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Choosing the centralised or decentralised route01 September 2005The machine designer specifying servo drives is faced by the choice between a centralised and decentralised drive technology. Changing from centralised to decentralised control, or vice-versa, will not automatically make a machine more effective or competitive. A thorough evaluation should be carried out, understanding that the choice influences hardware costs, commissioning time, panel building, communication costs and even system performance.How do we define a centralised system? Fundamentally the decision is based on where the control process is carried out. A centralised system will process machine signals in a central PLC or PC. This includes sensor signals and instructions for the drive in terms of speed, torque, position. There may be a motion control card built into the PC or alternatively a separate motion controller. The drive does no more than run its own control algorithms requiring little intelligent capacity; sometimes such drives are termed ‘dumb’.
On the other hand a decentralised system has the processing of machine variables done in the drive. Process variables such as pressure, registration, level regulation are handled by the drive along with sensor signals. Motion control functions such as speed, torque and position are also contained within the drive. Such systems are also referred to under the headings, “Drive-based Automation” and “Intelligent Drives”. Typically there will be a central PLC running logic functions such as start / stop and perhaps a HMI for parameter setting, but with the process control decentralised there is much less data traffic. The key word is “modularity”. A previously centralised system is split into several modules, which are all equipped with standard interfaces for power and signal transmission. Decentralised drive technology means that each module operates as an autonomous control unit and takes on “responsibility” for its own operation.
The traditional drive solution is a centralised one using a PLC, although the requirement for high speed signal processing and drive precision has led an increasing trend in recent years to industrial PC’s. The PC is a multi-function tool with capacity for programming and monitoring. Either way, centralised control makes sense for compact machinery or where most of the axes work together.
Examples are machine tools and robots. There are benefits in terms of programming time with only one unit to set. As the drives need little intelligence they can be lower in cost. In some cases it makes sense to use drives with a shared power supply in the form of a dc bus. Advantages here are reduced panel space and energy sharing / saving. Where there are a large number of axes, the centralised solution will generally be the more economic.
Decentralised drive systems have a wide range of benefits in certain cases. With the processing done in the drive, load on the central PLC / PC is reduced allowing a reduction in specification and cost (on occasion it can be eliminated altogether). Similarly there is a lower requirement for fieldbus communications, particularly high speed communications, which will reduce costs. Processing sensor signals in the drive reduces cabling costs – in one case a machine builder saved over £2,000 on slip rings.
Decentralised systems go hand-in-hand with machine modularity, often found in packaging and printing machinery. Modules can be added or removed to suit individual customers. The builder can even pre-test modules before shipping them. This can give particular advantages where machinery is large, for example process lines. Simplification of communications can result in faster signal response times and system
performance. Where safety technology is integrated into the drive, for ease of logic and fastest performance a decentralised system is the norm (although the advantages of integrated safety can also be applied in centralised systems).
One classic application that uses centralised control technology is in industrial robots such as those made by KUKA. Robots of this type have a minimum of 6 axes but sometimes up to 8. The KUKA KR series handles loads from 6 to 350kg and has a servo drives in the local control cabinet with a PC based motion controller. Centralised drive technology is used because all the axes work together and interpolation is required between them. With this number of axes, the ability to use a separate power supply and individual axis drives gives advantages of lower purchase cost and reduced panel space. A further gain is the ability to use a shared dc bus which reduces overall power demand.
The Swiss company Müller Martini manufacture rotary offset printing presses using decentralised drive technology. Their “Concept NT” machine is modular with up to 12 print towers. Each tower has a servo drive with PLC functionality plus four further slave drives. The “electrical shaft” concept is used to synchronise speed between the towers so the cost of a mechanical drive connection is avoided. Modules can easily be
exchanged or added to an existing machine installation and the user can assign individual functions to the print towers. The intelligent drives implement functions such as washing and plate changing, leaving a central system PLC in a supervisory role.
So, what factors should be considered in making a choice between the two technologies? Clearly if the machine benefits from being modular, for example to meet the flexible customer demands, then decentralised is the way to go. On the other hand, machines that have axes that work together, particularly
with interpolation, need to have centralised control. A high number of axes points towards centralised control, particularly if drives regenerate power that can be saved in a shared dc bus. However large machines with perhaps an even higher number of axes may benefit from decentralised control with gains in
cabling, local I/O, safety elements and commissioning time. Simple machinery can also benefit from decentralised control by permitting a lower specification PLC. Performance is not a good deciding factor. One thing is clear, designers should take advice from drive suppliers who can supply both technologies.
The Chief Executive Officer of Lenze AG, Dr Erhard Tellbüscher, recently commented on the status of the market: “The situation is not clear and it depends on the industrial branches from a technical point of view. We will see more decentralised technology in the future and this already is a particular trend. On the other hand, we see technology with PC based control and this will change the drive requirements in the market”. Contact Details and Archive...Related Articles...Most Viewed Articles... |
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