Analog display on the touch screen in the corresponding conversion plc

Analog value and the A / D conversion value of the conversion
the standard analog signal hypothesis is A0-Am (such as :4-20mA), A / D conversion value of D0 < br>-Dm (eg :6400 -32 000), set the standard analog signal is A, A / D conversion of the corresponding value
is D, because it is a linear relationship between the function A = f (D) can be expressed as a mathematical equation:
A = (D-D0) × (Am-A0) / (Dm-D0) + A0.
according to the formula, the value can be easily calculated based on D A values. Inverse transform the equation to arrive
function D = f (A) can be expressed as a mathematical equation:
D = (A-A0) × (Dm-D0) / (Am-A0) + D0. cite a specific instance of
to S7-200 and 4-20mA, for example, by A / D conversion, we get the
value is 6400-32000, that A0 = 4 , Am = 20, D0 = 6400, Dm = 32000, into the formula, too
out:
A = (D-6400) × (20-4) / (32000 - 6400) +4
assume the corresponding analog and AIW0, when AIW0 value of 12800, the corresponding analog electrical signal
is 6400 × 16/25600 +4 = 8mA.
another example, a temperature sensor, -10-60 ℃ and 4-20mA corresponds to T is temperature, AIW0
for the PLC analog sampling value, based on type directly into draw:
T = 70 × (AIW0-6400) / 25600-10
can directly display the temperature value T.
analog value and the A / D conversion value of the conversion more difficult to understand, to read the paragraph several times, with examples cited
will understand. To make things easier to understand, we give another example:
a pressure transmitter, when the pressure reaches full scale 5MPa, the pressure transmitter's output current is 20mA,
AIW0 value is 32000. Visible, each corresponding mA A / D value 32000/20, properly measured pressure
0.1MPa, the pressure transmitter current should be 4mA, A / D value (32000/20) × 4 = 6400. Derived from the
This, AIW0 values ​​converted to the actual pressure value (in KPa) is calculated as:
VW0 value = (AIW0 value -6400) (5000-100) / (32000-6400 ) +100 (units: KPa)
Programming examples
You can set up a small example system demonstrate analog programming. In this instance the CPU is CPU222, only
with a analog extension EM235, the first channel of the module to connect a transmitter with 4-20mA output
out of the temperature display instrument, the instrument's range is set to 0-100 degrees, 0 degrees output 4mA,journal, 100
degrees output 20mA. Platinum resistance temperature display instrument input connected to an adjustable potentiometer 220 ohm,
simple programming are as follows:
temperature display value = (AIW0-6400) / 256
compile and run the program, observe the program state , VW30 is the display temperature control instrument display is
not consistent.