Pmods made quite a showing at this year’s Digilent Design Contest. Six out of eight of the awarded projects used at least one Digilent Pmod to reach their final goal! So what roles are these handy little peripheral modules playing? What does each piece of hardware achieve and how do they work together?
All six of the projects mentioned are excellent examples of the Digilent platform, successfully integrating Digilent system boards and Pmods to achieve one cohesive vision. One of the projects I found most interesting is the Wearable Wellness System by Cristian Iulian Andries, Antonel Ovidiu Vantur, and Lucian Burlacu of Gheorghe Asachi University in Iasi, Romania. It also uses a total of four Digilent Pmods including the PmodIA impedance analyzer, which is itself a very interesting product. So let’s dive a little deeper into this project and discover exactly what role each piece of hardware is playing.
The Wearable Wellness System uses the Zybo ARM/Zynq FPGA development board, PmodOLED, PmodRF2 and PmodIA to design a system that can detect varying degrees of emotional stimulation. They rely on the fact that Galvanic Skin Response (GSR), also referred to as electro-dermal activity (EDM), increases with emotional arousal. This allows for the potential monitoring of stress responses by measuring changes in skin impedance. As stress increases, sweat glands produce more sweat, which in turn lowers the impedance of the skin. The locations on the body where this effect is most pronounced are shown in the image below.
In order to measure a subject’s GSR, the Wearable Wellness team designed a dual electrode impedance sensor mounted on a bracelet that the subject wears on his/her wrist, taking advantage of the high sweat gland concentration in this area of the body. The sensor is shown in Figure 3. Because impedance is an AC property, it cannot be easily measured like resistance. This is where the PmodIA impedance analyzer comes in. Each SMA connector of the PmodIA is connected to an electrode of the impedance sensor. The PmodIA has an on-board frequency generator that allows it to excite an external unknown impedance at a known frequency. By comparing the known output from the PmodIA to the wave form characteristics of the response signal (magnitude and phase) the impedance of the circuit under test can be determined, which in this case is the human skin. To read more about how skin can be compared to electrical circuits, check out the links I included at the end of this post.
The response signal is then sent to the on-board ADC, and a discrete Fourier transform (DFT) is performed on sampled data. The solution contains both a real and imaginary part which are stored in the on-chip data registers. The magnitude and phase of the impedance can then be calculated from these two data words.
In addition to the PmodIA, the measurement unit (the bracelet worn by the user) also integrates a SHT25 humidity and temperature sensor to account for environmental factors when attempting to determine the user’s stress level. In order to ensure that the measurement unit was small and discrete enough to be worn comfortably by the user, the Digilent chipKIT Cmod was chosen to perform the functions of sampling and sending sensor data to the base station.
Data is sent wirelessly from the measurement unit to what is termed the base station using a PmodRF module, a 2.4 GHz radio frequency transceiver module. Data is received by a second PmodRF located on the base station, and the data is processed and analyzed using the Zybo Zynq-700 ARM/FPGA board. Lastly, a PmodOLED organic LED display module is integrated into the base station design to display measurements of impedance, temperature and humidity. This allows real time monitoring of the subject’s level of emotional arousal. The complete system architecture is depicted in Figure 5.
If you found this project as interesting as I did, please visit the 2016 Digilent Design Contest website to read more about it, as well as to see the other five projects made using Digilent Pmods. All entries listed on the website provide a full project report and source code. There is also an associated Instructable for the Wearable Wellness System project.
Some additional resources on impedance and measuring impedance of human skin: