Android™ Application Development - Bahar Ali Khan

Chapter 10: Accessing Android Hardware
Creating a Speedometer
326
While an accelerometer won’t tell you your current speed, you can calculate a rough estimate by monitoring
changes in acceleration over time. In the following example, you’ll create a simple speedometer
using the accelerometers to determine the current speed based on acceleration changes.
The sensitivity and responsiveness of the hardware accelerometers will limit the accuracy and effectiveness
of this application, but the techniques it uses should give you a better understanding of how to use
the accelerometer sensors for something more useful.
Because accelerometers measure the change in velocity in a given direction, you can establish your
current speed by determining how long each acceleration value has been applied. For those mathematically
inclined, you’re fi nding the second derivative of the acceleration changes.
For example, if you accelerate at a steady rate of 1 m/s 2 after 10 seconds, your speed will be 10 m/s (or
36 km/h). When your speed becomes steady, your acceleration should return to zero. In the real world,
acceleration rarely stops and starts in an instant, nor does it remain constant, so you’ll need to adjust
your velocity calculations as the measured acceleration changes.
1. Start by creating a new Speedometer project with a Speedometer Activity. Modify the main.xml
layout resource to display a single, centered line of large, bold text that will be used to display
your current speed.
/>
2. Within the Speedometer Activity, create instance variables to store references to the TextView
and the SensorManager. Also create variables to record the current acceleration, velocity, and
the last update time.
SensorManager sensorManager;
TextView myTextView;
float appliedAcceleration = 0;
float currentAcceleration = 0;
float velocity = 0;
Date lastUpdate;