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PLC or drive bespoke programming?

21 November 2012

One frequently asked question in applications of a complex nature is: should I use the drive’s internal programming blocks or write my own blocks in the PLC to control the machine? The answer is not a straightforward one, as Colin Keating explains.

Variable speed drives are tuned in two ways: to match the drive to the electrical characteristics of the motor by auto tune routines (this is standard drive functionality these days), and to match the response of the drive to varying load conditions. These tuning mechanisms will ensure the drive-motor combination is optimised to maintain the desired speed.

Of course, applications vary in terms of their complexity and it may be necessary to undertake drive software programming at some point in the development of a control system.

Many drive manufacturers such as Siemens and Parker SSD have developed application software (often based on Macro type solutions) designed to meet most common drive applications and control requirements. However, it is not unusual for certain projects to require bespoke software to be written.

When building up a drive configuration engineers might need to employ many mathematical functions of varying complexity. Ranging from simple AND and OR gates to more complicated diameter calculation, tension profiles and PID (Proportional Integral and Derivative) tuning functions.

Given that there are two possible solutions, some experience is necessary if the best decision is to be made.

Using the drive internal function block programming 
Assuming the application can be controlled using the function blocks available, this option is often the quickest solution. Arranging the connectivity of prewritten function blocks with all their internal maths taken care of means the engineer must only select and connect the right blocks in an appropriate configuration (see the drive configuration diagram). Naturally, this requires good working knowledge of machine control and the control requirements of the application at hand. Nevertheless, it has definite advantages. We list these in Table 1.

As always, there are trade-offs to consider; different drive manufacturers employ different ways of programming the drives using their own software tools. For example, Siemens alone offers two programming options - ‘Starter’ (the simpler of their two offers) or ‘Scout’ (designed to engineer highly complex applications) . Parker SSD offers DSE Lite or DSE with similar characteristics. So familiarity (or expertise) with the programming software is a pre-requisite in order to build effective drive configurations.

Drive programming using PLC resident functions
Learning to use different drive programming packages with any level of expertise is a complicated task and it takes time to hone these skills. This alone will often justify an engineer’s decision to host the drive functionality software in the PLC (see PLC configuration diagram).

Often, control data can be passed to and from most manufacturers’ drives from the environment of the PLC with which the engineer is already familiar, thus limiting the range of software tools in which the engineer must be competent (see Table 2).

This does mean, however, that much more complicated mathematical functions will have to be coded by the engineer, often proving time consuming and difficult. In the main, PLCs are best suited to sequential control tasks, but when higher level mathematics is involved, more powerful processors should be specified and the structure of the functional software is considerably more complex.

Which programming method?
The choice of best control method is circumstantial. OEMs can benefit from using the PLC resident software option as they can have a common PLC with the flexibility to offer any type of drive to their customers.

For maintenance departments it would depend on the expertise of their staff. Having well documented and common function blocks (as in the drive resident software option) offers standard, documented software methods and the comfort of the drive manufacturer’s service and support resources. A ‘site standard drive’ is required here; if different types of drives and PLCs are used on a site then effective maintenance becomes that much more difficult.

Colin Keating is with Optima Control Solutions

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