Analog quantities, such as current, voltage, temperature, pressure, etc., are continuously changing in size. Industrial production, especially continuous production processes, often requires the control of these physical quantities.
As an industrial control electronic device, if PLC cannot control these quantities, it is a big disadvantage. To this end, various PLC manufacturers have carried out a lot of development in this regard. At present, not only large and medium-sized machines can carry out analog control, but also small machines can carry out such control.
PLC performs analog control, and it is necessary to configure A/D and D/A units for mutual conversion between analog and digital. It is also an I/O unit, but a special I/O unit.
The A/D unit converts the analog quantity of the external circuit into a digital quantity, and then sends it to the PLC. The D/A unit is to convert the digital quantity of the PLC into an analog quantity, and then send it to the external circuit.
As a special I/O unit, it still has I/O circuit anti-interference, isolation of internal and external circuits, and exchange of information with input and output relays (or internal relays, which are also an area of PLC working memory. Read and write), etc. Features.
The A in the A/D here is mostly current, or voltage, and some are temperature. The A in D/A is mostly voltage or current. The voltage and current change range is mostly 0~5V, 0~10V, 4~20mA. Some can also handle positive and negative values.
Here D, most of the minicomputers are 8-bit binary numbers, and the medium and large ones are mostly 12-bit binary numbers.
A/D and D/A have single-channel and multi-channel. Multiple channels occupy more input and output relays.
With the A/D and D/A units, the rest of the processing is digital, which is not difficult for PLCs with information processing capabilities. Medium and large-scale PLCs have stronger processing capabilities, which can not only add, subtract, multiply, and divide numbers, but also square, interpolate, and perform floating-point operations. Some also have PID instructions, which can perform proportional, differential, and integral operations on the deviation control quantity, and then generate the corresponding output. It can do almost anything that a computer can do.
In this way, it is completely possible to realize analog quantity control with PLC. The unit value of the control can be as small as 1/212 of the measuring range value, and most are sufficient.
PLC performs analog control, as well as units that combine A/D and D/A, and can be controlled by PID or fuzzy control algorithm, which can achieve high control quality.
The advantage of using PLC for analog control is that the switch can also be controlled while the analog control is being performed. This advantage is not available in other controllers, or the realization of control is not as convenient as PLC.
Of course, if the system is purely analog, using a PLC may not be as good as using a regulator in terms of performance and price. This should also be seen. work; etc., can be carried out.
The hardware structure of PLC is variable, the software program is editable, and it is very flexible when used for control. When necessary, multiple sets or groups of programs can be written and called as needed. It is very suitable for the needs of multi-working conditions and multi-state transformation in industrial sites.
There are many examples of switching quantity control with PLC. Almost all industrial industries need to use it in metallurgy, machinery, light industry, chemical industry, textile and so on. At present, the first goal of PLC is that other controllers cannot compare with it, that is, it can be used for the control of switching quantity conveniently and reliably.
