Functional safety and safety fieldbus

The evolution of communications in industrial environments

Automated industrial applications have evolved over 20 years. There are less than 10 years, the connection between each element of an industrial system was completely realized with a multitude of cables, where each information was conveyed by a unique cable.

Nowadays, industrial applications communications are increasingly made with networks. This category of industrial LANs is called fieldbus because closer to the machine, closer to the ground. In this context, the objective of a network ("communications system") is to ensure communication between devices connected to the manufacturing process (sensors, actuators, machines, ...) and their control. Network development field is related to the following parameters:

  • Fieldbus allows for communication entities at a cost and a relatively small sizes of the devices (smart devices) compared to the organs in which they should be integrated.
  • Fieldbus offers the advantage of flexibility of use and implementation, and reduces total cost of ownership during the lifetime of an application system.
  • Fieldbus creates another dimension in terms of distances. If the areas of parallel or serial transmission are in an area of ​​about ten meters, the fieldbus extends from 100 m to 5 km.
  • Fieldbus allows up to several hundred subscribers. In addition, several fieldbus can be connected to a system. The fieldbus is becoming the technology solution for connecting a range of simple or sophisticated products requiring harmonization of interfaces connections.
  • Fieldbus allows to take into account the architecture of critical cases in which exchange is performed through shared variables which are used by all the bus subscribers.
  • The fieldbus can convey more information and make that same information is available to all the organs that needs it. With its extensive features, it allows different devices to interoperate with each other.
  • The fieldbus can change and adapt configurations in applications where the manufacturing process is very long (several months to several years). It allows you to configure some distant organs.

However, the use of a field bus must in no way be detrimental transfer times information. In addition, the information provided must be reliable and safe. Approaches, focusing aspects, have led to different bus architectures field.

1 networks and transmission of information

1.1. The field networks in a communication system

The communications through Fieldbus covers both a communication system within the overall control system of an enterprise levels. The figure below details the communication scheme overall business.

Figure  : Fieldbus and global communication

  • Factory level 
  • Cell- level
  • Field level

Communications at the cell level are typically the one hand, between the cell level controllers and subordinate control and secondly, between controllers (PLC) and other organs.

At the sensors / actuator levels, the information flow is generally "vertical" ie between controllers and sensors / actuators.

The system for transmission of data must enable a reliable and efficient transfer of information in an imperfect environment. A High data integrity and high-speed transmission are often contradictory properties. In this case, the increased requirements for integrity can be achieved only at the expense of a reduction in the actual flow of information. Therefore, the requirements for the transmission rate and data integrity must be selected consistently with the accuracy of this system.

1.2. Network and protocol architecture: The reduced OSI Reference Model

The OSI (Open Systems Interconnection) is a reference model for open systems interconnection. It is a reference model for developing interconnection standards and cooperation of distributed systems. The OSI model defines a layered architecture and is applicable to all types of networks. A system is said to be open when it enables communication between devices of different types within the rules of communication in an OSI environment.

For transmission systems that require particularly short reaction (on networks with bandwidths of reduced transmission), an architecture for improved performance (EPA) has been designed. Frames based on this architecture using only three layers, namely the physical layer, data link layer and the application layer. The protocols that are based on this reference model are defined in EN 60870-5 standards series and also in EN 61784-X series.

 

Figure  : Fieldbus in the OSI model

Field networks are based on a protocol architecture, generally oriented OSI model (Open System Interconnection) model which is composed of seven layers.

In safety applications, the transmission of information must be secure. This notion of safety means two aspects:

  • integrity of the information provided; 
  • controlled transmission time information.

1.3. Measurements of the quality of a transmission information

The fundamental purpose of the communication function in monitoring and process control, is to achieve the maximum coherence of the system, that is to say, consistency between the physical state of a process and its image in the database of the transmission system.

In digital systems, the exchange of information is carried out digitally by a succession of "0" and "1". In stressed environments (EMI, potential differences between earth, component aging, etc.) , this succession of logic levels can be changed.

The data transmission must be done correctly in the presence of harsh environmental conditions. It is therefore necessary to ensure effective protection message against :

  • Undetected errors (on the bits and frames)
  • Losses undetected of information
  • The inclusion of unwanted information (message simulation by parasites, etc.),
  • Separation or disruption of coherent information.

1.3.1. Transmission of information and classes of integrity

The efficiency and the level of integrity of a coding system will be compared with integrity classes defined in the EN 60870-5-1,standard, standard related to remotely safety control systems .

EN 60870-5-1 gives requirements in terms of residual error rate or residual error probability. This concept, which is close to the one of the rate of error detection, is howeverdifferent because it is intended to "count" the residual errors. In the case of the transmission of information three characteristics must be taken into account: the quality of the transmission; the variable frame length; the delimiters of the frame.

  • The quality of the transmission involves an "error probability on binary elements (bits)."
  • The variable length of the frame leads to calculate a coverage taking into account the various possible cases.
  • The delimiters of the frame are information of start and end of the frame that allows to "synchronize" exchanges.

EN 60870-5-1 standard defines three classes of integrity of transmissions (see Figure: Classes integrity related to the transmission channel). This figure provides a graphical representation of the integrity of the transmission as a function of three parameters:

  • probability of error on the binary elements,
  • Hamming distance "d" of a code,
  • residual error rate R,

The curves defines the upper limits of residual error probability (or residual error rate R) depending on the error probability on binary elements (bits). These curves stop at an error rate on the bits p = 0.5, which corresponds to a reception random bits (received signal without noise). The slope of the curves for p <10 -4 represent the Hamming distance of the code "d" that is used.

Three classes of integrity I1, I2 and I3 were set for the data transmission. The use of each class depending on the nature of the data.

Figure  : integrity Classes related to the transmission channel

The quality of the transmission paths (that defines the probability error on binary elements - bits) should be monitored to ensure a lower limit that is acceptable for probability of error on the binary elements.

1.3.1.1.Probabilité error on the bits

This protection must include the physical characteristics of the transmission of information including:

· The source that generates the message to be transmitted.

  • The transmitter that puts the signal to be transmitted has a defined level (electrical, optical, ...).
  • The transmission channel which transports the information.
  • The receiver which converts the information into a message.

· The receiver that processes the received message.

Sources, recipients and transmission lines are the basics of transmission problems. In order to solve this problem it is necessary to choose the transmitter and receiver. For transmissions without channel transmission problems are negligible, it is otherwise in the case of communications transmission channels. Indeed, due to the disturbances present on the transmission channel, the information provided to the recipient is not always identical to the information provided by the source. It appears that transmission errors are defined by the term "bit error rate - BER".The transmission quality is qualified as better when the rate BER is low. The transmission quality is measured by means of the bit error probability. on binary elements

The received signal R (t) is the sum of the transmitted signal and the interfering transmission noise R (t) = S (t) + N (t). (N (t) is white Gaussian noise).

The probability of error by binary elements (or bit error rate - BER) is a characteristic of the transmission. This BER is identical to the average error probability. This is the ratio of energy per bit to noise density  .

Experimentally, the BER is defined by the ratio of the number of erroneous bits after demodulation and decoding on the number of bits transmitted during a given time interval.

A poor quality connection has a BER on the order of 10 -4  for a telephone line and 10 -7 for data transmission.

1.3.1.2.Codage and Hamming distance

A first means for defining the effectiveness of a coding system for a secure transmission is the "Hamming distance". This distance is used to study the similarity between two words of same length. This distance is more or less effective filter that allows the transmission of "mistakes" during transmission. These "errors" can be quantified by a measure: the residual error rate.

Coding objective is to detect and correct transmission errors. The choice of the detection / correction code is done from the minimization of the distance between the modulated and encoded signals. This distance is the HAMMING distance. This parameter is used to characterize the resemblance between words of equal length. This distance may be defined as the number of bits by which the two words differ. The Hamming distance is a function of the coding technique used and the length of words.

if we consider two words:

The binary Hamming distance  dH(Ci, Cj of two binary code words Ci, Cj  is defined as the number of bits by which two different combinations are different.

The HAMMING distance is defined as the weight of the word HAMMING Ci  Cj sum made ​​component by component (number of bits equal to "1" of this amount). HAMMING distance d(u,v) of these two words is calculated as the arithmetic sum of the digits modulo 2 that are of the same rank, taken in pairs.

For example: d(u v) with

d(u,v) = 2 because there are two different digits (digits 2 and 4).

The minimum Hamming distance is the minimum number of inversed bits required for one codeword turns into another codeword. Without encoding, the distance is "1", for the encoding with parity this distance is "2".

To calculate the residual error rate using formulas provided in the standard EN 60870-1 and this, for frame formats defined and for specific encodings, we need to know the HAMMING distance and the minimum HAMMING distance .

A minimum Hamming distance of an encoding system C(N, K) linear is defined by :

with: wH the number of codewords of weight H,  wH characterizes the ability of the coding system to detect errors and allows to characterize the performance of the code.

The minimum Hamming distance is specific to each detector error code. In order to know this minimum distance HAMMING, there are two solutions:

· To choose the frame formats defined in standard EN 60870-5-1.

· Either iteratively calculate this distance depending on the chosen code and the length of the frame. This is the solution that will be developped later to define the minimum distance-HAMMING specific to the CRC code and to the organization of the frame sequence information.

For BCH codes and CRC type wm is approximated as follows for m  d min:

A linear code C(N, K) of minimum distance dmin can correct with a binary decoding a maximum likelihood of any configuration comprising "t" errors such that dmin  2.t +1.

The code can also detect any configuration of "m"  errors so that  dminH+1 and simultaneously correct any configuration t  H errors such that dminH+t+1

1.3.1.3.Residual error rate

In order to measure the characteristics of a transmission network in terms of error detection coverage, it is necessary to integrate the error detection rate, the probability of occurrence of errors. The calculated result is called the "residual error rate."

The notation (n, j) below conforms with the standard EN 60-870-1. In these circumstances, and with the assumptions of the following paragraph for a CRC coding, we get:

Probability of occurrence of errors m from N binary elements

We will no longer talk of detection rate of errors, but of probability for there to be "1, 2, ... m" errors. The calculation of this parameter introduce the probability to have "m" errors (m = 1, 2, 3, ...) from "N" symbols (bits)  .

In the case of a binary symetric channel without memory where a disturbance is thermal noise origin (Gaussian white noise - which is the case of transmission networks) we get:

The number of combinations is calculated using the following formula:

When a code C (N, K) with HAMMING distance dmin is used in error detection, it is able to detect all error configurations that lead to a word receiveddifferent from the codeword (that is to say, the configurations of a weight of less than d min).

Considering the error of given configurations as equally likely, the probability of an error pattern of weight m  d min is a codeword is given by 

 

Or,

Hence, R =  (Ratio of the sum of non-detected faults on the set of all possible cases).

or else

The CRC (Cyclic Redundancy Check) prohibiting certain types of errors corresponding to the minimum HAMMING distance between two frames, we get the following general formula:

 

Effectiveness of the coding system

In order to ensure the integrity of the transmission in terms of information content, and to detect and / or correct transmission errors, techniques that consist to introduce redundancy into the message to be transmitted (at of the issuer) are implemented.

The coding of a signal allows to adapt this signal according to the physical device of the transmission channel. The coding takes into account the channel bandwidth and the signal to noise ratio.

The design of a transmission system must involve two parameters:

  • the modulation
  • the correcting coding.

The coding transforms a binary word Mi of K symbols {m i,k} in a binary word Cof N symbols  {ci,n} called codeword. The encoder introduces a redundancy which results in an increase in the symbol rate between input Mi and output of the encoder Ci . Encoder establishes a correspondence between the symbols on the output of the encoder and the transmitted signals. In the case of our study, the symbols in the input and output are binary.

The integrity control of the transmission is performed first by checking the frame format and also by controlling the accuracy of the verification key. The control of the format consists of checking the delimiters and the number of bits in the incoming frame.

There are several key verification, which can improve the safety level of the transmission by introducing redundancy into the transmitted frame. At the receiver level, the thing to do is just to check if the coding rule used in transmission is satisfied. This encoding allows to detect transmission errors.

The errors considered are:

  • Errors entering in the information data.
  • Errors entering in the verification key.
  • The errors on the delimiter and on the number of bits constituting the frame.

The code efficiency characterizes the number of additional information not relevant to the transmitted parameters but necessary to ensure its integrity. This efficiency is the ratio of the number of bits correctly transmitted to the total number of bits constituting the codeword or data frame:

with:

  • k = number of information bnary elements per frame,
  • q = probability to receive correct bits,
  • n = total number of bits per frame including frame delimiters and bits.

1.3.2. Response time of a communication system

Another characteristic parameter of the transmission of information is the time duration between the information is sent and the time where the information is used. Depending on the nature of the messages (monitoring the evolution of a parameter, stopping a cycle ...), and the types of reactions, the times durations must be limited.