Aktharuzzaman, Tanvir Noor Baig and K Siddique-e Rabbani ...

Microcontroller based FIM system
Design of a Microcontroller Based 4-Electrode Focused
Impedance Measurement (FIM) System
Aktharuzzaman 1 , T N Baig 1 and K S Rabbani 2
1 Biomedical Physics Laboratory, Department of Physics, University of Dhaka
2 Department of Biomedical Physics & Technology, University of Dhaka
Abstract: Hardware and software design was performed to implement the newly developed 4
electrode Focused Impedance Method (FIM) based on a microcontroller which involved
complex switching of current sources and amplifier inputs to the 4 electrodes, data acquisition,
analysis and display of output. The developed design and software were evaluated using a
circuit simulator software and the system performed the desired functions very well.
Keywords: Focused Impedance, Electrical Impedance, Instrumentation
1. Introduction
Focused Impedance Method (FIM) is a new modality of bio-electric impedance measurement
developed at the Biomedical Physics Laboratory of the University of Dhaka of which two versions
appear to have potential in physiological study and diagnosis, one with 6 electrodes (Rabbani et al
1998, 1999) and one with 4 electrodes (Rabbani and Karal, 2007). In the four electrode version the
electrodes are placed at the corners of a square matrix where each of the adjacent electrode pairs acts
as a current drive and potential is measured across the opposite pair. Four arrangements are possible
by rotating the configuration in steps of 90 0 . The four measured potentials, which are proportional to
the respective impedances (because of a constant driving current), are summed to give the focused
impedance value. This however needs a complex switching of electrodes which has to be done
automatically for any practically usable equipment and a microcontroller (μC) based measurement
system is attractive from a practical standpoint. This paper presents such a μC based 4-electrode FIM
system being developed at our laboratory.
2. Instrumentation of 4-electrode FIM system
A block diagram of the designed system is shown in figure 1. Here, two analog switch ICs, under
program control, are used to switch both the current drive and potential measuring circuitry to the
appropriate electrodes attached to the ‘body’. The measured potential is an ac signal which is
amplified, filtered, converted to an equivalent dc, and given necessary dc level shift to make the data
suitable for the next stage. All this is done by the potential measuring circuitry shown and the output
Constant current
generator (ac)
Switching
device
Body
Switching
device
Potential
measuring
unit
Latch
7-seg display
Figure 1: Block diagram for a microcontroller based 4 electrode FIM system
PIC16F877A Microcontroller with
AD converter
is acquired by the μC through its ADC input which converts the signal to the corresponding binary
digital value. For the four measurement sets needed for focusing, the appropriate switching is

Microcontroller based FIM system
performed by the μC and the corresponding output digital values are stored in its memory. At the end
of the sequence, the four stored values are summed and the result is displayed through a 7-segment
Display.
3. Circuit design, programming and simulation
Considering the availability, cost and performance a microcontroller (μC), PIC16F877A, with a built
in Analogue to Digital Converter (A/D Converter) was chosen to design the system. After preliminary
design, circuit simulation was performed using a software package named ‘Proteus 7’. The
programming of the μC was done in its ‘Assembly’ language. The different stages of the designed
system are discussed briefly in the following subsections.
Switching: SPDT analog switch IC ADG333A was used for the complex switching
operations under program control. A truth table for the switching sequence was developed
and optimised for this purpose, and the μC program was written based on this truth table.
Data acquisition: As mentioned earlier, the built-in A/D converter of PIC16F877A was used
for this purpose for which a program sub-routine was developed. It gave the right output
signals to perform the desired switching operations, to acquire the four output values in
sequence, and to store them in specified locations in memory.
Summing up the acquire data: Another programming sub-routine was developed to add all
the four acquired output values, in binary form. His routine also converted the binary data
into corresponding hexadecimal values and stored in another memory location.
Display operation: Four 7-segment displays with built-in hexadecimal to 7-segment display
converter were used for displaying the resultant output, in hexadecimal numbers. Due to non
availability of the right components the display was done in hexadecimal, however, the final
design would be done to display the result in decimal numbers. The internal output of the μC
had 16 bits, while the I/O port of PIC16F877A had only 8 pins. So the data transfer was done
in two steps using two octal latches, through multiplexing. Another programming sub-routine
was developed for this operation.
Simulation: The performance of each individual sub-routines and that of the total program
were examined together with the switching schemes using the simulation software as
mentioned before. Simulated analogue input values were put in and the displayed numerical
outputs were noted. All the results conformed exactly to the functions desired.
4. Conclusion
The design of the 4 electrode FIM circuitry together with the necessary program for the
microcontroller was a success, as verified through the simulation experiment using Proteus 7
software. The next step would be the implementation of the hardware for the 4 electrode FIM system.
The system could have been designed based on a microcomputer. However, the goal was to develop a
portable hand held unit, and therefore the microcontroller based design was chosen.
5. References
Rabbani K S, Sarker M, Akond M H R and Akter T 1998 Focused Impedance Measurement (FIM) –
a new technique with zone localization, Invited lecture, Proceedings, X International Conference on
Electrical Bioimpedance, Barcelona, Spain 31-34.
Rabbani K S, M Sarker, M H R Akond & T Akter 1999 Focused Impedance Measurement (FIM) – a
new technique with improved zone localization Annals of the new York Academy of Science,
Electrical Bioimpedance Methods 873 408-420.
K S Rabbani and M A S Karal 2008, A new four-electrode Focused Impedance Measurement (FIM)
system for physiological study, Annals of Biomedical Engg (Springer) 36 1072-1077.