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• [E2][T] Spin-off motor, PCB and UV LED?

  kelvinA • 4 days ago • 0 comments

  • Looking at my options, i think the best thing to do now is make a prototype that doesn't include a motor.
    • This would also make it easier to implement if someone just wants to have an LED lit, force sensitive but otherwise basic slider, similar in nature to the encoders installed in some hobbyist keyboards.
    • At least if literally everything else about the current concept works, it would be easier to justify £50 and up for a prototype motor or spending days making my own.
    • [E2] The proposal... fails. I need a haptic source, where the lack of which was partially responsible for the non-use of #AirBerries and SpaceExplorer 
  • I've created project #Tetrinsic Toroid [gd0152] which would contain the BLDC + PCB.
    • [E2: May 1] - Renamed to a trendier "Tetoroidiv" to keep with the coincidental tradition of an ever so slightly longer name on each subproject;

  • Move the UV LED out of Tetrinsic's domain
    • I've computed a new Tetent solution, and started on an Itinervate one too, and it seems that the LED would be better mounted on the enclosure or PCB mounted with Tetrinsic, and not something to bundle along with it.
    • The above is the potential Tetent solution side profile. The top/right orange part is the belt, and the left orange is an LED that just lines up with the ones on Tetrinsic to give the illusion that it's continuous. In actuality, this curve has the 21700 cell.. The grey lines is supposed to be a 3D texture of diamond squares.
    • [E1] Cleaner, 1:1 sketch:

  • Additionally, water resistance becomes mandatory.
    • I need to get "that 99.9% uptime". What I mean by this is that I want to be able to use it in 99.9% of reasonable situations. Thinking through thoughts during a shower or while running in the rain between locations is what I'd call a reasonable situation.
    • I think there's a notable and valuable gain from the difference between a 90%, 99% and 99.9% solution.
    • [E1] Unfortunately, the COB strip is IP20, so not off to a good start. 

• [E1][T] More accurate power estimates

  kelvinA • 4 days ago • 0 comments

  So I've added torque radius into the equation, which essentially is the effective radius of the force if  there was no gearing or reductions. This paints a very different picture now that there is a 2.6 : 1 reduction for getting the power from the motor and into the belt. 

  Additionally, my current understanding that, all else being equal, the proportionality of rotor radius, rotor length and number-of-turns is quadratic, linear and linear respectively. Thus, I've calculated the expected torque constant difference from the motor mentioned in the paper with a 40mm long rotor and 78 turns. There is barely enough space for a 50mm rotor and that would increase the torque constant to 43.5. This calculation assumes that a 4-pole rotor can be obtained. I'd expect less torque if using the 2-pole magnet I found on aliexpress.

  The $32 1656-18W motor also now has more favourable power consumption now.

  I'll have to see what the torque numbers are when at the new max expected speed of approximately 1200RPM.

  [Apr30] So I can see the amp-turns in action. I can fit 36 turns at 0.25mm diameter, 78 at 0.18 and 106 at 0.15mm, and with a 50mm rotor, they all get essentially 0.6W peak when putting in the resistances and compensating the amp. The only thing that changes is the minimum drive voltage, dropping down to 1.29V for the 36 turns configuration.

  Thus, I think the strategy needs to optimised from other perspectives. For example, If I aim for a minimum drive just under 3V, such as the 78 turns configuration, I could use the battery voltage directly. I'm already planning to read the currents drawn by the motor, thus I'd save on voltage conversion losses. With this in mind, the 18V version of the 1656 makes more sense:

• [R] 9.6mm V-Belt

  kelvinA • 5 days ago • 0 comments

  From the sketch I showed in the previous log, Fusion says that the belt length I need is 237.98mm... essentially 238mm. So I just searched "238mm belt" and coincidentally enough, such belts exist:

  I've seen these belts before. I just didn't know that they specifically came in 238mm loops.

  The black ones come with a thick looking groove on the underside, which is probably good for the sliding straights but not exactly ideal for the gear-teeth pulley.

  The orange ones are slightly cheaper, and because they transmit more light, it's much harder to guage if the underside profile is the same as the black, but it's probably safe to assume so. What I do know is that the black one is rubber and the orange is polyuetherane.

  Unfortunately, both of these are 3.2-3.3mm thick, which is almost double the thickness that I'm looking for.

  It looks flexible enough:

  Looking at what this belt attaches to, the minimum turn radius might be OK if the smaller pulley is 19.5mm:

  It also seems self-centering, meaning that I wouldn't need the flange and so I could probably tweak the gears to use something like this:

  12T+24T keeps ratios in nice integer values, though I can't find a tight-fit variant:

  The final height is 24.5mm, which isn't ideal but doesn't fail any downstream solutions.

• [M] Sketch around Tetrinsic Toroid

  kelvinA • 5 days ago • 0 comments

  So this is the wireframe sketch of the proposed Tetrinsic strategy. The belt has just been approximated, but the actual belt path would go though both rectangles in the centre plane (which represent the cross section).

  The plan is for the belt to roll on SLS printed, flanged 30T spur gears, and then connected to a crown gear (which I've never heard of until today) via an 18T gear. All gears are 0.5M. 

  Additionally, addressable LEDs exist in a 2.7mm COB strip which has 160 pixels per meter. The plan is to have one strip on each side of the belt. I'd consider this a "side-lit" approach.

  I've been able to find an Alibaba supplier of a 4-pole, 50mm long, N35 magnet which is $7/ea for 10pcs, along with $64 UPS shipping to the United Kingdom. Thus, the magnet is about £12.30/ea after VAT. For now, I'm going to try the 2x40mm N42 magnet approach.

  I also believe that I should be using the line / terminal resistance for calculating the minimum drive voltage and 0RPM wattages, as at least 2 coils will always need to be powered in a 3-wire BLDC to complete the circuit. The power numbers look steep but also more believable.

   VMOT = 5V seems like a good value as the LEDs also need this voltage.

• [R] 1656 18W BLDC and controller considerations

  kelvinA • 6 days ago • 0 comments

  I asked to see the datasheet of the $32 motor and this is what I received:

  The phase resistance is 1 / 2 of the terminal resistance (the resistance between two terminals, I presume), so it's 9 ohms. 

  At this point in time, I'm also asking around for the rotor magnet and it's sounding like 1 pole pair is much easier to make than 2 pole pair:

  It seems that Sinbad Motors has also gone with the 1 pole pair approach.

  Additionally, the STSPIN32G4 is only a gate driver. Basically, I'd need to provide 6 MOSFETS to actually drive the motor, and that's a lot of traces:

  It seems that the TLE9879 is similar in this regard:

  I thought that all that would be integrated, like this HT32F65C40F:

  Thus, it does seem that I'll just stick with the original controller plan of using tSPI, which would only need 3 pins since it'll likely be the only SPI chip on the PCB.

• [E1][B][M][R] Custom 16mm Slotless?

  kelvinA • 6 days ago • 0 comments

  From the centre outwards, you've got:

  • 2.5mm shaft
  • 2 pole pair magnet of 8mm diameter
  • 0.4mm air gap
  • 0.4mm spiral vase mode housing print (in PBT ideally, but ABS should be OK)
  • The space for the copper (1.805mm)
    • Coincidentally similar to the 1.85mm copper height in the 580KV motor.
  • Tolerance gap of 0.2mm
  • Outer lamination yoke that is 1.2mm.
  • I opened up the 580KV motor I got to find out what size was used.

  Copper foil?

  You might be wondering what this is. This is the approximation of 3mm copper foil stacked on itself. Since copper smaller than the magnet width does not provide any torque (so I've heard), it makes more sense for the tape to hug the edges.

  Assuming 

  • a 0.06mm thick tape, of which 0.03mm is copper
  • 90mm stator / rotor length
  • 11mm on each end to connect the straights together

  I got a copper tape length of 6060mm (6m) and a resistance of 12.1 ohms., which isn't great but kind of tracks compared to the 8.2ohm, $32 motor that looks like it's probably 55mm in total length and thus probably has a 45mm long stator. It obviously doesn't help that less than 50% of the CSA is conductive.

  Copper wire

  For the more traditional approach, it looks like there will be at least 15 * 8 = 120 turns (or 60 if I decide to double-up) using 0.2mm (32AWG) wire. Estimating for the extra space filling, it's probably going to be at or close to the 130 turns in the paper. It would also seem that a 3-pole-pair rotor would be ideal for this design. 

  Assuming that the average length to connect the straights is 9.4mm and that 138 turns are used, I get 27434.4mm (27.4m) per coil, which corresponds I get 14.7 ohms.

  Is that high? Maybe? However, if this design -- with rotor of length 90mm and diameter 8mm -- can match the 75mN.m obtained with the paper's motor (rotor of length 25mm and diameter 11.5mm), The minimum driving voltage and power consumption would suffice:

  If anything, I worry that the Kt would be too high and the minimum drive voltage would be lower than 6V. Additionally, manually winding dozens of coils for over 100 turns each sounds like it would be laborious. I could always double up the 0.2mm wire, but it seems that it would just be easier to work with 0.25mm wire:

  Here, it seems that at least 80 turns would be present, probably something like 82. If my tolerances are good, I could probably fit a 7th row and perhaps get to 90 turns. For 82, the total wire length is 16301.6m and the resistance is 5.4 ohms.

  Quick finger tests

  So, just to make sure I'm on the correct page, I put my scale against the wall and pushed my fingernail for Finger2-4 (see below) and the max reading across the tests were about 300g, thus I'm correct in aiming for 30mN virtual-endstop force.

  Secondly, I played some music, slowed down so that 1/8th of a beat corresponded to the time it took my finger to move 50mm. This is because 16 x PI = 50.3mm. Then, I used an online BPM tapper to get 1/4th, which was 220BPM. Thus, the top RPM I expect to see on the motor is >= 440. 

  Tube instead of laminations?

  The reason I want to know the top RPM specifically is because eddy current losses are a result of how fast the magnetic flux is changing. Considering that, unlike 99.9% of motor applications, this motor is working at no / low speeds of essentially 500RPM max (unlike the 10K that the authors of the paper called "low speed"), I'm wondering if the eddy current losses will be low enough to use a tube of magnet-permeable material. 

  Stainless Steel 304 could be a good choice, as it's readily available and, unlike things like mild steel, doesn't corrode in contact with water. I don't know if magnetic PLA is more or less magnetically permeable, but stainless steel probably would do a better job of dissipating heat.

  The paper says that the magnetic "air-gap" is essentially from the rotor magnet to the outer yoke and thus should be minimised, and that it's a balancing act between that and...

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• [E4][R] Frameless VS Slotless BLDC

  kelvinA • 04/26/2024 at 06:34 • 0 comments

  Introduction

  So I went over to the SmartKnob View discord to try and find some answers on motor options. The user K(at)B (the one behind the Ratchet H1)  noted the potential implications of airlines likely not wanting to take belts that emit magnetic fields. I also had the concern that these magnetic fields are likely to interfere, especially if a Tentrinsic for the thumb is used.

  He then suggested this:

  In practice, I'd expect to need 4 gears, not the two shown in the 3D sketch; 2 plastic 0.5M bevel gears and then 2 more gears that transfer the load to the pulley for the belt. He also suggested FSRs, and there happen to be long ones for an ok price, especially since an MCU ADC would be able to read it instead of requiring a fast but expensive 24-bit ADC. 

  This video shows that scissors can be used to cut the FSR to the required length if needed:

  [Apr 27: Edit 3]

  It seems that there's an even more ideal sensor, that works in the 20 - 500g range and is 10mm wide, which are both attributes that I'm looking for. There's also a schematic which essentially says that I can put this in a voltage divider along with a 10kOhm resistor and feed the output voltage to an MCU.

  [End of Edit 3]

  Thus, this was the start of searching for my motor options. Since 18mm Tetrinsic spacing seems more like "the standard value", and many motors seemed to be 16mm diameter, it makes sense to set the new maximum width of a solution to about 16-17mm.

  Things I learned about motors

  I think it's best if I fist provide some reference material that better helped me understand what to be thinking about when looking for a motor:

  • Lower KV motors are designed to be run at lower speeds and more likely to have minimal cogging.
  • KV seems almost irrelevant for coreless / slotless motors that have 0 cogging.
  • Torque for this application needs to provide 100 - 150gf continuously and over 300gf peak. 
  • This is because the former is needed when emulating an analog stick. This site suggests that 75g is "standard", 100g is "hard" and 150g is "strong". Due to potential frictional losses, it's probably better to slightly overspec the standard torque, and that's why I'm aiming for 100gf minimum. 
  • The latter is to emulate physical limit stops, and isn't expected that the motor will need to produce this torque for more than about 500ms.
  • Torque is proportional to current. That's it. Just current. 
    • It was a bit surprising when I found this out, hence the emphasis.
  • The voltage determines the maximum RPM of the motor. This is due to back-EMF generated when the motor is spinning. If it's not spinning, there's no back EMF. 
  • The voltage and phase resistance determines the max current that can be sent to the motor. It seems that the aim is to get a low PR as that means lower voltage, which results in exponentially less power used.
  • It seems that the torque constant (Kt), mN.m/A, is more important than the voltage constant (Kv). 
    • Smaller wire = more coils = lower Kv, but less current can go though said wire so the effect is cancelled out and thus Kv has no real connection to Kt.

  Frameless

  I noticed that, from looking at motors found for the Smartknob View, there were a few motors that were shorter than 15mm. The 2204 260KV motor, for example, is 13mm. However, looking at images inside the motor, it seemed that there was a notable amount of unused space inside. Some time later, I came across frameless motors, where just the stator and rotor magnets are sold: 

  Long story short, these are the only options available, but the good news is that I just need the 1 solution and the WK2806 seems to provide!

  • Phase resistance: 5 Ohms
  • Kv: 121
  • Nominal current: 0.81A
  • Nominal torque: 600nM.m
  • Stall current: 1.22A
  • Stall torque: 900mN.m

  Now, you may be wondering "Those seem to exceed what you need. Why not go for the 2205?". Well the issue is that the ball chain has...

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• [R] Coil All Around belt linear motor?

  kelvinA • 04/23/2024 at 07:33 • 0 comments

  The title is a play-on-words of GAAFET, standing for Gate All Around, Field Effect Transistor.

  Now that the #Coaxial8or [gd0144] is in a bit of a break period as I wait for the new heatblock design to be fabricated and arrive, I have been looking into Tetrinsic again. I still feel like if I had to choose 1 single project to "get past the post" (i.e. design, manufacture and implement into my daily life), this is it; unlike 3D printers and XR glasses, I unfortunatley haven't found enough information on the internet that suggests that some company would come out of "stealth mode" with a sufficient offering, and even if one did come out today, they'd most likely say it will start shipping "in months" but implications would mean that "in years" would be more accurate. On the other hand, I haven't been able to indirectly find this project through my search queries, so there could still be similar projects and solutions that I haven't come across yet.

  Anyway, I came across this video about linear shaft engineering:

  My strategy I thought up back in September 2023 was the U-shaped linear motor, just with the coils and magnets swapped around. This aleviates the cooling drawback they mentioned, but understandably still has the drawback they mentioned where only half the magnetic flux generated by the coils used (the other half is in the opposite direction). 
  

  The video states:

  Force (N) = Current (A) x Magnetic Flux Density (T)

   Thus, for a fixed current, more force is generated with a higher density. This also suggests that, for this application, it's better to drive the system with as low a voltage as possible.

  Looking into the DRV8311, they're just simple H bridges at the end of the day so it's not like the chip is doing anything fancy with the input voltage. Additionally, I found these answers on stackexchange. While doubling the voltage will double the torque, it would quadruple the power required. Thus, since the power for each Tetrinsic is limited to prolong battery life, and I mainly need the torque in very low - low speed situations (e.g. sub 300Hz haptics) higher voltages are actually worse, as are very thin (0.1mm) wires used to make the coil. Instead of 12V, it now sounds like it would make more sense to run everything on 3.3V, which would still allow for up to 16.5W per Tetrinsic.

  Additionally, I've read bits and pieces that should lead to improved performance, but this answer brings everything together. For example, it did seem that a thicker magnet would increase strength when reading Doubled Forces, which the user "cinaral" confirms. They also say that the thickness of wire -- thus its current carrying capacity -- roughly balance out with the amount of turns that can physically fit in the space, which sounds like a foil coil is still the more ideal strategy from a heat dissipation and manufacturing standpoint.

  As mentioned in the video, magnets emit their field in all directions. What wasn't mentioned, but probably just as important, is that the coil has it's highest flux density in its centre:
  

  The video proposes a shaft where like-poles are pressed together with no airgap inbetween:

  This configuration is probably to force the flux outwards from the magnets:

  It likely is also so that the shaft doesn't just act as one massive magnet. With this in mind, the plan is to move from a 10x3x2 to a 10x3x3 magnet so that I can place them in this configuration.

  Manufacture

  I already knew about linear shaft motors (but not their benefits over U-shaped ones), but the issue is that I still need to actually make a belt or a motor or both. With the U-shape solution, both of these can be made separately, wheras with a coil-all-around solution, one would have to be made in-situ.

  Additionally, I need to consider how individual magnets will come together to form a belt in the first place. Since the magnets are relatively small and the belt...

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• [A] Details page before 14 Jan 2024

  kelvinA • 01/14/2024 at 12:46 • 0 comments

  Details

  This project is intended for #Tetent [gd0090], #Tetrescent [gd0150] and #Leti [gd0149]. Tetrinsic is the "input element" and in the same category as a keyboard switch or controller joystick.

  Notable Tetent projects, sorted by project log count:

  • Input element: #Tetrinsic [gd0041] 
  • For Teti: #Tetent [gd0090] 
  • Wearable: #Tetent TimerSpy [gd0136] 
  • x86 PC Handheld: #Tetent UMPC [gd0149] 
  • Desktop: #Tetent TestCut [gd0139] 
  • Solar Powered: #Tetrescent [gd0150] 

  Examples of Similar Products

  I found this video which shows how the slider would ideally perform, just that you can press down on it and it can be set to allow movement "infinitely" in either direction.

  
  
  

  ---

  Tetrinsic is the merge of the above motorized sliding potentiometer and the SmartKnob View:

  ---

  The visible area is designed to be as minimalist as possible, so that things like an LCD backlight can be used for designs:

  
  

  The cool thing about Tetrinsic is that you don't have to remove a magnetic top layer (as seen in Flux) or hotswap out the switches (on a more traditional keyboard) if you want to change tactility. Just tune it to your precise tastes in software.

  Concept History

  Tetrinsic Concept3.2X2 (interwoven, dual BLDC motors)

  Unfortunately, Concept3.2 had a turning radius that was too ambitious. Thus, on August 10th 2023, it was decided that a redesigned Tetrinsic with dual motors and the ability to weave the Thumb Tetrinsic around the FingerN Tetrinsics would be the best strategy forward. This also allowed anything to be placed inside through the loop.

  From Aug 25th, the focus has been on designing for #Tetrescent [gd0150], where a solar cell is placed inside said loop.

  Tetrinsic Concept3.2 (single 3.2mm chain, dual TFT displays)

  After creating the new Tetrinsic PCB that uses the ESP32-S3-MINI-1U, on June 30th 2023 I eventually decided to increase the size of the ball chain to 3.2mm, make it so that there is only one path that is exposed and, to increase the usable length : body ratio, doubled the screens. This is the first concept CAD model that was electromechanically complete, made on July 18th 2023.

  Tetrinsic Gen 2X2 (smooth top wedge shape)

  Development started on Jan 12 2023 and moves the components so that the load cell is parallel with the LCD and that the motor does not protrude the top surface. The aesthetic is further improved whilst improving the ability to slide into a pocket (for TimerSpy). This was first called Tetrinsic Concept4.
  

  I then spent subsequent weeks turning this concept into a more fully-designed prototype, and added a photovoltaic solar cell variant. This concept was using 2.4mm ball-chain that slid on 1mm diameter stainless steel tubes. The above is more-or-less as far as I got before I started work on the PCB. Unfortunately, the design got a bit too large such that I couldn't come up with a solution for #Tetent TimerSpy [gd0136].
  

  Tetrinsic Gen 3X1 (wedge shape)

  The shape is to allow Tetrinsic to be mouned on the back of my hands and slide into pockets for TimerSpy and fit into the square prism shape of TestCut. A notable improvement is that an off-the-shelf load cell can be used, saving on build time, increasing precision and reducing displacement. 

  Development started from Jan 4 2023.

  Tetrinsic 2.0 (LCD backlit, dual 2mm ball chain)

  Tetrinsic 2.0 is an internal name to refer to the redesigned version I started developing from Dec 26 2022. It's not actually the second version of Tetrinsic, since I haven't actually made a first one yet. Think of it as Tetrinsic 2.0mm.

  This redesign should bring advantages such as lower sliding resistance, shorter allowable finger offset distance, shorter height, a full-bridge-configuration pressure sensor instead of an iffy half-bridge, a much more popular microcontroller (RP2040 vs M032) (thus, better software support) with more RAM and FLASH memory and,...

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• [E2][X] Plotting finger movements

  kelvinA • 12/23/2023 at 20:56 • 0 comments

  So I spent maybe 2 hours setting up the groundwork of this excel spreadsheet and then maybe 15 minutes actually puttinging in some values. This spreadsheet assumes I'm using #Tetrescent [gd0150] and in a mirrored (aka vertical) mode where for both sides, the placement of characters is Finger1->Finger4.
  

  I've tried plotting the position and force level in the black chart, and the change in position and force level in the white chart. For the most part, I can't really gather any insights so far. Perhaps it's just because I've only input the first sentence to see if I should continue or not.

  The fanciest part of the spreadsheet is the use of a lookup table, but the command LOOKUP didn't work. The good news is that we now live in the advent of AI:

  In this spreadsheet, I assume that a blank field for the char columns means no input, i.e. 0 force from fingers. Thus, my equations are:

  For position X: If the char field is blank, use the previous position
  (I'm just lifting my fingers), else find the corresponding position for the 
  char:
      =IF(ISBLANK(E4), B3, INDEX($AS$2:$AS$56,MATCH(E4,$AR$2:$AR$56,0)))
  
  For force F: If the char field is blank, the force is 0, else find the 
  corresponding force for the char:
      =IF(ISBLANK(E4), 0, INDEX($AT$2:$AT$56,MATCH(E4,$AR$2:$AR$56,0)))

   for the fingers, and 

  For position X: If char is blank and the previous position was 1, go to 2, 
  else if char is blank, do the same as the fingers.
      =IF(AND(R3=1,ISBLANK((U4))), 2, 
      IF(ISBLANK(U4), R3, INDEX($AS$57:$AS$70,MATCH(U4,$AR$57:$AR$70,0))))
  
  For force F: (similar thing to the position calc and finger force calc:
      =IF(AND(R3=1,ISBLANK((U4))), 2, 
      IF(ISBLANK(U4), 0, INDEX($AT$57:$AT$70,MATCH(U4,$AR$57:$AR$70,0))))

   for the dynamic spacebar thing (which could be the Thumb or Finger5). This is because the finger (in this case, Finger5) rests in the below white box of the layout:

  The user pulls back to column 1 --  similar to a trigger or that sliding, on-off toggle thing on the PSP -- and then the motor exerts a force to push the finger back to column 2. This allows me to have a virtual button without having to push down on the Tetrinsic. It's similar to how I envision the datahand-inspired layout to function.

  ---

  Now, the reason why I typed the phrase out first is so that I could also get pseudorandom typos integrated into it. For this monkeytype test, I had 2 errors. The first one has been recreated here, taking the Tetrinsic layout into account:

  [24 Dec: 20:00]

  So I've spent a while learning things like the =OFFSET command, so that I could have a dynamic start-end point for the charts without having to laboriously edit tens of values. BingChat initially generated code for =INDIRECT, but then I started moving colums around to make the spreadsheet more readable and the positions didn't update in the charts.

  I've moved things around so that everything is a lot more readable. Hopefully, this gives those finding out about this project a better understanding on how the typing system works.

  Then I needed another run. I decided to go with a quote this time, and despite it being 5am without having slept yet, I managed to 100-accuracy it.

  For both of these, I've assumed I'm proficient enough to realise when I can reduce finger travel, such as being able to keep Finger2 on "O" between rows 76 - 78.

  From doing this, I was able to make some slight optimisations to the layout, which can be seen below:

  It's still kind of tough to...

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