humidity sensor problem

[raul77]

so I have this (temp +relative humidity) sensor ( pfmini72) from mitchell instruments , the sensor works fine for temperature reading but when it comes to humidity it gives a reading of -378, -360, -342 and so on never crossing 0 , I connect the plc to a pc so that I can monitor the program that's how I can see the readings. it only moves by +18 as you see and these readings don't have a pattern so that I can modify the program, I know there is no pattern because I have a humidity measuring device a calibrated one and when it gives 20% RH , it gives once -378 and another time -360 and -342 and so on for the same value of RH and while the calibrated device keeps moving up and down in the RH measurement , the reading of the sensor always oscillate between two values , I mean it gives -378 reading for 20, 19 , 18,... 10% RH and so on. so the sensor output is 0-1 v for 0-100% RH so I disconnected the sensor from the plc and started to measure voltage from the sensor o/p and it gave correct o/ps like when it says .32 v, the measurement of the calibrated device is like 30 % RH which is very close and it's like that for all values of RH I encountered , so I don't think the problem is in the sensor. so I changed where I put the RH o/p of the sensor in the plc to the one that's connected to the temperature o/p but it's still the same. P.S. the plc is LENZE.

so any help would be greatly appreciated as I don't know how to deal with this humidity output, thank you.

 

[Lancie1]

[The Mitchell Instruments PFMini72 humidity sensor] gives -378 reading for 20, 19 , 18,... 10% RH and so on. [But] the sensor output is 0-1 v for 0-100% RH, so I disconnected the sensor from the plc and started to measure voltage from the sensor o/p and it gave correct [voltage] o/ps like when it says .32 v, the measurement of the calibrated device is like 30 % RH which is very close and it's like that for all values of RH that I encountered, so I don't think the problem is in the sensor.

It seems that you need to change and scale your PLC analog input from a current reading to a 0-to-1 volt reading.

 

[rootboy]

Your humidity output is 0 - 1 volt. Like you said, .32V is about 30%. How do you have your analog input card set up?

 

[raul77]

It seems that you need to change and scale your PLC analog input from a current reading to a 0-to-1 volt reading.

the temperature is in 0-1 volts and it reads fine in the same analog input, is that what you're talkin about?

 

[raul77]

Your humidity output is 0 - 1 volt. Like you said, .32V is about 30%. How do you have your analog input card set up?

okay I am really very sorry for the stupid question but how to know the set up of the analog input card??

 

[shooter]

your Lenze is -10 to 10 volt input so need an converter.

 

[raul77]

your Lenze is -10 to 10 volt input so need an converter.

what converter a function block or what??

 

[raul77]

anybody please

 

[Lancie1]

...how to know the set up of the analog input card??

If I were you, I would find the User Manual for the Lenze PLC analog inputs. Read that to find out the range of numbers that a voltage input is conveted into. Then set up a scaling computation in your PLC program that takes a 0 to 1 volt input and converts it to some X Minimum to Y Maximum number range. In other words, the same computation that must be done for almost every PLC analog input.

An alternate option is to buy an external Signal Converter, that will take your 0-to-1 volt input and convert it to a -10 to 10 volt range (that is apparently the full range of your Lenze PLC analog voltage inputs. That would have the advantage of using the full range of numbers, instead of only the small 0-to-1 volt number range.

 

[shooter]

You are needing two hardware converters example is WAGO jumpflex 857 series.


the input signal is for the first one is 0/1 volt output is -10/10 or 0/10 Volts depending on how you set up the input of the LENZE
the other has Pt100 input and also a -10/10 Volt output.

 

[raul77]

If I were you, I would find the User Manual for the Lenze PLC analog inputs. Read that to find out the range of numbers that a voltage input is conveted into. Then set up a scaling computation in your PLC program that takes a 0 to 1 volt input and converts it to some X Minimum to Y Maximum number range. In other words, the same computation that must be done for almost every PLC analog input.

An alternate option is to buy an external Signal Converter, that will take your 0-to-1 volt input and convert it to a -10 to 10 volt range (that is apparently the full range of your Lenze PLC analog voltage inputs. That would have the advantage of using the full range of numbers, instead of only the small 0-to-1 volt number range.

well it says :

analog input 1 :

1- level : -10v........10v

2- Resolution: 10bits + sign

3- Normalization : ±10 V ≡ ±16384 ≡ ±100 %

and:

Note!
Because of their normalisation, analog signals use an unsymmetrical resolution area. (-200 % ... +199.99 %):
External: -200 %........ -100 %.........0............+100 %............+199.99 %
Internal: -32768..........-16384.........0............+16384.............+32767


so how do I go about this If I want it to read from 0 to 1 v if it's even possible or do I have to get converters like shooter says?

 

[raul77]

If I were you, I would find the User Manual for the Lenze PLC analog inputs. Read that to find out the range of numbers that a voltage input is conveted into. Then set up a scaling computation in your PLC program that takes a 0 to 1 volt input and converts it to some X Minimum to Y Maximum number range. In other words, the same computation that must be done for almost every PLC analog input.

An alternate option is to buy an external Signal Converter, that will take your 0-to-1 volt input and convert it to a -10 to 10 volt range (that is apparently the full range of your Lenze PLC analog voltage inputs. That would have the advantage of using the full range of numbers, instead of only the small 0-to-1 volt number range.

okay I got it , I multiply the value of the analog input by 16384 and then divide it by 1023 but for the humidity value which is giving me negative readings like -378 equivalent to -2.3% in the code c0400 and -360 = -2.19 % , I don't know why it's giving me negative readings but the value of the voltage equivalent to -378 is lower than -360 but it's never stable and that's the problem , I think I need a number to subtract these numbers from and then I would have the scale set up.

any idea what I should be doing, thank you.

 

[danw]

Mitchell humidity output is 0-1.0 Vdc for 0-100% RH, correct? (first post)

Is the statement below for Lenze PLC analog input from the manual?

Because of their normalisation, analog signals use an unsymmetrical resolution area. (-200 % ... +199.99 %):
External: -200 %........ -100 %.........0............+100 %............+199.99 %
Internal: -32768..........-16384.........0............+16384.............+327 67

I have no idea how a ±10Vdc AI can 'see' ±200% of an input signal, but that's me.

If the statement for the values for the 'external' condition are true, then
0Vdc = 0 counts
10Vdc = 100% = (+)16382 counts

So 1.0 Vdc = (+) 163 or (+) 164 counts

Somehow, I'm sure you can scale
0 counts = 0 %RH = 0.0 Vdc
164 counts = 100 %RH = 1.0 Vdc

The resulting resolution is pretty coarse - 100%RH / 164 counts = 0.625 %RH per count

If the resolution too coarse for your purposes, then you'll have to get a signal conditioner.

If you get a signal conditioner, I wouldn't care about converting negative voltages - RH cannot be a negative value.

By the way, I never read this thead during the work week because your first post is one giant paragraph - all the information, all the sentences crammed together is very difficult to read. If it's hard to read and comprehend, I just go on to the next post.

Might I suggest that you separate your sentences with some white space to make it easier to read.

 

[Lancie1]

0 to 1 volt input range = 0 to 1638.4 counts

Humidity (%) = Analog Input Voltage X (100 - 0)/(1638.4-0)
Humidity (%) = Analog Input Voltage X 0.061035

Checks:
At 0 volts input, Humidity = 0 X 0.061035 = 0%
At 1 volts input, Humidity = 1638.4 X 0.061035 = 99.999 %

 

[danw]

Lancie is so right, I had an order of magnitude percentage error before morning coffee.