Driving towards decentralised intelligence

Decentralised motor control architectures leverage the benefits of distributed intelligence and provide ultimate flexibility. Darren Klonowski, applications engineer at SEW-Eurodrive, explores the options.

As manufacturers and plant owners strive towards maximum operational flexibility, decentralised motor control architectures are escalating in popularity. Mirroring the modular design philosophy increasingly adopted for control and automation systems, decentralised motor control systems involve the relocation of power components, plus monitoring and speed control units, to strategic positions near or on the motor.

Decentralised architectures can provide a number of cost and technical advantages to operators and systems integrators alike: not only do they offer a deal more installation and future-proofing flexibility than traditional motor control centres, but also the option of distributed intelligence across the plant floor. Although the initial capital cost of the system may be higher, the cost reductions associated with the planning, installation and maintenance of such systems lead to overall savings on the total system cost of 10 to 30 per cent (Figure 1).

To date, the automotive industry has led the transition to decentralised drive technology - the expansive nature of automotive manufacturing and the linear nature of the process make a decentralised approach particularly well suited. However, all systems composed of conveyors, or production lines with individual processing stations equipped by standardised machine elements, are perfect candidates for the application of decentralised technologies. Materials handling, water treatment, and bottling plants are other common examples of these.

A simple science

The key components of a decentralised drive system are the motor plus localised motor control electronics. Generally comprising either a motor starter or a frequency inverter for variable speed control, the motor control unit is housed on or close to the motor, and features standard communications and power supply interfaces.

Distributed motor control architectures often involve the integration of the motor control electronics onboard the motor. For more basic installations, an actuator/sensor interface (AS-i) allows connection of a compatible AS-i communications cable carrying both signal and control voltage for the system; the individual 'inverter motors' are then daisy-chained by a power supply.

For more demanding applications, field distributors can be integrated into the system. The field distributor acts as the optimum distribution unit, connecting the power supply, control voltage and a communications fieldbus to the drive components. The field distributor's fieldbus interface permits drive control through fieldbus systems such as AS-i, Profibus, Interbus, DeviceNet and CAN. This allows extensive diagnostic and visualisation options at each drive. Sensor and actuator signals are also connected to the communications interface within the field distributor.

Figure 2 illustrates three possible decentralised motor control system architectures incorporating field distributors, themselves daisychained together using the fieldbus communications.

Common cabling

In many applications, the process and production line is continually expanding or changing. This is where decentralised architectures come into their own; however, it is not always convenient or practical in such cases to accommodate the field distributor onboard the motor itself.

Prefabricated hybrid cables, which combine power supply, control voltage and a communication line within the same cable sheath, provide the connection between the field distributor and motor electronics.

The use of hybrid cables, connected via special plug connectors on the motor, reduces the number of cable entries and the associated installation start-up time, as well as the complexity of maintenance work. For regular servicing, or in the case of fault detection, the drives can be unplugged and replaced in a simple procedure. The interchangeability of machinery and subsequent adaptability of the production line is a major benefit of the use of hybrid cables in decentralised designs.

While most applications lend themselves to a scenario where the motor control electronics are mounted onboard the motor itself, it is sometimes necessary to physically remove them - for example, where high temperatures, chemical or gaseous reagents, potential robotic interference, or merely space restrictions are involved.

Under these circumstances, both the motor control electronics and the field distributor can be located together, near to but separate from the motor, providing free maintenance and start-up access. Supply and control voltage, plus data communication cables, connect to the integrated frequency inverter/field distributor assembly, which is again connected to the motor via no-fuss hybrid cables.

Clever control

Adopting a decentralised motor control approach also opens a wealth of possibilities in the form of distributed intelligence across the plant floor. The use of intelligent devices in the motor control electronics can rationalise the workload of the main controller and optimise system efficiency.

For each system to function reliably, it needs access to certain information and process status data. The decentralised intelligence philosophy rests on the idea of pre-processing information to and from sensor signals and actuators to reduce the activities of the central controller. This modular intelligence effectively reduces data traffic and dead times in dynamic control loops - plus simplifies the structure of the application software in the main controller.

A good example of the advantages of distributed intelligence is a positioning drive. Here, the base information - such as motor speed and rotor position - is held within the drive itself, which doesn't require any additional inputs or data transfer. Dispensing with fieldbus propagation times means responses are quicker and more accurately reflective of real-time operations.

In intelligent systems, the role of the central controller is consequently reduced to that of interlinking the individual parts of the system - a non-critical function in terms of timing. The controller merely issues the 'start' command for the positioning operation, depending on the status of the previous machine component in the sequence. Later on, the positioning drive signals to the controller that it has reached its target position and the controller responds by issuing the start signal for the next stage.

The main advantages of decentralised intelligence are the increased control quality, the reduced hardware costs through the sharing of existing computer hardware, and clearer control program structure. Building on this concept, developments in established fieldbus systems are now tending towards drive/drive communications, with the central controller acting in a supervisory capacity.

Further flexibility

The deployment of decentralised drive systems, with local intelligence, leads to a more responsive system on a grand scale. More flexible and intelligent structures, with significantly reduced start-up times, allow manufacturers to iteratively improve processes in response to market trends. An added benefit is the ability to accommodate localised shutdowns to the overall system. An isolator switch within the field distributor allows the isolation of individual drives - important from a safety perspective - while maintaining full system communications. For example, a safety zone can be delimited and monitored by three-dimensional optical scanning. At the breach of the scanning zone, either directly or via a safety relay, the control voltage can be switched off, immobilising the particular drive. Importantly, the supply voltage is unaffected, allowing continuity of all other functions. The direct switching option also allows for procedural maintenance.

More than just contributing to cost savings, the shorter setup times associated with decentralised motor control architectures yield shorter changeover periods - this promotes just-in-time production with small batch runs. The inherent flexibility and adaptability at the heart of decentralised system design provides future proofing by facilitating the introduction of new or next-generation machinery. A high level of serviceability results from the ease of sectional isolation, which can often avert a worst-case 'total line' shutdown scenario. In production processes with considerable manpower - such as car assemblies - the subsequent reduction in downtime can be a key factor.