Embedded Files
Current Source
Several discrete current sources allow setting the current source of the cape from 100µA to 2000µA in increments of 100µA. This is accomplished through specific switching using single-pole single-throw (SPST) and single-pole double-throw (SPDT) switches.
Current Sense
A shunt resistor and a differential operational-amplifier allow for integrated current sensing. Although not necessary during actual measurement, the current sensor is necessary for validation of the current sources and stability assessment.
Multiplexers
In order to configure ground, current, and voltage measurement at particular nodal outputs, multiplexers allow for configuration between 32 inputs and 1 output (or vice-versa). With this, 3 separate multiplexers are utilized to appropriately configure ground, current injection, and voltage measurement at designated sensor nodes.
Power Management
Powerboost 1000C
In order to accommodate integrated charging and voltage regulation, the Powerboost 1000C reduces complexity. This readily available device regulates the voltage of a dynamic 3.7V LiPo battery up to the 5V required for a Beaglebone, while retaining the ability to safely handle the LiPo battery.
3.7V LiPo Battery, 2500 mAh Battery
In order to configure ground, current, and voltage measurement at particular nodal outputs, multiplexers allow for configuration between 32 inputs and 1 output (or vice-versa). With this, 3 separate multiplexers are utilized to appropriately configure ground, current injection, and voltage measurement at designated sensor nodes.
12V Pololu Voltage Regulator
In order to enable the large dynamic range of discrete current steps desired, a 12V power rail is necessary. This rail ensures sequential voltage drops from the current sources does not impact the desired 5V compliance of the sensor nodes throughout operation. I.e. the voltage nodes can see between 0V and 5V.
Software
User Interface
Analog-to-Digital Converter (ADC)
The analog-to-digital converter (ADC) on the custom cape serves to translate all analog signals into processable data by the Beaglebone Black. It can reach a 16-bit precision at speeds of 500kSPS.
OLED Display (64x128)
An OLED display allows for user interaction through a clean and intuitive user-interface (UI). Because the display utilizes OLED technology, it consumes minimal energy allowing for extended operation.
Tactile Buttons
Large tactile buttons provide ease of navigation through the UI as seen on the OLED display. Consisting of a Select, Previous, Next, and Back button, the intuitive layout allows low overhead in operation.
D-Sub Connector
The external D-Sub connector establishes a universal connection to the sensing films. Sensors can utilize this particular D-Sub configuration or they can mate to an external D-Sub dongle. Because the connector is well sealed around its perimeter and mating interfaces, it also prevents dust and other unwanted ingress, protecting the internal electronics.
Micro USB, Type B
A female micro usb, type B port on the outside of the enclosure serves to both charge the device as well as download data. This allows for ease of access to the internal embedded microcontroller without disassembling the device.
Interior
Beaglebone Black Wireless
The Beaglebone Black is the brain of the mobile EIT device. With a 1GHz processor, the embedded Linux microcontroller has 69 configurable GPIO allowing for interfacing via a wide range of serial interfaces. This project utilized both SPI and I${^2}$C protocol in addition to standard GPIO inputs and outputs.
EIT Cape
This custom designed and manufactured printed circuit board (PCB) consists of all the main components needed to enable EIT sampling. Mechanically, the EIT is an expandable header board for the Beaglebone Black, otherwise called a "cape."
Battery Tray
To properly secure the 2500 mAh battery to the enclosure assembly, the 3D printed battery tray provides a form fitting spacing between the EIT cape and the battery itself.
Electronic
EIT Cape
Mechanical
Enclosure
Aluminum Chassis and Lid
Machined from a solid block of 6061 aluminum, the chassis and lid provide structural rigidity and protection to the internal circuitry.
Designed for a simple 3-axis milling operation, the full enclosure can be manufactured with minimal complexity. The lid features the ARMOR logo, which functionally serves as a Radio-Frequency (RF) window to enable wireless functionality. Moreover, the solid aluminum enclosure itself a Faraday cage around the electronics, shielding from external signals, and preventing damage from Electro-Static Discharge events (ESD).
Utilizing the buttons and OLED display, the user interface has an intuitive menu structure. With a consistent navigation language, the user can select various nodal sensing settings. The number of nodes sampled can be set in various common configurations, the current source can be either manually configured or auto set (sets the nominal nodal voltage to center around 2.5V), the sampling pattern can be set to either adjacent or across methods, and the sampling duration can be set to a designated time, number of samples, or a continuous operation.
Data Retrieval
The EIT device captures nodal data through user settings, which consequently configures the C code to interface appropriately with the hardware. With data captured on the onboard 16 GB expandable micro SD card, the user can then retrieve the data via wifi or usb protocol. This allows both wireless and tethered retrieval.
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