The control characteristics of PLC analog _PLC analog classification -

Control characteristics of PLC analog error

1, the

analog is continuous in time, the values.

value of analog quantity in a certain time interval, is called sampling. The value is the resulting from the sampling discrete quantity. Obviously, the dispersed quantity is discrete in time, that is only at the instant of the sampling on behalf of the analog quantity, the analog values do not represent other moments. But the value is continuous.

with the numerical approximation of discrete quantity, namely discrete quantity and actual closest to the digital quantity, known as quantitative. After quantitative discrete quantity is called the digital quantity. Digital quantity on time and the values are discrete.

PLC can only deal with digital quantity, and to use it to control analog, must first to the analog sampling and quantization.

it is to quantify, so after the quantitative error value and analog value always exist, but this error is controllable. Way to choose the appropriate input and output module of digits. Such as using 8-bit input module, and its quantitative value can only be 0 to 255 ( Hexadecimal FF) Between the integer. Therefore, one of the resolution of 256 points. Choose if the digit number of, also has a high resolution and high precision. Unit number, module is also expensive. Above 16 with double word command even when processing, but this will also increase the resource cost and processing time.

error can be controlled is one of the most important advantages, but there is also a reasonable 'degree', should be under the guarantee accuracy requirements, makes every effort to reduce the digits.

2, intermittent

in order to ensure that sampling signals can be less distortion, restore to the original continuous signal, according to the sampling theorem, the sampling frequency shall generally be greater than or equal to the double frequency of the system. Maximum frequency is the amplitude frequency characteristics on the system frequency amplitude is zero.

3, delay

the inertia of actual system itself and the movement transmission has a process, a certain delay. Using PLC control, sampling, information processing and control output is also a process, more delay. After the implementation of a new round of control function, cannot be expected to immediately respond. So, just because at that time can't get the desired response, just blindly change control function. That is likely to make the system unstable. Again, such as, using PID control, the operation interval should not be too short, there is no special measures, such as the interval at least should be greater than the scan cycle of the program, etc.

classification of PLC controller analog

1, according to the analog control method, a single loop feedback control, cascade control, etc.

2, according to the control points, has the open-loop control system, closed loop control system and compound control system.

3 points, according to the element type, has the mechanical system, electrical system, electrical system, hydraulic system and pneumatic system.

4 points, according to the system function, system with temperature control, pressure control system, traffic control system, position control and speed control system.

5, according to the performance of the system, a linear system and nonlinear system, time-invariant and time-varying systems.

6, according to the output points, has a fixed value control system ( Regulator) , servo system and process control system.

7, according to the analog input methods, has analog input unit ( AI） , pulse input, PI） 。

8, according to the method to control the analog output, analog output unit ( AO） And the ON/OFF switch ( 做) , pulse output ( Different pulse width) Output ( PO） 。

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