Analog input with amplifier

Hello guys ! I'm just new here, and I have a couple of questions about LOGO!.

First at all, let me introduce myself, I'm finisihing electronics degree and my final degree project consists of monitoring an isolated photovoltaic installation with logo.

My problem is with amplifier block, because I need to mesure a current and an analogic input is between 2.5V and 3.18V, which means 0A to 50A DC (just for start). 

I tried everithing and only ehat I get was 2.5 to 3.5, actually, 25 to 35.

And my last question is, how i need to connect the Vout to the logo ? what I mean is, do I need to connect the Vout and the gorund as a reference provided both by a sensor ??

Thank you in advance.

It would be easier to help, if you offer the type of the sensor maybe with a link, a schematic of your connections and your actual logo program. You can upload all files a zip archive.

If you have a current sensor with normally 0..(5V)10V output, then you can have a problem with the input resistance of the Logo analog inputs (~70kOhm). I know sensors, which need a load resistor >= 1MOhm. Then you can connect correctly, but it will not work.

Regards, Scorp

Thank you for your answers, very helpful. 

I am using a Siemens Logo! 8 12/24 RCE - 6ED1052 - 1MD00-0BA8 with logo soft comfort V8 software.

I attached sensor's datasheet and diagram connections, actually what it is supossed I need to do.

It is just a first schematic. I need those sensors because I need to monitorize power consumption and power coming from photoltaic cells, just first steps. 

The idea is, working during whole development whith that coneections, pretending is a photoltaic installation and last steps work with a real one. Of course the sensor will be a bigger ones. But they will remain the same type, just increment current range.

I see 2 current sensors connected to I1 and I2, this will be AI3 and AI4 of the Logo, if you select 4 analog inputs. Otherwise you have to use I7 and I8 with 2 selected analog inputs in the properties to have AI1 and AI2.

I think, L1 and N on the picture is L+ and M in your Logo. Then you have to connect the sensor cable as follows:
red to +5V of power supply
black to 0V of power supply
yellow to I1/I2 or I7/I8
If there is no connection between 0V of 12V supply and 0V of 5V supply, then you need an additional connection between them.

You would have a bad accuracy as ixo65 has already written, if you only use a little part of the 0..10V analog input. You can calculate it by yourself, 0..10V will be converted to a value 0..1000. Additional you have a tolerance of 1.5% of the maximum value. This is a value of 15 in the Logo. If you have an input voltage of 1V, the value will be 100 +/-15 = 15%.

If you need a better accuracy, then use an other sensor or an precise amplifier with capability of adjusting gain and offset.

Regards, Scorp

abe708

I am using a Siemens Logo! 8 12/24 RCE - 6ED1052 - 1MD00-0BA8 with logo soft comfort V8 software.

I have been thinking about your explanations, and I think I would start with these first, and maybe I will change for another one with better accuracy or just get current with another sensor, different type I mean if finally I want that the system work properly in a real situation.

If I understood well, when I set the gain and offset of the amplifier block, to the maximum and minium numbers [25, 35], (31 is not possible), here is where am I loosing precision ? or also with a load input impedance ? 

Thank you for information.

@abe708

If you have a very little picture on the PC and can not see any details on it, then you can scale the picture to 1000% without problems, but you still can not see any details. You have the same problem with the amplifier block or the arithmetic block of the Logo.

The only solution is a current sensor with fitting output to the Logo, this will be 0..10V. If you want to measure max. 50 A, then the resolution will be 50mA (50A/1000). But the resolution is not the accuracy, this depends on the tolerance of the sensor and the Logo input.

A better way to help are more details, what you want to do with the current values ? Whatever you take as sensor (50A or 5000A), do you want to switch something ? Which accuracy do you need ?

Regards, Scorp