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!!

BITSAA Talk on 7th March at BITS Goa

I have had a few requests for the slides from my talk on 7th March. For whatever it's worth, they're embedded here.

In short, the message of the lecture was supposed to be --
Try to make positive contributions in whatever research discipline on campus is nearest to your interests. Co-operate and help your fellow students. Most of all, don't let the small-mindedness, petty quarrels, and unnecessary one-upmanship which sometimes plague engineering campuses drag you down.

If that message reached five people out of the group that attended, I would consider it a success and a favour to me.

A cool web-based circuit simulator!

Here's a simple circuit simulator that is browser based. No need to download anything, just draw a circuit and it's ready for testing!

https://www.circuitlab.com/

Simple and quick, isn't it? 

A Service-Learning Model

A learning model that is increasingly becoming popular among educators, is service-learning based engineering education model. This model has a tremendous potential when applied to engineering curriculum in developing nations such as our country. Our nation has its own social and economic issues. A small percentage of people are both socially aware and financially capable so that they help communities in their humble ways.

How can we, engineering students, help solve such problems? How can we help underprivileged communities?

The answer comes from the fact that the communities are facing problems, some of which can be solved by applying smart engineering solutions designed by students as they learn with the guidance from people from academia and industry. 

To elaborate on this, let's discuss three critical elements of this model:

1. Community problems: These are public/private institutions that help communities with their services. Some examples are: public libraries, night schools, science museums, construction workers' associations etc. They face a number of problems and they have many agenda items to improve the quality of life of their members. We, as students, can identify problems that can be converted to small engineering problems and with the help of faculty, industry people, can  work on solving them. Some examples of such problems could be: designing a software database for library books to be accessed via the Internet (CS, IS), designing a solar power and storage system to be applied for night schools (EEE, EnI), building science/biology demonstration models for school children (ME/BIO),  building battery-operated chairs for handicapped (ME,ECE,EnI).

2. Engineering solutions: While solving such problems, we learn actual implementation of the theoretical ideas learned from the courses. Not only the technical knowledge, but we learn a great deal about team building, finances, public relations, and most importantly professional approach to solving problems. Applying such practical solutions to real problems makes this model an extremely powerful tool. We can take those projects as LOP,COP to get suitable credits for our work. When we deliver the product to communities, we actually contribute serving the communities!

3. Industry reviewers: Our industry is more than willing to donate money for social welfare. They need a proper established channel. Funding such projects can be an excellent way not only to empower the communities by the delivery of products, but to help engineering students learn practical tools along the way. Industry can send the reviewers to critique the design and monitor the progress. This relationship is extremely helpful in terms of getting funded projects, practice school stations and finally placements!

This model is developed by educators in Purdue University, West Lafayette, IN, USA and is currently running successfully since last 10+ years (https://engineering.purdue.edu/EPICS). It has numerous imitations all over the world. If the BITS administration decided to apply this model, they will not only increase the level of engineering education, but help serving communities across the nation.

As students at BITS, we can go and talk to communities and take up small projects, work on it and make a difference as well. This will be our share of service to our nation while adding value to our own engineering education and opening doors of countless opportunities through industry contacts.  

The Electric Vehicle Revolution

Technology is changing. Horses transformed into gasoline cars. After a century domination gasoline vehicles are now changing into hybrid vehicles and within few years a new era of pure electric vehicles will become mainstream. Hybrid vehicles consist of an additional prime mover along with the engine. It can be an electric motor or a fuel cell or even a CNG gas tank. Pure electric vehicles on the other hand completely replace the engines with an electric motor.

Who Killed the Electric Car?

Electric vehicles are not new. More than a decade back General Motors (GM) came up with a vehicle with an electric motor in it instead of an engine. They called it EV1. The technology was new, the market and people were not ready for it. GM saw the future. The EV1 was made available through limited lease-only agreements. This was more for the “early adopter” to “try out” this new technology. People liked it. Obviously they would.. zero emissions, good torque and a super silent car.. why wouldn’t anyone like it!

But then, what happened. You can see the picture below. GM said,  “Naah! This is a totally unprofitable niche of the auto segment. Not worth investing in it” And they literally crushed ALL the EV1s produced. To your surprise they even took away cars from the customers that wanted to keep them desperately (at any cost). All cars were crushed, stock piled and the program was closed!

EV1 Details

The revenge of the Electric Car

So what happened back then? The market was not ready? The technology was not ready? The infrastructure was not ready? Or was it just the mentality of the automakers that were used to producing gasoline cars for nearly 100 years? There were many such reasons for the death of the electric car. But now emission restrictions are becoming more stringent. 2025 US emission restrictions would definitely force the auto OEMs to produce at least hybrids if not electrics. Electricity is getting cheaper. Electronics is encroaching in the auto industry in every segment, battery technology has improved from lead acid to lithium ion, motor controls have be thoroughly developed in last decade. All these small factors are gearing up and coming together for an electric revolution. Each auto OEM, if you look closely, has at least one hybrid variant (Toyota Prius, Ford Focus etc). A couple of them have pure electric variants (Nissan Leaf) and there are also pure electric companies getting built from ground up (Tesla Motors, Mahindra Reva etc) producing only pure electric cars.

So.. the future is electric. And you guys should gear towards it if you are inclined towards automotive.

What you need to be up to date!

Automobiles now, along with Mechanical engineering, are heavily dependent on Electronics and Electrical engineers. Good EE and ME background, solid conceptual understanding definitely helps. But to be specific for this domain you should be aware of basic powertrain components along with obviously the chassis (vehicle engineering) and the body (design and styling) of the car. Powertrain components here are the electric motor, the inverter and the battery pack. Decent understanding of modeling these components (systems and dynamics), controlling these components (control systems) coding firmware (Embeded software development) for operating these systems and diagnostic/ safety related point of view will be super helpful.