Standardisation in programming language for motion control brings
significant benefits to users, allowing vendor independence, giving the
freedom to choose the best equipment, at the best price in order to meet
their needs. PLCopen now provides the accepted standards in programming
for general-purpose motion control. Gareth Jones reports
TPLCopen (www.plcopen.org) is a standards organisation that is vendor and
product independent, focusing on developing standards for software in
industrial automation. The organisation has had major success through the
standardisation of PLC programming languages - the IEC61131-3
specification, for example, which has now been widely adopted. Now,
PLCopen has turned its attention to the standards for motion control and
has obtained general agreement on a standard for motion control functions
blocks. Standardisation allows users the freedom to choose and change
automation suppliers without needing to learn new programming languages.
A task force consisting of automation users and automation vendors has
developed the PLCopen specifications, ensuring that the requirements of
users are well catered for.
PLCopen motion control function blocks take standardisation to the next
level. Previously, there were often large discrepancies between the
methods and functions provided by motion control suppliers, each with a
unique technique for controlling the motion with programming languages
such as Basic, using bespoke commands, or graphical interfaces. The clear
advantage to the designer/software engineer is a standard language that
is easy to use, saving development and programming time (and money) and
providing vendor independence.
Users will always choose open standards in place of similar proprietary
schemes. Up until now, if a user were to develop a solution for a
machine, they could choose open standards to define the drive and network
to give them vendor independence. For example this might comprise a
Sercos network and Sercos enabled drives.
But the programming method of the motion control remained proprietary
and, to make matters more complicated, the motion control is where most
of the development time and intellectual property is concentrated,
meaning that the tie-in to the manufacturer is comprehensive and
therefore less than satisfactory to some users. PLCopen motion function
blocks address this issue and so are now gaining widespread acceptance
alongside IEC61131-3 programming tools. This gives users greater
flexibility than ever before in their choice of drives.
PLCopen puts in place a superb framework for motion control. In the event
of a change of supplier, the user may not be able simply to copy and
paste large swathes of software from one vendor platform to another.
However, the key building blocks of the software remain mostly the same,
making the task of migrating to a new supplier much easier - and
considerably faster! The variation in features and functions offered by
different motion control platforms does, however, require that the
standard allows for some vendor specific functionality.
The current trend in motion is clearly away from centralised motion
controllers, towards intelligent networked drives with onboard motion
controllers (or often a hybrid system, combing both centralised and
decentralised control). IEC61131-3 programming tools and PLCopen motion
function blocks offer users a massive benefit in terms of allowing the
freedom to put the intelligence in a system, where it gives the best
balance of performance and cost.
On both counts, performance and cost, this value equation tends to fall
in favour of a full, or partial, distributed control philosophy, where
the intelligence on-board the drive has the key advantages of close
intimacy with the process. This means that the onboard position
controller has immediate access to drive parameters, encoder position and