The principle of the functional construction of a discrete control device
The generalized matrix structure diagram (Figure 6.23) can be implemented using the input logic converter LP 1 of the input effects vector u (k) and the current state vector of the control device x (k) into the transition state vector x (k + 1), the memory element P of the current state ar (Nr ) and output logic converter LP 2 vectors of input effects and (k) and the current state vector x (k) in the output vector y (k).
The TG clock generator is used to specify the quantization pulses in time, in accordance with which the calculated state of the transition x (k + 1) is recorded in the device memory.
Fig. 6.23. Hardware implementation of a discrete control device
The control device considered is called a sequential synchronous machine. The machine operates in discrete time. Let the automaton have the internal state x (k) at the time and the input state and (k). Equation (6.16) realized in the input logic block specifies the so-called transfer function of the automaton, according to which the state of the automaton x (k (k + 1) T.
Equation (6.17), realized in the output logic block, specifies the output function, according to which the current state of the output y (k) is determined by the total state of the automaton. At the moment time (k + 1) T on the rising edge of the pulse C from the clock generator, the calculated state of the transition to memory machine. After this, a new phase of the calculation of the next state of transition and exit follows.
Thus, at discrete instants of time kT, where to = 0, 1, 2 ,. ., there are discrete changes in the state vector and the output vector, which is the main feature of any discrete control system. The hodograph of the state vector (Figure 6.24) is a collection of discrete points (discrete) in the state space into which a discrete system successively passes with each quantization clock cycle. A sequential synchronous machine can be considered as a "black box" that converts a sequence of input "packets" numbers for m -numbers in the packet in the output sequence of "packets" numbers in l -members in the bundle where m and l is the dimension of the input and output vectors, respectively.
Fig. 6.24. The hodograph of the state vector
Hard logic devices and programmable logic devices
Today, automation specialists have various hardware to create discrete control devices. These tools can be divided into two large classes: hard (installation) and programmable logic (Figure 6.25).
In the first case, a parallel solution of the system of difference equations describing the operation algorithm of the control device is made (Figure 6.25, a), in the second case, a sequential solution using a special processing device data - the processor, which with the help of a pre-compiled program informs the sequence of necessary actions for the implementation of the control algorithm (Figure 6.25, b). First, the processor monitors the input and memory sequentially their sos oyaniya in memory. Further, based on the information on the current state of the control device (also stored in memory) and the state of the inputs, the state of the outputs and the transition state are calculated. The current state is replaced by the state of the transition, sequential output of control actions new reading of the state of the inputs, etc.
Fig. 6.25. Control Devices
Electromechanical elements - relays and contactors, as well as integrated microcircuits of small and medium degree of integration (noncontact logic) are used for the implementation of hard logic devices. Today, more than 50 % of all discrete control systems used in the industry - hard logic devices. However, they have drawbacks: the lack of flexibility when making changes, the introduction of new functions and the difficulty of implementing complex control algorithms. These shortcomings can be eliminated by replacing hard logic devices with programmable devices. The threshold of cost-effective use of programmable devices is constantly decreasing and has reached already small systems containing no more than 20 relays or contactors.
In general, logical converters should contain blocks of digital multiplication, summation and subtraction. Their hardware implementation, although possible, turns out to be cumbersome and inefficient, in comparison with software control devices, where the same operations are performed using the appropriate commands. Therefore, control devices, at the input of which digital codes arrive at the output of which digital codes are generated, are mainly performed in the form of software control devices, i.e., flexible logic.
Depending on the complexity of the tasks being solved, these devices are divided into: digital regulators (digital filters) and microprocessor controllers for direct digital control of power converters and the drive as a whole; control microcomputers for controlling electric drives and technological equipment; Unified block-modular microprocessor systems for a wide range of applications; multi-level multi-microprocessor (multiprocessor) hierarchical control systems with distribution of management functions at individual levels of the hierarchy.
The structure of the analog-digital position control system was previously mentioned, where the functions of the digital position controller are performed by a microprocessor system that generates a speed reference signal at the input of the analog speed control subsystem (see Figure 6.19). This structure is typical for the drives of CNC machine tools. It is possible to replace all controllers - speed (PC), current (RT) and control system (SS) with a power converter (SP) with digital control devices. The entire control system as a whole will be called the microprocessor controller of the DC drive. The controller can have an independent design or be integrated into a power converter. Communication with the control system of the upper level can be carried out via a serial or parallel communication channel. It is also possible to install a group of controllers (by the number of drive axes) in the construction of the control computer to its bus system (to its interface). The latter option is characterized by an increase in the speeds of information exchange between controllers and computers.
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