Discrete Control Automata and Logic Converters
In the simplest case, all input and output variables change only at two possible levels - the logical unit & quot; 1 & quot; (high or & quot; enabled & quot;) or a logical zero of & quot; 0 & quot; (low or "off"), that is, they are so-called binary or Boolean functions. Their hardware implementation is possible with the help of relay-contactor or non-contact equipment. In the latter case, logical elements, encoders, decoders, multiplexers, programmable logic arrays, permanent memory devices and other integrated circuits of small, medium and large degree of integration are widely used in practice. Triggers, registers, and counters are used as memory elements. The hard logic control device that processes input binary signals and generates output binary control variables will be referred to as a discrete control automaton.
The functions of a discrete control automaton become complex with a large number of input and output variables (several tens, hundreds). A programmable logic device that performs the functions of a discrete control automaton is called a programmable controller. Modern models of industrial programmable controllers can not only perform logical functions, but also perform arithmetic calculations, performing additionally the functions of digital regulators. Therefore, in their composition, in addition to the input/output modules of binary signals, the input and output modules of digital and analog signals can be included.
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If the output binary signals of the control device are determined only by the state of the binary inputs, then such a control device is called a combinational logic circuit or a logic converter. It is obvious that the system of equations (6.16), (6.17) degenerates into one matrix equation:
which can be replaced by a system of l logical functions:
The considered classification of discrete control devices by the complexity of the tasks they solve is shown in Fig. 6.26. In Fig. 6.27 there is an additional classification of discrete control automata, which, in turn, are divided into two large classes: synchronous and asynchronous.
Fig. 6.26. Classification of Discrete Control Devices by Complexity of Solved Problems
In synchronous machines there is a time-quantizer (clock generator), and the state of the automaton changes at strictly defined times (more precisely, it can only change at these instants). Asynchronous machines do not contain a clock generator. Their state changes after the change in the state of the inputs. Such automata must be stable, namely, all their states must be stable. This is possible if, upon transition to some new state x (i) under the influence of some input signal the output from this new state It is possible only when another signal other than arrives at the input of the machine. Note that asynchronous automata are implemented, as a rule, on relays and contactors. In slot machines on contactless logic elements, it is not difficult to synchronize. In this case, the stability condition is automatically achieved.
Fig. 6.27. Classification of discrete control automata
If the output function depends only on the internal state of the machine and does not depend on the state of the inputs, then such an automaton is called the Moore automatic machine, otherwise Miles. In the theory of automata, it is proved that, due to the transition from one state variable to another, it is possible to convert Moore automata to Miley automata and vice versa.
Advantages of microprocessor control systems
Compared to analog systems, microprocessor systems have several advantages:
1) flexibility. The ability to reprogram changes not only the parameters of the control system, but algorithms and even structure. The hardware part of the system remains unchanged. In analog systems, it would be necessary to re-link the links, include new ones, etc., that is, change the hardware;
2) removal of all restrictions on the structure of the control device and the laws of control. The digital control system can provide, in the simplest case, results not worse than analog (continuous), when the traditional control algorithms obtained for continuous systems are approximated by discrete ones. However, the potentialities of digital systems are not used: the simplicity of realizing nonlinear, optimal (for example, start-stop), adaptive algorithms; the possibility of constructing interconnected multi-structural control systems. Given these opportunities, the quality of digital systems can significantly exceed the quality of continuous systems. For example, the actual creation of speed limits for control systems;
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3) self-diagnostics and self-test control devices. The ability to verify the integrity of mechanical components of the drive, power converters, sensors and other equipment during periods of technological pauses, i.e., automatic diagnostics of equipment condition and early warning of an accident.
4) more high accuracy due to the lack of zero drift typical for all analog devices (zero drift prevents accurate measurement of small mismatches in analog systems). Thus, digital speed control systems provide an increase in accuracy by two orders of magnitude, compared to their continuous counterparts;
5) easy to visualize the process parameters controls by applying digital indicators, display panels and displays, organizing a dialogue mode of information exchange with the operator for management purposes
6) high reliability, smaller weight, dimensions and cost.
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