5501-471 Woodward Digital Input Module

Description

Nowadays, PLC is not only used for on-off control, but also for analog and digital control. It can collect and store data, and also monitor the control system; It can also connect to the internet and communicate, achieving large-scale and cross regional control and management. PLC has increasingly become an important role in the family of industrial control devices.

1 for on-off control
The ability of PLC to control switching values is very strong. The number of input and output points controlled can range from a few dozen or dozens to hundreds, thousands, or even tens of thousands. Due to its ability to connect to the internet, the number of points is almost unlimited, and can be controlled regardless of how many points it has.

The logic problems controlled can be diverse: combinatorial, temporal; Immediate and delayed; Not requiring counting, requiring counting; Fixed sequence, working randomly; Wait, everything can be done.

The hardware structure of PLC is variable, and the software program is programmable, making it very flexible when used for control. If necessary, multiple sets or sets of programs can be written and called as needed. It is very suitable for the needs of multiple working conditions and state transitions in industrial sites.

There are many examples of using PLC for on-off control, such as metallurgy, machinery, light industry, chemical industry, textile, and so on, which are needed in almost all industrial industries. At present, the primary goal of PLC is that it can be easily and reliably used for on-off control, which is incomparable to other controllers.

2 for analog quantity control
Analog quantities, such as current, voltage, temperature, pressure, etc., vary continuously in magnitude. Industrial production, especially continuous production processes, often requires control over these physical quantities.

As an industrial control electronic device, if PLC cannot control these quantities, it is a major drawback. Therefore, various PLC manufacturers are conducting extensive development in this area. At present, not only large and medium-sized computers can perform analog control, but even small computers can also perform such control.

PLC should be equipped with A/D and D/A units that convert analog and digital quantities for analog control. It is also an I/O unit, but it is a special I/O unit.

The A/D unit converts analog signals from external circuits into digital signals and then sends them to the PLC. The D/A unit converts the digital quantity of the PLC into an analog quantity and then sends it to the external circuit.

As a special type of I/O unit, it still has characteristics such as anti-interference of I/O circuits, isolation of internal and external circuits, and exchange of information with input/output relays (or internal relays, which are also an area of PLC working memory. Read and write).
The A in A/D here mostly represents current, voltage, and also temperature. A in D/A is mostly voltage or current. The range of voltage and current changes is mostly 0-5V, 0-10V, and 4-20mA. Some can also handle positive and negative values.

The D here is mostly an 8-bit binary number for small computers, and a 12-bit binary number for medium and large computers.
A/D and D/A have both single and multiple channels. Multiple input and output relays are used.

With A/D and D/A units, the remaining processing is all digital, which is not difficult for PLCs with information processing capabilities. Medium and large PLCs have stronger processing capabilities, not only capable of adding, subtracting, multiplying, and dividing numbers, but also capable of square root, interpolation, and floating-point operations. Some also have PID instructions that can perform proportional, differential, and integral operations on the deviation control, thereby generating corresponding outputs. It can calculate almost anything that a computer can.

In this way, it is completely possible to achieve analog control using PLC. The unit value controlled can be as small as 1/212 of the measurement range value, most of which are also sufficient.

PLC performs analog control, and there are units that combine A/D and D/A, which can be controlled using PID or fuzzy control algorithms to achieve high control quality.

The advantage of using PLC for analog control is that while analog control is being carried out, the switching value can also be controlled. This advantage is not possessed by other controllers, or the implementation of control is not as convenient as PLC.
Of course, if the system is purely analog, using a PLC may not be as cost-effective as using a regulator. This should also be seen.

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