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Motorized longboard


Project Overview:

Image that you can do your favorite downhill tricks… going uphill. Or carve and carve… without ever pushing. A longboard without ever pushing can going uphill and faster than a bike. You can jump of your board, walk upstairs and keep using your board to climb uphill. This is impossible for bicycle and car, but can be achieved by an electrical longboard.It is designed to be portable so you can carry it everywhere even to classroom.

This project is separated in following areas:
  • General design
  • Mechanical and power transmission design
  • Electric speed controller optimization
  • Radio control system
  • Battery safety
  • Power management
Weekly Update

Photo:





Members:

  • Theo Long 
    • TheoLoong@vt.edu

Mentors:

  • Conor patrick

Current Status:

        Looking into Battery BMS and better speed controller.
        Still figuring out the battery solution and look for the reason why current lipo battery has low voltage shutoff
        Come back from summer break and start working on the battery solution.
        One lipo battery die completely and the other one barely survive.
        both timing belt are broken at about hitting 300 miles. Need more belts to replace.
        ordering components for first maintenance. 
        both timing belt are broken at around 160 miles/ 3months. Just replaced with new timing belt
        testing prototype
        Finished first Prototype report

Yearly maintenance
renewing the board, rust removal, replacing consumables.  

Demo:

The first prototype

Educational Value:

  • This project supports further interest in 3D modeling and design.
  • This project is a practical application of what we have learn in class such as Inventor, CAD and problem solving etc.
  • This project shows that by simply combining technology can create something new and useful for our life.
  •  Learning how to design a electric vehicle to be safe.
  • Learning how to use lipo battery in a safe way.
  • Learning how to design a custom drive train and power transmission.
  •  Learning how to design and prototype for cost-effectiveness.
  •  Skills involved in this project such as 3D design and product design are very helpful for future career.
  • Applying physics such as rotational kinematics calculating torque and work in real life.
  • Involve simple wiring, soldering and hand-on skill.

Problems found in the first prototype test:(New)


    The first prototype was assembled during mid September 2015. Problems was found by testing the prototype. The way of testing the prototype is be a rider of it every day. During weekday, I ride it to class and around campus. During weekend, I try to ride the longboard instead of driving my car. So far I have put approximate 100 mile on the longboard.  The following is the report including problems I encountered and improvements I have made or may apply; 
    This project is separated in following areas. Therefore, report will be separated in following subtopics.  
  • General design
  • Mechanical and power transmission design
  • Electric speed controller optimization
  • Radio control system
  • Power management

 General Design:

        The purpose of this project is to provide a motorized longboard that is portable, stable and durable for each day commuting. To achieve this goal, the following conditions is needed to be consider:

  • Weather
  • Terrain 
  • Battery life (miles per charge)
  • Weight


Weather:

This longboard is need to handle different weathers (except snowing, longboard cannot run on snow). Raining is the main issue for this longboard. As the first user of this longboard, I found really inconvenient when it raining outside or the ground is wet. Therefore, the longboard must be water proof or at lease splash proof. The original design was not a water proof design. Although the motor is water proof, other electronics such as radio receiver and speed contorller are not. The first solution will be making all components stay under the longboard, so water can only damage the longboard from the bottom. Riding a longboard on wet ground will result in splashing waters into electronics and damaging them. 

In order to make components stay under the longboard with a balanced weight, I used CNC mill to make a nice and deep slot on the longboard. Now electronics can be install evenly on the front and back and connected by wire that in the longboard. Speed controller cases and battery cases was also design and 3D printed so that components can stay under the board firmly. (pictures)

To solve this problem, I used liquid tape and water proofer. Liquid tape, or plastic dip is used to insulate and protect electrical splices and connections. It is also a water proofer for electronics. By applying this product, speed controller can even work under water. I applied some liquid tape to protect the speed controller and seal the gap between longboard and speed controller mound. I also applied some on connection to prevent disconnection due to shacking longboard cause by rough terrain. Water proofer spray was also used as a final coating for the bottom of the longboard. (pictures)

Terrain:

There are mainly four terrains in cities: pavement, concrete. asphalt and grass. Longboard wheels are designed for asphalt and concrete. Therefore,       pavement and grass field are my mainly concern. Based on my test, my longboard can run on flat grass field pretty well.

There are two potential dangers  when riding a longboard on pavements and bad grass field. First, when there is a gap cause (such as expansion joint and cracks between pavements), wheels can be trapped by the crack. Rider can be launched off the longboard if wheels are trapped. Anther danger is when longboard is passing though cracks with 80mm wheels, strong vibration will be caused. This vibration can physically damage electronic device and connections.The possible solution for this problem is using 8inch mountainboard wheels. Mountain wheels have inflated tire and large diameter. These property can reduce risks and vibration cause by passing through cracks and increase user experience. (video for terrain test).

Battery Life:

I have not done detail test yet. But for my daily usage, I single battery (6s 5000mAh) can run about 7 miles. Battery life also depends on users' skate habit and terrain. This data was obtain from commuting on Virginia Tech Campus.

Weight:

acceptable weight but heavy if carrying for longtime. Motors and battery contributes most of the weight.

Mechanical and power transmission design:

    After approximate 100 mile test, all mechanical parts are still functioning. One things need to be mention is the gear hub I used for my longboard is 3D printed with ABS. During my prototype test, flanges of gear hub is somehow broken and fall off. However, the timing belt and pully still functions pretty well. This means ABS printed gear hub is very durable. Motor mount and motors from Enertion works great. 

    One problem about mechanical design is when the longboard is turning, motors start scratching wire under the longboard. this is pretty dangerous because after longtime, wire could be exposed and short electric components such as battery. The reason cause this problem is not enough clearance between motors and the longboard. This could solve by replacing thicker risers. (pictures)

    In conclusion, mechanical design turns out to be very stable and durable.  


Speed controller:

I choose the existing speed controller for longboard, the filer esc. I discovery that their ESC is built by using two RC car speed controller. The only different is that these car speed controller use heat sink instant of fan cooling. It has some good programmable options such as brake percentage and start up power percentage. It is reliable and supports up to 8 cells battery. It's throttle can also be programmed in linear trend or exponential trend.


However, the bad side for this speed controller is that it is design for RC car. It has a unstable start up torque output. For example, if rider try to start the longboard when facing up hill, the longboard will output inconstant torque. If the board fail to start up, motors will stop even you pull the trigger completely off. Although this can be address by a push jump longboard trick, tis ESC is not very user friendly for new riders.


VESC speed controller, a open source electric skate board speed controller, is one of the solution to this problem. http://vedder.se/2015/01/vesc-open-source-esc/. It can be programed freely and can output constant torque, which is what skater is looking for. However, this is needed to be dig in and see if this works at the author illustrating. The next step of this project will be researching about VESC and validate it as a electric skate board controller.



Radio Control system:

So far I am using regular RC pistol controller to control my motorized longboard. This is a fast, cheap and reliable short-term solution. This controller is big, heavy, not rechargeable and has a lot of features that is not necessary. Controller for skateboard does not need long range and multi-functional. The following features are the design goal of skateboard controllers.
  • Light weight
  • compact size
  • Last long battery life
  • Analog chanel (only throttle is needed, additional feature such as turing light can be added))
  • Battery life indicator (optional, for skateboard battery and controller battery) 
  • Rechargeable 
Skateboard controller is also a sub-project. Here is the project page for skateboard controller:https://sites.google.com/a/vt.edu/amp_lab/projects/motorized-longboard/motorized-longboard-remote-control

Power Management:
Currently I'm using a hobby balance charger. I will need to pull out my battery from the longboard and hock it to the balance charger. This is a safe and easy existing solution. The next step for improvement is looking into onboard charger. Taxes instrument has similar solution that can balance cells and can charge on the same board. 







Theory Research:

https://docs.google.com/spreadsheets/d/1SJwraL45K7frAA6vHU7z7RTrxvr8YPZXTcyv31NhzNU/edit#gid=5
This table calculate the top speed of the board in ideal situation. 
Top speed is under the situation of no friction and leveled ground. The data in here is used as reference.
In real life, static friction between wheels and ground is involved. Static friction depends on load, weather and contact surface material.
In daily life, wheels will most likely contact with asphalt and concrete ground.
http://www.engineeringtoolbox.com/friction-coefficients-d_778.html
According to EngineeringToolBox, we will have maximum static friction coefficient when rubber wheels contact with dry concrete. The static
friction coefficient will be 0.85 μs.
Assume the amount of load is 100kg (with rider, board and electronics), friction force = mgμs = 833.85N
Assume the board is traveling with rider in a constant speed, net force is 0. Motors must provide 833.85N in order to travel in constant speed.
Torque = force * radius. The radius of longboard wheels is 40mm, or 0.04m. Torque = 833.85N * 0.04m = 33.35Nm
If the longboard is running in 20km/h, the speed of the wheel would be 1372.35RPM
With two 2500w motors, the total watt would be 5kW.
Torque = 9.5488 * power(kW) / Speed (RPM) = 35.98Nm
So, two 2500w motors theoretically can provide enough torque to push the board with 100kg load in a constant speed of 20km/h.


Wiring Diagram : 



Design and component Decisions:

Decision 1:  Use 270 190kv brushless motor (Updated)

The new Motor:
Exertion provide 50mm 190v motors. The lower kv, the higher torque. 
With higher torque, user can experience a smoother acceleration path and more power.


    For dual motor setup, a 50mm size motor would work better. A smaller motor allows me to design a smaller motor mount and keep things higher off the ground.
    These motors are called outrunners, meaning the motor case spins with the shaft. Other than size, the KV of the motor is important. This can be as low as 138kv and as high as 270kv. The lower the number the better the torque. Higher KV motors have higher RPMs, but won't have the power to get you started.
    The number one thing when looking for an outrunner brushless motor is large size motor with the lowest kv motor possible.The longboard need a kv below 300kv but preferably below 200kv. I am looking for torque/power not high speed. I can always gear it higher to get more speed but I can make a high kv motor perform better at low speeds. When I think about it you way much more than the average RC car. 
    


Decision 2: Gear ratio and time pulley

https://docs.google.com/spreadsheets/d/1SJwraL45K7frAA6vHU7z7RTrxvr8YPZXTcyv31NhzNU/edit#gid=5
This table calculate the top speed of the board in ideal situation.
(click to view the full size image)

(click to view the full size image)

T5 time belts have 5.00mm pitch. A lager pitch can have more surface contact areas than small pitch. T5 is designed to be a high torque time pulley. For motorized longboard, I'm looking for torque/power not high speed. The 36 teeth time pulley has 6 taped hole. This is more easer for attaching to the wheel.
The width is usually 9mm, but I can also go with a 15mm width setup. That takes up more room though. So I choose a 10mm width time pulley and belt.


Gear ration can be change according to different situation. Targeting time pulley has 55teeth and the gear options are 12, 14, 15, 16, 18, 20, 21 and 22 to 36. The distances between two time pulley are range from 66mm to 77mm. Motor mount should have mechanism that can adjust the distance in order to fit these gear ration

The final solution: I design a gear hub for the longboard and 3d printed it. It works great and very durable. Metal gear will be great but ABS printed gear is also very durable. So far (after 100 miles) I don't need to replace any gear.

Decision 3: Speed controller (updated)

Regen Braking:

For battery safty and lifespan, I want to cut the battery power off to prevent over discharge. However, losing power in a sudden is very dengerous. I'm currently looking into regen braking that can brake the motor even the battery is cut off.
Here is a interesting reference for this topic.
http://ww1.microchip.com/downloads/en/DeviceDoc/Regenerative%20Braking%20of%20BLDC%20Motors.pdf

Option1: Car ESC

For safety, Motorized longboard need breaks. Boat speed controller usually design to be water cooled, so it is not suitable for longboard. However, for airplane esc, they don't have any braking and can be a problem getting the motor started from a stop. I will also need a way to power my receiver. This takes up extra space and is just more to worry about car esc. Car esc is design for terrestrial vehicles so  is more suitable for electrical longboard.  Car ESC's are what most using currently. I will get into this with batteries, but the ESC needs to be able to handle 6s or about 22 volts. For Car ESC's this means at least a 150 amp version. 

 .





Option2: Boat ESC

Boat ESC also has breaking function. However, the braking does take longer for you to come to a complete stop, but it should slows rider down fast enough that it shouldn't be an issue. The brakes also don't squeal like a car ESC does, which makes it stealthier. You aren't going to wake up the neighbors when you are out riding late at night. Throttle curve seems to be exactly like the car ESC, more like the Hobbywing version than the Hobbyking one though.

Boat ESC is much cheaper and smaller than car ESC. It is worth to try out boat ESC on e board.



In the first prototype, I used Fliers skateboard esc. It is a existing solution so I choose it and see how well it worked. 

Decision 4: Battery pack

I was thinking a lot about why lipo battery (even the good one with BMS in it) doesn't have a circuit breaker. I think that is because they want it can be over discharge when necessary, and this make sense. Let say if I'm riding my longboard going down hill. If the the power cut off in a sudden, I will have no brake at all. Just like when drones are running low, over discharge can still make it land with the cost of damaging a lipo battery. And in tesla cars, they don't cut power right away if you are running low. You don't want to lose control on the high way. However, This problem may be solve if a ESC can brake the motor without power source. Please reference to the previous session.

BSM:


And yes, over discharge can be easily triggered by improper use. But this should not be the reason to break the power right away, as damaging a lipo is way cheaper than damaging a people. 

So instead, I think I should work on how to prompt user about the low battery and help user make a good estimation so the user will know before going for his trip. 

Note:  We are aware of the energy limit and safety implications of using a large LiPo battery, so we are currently still designing the battery portion.

A 10000mah battery with only 2cm thickness. It won't  appears to be a giant block of battery when attaching it under the deck. A thin battery pack also reduce the chance that battery is damaged by hitting the ground while flexing. Thus, it is very suitable for longboard.

If I want to make a battery that is thin and powerful, I need either parallel or series. If I parallel 2 battery, it means I have 2 balance charge plug. More thinner I want, more battery I have to use and there are more connection. This would be difficult for balance charge the battery and maintenance. I single battery that has the dimension I want is the best choice.

Alien Power Systems is a small family operation. Alien Power System now works in collaboration with professional RC racers, CNC engineers and software/hardware engineers to continuously improve our products. They emphasis on providing a very personal customer service and will be more than happy to answer any queries or provide assistance in choosing the correct product. That is the reason why I choose it. Many of my friend's airplanes are using their custom brushless motor and they worked really well.

(updates): Alien Power System is a private company that do custom cells. However, They can't not provide any details for the battery.
                   As discussed previously in this session, a BMS is required. Tattu smart battery integrated with BMS and it became a idea solution to test.

Decision 5: longboard Deck

The general longboard deck was made of Maple trees. Canadian Maple trees take around 40-60 years to mature before they can be cut down to make shake boards. However, bamboo can grow two feet a day. Also bamboo deck is more durable and flexible than traditional maple longboard deck.

YouTube Video

BAMBOO vs MAPLE skateboard stress experiment

Carbon fiber deck is preferred for it's weight and look. In the first prototype test, I choose a $50 bamboo deck for testing.

Decision 6: Wheels

A longboard wheels that have lager diameter can avoid time pulley or time belt from damaging by touching the ground. Also, Abec 11 flywheels  have spoked which means its easier to attach a large drive cog to them and  still be able to rotate/change the wheels for wear.   81a is a good durameter for speed riding.

In the first prototype, I choose Orangatang Kegel rather than Abec 11 because Kegel has holes that I can use to bolt my gear hub.

Decision 7: Longboard Trucks


A 44 degree trucks have high stability while providing a larger flex degree. Caliber truck is the only truck that has a square hanger. This is more suitable for motor mounting compare with other longboard trucks.

Decision 8: eBoard kits

This website provide electric longboard kits for DIY. These kits include electric component and motor mount. 
Their kits has 600 days warranty and life time service. 
By using their product, I can understand how existing product works and their pros and cons.
I can improve my design by trying out their product
http://www.enertionboards.com/buy-electric-skateboard-kits-build-your-own-eboard/



Mechanical Design:

Motor mount:

(click to view the full size image, dwg file was provided at the bottom of the page)  

This motor mount was design for Caliber's square truck. The distance between gears is adjustable to fit different gear ratios and belt tension 


Wheels mount:

This wheel mount was design to attach the time pulley with Abec 11 90mm wheels.
(click to view the full size image, dwg file was provided at the bottom of the page) 



Design feature:

Single side assambly diagram:

(click to view the full size image, dwg file was provided at the bottom of the page) 


Assembly without motor cover:

(click to view the full size image, dwg file was provided at the bottom of the page) 
(click to view the full size image, dwg file was provided at the bottom of the page) 
(click to view the full size image, dwg file was provided at the bottom of the page) 



Assembly with motor cover:



Flexibility:

By testing an existing longboard, I found the biggest flex angle between truck and board is 14 degree. I took this data while design the motor mount.
While turning, motors and pulley will not hit the board and damaged itself.

Material:

The first prototype of motor mount will be 3D printed. After serial test, the motor mount will modify to a CNC aluminum cut design.



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Theo Long,
Nov 23, 2014, 9:18 PM
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Theo Long,
Nov 23, 2014, 9:18 PM
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Theo Long,
Nov 23, 2014, 9:19 PM
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Theo Long,
Nov 23, 2014, 9:19 PM
ċ
Theo Long,
Nov 23, 2014, 9:19 PM
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