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Thread: Scooty-Puff

  1. #11
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    Re: Scooty-Puff

    The controller PCB has been reworked several times since the last post, but thinking I'm finally starting to settle on things. Switched to JEDEC MO-299 (HSOF/PSOF) package instead of TO-263 since it is a bit smaller overall and same price (80V, 3mOhm, $2.50~2.60 each at 10+). The VIN+, PHASE, and VIN- connections are separated 20mm on center vertically and each half-bridge is separated 24mm on center horizontally on a 100mm x 100mm board (with M3 mounting holes on a 94mm square) with the PHASE connections centered at 50mm vertically. The top 20mm is taken up by the resonant snubbers and the bottom 20mm is currently blank with all signals routed to a 2x24 header (2mm header pitch; two headers spaced 18mm apart).

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    The power connections are 5mm plated vias under the MOSFETs (for VIN+ and PHASE) and in free board space (for VIN-). They get a Cu-110 plug press-fit into them. The plugs are 6.35mm rod cut 6.35mm long with the press-fit side having 12 divots spaced 30 degrees apart milled out by a 1/8" endmill at a radial distance of 3.925mm to a depth of 1.5~1.6mm. The other end of the plugs are milled to a square 5.675mm on a side to a depth of 2mm and a centered hole 1.6mm in diameter to a depth of 3.75mm (hand tapped to M2 or self-tapped by screw). The mating surface for the plugs is a 2mm deep cutout made from a 1/8" endmill tracing a 2.5mm square. The VIN+ and VIN- buss bars are simply 0.25"x0.5" Cu-110 bars with 2mm holes spaced 24mm on center with a 4.2mm through-hole on either end for M5 screws to attach power leads. The PHASE connectors are 0.5"x0.5" Cu-110 cut ~0.5" tall with the plug mating surface milled into one end and a 2mm through-hole centered in it. The opposite side gets the 2mm hole expanded to 4.2mm for a depth of ~8.3mm to accommodate the M2 screw's head and for an M5 screw to attach power leads. Should wind up with the VIN+ and VIN- buss bars hidden under polycarbonate or delrin plate with cutouts to keep the PHASE blocks nicely isolated (and provide additional resistance to blocks rotating). The M2 screws to attach the VIN+ and VIN- buss bars are 10mm long and the M2 screws to attach the PHASE blocks are 6mm long.

    If the M1585 (15mm x 8mm x 3.5mm N50) magnets from supermagnetman are actually going to be restocked, then each 32mm long rotor will be made from 20 of them on a stack of six 5.5mm hardboard spacers from ponoko for a ~$20 rotor. Would be ~$30 each for an entire motor including donor stators, new windings, new shafts, and new bearings. Cost likely increases a bit if I have to make new endcaps to clear the windings instead of reusing the original castings.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  2. #12

    Re: Scooty-Puff

    What gauge wire will you wind with?

    Also: Are you seriously making your own plugs out of stock? That's both awesome and crazy :-)

  3. #13
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    Re: Scooty-Puff

    Quote Originally Posted by jwatte View Post
    What gauge wire will you wind with?

    Also: Are you seriously making your own plugs out of stock? That's both awesome and crazy :-)
    Still same as original post: 27mil Cu-110 sheet cut 4.8mm wide insulated on one wide face by 6mm kapton (~12AWG).

    Yes, making my own plugs and mounting blocks. Should be easy enough to make the plugs and blocks after building a few jigs. A drill press would even be sufficient for the press-fit side of the plugs after making a small 12-position rotary mount for the 1/4" rod. Cutting the square on the other end of the plug can be done using any of several different methods. The cutout for preventing the mount blocks from rotating on the plugs is the only operation maybe requiring a proper mill.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  4. #14
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    Re: Scooty-Puff

    Onto the topic of brakes...
    Conventional disc brakes are essentially two pistons stuffed in opposite ends of a common center-fed cylinder to push in opposite directions: one piston acts on inboard pads and other is part of caliper structure that acts on outboard pads. Since I could not stuff decent brakes inside the 4" rims even if I wanted to, I had been thinking of making the brakes as multiple pistons in their own cylinders on both sides of the rotor. A little more thought, and I can make 'normal' style brakes easily enough with even less material and fewer machining operations. Regardless of design, all pistons in a wheel are controlled by a hydraulic pump in series with a spring pressurized reservoir: brakes are engaged by pumping fluid from reservoir to cylinders, and brakes are disengaged by pumping fluid from the cylinders into the reservoir. When power is lost, a solenoid releases a needle valve to create a direct connection between the pressurized reservoir and cylinders to engage the brakes. Compression of the reservoir's spring has to be manually adjusted with time to maintain the minimum fail-safe system pressure as the 0.25" thick pads wear down.

    Thanks to the ever wonderful Shigley's Mechanical Engineering Design, the brakes should be very, very much overkill. Since I cannot get custom molded brake pads cheaply and do not want to waste material bought in rectangular strips from McMaster, I'm just making the pads from a collection of 1" squares bonded to a backing plate and approximating them as circle/button/puck pads. Given a 180mm rotor with the pads centered at a radius of 67.5mm (should provide ~10mm clearance from edge of rotor), a maximum material pressure of 200[psi]~1.4[MPa] (actual material rated to ~4000ft/min at 250[psi]), and a coefficient of friction of 0.47, then: the effective radius is ~65.4mm, the average pressure is ~0.58[MPa], the actuating force is ~245[N], and the braking torque is ~7.5[N-m] per individual pad. With two 1" cylinders per pad at a fail-safe system pressure of ~76[psi], the force on each pad backing plate will be ~120[lbf] or ~530[N]. With either two or three 1" pads per backing plate and two plates per rotor, then the stopping torque per wheel should be ~32.5[N-m] with an average pad pressure of either 0.52[MPa] or 0.35[MPa] which are both well below the maximum pressure survivable by the braking pad material. For reference: Chibi-mikuvan only puts out ~13[N-m] at the wheels, so the brakes of a single wheel could easily stall the motor twice over and I'm planning for a brake on every wheel.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  5. #15

    Re: Scooty-Puff

    You're going to drive this for tens of thousands of miles, huh? :-)
    I have the feeling that, for this competition, the pads could be made from hardwood and they'd still be OK.

    How does the pumping back/forth for applying/removing brakes work? How fast response do you get?
    Also, what material in strips from McMaster?

  6. #16
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    Re: Scooty-Puff

    Quote Originally Posted by jwatte View Post
    You're going to drive this for tens of thousands of miles, huh? :-)
    I have the feeling that, for this competition, the pads could be made from hardwood and they'd still be OK.

    How does the pumping back/forth for applying/removing brakes work? How fast response do you get?
    Also, what material in strips from McMaster?
    Basically, a fun test bed for future robot and/or moped parts. As for PRS, there is a brake test integrated into the qualifying run (one untimed lap, then one timed lap ending in a complete stop within 18 feet of the finish line) and people have failed it before.

    The pad I'm looking at is 2" wide 1/4" thick metal-free brake and clutch lining in a 2 foot length (mcmaster.com/#6175K828) with pistons made from 28mm lengths of 1" diameter 1144 steel rod with three 0.75" steps to hold a pair of 210 o-rings and an optional 1/8" o-ring reinforced piston cup seal, and a short 0.25" diameter step prevent the two back-to-back pistons completely blocking flow into the cylinder. The pistons will be center tapped to M5x10 on the 1" diameter end for flat head socket cap screws to hold the backer plate to the pistons.

    Nothing has been built yet, as still fleshing out the design; this thread is partly so I don't forget details (my usual documentation method ends with me trying to piece together incoherent notes scattered all over my workspace and computers). The intended motor to drive the pump is a certain 2728 1000kV dual-shaft outrunner from Hobbyking run at 12V (200W peak power) with pump and appropriate gearing yet to be determined. The fluid flow rate required to get from 2mm clearance between rotor and both brake shoes to initial contact in 100ms is ~4.053e-6[m^3/s] but requires only enough pressure to overcome the losses in the connections between the pump and cylinders, so the energy requirement is quite small (e.g., if 10[psi]/0.07[MPa] in losses, then a hydraulic power requirement of ~3[W]). Maximum pump pressure required would be ~230[psi]/1.6[MPa] to fully charge the reservoir/accumulator to fully retract the brake pads (much harder to disengage brakes than engage them). So, worst case power requirement in normal usage would be releasing fully engaged brakes while trying to do a quick start (reservoir/accumulator pressure increasing from minimum of 0.53[MPa] by ~0.042[MPa] per 0.25mm of pad travel).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  7. #17
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    Re: Scooty-Puff

    You can't use the electric motor to brake? Or would that not work? My son's Dune Racer stops pretty fast, but he only weighs 42 pounds.

  8. #18

    Re: Scooty-Puff

    The rules require positively actuating friction brakes, that are not a stick contacting the ground.

  9. #19
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    Re: Scooty-Puff

    So, yeah. The magnets I was looking at for the replacement rotors cannot be back-ordered, so spent a while trawling ebay for magnets in any of the size ranges I can easily use. Sadly, the only 8mm wide magnets I could find were all 25mm long, which is not too useful for a 32mm long rotor. Ordered a 20 piece lot of 30mm x 5mm x 3mm N50 magnets of unknown magnetization for $7, but should be able to form a 10-pole halbach array as long as they are not magnetized along the 30mm length. The magnet coverage should be very similar to the 10-pole rotor using 8mm wide magnets (~72%), although not entirely sure how the actual field strength and distribution will compare in practice. In theory, arranging them as a halbach array should direct the vast majority of the magnetic field to the exterior of the rotor, but the gaps between the rectangular magnets will likely result in quite a bit of leakage into the interior of the rotor.

    As for brakes, the rough parameters I was using to find/design the hydraulic pump were ~5100[mm^3/s] (5mL/s) at up to 1.7[MPa], which would be a flowrate capable of 0.25[mm] pad travel in 0.1[s] with the accumulator fully charged. That left me quite tempted to try making a tiny axial piston pump for controlling the brakes since it would require no valves (unlike a diaphragm pump) or thin sliding seals (unlike a rotary vane pump) for unidirectional operation and no valves or rotor reversing to change flow rate or flow direction (just change angle of swash plate for continuously variable speed and direction). Thinking UHMW-PE, acetal, or PTFE rod (0.75" for rotor and 1" for end caps/bushings to keep the pump from pushing itself apart) with the rotor having ten 3mm bores at 6mm radial distance and a 6.35mm diameter step on the end that mates to the input/output port endcap (opposite the pistons and swashplate). Quite certain the pistons will be 3mm O1 tool steel; possibly with one end ground flat and the other ground round to mate to guide slots in a washer between the round end of the pistons and a disc magnet acting as a swash plate. If the magnet is not strong enough to make the pistons retract, then would have to add some other sort of conventional link between the pistons and the guiding washer. The small diameter of the piston mounting circle means a bit limited in possible angles of the swash plate without a means of varying the radial distance between the pistons and center of the washer in the plane of the washer. Any radial forces on the piston will cause wear of the bore which will cause sealing problems. Limited angle of swash plate leads to limited displacement of pistons, which requires higher rotational rate to compensate. Hoping to achieve at least 30 degrees from vertical for a total displacement of ~7mm per piston (~494[mm^3/rev]); 45 degrees would be even better with total displacement of ~12mm per piston (~848[mm^3/rev]). With a 17:1 gearbox on the 1000Kv motor at 12[V] and 2[A], the pump should run at up to ~700[RPM] with a peak flowrate of ~9960[mm^3/s] and peak pressure of ~1.55[MPa]. That is assuming I can actually get a decent seal between all the parts.

    As for the lasercut cycloidal gearbox, decided to rework it to be much smaller (30mm square) and cheaper since I do not need super large torque capacity for the pump. Might also try getting an alternate version printed on 3dhubs, but probably going to be kinda hard to beat ~$3.70 per stage from ponoko (without prime). Have not done any FEA, so not certain 2.3mm delrin will survive very long (entirely 2.3mm delrin except for eight 2mm pins in output disc, 1/8" hex shafts, and four M3 screws to hold it together; twelve full stages per P1 plate). Jumps up to ~$9 each when made from any thicker delrin. Hoping I will be sufficiently satisfied with the designs to finally place the ponoko order for the gearboxes, rotor 'laminations' (5.5mm hardboard), and Ripley's new torso some time early next week.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  10. #20
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    Re: Scooty-Puff

    For anyone interested: yes, it is possible to completely remove the bearing holder bracket/hub from certain harbor freight wheel rims without a lathe. Lots of pounding on the lip of the tube with a cold chisel to get it folded back off the rim half, but no power tools required. Still wise to wear ear protection. Might take a wire brush to the hubs to get rid of the little weld splatter balls that were hiding under the bracket and give them a new coat of paint. This pair of hubs/tubes/tires is hopefully going to be drive wheels on a certain low-speed outdoor robot, but the rims fit multiple tire sizes that can actually survive road use.


    Since pump pressure can easily be increased by increasing motor current and flowrate can only be increased by higher swashplate angles (limited by swashplate design) and/or higher speeds (limited by auxiliary system voltage), dropping the gearbox down to an 11:1 reduction. That would result in ~1090[RPM] for a maximum flowrate of ~15400[mm^3/s] if ever able to manage a full 12[mm] stroke and pressure of ~0.505[MPa] when running at 12[V] 1[A] with theoretical efficiency of ~65%. To reach maximum pressure of 1.7[MPa], have to increase current to ~3.4[A] which is still well below the motor's maximum rating. Smaller gear ratio also lets me shrink the gearbox further down to a 25[mm] square with seven full gearboxes (two endplates, two gear+pin plates, one spacer plate, and two output disc plates) per P1 plate and <$3 per fully assembled gearbox using 2.3mm delrin, 1.6mm pins, and double slotted 4mm shafts with 1.6mm pins as key stock. Not sure if I will have to add bearings and larger endplates to ensure the shaft actually stays centered; downside of only 0.5mm eccentricity of driving cams on the shaft is very little permissible radial movement of the shaft.


    Because I'm so very twisted, I've been considering a hemispheric drive system as continuously variable transmission on each drive wheel. Normally, the rubber hemisphere is on a two axis gimbal to let the contact patch of the hemispheric 'wheel' produce a driving force in any direction (and many different magnitudes/speeds) in two dimensions without changing the rotational rate of the hemisphere (or the motor driving it). Thinking of putting the hemisphere on a single axis gimbal and using the brake rotor as the output side of the CVT. A 60[mm] diameter hemisphere and 180[mm] diameter brake rotor could result in a rather impressive range of speed reduction. Centering the gimbal at 80[mm] from the rotor/wheel axis and managing to give it a range of 5~60 degrees would get ratios in the range of 30:1 to 1.5:1. Need to add bearing(s) on the other side of the rotor opposite the hemisphere contact patch to keep the contact force high without deforming the rotor or causing large bending moments about the rotor/wheel axis. Would prefer to mount it on a near clone of the brake caliper to easily adjust the applied forces on the rotor to adjust for wear and permit free-wheeling.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

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