Executive summary

This white paper sets out to identify the pros and cons associated with centralized and decentralized drives for motion control applications. The intention is not to favor one approach over the other, as in many cases each will have applications to which they are best suited. Instead, the idea is simply to make machine builders and engineers aware of the specific advantages and disadvantages that can help bring about project success.

Introduction

Adopting a centralized strategy means that the drive, along with all of the other necessary motion control components, is housed within a cabinet, the likes of which can be seen alongside machines and production lines worldwide. In contrast, taking a decentralized (or distributed) approach sees the drive technology relocated from the control cabinet to within far closer proximity of the motion control process – sometimes even integrated with the actual motor itself.
Decentralization can of course occur on many different levels, from an individual soft starter or drive located at the motor, to an entire decentralized system, which may comprise, for example, a VFD, overload protection, motor disconnect switch, I/O and bus module. In many cases, all of this equipment can be provided as part of a single package from one supplier.

Centralized and decentralized drive architectures have their virtues and drawbacks. Deciding which is best depends very much on the specific application. Indeed, mixed architecture systems are fairly commonplace within industry, especially when drives exhibit some commonality of features, thus demonstrating the co-existence of both approaches.

The following chapters outline some of the most notable pros and cons associated with centralized and decentralized drive strategies, focusing on areas that include cabinet/control panel size, application sizing, options, and modularity.

Cabinet/control panel size

As is often the case in an increasing number of industrial applications, space is a limiting factor. With a centralized approach – if space permits – then all of the drives can be located within one cabinet, simplifying diagnostics and maintenance. Similarly, it might be possible to centralize certain services, such as cooling, power distribution, and security. Clearly, performing visual diagnostics is also far easier when adopting a centralized motion control strategy.
Of course, all of this centralization comes at a price: the footprint or dimensional envelope of the cabinet increases. With many factories and plants compromised for space, machine builders are increasingly less keen on bulky control cabinets. Being able to promote machines with a ‘compact footprint’ has become a major USP for OEMs in recent years. There may also be a size issue when industrial facilities look to upgrade or extend existing machinery, perhaps to eliminate a bottleneck or boost efficiency, for example. In such situations, engineers often find that space is limited or reserved for capacity expansion in the future.
Another key factor here is that the cabinet control panel represents a significant cost in terms of material and labor, as it is typically engineered specifically for the application. Therefore, the cost associated with the design, build, and installation of a centralized system has to be a major consideration in overall machine expenditure.
Housing the drive systems in a cabinet does of course offer full protection from the external environment of the plant or workshop. However, as heat loss is generated centrally, effective cooling is required inside the control cabinet.
When deploying a decentralized drive strategy, the dimensions of the main electrical panel are typically extremely compact, while set-up costs are lower in terms of both material and processing. Another advantage is that distributed drives are wired with cord sets to reduce the possibility of error and shorten commissioning time.
Naturally, a decentralized approach is particularly suited to simple stand-alone motor control, but not exclusively. As the drive can be mounted on or near the machine/motor, the benefits of size reduction are clear to see in comparison with an equivalent centralized system. Reduced costs also result in thanks to eliminating the need for a customized control panel, not to mention the manpower required for system assembly and installation. In addition, wiring lengths are reduced, while further benefits include improved EMC behavior and the widespread distribution of heat loss, reducing the requirement and cost of a centralized climate control system.
While some assume that this type of architecture is insufficiently robust to provide a high degree of safeguarding against the surrounding environment, the contrary is in fact true. Many of the latest decentralized drives offer embedded features and a high level of IP66/NEMA 4X protection to permit installation directly on the motor or nearby. The rugged design of decentralized drives can guard against penetration by dust or jets of water and offer protection for technicians and other personnel against contact with live parts.
There are few drawbacks with decentralized drives in this area, although visual diagnostics and maintenance are sometimes more complex because the drives are often located in places that are difficult to access.

Application sizing

A major advantage of centralized solutions in terms of application sizing is that motors are not subject to any derating. To provide a commonly used definition, any adverse operating conditions require that the motor performance be derated. Such conditions can include ambient temperature above 40°C, motor mounting position, drive switching frequency or the drive being oversized for the motor.
Derating is a design process that can make a significant contribution to reliability. With a centralized approach, smaller motor dimensions and lower rotor inertia are typical, while the achievable performance is higher than decentralized solutions. With regard to shortcomings, those opting for a centralized solution need to take longer cable lengths into account, not forgetting that each motor is connected to the cabinet via two wires, one for power and one for feedback.

Anyone thinking that decentralized motion control solutions do not lend themselves to optimized application sizing would be mistaken. For example, in certain cases, it is possible to employ a standard cabinet for basic functions and add optional independent modules (with their electronics on-board) without having to modify the original cabinet.

In some instances, the use of decentralized drive-based control may be necessitated by machine size. It is possible to eradicate long motor cables from a central control cabinet by bringing power to the decentralized drives in a daisy chain, drive-to-drive fashion, or by using a drive with an integrated power supply. Furthermore, decentralized drives can enable even large and complex machines to be more clearly structured, a particular advantage in applications for sectors such as automotive and intralogistics, for example.

There is, however, one potential handicap for decentralized solutions in regard to application sizing. In motors with integrated drives, a derating of the motor due to heat exchange with the drive must be accepted. To counter this issue, for the same T,n performance, the motor with the integrated drive will be bulkier in design (T,n is the relationship between rotating speed and torque). It should be noted, however, that this requirement is not necessary for nearby decentralized drives.

Options

When it comes to options, centralized solutions tend to score best due to the greater customization of modules inside the control cabinet. This type of framework solution grants the potential to add options in the field at a later date. Although the most common options – such as STO (Safe Torque Off), Safety Bus, I/O and real-time Ethernet protocols – are available for decentralized systems, they are limited in comparison with framework solutions due to restricted space. Typically, options are installed by the OEM and it is generally not possible to add or remove them in the field.

Modularity

Centralized systems are seen as less modular than their distributed counterparts, even though there is more flexibility regarding heat dissipation systems based on air or liquid cooling. Generally, the modularity of the control panel is not related to the modularity of the overall system.

In contrast, decentralized configurations are extremely modular. Here, the architecture of the drives can follow the mechanical modularity of the machine or system, with the clear advantages this brings.

It is probably fair to say that today’s machines and factories are increasingly created based on modularity, predominantly because modular systems facilitate reduced development costs and shorter delivery times. Ultimately, it would appear that the industry is looking to realize its options without expanding its control cabinet.

Decentralized drives can be located where they are needed and, thanks to integrated I/O, can solve demanding tasks without adding terminals, saving time and money.

Networking capabilities

Many of the latest drives offer optional communications networking and I/O modules that are fast and easy to install, thus allowing adaptation of the standard drive to individual user applications. Plug-and-drive communication via real-time Ethernet networks is also increasingly commonplace with today’s drive technology. The use of plug-in interfaces for protocols such as Profinet and EtherCAT permit the seamless integration of drives into existing communications networks at the end-user site.

For those considering a distributed approach, using a decentralized PLC module alongside decentralized drives reduces the load on the higher-level controller and can even, in certain applications, create the basis for modular machines that are truly free of control cabinets. Such PLC modules will typically feature RTOS (Real-Time Operating Software) to ensure decentralized intelligence with networking capability provided by a selection of communication protocols. The upshot is that design flexibility and suitability for modular machine construction is assured.

Investment costs

It is difficult to draw a direct savings comparison between centralized and decentralized drive solutions, not least because every application is different. In certain straightforward, isolated cases the decision between centralized and decentralized can be clear-cut. However, evaluating the options for a production line, where every workflow step is dependent on other devices, means that the decision becomes considerably more complex.

Some who support or promote purely distributed systems argue that 30% or more can potentially be saved against the comparable cost of a centralised solution, which can sometimes entail more engineering time, more components and wiring, larger panels and PLCs, and slower installation and commissioning.

However, always check with a drive technology specialist for specific application advice as each project has its own requirements that can affect decision making for those seeking an optimum result.

Product solutions

Among the extensive number of product solutions available from Servotecnica is the AMK iC/iX series of distributed brushless drives. The decentralized AMKASMART iC servo drive with integrated power supply, for example, is optimized for use in single-axis applications and modular machine structures. Thanks to the integration of a power supply module, the need for a control cabinet is practically eliminated to facilitate a flexible machine solution.

The AMKASMART iX decentralized servo drive, just like the iC, is designed for rotary and linear synchronous and asynchronous motors of various kinds. However, in this case, the power supply and communication are looped from module to module.

With AMKASMART drives it is possible to combine the various distributed solutions of AMK on the machine or production line, and therefore have the possibility of choosing between decentralized and centralized solutions, or use the two types together. After all, there is no one-size-fits-all solution.

Further decentralized products available from Servotecnica include the AMKASMART i3X (three decentralized servo controllers in one housing); the AMKASMART iDT5 and iDT7 servo motors with integrated servo drives; and the AMKASMART iSA controller that features an incoming power supply to facilitate automation without the need for a separate PSU.

by Riccardo Francazi
R&D Manager

Managing data communication at inputs and outputs level in automated machinery systems. That’s what IO-Link is for. It is economical, simple and efficient. Well… this being said… readers expect to know more. There is a lot of documentation on the use of IO-Link for the management of Input/Output traffic, but very little yet on its use in the motion control field. Not forgetting its actual limits in that field, this article gives some highlights about motion control applications of IO-Link.

Talk to IO-Link’s users and ask them about its key points. Consistently, you will hear about the fact that this is a simple and robust solution, economical all over the life-cycle of the machine, easy to integrate, that it is simplifying installation, and commissioning, as well as usage and operation, that it is enhancing maintenance… By the way, IO-Link is very suitable for simple motion applications, and this is particularly what we want to mention here. But first, let’s see what is the IO-Link.

Not a fieldbus… Universal… Ready for Industry4.0…

In reality, IO-Link is a standard – the IEC 61131-9 – specified by the IEC international organization. It specifies a Single-drop Digital Communication Interface – SDCI – for small sensors and actuators. Whatever the controller (and the industrial-data communication protocol – fieldbus) which is used for the automation of the system, this “part 9” of the “IEC 61131” standard specifies a unique and universal SDCI technology suitable for applications using small sensors and actuators (which are so largely used in machinery). Yes, the IO-Link organization is a member of the PROFINET consortium, but the users do not have to adapt their sensors/actuators system (connectors, cables, hardware devices and software) to any specific fieldbus protocol. As often heard, “IO-Link is a universal cross-protocol solution” and fits into any architecture based on Modbus, PROFIBUS, EtherNet/IP, AS-I, etc.

Its goal is simple and clear: extend the traditional digital input and output interfaces towards a point-to-point communication solution (whatever the fieldbus which is involved at the PLC level).

On the field, over the last meters running through the heart of the machine (down at the sensors and actuators level), this technology supports bi-directional transmission of process-data, service-data and events. For both masters and devices, it is based on a protocol specified in accordance with the ISO/OSI reference model (physical layer, data link layer and application layer). It enables the transfer of control data and parameters down to devices, as well as the delivery of process data and diagnostic information from the devices up to the automation system.

Physically, IO-Link is based on a simple, robust and proven-for-long technology: the classical 3-wire connection used for the simple sensors and actuators, without any additional requirements regarding cabling. In the words of its promoters, it is “the further development of the existing, tried-and-tested connection technology for sensors and actuators.” And it does not cover the need for communication interfaces and systems based on multiple point or multiple drop linkages. This said, IO-Link is by essence used in factory automation, and it is largely used with simple sensors and actuators, in applications which include small and cost-effective microcontrollers.

In other words, one of the key benefits of the standard is that the sensors/actuators (inputs/outputs) level can remain the same, whatever the higher-level controller of the machine and/or the controller of the complete automation system. This leads to less design and engineering hours, to lower number of repair and spare-parts to keep in stock, to less “knots in the brain” and more peace of mind. The fact is this standard is universal and easily enters many applications!

As some users say, “IO-Link is revolutionizing communication at the field level.” Data from all levels of the machine and of the complete system is made available, entirely in line with Industry4.0 strategies. Being Industry4.0 ready brings today – and will definitely bring in future – the potential for implementing improved (and even completely new) machinery functions. Better and more economical production technologies are already anticipated for future with IO-Link. The fact is that this standard provides and guarantees long-term investments!

Economical… Easy to integrate, implement, use, diagnose and maintain…

IO-Link is cost effective. Before going on the field with the material and physical solution, it helps for reducing the number of engineering hours, shortening the design studies, reducing the preparation of the commissioning. When arriving on the field, the commissioning itself is faster. As mentioned above, IO-Link uses standard cables. It also helps reducing spare-part inventories notably thanks to intelligent multi-purpose devices.

IO-Link also simplifies diagnosing the sensors/actuators network, and organizing maintenance campaigns. Expanded diagnostics functionalities allow for remote diagnostics down to the field-device level, for cable break detection, and for device-specific diagnostics using IO-Link. And because it is simple and robust, easy and fast to maintain and repair, IO-Link contributes to increasing the runtime of the machinery systems. Typically – and not going too deep into details here – each field-device is described via an “IO Device Description” file (IODD). This file contains information such as the manufacturer of the device, its model number, serial number, the device type. It also contains parameters related to the application. These parameter values can be changed remotely (via the master), and so the machine can be adapted remotely and on-line, in order to fit the next coming production batch. At the same time, while parameters of a link are residing in the master (hardware), these parameters can also be reset, adjusted, modified dynamically during the production process. And if a master unit needs to be replaced for whatever reason, it simply requests pre-configuring (in the office or in the lab) and replacement (on the field). In such cases, it is just a “mechanical change” to operate in the cabinet of the machine. Powerful, isn’t it? And Easy.

In summary, IO-Link is factory automation and machinery oriented. Innovative IO-Link machine-concepts ensure simplified installation. It standardizes interfaces and cabling systems (parallel, analog, digital) to a unique type, and modular-machine concepts are inherently supported. Function modules and tool-assistance allow for a highly automated parameter setting. This leads to faster and easier tasks from design to commissioning, and maintenance.

And, of course, all these simplifications dramatically lead to the reduction of documentation and of training costs.

Motion Control

Quite a lot of information is available for typical IO applications. Much less easy is to find something to read on applications for motion control.

Well, from technical literature producers point-of-view, motion control is of great interest when it comes to advanced motion control applications dealing with high-speed and/or cycling positioning… and/or applications typically requiring a lot of interpolation calculations… and/or applications involving electrical gears and/or cams…

But on the other side, while motion control manufacturers and vendors can serve those complex applications, many users simply look for designing simple applications. Here, as IO-Link supports only quite slow automation cycles, it can perfectly serve such applications.

In those applications, sensors are measuring and controlling process values and parameters such as: angles, distances, frequencies and pulses, levels, positions, pressures, rotation or slide counts, rotational or linear speeds, temperatures, etc.

Measurement ranges and thresholds have to be set variably according to various production batches. While those changes used to be made manually (with the risk of human error to be handled and corrections to be managed), IO-Link allows for them to be set remotely. An IO-Link master is capable to recognize the connected sensors, actuators, displays. Before running a production, it will check for the configuration parameters of all the devices. While not complex in terms of speed or interpolations, many motion control applications get a lot of parameters involved. And there, the dynamic setting of the application shows a real benefit.

The global machine market is hotly challenged. Competition is high! In such a contest, the jury is composed of… the users! And users look here for reliable machines. And they expect that these machines can quickly pay for themselves.