analogue control could be replicated in a digital environment. In fact analogue functionality did not always get fully transferred into a digital format, a significant amount of simplification was sometimes used.
Similarly, digital processing in a variable speed drive is not always the same as in a PLC, so replication of an analogue function in a digital drive could be different from in a PLC.
The crux of the matter is that multiple set point systems, closed loop control, field weakening and general processor and signal buffering are not always simple to configure in a PLC, but they are essential for drive performance. Such functions are often easier to set up in a programmable drive, which is designed specifically to control motors, whereas PLCs are designed as general purpose controllers so tend to be jacks of all trades but masters of none.
In reality today many applications are best addressed with digital based drive control. However, analogue control to set up a drive with selection from DIL (direct in line) switches and small potentiometers makes installation very easy. The addition of a buffer card can enable control of linear ramps, differential amplifiers, summing, subtracting, clamping and load cell monitoring. This is particularly appropriate for the many standalone machines that will always be found throughout industry - and there is nothing to stop such machines being subsequently integrated into wider plant control networks.
Many application requirements such as speed adjustment, flow control and winder tensioning are analogue in nature, so it is advantageous that controls for such functions were originally perfected in the analogue sphere and then transferred into the digital world. This ensures that key process functionality is present. For example in winder controls static tension, taper tension and diameter calculation all need to be included. Simple blocks for pre-set speeds, raise lower pushbuttons and PID control should all be easily accessible and simple to use.
Plantwide integration
One of the great advantages of the move to digital technology is that it has accelerated integration of related machines into sophisticated multi-function plants. Individual machines' drives and controls can now be connected through various communication protocols such as Profibus, Profinet, Canbus and Ethernet (to name but a few). This allows them to share operational data and thus optimise overall plant performance.
With the development of Industry 4.0 and the Industrial Internet of Things (IIoT) we are starting to see drives become predictive and preventative, which should in the long term makes plants more productive and efficient. There is still some way to go in achieving this ideal, but digital drives will be instrumental in making it happen.
Finally, plant operators sometimes think that their existing machines cannot cost-effectively be integrated into wide area control systems, particularly if they use DC motors and drives rather than AC. Such assumptions are usually incorrect, DC drives can be digital and can be integrated into digital control systems.
In short there is no reason why the great advantages of 21st century control technology cannot be applied to existing plant and machinery.
For more information, please contact:
Sprint Electric Ltd
Mr. Neill Drennan
Peregrine House
Ford
Arundel
West Sussex
BN18 0DF
Tel: +44 (0) 1243 558080
Email: Neill.drennan@sprint-electric.com
Web: www.sprint-electric.com
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