New!

The PLCopen Motion Control Library :
changing the landscape of industrial control

Eelco van der Wal
Managing Director PLCopen
evdwal@plcopen.org

ABSTRACT

The industry has to change to fulfill the changing requirements of the consumers and the governmental rules. These requirements can be solved by using new technologies, combining them with existing ones. In first line this is focused to the consumer oriented suppliers, like in the food & beverage or pharmaceutical sectors. These transfer their requirements to their machine suppliers, which in turn transfer it to their control suppliers. The controls play a crucial role in fulfilling the requirements. To be more specific: the application software is crucial. And to make that effective, standardization is needed.

This paper presents such a standard: being implemented across different technologies, new and existing ones, and being well supported by a large representation of the industry, this software standardization will certainly change the landscape.

INTRODUCTION

Changing consumer needs require changes at the producers: smaller batches, differentiation in packaging, differentiation in products. Also, changing governmental rules, focused to protect the consumers by making the suppliers responsible and liable, requires producers to change their way of production. On top of this, they have to produce more efficient, for instance to serve their shareholders.

CHANGING REQUIREMENTS IN FOOD PACKAGING

A typical example of the effect of these consumer changes is the packaging industry. Packaging comprises many aspects: bottling, flow wrapping, cartooning, form-fill-seal, labeling, palletizing, robotics, material handling, weighing, inspection, and bagging comprise a wide variety of application domains within a packaging plant. Each application provides a unique set of requirements specific to both industries and regions. Pharmaceutical plants must track products accurately through every stage of the manufacturing process. Food and beverage suppliers are also concerned about product tracking as well; however, the ability for a plant to respond to wide variability in container sizes and labeling is absolutely imperative today.

These changes effect a large industry. John Gregg, Vice President R&D Kraft Foods at the PackExpo 2000, Chicago IL., showed some interesting figures. Shipment of packaging machinery in 1999 in North America was $4.85 billion, up +7.3% over 1998. This is more then 10% of the value of the retail food sales in North America, which is around $460 billion. Overall the North American food industry is around $460 billion for retail foods and $ 358 billion. for foodservice, totaling to a $800 billion industry. This is around 11% of the output of the entire North American economy.

Let us look at an example: consumers nowadays want a huge number of varieties in coffee. Overall, changes in consumer needs/wishes include customization, personalization, and convenience. These changes happen in the distribution and storage along the chain (for instance just-in-time, and additional selling places), as well as the food packaging industry. Consumer changes like these can make huge portions of the existing packaging equipment obsolete very quickly.

ONE STEP BACK IN THE CHAIN: MACHINE BUILDERS

To cope with these consumer changes, the packaging industry is putting pressure on the leading packaging machine builders to better fulfill their needs in:

Smaller footprints
Faster startups
Flat to lower cost per function completed
Higher speeds
Improved efficiency
Faster changeovers
Better quality package
Reduced waste
Improved reliability

ANOTHER STEP BACK – THE EFFECT ON EQUIPMENT SUPPLIERS

Packaging control systems deals with:

Human Machine Interfaces
Logic and Sequencing Control Systems
Motion Control Systems including drive technologies
Network Architectures For Business System Integration
Interface technologies to drives and other actuators and sensors

All these aspects are controlled by software, playing an ever increasing role. Since these multiple environments need to be integrated, standards and open platforms are a pre-requisite. As the investment cost in the application software is ever increasing, a essential part are the programming languages to create, maintain, and operate the application software.

EFFECT ON CONTROL SYSTEMS

To cope with these new requirements in packaging effectively, from a motion control point of view, a mechatronics design is needed. This means that mechanics in the machine are replaced by electronic solutions in the form of digital; motion control. For example a rigid CAMshaft is replaced by a combination of multiple drives/motors. The control software in these mechatronics solutions provide the flexibility here.

To solve this cost effectively, the packaging machinery must take advantage of the latest software, networking, and operating system technologies to enable machinery to be flexible enough for today’s manufacturing requirements. Manufacturing plants need equipment that is less complex, more flexible, easier to maintain, and with smaller footprints. These requirements translate into substituting today’s mechanical solutions with electronic and software based solutions. Motion control technology combined with industrial software is capable of vastly reducing the complexity and size of every domain of packaging machinery today.

MOTION CONTROL – A CHANGING MARKET

All these changing requirements are heavily reflected in the control architecture. Motion control plays an ever increasing role in this. No wonder that the motion control market is changing.

The global motion control market is estimated around $3.6 billion, according to UK-based Datamonitor. The top eleven markets for motion control together account for 90% of this market. Servo systems currently account for almost 70% of the global motion control market.

The primary customers for motion control systems remain machinery builders, mechanical handling and warehousing companies, electrical panel builders and specialist motion control projects companies. The leading global industrial markets still account for the overwhelming proportion of motion control systems business. However, the end-user market is becoming even more diverse and now encompasses travel and service industries (e.g. baggage handling and postal delivery) process industries and manufacturing industries. Even the entertainment industry can be an end-user, for example in the control of theater scenery and lighting. In addition, there are now indications that the end-user is becoming more likely to implement a motion control application directly.

Compared with other factory automation products, the global motion control market is extremely fragmented. Although the major automation companies have a significant presence an astonishing number of smaller companies have large shares of their national or regional markets, often focused to niches. However, new forces are changing the structure of the market, with consolidation sweeping up many of the well-known motion control brands into the global players. Successful small companies may in time be taken over or bought out by larger concerns. Over the last few years, consolidation has swept up many of the well-known motion control brands into the global factory automation companies. Others are being absorbed into pan-national groupings by major holding companies.

THE NAME OF THE GAME – SOFTWARE

Motion integration issues have emerged to the forefront, along with maintainability and connectivity to automation solutions. Unfortunately, the motion control market is a very fragmented market, providing a wide variety of incompatible systems / solutions. In practice this means that the architecture and the software tools for development, installation and maintenance will differ widely. This incompatibility induces considerable costs: applying different implementations is confusing, engineering becomes difficult, and the software is not reusable across platforms. Overall, this means that there is too little standardization in this market. Standardization that provides flexible solutions that are open to new developments. Meaning standardization not only in the programming languages itself, like done within the worldwide IEC 61131-3 standard, but also in the software interface towards different motion control solutions, like distributed versus integrated. In this way, the benefits of software standardization are clear:

	Less hardware dependence
	Higher level of reusable code
	Transparent programs
	Lower commissioning, installation and maintenance costs
	Wide industry acceptance
	And last but not least: reduction in training costs

With the ever increasing processing power, different solutions are possible: centralized systems versus distributed systems based on multiple intelligent drives.

The first intelligent drives were intended to off-load the main processor by closing most of the loops, including the fine interpolation, on the processor integrated in the drive itself. With the ever increasing processing power, one now sees an excess in processing capacity on the intelligent drive. This additional power can be used to run one or more tasks that were primarily done until now on a central controller, like a PLC. For this, the suppliers needed a software environment for their PLC tasks. This of course was provided by the only available standard IEC 61131-3. In addition, the supplier needed a coupling for the PLC tasks and the motion control tasks. By connecting several drives via a network, one can in this way easily construct a full system, in the sense of motion control tasks combined with PLC tasks, the latter for product identification, tools change or oil supply, etc., and by adding a simple operator panel as human machine interface. Also, the networking solutions are nowadays focussed to the much faster Ethernet solutions (unfortunately, there are more than one protocols to run on it).

On the other hand (or extreme), this increase in processor power is also valid for the controllers, creating options for a complete centralized approach. One see this very clearly in PLC systems with integrated motion control, becoming more and more standard available. Also, one sees this very clearly in PC-based control: the first step was to add one or more PLC-tasks in software. Nowadays, one can even add one or more motion control tasks on the same hardware, creating the possibility of very tight control, and adding axes at very low costs. The interface towards the drive is either analogue or pulse-width-modulation, PWM, directly to the power stage of a ‘bare’ drive (besides of course feedback from one or more encoders). The essence is that they run as software on one processor: the additional intelligent motion control cards are becoming obsolete in these systems. There are nowadays several soft-CNC solution constructed in this way, offering a complete environment with HMI, multiple axes motion control, and PLC tasks on one platform, and as said, in most cases even without dedicated motion control boards.

Also the PC-based control environment needed new environments. The standard Windows API is not widely accepted on the factory floor – especially not in maintenance. They also adopted IEC 61131-3 to provide an alternative representation. They also had to link the different tasks.

THE ROLE OF PLCopen

This changing environment, combined with the ‘classical’ PLC environment integrating more and more motion control into their controls, created a basis for standardization: let us not all invent our own wheel, but do it together. This vision is nowadays shared among many different suppliers as part of the organization PLCopen – and resulted in the definition of a PLCopen Motion Control Library. It is a standard in the programming environment, to harmonize the access of motion control functionality across platforms.

Effectively this standardization is done by defining libraries of reusable components. In this way the programming is less hardware dependent, the reusability of the application software increased, the cost involved in training and support reduced, and the application becomes scalable across different levels of control. As such it is based on IEC 61131-3 Function Blocks. With the standardization of the interfaces and the functionality, and implementation on multiple platforms, the generated application program is much more hardware independent, and so re-usable across platforms. Due to the data hiding and encapsulation, it is usable on different architectures, for instance ranging from centralized to distributed control. As such it is open to existing and future technologies. Overall, the standardization is expected to cover around 80% of the motion control market.

The task force has defined the following goals for the definition of the motion control function blocks:

Simplicity - ease of use, towards the application program builder, installation and software maintenance
Efficiency - in the number of blocks, directed to efficiency in design (and understanding)
Consistency - conforming to IEC 61131-3 standard
Universality - hardware independent
Flexibility - future extensions / range of application
Completeness - not mandatory but sufficiently

The definition has been done via the definition of the state machine and the definition of a basic set of Function Blocks for single axis motion and a set of multi-axes Function Blocks.

To the user, the Function Blocks are crucial. These are the software equivalent of electronic chips. They contain inputs and outputs, with the associated names and data types. Each Function Block contains code (like a small program) to give it its functionality (like the components in a chip), and map it to the corresponding (motion control) environment. The user only sees the interface, being the inputs and outputs. The code itself is hidden – this is called data encapsulation and hiding, and is crucial to separate the different levels of programming and maintenance. Below a small example of two Function Blocks operating on the same axis. Access to the axis itself is via Axis_Ref. This is a data-structure describing the drive itself. All Function Blocks have access to this reference. In this way the internals of the drive or architecture are hidden to the user, providing hardware independence: distributed, integrated, or centralized controls, for the user all architectures are accessed in the same way.

Overview of the defined Function Blocks

Without going into details, the following table gives an overview of the defined function blocks, divided in administrative (not driving motion) and motion related sets.

Administrative		Motion
Single Axis	Multiple Axis	Single Axis	Multiple Axis
Power	CamTableSelect	MoveAbsolute	CamIn
ReadStatus		MoveRelative	CamOut
ReadAxisError		MoveAdditive	GearIn
ReadParameter		MoveSuperimposed	GearOut
ReadBoolParameter		MoveVelocity	Phasing
WriteParameter		Home
WriteBoolParameter		Stop
ReadActualPosition		PositionProfile
Reset		VelocityProfile
		AccelerationProfile

Example : the same Function block instance controls different motions of an axis The figures below show an example where the function block FB1 is used to control “AxisX” with three different values of Velocity. In a Sequential Function Chart (SFC) the velocity 10, 20, and 0 is assigned to V. To trigger the Execute input with a rising edge the variable E is stepwise set and reset.

1: Single FB usage with a SFC

The following timing diagram explains how it will work.

Certification

Included in the document are the rules for compliance and certification. Basically this is a self-certification, from which the results per supplier are published on the PLCopen website www.plcopen.org . Certified companies are allowed to use the logo below, with additional number, date and number of supported compliant function blocks:

CONCLUSION

Changing needs at consumers put a high pressure on the consumer related industry, as well as on their suppliers. Fulfilling these needs, requires changes in the control architecture: moving from mechanical solutions to mechatronics. This makes the role of software extremely important, combined with motion control.

Standardization in the fragmented motion control area is crucial. And such a standard is available: the PLCopen Function Blocks for Motion Control. As such it is destined to change the face of industrial control. The specification is freely available on this website as downloadable file (in pdf format).

Through the independent organization PLCopen a wide user acceptance is realized, and the first implementations are already available. Also, cooperation’s with other organizations and workgroups, like the OMAC Motion Control for Packaging, is continued on an on-going basis.

For this page as Portugese WORD-document, click here.
For a PDF about this subject, click here.