This is an explanation of another one of those ambitious projects which I really want to do, but I have neither the experience nor the people to actually do it with.
I hate rough inclines: they kill my legs. The number one detractor to riding a bike in my childhood was that in my neighborhood, there are some very steep inclines. It made riding a bicycle not a very pleasant experience, and my father never wanted to bring me to a park for me to ride my bike on, so in the end, I never really used my bike.
However, given the fact that using a bicycle is the only practical mode of rapid transit in the city where I attend college, I want to actually start riding a bike again. And after a year or so of riding that bike, I want to make the riding experience cooler.
First, I want to retrofit a brushless DC motor to the drive shaft; something rated for around 600 W of power output. If it is not possible to attach it directly to the hub, I’ll attach it to the shaft with a belt; ideally, a belt with the quality of a timing belt. But I hope I don’t have to do this, because if so, I’d have to play with the tension, pitch, and so on of the belt, which would be problematic.
Next would be the electronic speed controller and charge controller. I want the controllers to automatically switch to a regenerative mode for slight brakes by bypassing the ESC, inverting the poles of the motor, and taking the current straight to the charge controller. Then, on pedaling, the controllers should switch back to drive mode. This behavior would be directed by the main controller, since regenerative braking is a non-essential feature.
Speaking of a main controller, what exactly is it? The main controller is the Arduino or whatever microcontroller I decide to use that is wired to the ESC and charge controller, but is not required to be run in order to operate the bike in case of a fatal error or low battery charge. It would run a real-time operating system with prioritized continuous tasks and many, many interrupt routines. These would be its high-level tasks, in order of descending priority:
- Emergency brake applicator. Continuously checks “emergency stop” button, dead man’s switch (clipped to clothes, but the clamp is limited enough such that it cannot be clipped to the handlebars or other part of bike; then the other end of the clamp is magnetically attached to a port on the control box), or >95% application of brakes while moving at a formidable speed.
- 10 Hz alternating pulse. This signal is generated and passes through some kind of failsafe circuit, which then determines whether or not the ESC should be enabled. The alternating pulse ensures that the main controller is not “frozen” on an operation that could prevent it from stopping the motor. This assumes that as long as the pulse is alternating, the controller is working as intended.
- Speedometer. It simply samples the speed at which the back wheel is spinning and determines the current speed.
- Speed regulator. This task scales back the output DC current based on how close the bike is to the speed limit. This task can be overridden, but it’s not a good idea to do so.
- Brake detector. This task detects the brake application percent. The actuation of the brakes is completely analog, but if it is significant, the main controller can signal to go to regenerative mode.
- Pedal detector. This task simply detects how much positive force is being applied on the pedal and sets the target DC current proportional to this force (clamped, of course).
- Odometer. It uses the same sampling metric as the speed counter, but it increments the distance by the circumference of the wheel. After around .2 miles, it writes to the EEPROM. I suppose I could use a pointer to level the wear on the flash, or I could use a preexisting file system designed specifically for microcontrollers.
- Display driver. This assumes that there exists a layer of abstraction between the UI and the display itself.
- Sound driver. Just for basic beeps and boops.
- Main UI. This handles button interrupts (the calls of which are passed to the foreground user task), the failsafe UI (if all user-mode UI tasks are dead), and the UI toolkit itself.
- Foreground user task. Dashboard, options, etc. Must not directly control motor operation.
- Background user tasks. Battery icon, clock, etc. Must be non-critical.
The e-bike’s main controller would require a key for operation and then a simple on/off SPST switch located in front of the handlebars. The display would ideally be a Hitachi HD44780-esque LCD, but it could also be the Nokia-style LCDs, although these might be a little too small. There will be six buttons: on the left below the display, there will be four directional buttons laid horizontally (in a style familiar to Vim users or Dance Dance Revolution/StepMania players), and on the right, a back button and an enter button. The display and controls need to be water-proofed.
Instead of using heavy deep-cycle lead-acid batteries, I’d just opt for using LiPo cells, which are ubiquitous in hobby usage for high-performance electronics. Industry professionals are not fond of LiPo cells because they are comparatively more dangerous and volatile than other types of cells, and this increased risk cannot be tolerated in mass production. However, since I am not mass-producing e-bikes, it should be OK to accept the risks and enjoy the power of lightweight LiPos, as long as their charging is supervised closely.
This e-bike also needs a brake light, signal lights, and an LED headlight with a white color temperature rather than blue.
That’s all I want the bike to do. All of this, but I want to keep it street-legal and be able to prove that it can be safely ridden in busy streets under the consideration of various fail-safe mechanisms, including a speed regulator that requires manual override.
Sadly, I don’t know if I will ever be able to make this contraption.