New safety fieldbus technology and its application in the automotive industry

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New safety fieldbus technology and its application in the automotive industry [Copy link]

Safety control technology has always lagged behind standard control technology. To illustrate this point, let's review the development of automation control systems over the past few decades. Thirty years ago, almost all control systems used relay control, but this method was expensive to install and maintain, and it was troublesome to reconfigure the system and was very inflexible. For these reasons, PLC and other more "intelligent" control technologies gradually replaced relay control, because PLC reduced hardware configuration, simplified installation and maintenance, and could adapt to production process-based control tasks very flexibly. With the further expansion of production scale and the continuous advancement of production technology, the complexity of control systems is also increasing. The era of new open fieldbus technology has arrived, which not only enables equipment from different suppliers to coexist in a bus system, but also simplifies wiring and speeds up the spread of information on digital networks. But in fact, the standard fieldbus system cannot be regarded as a perfect bus system, especially in the aspect of "safety control". There are big loopholes.

Take automobile manufacturing plants as an example. Especially in welding workshops and assembly workshops with a high degree of automation, the emergence of a large number of welding robots and assembly manipulators has created many dangerous areas that need safety protection. Therefore, the use of safety sensors such as area protection scanners, safety light curtains, safety locks, safety carpets, emergency stop switches, etc. has become very common. However, the control system connecting these safety devices is still in the low-level control state of the non-bus control system of "safety relays" or "safety PLCs", which greatly limits the development of production process control systems. However, if these safety devices are connected to non-safe fieldbus systems (such as standard Profibus, DeviceNet and other bus systems), there will be relatively large safety hazards, which is a non-safe control state. Figure 1 shows the wiring method of safety products.

Figure 1 Wiring method of safety products

Based on this situation, several large automation system companies in the world (such as Siemens, Rockwell, etc.) have joined forces with several well-known safety sensor manufacturers (such as SICK, OMRON) to jointly develop open and mutually compatible safety bus systems, such as AS-Interface safety, Profisafe, DeviceNet Safety, interbus safety and Safety bus p. These safety bus control systems have greatly accelerated the pace of control system development. Figure 2 is a standard Profisafe bus system structure diagram.

Figure 2 A standard Profisafe bus system structure diagram

The main difference between the safety bus

system and the standard bus system is the different standards of the communication protocol. The safety of the safety bus system is an extension of the standard protocol, which allows standard and safety devices to run on the same network. Its uniqueness lies in the fact that the implementation of safe communication does not require expensive hardware devices such as gateways and bridges, but can be achieved only through the software layer. Figure 3 shows the protocol structure of the safety bus.

Figure 3 Security bus protocol structure

A standard safety bus system should include:

• Safety PLC with dual CPU;
• Safety bus protocol;
• Safety bus interface I/O and safety sensors.
  

Initially, people generally believed that it was a risky behavior to change the nature of the bus system only by changing the software, but it is precisely the inherent safety protocol in the safety bus system that ensures the safety of the entire system. Functions such as data redundancy and interleaved mutual inspection stipulated in the protocol ensure high reliability of data transmission and fast and timely fault diagnosis.

The safety bus system can not only be used alone, but also directly connected to the standard bus system without other auxiliary equipment.

Advantages of the safety bus system:

• The transmission medium with excellent performance has been tested in harsh environments such as "high noise interference";
• Automatic check function for overlapping "node addresses";
• Built-in "data link layer" review function;
• "Priority" functionality established through configuration;
• Very low bit error rate (≤10-7, equivalent to only 1 error in 150 years under full load operation).

The safety fieldbus is currently in its infancy. The Profisafe safety fieldbus has completed the formulation of the protocol and has begun product promotion and trial in Europe. Siemens plans to start promotion in China in 2004. The DeviceNet Safety safety fieldbus is currently in the stage of protocol improvement and will start market promotion one step later than Profisafe.

Application of safety bus system in the automotive industry

(1) Welding robots

There are a large number of welding robots in the welding workshop of automobile factories. Mechanical safety fences, safety door locks, laser area protection scanners, emergency stop switches and other safety protection devices need to be installed around the welding robots. These safety protection devices are connected to the safety bus system through the "safety bus interface" (as shown in Figure 4).

Figure 4 Application of safety bus in welding workshop

(2) Fully automatic punching machine

In the working area of the fully automatic stamping machine, mechanical safety fences, safety door locks, safety entrance and exit safety gratings with shielding sensors, emergency stop switches and other safety sensors are usually installed. These safety protection devices are connected to the safety bus system through the "safety bus interface" (as shown in Figure 5).

Figure 5 Application of safety bus in stamping production