Mitsubishi FX5 Strain Gauge Problems

[Viv-V]

Hello,

I'm currently working on a project where I need to read values off my 0-3kg Strain Gauge and display them on my HMI display.

I've connected the SG to a Mitsubishi A/D module per the V1+ input and COM, wrote a test program in GX-W3 and displayed the values to the HMI.

Problem is the values do not make sense. When I display them I get a 10 character number, where the last 8 digits change slightly upon detection of force. It also jumps erraticaly (not at all incrementaly) over the value 0 to -10 digit number, upon the reversal of force on the sensor. How could I fix this?

Other info:
-PLC: FX5UC-32MT/DSS-TS
-A/D: FX5-4AD-ADP
-SG is wired directly to the A/D, no external AMP
-The Strain Gauge was tested on an Oscilloscope prior to being connected to the PLC, and was working fine.
-A/D configured in GX-W3 to -10v - 10v range. I tried using the scale function, but with it turned on, the display did not work at all.
-GT-D3 used for the display, with Signed 32bit display option, reading from D10 register.
-Program and Strain Gauge info in attached pictures.

Thank you for the help

 

[Liam Moran]

A strain gauge is typically supplied with a constant external voltage. What you will be measuring will be only millivolts. The sensitivity of your unit is quoted as 1.0 - 1.5mv/v. Therefore if you use a 10 volt supply, the maximum output from your loadcell fully loaded wil be 10-15 millivolts.

 

[Liam Moran]

If you are using a conventional PLC analog input then you really need to get a loadcell amplifier to bring the strain gauge output up to a level for the PLC to read.

 

[drbitboy]

Where is the decimal point in the displayed number?

What are typical values in D0 and D10?

Can you give an example of a simultaneous set of the three displayed values of the measurement: D0; D10 (should be the same as D0, but floating point); what the HMI is displaying?

 

[Viv-V]

If you are using a conventional PLC analog input then you really need to get a loadcell amplifier to bring the strain gauge output up to a level for the PLC to read.

Yes I'm think you are right. Do you have any recomendations?
I was thinking of using a HX711 and maybe an Arduino to handle the aplification on a limited budget, since it's for my thesis.

 

[Viv-V]

Where is the decimal point in the displayed number?

What are typical values in D0 and D10?

Can you give an example of a simultaneous set of the three displayed values of the measurement: D0; D10 (should be the same as D0, but floating point); what the HMI is displaying?

I can't give you precise values for those since my lab is under maintenance until monday.

What I can tell you is, that the HMI gives vastly different results based on the type of display option selected. The ones that give me somewhat useful values are:
-Real (those values range from -8 to about +12; without any decimal points) and:
-Signed 32bit (10 digit values without any decimal points)

Reading measurements of D0 and D10 seemingly makes no difference (same result on HMI)

 

[drbitboy]

You need to incorporate the load cell amplifier as @Liam Moran says.

As to the 10-digit numbers, if

• the raw values in D0 range from -16 to +16 (integers), and
• you used INT2FLT to convert those values to -16.0 to +16.0 in D10,
• but had the HMI read the bits (cf. item (i) below) in D10 as 32-bit signed integers

then your results make perfect sense; see here; the correlation ([D0],[D10 float bits displayed as integer]) probably looked like this:

$ python

Python 3.8.13...

>>>

>>> import struct

>>>

>>> for i in range(-16,17):   print((i,struct.unpack('i',struct.pack('f',float(i)))[0],))

...

(-16, -1048576000)

(-15, -1049624576)

(-14, -1050673152)

(-13, -1051721728)

(-12, -1052770304)

(-11, -1053818880)

(-10, -1054867456)

(-9, -1055916032)

(-8, -1056964608)

(-7, -1059061760)

(-6, -1061158912)

(-5, -1063256064)

(-4, -1065353216)

(-3, -1069547520)

(-2, -1073741824)

(-1, -1082130432)

(0, 0)

(1, 1065353216)

(2, 1073741824)

(3, 1077936128)

(4, 1082130432)

(5, 1084227584)

(6, 1086324736)

(7, 1088421888)

(8, 1090519040)

(9, 1091567616)

(10, 1092616192)

(11, 1093664768)

(12, 1094713344)

(13, 1095761920)

(14, 1096810496)

(15, 1097859072)

(16, 1098907648)

​

It's all about the bits.

 

[Viv-V]

You need to incorporate the load cell amplifier as @Liam Moran says.

As to the 10-digit numbers, if

• the raw values in D0 range from -16 to +16 (integers), and
• you used INT2FLT to convert those values to -16.0 to +16.0 in D10,
• but had the HMI read the bits (cf. item (i) below) in D10 as 32-bit signed integers

then your results make perfect sense; see here; the correlation ([D0],[D10 float bits displayed as integer]) probably looked like this:

$ python

Python 3.8.13...

>>>

>>> import struct

>>>

>>> for i in range(-16,17):   print((i,struct.unpack('i',struct.pack('f',float(i)))[0],))

...

(-16, -1048576000)

(-15, -1049624576)

(-14, -1050673152)

(-13, -1051721728)

(-12, -1052770304)

(-11, -1053818880)

(-10, -1054867456)

(-9, -1055916032)

(-8, -1056964608)

(-7, -1059061760)

(-6, -1061158912)

(-5, -1063256064)

(-4, -1065353216)

(-3, -1069547520)

(-2, -1073741824)

(-1, -1082130432)

(0, 0)

(1, 1065353216)

(2, 1073741824)

(3, 1077936128)

(4, 1082130432)

(5, 1084227584)

(6, 1086324736)

(7, 1088421888)

(8, 1090519040)

(9, 1091567616)

(10, 1092616192)

(11, 1093664768)

(12, 1094713344)

(13, 1095761920)

(14, 1096810496)

(15, 1097859072)

(16, 1098907648)

​

It's all about the bits.

You are absolutely. Now that I read your explanation it makes perfect sense. Seems like I missed the forest for the trees. Thank you for the great reply!

I definitely need to search for a suitable aplifier now...

 

[drbitboy]

the maximum output from your loadcell fully loaded will be 10-15 millivolts.

With a signal range of ±10mV, and with the FX5-4AD-ADP resolution of 0.625mV/count, there would not be a reading that would place a value into D0 (integer) of beyond a couple dozen counts, which is in the ballpark of the -8 to +12 displayed when D10 is interpreted by the HMI as a REAL.


You could always apply a voltage of around 10kV to avoid the amplifier .

 

[drbitboy]

Seems like I missed the forest for the trees.

Meh. I always think about the bits first, so it's easy for me; as soon as I saw the INT2FLT instruction I knew what happened. At least you understand.

"It's only ones and zeros; it cannot be hard" - Jouni Ryno