Overview of Project Context
The ARMOR Lab at UCSD is interested in developing densely distributed sensing skins/thin films capable of measuring damage and deformation to material or structure. These nano-engineered films have electrical properties that vary linearly with strain. By measuring changes in the electrical properties, strains and stresses in the material can be determined. The ARMOR Lab has been successful in interrogating the films on the laboratory bench-top with bulky equipment, but to be deployed in the field (e.g. a Kevlar vest, prostheses) the data acquisition system needs to be smaller and portable. This is the challenge for our group. We have been asked to design a small, battery-powered system capable of providing a constant current to the material, measuring voltage across an array of sensing nodes with reasonable fidelity, and storing the data for later extraction and post-processing.
Project Objectives
High Priority
1. The data acquisition device must be completely portable, capable of operating off a battery for 20-30 mins of measurement time.
2. The device must be able to inject a stable 0 - 2mA, DC current into the thin films while also monitoring and recording this injected current.
3. The acquisition aspect of the device must be capable of measuring 12 nodal voltages sequentially at a rate of 10Hz.
4. Data acquisition must alternate nodal measurements automatically, independently measuring each voltage node once for a cycle of measurements.
5. Nodal ADC measurements must allow a .5V range within a 16 bit accuracy.
6. Data acquisition will be stored locally on the device until retrieval.
Secondary Objectives
1. The device ideally will have a form factor similar to that of a phone in X and Y dimensions, but potentially larger in the Z direction.
2. The device should be robust against general field handling (dropping, transport, etc.)
3. The device can have a higher sampling rate, for greater time response granularity.
4. The device can be capable of a bit accuracy greater than 16 bits with the ability to measure a .10V range.
5. The device can be capable of greater than 12 measurement nodes.
Other Constraints and Objectives
Other constraints on the device include: power limiting circuitry, capable of preventing damage to the equipment in the event of a user error, the ability to use a simple AC/DC power supply to charge the device along with battery regulation circuitry necessary for maintaining the integrity of the battery, easy nodal connection hot swapping via a simple external connection (ribbon cable, etc.), and no disassembly required to upload data.
WOW Design
The ideal design for this project is an incredibly robust and accurate data acquisition device. This device would be capable of easy transport and charge on the go, capable of storing vast amounts of data, and capable of probing multiple film skins simultaneously. With a simple, easy to use interconnect between the device and the nodes, the DAQ will probe 16 nodes at a high sampling frequency within a 16 bit accuracy. Nodes can be easily and consciously probed, with feedback given by the system on current readings, battery life remaining, and other metrics of performance relevant to the user. The device should be fairly low cost, allowing for a larger scale deployment if desired. Controlling of the sensor measurements would also be variable, subject to the user's input and desire.
Risk Reduction Objectives
Initial team discussions concerning development of the War Fighter portable sensing node have focused primarily on the development of a portable current supply and data acquisition device. This is a major concern and an area of substantial risk for the project since both components are instrumental in providing proper function of the sensing skins. Kenneth Loh and his group have successfully performed real-time data acquisition on the sensing skins using bench-top power supplies and a PC. While this provides a great starting platform, moving to a portable system that is reliable and compact is going to be difficult. In order to mitigate risk and ensure the project moves forward in a timely manner the following risk reduction objectives will be carried out during the last three weeks of this quarter:
1. To determine full design constraints
• We need to figure out acceptable current tolerances for the current injection source.
• We need to discuss acceptable operation time and tolerances for device
2. To develop a reliable battery operated current source
• Capable of reliably providing up to 2mA of DC current for 10s intervals
• Building a breadboard prototype of this current source circuit
• Battery monitoring and charging will be figured out later (lower risk)
3. Source a compact micro-controller platform that can be expanded
• Capable of memory storage
• 16 or 24 bit ADC
• Multiplexing capabilities
Intermediate Milestones
Weeks 8-10 of Winter Quarter
1. Decide on a platform for data acquisition
• How many channels are required for the ADC to effectively sample at least 12 discrete nodes within the given sampling frequency requirements.
• Decide on the micro-controller platform to use.
• Decide on the battery technology to power the DAQ.
• Decide if we are going to do DAQ switching between nodes internally on the micro-controller board or externally with a switching circuit.
Weeks 8-10 of Winter Quarter
1. Decide on a method/circuit to use for the required constant current injection
• Decide on a power source (battery directly or DAC) to power the current injection within the desired operating tolerance.
Weeks 1-3 of Spring Quarter
1. First Prototype
• Design the first preliminary housing for the system including all desired functional electrical components.
Weeks 3-7 of Spring Quarter
1. Iterate on and validate design for prototypes with actual films
• Potentially work to consolidate circuitry to custom PCB design if required for space.
Weeks 8-9 of Spring Quarter
1. Final design validation and proof of successful operation
Individual Component Analysis Topics
Current Source Circuit
Because the current source is an integral component in the fundamental observation of these films/skins, the requirement to maintain a very stable, low amperage current despite the load circuit requires more convoluted circuitry. This will likely involve some form of feedback, as in a Howland current source circuit.
Data Acquisition
As the major functionality of this device, analyzing the necessary requirements for data acquisition, the different technologies available for compact acquisition and storage, and the trade off between resolution and sampling rate must be fully vetted in order to select our micro-controller platform.
Power Management
Because the device will be portable, the power management circuitry will play a key role in the fidelity of the system. Namely, we must analyze the battery technology requirements along with the mitigations necessary to cope with changing voltage in the batter over time (i.e. maintaining accuracy of the system).
Material/Enclosure Robustness Requirements
With the device deployed in the field, it must be capable of withstanding transport and potential impact events without compromising the system’s integrity. Thus, material mechanics and selection will play a key role in the design of the system enclosure.