7.3. COMPUTERS AND PROCESSES

Process engineering includes activities related to the implementation of the industrial processing of physical materials, design and construction of processing plants in the chemical industry, mining, metallurgy, processing waste and elsewhere. The effectiveness of this activity is now almost unthinkable without the use of a digital computer or as a separate entity or part of the whole complex system.

Computer and process technology can be used only for its supervision or to automatically manage the entire process. Which of these methods will be applied depending on the method of data entry into a computer, and their treatment and how to use the results management process. Thus, one should distinguish between supervision process (sensing relevant parameters) and control it (setting parameters) either directly (automatically) or indirectly. According to the method of entering data into the computer, and their use to manage the processes differ:

• On - line systems
• Off - line systems

The division is primarily related to input data into a computer, although it can be applied to the way a computer controlled process.

On-line data entry is done by directly connecting data sources to a computer through electrical and electronic circuits. Given the high speed signal transmission and response time of arrival computer data from a source to a computer is the order of microseconds. Then it says that the data is processed in real-time and data transmission time is usually negligible.

Off-line data transmission takes place and the two phases. The first data from a source written in some removable media (floppy disk, cache, paper tape, etc.), and only then with these media transmitted or entered into the computer. The time between data collection and entry, if necessary, for an hour or two, a week, a month, a year or never. Of course, this system is unsuitable for fast and direct supervision of processes.

Conceptual diagram hardware elements in the management process is shown in the following figure.

Figure 7.3.1 Devices and circuits in computer process control.

Circuits and devices in the figure can be divided into three basic environments:

• Managed system
• Environment to connection
• Digital computer

The Figure shows that is displayed on-line management system. The off-line control system according to the executive part of the unit would be in operation periodically.

Digital computer is the most important element of this whole structure. The principles of his work describes all the previous chapters, and is now incorporated into the system shown in process control. Will they manage to perform a PC or microcontroller will depend on the complexity of the task and the frequency of the need to change the mode of the process or its control.

Managed systems are so different that they can not be considered in general, and their differences will require different elements in the environment to connect. The role of sensors (transducers) and operating mode A/D and D/A converters are described in the previous chapters. A/D and D/A converters are also shown here outside of the digital computer as one of the possible ways of their implementation. Naturally then, the computer interface may be different from the serial (RS 232c) and parallel (CENTRONICS), USB or other type with a special card.

Managed system

Containing the sensor and actuator (executive device). The physical size of the sensor as temperature, humidity, pressure, or other converting the analog signal. As the output of the analog sensor signal usually does not fit through the parameters as standard input to the A/D converter is necessary to make its analog processing.

If simultaneously monitor multiple parameters will be used multiple sensors and A/D converter with multiplexed inputs, of course, an analog processing of each.

Using sensors indicate that it is on-line management system. But there are times when the system can not monitor the sensor. For example, if the measure of water pollution presence of leeches. As soon as leeches escaping water is polluted. This system is one of the off-line tracking system because the presence of leeches in the water can not be measured. A similar system is set for hot drinks. Very simple sensor (a key) identifies what he wants to drink. The computer on the basis that miss by executive Device certain amount of water, sugar and coffee in a bowl. No part of this system is still online. Namely, in the absence of the vessel or any of the components of the computer will not allow to execute the desired operation for the appropriate message to the lack of attention.

Modern cars already have quite a number of sensors that warn of the condition of the car, or a possible malfunction or wear of parts and how in some ways warn drivers (light, sound or message). Based on the embedded software in the computer cars will follow the message on which the decision of just the driver or driving a car will make it impossible, for example if the driver seat belt is not fastened.

Thus, there are two basic modes of operation.

• Closed loop control
• Open loop control

While the data may show a man in an appropriate manner, the loop is still closed if a man take some action, but not electrically closed. So control is not fully automatic. The second is when the process is done in an environment of measurement (monitors) and not taking of any action, whether decided by a computer or human. The loop is then opened, and no executive devices.

To the process that takes place in an environment could act must be executive that directly affect the environment, such as: circuit breaker, solenoid valve, electric motor and more. Executive devices are like sensors and highly specific for their proper operation to harmonize output from the D/A converters for them with some sort of device for suiting naturally aligned and designed for managing the device with which it is connected.

Processing of analog signal

Electrical signals coming from the sensors will often be a continuous variable voltage but can be variable pulse width or size or variable frequency AC signal or stages. On the other hand, the signal can be very small or large in relation to the standard input of the A/D converter. Moreover interdependence condition sensor changes observed (e.g. temperature) and the output value of the sensor can be linear or nonlinear. All this again is not the same for each sensor performance although they are intended for the same purpose. In short, sensors and signals that give very different, and enters the A/D are mostly standard. In order to enable the A/D converter recognizes the sensor sends him to first to complete electronic processing of sensor signals.

Circuits for electronic processing of analog signals are a special area, but in general the rule is that it is good to take the sensor with a manufacturer who offers devices for electronic processing, because then their work is mutually consistent. The task of the device for electronic processing is reduced essentially to the fact that the accuracy of the information (data) on the size of the measured parameter as little damage, and if necessary, improved. Although modern software in a digital computer that performs supervision, can make corrections characteristics of sensors, it will generally meet only when all the elements of the sensor to the program support from the same manufacturer.

But from the description of controls is a large diversity of electronic devices for analogue signal processing when using multiple sensors to monitor an environment. In such a situation, they will have to use the A/D converter with a multiplexer. Complexity transducer will depend to what extent are the outputs from analog circuits for signal processing to be harmonized. If compliance is good to satisfy the A/D converter with a multiplexer at the entrance and shared further conversion electronics (amplifier, filter, S&H, ...) and if you did not have to use separate systems conversion in the A/D converter before the entrance to the multiplexer which raises the price of the converter.

In the case of monitoring multiple sensors very rarely happens that enters the multiplexer sequentially exchanged at regular intervals. Time intervals between sampling may be different if, for example, monitor the temperature and pressure in a boiler. When and where to take the time to decide the sample program support based on her default parameters.

Digital computer - microcontroller

The most important element of the on-line system for process control basic reason for its application is high speed operation, high reliability and processing power. Digital computer hardware components have already explained in the chapter on the PC system. But diversity is reflected in the program support, which monitors for a process. The design of software involves a detailed study of the measurement environment, or processes that are in the middle of the event. Thus the uniformity of software out of the question, unless designed by the manufacturer of laboratory equipment. In all other cases, the design of software is fully compatible only with the process being considered and for which the equipment is designed, then develop a measurement or control algorithm compliant with the character of the process is what the on-line system should do.

The algorithm does not need to create a constructor on-line system for its user, or if the system is complex, it will create together. On the basis of this algorithm using a language processors will be made using the client software for the PC (phase development program support) to make the computer capable of carrying out measurement and programming assignments. If the task is to change enough to change the program and that part of the electronic and control devices remain the same.