Centralised or decentralised - which way to go?

06 January 2015

Arne Linder, product manager at KOLLMORGEN, looks at the benefits and barriers to the use of centralised and decentralised servo technologies.

Decentralising servo technology can offer machine and plant construction savings during installation. Additional advantages could include reduced cabinet heat loads and more straightforward drive architecture. However, when deciding between an integrated motor/drive solution or a detached motor and drive there are many things that first need to be considered and it is important to have a good understanding of the different technologies.

The centralised architecture – In contrast to horizontal conveying, where decentralised servo drives are a common sight, centrally located servo drives still dominate the market for highly dynamic and precise motion control. Servo drives, along with other control components - sometimes with a full blown IPC - reside together in a control cabinet protected from the outside world. Connection to the motors is achieved via a star shaped structure, each having control and power cables. Because heat loss is centrally generated, effective air conditioning is needed in the cabinet.

The decentralised alternative – Decentralised servo technology follows the basic principal of shifting the individual motor control from the central control cabinet and more closely locating them to the process. This architecture requires a robust design that can provide a high degree of environmental protection. Its main advantage lies in a reduction in motor cabling. Two other advantages include improved EMC-behaviour and the widespread distribution of heat loss, which reduces the need for a centralised cabinet climate control system and the additional costs associated with this.

Packaging machinery and machines for the food and beverage industry are moving away from meter long cabinet walls towards the installation of small, highly self-protected units within the machine frame. 

Against this backdrop, practical experience shows that a decentralised structure can be advantageous, especially with spatially distributed single axes. In contrast, machine modules for synchronised axis, physically close together and centrally located drives in a cabinet, are generally the preferred structure.

Typical example
The potential of decentralised technology can be seen in an eight-axis metal forming machine. The first axis is located 5m away from the control cabinet with each additional axis located 3m further. A centralised control system would be characterised by a central control cabinet housing the drives with each motor having separate shielded power and feedback cables adding up to 248m of cable.

Instead, a combination of a single KOLLMORGEN AKD-C power supply module and eight decentralised servo controllers would reduce the cable requirement to 34m. The calculation - a single 5m hybrid cable supplying power and field bus for axis control is connected between the power supply module and the first decentralised controller. A single hybrid cable 3m long connects each additional drive for a total of 21m. Because we assume that each motor is located 1m away from each decentralised AKD-N drive KOLLMORGEN provides a one-cable motor connection technology, only 8m of additional cable is required. 

Overall, the decentralised system reduced the cable requirements from 248m to 34m - a saving of 86%. Another benefit of the relocation of drives is the reduction of power dissipated in the control cabinet. This effectively reduces the requirements for air conditioning offering savings for both the OEM and end user. The control cabinet air conditioner can be reduced in size or completely eliminated.

Increased design freedom
Sample calculations show that the decentralised servo technology can save space in combination with the single-cable connection technology between motor and controller. This results in smaller cable trays, lighter drag chains, and more compactness giving greater design freedom.

A way to decentralise is with the use of a hybrid integrated solution. Disadvantages of this ‘piggyback’ solution can include drive derating with increased ambient temperature. The higher the ambient temperature the more performance reduction occurs in order for the drive to self-protect from overheating. This relationship ensures that the motors must be larger than otherwise would be necessary, to give the required performance within acceptable temperature limits. Typical servo tasks, such as rapid acceleration and deceleration during positioning, can be especially difficult in the design of hybrid solutions due to heat dissipation problems. 

Separating the motor and drive at this point, however, prevents inherent design-related derating. This solution provides the basis for smaller motors in combination with better energy efficiency. In addition, integrated combinations are usually focused on a single motor type, limiting flexibility in the machine design.

KOLLMORGEN brushless motors can be connected to the decentralised AKD-N servo drive. These motors include conventional or direct drive rotary and linear direct drive types, providing true design freedom and optimum performance.

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