Summary of Rear Wheel Tachometer using arduino
### Summary Movable Party aims to generate power via hub motors on stationary bikes, where speed determines output. To measure this, the project uses a non-contact tachometer counting wheel revolutions. Due to friction and alignment issues with mechanical switches, and concerns regarding optical systems in ambient light, the team selected magnetic sensing. This involves attaching a magnet to the wheel and using a fixed sensor on the fork to detect passing magnets without physical contact.
Parts used in Movable Party Rear Wheel Tachometer:
- Hub motor attached to rear wheel
- Magnet (attached to wheel)
- Sensor (attached to fork)
- Reed switch or Hall effect sensor
- Bike computer circuitry for revolution counting
One of the goals of Movable Party is to provide an interactive experience for audiences/participants. Since power will be generated from a hub motor attached to the rear wheel of each bike (see this post), the speed of the rear wheel directly translates to the amount of power generated. Detecting how fast the rear wheel is moving thus seems like the most obvious/important piece of data to capture from these stationary, power-generating bikes.
Tachometer
The easiest way to detect the speed of the rear wheel (the way most bike computers work), is to make a tachometer–simply put, a revolution counter. By counting the number of revolutions over a certain period of time, we can determine how fast the wheel is turning. Basically, you want to count the number of times a specific spot on the wheel crosses a point of measurement, and divide by some time constant (e.g. Revolutions per Minute, RPMs). Electronically, you are triggering a switch every time this point on the wheel passes a fixed sensor.
Because the wheel is turning and the sensor is fixed, using a mechanical switch where there is contact every rotation will not work very well. For example, the mechanical switch will cause friction every rotation. Also, maintaining perfect distance between the wheel and the switch will be difficult. Instead, there are two common ways of detecting rotation that do not require contact between the sensor and the wheel–optically and magnetically. Due to my own personal familiarity with magnetic circuits, as well as the expense of using an optical system and concerns about the functionality of that system in ambient light, I have decided to pursue magnetic sensing.
Sensing Changes in Magnetic Fields
To construct a tachometer using a magnet, you attach the magnet to the wheel, and attach a sensor to the fork (at a fixed position). As the magnet passes by the sensor, it triggers a switch that allows you to count the number of revolutions over a period of time. There are two types of sensors that accomplish this function: reed switches and Hall effect sensors. The reed switch is a simple switch that is closed (or opened) using a magnet.
For more detail: Rear Wheel Tachometer
-
How does the speed of the rear wheel relate to power generation?
The speed of the rear wheel directly translates to the amount of power generated by the hub motor. -
What is the primary method for detecting how fast the rear wheel is moving?
The most obvious way is to use a tachometer that counts the number of revolutions over a specific period of time. -
Why was a mechanical switch not chosen for this project?
A mechanical switch causes friction every rotation and maintaining perfect distance between the wheel and switch is difficult. -
Which two non-contact methods are commonly used to detect rotation?
The two common ways are optical detection and magnetic detection. -
Why did the author choose magnetic sensing over an optical system?
The decision was based on personal familiarity with magnetic circuits, the expense of optical systems, and concerns about functionality in ambient light. -
What components are required to construct a tachometer using a magnet?
You need to attach a magnet to the wheel and a sensor to the fork at a fixed position. -
What types of sensors can be used to accomplish magnetic sensing?
The two types of sensors mentioned are reed switches and Hall effect sensors. -
How does a reed switch function in this context?
A reed switch is a simple switch that closes or opens when triggered by a magnet.
