Hello everyone,

I'm using Siemens S7-1212C AC/DC/Rly and I'm very new at PLC Programming. I'm using LAD Language to read my Wegapuls WL 61 Ultrasonic Sensor. The sensor has 4 - 20 mA analog output and I'm using 500 Ohm resistance to convert 1 - 10 V. To this point everyting has fine. 

My problem is my sensor show 9.95 Volt at 0 meter and its reduce when I lift the sensor. For exapmle;

0 meter --> 9.95 Volt

1 meter --> 9 Volt

2 meter --> 8.2 Volt...

I need to reverse this on program but I cannot find how can I do this? Please help, I'm very new at PLC programming I declare a function block that contain NORM_X and SCALE_X but i find it on the internet and don't know what to do it.

Really more of a math problem than a programming problem.  I assume that you want 0 to be 0 distance and full output to be the maximum.

Remember your line equation y = mx + b?

y = distance, x = volts

m = delta y / delta x = (2-1) / (8.2 - 9) = -1.25 m/v

Switch the sign so that now your m = 1.25 m/v

So now your new equation is y = 1.25 x + b.  Set your b to give you 0 at 0 distance

You can use the Calculate block for this. 

Jeff Casagrande

Really more of a math problem than a programming problem.  I assume that you want 0 to be 0 distance and full output to be the maximum.

Remember your line equation y = mx + b?

y = distance, x = volts

m = delta y / delta x = (2-1) / (8.2 - 9) = -1.25 m/v

Switch the sign so that now your m = 1.25 m/v

So now your new equation is y = 1.25 x + b.  Set your b to give you 0 at 0 distance

You can use the Calculate block for this. 

Dear friend Jeff,

Indeed helpful notes, but....

I think part of the problem for Blake is how the 0 to 10 V is represented inside the PLC. As you know the PLC does not use "volts" inside its program. Would you please be so kind as to guide him in this regard?

Frankly, I would suggest
(1) use scaling to get analog reading to a REAL number representation
(2) do math to rework the number to what is required.
... IMO this way would be less involved in calculations.

My thanks for your help in this matter.
w

I agree, I should have used normalized numbers for the formulas, assuming they are normalized first.

The exercise is left to the student.  

The Normalized volts will be a number between 0 and 1, so m = (2-1) / (0.82 - 0.9).  M = -12.5

New equation is distance = 12.5 x + b

at distance = 0, x = .995 so b = -12.4375

So in the Calculate block, Out = 12.5 * Normalized_Input - 12.4375

..maybe should start at the foundation first
(not knowing what you knowledge level is)

https://support.industry.siemens.com/cs/ww/en/view/39334504

The analog value is probably read from (default) %IW64.
Therefore %IW64 is a number between 0 and 27648.

To invert you can do a subtraction with 27648 - %IW64.

From the provided link it is possible to convert to engineering units.

Sorry fo the rushed notes.. is it however helpful?
w

That would be much simpler, I didn't think of that, but I tend to overcomplicate things.

So, step 2, convert this integer value to a REAL using the NORM block.  result is a real value between 0 and 1.

Step 3.  Use the SCALE_X block to convert to engineering units.

In general, I prefer to use the CALCULATE block, since there are no restrictions other that data formats.

blake123

I want especially thank to you William_B. As Jeff Casagrande say I know linear equation more than him. But as I say, I'm very new at PLC things and I can't see CALC bloc. I solve my problem and really thank to everyone of you. This is too easy that I imagine.

Thank you for a very welcome & positive feedback.
As to the CALCULATE instruction...