Servo technology - centralised or decentralised?

Often the answer to a question is neither A or B, but instead C! This is true for decentralised versus centralised servo technology. In this case, C often represents mixed architectures, a coexistence of both types. When the drives have many common features, the two approaches can be easily combined.

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 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.

Decentralised servo technology follows the basic principle of shifting individual motor control from the central control cabinet to be closely located to the process. This demands robust design with a high degree of environmental protection. The advantage lies is reduced motor cabling. Two other advantages are improved EMC-behaviour and the widespread distribution of heat loss, reducing the cost or need for a centralised cabinet climate control system.

Machine building trends

With packaging machinery for the food and beverage industry, the trend is away from metre-long cabinet walls, but instead installing small, highly self-protected units within the machine frame. The trend is towards modular production, necessitating distribution or combination of functions. This is encouraged because production equipment consists of a main process and ancillary tasks - compatible modules comprising conveyor systems, sorters and handling units. This provides a path for easy expansion of the machine. Practical experience favours a decentralised structure, especially with spatially distributed single axes. In contrast, for machine modules with synchronised axes in close proximity centrally located drives in a cabinet are favoured.

The AKD -N series Kollmorgen drives provide IP67 protection and connection via an 11mm diameter hybrid cable to the central supply module in the control cabinet. This single cable provides power and communications without the need for any additional cabling. Each AKD-C supply module can support two strings of AKD-N drives up to 4kW each, and up to eight AKD-N drives per string. Only one cable is necessary between the distributed servo controller and its connected motor.

Distributed drive technology as a hybrid

An alternative way to decentralise is by a hybrid integrated approach. These are combined motor and servo controller units without the need for external wiring. This so-called "piggyback" approach has the disadvantage of drive derating with increased ambient temperature. The higher the ambient the more performance reduction occurs in order for the drive to self-protect from overheating. This relationship ensures in practice that the motors must be larger than otherwise necessary to give the required performance within acceptable temperature limits for the electronics. Typical servo tasks such as rapid acceleration and deceleration during positioning can be especially difficult in hybrid designs, due to the problem of effectively dissipating heat.

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