Difference between revisions of "Electric Bobby Car Build"

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[[Image:electric_bobby_car_build_wheels_rear_plate_slots3.jpg|400px]]
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The screw holes needed to be drilled with a pilot hole of ''Screw Diameter x 0.9 = Drill bit size'', e.g. M8 x 0.9 = 7.2mm, or the closest one available (7mm). Then using a taper tap to gradually cutting out the required threads. Normal through holes use the required drill diameter, e.g. 8mm drill for 8mm hole.
The screw holes needed to be drilled with a pilot hole of <code>Screw Diameter x 0.9 = Drill bit size</code>, e.g. M8 x 0.9 = 7.2mm, or the closest one available (7mm). Then using a taper tap to gradually cutting out the required threads. Normal through holes use the required drill diameter, e.g. 8mm drill for 8mm hole.


[[Image:electric_bobby_car_build_wheels_rear_plate_screw_holes1.jpg|400px]] [[Image:electric_bobby_car_build_wheels_rear_plate_screw_holes2.jpg|400px]]
[[Image:electric_bobby_car_build_wheels_rear_plate_screw_holes1.jpg|400px]] [[Image:electric_bobby_car_build_wheels_rear_plate_screw_holes2.jpg|400px]]

Revision as of 11:41, 9 December 2019

Electric bobby car build final.jpg

This is the electric motor upgrade build (not a chronological log) of a Bobby Car using parts from two Hoverboards/Swagboards. The end result will have four motors, one per wheel, two Xbox 360 potentiometers as throttle and brake triggers on the stock steering wheel, and two push buttons to turn on and off each controller separately (e.g. FWD, RWD, AWD). Inside the Bobby Car all the equipment resides, e.g. the controllers, battery, and cabling. In addition to elements to improve the strength of the base.

Most of the front wheel steering assembly had to be replaced by new 3D-printed parts. The same is true for the rear wheel base as it was replaced with an aluminum mounting plate.

The controllers was flashed to with a new alternative firmware developed by Larsmm, which is a fork of NiklasFauth original base. This firmware provides different four driving modes with different speed, acceleration and features, and smooth acceleration and breaking ramps, an a cool turbo ramp!

Base preparation

Main equipment bay

A slot had to by cut out of the bottom to allow internal access and functions as the main equipment bay. It measures 78 x 148 mm, just enough to fit the controllers and battery. Rounded corners to minimize stress areas.

Electric bobby car build base cut internal bay1.jpg Electric bobby car build base cut internal bay2.jpg Electric bobby car build base cut internal bay3.jpg

In the final stage it is covered by a see-through Lexan plate.

Rear wheel slots

Two rear indentations were cut out to make space for a wooden structure which later would be used to mount a 12mm aluminium wheel plate. These were cut just at/below the stock axle hole and are at the same height as the new front wheels when installed, so the base would be level.

To judge the right cut height, it is an advantage to first prepare the rear wheel base and use it for make an educated approximation.

Electric bobby car build base cut rear slot1.jpg Electric bobby car build base cut rear slot2.jpg Electric bobby car build base cut rear slot3.jpg Electric bobby car build base cut rear slot4.jpg

To remove sharp edges and to release stress areas, the edges were treated with a heat gun.

Electric bobby car build base cut heatgun.jpg

Rear wheel structure

The internal rear wood structure consists of one cross beam and six smaller wood pieces stacked on top of each other. The load of the driver will be nicely distributed to the wheels and ground, easily increasing the max. driver weight to over 100kg!

Each wooden piece was fastened to the parent piece using wooden screws, making sure to use a different screw pattern for each layer to avoid conflicts.

Electric bobby car build base rear structure1.jpg Electric bobby car build base rear structure2.jpg Electric bobby car build base rear structure3.jpg

Mounting the wooden structure to the plastic base wall was done by nine 4 x 30mm wooden screws (pilot holes pre-drilled), linking each layer of the internal structure. Using washers to distribute the load as wide as possible. If the wheel touches the head of the screws, file them down a bit.

Electric bobby car build base rear mounting structure1.jpg Electric bobby car build base rear mounting structure2.jpg

Rear wheels

All the stock parts of the Bobby Car front and rear wheels was removed, only a few parts of the steering was reused. Two Hoverboard wheel brackets and screws were also reused.

Electric bobby car build wheels rear cad overview.png Electric bobby car build wheels rear plate design.png

Wheel base plate

As much of the driver's weight will be over the rear wheels, the wheels had to be mounted on an strong 205x70x12mm 6061 aluminium plate. Slots and screw holes were routed and drilled/tapped out, respectively, using regular home/garage gear. Additional screw holes were added for future expansions, like a hook for a trailer, photo interrupter sensor, and lights.

The plate was prepared by outlining the design on surface. The holes were first counter-sunk, then drilled. Sharp edges were deburred using the counter-sunk drill bit. Holes to fasten the plate to the wood structure were counter-sunk for a flush finish.

Electric bobby car build wheels rear plate holes1.jpg Electric bobby car build wheels rear plate holes2.jpg Electric bobby car build wheels rear plate holes3.jpg

The plate was cut to length with a jig saw, at 205mm.

Electric bobby car build wheels rear plate length1.jpg Electric bobby car build wheels rear plate length2.jpg

To cut out the wheel axle slots, a Bosch router with a HSS straight two flute Ø8mm milling cutter was used (see drawing for dimensions). I did 1-2mm cutting depth at each pass and multiple finer passes to end up at the final smooth finish. Used WD40 and low RPM on the router to avoid overheating the cutter.

Electric bobby car build wheels rear plate slots1.jpg Electric bobby car build wheels rear plate slots2.jpg Electric bobby car build wheels rear plate slots3.jpg

The screw holes needed to be drilled with a pilot hole of Screw Diameter x 0.9 = Drill bit size, e.g. M8 x 0.9 = 7.2mm, or the closest one available (7mm). Then using a taper tap to gradually cutting out the required threads. Normal through holes use the required drill diameter, e.g. 8mm drill for 8mm hole.

Electric bobby car build wheels rear plate screw holes1.jpg Electric bobby car build wheels rear plate screw holes2.jpg

Installing plate

Once the wooden structure and plate was ready, pilot holes were drilled (to relieve pressure and avoid cracking) and the plate was installed using 6 x 45mm counter-sunk screws. This provided a very stabel platform to mount the wheels.

Electric bobby car build wheels rear installing plate1.jpg Electric bobby car build wheels rear installing plate2.jpg

Mounting wheels

Both wheels are mounted to the base plate by sandwiching the axle between the stock Hoverboard bracket using the existing four M8 x 30mm screws. The tip of the screws won't protrude once fastened.

Electric bobby car build wheels rear mounting1.jpg Electric bobby car build wheels rear mounting2.jpg Electric bobby car build wheels rear mounting3.jpg

Axle cover

The axle and motor wires are directly exposed and could be damaged. A 3D-printed cover (include in the CAD package) screws in place over this area. Water drainage holes provided.

Electric bobby car build wheels rear cover1.jpg

Wiring

The wiring coming from the motor was feed through the base plate and then inside the base through a small hole just underneath the front end of the wheel plate.

Electric bobby car build wheels rear wiring1.jpg Electric bobby car build wheels rear wiring2.jpg

The wires were protected from abbreviation by a cable sleeve and heat shrink cover.

Front wheels

The front wheel assembly was modeled and new parts designed to incorporate the motors into the stock front wheel structure. Wheel clearance height was increase/raised by 10 mm to all the bigger radius Hoverboard wheels to fit (155mm vs 170mm).

All parts were designed/3D-printed (ABS) with strength as one of the main principles, primarily using long through screws and 70% infill to compress and distribute, respectively, much of the stresses experienced on the parts.

Electric bobby car build wheels front cad overview1.png Electric bobby car build wheels front cad overview2.png

Full aluminium CNC parts or metal support inserts would be a future upgrade here.

Steering wheel assembly

The assembly consists of four main parts and six screws along with six nuts to keep it all together. See the BOM for full break-down of the screw selection. A skateboard bearing lessens some of the pressure and tear on the upper steering joint.

Electric bobby car build wheels front assembly1.jpg Electric bobby car build wheels front assembly2.jpg Electric bobby car build wheels front assembly3.jpg Electric bobby car build wheels front assembly4.jpg

Cross bracket

Both wheels are kept in alignment by the stock front cross bracket. This is screwed onto the base frame. The bracket had to be reinforced using eight M4 x 30mm screws which connect to a counter plate with the nuts inside the base, instead of the stock two threaded plastic screws. No washers on the four corner screws, for space reasons.

Electric bobby car build wheels front bracket1.jpg Electric bobby car build wheels front bracket2.jpg Electric bobby car build wheels front bracket3.jpg Electric bobby car build wheels front bracket4.jpg

Wiring

The wiring coming from the motor was feed through a small hole just in the middle of steering range motion to minimize cable stress/bending.

Electric bobby car build wheels front wiring1.jpg Electric bobby car build wheels front wiring2.jpg

The wires were protected from abbreviation by a cable sleeve and heat shrink cover.

Steering

Much of the steering assembly was redesigned from ground, except for the steering wheel itself. Much work went into figuring out how the throttle and brake trigger would mount and wiring would be routed. Addition base reinforcement was also needed.

Electric bobby car build steering cad overview1.png


Base rod support

As there will be a lot of load on the steering rod, additional support had been added to the top and bottom side where the rod protrudes through the plastic base.

The top and bottom support brackets were fastened to the base using slotted M4 x 25 mm mushroom screws and nuts.

Electric bobby car build steering rod support top1.jpg Electric bobby car build steering rod support top2.jpg

Inside the bottom bracket there is also a ID Ø10mm ball bearing to make the axial steering motion smoother.

Electric bobby car build steering rod support bottom1.jpg Electric bobby car build steering rod support bottom2.jpg

Rod bending

The new rod was based on the stock dimension and bend, just longer on one end to put the steering wheel at a comfortable position.

Using a Ø10mm stainless steel rod, one end was heated using a simple butane camping gas stove to make it pliable. Stuck it in vise and bent it to the approx. radius. Then put the entire bend into the vise and squeeze it into the right dimension, using the stock as a template. If the bent end is too long, saw it to the correct length. This took almost 2 hours.

Electric bobby car build steering rod1.jpg Electric bobby car build steering rod2.jpg

Rod length and holes

The original rod was too short, so the steering wheel had to be raised and consequently the steering rod had to be lengthened by 155 mm.

Finding the best height can be done by trying the rod on the Bobby Car with the steering wheel next to it. Mark the spot and cut the rod - be sure to include the inserted length too.

To allow for different steering wheel heights/accommodate more drivers, I made three different mounting holes, Ø5mm each. This also requires a through hole in the plastic on the steering wheel column. As I was going to use a small 300x300mm cotton cushion I included that in the measurement.

The holes were spaces in 15mm intervals:

  • 15mm from end
  • 30mm from end
  • 45mm from end

Electric bobby car build steering rod mounting1.jpg Electric bobby car build steering rod mounting2.jpg Electric bobby car build steering rod mounting3.jpg

To keep the rod from falling through on assembly or pop out of the steering swing arm, an outer Ø32 x 140mm white PVC tube keeps the steering wheel at the correct position and protects the wiring going to the controller.

Electric bobby car build steering rod tube1.jpg Electric bobby car build steering rod tube2.jpg

Throttle and brake

Using Fisch's method for the throttle and brake levers by mounting them on the front end (10:30 and 1:30 o'clock) on the steering wheel. A special 3D-printed holder keeps the trigger mechanism, potentiometer and support board in place. All put in place on the steering wheel using hot glue. An upgrade here would be a through screw.

Electric bobby car build steering throttle brake mounting1.jpg Electric bobby car build steering throttle brake mounting2.jpg

Wiring the potentiometer is simple, 3V3 (red), analog out (yellow, mid-pin), and ground (black). The wires are tucked away underneath the steering cup. When pressing the trigger the voltage rises gradually to 3V3.

Electric bobby car build steering throttle brake wiring1.jpg Electric bobby car build steering throttle brake wiring2.jpg

Wiring

Both triggers connects to a modified break-out board - it just makes it easier to plug everything together using JST XH B4B connectors. It is located inside the steering wheel column. The real active break-out board is located next to the controllers.

The lead was feed through a drilled hole in the steering wheel column and the lead from the break-out board comes out of a hole next close to the steering rod.

Electric bobby car build steering steering wiring1.jpg Electric bobby car build steering steering wiring2.jpg Electric bobby car build steering steering wiring3.jpg

Electronics

Break-out board

This is board makes it easier to finish the build. It has easy to connect single JST XH B4B socket for the throttle and brake lead, two sockets for each controller, analog signal filtering, master and slave throttle op-amp isolation - where one driver board can be turned off without interfering with the other. It tucks away inside the steering wheel under the red beeper cap.

Inspired by the Jan Henrik break-out boards.

Bobbycar hoverboard throttle breakout schematic.png

Bobbycar hoverboard throttle breakout pcb top.png Bobbycar hoverboard throttle breakout pcb bottom.png

Motor wiring

Connecting the front motors to controller is straight plug and play, but the rear main motor wires needed to be extended 10 cm to reach the controller. Simple cut-and-solder in 16 AWG extension wires, keeping the existing bullet connectors.

Note that if the motors will not turn around correctly when throttle is applied, try to swap two of the leads and make sure every connection is solid.

Electric bobby car build motor wiring1.jpg Electric bobby car build motor wiring2.jpg

The thinner motor hall-effect wires were easily extended using custom made 15cm JST SM extension wires with connectors on each end.

Electric bobby car build motor hall wiring1.jpg

A future upgrade would be to increase the gauge of the battery and motor wires, as they are already at their limits.

Mounting controllers

The controllers are essentially identical, except for slightly different caps, power transistors, and wiring method. Both are from Hoverboard productions in 2015. Both rock the same STM32F103 ARM Cortext-M3 MCU.

Electric bobby car build controller1.jpg Electric bobby car build controller2.jpg Electric bobby car build controller3.jpg

Both controllers are mounted back-to-back using four M4 x 30mm screws and nuts with a stand-off spacer in-between, and installed vertically using a custom 3D-printed bracket and two slotted M5 x 16mm mushroom screws and nuts.

Electric bobby car build controller mounting1.jpg Electric bobby car build controller mounting2.jpg Electric bobby car build controller mounting3.jpg

Battery

This build is using the stock 18650 10S2P 36V Lithium-ion battery from the Hoverboard. It has a BMS built-in. Although a 12S2P 44.4V battery is supported by the controllers, that is for future upgrades.

The battery is kept secure by a hook-and-look strap around the battery and through two slots in a Lexan cover plate.

Electric bobby car build battery1.jpg File:Electric bobby car build battery2.jpg

Power buttons

As this is a four motor, two controller setup, I wanted to have the possibility to power-on each controller separately. This would allow for FWD or RWD, depending on the occasion. This requires the custom design break-out board in order to isolate the two controllers so they would not power each other or block/pull-down the throttle/brake signal when one of the controllers were powered down.

Two momentary LED push non-latching buttons were prepared with a four-pin JST XH B4B connector. The corresponding socket were connected to the 15V regulator on the controller and power-on pins. A 2k resistor were put on the button LED power pin to lower the illumination. A Ø12mm hole was prepared and the buttons installed next to the steering rod.

Electric bobby car build power buttons1.jpg Electric bobby car build power buttons2.jpg

Disconnect plug and charging

A 3D-printed bracket holds the disconnect plug (cuts all power) and charging port in place just rear of the equipment bay slot, by Fisch's inspiration. Fastened using three shortened M4 x 25mm screws and nuts.

Electric bobby car build power charging1.jpg Electric bobby car build power charging2.jpg

Firmware

Compile firmware

The larsmm firmware needs to be configured, calibrated and recompiled in order to be used. I used a VMWare image to set up the ARM toolchain.

git clone https://github.com/larsmm/hoverboard-firmware-hack-bbcar
cd hoverboard-firmware-hack-bbcar
pico Inc/config.h
make

The new firmware will reside in build/hover.bin

Flash firmware

The Hoverboard controller board expose the programming port, requiring only three wires or a header to be soldered.

Programmer Board Wire color Note
3V3 3V3 Red wire Leave unconnected, 3V3 provided by board
SWCLK CLK Blue wire -
GND GND Black wire -
SWDIO DIO Yellow wire -

Electric bobby car build flash port1.jpg

Make sure the power-button is connected as this is required to put the MCU into DFU flashing mode. Connect the ST-Link V2 Mini programmer to the mainboard.

Power off, restore power, press and hold the power-button while executing the following commands.

Unlock MCU:

openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg -c init -c "reset halt" -c "stm32f1x unlock 0"

Flash MCU:

openocd -f interface/stlink-v2.cfg -f target/stm32f1x.cfg -c "reset halt" -c "stm32f1x mass_erase 0" -c "flash write_bank 0 hover.bin 0" -c "reset run"

BOM

General parts

Quantity Description Destination
1x Big New Bobby Car Main base
1x Xbox 360 Game controller Throttle and brake potentiometer
2x Hoverboard with STM32 MCU Wheels, controllers, batteries and misc.
1x ST Link V2 compatible programmer Flash new firmware
1x Aluminium 6061 205x70x12mm plate Rear wheel plate
1x Several two-by-four wood pieces Rear wheel structure
1x Round Stainless Steel Rod Ø10mm x 100cm Steering wheel rod
1x Plastic PVC Ø32mm x 50cm tube Steering wheel height and wire protection
4x JST SM-2.54 22AWG Wire Male and Female Connectors Hall-effect extension cable
1x XT60 60A connectors Power plug and power cable
1x Silicone Wire 14AWG red Power cable
1x Silicone Wire 14AWG black Power cable
2x Deep Groove Ball Bearing 608ZZ ID Ø8mm Front upper wheel join load distribution
1x NMB Steel Bearing Motor Grade Bearing ID Ø10mm Steering rod lower load distribution

Screws

Quantity Description Destination
16x 4 x 30mm Screws Chipboard Pozi Pan in A2 Stainless Rear plastic body to wood structure
16x M4 Washer Form A in A2 Stainless - ISO 7089 DIN 125A Rear plastic body to wood structure
12x 6 x 45mm Torx Countersunk Chipboard Screws in A2 Stainless Steel Rear alu-plate to wood structure
8x M8 x 30mm screws for motor axle bracket - use existing Hoverboard screws, 1.25mm pitch Rear motor bracket to alu-plate
8x M8 x 40mm Socket Low Head Cap Screws 3mm profile A2 DIN 7984 Steering assembly motor clamp
2x M8 x 25mm Low Socket Head Cap Screw 5mm low head in A2 Stainless - DIN 7984 Steering wheel inner stability top plate core
2x M8 x 80mm Socket Head Cap Screw Full Thread cut to 74mm in A2 Stainless - ISO 4762 DIN 912 Steering wheel inner stability body and bearing
2x M8 x 40mm Cap Socket Head Cap Screw 8mm head depth in A2 Stainless - ISO 4762 DIN 912 Steering wheel inner stability arm core
14x M8 Nyloc Nut Type T Thin 8mm in A2 Stainless - DIN 985 Nuts for steering wheel assembly
8x M4 x 30mm Socket Head Cap Screws ISO 4762 DIN 912 in A2 Stainless Steel Reinforcement screws for cross bracket
4x M4 Nyloc Nut Type T Thin in A2 Stainless - DIN 985 Reinforcement screws for cross bracket
4x M4 Washer Form C in A2 Stainless - BS4320 Reinforcement screws for cross bracket
1x M5 304 Compression spring rod Steering wheel to steering rod lock screw
4x M4 x 25mm Slotted Mushroom Screw in A2 Stainless Steering rod support brackets top and bottom
4x M4 Locking Nuts Nyloc Type T Thin in A2 Stainless - DIN 985 Steering rod support brackets top and bottom
4x M4 Washer Form C in A2 Stainless - BS4320 Steering rod support brackets top and bottom
4x M4 x 30mm Socket Head Cap Screws ISO 4762 DIN 912 in A2 Stainless Steel Motor controller board assembly
4x M4 Nyloc Nut Type T Thin in A2 Stainless - DIN 985 Motor controller board assembly
2x M5 x 16mm Slotted Mushroom Screw in A2 Stainless Motor controller board assembly mounting to plastic body
2x M5 Locking Nuts Nyloc Type T Thin in A2 Stainless - DIN 985 Motor controller board assembly mounting to plastic body
2x M5 x 16mm Slotted Mushroom Screw in A2 Stainless Bottom cover plate
2x M5 Locking Nuts Nyloc Type T Thin in A2 Stainless - DIN 985 Bottom cover plate

References