Projects

In a stationary plasma thruster, the anode unit (also called a discharge unit) propels the ions of inert gas by applying the electric field in the channel. This electric field is generated by a high-power isolated DC-DC converter which boosts the battery voltage in a satellite.

In the electric thruster, a stream of ions leaves the thruster which is needed to be neutralized for stopping the net charge build-up on the spacecraft. For this purpose, a stream of electrons is supplied by the thruster's cathode unit. The cathode is heated to a high temperature as it works on the thermionic emission principle. There is a free space between the cathode (ejects the electrons) and anode unit (ejects the ions.). To start the thruster, the electrons are needed to be sucked out of the cathode body. This is done by applying a high voltage pulse just in front of the orifice of the cathode. This high voltage pulse is supplied by a DC-DC converter.

Once the thruster starts the plasma path is established for the electrons to move, however, the temperature of the cathode is still needed to be maintained. Using the heater, for maintaining the temperature is very inefficient. The most popular way to do this is to form a plasma in the cathode by injecting a small amount of the inert gas. The electrons passing through the cathode hit the gas which generates the plasma. The ions in the cathode plasma will hit the walls of the cathode insert and maintain its temperature. The minimum current required to achieve the thermionic emission condition depends on the type and the geometry of the cathode. If the required current is more than the anode current then the extra current is supplied by another constant current DC-DC converter called Keeper. In high power SPT, the keeper may not be required at all especially in the case of a barium oxide impregnated tungsten cathode which has lower work function compared to LaB6 cathode.

For a fixed amount of fuel, the velocity change in the satellite/rocket is proportional to the exhaust velocity of the ions. The exhaust velocity can be increased by increasing the anode voltage which is responsible for pushing out the ions.

The output voltage of the anode power supply is limited by the breakdown voltage of the rectifier diodes. Wide band-gap diodes are still not mature for space applications. Seriesing multiple converters is one of the ways to increase the voltage, yet it is not an optimum one. Therefore, for this project, a novel method is used to scale-up the output voltage while achieving soft-switching conditions for the switching devices.

In a switched-mode power supply, the switching losses are the main contributor to the power loss. All the transistors have a finite response time for state transition. During the state transition, the voltage and current of the transistor are not zero altogether which leads to power loss. Transistors have a parasitic capacitance at the gate terminal which needed to be charged/discharged in order to turn on/off the device fully.

In comparison to the MOSFETs, Gallium Nitride FETs are smaller in size, hence, they have lower parasitics. They can be switched at a very high frequency leading to a very significant size and weight reduction which is of utmost importance for a space mission.

The GaNFETs have already dominated the industry, however, the Radiation-Hardened GaNFETs have been released very recently (by Renesas). In the near future, we will be able to see next-generation power converters in the satellites.

The propellent is always fed to the thruster at constant pressure and flow rate. Propellent is stored at high pressure in the tank. A pressure regulator's job is to reduce this pressure to the required level and maintain the pressure.

The course correction in the pressure is done by repeatedly turning on/off a solenoid valve in a close loop. The fine correction is done by a piezo-electric proportional flow control (PFCV) valve. Solenoid valves are controlled by applying forward and reverse voltage pulses to the coils, it can be accomplished by using an H-bridge circuit. Because piezo valves require high voltage for operation & act as a capacitive load (very low power requirement), a flyback converter is an optimum choice.

These pressure regulators are intended to be used in electric propulsion systems and cold gas propulsion systems.

According to Maxwell's equations, a changing electric field produces a magnetic field, and changing magnetic field produces an electric field. Therefore, in a plasma state, the movements of the charged particles are governed by the electromagnetic forces induced by themselves. From the above argument, we can guess that there are many types of instabilities in a stationary plasma thruster (SPT) also.

The movement of plasma in SPT reflects as discharge current oscillations which in turn leads to the voltage oscillations. These oscillations needed to be maintained within a specified limit for the thruster's operation. The most dominant part of these oscillations is called Breathing Mode oscillations.

There are two things that take place in the thruster. First, neutral xenon gas coming out of the anode channel gets ionized, and second, these ions are pushed out of the channel by the electric field. The discharge filter has an LC component or these oscillations are taking place at a high frequency so the parasitic LC of the harness itself will play a role in governing these oscillations. When the ions leave the channel in a large quantity, the current is supplied by the Capacitor. The voltage across the capacitor, hence, the voltage across the anode reduces. The charging of the capacitor is delayed by the inductor. During this period, due to the smaller electric field, the ions leave the channel at a slower rate resulting in a current drop and accumulation of the charges. Once the voltage increases, the ions again leave the channel in a large quantity repeating the process. This is the process behind the breathing mode oscillations. It is explained by the predator-prey cycle which exists in nature and keeps the population steady.

There are two jobs for the discharge filter, First, it needs to reduce the amplitude of the current oscillation. The current fluctuations can be reduced by tuning the phase of the filter. Second, the thruster will quench if the voltage across the anode unit becomes too low. To avoid this, the value of the capacitor needs to be selected accordingly and the filter needs to be placed near to the thruster as close as possible.

An electromechanical actuator with built-in digital drive electronics is advantageous in terms of size and controllability. Drive electronics uses Texas Instrument's TMS-320 digital signal processor and an H bridge power amplifier. The control electronics is retrofitted into the existing actuators.

The checkout system uses a digital oscilloscope to capture the test data. This test data is then post-processed using Python. It can automatically create the plots of interest out of the large quantities of the data. It can also highlight any kind of anomaly, the deviation of the parameters from the nominal, generates test result report, and stores the useful data only. All the results are displayed on a visualization dashboard web-app.

Fine-tuned and trained the Inception V3 Deep Convolutions Architecture for image captioning project and gained theoretical & hands-on experience on Convolutional Neural Network, word-embedding, and Long-Short-Term Memory Network.

Among the several approaches to design a depth camera, Time to Digital Converter (TDC) approach is the most promising one. In this method, a short light pulse is sent by the transmitter and its reflection is detected by a photo-diode. The total time of flight is directly converted to binary output by a Time to Digital converter in the pixel itself.

Time-domain Successive Approximation TDC architecture is designed for this project. It uses some predefined delay blocks designed by using current starved inverters. The delay is stabilized using Delay Lock Loops. Path swapping is used to make the delay blocks immune to the temperature and parasitic variations. A Corner simulation has been carried out for the TDC and its layout is generated.

The continuous level sensor available in the market are either expensive or does not offer a significant range and resolution. For smart agriculture applications, especially in India, if the cost of the device is going more than 5$ then the farmers will not utilize it. I have built an IoT-based level sensor (resolution 1cm & range - 1 m) that can be used as a flow meter (resolution can be configured) under 2$. The readings can be accessed on the cloud. The sensor can be internally triggered periodically or it can be triggered externally to reduce the power consumption. It has been deployed in the field.