Fast Start Up (FSU) for Profinet IO
by: Kostyantyn Shcherbina of Hilscher and Jens Bürger of Murrelektronik

Fast Start Up (FSU) was specified for PROFINET IO to make it possible for an IO Device on the network to go instantly into a ‘power on’ state following cyclic data exchange with an IO controller. Such functionality is a high priority requirement for some important applications in factory automation. With start up times lower than 500 ms nothing stands in the way of PROFINET IO's utilization on robots with ATC.

Problem and Solution

Industrial robots have become very important for efficient factory automation. Modern robots and their ability to change tools automatically increase the flexibility of production lines as well as reduce the number of robots per cell. This leads to a reduction of costs and floor space.
With good planning, tool changing may also shorten the production cycle and increase productivity.

The fact that a production cycle of a typical line can be lower than one minute results in the requirement for the tool changing time to be as short as possible. Every delays while uncoupling one tool and coupling another should be minimized. A tool, and thus the whole robot, is only ready for operation if all corresponding IO peripherals are in cyclic data exchange with the PLC. This means that the time required for the establishment of the communication between the IO Device of the tool and a PLC can negatively impact productivity.

In 2004, AIDA (the automation initiative of German automobile manufacturers) made a decision to use PROFINET IO as the basic communication standard for their factory automation. At that time Industrial Ethernets suffered from the handicap of very slow start up time compared to traditional fieldbuses. Typically, start up times were in the range of seconds. This disadvantage prevented the use of Industrial Ethernet systems with robots with automatic tool changing (ATC).

Technology

FSU is described in the latest specification of PROFINET IO. The basic rule defines that the time between the ‘power on’ and receipt of the first cyclic input data should not exceed 500 ms.
Because during the first start up the IO Device is parameterized and configured the benefits of FSU may be used only from the second start up.

The protocol optimizations are standardized in the following ways:

• use of fixed transmission parameters (only for copper wires), instead of automatic detection, which reduces the start up time by up to three seconds.
• the network address is not passed to the IO device on each cycle, but only at first start up. The parameters are saved within the permanent memory of the IO device. On every following start up the parameters are retrieved from the permanent memory and reused. This optimization may save several seconds.
• IO Devices announce their readiness to establish communication instead of waiting for the IO Controller to search. It is possible to save up to one second this way.
  

These enhancements remove any protocol-specific delays. Furthermore the start up of a device can be optimized through the use of suitable hardware. Full details of FSU can be found in the corresponding standards or case literature.

Comparison with alternative solutions

The transmission of signals in present-day ATCs is mostly based on approved fieldbuses like PROFIBUS, DeviceNet and INTERBUS. The typical start up time for conventional fieldbus systems lies in the region of a few 100 ms.

For a long time Industrial Ethernet communication systems were not able to keep up with the conventional fieldbuses in terms of start up time. The market did set new performance requirements which resulted in the development of compromise solutions. The most noticeable among them are gateways and wireless.

For some time PROFINET IO - in contrast to the other common Industrial Ethernet systems - has standardized the way IO Devices can reach start up times of less than 500 ms. Devices on the market guarantee times of approximately 400 ms. However, considerably lower times can be easily reached.

The recently-established Gateway concept enables fast ATC for fieldbuses. The Industrial Ethernet systems handle vertical communications while the fieldbus handles horizontal. The crucial drawback is increased complexity due to the utilization of two technologies, which has a noticeable, negative, effect on maintenance and service.

Wireless solves the problem of a fast start up either through keeping the communication up all the time or utilizing the technique of communication establishment shortly before the tool is coupled.

The advantage of wireless is the absence of idle time during start up or cable induced problems (for instance, line disruption or short circuits). This may help to save costs. However, the disadvantages are increased safety risks and interferences, which lead to higher jitters, cycle times, additional planning effort and limited amount of transmitting devices in the local area.

Therefore, wireless should be used only where:

• cable usage is uneconomical or impossible.
• the disadvantages are negligible.

User opinion

One of the leading German car manufacturers is currently planning its future production line and has to choose the communication peripherals for his robots with ATC. Typical operations are welding and gripping. Car body production for a robot cell is as low as 45 seconds, including the delay for tool change. That emphasizes the crucial role of the communication components. This manufacturer has clear views:

• FSU devices with a start up time less than 500 ms are required.
• gateway concepts will not be used.
• the use of wireless components will be limited because the cycle times are usually not satisfactory.

Conclusions

With start up times lower than 500 ms nothing stands in the way of PROFINET IO utilization on robots with ATC. Based on AIDA’s choice and the advantages in comparison to other solutions PROFINET IO will soon become an approved standard for ATC systems.

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