Electric Bicycle Anti-Lock Braking System

Electric bikes are becoming more and more popular around the world. Many injuries and accidents occur on electric bikes due to front wheel traction loss. These bikes are heavier and faster than traditional bicycles and usually come with powerful brakes. When improper braking technique is used, it is easy to lose traction on the front wheel especially in unfavorable conditions. This can lead to otherwise preventable injuries to unsuspecting riders who may have made a small input error. This ABS system is designed to stop the front wheel from locking preventing this type of accident.

Overall system design

This project is a collaboration with my brother Ariel Schanke. He will be doing CAD/manufacturing for the mechanical side of this project and CAD versions of this will be put up as soon as that is done.

The system will generally follow this feedback loop:

Cutoff is shut and wheel has traction (Normal operation):

The system waits for the slip ratio goes higher than our threshold. Then the solenoid by the accumulator opens to relieve pressure in the system.

Cutoff is open and wheel is slipping:

Once the accumulator is open, the system will keep it open until the wheel has traction again and our slip ratio is below the threshold (pressure is removed)

Cutoff is open and wheel regains has traction:

Shut the cutoff solenoid by the accumulator, pump fluid back into the system (pressure is normalized) and we go back to our first state.

As with many car ABS systems we will be using an accumulator since there isn't really anything as reliable for removing fluid pressure. We considered using a solenoid to directly remove and add pressure, but there would be multiple drawbacks to using a system like that. A major one being that the solenoid spring would have to be strong enough to overcome any braking pressure which means it would be inefficient and slow to activate.

Current design for the accumulator/solenoid cutoff

Design for the electrical system

We are using an AVR128DB48 microcontroller to interpret wheel speed and accelerometer data and modulate the solenoid and pump.

The AVR microcontroller, wheel speed sensor and accelerometer are powered by this Mini Buck converter. 600mA should be enough power for this part of the system.

The pump and C5-272-B-1 solenoid will be controlled by logic level outputs from the microcontroller via IRLZ44NPBF MOSFETS (low side) since they need more power than the microcontroller can directly control or output. Both have a 1N5822 freewheel diode to prevent damage from back EMF.

The LIS3DH accelerometer is connected to the AVR via TWI. The configuration of TWI and data settings are in the datasheet.

The T-Slot Photo Interrupter has an open drain output so it has a pull-up resistor to 5V.

Slip Ratio

The slip ratio of the wheel under braking is given by: 1 - (r*ω/V) (Credit Section 2.2.3)

V - forward velocity of wheel/vehicle

r - radius of wheel

ω - rotational velocity of wheel

rω/V is the ratio of the velocity of the contact patch (or any outer part of the tire) to the forward velocity of the wheel.

When rω = V the wheel is rotating freely and the slip ratio is 0. And when the contact patch velocity is much smaller than the forward velocity it will be non zero meaning the wheel is slipping. If rω = 0 then the slip ratio is 1 and there's a lockup if V is nonzero.

As seen in the graph there's a sweet spot where we have the highest kinetic friction coefficient μ with the ground depending on the surface. An interesting future addition to this project would be to use a small AI core to determine road surface and then adjust the target slip ratio based on the surface.

Using and accelerometer and the wheel speed we can determine the slip ratio and add/remove brake pressure accordingly thus maximizing how effective the brakes are.

TWI Driver For The AVR128DB48

I couldn't really find a perfect driver for the AVR to connect to the LIS3DH accel so I created a driver with the help of existing AVR TWI drivers.

The code sets up TWI with the correct address for the LIS3DH, initializes accel settings and accesses the data registers.

The LIS3DH is a register mapped device so the code basically writes and reads registers on accel for operation.

I have this example listed on GitHub that outputs the accel data to USB serial.
AVR TWI LIS3DH

Current Challenges:
Creating a leak-free hydraulic brake system is particularly challenging mechanically. Pressure can get quite high in these systems (over 2000 PSI).

Pump design is new for both of us so choosing the right motor and design is something that is currently being researched. The current design idea is to use a piston pump with an eccentric cam on the motor to move the piston.

Manufacturing costs might get expensive (relatively) for this one off system. Our machining experience is limited and we won't be able to manufacture all of these parts on our own. It's likely we will use PCBway.

Braking systems cannot have air in them so we need an effective and easy way to "bleed" the brakes and rid air from the system.

We are both currently juggling many things as college is in session. Progress may be staggered due to limited time availablility.