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Saturday, May 9, 2009

Tension-based Throttle Improvements

I've decided to change the tensioner itself to an idea borrowed from AussieJester's work on this thread on the FreakBike Nation forum:
specifically this post:
where he's used a brake lever (wheel end) from a bike, along with it's spring and pivot point, with a derailer tensioner wheel bolted in place of the cable retainer, to create a chain tensioner.

(pic used with permission)

A quick GIMP mockup pic:

This would greatly simplify my tensioner's construction and reduce it's size and weight, compared to the one I originally planned in this post:

So now the tension-based throttle is simply this:

Mounted on the lever is a magnet, which passes by a frame-mounted hall-effect sensor wired to the throttle input on the controller.

The harder one has to pedal, the more pull/tension there is on the top of the chain, and thus the higher the lever is pushed, closer to the sensor, causing more motor power output, which helps ease the tension on the chain by reducing the power needed from the pedals.

Since the amount of tension desired might be different for different people or bikes, there can be an adjustment of where the magnet is along the lever (and thus how much power the motor outputs and when).

The magnet itself was originally intended to be moveable on the steel lever simply by sliding it around, to calibrate the throttle. Using an aluminum brake lever necessitates some changes to that, but can still work the same way. Probably the magnet will be mounted to the brakepad-adjust slot, if I do have to move it.

Alternately, this adjustment can be done via a scaling control in the controller electronics, but that would require extra changes outside the scope of most people's capability, for those who are using this blog to make their own DIY e-bike.

For my original design, that's what I would have been doing, though, by using an op-amp just after the Hall sensor output, with an adjustment knob either on the throttle mounting box, or on the handlebars.

Since my first design was expected to have (and has had!) breakdowns, I also wanted a manual throttle override so I could use the motor itself to get home if I found myself unable to pedal due to a system failure (which has happened, unfortunately before I had the motor on there!). This would be done by using the cable-control throttle off an old Honda Spree scooter, and the cable would pull the tensioner up as if the chain were pushing it up, causing the motor to receive power as a constant based on throttle position, rather than the feedback-based control provided when the chain tension moves the lever.

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