Drives software programming – using PLC or drive custom programming?

Automation Integrator Guide: Drive programming applications vary in difficulty, and some projects require custom software. Should one use the drive’s internal programming blocks or write one’s own blocks in the PLC to control the machine? Guidance follows.

11/09/2012

A variable-speed drive’s purpose in life is to control the speed and torque of a motor. In essence, it takes ac electricity from the mains supply and converts it to drive a motor in a controlled way. Variable speed drives are tuned in two ways: 1) to match the drive to the electrical characteristics of the motor by auto tune routines (standard drive functionality these days), and 2) to match the physical load conditions (by drive commissioning engineers) to match the response of the drive to varying load conditions. These tuning mechanisms will ensure the drive-motor combination is optimized to maintain the desired speed.

Applications vary in degree of difficulty, and sometimes require complex and often time-consuming drive software programming. 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 that custom software be written.

One frequently asked question in applications of this nature is, “Should the drive’s internal programming blocks be used or should custom blocks of programming be written in the PLC to control the machine?” The answer is not straightforward.

When building 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-derivative) tuning functions.

Given the two possible solutions, some experience is necessary to make the best decision

Using drive functions

Option 1 is 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 internal math completed means the engineer must select and connect the right blocks in an appropriate configuration (see drive configuration drawing). Naturally, this requires good working knowledge of machine control and the control requirements of the application at hand. Nevertheless, it has definite advantages, which are listed in Table 1.

As always, there are trade-offs to consider—for example, different drive manufacturers employ different ways of programming the drives using their own software tools. Siemens alone offers two programming options: “Starter” (the simpler of its two offerings) 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 prerequisite to building effective drive configurations.

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Table 1. Internal drives blocks: pros and cons
Drive programming software written and supported by drive manufacturer.   Each manufacturer has its own software to learn. Some are quite complex!
Quick to build a configurationIntegration to HMI and PLC is more difficult as more data needs to be transferred to the PLC.

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Some software packages have built-in trace features like an oscilloscope. This is useful when testing.

Using PLC functions

Option 2 is to do drive programming using PLC resident functions.

Learning to expertly use different drive programming packages is a complicated task that takes time. This often justifies an engineer’s decision to host the drive functionality software in the PLC (see related PLC configuration drawing). 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 with which the engineer must be aware (Table 2).

This means, however, that much more complicated mathematic 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 needs; they do this very easily and reliably. However, when higher level mathematics is involved, much more powerful processors should be specified and the structure of the functional software is considerably more complex.

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Table 2. Using PLC programming blocks: pros and cons
Common control for any drive manufacturer  Complex math requires significantly more engineering.
Once written it is quick to implement as all systems are the same.Complex math requires much more processing power than simple code. This pushes up the PLC cost.
Only simple signals are sent to the drive (start, stop and speed).

None

HMI Integration is easy as all important numbers already exist in the PLC.None

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Decide programming method

Which programming method should be used? The 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 engineers. Having well-documented and common function blocks (as in the drive resident software option) would offer standard, documented software solutions and the comfort of the drive manufacturers’ 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 is a more difficult task.

As an independent systems integrator, Optima Control Solutions has no drive or PLC manufacturer obligations to meet. Customer recommendations and designs are based on customer circumstances.

- Colin Keating, sales and application engineer, and Hristina Stefanova, marketing executive, Optima Control Solutions Ltd. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, mhoske@cfemedia.com.

Go Online

At www.controleng.com, search “drive programming”

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