PITSTOP | Jan’16
Contents 4
FS Experience Design & Improvements: Mechanical
8
Electronic Differential
9
Rocker Assembly
10
Gear Box
11
Steering Assembly
12
Brakes
13
Wheele Assembly
14
Cooling System
15
Tyres
16
Side Pod
Design & Improvements: Electrical 18
Battery Management System
19
Low Voltage Safety
21
Data Aqusition
20
CAN
3
4
Maithili Patel
Design Engineer, Powertrain visited FSUK’15
FS
is the most well-established motorsport racing themed design competition that numerous universities from across the globe crave to ace. It was bound to be a one-of-its-kind experience and so had been repeatedly expressed by all those who had been there. This intrigued me and with high hopes I started off to UK. Albeit all the alluring words I had heard, the experience there was way larger than I had expected. Our trip started with a stay at Hatfield, Hertfordshire, where University of Hertfordshire was going to host us for the pre event prep. An excited bunch of 9 of us reached there 2 weeks prior to the competition all bucked up for the finishing touches and testing. However, this excitement soon turned into concern as we uncrated the car to find it drenched in water along with all the electrical components. The following couple of days were cascaded with findings of more and more components that needed to be repaired or replaced, the most important one being the motor controller. We however held up our spirits and faced all hurdles as they came by. Rushing to the lab early in the morning and return-
ing past midnight in the chill had become everyday business, and surprisingly I enjoyed it thoroughly. We successfully managed to fix the car and and get it up and running with continuous effort and a lot of help from the UH Racing team. The two weeks probably gave me more practical experience than the entire year spent behind the car had. The stay at Hertfordshire gave a hint of the competition lying ahead, but the moment we touched Silverstone, the enormity of the event and the sheer number of motorsport enthusiasts were bewildering. The campsite filled with hustle as the teams pitched in and the paddock bustled as all teams set up their pits readying for the event. The shiny masterpieces of race cars in the adjoining pits and the atmosphere was exhilarating to say the least. The top teams of which only photos, articles and newsletters I had seen, being there right in the next pit with their brilliant creations felt like fantasy stories coming alive. The technology these teams put into reality is the kind no-one would imagine students to achieve. A glimpse of the competition and every last second put into designing, building, assem-
PITSTOP | Jan’16
bling, testing and repairing the car seemed worth it. The next couple of days were for scrutineering and other qualifying tests and presentations. We finished with most of the scrutineering and rain test with ease on the first day itself. Next were the cost presentation and the design presentation where the design judges were happy with the numerous innovations we attempted at. Then was the brake test, our only hurdle on the way to dynamic events. We all went there with high hopes as well as little apprehension as we could not do brake tests due to the glitches at Hertfordshire. Our apprehension actualised as the car just refused to brake. We took it back to the pit and worked hard bleeding brakes and troubleshooting possible issues. As time progressed we were missing one after the other event; acceleration, skidpad then autocross. We gave it one last shot before the night clutching at the reminiscent hopes of participating in endurance the next day but luck wasn’t on our side and the brakes just refused to lock. Here ended our journey of FS 2015. This dampened our spirits. However the endurance event was going on and we joined in as the
stands filled to see the top teams compete. Seeing these beasts run and manoeuvre perfectly around those tight turns and slaloms on the damp track set us awe-struck and also inspired us to level up to them the next time. The whole 3 weeks of the trip are etched into my mind with its highs and lows, the pressure, excitement, nervousness, challenges and most importantly the thrilling air of Silverstone UK. The entire event unraveled at a lightening pace as all the teams, be it the best ones or the first timers, fought for acing each event - many of them going down at every stage, while others emerged victorious - and in a blink of the eye, the four days were over. The competition taught a lot about not only the technical aspects, but also the importance of team-work, presentability, perseverance, organisation and working under pressure. The FS absorbs you into sleepless nights full of efforts put into building a single prototype for a year full and leaves you gratified for all the toil and trouble it got you into!
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8
Electronic Differential by Maithili Patel
P
roper handling and monitoring of cells of Ltium polymer chemistry is very essential for a smooth running car. The Battery Management System (BMS) manages the safety parameters, that is, cell voltages and the temperature of the battery sytem, whenever the high voltage system is active. If any violation is detected, the system must be shut down. A custom BMS is designed for the car. In electric drive if all wheels are driven by separate motors, it opens a perfect opportunity to control each motor separately. This allows us to control the yaw rate (rate of turning) of the car and reap maximum traction. This is implemented by an algorithm that takes various sensor values as input and sends appropriate signals to the motor controller.It uses sensor inputs like steering angle and rpm sensor to calculate the yaw rate that the car should ideally have. Then it minimises the deviation from the actual yaw rate using a PID controller. It does this by giving a differential torque to each motor giving a net angular acceleration to turn the car. The difficult part now becomes the tuning of parameters of the PID controller which is done using a vehicle model coupled with a driver model which simulates the behaviour of the car on a path which is fed into the model. The driver model used for this purpose is indigenously developed and improvements are made every year to model driver behaviour with greater accuracy. It can now be tuned to include characteristics of different drivers. Also the vehicle model has been entirely changed this year to improve its accuracy. This model now includes accurate tractive limits and previously ne-
glected roll and pitch motion of the vehicle. Also for the first time an extensive testing is being done on the previous car EVo4. This provides useful testing data for observing the actual effect the e-diff has on the car. Testing data is useful to validate the models and look for areas that need improvement. A battery model shall be added in order to include the detrimental effect of reduced voltage of the battery on car performance and also accurately predict the energy requirement in the battery pack. Regenerative braking would also be implemented which effectively saves energy by returning energy back to the battery pack as the brakes are applied. The model can also be used to simulate the behaviour of the car on the competition tracks and also the effect of various car parameters, of e-diff as well as those of suspension and gearbox, on the same. The end result of this exercise when implemented accurately is increased stability and manoeuvrability of the vehicle and maximum utilisation of the traction available.
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Rocker Assembly
by Gauhar Singh
S
uspension is the system of tires, tire air, springs, shock absorbers and linkages that connects a vehicle to its wheels and allows relative motion between the two. The rocker assembly consists of the spring, damper ,rocker ,pull/push rod and the anti-roll bar. The goal of the of the suspension is to absorb and release the energy drawn by the tyres at a controlled rate so as to make the ride both comfortable and responsive at the same time. The suspension controls three modes of motion in particular , vertical motion , roll and pitch. Both pitch and roll are due to weight transfer across the vehicle and it is the overall weight distribution which determines the how the vehicle will perform while cornering. After designing the suspension , the next and most important step is testing and tuning the car’s suspension . Tuning the suspension involves experimenting with different tire pressures, spring rates ,anti roll bar stiffnesses and damper rates to suit the driver and the track conditions.
Being part of the team was an enlightening and fun filled experience . Woking with the team taught me how complicated and intricate it can be to design a something as common as a car from ground up. This challenging and overwhelming experience helped me grow as a student and a budding engineer and further promoted my adoration for cars. After designing the car for the better part of last year , I am thoroughly excited to start building and testing the car for this year’s completion .
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Gear Box
by Aakash Abhishek
G
earbox is the principal component of transmission system. It provides speed and torque conversions from engine/motor to the wheels. In race car applications, the gearbox reduces the rotational speed of the prime rotor of the motor and simultaneously increases the torque of the motor by the same ratio. This ratio is called the reduction ratio of the gearbox. In commercial cars, multiple reduction gearboxes are used. But since we use electric motors with specialised performance characteristic (Torque-Omega characteristic) over a wide range of speeds, we use single reduction gearbox which is optimised to give maximum accelerative performance. The two main events at the competition are acceleration run and endurance run. Both these events demand high acceleration. The optimal gear ratio was
calculated to be 5.28 which would give the maximum acceleration of the car. After further consideration regarding the ease in manufacturability of the gears, a reduction ratio of 5.33 was finalised. The gearbox design had to be optimised for minimum weight and space. We had two basic designs in our mind. A simple planetary design and compound gearbox design. By comparing the approximate weight of the two designs, we came to a conclusion that a simple planetary gearbox would be lighter. This design served all our requirements of weight, space and manufacturing. It took a lot of innovative thinking and effort from the whole subsystem to design the present gearbox. I thoroughly enjoyed the design phase.
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Steering Assembly
by Abhinav Anand Mishra
“A
ll intentional turns are initiated and controlled by deliberate turning of front wheels. Therefore, response to driver’s steering motion must be precise, linear and consistent.” says the veteran Carroll Smith. The design of the steering geometry starts with the fact that, even if the geometric center of vehicle’s path of curvature (simply stated as centre of curvature) is located on an extension of line of vehicle’s rear axles (rear axle means the line joining both rear tires), both front tires of the car have to turn by different angles in order to avoid slipping of the tires as both tires turn at different radii. The above process becomes a lot more involved mathematically with the inclusion of slip angles at high speeds. The steer angles are obtained using the ‘Two Tr a c k Roll Axis Model’. The geometry depends hugely on the viscoelastic properties of the tire which can be studied by semi-empirical methods. The companies manufacturing the tire provide the tire test data. This data helps us model the lateral force, longitudinal force and aligning moment as a function of vertical load, inclination angle, slip angle and slip
ratio. The model uses this data to determine the angles of turn of both the tires for different radii of the car’s turn. A rack and pinion steering mechanism is used. After obtaining the angles, the steering geometry to achieve these angles is prepared by solving the constraint equations using Microsoft Excel as it allows for dynamic updation of data to give better control over how the quantities should vary. Different positions and lengths of the steering rack as well as steering arm a r e tested to get the best possible steering angles, also keeping in mind the needs of chassis, brake and rocker assembly. The various parameters like kingpin inclination, caster angle, camber angle, toe angle, mechanical trail, scrub radius and pneumatic trail affect the forces driver has to apply (Please don’t worry if you don’t know these terms. These are minor tweaks in position of tires and suspension incorporated to tune the performance. Just google these to know more) and these are used to calculate the moment driver applies at different radii of turn. The next phase involves design of different components using Solidworks and analysis of the components in ANSYS for predetermined forces and constraints, ensuring a minimum safety factor, and at the same time having the minimum weight.
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Brakes
by Saaransh Pandoh
B
rakes are designed to slow down your vehicle. Brakes are essentially a mechanism to change energy types. When you’re travelling at speed, the vehicle has kinetic energy. When you apply the brakes, the pads or shoes that press against the brake drum or rotor convert kinetic energy into thermal energy via friction. The cooling of the brakes dissipates the heat and the vehicle slows down. This is all to do with The First Law of Thermodynamics, (popularly known as the Law of Conservation of Energy. Since brakes are prime component responsible for safety of the driver, the brake assembly has been designed such that it is able to withstand a force of 2000 N. Further analysis of a braking system has been performed for maintaining the right brake balance important for system’s optimisation. Optimum force distribution is found using tyre data provided by FSAE Tire Test Consortium (FSAE TTC) in order to prevent accidents involving locking of wheels and loss of directional stability due to premature Rear brake lockup. The optimum brak-
ing forces for the straight-line, level-surface braking process are analysed and actual force distribution of front is to rear is kept such that both of front and rear wheels lock simultaneously. Disc brakes are the most popular braking systems due to their efficient heat dissipation. Heat analysis of brake discs has been conducted to ensure that discs are able to withstand high temperatures experienced during the run. The heat transfer problem of a brake disc is highly transient which involves conduction, convection and radiation and steady state is not achieved. During complete stop, a large amount of heat flux is given to the disc for a very short time. Detailed analysis has been performed in order to prevent the failures occurring due to thermal stresses. Experience during design was completely fulfilling and equally enriching. And now I look forward to manufacturing phase where we are all geared up to test our car to its limit and tuning it to its optimum level.
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Wheele Assembly by Saumil Shah & Yaswanth Gonna
C
onnecting the suspension system and the drive shaft to the tires; this functionality is provided by the wheel assembly. Wheel assembly transfers the sprung weight of the car to the tires through the suspension arms (A-arms) making its structural rigidity & stiffness the most crucial designing parameters. Another major aspect is its weight reduction, as it will have the highest impact on reducing the inertia of the car which opposes turning, being located farthest from the car’s center of gravity (COG). Hence, the weight of wheel assembly should be low enough but high enough to handle extreme loads. We need to find this optimal. The wheel assembly components include the upright, hub, UBJ (upper ball joint) clamp, shims, sensor disc & sensor mount and the tripod housing. The UBJ, LBJ clamps attached to the upright are connected to the A-arms through a ball joint. Shims can be placed between the upright and the UBJ clamp in order to change the camber. Upright, being connected to the suspension system, needs to be kept stationary and the hub, onto which the tire is mounted, needs to have a rotational degree of freedom. This arrangement is made possible using deep groove ball bearings between the hub and the upright. The sensor disc and RPM sensor on the front hub measure RPM of the wheel. Also, as our car is rear-driven, the rear hSaumil Shah & Yaswanth Gonna ub has a tripod housing press fitted inside its central cavity for the drive shaft.
The weight bias between the front and rear of the car, the max. acceleration and deceleration along with maximum cornering force, all these are considered to create extreme cases & act as thresholds which the designed parts must cross in order to be deemed as reliable. For weight optimization, a software called HyperMesh (from the software package HyperWorks) is used which is fed with extreme case data and on simulation, it gives us the stress distribution over the part. In turn, this data is fed into OptiStruct (another software from HyperWorks package) which removes material from those regions where the load paths are not present, thus neither compromising on the functionality or the stiffness of the part as well as achieving reduction in weight.
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Cooling System
by Kanishk Jain
C
ooling system ensures that during continuous operation, both the motor and motor controller stay below 60 degC, beyond which the motor shuts down automatically. This is important since over-heating of stator coils may lead to reduced efficiency and increased chances of failure. Cooling assembly mainly comprises of a cross flow air to water heat exchanger – radiator and a centrifugal water pump, along with the required routing through motor controller. Water is used as refrigerant which is pumped through the closed circuit heating up, and then is cooled through convection across radiator. Through simulations and analysis of test run data from last year, maximum heat load was calculated to be 4.5kW, and the average temperature of the power module of the motor controller was found to be 45 degC. To reduce the size of the radiator at this heat hold, it was decided to place them in sidepods. This had an added benefit of reduced weight, much compact assembly and better aesthetics. Heat load and air speed was used theorize approximate size of the radiator and flow rate of water needed, which came out to be 170mm*100mm and 9 lpm respectively. This was further tested on the
vehicle and the final size was 180mm*110mm, due to better manufacturability and performance. One such radiator will be placed on both the sides inside sidepods. Accordingly, the pump was decided which could deliver the flow rate for Pressure drop of 1 bar, and at the same time operate continuously during the endurance run for atleast 45 minutes. To reduce space, complexity and pressure load across the pump, independent routing of ½ inch pipes was finalised for the cooling assembly, i.e. two separate cooling system with pump and radiator for motor and motor controller on each side.
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Tyres
by Partha Sarathi
I
magine walking with wrong size shoe on your feet…that’s how bad the car feels with the wrong tires. The car expresses its forces through them. So we better ensure that we extract the maximum possible traction from them. The tires in the passenger cars are very different because they are made to last thousands of kilometres while a typical racing slick used in formula student lasts for ~160Km. The passenger car tires have grooves in them which enable them to travel on wet roads (google aquaplaning). Slicks don’t have those grooves thereby the deformation is reduced. So they are made with a softer compound which results in better grip but faster wear. We can design a car for a tire or a tire for a car, both approaches having their own merits. To do either of them first we need to understand the tires. To understand them we simulated the tires with an in-house developed vehicle dynamics simulator. To run a vehicle dynamics simulator we need a decent mathematical model of a tire. There are many ways to model the tires which are available
in the research literature and Pacejka model is the most convenient. To get the models we obtained the tire testing data from Tire Testing Consortium (FSAE TTC). After much effort we were successful in extracting the tire models smoothly from the test data and our procedures are now comparable to commercially available solutions for tire modelling. After analysing the models for different tires we decided to go with the Hoosier 20x7.5 as we had a good experience with them in the last season too. Once the design is done and the car is manufactured then comes the testing. Tires can be modelled accurately only to an extent and their properties change depending on the wear. So we need to always improve our tire property estimates updated with the data obtained from the tests. This would be pretty helpful when we are trying to analyse the data of Electronic differential testing as the behaviour of the car heavily depends on tire properties. We believe that there is a lot of stuff unexplored and unexploited in tuning the car like camber,toe,tire pressures,etc. We are determined to get a perfectly tuned car this season.
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Side Pod by Chahal Neema
T
he implementation of side pods for cooling down the water in the radiators is done for the first time on our car. The objective of designing the side pods is to increase the air flow rate across the radiator, at the same time minimizing the drag due to the increased turbulence and adverse pressure gradient across the radiator. A narrow inlet which widens in the middle and again tapers down at the back is the basic design geometry. A near actual shape was reached by following well established literature, and fine tuning was done through aerodynamic simulations in ANSYS Fluent.
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Battery Management System
by Swetapadma Sahoo
P
roper handling and monitoring of cells of Ltium polymer chemistry is very essential for a smooth running car. The Battery Management System (BMS) manages the safety parameters, that is, cell voltages and the temperature of the battery sytem, whenever the high voltage system is active. If any violation is detected, the system must be shut down. A custom BMS is designed for the car. The main features of this year’s BMS design are:1. Cell Voltage monitoring: Multiple multi-cell monitoring ICs are used for this purpose. The ICs communicate with a master controller, which broadcasts addressable communications. There is a transition in terms of the communication protocol used- from SPI to IsoSPI, which is suitable for long distance communication.
2. Isolated communication: The IsoSPI signals are converted to SPI signals before interacting with the master controller.Multiple devices are connected to a single isoSPI master by multi-dropping them. A pair of transformers are used to isolate the isoSPI signals. 3. Temperature monitoring: Thermistors with negative temperature coefficients are used as sesors to monitor the temperature of the battery. 4. Balancing: Another important feature of the BMS is passive balancing with programmable tier for each cell, which enables better balancing, and therefore extend the lives of the cells.
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Low Voltage Safety by Sudhir Kumar
L
V safety (an acronym for Low Voltage Safety System) particularly deals with the low voltage control unit of the car. Under this subsystem we ensure 4 main things listed below: 1. Checking for real time errors and if needed cutoff the power supply 2. Giving signal to connect our power system of 400 V to the motors 3. Giving lv ready signal to follow up the commands to move motors 4. Getting feedback from harness and various important signals to our diagnostic using CAN protocol to have a close look on the functionality of whole running system. Coming to design part our board can be broadly divided into 3 major blocks naming Error block, Control block and Diagnostic block.
1. ERROR BLOCK: This block consists of checking 5 errors named as BPS error, GFD error, BMS error, Motor controller error and E-diff error. This block checks error like if brake and throttle are not pressed simultaneously beyond certain extent, if high voltage and low voltage ground are separated or not, if there is not any error in battery system regarding the temperature and voltages of cells, motor controller board and e-diff board are working fine or not. These all checkups are done simultaneously and any error may cause hazards, so it generates a signal ERROR FINAL TO RELAY causing the power supply to cutoff.
condition is violated means if shutdown sequence is not complete or any error is there, power supply to motors gets cutoff immediately. 3. DIAGNOSTIC BLOCK: from the various feedback signals on board and off board we monitor signals using micro controller through CAN protocol for diagnostic purposes to figure out exact real time source of problem if occurs during the run. Also microcontroller is used to generate ready signal after which only E-diff board can give commands to run the motors otherwise motors will not be moving.
2. CONTROL BLOCK: After checking all the errors and ensuring shutdown sequence is complete the board finally gives signal to pre-charge the motor controller and takes feedback to connect the power supply to motors. During the run if any
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Data Aquisition
by Neil Adit
T
here are enormous number of sensors installed on the car and various signals being sent across from one subsystem to the other. With such an immense amount of inter communication and data being sent continuously, we need a system which can keep track and show us real time happenings in the car. That’s where DAQ comes into play. Electronic Differential Data
Powertrain Data
Others
Wheele RPM x 4
Battery Temperature
Suspension Sensor
Brake Position
Battery Voltage Current
Brake Line Pressure
Throttle Position x 2
Available Energy
LV Diagnostics
Steering Angle
Cell Performance
Shutdown Sequence
Yaw Rate Data
Mortor Temperature
Time Stamp
DAQ (Data Acquisition) can be divided into 3 major components of work. 1. Real Time Data Logging – Take data from CAN buses and UART and store them on a SD CARD with a time stamp. 2. Transmitting data wirelessly over long range also called long distance telemetry – This is done using a device called XBee which works on a wireless protocol called Zigbee. In simple terms it transfers data from one point to the other wirelessly. We send data using an Xbee on the DAQ Board and receive data using another Xbee attached to a remote laptop. 3. Viewing data on a remote laptop with a Graphical User Interface (GUI) – We design a GUI to view the data in a more user oriented manner on the laptop, say a speedometer for seeing the speed of the car.
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CAN
by Rohit Rothe
A
ny car needs a control unit which directs its systems to work in unison and get the car running. These systems are a network of sensors which should be able to communicate with the control unit. This communication between the sensors and the control unit and between the control unit and the motor controller is ensured by the CAN protocol. CAN has many advantages over the conventional protocols like UART. It has very good error identification and rectification system, an ID based communication platform and priority based channel which makes it better than the other protocols. There is a single bus running throughout the car on where all the sensors put their data with the corresponding Id. This reduces the size of the harness in the car and simplifies the design and makes debugging a lot easier. In our car we have used 3 CAN buses – one for control unit to the motor controller(500 kbps), one from the control unit to the sensors except yaw sensor(500 kbps) and one from the control unit to the yaw sensor(1 Mbps). The function of the node is to convert the analog or digital data being sent by the sensor into a CAN message and transmit it on the bus with a unique ID. This data is extracted by the Data Acquisition system and is stored in a Flash memory. The data is also used by the DASH system for displaying relevant data on the dash board. The microcontroller being used on the node is PIC18F25K80 (Microchip). The sensors installed on the car are brake sensors, throttle sensors, suspension sensors, RPM sensors, yaw sensors, temperature sensors and steering sensor. In case of the RPM sensor the input is a wave consisting of 1’s
and 0’s. We then extract the frequency of the wave. The data on the bus is then used by the Electronic Differential system to govern the functioning of the motors. The CAN node has a UART programming facility by which we can programme the baud rate for CAN using UART data sent by the programmer. For proper functioning of the car all CAN nodes have to work efficiently otherwise the data communication will stop. Our main aim while designing was to make the node as small as possible to go in tune with the agendas of the mechanical systems. The node will be mounted in a small I/P box of dimensions 65*50*35 (mm). Two LEDs will poke out of the box so that we can monitor the functioning of the node. This will be a large improvement over the previous year’s design as previously we used to directly read the analog value from the sensor directly on the microcontroller. The design phase was a thought-provoking and enjoyable experience like no other. The design looks splendid and I am looking forward to see it materialize into a beautiful machine.
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