How to program a formula
Posted 08 July 2011 - 05:36 AM
I'm building a little test unit that will interface with our refrigeration equipment and it will carry out certain tests.
I need to be able to program the following 2 formulas and simply display the answer on the LCD of the XD12.
The X value will actually be from a 0-10V input connected to a pressure transducer. The answer (Y-value) will be displayed.
Y=0.0000002X^3 - 0.000253X^2 + 0.167X - 44.46
The only X values that will go into this formula are between 29.5 and 492.5 as these I'll manipulate from the 0-10V input.
Y=0.0000042X^2 + 0.038X - 13.05
The only X values that will go into this formula are between 976.2 and 2766.6 as these will be manipulated values from the 0-10V input.
If it helps, these 2 formulas are close fit graphs to a pressure/temperature table for R404a refrigerant.
If it's impossible to program these formulas, how would I go about entering a pressure temperature chart from 7.6kPa to 2766.6kPA, and have the PLC read a corresponding temperature from the table, and be able to take a linear average for values between the table values.
Posted 14 July 2011 - 04:53 PM
There is no way that you can load this formula into the Millenium 3.
The Millenium 3 doesn't have any floating point arithmetic. All the numbers processed by the controller are 16-bit integers. The value for these numbers can range between -32768 and 32767.
You can somehow manage values with a decimal point by using a Display block (e.g. 1205 displayed with 2 decimal places is shown on the screen as 12.05).
The only thing that you can do in this case is to use a lookup table (use block Y=F(x)).
Could you please clarify what do you mean by "be able to take a linear average for values between the table values".
What exactly do you want to do?
Posted 06 October 2011 - 05:58 AM
I actually got it right in my program.
M04 and M05 labelled LPPT and HPPT are the macros where I actually do this.
It takes a 0-10V input from a pressure transducer and converts is to an actual pressure reading in kPa.
Then I have 2 formulas like the one I presented that converts that pressure to a temperature for R404A refrigerant.
The formula presents the relationship between the 2.
So with pressure as my input to the macro, my output is temperaute according to formula.
Posted 06 October 2011 - 06:12 AM
What I mean here was only applicable to a look-up table...
If I create a lookup table like the following...
Input 1 = Output 15
Input 5 = Output 75
Input 10 = Output 200
Looking at a table like that we can see that the relationship between the input growth and output growth is not linear.
So what happens if I Input 3? That's not a value in the table. So how is it's output calculated?
3 is pretty much halfway between 1 and 5. Will the output be a number halfway between 15 and 75?
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