Project Summary:

My goal is to design and test an electromagnetic bicycle brake, capable of slowing or speed limiting a bicycle.

Intro

Problem: Riding around college hill, with its numerous steep slopes and stop signs puts a large load on my bicycle's linear-pull brakes, this necessitates frequent brake adjustment and pad replacement. Many e-bikes incorporate regenerative braking, however the braking system is inseparable from the e-bike system. This means carrying around heavy batteries, a motor controller, and the physical interface between the motor and the wheel. A mountain biker, or a person who lives in a hilly area, would receive only limited braking power from an e-bikes motor and little benefit from the additional weight and features of an e-bike.

Goal: I want to design, test and implement  an elegant electromagnetic (EM) braking system. The system will be designed solely to retard the motion of a bicycle, thus removing many of the complications of an e-bike system. My system should be able to:

• produce at least 500W of braking power
• brake a bicycle on level ground from 15mph to 4 mph
• allow normal operation of the bicycle when not in use
• Present only the most minor electrocution risk while in use

Background: There are many current e-bike systems on the market today which feature regenerative braking, both OEM and retrofit kits. However these systems are designed primarily to provide power, and thus have batteries, motor controllers and motor/generators. This type of system is far more complicated and heavy than necessary for a pure braking application. My research has found no EM bicycle brake systems or patents, excepting the multipurpose e-bike systems discussed above.

Chart detailing differences between an EM bike brake and an e-bike system

Intellectual Property:

None pertaining to EM bicycle brakes found.

My Proposed Solution:

     Design a front bicycle wheel with ~10 permanent magnets attached, evenly spaced, to a special magnet holder attached directly to the rim.

The fields of the magnets should be parallel to the wheel axle. Design a pickup, which will mount to the front fork of the bicycle, to contain the induction loops and connect to the control circit. The pickup will have to position the induction loops extremely close to the magnet holder.  Design a controll circuit to dissipate the energy generated through high power resistors and control the braking force. The circuit should be designed to maximize safety. Additional safety equipment may be required, such as a grounding wire and insulated mounts for some components. Build and test all components.

Theory

Faraday's law governs electromagnetic induction:. The Emf, or induced voltage, is equal to negative the change in magnetic flux over time. In the case of multiple loops of wire, the voltage is multiplied by the number of loops. For the N42 magnets in my test rig, the field at 1.5mm above the surface is rated at 0.27 T. The design condition is a bike moving at 15mph on a flat surface, equal to 6.7m/s. A mountain bike tire (D=0.55m), C=πD=2.07m, thus at 6.7 m/s the bike wheel will make 3.2 revolutions per second. Because there will be ten magnets evenly spaced around the rim, it will take only 1/32 sec=0.031sec from magnet to magnet. Because the field strength falls off exponentially and the gap between the magnets is about 9 times the width of the magnet I will consider the field to be 0 as soon as the whole magnet has passed 1 magnet width past the loop, thus the time from Bmax to B=0 is approximately 0.0062sec. Thus is the theoretical voltage in the instant that a magnet passes by the inductor loop.  I plan on using 10Ω resistors (Ohmite Corrib 280Series).  So using V=IR, 43.5V=Ix 10Ω  thus the instantaneous current induced is I=4.35 amps. Instantaneous power P=IV=4.35x43.5=189W.

These instantaneous calculations imply that the goal of 500W is achievable with a few pickups. However, because there are only ten magnets on the rim and thus, at 15mph, there are only 32 of these instants every second, over 90% of the time is not an instant when a magnet goes whizzing by the inductor. The theoretical treatment of this problem is to consider the Root Mean Square voltage to calculate time averaged power.    Where and C is the crest factor. For a sine wave C=rad 2=1.414, but for a spiky voltage with long intervals between spikes C= where T is the period and t1 is the width of the spike.

Prototype 1:

The first prototype is a test rig demonstration of my EM brake. The rig itself only serves as a mount for other components. A scale bike wheel (from a children's bike), outfitted with a scaled-prototype special magnet holder, will be mounted on the rig. A scaled-prototype pickup will also be mounted on the rig. The test rig, and scale components will be used to:

• test the workability of my EM  braking concept
• measure the braking potential of N42 magnets when used in my brake
• observe the waveform of the generated current
• test various controll circuits

Test Rig without wheel installed. The bolt seen on the base of the near arm is removable, allowing the arm to pivot for easy wheel instalation.

Wheel with magnet holding rim attached

Below is the 0th prototype pickup, made because the 1st prototype pickup was curing for 24 hours. It is a coil with four loops, .75"X1.375"

The test rig ready to run for the first time. (with 0th prototype pickup installed on near arm)

The firts test run:

Which produced a whopping 25 millivolts! This is ~1/2 the theoretical peak voltage. As measured by a HP 973A multimeter, set to AC mode. Unfortunately I do not know what crest factor the multimeter is using in its algorithm, so this data is not usefull. Also the gap between the pickup and magnets is not close enough to 2mm, resulting in low power output. At this rate I am 1.25x10^-7 of the way to my goal of 500W!

The first prototype induction coil was scavenged from prince lab. It is enameled copper wire wound into an inductor with Rinner=1.5" and Routter=3". The coil has ~240 loops and a resistance of ~5 Ω. I mounted this inductor coil on the test rig and attached it to an oscilloscope. The voltage induced over one rotation of the wheel [one period] is shown below. 

The induced voltage peaks are around 1-1.2 volts. The speed or rotation, as calculated from the oscilloscope time data, is 76.8 RPM. Based on the dimensions of the inductor the flux density is calculated to be 4.56x10^-4 Wb/m^2. The width of the peaks =~0.03s. thus dphi/dt=0.0152, times n=240 turns, means the theoretical peak voltage is 3.65 volts. My system achieved about 1/3 the theoretical potential. This is most likely due to the large outer radius of the induction coil, the coils at the edge were farr enough away from the magnet that the field they expierenced was fairly weak. 

Prototype 2:

The second prototype will be full scale. A Bicycle rim will have the magnet holder welded to it, the magnets will be attached to the holder (Epoxy? they advertise ~1000PSI?). A front-fork mounted pickup will be installed on this prototype. Braking capable circuitry will also be attached to the bike. Below is the design for 1/2 of the special magnet holder, it will be manufactured using the EDM, and the pockets will be etched. The half arc design makes construction and welding easier.

See How My Electromagnetic Bike Brake Project Continues