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Platform Types
Skateboard I-Beam based prototypes
Propulsion Sub-Systems
Testing and Usage



Skateboard platform using I-Beam

Using standard aluminum I-Beam stock for rapid prototype development was settled on in early 2020. The main factor for determining the ideal I-Beam cross-sectional size is being able to physically place necessary components in practical and safe (protected) places on the bogie itself. Both the controllers and batteries are subject to physical damage, and this would have expensive (controller) and dangerous (batteries) consequences. The I-Beam also has to be long enough to accommate the front and rear "trucks" (axle assemblies), as well as the track-switching assembly. The current length being used is approximately 17.7", since that's a convenient size for being held in our CNC jig.

Shown below is a bare prototype with 2.5" I-Beam and 60mm wheels, shown on Highland 30' test track. Will likely try the next size up (3") I-Beam next. The nominal I-Beam measurements used here refer to the top/bottom widths of the I-Beam, not its height (which is greater). Both trucks shown here are passive (non-drive) 60mm wheels, and are spaced (shimmed) out to the end of the axles using copper pipe pieces about 1" long. The trucks we are using on both ends of these bogies are described as "rear trucks" and are designed to accommodate 83mm wheels with built-in hub motors. No axle spacers are needed when these hubmotor wheels are used.


Below you can see both the 2.5" (in the foreground, with trucks attached) and the 3" size (in the back) I-Beam assemblies. The VESC 6.6 dual controller PCB will only fit within the rib cavity on the larger 3" I-Beam. The lipo brick battery will fit "inside" either size I-Beam. Note that all trucks use an elastomer-type bushing as a shock absorber, and as delivered from the factory the axle alignment is not necessarily parallel to the truck's top mounting platform. Using a M14 wrench to loosen the shock absorber assembly while the skateboard is on a level surface, and then re-tightening the assembly will help all 4 wheels being on the surface at the same time.




The configuration of the front and back guide wheels is being tested here, using available 60mm passive wheels. The Highland track has a 190mm gap in the bottom of the track, so the guide wheels will be adjusted for that initially. The San Jose track has a narrower gap of 160 ??? mm. The side-to-side adjustment of the guidewheels appears to work best when they are set to just not touch the edge of the rail when the bogie is perfectly centered on the track. On the Highland track, the wheels run on the inside of "L" shaped aluminum stock which has a radiused 90 degree turn to the vertical, so the hubmotor wheels tend to avoid climbing the radius. This tendency helps the motorized wheels run in the center of the track. Non-motorized wheels have a narrower profile (side-to-side), so they never get that far to the edge of the track in the first place.



Photo of two bogie prototypes on Highland track. The closer one has the larger 3" I-Beam format and shows the guidewheels mounted. (Note the 14 pound bowling ball hanging from an earlier generation single motor bogie further down the track.)


Five second video showing the bogie being pushed down the Highland track. This version of the bogie is using 83mm wheels in all positions. This is just testing for smooth running and adjusting the guide wheels -- not testing the motors. When the guidewheels are properly adjusted and the track is smooth, the bogie will easily coast 20 feet. This shows that there is relatively little "cogging" associated with these BLDC motors.


August 1, 2020: Showing the I-Beam bogie powered up, controlled by a center-neutral joystick (5K pot) acting as a two-way throttle. I used the VESC-TOOL app to configure the dual controller's input to be "ADC" (vs "PPM") and set the mode as "Current Center Reverse". The joystick is connected to the 3.3V positive, negative/ground, and the ADC1 pins on the primary Comm (JST-PH-8M) port on the dual controller. Pushing the stick up makes bogie go in one direction, pulling it back makes it go in the other direction. No pressure on the joystick leaves in its center position, which is 'throttle neutral'. Part of the configuration for the controller's "Input" mode includes a calibration process for this particular throttle device. This makes sure that there is no movement when the joystick is at rest, and that there are no "dead zones" in the joystick movements.

Obviously, this hardwired method is NOT a practical way to control the bogie, but this is the first 'live motors' test on the Highland Track. Next up is a little bit of wire management and other cleanup, then this little bogie is ready to go West. Do YOU know the way to San Jose?

This is the I-Beam prototype #1 ready for shipment to San Jose, 6 Aug 2020. The 83mm hubmotors have been electronically matched to the onboard dual VESC controller. The controller's Setup here has been temporarily configured as an analog (ADC1) joystick throttle for testing purposes. The joystick throttle and its JST-PH-8F connector (white wire bundle) will not be included in the shipment, since the bogie will be controlled via a remote (wifi radio) control system on the San Jose Spartan Superway track. The 6S lipo battery (attached via a red bungee cord) shown here, plus a spare, will be shipped (via ground) in a separate box. The SJ Spartan project will obtain an appropriate 'hobby balance charger' locally to maintain these batteries until a more permanent bogie power system is developed.

For a short test run, see short video below the following photo.

Remember that in the video above the bogie is actually running upside down, and a yoga mat isn't the same as an actual track surface.




Using CNC for some bogie machining tasks...

A chunk of 1.25"x7"x0.3" scrap aluminum being made into a guidewheel mounting bracket. Here we used the HandiBot CNC machine to cut the adjustment slots in the ends of the bar stock. The slots are 1" long and designed to accommodate a stub axle made from 3/8" all-thread, part of which is machined down to 8 mm to fit in the standard skateboard wheels used as the guidewheels. Sideway adjustments are made via tightening the stub axle in the correct location within the slots. Depending on the loads the guidewheels are subject to, it might be adequate to simply use M8 (5/16") bolts as the stub axles for the guidewheels. We'll see if we break anything.
Nothing broke, but it turned out that the thicker aluminum guidewheel bracket had the undesirable side effect of raising the guidewheels higher and leaving less of the wheel to contact the rail. The next iteration of the bogie is using a thinner aluminum, but in an "L" shape, to support the guidewheels. Also, we're using the 5/16" all-thread to fashion the stub axles, and it appears to work just fine (tested with a freely swinging 180 pound weight hanging from the bogie, but on a straight track).



Typically it's a greater effort, in terms of both time and materials, to establish a proper setup of the material to be machined than to do the actual machining itself. Doing many repetitive operations, with extremely repeatable accuracy, helps to make this approach still advantageous.



Photo of the smaller, 2.5" I-Beam jigged up for machining with the HandiBot. The I-Beam is clamped into the vise below to prevent downward deflection when the cutting bit bites into the material, and is kept from moving horizontally by the two boards clamped against it along its length. Rather than having the CNC's x/y=0 (hardware home position) be in the very bottom left of its machining range, I find it easier to reference all measurements from the physical center of whatever piece is being machined. This gives greater freedom to clamp the piece wherever it will be held securely, and then to simply 'home' the CNC on the center of the workpiece. For all rectangular pieces this is easily done by scribing an "X" to find the center and then marking it. Here I used a countersink on the drill press to make it easier to see the center when the HandiBot is over the workpiece. In this particular instance, there are two 'centers' on the top of the I-Beam, one for each of the trucks and its associated machining operations.


Nothing beats a great drill press when it comes to just shooting a bunch of holes. What I've been doing is using the CNC with a carbide single flute upcut 3/16" bit to machine 'pilot' holes down to 0.1" and then finishing the holes on the drill press with a stock HSS bit, using WD-40 to get cleaner cuts. Since I sharpen my own drillbits and the drill press has a huge motor and a bullet-proof spindle, this saves wear and tear on the expensive CNC bits and machine. In addition to having very accurate pilot holes, you also get a perfect reservoir for the cutting fluid.




Highland Bogie Test Track

Cross-section dimensions of 30 foot Highland bogie test track





San Jose Bogie Test Track

Several photos of the 10 meter San Jose bogie test track (photos courtesy of Bill James)






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