Centralized and decentralized drives the pros and cons
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.
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.
|Cabinet/control panel size||Centralized||Decentralized|
|Cost of material||✓|
|Cost of labor||✓|
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.
|Derating of Motor||✓||✓ (with nearby)|
|Motor cable length||✓|
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.
|Safety via Bus||✓||✓|
|Scalability of the options||✓|
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.
|Flexibility dissipation system||✓|
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.
|Performance of PLC||✓|
|Comunication ethernet RT||✓||✓|
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.
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