• The Key to building a succesfull robot

    2013-11-02 Author: Mikael Tulldahl
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    Adjustability

    It’s key to make things adjustable in your hardware. A good example of how devastating it can be to forget this can be seen in this clip:


    the robot to the right have the wheels too high up, so the magnets are in contact with the dohyo, and the wheels just spin in the air. probably they adjusted the height once on a table, but with the forces from the magnets, they chassi bent and needed re-adjustment. important things to make adjustable: plow height, motor mount, wheel height. with tons of unknown parameters, it is impossible to design something that will have the right settings directly

    Tolerance

    you find yourself grinding and fileing things a lot of the build time on your first robots, and the reason is the optimism about how things will fit in eachother. if you cnc-cut things, a “coincident” contact surface in CAD will have high likelyhood to cause problems.

    Iterative design

    go through many iterations, ask others about their opinions and don’t stick to one design just because you spent time developing it, if it turns out to be a bad design making a detailed design will take more time to implement, but will save time in the end, by not just having rectangular blocks stuffed together you will realize problems before you manufacture.

    Design first, then buy&build

    design and browse for parts simultaneously. don’t buy stuff before the design is locked down, unless it’s cheap. try to make design decisions based on available hardware.

    Spare parts

    get spare parts for everything. things will break, and they will break when there is no time to order new ones. a lot of robots have been stranded because one part got broken. it’s worth the extra cost in the time you will save.

     

  • Lilla Lisa Specs

    2013-11-02 Author: Mikael Tulldahl
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    Motors

    Turnigy L3040A-480G Brushless Motor

    Model: Turnigy L3040A-480G Brushless Motor

    Specs: kv :480 rpm/V

    max current: 52A.

    suitable for 6s system (22.2V)

    You can calculate the torque constant, Kt, given the kv:

    Kt=9.5/Kv=9.5/480=0.02Nm/A

    at 52A: M=I*Kt=52*0.02=1.04Nm

    gear ratio of 10:35 gives:

    Mout=1.04*3.5= 3.64Nm

    M=F*r –> F=M/r=3.64/0.02=182N

    two motors: 2*182N=364N

    so it is able to push forward with about 364N at full current at both engines. however we dare to push much more current for a short duration

    Transmissiontransmission

    the motors have a 5mm axle, we picked regular rc buggy pinion steel gear for use on the motor shaft, 10t and mod1 the motor gear has a direct drive on the wheel gear, a 35t mod1 gear from sdp-si of stainless steel. the wheel gears were modified (not the teeth)with cnc to minimize weight and to fit nicely with the rims. the rims are cnc cut from both sides from a piece of Nylon. nylon turned out to be very hard to work with and i had some failed pieces before I figured out a good manufacturing method. the wheel gear is slided on the hub, and fastened with a handful of m3 screws threaded into the nylon. a mold was milled from 8mm sheets of PC and used to mold polyurethane on the rims. a wheel diameter of 41mm was used. with a 4.5mm thick polyurethane tire. the tire is expected to compress about 1mm as they touch the doyho.

    wheel curing polyurethane tires SAMSUNG

     

    Batteries

    1.3 Ah 6s nano-tech

    we have one individual battery pack for each motor, each is a 1.3Ah 6s(22.2V) 45c – 130c turnigy nanotech

    http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=22907

    Motor controllers (ESC)

    the motor controllers are a custom design by Benjamin Vedder. they turn out to run our motors perfectly sensorless, but we will anyway probably

    put in sensors in the motors, by putting hall-sensors between the coils.

    MagnetsSAMSUNG

    magnets were sourced from http://www.magnetvaruhuset.se/

    we chose the type “super-flatgripare” with threaded tap. these are high quality and very strong!

    diameter: 16mm

    rated attraction force: 10kg

    depth:4.5mm

    treads: M6

    price:28SEK each

    after experimentation i’ve evaluated them to give a force of up to about 7 kg each on the dohyo. however, the force is drastically decreasing with increasing distance, e.g. 0.1mm apart from dohyo might just be about 2-3 kg force left. there is space for up to 35 magnets, but in reality about 10 slots are not used ( could be used if more magnetic force is found needed). The “center” of magnets lie about 1/3rd of the distance between wheels and plow, measured from the wheels

     

  • Linefollower airflow simulation

    2013-10-14 Author: Mikael Tulldahl
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    i did a quick and dirty check on how the undercarriage shape affects the pressure and thus the downforce. what i wanted to know was: can you assume that the pressure difference the fan creates is maintained under the entire hood, or is the vacuum decreasing towards the edges – and what kind of downforce can be expected?

    i’ve learnt that the shape of the underside of the hood plays a major role in the downforce generated, as you can see in the picture bellow, the fan creates a difference of about 300Pa, but the major part of the undercarriage is only experiencing a mere 60 Pa pressure difference. more investigation needed!

    downforce

  • Linefollower beginning calculations

    2013-10-14 Author: Mikael Tulldahl
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    since competing at RobotChallenge 2013 in Vienna with Sleipner and Lilla Lisa, i’ve got more and more excited about building a linefollower.

    initial design-goals ( preliminary)

    200g mass

    3g acceleration sideways as well as accelerating/deaccelerating (all the way up to top-speed)

    more than 5m/s top speed

    vacuum fan. probably a 35-27mm EDF

    size of max 25 x 25 cm to be able to compete in Robot-SM, Stockholm Robot Championship, Robotex and RobotChallenge

    (just for calculations) wheel radius: 1cm

    2s lipo

    rought estimate of suitable components:

    rpm on wheels at 5m/s: 4800rpm

    since 2s lipo –> ca 7.4 volt system –> 4800/7.4=650kv on motor or equivalent with gearbox

    power out of engine at 5m/s, 3g(=30m/s^2): P=F*V=m*a*V=5*30*0.2 =30W = 15W per motor

    since there is no lightweight 650kv, 15w motors, i’ll have to use a gearbox. i like brushless outrunners, there are lots of variations but i’ve found some ca 10g 15W motor with about 2000kv, so a gear ratio of about 3:1 is needed.

    2s lipo ,350mah, 65c 130c burst = 22A continous= 170W kont, 34g. http://www.hobbyking.com/hobbyking/store/__19117__Turnigy_nano_tech_350mah_2S_65_130C_Lipo_Pack.html

    ESC wheel engines: 2s, 10A, 8g http://hobbyking.com/hobbyking/store/__23766__HobbyKing_Brushless_Car_ESC_10A_w_Reverse.html

    ESC EDF: 10A, 9g http://www.hobbyking.com/hobbyking/store/__659__TowerPro_9g_w12A_Brushless_Speed_Controller.html

    EDF: 27mm, 2s, 5.5A, 40W, 8g

     

    wheel motors: http://www.hobbyking.com/hobbyking/store/__5358__18_11_2000kv_micro_brushless_outrunner_10g_.html

    CAD for motors: https://grabcad.com/library/micro-brushless-motor-1

    axle:

    bearings: http://www.kullager.se/sv/product/501/Kullager-623-3x10x4

     

    so far it seems as if 200g total is a reasonable and achievable goal. i will research more what kind of “diffusor” style undercarriage is the best shape to get as much downforce as possible with the EDF as a vacuum fan.