The advantages of decentralised control

June 12th, 2014, Published in Articles: Vector

 

An overview of decentralised motor control systems, their applications, savings incurred, programming, motor brake control, reliability and commissioning.

Fig. 1: The Movifit decentralised solution from SEW Eurodrive.

Fig. 1: The Movifit decentralised solution from SEW Eurodrive.

In decentralised control systems, motor control functions are removed from a central control cabinet and distributed out on a machine so that control devices are located closer to the machine’s motors.

This form of control was first implemented in Europe in the early 1990s. The automotive industry was the first to adopt it in the USA because it simplified production line changes and reduced engineering costs. It is now considered in many applications such as material handling and logistics.

Decentralisation exists on many levels, from motor starters or drives located at the motor to fully decentralised systems with variable frequency drives, overload protection, motor disconnect switch, I/O and bus module, all combined in single packages installed on or near the motor. This package is often called a field distributor.

Advantages

Decentralised motor control offers many benefits. Chief among these are significant cost savings in engineering time and materials, including less wiring and smaller control panels, reduced installation times through drive and motor pre-commissioning, improved machine performance, less maintenance and fast replacement of electronic components without wiring.

Applications

As a rule of thumb, decentralised control is best for applications involving ten or more motors. These include the large conveyor systems used in material handling applications such as logistics or packaging or automotive assembly.

Typically, such systems cover large amounts of floor space and require extensive wiring with a centralised control scheme.

Applications that require multiple motors and take up large amounts of floor space are best suited for decentralised control systems. These include any conveying process such as rotary and lift tables, as well as automotive assembly, food and beverage processing, packaging, warehouses, logistics and other material handling applications.

Saving money

In field tests conducted by SEW Eurodrive, total savings of up to 30% were achieved compared to centralised control systems. The savings were due to reduced engineering time, fewer components and less wiring, smaller panels, smaller PLCs, and quicker installation, debugging and commissioning.

An example of a typical application is a bottling line using 183 m of conveyor and 50 1 HP motors. In wiring alone, for example, a decentralised control system would reduce the amount of power cable. Field installation time would decrease from 300 hours to just 40 hours because there are fewer components.

The estimated cost for building and installing a centralised control system for the bottling line, including panel construction, cable, cable tray, components, drives and labor, is
$150 000. By using decentralised control, the cost for the same system would be reduced to $90 000.

Panel building costs

Panel building costs are particularly significant in centralised systems because of the large number of components required, including overload protection, drives and bus gateways, as well as the larger panel size needed to accommodate these components. More heat is generated as more electronic components are added and costly fans and heat sinks are required, adding to the cost of building a control system. These also add to the energy costs for operating the system.

Reduced wiring

Complex wiring systems that run from the control cabinet to the motors are eliminated in a decentralised motor control system. With fewer wires, there is less potential for cross-talking noise and EMC interference, a major source of machine downtime. This means less wire to install and less rewiring when production lines must be changed or equipment must be upgraded to add sensors or other control devices.

Maintenance cost

Significantly less maintenance is required for a decentralised control system. Commissioning is less complex, there is less wiring to change or repair, and it is easier to troubleshoot and replace a failed component rather than an entire system if problems arise.

Programming

Programming should be simpler, making this another source of significant cost savings. In contrast to a centralised control system which requires a large PLC to accommodate the complex programming needed to govern the entire machine, each field distributor carries only the code necessary for a specific set of activities in a decentralised control system. The PLC can be smaller and less expensive as all control functions can reside in the drive or field distributor. The PLC’s primary function becomes system monitoring over fieldbus or Ethernet.

Systems compatible with the IEC 61131 standard provide a wide choice of programming languages, including function block diagram (FBD), sequential function chart (SFC), structured text (ST), instruction list (IL) and ladder diagram (LD). Most drives today ship with pre-programmed application routines, reducing programming and debugging time and making commissioning much faster and simpler.

Fieldbuses

Depending on the unit selected for the application, a fieldbus is not always required with decentralised control. Many fieldbuses are, however, available with decentralised control. The fieldbus allows for a more extensive and complex exchange of important data which can include control data, speed, ramp, unit status and other information.

The use of fieldbus networks for monitoring and control is widespread in Europe and in the automotive industry around the world, and is now being adopted by many other industries, particularly for material handling applications. Many suppliers offer a wide range of common fieldbus options, including Modbus Ethernet TCP/IP, Profibus and Profinet, EtherNet/IP, DeviceNet and ASi Interface. Binary control using discrete I/O as an alternative to a fieldbus is also possible.

It is necessary to use a PLC when using a fieldbus. The size of the PLC and programme can be reduced if the decentralised unit has programming ability allowing control routines to be carried out in the drive. This decreases the programme size of the PLC and the scan times of the system.

Motors and drives

It is not true that drives used in decentralised control systems should be oversized to withstand 150% of the normal motor operating current. The drives in a decentralised control system should, in fact, be sized exactly the same as in any control system. A 1 HP motor would need a1 HP drive. In most cases, the motor and drive for the application must be sized. The motor and drive may only have to be oversized in high cycling operations where a high starting torque is required, but this application type is very rare.
Most decentralised systems have integrated UL-rated motor disconnect switches unless an IP 69K-rated system is being built. This requires that a UL-rated disconnect be placed away from washdown areas.

Variable frequency drives or starters are only needed in some applications. Starters and even soft starters will not modulate starts and stops as well as a variable frequency drive. There is very little price differential between starters and drives today.

An SEW decentralised control system such as Movifit enables one drive to control up to three motors. The three motors are addressed as one node on the fieldbus, but each is controlled individually by the drive. With 62 potential nodes, an SEW system can control up to 186 motors. Other systems require a drive and a fieldbus node for each motor. This means that a maximum number of 62 motors can be controlled on those systems.

Motor brake control

Motor brake control is integrated into these systems while separate brake relays must be added to other systems. A brake resistor can also be used to dissipate regenerative energy created when a motor is decelerated.

Reliability

Use components that are designed specifically to withstand industrial environments, not just a drive placed in a metal enclosure. Some manufacturers use a corrosion resistant aluminum alloy rather than standard aluminum for the housing. The housing must be rugged so that it can stand up to the abuse typically received by equipment in factories, including contact with moving equipment such as forklifts or corrosion, dust, oil and moisture. System components must also be compact enough not to stick too far out into the aisles between machines.

IP rating

A minimum IP 65 rating is needed for industrial environments. An IP 69K rating is recommended in washdown environments.

The IP 69K or hygienic version of decentralised control systems is designed to resist corrosion on the outside and to stay dry on the inside. Sharp edges that tend to collect contaminants and moisture are eliminated and coatings will not scratch or wash off under high-pressure water sprays.

Commissioning

Commissioning is as simple as setting the fieldbus address of the device with systems using optional dip switches. Drives can also be programmed by means of software.
Commissioning typically takes much less time, even when programmed with software, compared to a centralised system with complex programming because the control code is divided among a system’s drives programming.

Software is used to commission the system except where a dip switch option is provided, which simplifies commissioning. Simply set the dip switches for control and fieldbus address.

Maintenance

Troubleshooting is easier and less labour intensive since technicians no longer have to be stationed at both the control panel and the machine to identify and correct problems. There are also fewer wires and external components, so diagnostics are less complex. Failures are easier to troubleshoot because many manufacturers locate an LED display on the front of their units. Look for a modular system design that allows failed components to be replaced easily without sending the complete unit back to the supplier for analysis or repair. It is much more cost efficient to keep spare components rather than complete spare units on hand. Also ask for pre-wired plug connectors, which reduce maintenance time and replacement costs. Faults can be reset remotely over fieldbus or manually at the machine.

Other considerations

Overload protection against excessive heating due to motor overload is a UL requirement for any control system. Some manufacturers integrate overload protection into their control board technology. With other systems, you will have to add an overload relay.

PLCs and safety networks

There are many safety system options, including safety PLCs and safety networks. SEW Eurodrive offers a safety concept which meets EN945-1, category 3 by disconnecting the 24 V safety circuit from the drive, eliminating the need for contactors. This safety architecture presents an opportunity for huge savings since only one safety relay is required instead of a safety relay and two contactors for each drive.

Connection technology

Suppliers such as Harting, Phoenix Contact, Amphenol and Woodhead provide many connector options. The user typically had to wire a drive to the motor during installation, but with new connector options and plug-in cables, all manual wiring by the customer is eliminated, which reduces installation time, cost and wiring errors.
One of the most common standard options available with decentralised control systems is a keypad for manual operations, which also serves as a maintenance start/stop for diagnostics and repair. In addition, a wide variety of connector options is available for faster installation and replacement of motors.

Cabling

Shielded cable is required for the 460 motor leads from a panel-mounted drive to protect surrounding equipment from EMC noise problems which often arise from pulse width modulated (PWM) signals sent from the drive to the motor. In a decentralised system, shielded cable is not required for the input power and is only necessary for the short distances between the drive and motor. Standard cables are used for all other purposes. No shielded cables are required where the drive is actually mounted on the motor.

Sensors

Almost any sensor can be used in a decentralised control system, including proximity and photoelectric sensors. Many systems treat each I/O point as a separate node on the fieldbus, which makes the fieldbus system larger. In some control systems, only the fieldbus distributor has its own node, so the user can add more devices such as sensors without using additional nodes.

Contact René Rose, SEW Eurodrive,  Tel 011 248-7000, rrose@sew.co.za

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