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  • Motor assisted Trike

    Discussion in 'The main mechanical design forum' started by SMADaddy, Dec 16, 2012.

    1. SMADaddy

      SMADaddy New Member

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      Dear Forum members:

      My son is afflicted with a form of muscular dystrophy called spinal muscular atrophy (SMA). For that reason, I want to build him a modified tricycle where he can enjoy normal activities of riding outside. I'm deciding on a trike because Radio Flyer has off the shelf models I can adapt and modify to hopefully fit the needs.

      Riding is very helpful for my son to maintain the "strong" muscles he has. So I am trying to find or design a system that will allow my son to propel the trike forward. My son can apply a very minimal amount of pressure from his feet to the trike we currently have. The force/pressure his legs generates cannot move his own weight, even on a declining slope. So I'm hoping with some guidance for you, I can find a motor and system that can detect his minuscule force applied to the peddles and propel the trike forward.

      I have no formal training back ground, but I like to dabble in all sorts of mechanical stuff. I have a good amount of mechanical tools to build/modify the trike as necessary. Any tips or tricks to assist me and my son?

      With thanks and gratitude,
      Kevin
       
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    3. Erich

      Erich Well-Known Member

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      This is the way I would approach the problem.

      You need a DC motor servo system. This has 3 parts, the motor, an electronics box and a position feedback element.

      There are many different types of feedback elements, an optical encoder is one.

      Typically the encoder is mounted on the shaft of the motor. For your application you don't want to do this.

      You want to connect the motor to the rear wheel.

      Disconnect the pedals from the rear wheels.

      Attach the encoder so it spins with the pedals.

      Rig up a adjustable friction brake on the pedals. Think exercise bike.

      When your son starts pedaling, the encoder changes position. The servo electronics starts to spin the motor to catch up to the encoder. As long as he spins, the motor will spin trying to catch up. The faster he spins the faster the motor will go.
      If he stops the motor stops. If he back pedals the trike will go in reverse.
      You adjust the friction brake on the pedals to a level that he is capable of. Up hill, down hill , it don't matter his effort is the same.

      This is just the bare bones of a design, but it should give you a starting point for research on the details.

      Good luck, and I hope you son can stay strong.
       
    4. Randall Wink

      Randall Wink Member

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      I think I would approach this from the same general direction with a couple of refinements. You said that your son doesn't have enough strength in his legs to pedal himself so I would think about some sort of analog switch attached to the pedals to measure the amount of force he's exerting and compensate with some additional amount from the motors. I would set an adjustable upper limits on the amount of force the motors could exert to prevent a jerk on starting or stoping. I would use a position sensor on the axle shaft attached to some stationary portion of the tricycle with the pedal switches to determine if he's trying to go forward or backward. The adjustable upper limits would allow YOU to adjust for his additional body weight as he grows and you might add some sort of lower limit that he has to make before it will work to help build whatever strenght he has.
       
    5. TWojtacki

      TWojtacki New Member

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      Try going to http://www.instructables.com/ and search under "motor assisted trike" and you should find some DIY help there for your project...Also you could try searching the Make site (http://makezine.com/) for similar projects...Both of these sites should have enough information for you to work up a bike project...
       
    6. srdfmc

      srdfmc Well-Known Member

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      Hi Kev,

      Obviously you need a motor.

      I will suspect that an Electrical Motor (EM) on the front wheel axis will do the trick.

      Obviously again, you want your little angel to exerce himself doing so. So the EM does only to hve to compensate for the little lack of strencht your boy need.


      So this is easy to setup. No need for fancy stuff. What you need to hve is a strenght unrelated linking device.

      In the fighter jet things like that are done since long. This kind of stuff is generally seen on control (ctrl) yoke enabling a young pilot to master the ctrl of his plane even in hard G turn (where the effect of gravity decrease your ability to preciesly ctrl your inputs on the yoke).

      As promised, you don't need fancy stuff. You just need a sensor that will feel the amount of time and strenght your boy will apply on teh pedal. Let's say tht you are at ease with maths.

      What you need is a sensor that can detect S=(F (t,alpha) where
      S is the signal
      Alpha the angle made by the pedal around its axis (minimal - less than 5 or 10 degree)
      t being the time the deflection is applied to the pedal

      So effectievly, your boy's feet will move with his legs whyle he rides his Toptech trike adding additional sensitivity to his muscular exercise.

      By carefully programing the ctrl law (trough basic electronics - don't pay more than 100$ for the hardware - carefully setting the ctrl law will certainly needs time hence money -sry) you 'll got the sensor perfectly suited for your little angel.

      Regards,

      SRDFMC
       
    7. Tiha

      Tiha New Member

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      If you want to build off-the-shelf, without designing an electric control-drive system, just buy a electric bike kit and install it on the trike you want. This electric bike kit consists of a brushless motor mounted on the hub of the traction wheel (need to rebuild the wheel with the appropriate length spokes, which a bike shop will do for you), a control system and a battery. All should be around 10-15 kg in total. They should be available anywhere you live in the world. Very easy to install.

      There are versions available with different control logic: the rider pushes a button to start, a sensor mounted on the pedal/chain-wheel senses the rotation and starts up, check out which is best for you.

      Just go to a bike shop and try out an electric bike to see if this is what you want.

      Regards, Tiha
       
    8. vondeliusc

      vondeliusc Member

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    9. wacio

      wacio New Member

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      There is electric assisted bicycle sold at Menards for about $500. It comes with pedal mounted cyclist effort sensor. I think that if you bought it you could easily combine it with your trike. It also comes with lightweight lithium ion battery with Panasonic (quality) cells. Let us know how this goes for you.

      Regards.
       
    10. srdfmc

      srdfmc Well-Known Member

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      Sry Shmit but this can't work. Due to weight concern, grd friction and weakness of the child leg muscles you' ll see that he won't be able to rev up. Think abt the overall weight of the system and what has been described in the statement. Due to thegeometrie the power vary on each stroke like a sinusoid. It is not the most practical way to drive a motor with a linear rule!
      Try to imagine what it would be drving your car with an accelerator's pedal reacting with such a sinusoidal law ;)
       

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