Is a strain gauge the right way to implement this? Will the strain change significantly (enough for measurement and control) between the footbeds depending on where your weight is? Is there a different type of pressure sensor which would be better? Like a weight sensor (like in electric scales for instance?) edit - TIL electric scales use strain gauges also…you’re on to it, I’ll shut up now 
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Some things like that get pretty cost prohibitive. They can also be pretty sensitive to shock.
I’m thinking maybe a solution like what onewheels use is better, technically those are just large strain gauges too, and they take advantage of a hard surface, similar to this.
If it flexed more, it might work. I do think that it will always be a problem of filtering the signal properly too. There have been a couple boards that did it successfully and I think most of them used a footpad similar to one wheels
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I think that load path is the correct mental framework here, not moment arm. But yeah.
I destroyed all of my spare strain gauges during soldering, I have to order more. 
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I meant that the moment arm from where you were standing, like when you’re in the center, it flexes more. There is a tiny moment arm between the foot pads and the trucks
Yes, strain gauges are frequently used in industry for measuring structural deflections and vibrations. The options for directly sensing the force would be load cells or force sensing resistors as far as I’m aware, but neither are rated for shear or vibration loads, so I think they would break in relatively short order while on a skateboard in the real world. Like I said a few posts ago, based on the published figures, I should only need a gain of around 20 to get a signal in the 5V range that an Arduino can measure. The gain on these amplifiers goes up to 10,000, so the fact that I’m not getting much response means that I borked something up good.
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Let me clarify what I meant. The sensors are oriented so that they only detect longitudinal strain, so it is only longitudinal bending moments apply to them. The moment arm in question is the distance between my foot and the axle (distance between foot and wheel arrows), which is where the ground pushes back up on the board. Putting my feet on the tube versus the edges of the foot pads creates the same bending moment in that direction. It just changes the path that the forces take to get to the tube.
My thinking when I designed this was that, since the vertical walls of the foot pad box have orders of magnitude less stiffness than the tube, the front and back walls would not impinge on the tubes bending. So it could deflect as shown in the picture while the foot pad box remained more straight due to the high longitudinal stiffness of the sidewalls. And the top of the deck would bend inwards and become a little bit bowl-shaped to accommodate this.
If the foot pad does restrict the tube’s bending, then the section of the tube inside the foot pad could remain straight, and I need to put the sensors outside the foot pad in order to detect any bending.
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How do you lean into turns and not have this picked up as a signal to accelerate?
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Good question. Having sensors on both sides of the tube and taking the average should mitigate any change due to asymmetry like you mentioned. However, in order for one side of the tube to bend while the other does not, there would have to be some kind of twist along its length, but it’s extremely rigid with respect to twisting. So I don’t see it as being a huge issue, even if you load it up from one side, both sides will bend.
So even if this is mitigated with the differential between two sensors. How would you freewheel around a turn with sand/gravel . Every turn is alittle different. Some turns are a blast to lean into and accelerate and sometimes require a feather touch of the throttle to keep from bustin yo ass. Every turn in the city is something new and requires split second reactions.
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Thanks for the heads up. I thought this might be necessary as well, it’s easy to program in a dead band. I can set it to be whatever I need to freewheel reliably.
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So:
After some ~extensive probing~ I have confirmed that everything is connected to the correct pin on the amplifier, the strain gauge bridge is assembled correctly, and every junction is at the proper voltage level that you would expect while in operation. Plus, turns out that the 79mV zero load output I thought was weird was within spec.
I also noticed that I can get large output readings when I press my toes directly over the spot some of the gauges are sitting on the foot pad, I assume since the panel is thin there it will flex locally under those conditions. I think this is more evidence towards the idea that I put the sensors in a poor location where they don’t get flexed very much, so I ordered some more.
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I’ve gone crazy enough from the boredom of waiting for new strain gauges to arrive that I finally reached the point where hours of hand sanding was preferable to doing nothing. So, I did a dry fit of the motors and drives, marked the point of motor bite, and used some sand paper wrapped around a cardboard tube to carve out these scallops.
The angle-ometer on my phone tells me that the board can lean up to 20 degrees, but these still collide at 17. Since I can’t go much further without interrupting the deck’s profile, I’ll flush mount the rear truck next.
EDIT: Also, carving a gouge through half of my structure may be a bad idea idk
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You can also wrap the sand paper around the wheels. Direct fitment check.
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The issue is motor bite instead of wheel bite (I made the nose and tail plenty long, but the rear is pretty low due to the adj. baseplate), but that’s a good tip, I’ll keep it in mind. Probably not good to dirty up the motors with that strategy, I’d imagine.
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Having spent all damn day carving this out with a dremel cutoff wheel, I would advise anyone considering the same to do literally anything else.
The cut goes halfway through the deck, raising the rear by 8mm, and making it basically level with the front. I did a fit check, then epoxied over all the exposed wood to keep up the waterproofing.
With my trucks as loose as I can get them, my full body weight gets it about 2 or 3mm away from motor bite, which I’ll call a success. Might bite if I whack a pothole, but c’est la vie.
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Haha, do you not have a router?
That’s a 2 minute job w/a router & 3D printed/otherwise fabbed template.
this hurts me personally and I have nothing to do with this
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nopeeeee. I don’t even have a dremel, I have a prestigious WLXY Mini Drill Tooling! It’s a $25 rotary tool from Hobbyking that runs off a 3-cell lipo battery. It’s actually not horrendous, considering what it costs, but the motor does overheat on bigger jobs like this.
Took 2 hours instead, cuz those grinding wheels suck for cutting wood. Going out to buy a different attachment would have taken 2 hours for me also, so meh. If I have to do this again I’ll do it properly.
Some major tool shaming going on here, i agree. Tsk tsk.
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VIIIIIIIIIICTORYYYYYYYYY!!!
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