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FDC1004: capacitive measurement of human-body-water

Meik Nyuken
Prodigy 20 points
Part Number: FDC1004

Hey there,

my name is Maik and I´m from germany. Sorry for it in advance, but my english is not the best.  During my studies, I have to work on a research project. My theme is the development of a sensor to determine the relative amount of water in the body. This has to be done over a capacitive change. For this I first got the FDC1004 concerned. We have to complain about the software correctly. The programming language is python. Here is our source code:

import smbus
import time


FDC = 0b1010000 # 7-bit Adresse für FDC1004 Slave

MEAS1_VAL   = 0b0000010110000000    # 16-bit Schlüssel zur Konfigurierung der Messung -
FDC_VAL     = 0b0000110110000000    # 16-bit Schlüssel zur Konfigurierung des FDC

MEAS1_MSB  = 0x00   # Zeiger für Register MSB Messung Kanal1
MEAS1_LSB  = 0x01   # Zeiger für Register LSB Messung Kanal1
CONF_MEAS1 = 0x08   # Zeiger für Register der Konfigurationseinstellung Kanal1
FDC_CONF   = 0x0C   # Zeiger für die Konfiguration der Messung

fdc1004 = smbus.SMBus(1)   # Erzeugt eine I2C-Instanz und öffnen des Busses/ 1 für eine der beiden I2C schnitstellen (den mit NR1)
 
#Messung Konfiguration
fdc1004.write_word_data(FDC,CONF_MEAS1,MEAS1_VAL) 
fdc1004.write_word_data(FDC,FDC_CONF,FDC_VAL)
time.sleep(0.2)


#Messung
  
MSB2 = fdc1004.read_byte_data(0x50,MEAS1_MSB)
LSB = fdc1004.read_byte_data(FDC,MEAS1_LSB)

print ((MSB), hex(LSB))

The central processing unit is the Raspberry pi model 3. Now, to our problems with the software. In our view, all the configurations we have chosen are written into the respective registers. When configuring, we do not know exactly how the byte for writing and reading is to be sent as described in section 8.5.2 of the data sheet. Also we do not know exactly how we should continue to program the data of the FDC1004 correctly. Could we possibly get a source code from you to have a template? The programming language would not matter.

Since we did not get any further on our own software, we still bought the FDC1004 EVM. We thought that we could at least carry out the measurements. Unfortunately this did not work as planned.

On the two pictures you can see our measuring plate. On the left picture the shield electrode is shown. To the right, the other side of the measuring unit is shown with two additional electrodes. This is attached to the forearm by means of a bracelet, in order to detect liquid changes there.We now only do not know exactly whether this is the correct or suitable construction of the measuring unit at all. And we also do not know how we should connect the respective electrodes with the FDC1004. In addition to that, we do not know how to configure the FDC. I know these are quite a few questions at once but I would be grateful for any info.  

Best regards

Maik Nuyken

  • 0
    TI__Expert 6480 points
    Hi Malik,

    How are you measuring the liquid? Is there another fixture that sits on top of the sensing electrodes that fills up with liquid? Or is liquid flowing in between the sensors through the unit that it's attached to? Could you show me the measurement setup so that I can better understand the system and give more helpful feedback?

    I'm not sure if you've been able to look at some of the application notes we have on the FDC devices but here's a couple that talk about liquid level sensing and proper shielding techniques:
    - www.ti.com/.../snoa925.pdf
    - www.ti.com/.../snoa926a.pdf

    In terms of software, we have some arduino code shown in this other E2E thread that might help.
    - e2e.ti.com/.../495744

    Thanks,
    Rachel
  • 0 in reply to Rachel Liao
    Prodigy 20 points

    Hi Rachel,

    thanks for the quick reply.The sensor should be attached as shown on the left image. The right picture shows a schematic representation of the planned measurement setup.

    So I just want to measure the capacitance of the condenser, which rests directly on the skin. The water that is in the cells serves as the dielectric. (This is at least my plan). I do not know if the selected construction is the best way to determine the amount of liquid as accurately as possible. Is there any other structure that would be more suitable? In the moment I use the one electrode as Cin1 and the other as GND. I am referring to the application of the sensor as a rain sensor like it is described in one of your documents. But anyway I come only to a change of 3-4pf when I put my finger on Cin1. If I drop some water on Cin nothing happens.

    Best regards

    Maik Nuyken

  • 0 in reply to Meik Nyuken
    TI__Expert 6480 points
    Hi Maik,

    Is the goal to measure the capacitance change from no skin to skin? Otherwise, I'm not sure capacitive sensing would be appropriate for this application. If what is being measured is the fluctuations of water in the cells, the signal won't be large enough for our device to detect.

    Thanks,
    Rachel