Service-Learning Model: An Example

This article is a follow-up of the previous "A Service Learning Model" post. The model suggests interdisciplinary engineering students working on real-life projects that benefit the communities.

Earlier we talked about how this model is useful in three ways:
1. BITSians learn to work on real-life projects that have customer requirements, deadlines and the most essential aspect of engineering: design, build and test.
2. The needy communities benefit from the delivered products, so BITSians help the society around them.
3. BITSians make industry connections so that they share a great relationship at the time of internships, placements.

As an added advantage, such project provides a great platform to strengthen students' skills and resumes. This model, truly has an enormous potential to become one-of-its-kind in India to be implemented exclusively by BITS, just like our renowned Practice School program.

Now, let's consider an example of such a project:
   
Primary/High school children learn basics of physics, biology and solar system roughly in their 1st to 7th standard. It'll be an interesting exercise to create models to teach them basic concepts through interactive mechanical models with which they can play around. To take an example, one project can be to demonstrate operations of a biological cell with the help of a workshop-fabricated model with microcontroller-based sound system, LCD displays and rotation mechanism. This model can be an excellent exhibit in a local science museum. This will benefit the children around BITS campus.
  
 Let's break it down to see what the project components are. Firstly, this project has a great interdisciplinary aspect to it. BITSians from various engineering and science major can contribute. The mechanical subteam can handle: the choice of material, fabrication of the model, fitting various parts to it and mechanical movement capabilities. The electrical subteam can take up microcontroller programming, motor and LCD interfacing and power management. The biology subteam can help with the biological processes within a cell and help draw a skeleton of the model and its intended functionalities. Folks interested in project management can work on the plan, timeline, keeping everyone on track and community/industry communication. They can also apply for industry grants and gather support from local administration.

----There are tons of exciting tasks with each project and each task cultivates a skill that is very useful in post-graduation (industry) life.

More on this model with some more examples in future posts.... Stay tuned!


 

The Electric Motor

Hey Guys,

The third most important part of an electric vehicle is the electric motor. There are tons of books and online material on electric motors and hence I will not go into the mathematical details of it. You guys will learn the basics of electric machines in your ES2 (Electrical Sciences 2) course and I urge you to do that sincerely (even if you do not like the text book! :))

Today I will try to give you some pointers on electric motors so that we can  directly jump into modeling of all these components

You may want to be conversant with the following if you want to excel in this field

1. AC Induction Motors (3ph, Single phase)
2. Brushed DC motors and Brushless DC motors
3. Permanent magnet motors
4. Differences between the above three
5. Theory of Induction (stator/ rotor construction, importance of air gap, winding patterns etc)
6. Equivalent circuits of induction motors (very important)
7. Finite Element analysis of Motors (to calculate fundamental parameters like resistances and inductance which are used in motor control)
8. Motor Control Basics

A few of the good resources that I know are:

1. MIT Open course ware for Electric machines:
2. NPTEL: Electrical Machines 1, 2,3courses: 

     Books:

1. Electric Motors and Drives by Austin Hughes
2. Induction Motor Control Design by Riccardo Marino et al. (advanced book)
3. Motors and Drives (A practical Technology Guide) by Dave Polka  

I would say this should get you started on electric motors. Your aim should not be memorizing equations and solving problems, but to understand the induction concept and modeling a motor in MATLAB. I will briefly go over MATLAB modeling later

The Motor Controller

Before we looked why electric vehicles are important, the architecture of an electric vehicle and started looking at the components of the electric powertrain. Last time we saw the battery management controller and its functions. Today we will glance over the motor controller.

Similar to the BMS (Battery management system) which manages the interaction of the battery with various other components of the powertrain, the motor controller manages the interaction of the motor with the battery and rest of the vehicle. In most of the electric vehicles the motor used is 3ph AC induction motor. You should be studying this in your ES2 (Electric sciences 2) course in second year. AC motor is used because it has many advantages, the most important being high efficiency and low maintenance. Now this being a “3ph-AC” motor, it requires 3ph AC current whereas the battery outputs DC current. Hence there is a need of an “Inverter” in between the battery and the motor which converts DC current into AC current and the role of the motor controller is to control the functionality of this inverter.

The control action

The most important function of the motor controller is the 3ph AC current generation. Without going into detail mathematics, I would say that this is achieved by transforming the  current vector by a series of transformations, controlling their magnitudes by tuned controller gains and feeding them to the IGBTs of the inverter to convert them in 3ph-AC. 

So to be more specific the motor controller does the following things

1. It measures the fed back current and the speed (RPM) of the motor via appropriate sensors. 
2. It takes in these values and applies a few transformations transform these quantities into quantities that are easy to control. The controller uses Clarke and Park transformations.
3. The PI (proportional-Integral) controller compares these values with the ideal values which are generated from the pedal position. (More the pedal is pressed, more velocity/ power is requested and hence more magnitude of AC current is needed). 
4. This controller outputs corresponding voltages that would be required to generate appropriate PWMs for the 3 phases. 
5. These voltages are then passed in a SVPWM (Space vector PWM) algorithm coded inside this controller which gives out the 3 ph PWM signals, which are in turn fed to the IGBTs with the DC current from the battery to generate the correct 3ph AC currents. This current is then fed to the motor windings to generate torque.

Hence the motor controller is a closed loop controller which ensures the correct 3ph currents are generated to achieve the requested torque and power levels.

If you guys want to go deeper into the design of motor controller you should be aware of the following things

1. Clarke/ Park transformations
2. Feedback controller design (PI, LQR)
3. Space Vector PWM techniques
4. Inverter (IGBT) Electronics
5. Field Weakening algorithms etc.

I will try to give you brief information on these to give you a head start for the intereseted. Tons of information is available on motor control techniques online. This is a very well developed area and it should not be difficult to find research papers on this. I think Texas A&M university in the US has a good research lab on motor controls.

April Recap!

Hello Again!

We have come to the end of April and from this month I will try to give you a recap of what we saw in the current month and a sneak peak of what’s coming in the month ahead. In this case it will be easy for you to browse through the blog and pick only the right articles you need :) and filter out the rest!

We would like to provide you guys some insights on whats going outside and some handy tools which could help you. Hence your feedback is certainly appreciated. You can email us or just comment on the blog about the things you would like to see on it and anything specific you would appreciate. And even those things that are boring! We will try our best to keep the variety and answer to all of you that communicated with us. ...and its free! :)

You all can get in touch with me on bits2world@gmail.com.

So, in the month of April we focused on variety of areas:

1. Electric Vehicle Infrastructure: We looked at how electric vehicles are going to become important in future. We glanced over the basic architecture of an electric powertrain and its components (link) and then looked at the functions of the most critical component a.k.a the battery management system (link). For you guys, it will be definitely beneficial to be aware of the latest developments in this domain as this will be an important area in future where companies will be moving. 
2. We looked at some random stuff on electronics and how various courses from EE can help in various fields in electronics. Mandar also gave a general advice on how you should consciously build your profile in electronics and what are the good-to-have tools with you (link). We also linked you to a tool which could come handy in circuit simulations for all you EE guys out there (link). 
3. Service Learning Model (link) was something different and peculiar in the sense that interested students can take up engineering projects for community welfare. This happens in US and a profile in such things could be a gateway to a research lab.. who knows! :)

Lot of stuff. So what’s coming next! Let’s have a sneak peek into the month of May.

I plan to continue giving you inputs on various components of electric vehicle. We will look at the motor, the inverter and its controls. We may also look at modeling such components using industry standard tools and techniques.

Neeraj will try to give you a few more inputs on the Service Learning Model and may be a few project ideas that you can carry out.

We may introduce you guys to management! :) I know.. half of you don’t give a crap about engineering, but have tremendous ability to manage and schedule projects and events..to make the engineers work!..haa!. Let’s have some inputs on that side too from some managers working in the industry.

Looking forward to hear from you!

The Battery Management System Design

I will try to glance upon the different important controllers of the electric powertrain system. Let's start with the battery controller a.k.a battery management system.

The brain of a pure electric vehicle is its battery management system. I will briefly go over the responsibilities of this controller in this article so as to give you an idea of what all this battery management system (or BMS) constitutes of. This could prove as a background in case you dive into this field in future.

As described in my previous article the battery constitutes of cells in parallel and series. There could be thousands of cells in a car battery which together produce hundreds of volts and a significant amount of current. Critical parameters of each cell are its voltage, current and temperature. Any one of these if goes out of control, can lead to a thermal runaway which can lead to an explosion. The entire battery is kept in control by its battery management system. The BMS is responsible for variety of function some of which are mentioned below.

State of Charge Estimation:

The BMS is responsible for estimating the charge remaining in the battery and hence the range. Hence the more effective the BMS estimates the range (taking into account factors like terrain, temperature, weather etc) lesser will be the range anxiety of the customer! There are various techniques used to calculate the state of charge in a battery (like coulomb counting etc)

Temperature/ Voltage/ Current Sensors:

It is very critical to measure voltages, currents and temperatures of all these cells. How these three quantities are measured depends on each manufacturer of the battery pack. Usually there is one current sensor measuring the current that the battery consumes or delivers. There are multiple voltage and temperature sensors at various locations to measure these cell variables at throughout the battery. All these are inputs to the battery management system (BMS) and the control logic in the BMS makes sure that these stay in bounds.

Battery Cooling Circuit:

It is also critical to sufficiently cool the battery. In an electric vehicle, batteries are generally cooled by separate pumps or via the radiator. The inlet and outlet temperatures are measured by two thermistors and are fed to the BMS which in turn controls the cooling fluid.

Contactors:

Contactors are nothing but switches that connect the battery to the motor/inverter or charger. Battery contains high voltage all the time. But when a car is not being driven, it’s not necessary to supply this voltage and hence current to the inverter. When a person puts the car into a state where he wants to drive it or charge it (where either the battery outputs or consumes current), these switches close and the loop (battery-charger or battery-motor) is complete. BMS decides when to close the contactors and let the battery drive or charge the car

Safety Monitoring:

BMS is also responsible for keeping the battery in safe condition. In any unsafe condition the BMS will not close the contactors and hence the high voltage will remain contained in the battery itself. Unsafe conditions can include any of the following

1. Over heating of the battery, over/under voltage and over current conditions: BMS open the contactors and isolates the battery from the car if any of the voltage/current/temperature sensors report the above mentioned conditions. 
2. Over charging: BMS stops charging if it detects an over charged cell. Overcharging can significantly reduce the battery health and lifespan. 
3. Unequal charging of the cells: BMS throws appropriate alerts if different cells are charging at different rates. It is also responsible for discharging/ bleeding the over charged cells (if any) and maintain the charge of all the cells to same level. 
4. Over discharging of cells: Over discharging cells also create problems for battery health. BMS prevents that. 
5. Grounding/ Isolation problems: BMS always keeps checking if the battery is properly isolated from the vehicle of not. In case of a short/ thermal runaway BMS makes sure that the damage is contained in the battery and does not propagate into the car. In case of a crash (head on collision) the BMS has provisions to cut the current supply from the inverter and isolate the battery.

For each bullet mentioned above, BMS has its own state machine and probably one person dedicated for its design in the industry! 

So what’s there for you guys here: Mechanical engineers play a crucial role in conceptualizing and modeling the BMS. EE, E/I engineers are generally responsible for designing a PCB that functions as a BMS controller while software engineers write the firmware that does the control. Hence it’s a super interdisciplinary board in an electric car.

I will finish with an interesting video clip explaining the BMS in short. Hope you like it :)

Interesting video clip on the BMS (Click Here)

  

Of Negative Frequencies

A question is often (well at least once in each DSP course) asked by DSP students. (Quoting from Quora).

What negative frequencies actually mean physically?  Considering the fact that bandwidth is specified by the positive part only, do negative frequencies exist or are they just a mathematical side effect?

Here's my answer, with some edits.

Consider a sine wave. What is its Fourier transform?

Fig. 1  A sine wave and its Fourier transform (Image source)

A negative ordinary frequency of $-10$ does not mean a sine wave oscillating at $-10$ cycles per second. It refers to a complex exponential $e^{-j2 \pi 10t}$ When we add another exponential with positive exponent $+10$ we get a real sinusoid which oscillates at $10$ cps. This also explains your bandwidth confusion ("Considering the fact that bandwidth is specified by the positive part only.."). If by some magic, the formula for addition of complex exponentials were $e^{j\omega t} + e^{-j \omega t} = 2 \cos \sqrt \omega t$  then bandwidth would have been specified by the "square root of the positive part".

For real signals, these "negative frequencies" do not occur alone and always are paired with a corresponding "positive frequency" so that the resultant signal is real. The impulse which you see on the negative X axis in the right hand figure does NOT represent a sinusoid but a complex exponential. This is what one may lose sight of while talking about negative frequency.

The Electric Vehicle Architecture

Hey Guys,

Last time we looked at the fall and rise of electric vehicles. I realized that there is a decent interest amongst the student community to learn more about electric vehicles. I am also very glad that there are groups in BITS-Pilani Goa campus working on electric car prototypes and I am sure other campuses would also be making such contribution. I strongly believe that such experience in college will definitely benefit students who are interested in pursuing career/ higher studies in this domain or even in fields like power electronics/ motor controls and firmware for automotive powertrain systems.

For the people out there who are not really up to speed but are super interested and would like to get into this, today I would like to give a brief architectural overview of electric vehicles. The major difference between gasoline and electric vehicles is that there is no internal combustion engine in electric vehicles. This is replaced by an electric motor (AC or DC) and this motor is charged by a battery on the car. So, as gasoline engine cars require petrol or diesel as a fuel, these cars need charge or current. People who know the complexity of an engine would instantly realize the simplicity of electric car.

As you can see in the figure below the electric car consists of 4 major components.

A Battery: A battery is the heart of the car. This is the energy storage device (analogous to fuel tank in normal car). This stores DC voltage when it’s charged. The voltage levels are generally in 100s of volts (~400V). This battery has certain number of cells in series and parallel. The number of cells in series and parallel is decided on how much voltage (~range) and how much current (~power) you want in your car respectively.

A Charger: (not shown in the figure) It is used to charge the battery up to the desired voltage. This takes in AC voltage from the wall, rectifies it and then charges the battery. The charger is rated at certain KW which determines how much current it can consume which in turn dictates how much time you will need to charge the battery. You will study the concepts used in charging in ES1.

A Motor: Usually a 3ph AC induction motor is used in electric cars (due to efficiency/ reliability reasons).It takes in 3ph AC current and produces torque due to magnetic induction across the stator and rotor. The shaft of the motor is connected to the halfshaft of the car through a reduction gear box. You guys generally study these in ES2.

An Inverter: Inverter is a power electronics device that is needed in between a battery and the motor to convert the DC current that the battery produces to AC current that the motor consumes. It has IGBTs as switches which switch at high frequencies like 10KHz while converting DC to AC.

Having said this there are a couple peculiarities of electric vehicles:

Regenerative Braking: When you apply brakes you are essentially applying negative torque to the motor to speed it down. That means the motor consumes negative current which you can visualize as a current going from motor to battery through the inverter. This results in charging the battery. This phenomenon makes electric vehicles very attractive in the sense that you can recharge the battery using the energy spent in braking.

Max torque at zero speed: If you see the motor torque speed characteristic as shown below you will realize that we can extract maximum torque from the motor even at zero speeds. This means that we will not need a gear box to shift to the right gear to attain right amount of torque from the motor at various speeds (as we require in an engine). Hence you can achieve high amounts of accelerations/ power from the vehicle from dead stop positions.

Below is a very interesting link of a drag race between BMW M5 (known for its acceleration) and Tesla ModelS (electric sedan company that I work for :)). See how Tesla gets a head start due to full torque at zero speed!!

Drag Racing: Tesla ModelS vs BMW M5!!