PM154 3BSE003645R1 ABB

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 deficiency.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 carries on the analog quantity control,must be equipped with the analog quantity and the digital quantity mutual conversion A/D,D/A unit.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 converts the digital quantity of the PLC into an analog quantity,and then sends 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 ofPLC working memory.Read and write),etc.Features.

The advantage of using PLC to control the analog quantity is that the switch quantity can also be controlled while the analog quantity is being controlled.This advantage is not available in other controllers,or the realization of control is not as convenient as PLC.

模拟量，如电流、电压、温度、压力等等，它的大小是连续变化的。工业生产，特别是连续型生产过程，常要对这些物理量进行控制。

作为一种工业控制电子装置，PLC若不能对这些量进行控制，那是一大不足。为此，各PLC厂家都在这方面进行大量的开发。目前，不仅大型、中型机可以进行模拟量控制，就是小型机，也能进行这样的控制。

PLC进行模拟量控制，要配置有模拟量与数字量相互转换的A／D、D／A单元。它也是I／O单元，不过是特殊的I／O单元。

A／D单元是把外电路的模拟量，转换成数字量，然后送入PLC。D／A单元，是把PLC的数字量转换成模拟量，再送给外电路。

作为一种特殊的I／O单元，它仍具有I／O电路抗干扰、内外电路隔离，与输入输出继电器（或内部继电器，它也是PLC工作内存的一个区。可读写）交换信息等等特点。

用PLC进行模拟量控制的好处是，在进行模拟量控制的同时，开关量也可控制。这个优点是别的控制器所不具备的，或控制的实现不如PLC方便。