1. Les fosses de la zone littorale
1.2. Structuration des ensembles sépulcraux
Data Transmission
Modern process analyzers provide a great deal of information, both about their analytical results and about the status of the analyzer itself. The oldest and still most prevalent method is a local display of results through an analog output. The drawback with an analog system is the lack of analyzer status information. With the development of microprocessor-based analyzers and control computers, the analyzers are able use serial communication to directly communicate with the control computer.
Analog Data Transmission - Direct analog connection to the control computer has been the standard for many years. In its simplest form, analog transmission is accomplished by causing an electronic amplifier to mirror the process variable. Figure 58 shows an analog signal connected to a
recorder. When the analog signal is plotted on the recorder over time, a “trend” is developed.
Figure 58: Analog Circuit
There are several problems in using analog data transmission with a gas chromatograph. Gas chromatographs typically read multi-components on more than one stream. In this situation, each component has an analog output for each component on each stream. The control computer has to have a similar number of analog inputs available. The amount of hardware increases with the number of analyzers. The other problem is that there is a fixed range for each component.
Figure 58 shows an example of an output ranged with a chromatograph. If the analyzer results fall outside these ranges in an upset condition, the results can no longer be seen on the trend. For these reasons, the use of serial communication may be a better choice.
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Example:
A chromatograph is used to measure the concentrations of methane and ethane in a natural gas stream. Two current amplifiers are dedicated to the task of transmitting the results of the
measurement. Both current amplifiers are capable of delivering a current no lower than 4 mA and no higher than 20 mA. If the methane ranges from 70% to 90% and the ethane ranges from 5%
to 20%, the outputs of the current amplifiers represent the actual measurements as follows:
Amplifier 1
4 mA means methane at 70%
12 mA means methane at 80%
20 mA means methane at 90%
Amplifier 2
4 mA means ethane at 5%
12 mA means ethane at 12.5%
20 mA means ethane at 20%
Notice that 12 mA is the mid-range of the current amplifiers (that is, halfway between 4 mA and 20 mA) and is therefore used to represent the halfway point of the process variables being measured.
Serial Data Transmission - The availability of computer electronics has made serial communication a popular and rapidly growing communication technique. This technique provides a means to overcome the limitations with analog communication systems. Furthermore, serial
communication permits an analyzer to both send and receive data.
The essence of serial communication hardware is a single digital electronics device, an electronic voltage or current amplifier. The electronics are capable of assuming only two conditions - off / on or lower / higher voltage level. By introducing a time element into the picture, the same electronic hardware can be used to express an unlimited amount of information either into or out of an analyzer system.
Figure 59 demonstrates this technique. At a single point in time, the electronic hardware is in only one condition - high or low. But if a timer is started and the pattern of high / low voltages coming from the electronic hardware is watched, the pattern itself can be used to represent an item of complex information. For example, as shown in the figure, in the ASCII standard code interpretation, a pattern of 7 states is used to represent the alphabetic letter “C”. Thus, by starting a timer that synchronizes the signal source (such as the analyzer) and the listener (such as a printer) and by observing the pattern of signals that occurs during a specified time interval, it is possible for the analyzer to send the letter “C” to the printer. Similarly, if the timer is then restarted so that a different pattern is created by the analyzer in the next time interval, it is possible for the analyzer to send yet another letter to the printer. As shown in the figure, this could be the letter “B”.
Figure 59: Basic Serial Representation
This technique makes it is possible for the analyzer to send written information. The key elements are (1) that the analyzer and the printer “agree” on the binary patterns that will represent the various letters, (2) that the printer and the analyzer use a consistent and identical time standard, and (3) that enough time is allowed to pass for information to be transmitted from one device to the other.
Hardware Variations
In the next section, some practical considerations of serial communication will be discussed;
however, it is first appropriate to consider some of the variations in hardware that are used for this purpose. Several systems are in common use. The older of these systems are:
• RS-232-C voltage hardware
• 0/20 mA current loop hardware
Most newer systems are variations of these two systems.
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RS-232-C Convention - The RS-232 system is a convention that is generally accepted as a standard hardware technique throughout industry. It uses voltage amplifiers to create the two voltage signal levels that are needed to represent the two binary conditions. The basic hardware has three signal wires, which transmit, receive, and ground. The transmit line sources voltages relative to the ground line, and the receive line observes voltages relative to the same ground line.
The voltages in use are:
-3 to -15 volts means binary 1, or set, or on, or high +3 to +15 volts means binary 0, or reset, or off, or low
>15 volts may cause hardware damage
-3 to +3 volts is indeterminate and may cause error
The binary conditions are often expressed in three ways as indicated. It is significant to note that the high / low binary condition is the opposite of the actual voltage signal.
The RS-232-C convention includes several optional signals besides transmit and receive. These include:
• RTS - Request to Send • DSR - Data Set Ready
• CTS - Clear to Send • DTR - Data Terminal Ready
These optional signals are also defined and are used for control of communication devices such as modems. A more complete discussion is beyond the scope of this module.
Physical wiring is also often standardized under the RS-232 convention. A “D” shaped cable connector is used. In this system, pin 2 is the transmit line, pin 3 is the receive line and pin 7 is the common. The other pins are also defined in a standard manner. In general, a voltmeter or an oscilloscope can be used to observe RS-232 signal activity.
Current Loop Convention - This technique was originally established as a means of operating mechanical teletype machines. As long as a signal current was flowing, moving components in a teletype machine would remain in one physical location. When the current in the signal line was interrupted, mechanical components would release. At rather slow speeds, it was possible to cause these mechanical fingers to activate appropriate keys on the mechanical typewriter of the teletype. The convention establishes two current levels as the operating levels that represent binary states:
1. 20 milliamps means binary 0 or space or reset 2. 0 milliamps means binary 1 or mark or set
While the technique has almost no present use in teletype activation, it has been retained as a de facto industry standard for several reasons. First, there was a large installed base of teletype equipment in the word and it was suitable for equipment manufacturers to make new equipment that was compatible with the old. Second, the current loop offers higher signal noise immunity that the newer RS-232-C convention. In general, there is wide latitude around the nominal 0 and 20 mA setpoints of the current loop convention. Third, current loops can be powered by current amplifiers that can operate into longer transmission lines. The voltage amplifiers of the RS-232 are limited to voltage losses that occur in the resistive wiring. The RS-232 specification itself limits line length to 50 feet (18 meters). Fourth, multiple receiving devices can usually be installed in series on a current loop. Multiple devices installed on a voltage line like RS-232 can overload the voltage drive capabilities of the electronic amplifiers. Fifth, current loop systems do not have to be referenced to a common voltage reference such as ground. Consequently, current loops are more likely to be immune to ground loop problems.
Next to these advantages, there are some disadvantages of current loop systems. Such systems do not have well defined control signals to permit more complex device control such as those needed for modems. Because of electronic rise and fall times, current-loop based systems may be
constrained to operate at slower speeds.
Other Hardware Variations - Most modern communication systems support RS-232-C and current loop conventions in standard configurations. Most modern systems also implement newer
conventions including a later military specification, RS-422, and others. These newer conventions are based on voltage amplifiers that operate with differential line drivers (see Figure 60). Such systems usually depend on a 0 volts / 5 volts signal differential that switches polarity between two signal wires. A ground line may be provided as a noise shield, but this line is not part of the transmission loop. As a result, ground loops do not occur.
Figure 60: Voltage Amplifier for Serial Communication
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Additionally, modern transmission line drive circuitry supports a “third” state besides the binary high /low. This third state is “off” and in this condition, a transmitter on a transmission line is apparently “disconnected.” This operating mode permits several different devices to be connected to a single communication line in a “multi-drop” configuration (see Figure 61).
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Figure 61: Pareto Chart
Practical Considerations of Serial Communication Techniques
Serial communication systems are in common use because they offer the following advantages:
• The capability to combine both input and output functions into a system that is common with maintenance and control functions.
• The capability to transmit and receive large amounts of information with very little hardware cost.
• The ability to use standard hardware to connect two different systems from two different manufacturers.
However, for each of these advantages, there is a corresponding disadvantage. These are:
• The difficulty of training human operators to understand the function that is being performed at any one time.
• The requirement for very specialized hardware and potentially complex software, both of which lead to relatively high expense.
• The lack of standardization at any level above hardware, including software protocols and application software.
In systems in which an analyzer is to communicate with another computer, care should be taken and analog or discrete digital options should be considered.
The following sections detail two of the most common problems that occur with serial communication.
Software and Protocol - As was stated in a previous section, successful serial communication requires agreement between the sending and receiving devices on the codes that will be used to represent information and the timing and time intervals through which the codes will be sent.
In most systems, standard time intervals are defined and accepted in the industry. Expressed as
‘baud rate’ these frequencies specify the number of binary bits of information per second that are sent from one device to another. Standard baud rates are 300, 1200, 2400, 4800, 9600, and 19200 baud. Some other variations are occasionally used.
Timing is often controlled by a bit synchronization sequence. For instance, if an alphabetic character requires 7 binary bits to represent it, then the sending device precedes these seven bits with a “start” bit to “wake up” the receiving device. Following the seven data bits is usually an error control bit, called parity, and one or two end bits. The start and end bits allow a known time interval for both transmitting and receiving devices to synchronize.
Protocol agreement is much more difficult to establish. At the most rudimentary level, certain industry standards have been adopted which allow representation of alphabetic characters in a predetermined form. The most commonly accepted standard is ASCII.
Alternatively, raw computer information is often transmitted as an image of the computer’s memory (i.e., in its direct binary form). Such information may be “received” by a device and still be of no value if the receiver does not “understand” the data. Such understanding is accomplished by the software in both the sending and receiving devices.
Additionally, there must be rules as to which device may “talk” and which device must “listen” at any given point in time. There must be agreement about how long one device may talk before it must listen. These rules are implemented in the software in the sending and receiving devices.
This software is referred to as communications “protocol.” Protocol software is usually complex and it is usually customized for each application because very few industrial standards exist.
Fortunately, the analyzer industry has made some progress toward mutual agreement on protocol standardization. One of the more commonly accepted protocols was originally published by Gould, Inc. as part of the Modicon line of programmable logic controllers. The original protocol and various adaptations of it are referred to as the Modbus* protocol. (Modbus is a registered trademark of Gould, Inc.)
Hardware - Although hardware conventions are well established and standardized, there are many practical considerations in dealing with communications hardware. Normally, specialized
equipment is required to test or troubleshoot such systems.
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As mentioned earlier, time is required to pass information from one device to another in serial fashion. In other words, information transfer from one point is delayed until data transfer from another point is completed. This is one of the biggest disadvantages of serial communication systems. Transfer is neither continuous nor is it instantaneous.
To overcome the time delay problem, manufacturers have improved electronics to allow very high speed data transfers. Faster data transfer has reduced problems of delays in communication systems, but it has not eliminated them. For example, if several analyzers want to send their information to output devices, the analyzers must transfer data one at a time. Ultimately, the last analyzer to send its data may have to wait a long time. If a communications system is made faster, this delay is reduced, but it is never eliminated.
In the process of making the systems faster, manufacturers have also made the hardware more difficult to maintain. It is not possible to use a voltmeter to observe communication that occurs at high speed. The best that can be done is to use the meter to troubleshoot simple short circuits and open lines in the communications wire. Additionally, although an oscilloscope might be used to observe voltage changes at high speed, an oscilloscope does not have the software to recognize and understand binary patterns. Specialized equipment, such as a serial data analyzer, may be used for this purpose. Although a serial data analyzer makes trouble shooting possible, it introduces a new problem. Now, maintenance personnel must understand the software protocol that the machines are using.
Despite these difficulties, the advantages are strong and development continues on serial digital communication techniques. Problems are continuously being resolved and these systems are being made easier to use.
Use of Personal Computers
Data Processing and Reporting
The introduction of the microprocessor during the early 1970’s drastically improved
instrumentation and process control. More specifically, the microprocessor provided a better means of programming the complex functions that the GC performs during each cycle (i.e., sample injection, column switching, peak detection, etc.). Manufacturers continue to develop more powerful and sophisticated GCs while improving the analyzers’ reliability and redundancy.
One of the major areas of improvement involve the development of GC networks. The networks today allows the users to communicate with all of the analyzers in the system.
With a personal computer connected to the analyzer network comes the ability for the analyzer, to not only receive and transmit information to the control computer, but for personal computer to receive and transmit the same information. All of the analyzers status and component information can now be gathered by a personal computer and used locally by the user.
Personal computers have increased in power and capabilities to the point where they allow the user to perform the following functions:
• access the system
• backup and restore each analyzers program
• store analyzers results
• create historical trends
• monitor the overall system
Some computer systems allow the user to perform most maintenance tasks directly from the personal computer. Multiple personal computers can be connected to the system or remotely by way of phone lines and modems.
Analytical monitoring systems allow live links to the analyzer network system, which allows live trending. With certain software systems, live information from the analyzer system such as process and calibration information can be gathered. With this information .procedures and analyzer documentation can be maintained at the personal computer. Statistical quality control (SQC) procedures and data gathering can now be automated by the personal computer.
SQC Techniques
Statistical Quality Control (SQC) techniques are not only being applied to process control systems, but they are now being applied to the analyzers. SQC helps to take out the guess work of when the analyzer may need maintenance or how often the analyzer should be calibrated. With SQC, measurement data is collected, statistically analyzed, and displayed to prove the analyzer’s capability, or Cp. Cp is a number that shows the capability of an analyzer to repeat measurement on a consistent process stream. The specifications for GC analyzers are determined by the potential use of the analyzer, type of process stream to be analyzed, etc. These specifications are given in percentage of full scale. For example, if full scale is 10 units, the analyzer will be run near 5 (halfway) with a specification of ±0.5%. This means that the measurements should repeat within a band of 0.1 units.
With the availability of personal computers connected to the analyzer network, the measurement data is gathered automatically. The technician’s can now have instant access to the analyzer statistical information. The use of a personal computer to automate the SQC task frees up the user to better maintain the analyzer system.
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Understanding the measure of variation, or standard deviation, is necessary to understand Cp.
Standard deviation can be thought of as the average deviation from the average of a set of values.
While this definition of standard deviation is not totally correct, it gives a good idea of how variation is measured. The formula for the average and standard deviation, σ, of a number is shown in Figure 62.
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Figure 62: Normal Distribution
The formula says that standard deviation is the square root of the sum of the differences from average squared, divided by the number of data points. The variable “n” is the number of data points and Xi is the ith data point (reading from a cycle). A distribution like the normal
The formula says that standard deviation is the square root of the sum of the differences from average squared, divided by the number of data points. The variable “n” is the number of data points and Xi is the ith data point (reading from a cycle). A distribution like the normal