P15610: Digital Microfluidics Control System
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Subsystems Design

Table of Contents

The documents for the Subsystems Design can be found here Subsystem Design Documents.

Subsystem Identification

The first step of the subsystem design process was to identify each subsystem and the way that they will interact with each other. This will help the team to determine required specifications for each subsystem, as well as they way that risks to one subsystem contribute to risks elsewhere. Each of these identified subsystems will be independently considered and analyzed.
Subsystem Interactions

Subsystem Interactions

Responsibilities and Subject Matter Experts

Due to the size of the team and complexity of the project, the main responsibilities associated with each subsystem need to be distributed throughout the team. Each subsystem has been assigned to multiple team members as shown below. If the team members need assistance with component design or justification they will seek the identified Subject Matter Expert(s).
SME Matrix

SME Matrix

Subsystem Specifications

In order to accomplish the specifications of the entire system, each subsystem will have its own specifications that it will need to achieve. The following is a breakout of the identified subsystem specifications.
Subsystem Specifications

Subsystem Specifications

Risk Assessment

Risks to the entire system are often the result of failures in individual subsystems. In order to mitigate the risks to the system, risks need to be evaluated in each subsystem. The risks identified for each subsystem are as follows.
Subsystem Risks

Subsystem Risks

Housing

The team plans to purchase and modify several off-the-shelf electrical enclosures, and assemble these individual enclosures into one final system housing. The team approached Subject Matter Expert Prof. John Wellin for assistance in determining an enclosure, and were recommended to consider Automation Direct as a vendor. The current plan is to purchase a quantity of 4 of the SC101204 Wall Mount Enclosures, and utilize these as the base components of the housing. The spec sheet for this part can be found here. The team plans to make a large amount of modifications to these enclosures in order to allow easy access to components and inter-enclosure assembly.

A mockup CAD model of the fully assembled housing can be found below.

CAD Model of Single Box

CAD Model of Single Box

CAD Model of Assembly

CAD Model of Assembly

Two standard CPU fans will be required in order to cool the interior of the housing and protect the electrical components of the system from over-heating. The fan chosen for this purpose can be found here. The fans will be mounted on the back side of the middle and bottom enclosure, as shown below.

60mm CPU Fan

60mm CPU Fan

CPU Fan on Housing

CPU Fan on Housing

The critical components of the grounding system will be a 104-PR-5A Miniature Circuit Breaker. This will provide a central overcurrent protection for all system components. The datasheet for this part can be found here

Circuit Breaker

Circuit Breaker

Test Plan

Parts necessary

Control System & I/O Boards (Top Level Enclosure)

The I/O Board will be split into an Output Board and Input Board, both controlled by the Control System.

The Output Board will be identical to one created by the DropBot team from Wheeler MicroFluidics Laboratory, with the exception that it will switch between two possible inputs: a "write" input, and a "read" input. Control signals from the Control System will decide which outputs (of 40) are provided with these signals. The BOM required for this board can be found here.

The Input Board will attach to all 40 pins of the output board. When a "read" control signal is sent, the voltage on one pin will be passed to a conditioning circuit that uses operational amplifiers to turn the capacitance of the pin (and thus the presence, absence, and state of fluid on the DMF chip) into a voltage between 0 and 5 Volts, which will then be accessed by an ADC in the Control System for measurement.

To provide independent signals for 40 pins without requiring multiple micro-controllers, shift registers will be used. The planned size of the Control System and I/O Boards is shown below.

I/O Board Projected Physical Size

I/O Board Projected Physical Size

To measure the capacitance of an electrode on the DMF chip, a sinusoidal voltage will be applied through the output board. For a non-changing capacitance, the current through the electrode is directly proportional to the capacitance and the frequency and amplitude of the voltage in; this current is then (through the use of the Capacitance Measurement Circuit below) turned into a voltage.

Capacitance Reading Circuit Diagram

Capacitance Reading Circuit Diagram

Capacitance Measurement Circuit Diagram

Capacitance Measurement Circuit Diagram

Control Board

The control board, an Arduino attached to a frequency generator with digital controls, is most straightforward to test by programming the Arduino. First, the Arduino will be instructed to output high voltage on certain pins, and an oscilloscope will measure those outputs. Next, the Arduino will be instructed to control the frequency generator to provide certain signals, and the oscilloscope will be used to assure those signals come from the frequency generator. Lastly, the Arduino's ADC will be fed a stable DC input, and then this value will be read from memory.

Parts necessary

Input Board

The input board controls the measurement portion of the chip and allows capacitance to be directly observed. The side-effect of having this is that the high voltage of the DMF chip can potentially feed into the measurement circuits, which would probably destroy those components. Due to this fact, the input board must be designed very carefully in order to dissipate the high voltage outputted to the DMF chip.

Test Plan

Breadboard a small version of the whole board and test the circuit elements Test single FET with required voltage

Test two FETs with their sources tied together

Test more than two FETs with sources tied together

Test high voltage and high capacitive load on single FET switching into read mode without a high voltage dissipation circuit

Test high voltage and high capacitive load on single FET switching into read mode with a high voltage dissipation circuit

Single FET Test Circuit

Single FET Test Circuit

Preliminary Voltage Test Results

Preliminary Voltage Test Results

Parts necessary

Output Board

The model of the output board is identical to the High-Voltage Switching Board of the DropBot project, available here.

Test Plan

The output board's shift registers will have a very slow (~0.5Hz) clock signal attached to their clock inputs, and one or two bits of the signal input will be high (these will be done with a DC power supply and signal generator). The "high voltage" input (which in the full project will be 120Vrms) will be attached to a steady 5VDC. The outputs will be "chased" by a string of four to six LEDs, to visually indicate that the high voltage signal can flow through the output board to the correct output pins.

If the previous test is successful, the high voltage input will be changed to 5V peak-to-peak AC, and the clock signal disconnected. A voltage divider will be placed on a pin output which receives the high voltage input, and the divided voltage measured by an oscilloscope. The input high voltage AC will be increased, assuring circuit functionality, until reaching 120Vrms. The AC voltage will be provided by a signal generator and a transformer.

If the previous set of tests is successful, a two-input oscilloscope will be attached to the output of one pin output, and the input clock signal to the shift registers. The time between the rising clock and the appearance of sinusoidal DC signal at the output will be quantified and checked to be between 0.25ms and 0.5ms.

Opto-Isolator circuit for HV Output

Opto-Isolator circuit for HV Output

Parts necessary

Amplifier & Signal Generator (Middle Level Enclosure)

The Amplifier and the frequency generator will be similar to the one designed by the DropBot team. The frequency generator is controlled by the Arduino and capable of voltages 20V p-p and frequencies ranging from 100 Hz to 100 kHz.

The amplifier is Class AB, that will provide up to 200 V p-p, be able to drive capacitive loads of 10 nF with a gain bandwidth of 100 kHz.

The Bill of Materials for the signal generator board can be found here.

Middle Level:Amplifier Layout

Middle Level:Amplifier Layout

Frequency Generator

The frequency and amplitude will be controlled by the Arduino, and manually confirmed with an oscilloscope. If the output voltage is too low, the amplifier will accommodate for the loss by increasing the gain.

Signal Generator PCB

Signal Generator PCB

Test Plan

High Voltage Amplifier

Test Plan

Test current limiting circuit

Test linear regulator circuit, the circuit is only active when FAULT input is LOW.

Test the High Voltage Amplifier circuit with input connected to signal generator.

High Voltage Amplifier Schematic

High Voltage Amplifier Schematic

Combine current limiting circuit and regulator circuit, test the output voltage of the combined circuit.

Combine all sub-circuits above to create an amplifier system.

Test the high voltage inverting circuit separately. Output voltage should be in the range of 100 Vp-p to 200 Vp-p depending on the varying input voltage values.

Step Up Transformer(High Voltage Inverter)

Step Up Transformer(High Voltage Inverter)

Combine signal generator, amplifier system, and transformer. Vary the frequency (100 Hz ~ 100 kHz) of the signal generator and test the circuit for alternating output voltages, record data. Calculate amplifier frequency response up to 100 kHz.

Equipment

Power Board (Bottom Level Enclosure)

Power Board Layout

Power Board Layout

Power Distribution Diagram

Power Distribution Diagram

Test Plan

DC Power Supply: Utilizing a computer power supply, a digital multi-meter will be used to manually check the DC voltages at each output. When the PSU is turned on, there will be LED’s indicating which output rails are connected. There are short-circuit safety measures embedded in the power supply that will prevent damaging our devices.

Equipment

Project Plan

The following is the team project plan for the next phase, Detailed Design and Component Selection. This will be the basis of the team's activities for the next 3 weeks.
Project Plan for Phase 4

Project Plan for Phase 4


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