Energy management key during plant downtime

Paul Boughton
Mark Freeman looks at implementing active energy management in automated plants.

By selectively switching off any energy consumers that are not currently in use, the demand for energy and the associated costs can be drastically reduced. This is particularly relevant to downtime in production. During breaks in production, many energy consumers continue running even though they are not required. This is where PROFIenergy can be applied, switching off these units during breaks in production in order to save energy and money.

Current situation

Diagram 1. shows, on the left, a typical operating curve of a welding robot that was tracked over a period of several weeks. It can clearly be seen that even during the non-production time at the weekend, the energy consumption of the plant is still approximately 60 per cent of the level during production.

Until now, such loads have not normally been switched off. The reasons are complex: in particular, additional hardware would have to be installed externally on the machines.

Many users balk at the engineering work required for this. If, however, an 'energy-wasting' component could be directly switched off by the higher-level controller, additional hardware would be unnecessary.

The user would only have to send a command from the user program to switch off the unit and subsequently another to switch it on again (Diagram 2).

At present, external cabling (for example, a distributed station equipped with relay modules) is necessary in order to switch off the energy consumer. However, in future this will not be necessary as PROFIenergy commands can be used instead.

The energy consumers themselves "speak and understand" PROFIenergy, as they have corresponding energy-saving measures already integrated into them.

By relocating this switching to the device, manufacturers can decide how to optimise their devices in order to save the maximum amount of energy. Hard, external switching means that some loads must continue running during short breaks, as they would otherwise not be ready to resume operation promptly at the start of production

However, with PROFIenergy, the manufacturer has the flexibility to switch off certain parts of the machine rather than the whole unit.

This means that during short breaks only the parts which are quick to reinstate can be switched off, where as the entire unit can be switched off during longer periods of downtime. This means that the manufacturer can maximise the energy savings.

PROFIenergy is extremely easy to use. Using PROFIenergy, a command can be sent from an industrial PLC to the device or machine, instructing it to the length of the break.

The device then decides for itself which parts can be switched off, in order to be ready for operation at the end of the break. It is irrelevant whether this device is an individual component, such as a drive or IO station, or a complex device such as a machine tool or robot. The command structure is always the same (Diagram 3).

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Co-ordinated start-up

In order to make the device ready for operation again, the user sends a starting command. This means a co-ordinated startup of the plant is possible, dependent on the application in question.

And in order for the user to switch on devices with different start-up times in a coordinated manner, these devices inform him of their startup time. This allows the user to calculate exactly when the activation command is to be sent. (Diagram 4).

In this way, PROFIenergy meets the demand for simple operation. This is the basis for wider acceptance. It also minimizes the engineering work required.

The user only needs to incorporate the two commands into his program. This enables him to make a hard and fast distinction between the actual control logic for the process and the energy management.

Function blocks

Device and system manufacturers also have the opportunity to support the user by means of corresponding function blocks that, for example, conceal the processing of the communication mechanisms.

In order to take into account the user's requirement for retrofitting capability and compatibility, these commands are mapped onto existing, tried and tested PROFINET services. In this case, onto records. These acyclic services do not occupy any additional addresses in the process image. In other words, the useful data quantity structure of a PROFINET device is not modified by PROFIenergy. This means that PROFIenergy can be used in existing program libraries and factory standards, such as those that exist in the automobile industry.

Likewise, the device manufacturers can upgrade their existing components to PROFIenergy by using a firmware update. This facilitates a swift implementation - even into existing products.

The main benefit for the end user is saving energy and the costs associated with it. Measurements revealed that even during non-productive periods many plant sections still use between 40 and 60 per cent of the energy that is required during actual production (Diagram 1).

Non-productive periods

Whereas in operation the focus is on energy-saving products such as motors with a better energy efficiency class, PROFIenergy allows savings to be made predominantly during the non-productive periods. In this case, it is a matter of partially or completely switching off unnecessary systems. Users can expect energy savings of up to 80 per cent.

A further key factor is the sensible deployment of the energy available during production. Here, too, certain auxiliary processes can take a break during production. PROFIenergy thus has the task of managing and making most efficient use of the energy available. Partial load operation must be possible.

Selective measures are already taken today, in order to shut down plant sections during breaks. These are, however, very application-specific.

Frequently the actual control task and the energy management are amalgamated. This all makes the program generation, but especially the maintenance of such software, very difficult.

In addition, external hardware is necessary for the execution of the actual switching process and this hardware must be configured, installed and maintained.

The cost of doing this often outweighs the actual savings, so many users do not take such measures PROFIenergy also offers an economical solution here. This is a matter of a pure data interface, as the switching functions are already integrated in the devices.

In addition, there is a clear separation between the program sections for the energy management and the control logic. Both parts are clearly separated from one another and can be tested and put into service independently of one another.

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Structured programming

In the long term, the care and maintenance of the user program is significantly simpler due to structured programming. This enables users in turn to integrate PROFIenergy easily into their own (factory) standards and program libraries.

Mark Freeman is Product Manager, Siemens Industry Automation & Drive Technologies, Manchester, UK.