Communication networks & Wireless connections

Wired communications

The use of local networks (bus systems) in the field of automation technology has become standard in the last few years. The manufacture of high quality products at low unit and production costs is no longer possible without using modern control units and comprehensive communications. Production facilities are only profitable if high availability can be guaranteed. For this reason, on top of everything else, in automated production employees have to be integrated into the manufacturing process. It is the individual worker who can best position parts, detect faults before they manifest themselves, speedily rectify errors or even carry out maintenance and service work during the ongoing production.

 

Whenever people are involved, every possible risk to life and limb must be eliminated. Either no error may occur in the first place, or any potentially hazardous operation performed by the automation mechanism must be detected before real danger to a person arises. For this reason, the communication networks used to transmit data relating to controls, sensors or actuators have to operate absolutely faultlessly.

 

Safety-related bus systems have to be distinct from traditional bus systems in that they detect all sporadic and systematic errors. Moreover, in all cases they include techniques that allow comprehensive failure control.

 

The following table illustrates all errors known today that occur in bus systems during data transfer. For every error effective measures can be integrated into the network concerned. Since some measures are capable of detecting more than one error at a time, the contractor can choose the package of measures which suits him.

 

Safety-related network requirements

Calculations of error probabilities are based on empirical values of digital and analogue data transmission. The following table gives an overview:

  

Bit error rate Transfer system

> 10-3

Radio
10-4 Phone cable
10-3 to 10-5 Twisted-Pair cable
10-6 to 10-7 Digital phone
10-9 Coaxial cable
10-12 Fibre optic cable

 

Depending on the possible error rate a certain residual error probability exists that false information may be identified as correct. According to industrial standard IEC 61508 (depending on the intended category), the following values are still acceptable.

SIL

Residual error probability for continuous operation calculated in errors per hour [errors/hour]

3

<10-8 ... <10-7

2

<10-7 ... <10-6

1

10-6 ... <10-5

 

Experience has shown that within a safety-related network it is acceptable for a maximum of 1% of the error rate of the entire system to be due to the error probability of the network itself.

 

Errors within the software, systematic faults, malfunctions of the hardware, etc. are much more prominent. Hence, the data transfer should be better than the SlL assumption by a factor of at least 100. Thus, the residual error probability has to be better than 10-9 in order to attain category 4 (SIL 3).

 

Safe wireless linkages

Safe wireless data links are used in all plants that are equipped with mobile items. Typical plants or systems are:-

  • cranes
  • transport systems for goods
  • pilotless trains
  • railways
  • ships
  • robots

 

The data transfer has to fulfill all safety requirements. Adherence to the guidelines for safe bus systems is therefore necessary. In contrast to wired plants, wireless and infrared linkages exhibit the following properties:-

  • problems in availability (possible disruption or blind spots)
  • possibly increased interference level with a number of bit mutations within the data package

 

Depending on the individual application, standard transfer systems can be used on the basis of IR or wireless LAN. The necessary boundary conditions of the installation have to be determined and ascertained with regard to the standard safety features.

  

 

Our services…

innotec advises you in selecting the appropriate communications network depending on the desired topology, the maximum spread, the desired number of participants, etc.

innotec develops with you a safety layer for your specific company protocol.

innotec carries out all calculations required to determine the safety parameters for the communications network.