- Introduction
- Electrical connection
- How data is collected
- Passing Settings to Arduino
- Function of LED pins
- How data is printed
- Run, Pause and Stop Data collection
- Important References and Links
- How to run Arduino code?
- How to run Matlab code?
This code primarily collects the data that is required to generate an Electrical Impedance Tomography (EIT) Image. It also collects the resistance between any two electrodes. The arduino code works only on Arduino Mega. The Matlab code works only on Matlab versions 2019a and above.
The hardware setup consists of 16 electrodes (arranged evenly in a circular fashion) connected to a sensor / conductive material. The electrodes are connected to the 16 analog pins of the arduino mega and also the 16 channels of the three DG407 Muxes.
The 'D' pin of mux0 is connected to a constant current source.
The 'D' pin of mux1 is connected to ground.
The 'D' pin of mux2 is connected to VCC and a constant resistor. (Used to measure resistance, using a voltage divider circuit)
One can collect two types of data from this code:
-
Voltage Data for EIT Image
Current is injected between any two adjacent electrodes and the voltages from the remaining electrodes are measured. Then the current is injected between the next two electrodes. This cycle starts from (electrodes) [0,1] and ends at [15,0]. For this type of data collection the muxes 0 and 1 are enabled.
-
Resistance Data between any two electrodes (the data that is actually collected is the potential difference) - Using a Voltage divider circuit.
To measure the resistances between any two electrodes, (using combinatorics it is nC2, n = 16 electrodes, a total of 120 readings), the muxes 1 and 2 are enabled. This circuit works as a voltage divider circuit. For this we switch the muxes 1 and 2 from (0,1),(0,2)...(0,15)(1,2),...(14,15) and we measure the voltage difference between those pair of electrodes.
Note: In both the cases mentioned aboue, the value is printed out as a value from 0 to 1023, one has to convert that data according to the Analog Reference that was given. The analog reference for the voltage divider circuit is 5V and for the EIT, it is what you give initially (more on this in section IV, V and X).
The communication with arduino is purely SERIAL. So send these settings via serial and NOT I2C or any other modes of communication. You can customize data collection by sending these values in a specific order to the arduino (after it has been connected serially, ofcourse). The order in which you have to send is:
| Setting | Range |
|---|---|
| num_repeats | [0, 255) |
| inj_pattern | [1, 15] |
| analog_ref_voltage | [1, 5] |
| prescaler_value | [1, 7] // Refer here |
| sending_order | [0, 4] |
| delay_after_mux_on | [0, 65,535) |
where,
num_repeats :
Number of times "analogread" is called (for both voltage and resistance data)
inj_pattern :
Current Injection pattern. Not the measurement pattern. Google EIT injection pattern.
analog_ref_voltage :
You can tell arduino the reference value to compare against when performing analog read. For the
resistance data collection this value is set to 5V. But for Voltage data collection, you can set
any of the available reference values. Refer [here](#important-references-and-links) for more details.
prescaler_value :
You can tell arduino at what speed to collect the data. This is determined by the prescaler value
of the analog read function in arduino. Refer the table in Section X, to see acceptable values.
delay_after_mux_on :
You can specify the amount of time (in microseconds) the arduino can start collecting data after
the muxes have been turned on.
sending order:
| 0 | 1 | 2 |
|---|---|---|
| Voltage | Resistance | Resistance |
| Voltage | Resistance | Voltage |
| Voltage | Resistance | Resistance |
| Voltage | Resistance | Voltage |
| . | . | . |
| . | . | . |
| . | . | . |
In which,
Voltage :
The entire voltage matrix (14x16) is sent. This matrix is collected when the current is switched
between all 16 pairs of electrodes.
Resistance :
The potential difference between any two electrodes. All the 120 values are sent.
Note: There are no checks performed on these values, please ensure that you send the approporate values within the bounds.
The 2 LED pins that are connected to digital pins are mentioned in the variable "led_pins". It is specified as {Green_LED, Red_LED}.
Green_LED:
Shows that the serial connection has been established.
If ON: Active Communication
If OFF : Communication Disconnected
Red_LED:
Denotes the data collection status.
If ON: Data has been collected
If OFF: Data is being collected or Arduino is in "Pause" or "Stop" data collection state
The values of any variable Voltage_1 or Voltage or Resistance (see section IV) are sent with a ' ' (space) in between them. After all the values of the variable are finished printing, then a newline and carraiage return character is sent. This is done to distinguish between the two subsequent variables that are sent.
The start time (in milliseconds) is sent first (in a separate line) before any other variable is sent.
Voltage:
The Voltage values of electredes 2 to 15 are sent (0 indexed) - [current passed through 0 and 1] then
voltage values of electrodes 3 to 15 and 0 are sent - [current passed through 1 and 2], and so on...,
so a total of 14x16 = 226 values are printed with a ' ' (space) between successive values.
Resistance:
The potential difference between (0,1),(0,2)...(0,15)(1,2),...(14,15) electrodes are printed.
Note: The values of these variables are separated by ' ' (blank space). The variables are itself separated by a newline and carriage return.
While the arduino is collecting data to
| Action | Data to send |
|---|---|
| Pause | 0 |
| Stop | -1 |
| Resume after pause | 1 |
| Warn about possible circuit failure | -2 |
Note: When "-2" is sent the green and red led's flash together at a constant frequency and arduino goes to "Stop" state
Reference Table for Prescaler:
| Prescale | ADPS2,1,0 | Clock (MHz) | Sampling rate (KHz) | Data to Send |
|---|---|---|---|---|
| 2 | 0 0 1 | 8 | 615 | 1 |
| 4 | 0 1 0 | 4 | 307 | 2 |
| 8 | 0 1 1 | 2 | 153 | 3 |
| 16 | 1 0 0 | 1 | 76.8 | 4 (Good Choice, but small errors near 5V) |
| 32 | 1 0 1 | 0.5 | 38.4 | 5 (Excellent Choice) |
| 64 | 1 1 0 | 0.25 | 19.2 | 6 |
| 128 | 1 1 1 | 0.125 | 9.6 (default) | 7 (Default Value) |
List of all analog reference values: (for Arduino MEGA)
DEFAULT : Default analog reference of 5 volts (on 5V Arduino boards) or 3.3 volts (on 3.3V Arduino boards)
INTERNAL1V1 : A built-in 1.1V reference
INTERNAL2V56: a built-in 2.56V reference
EXTERNAL : The voltage applied to the AREF pin (0 to 5V only) is used as the reference
The value that you have to send to arduino for
INTERNAL1V1 : 1
INTERNAL2V56 : 2
3.3V : 3 (Note: This voltage has to be provided to the external pin)
EXTERNAL : 4 (Note: This voltage has to be provided to the external pin)
DEFAULT : 5
Link to MUX Datasheet: https://www.intersil.com/content/dam/Intersil/documents/dg40/dg406-407.pdf
For list of baud rates see Page 226 in
Copy and paste the contents of "/lib" to "C:\Users\user-name\Documents\Arduino\libraries" and then upload the "EIT.ino" in "/Arduino/src/EIT" folder to Arduino.
Specify the necessary settings in all the files in "Matlab/Settings/". Make sure EIDORS v3.9.1 or above is installed. Run the "main.m" code in "Matlab/Main"