How Does The Nike + iPod Sport Kit Accelerometer Work?

This accelerometer is technically called a piezoelectric accelerometer. The term piezoelectric, when applied to a particular material, simply means that it produces electricity whenever it changes shape. That is, if a material is piezoelectric, then applying pressure or force on it, causing it to become deformed from its original shape, would cause it to produce electrical current. (The reverse is also true: applying electricity to a piezoelectric material could cause it to change shape slightly. But we are interested in the other kind of piezoelectric behavior, at the moment.)

Thanks to modern technology, piezoelectric materials can now be modified or even synthesized to particular specifications. For example, the amount of electricity produced could be made to vary, depending on the direction of the pressure applied to the piezoelectric material. Modern manufacturing methods also make it possible to create piezoelectric sensors that are very small, but also very accurate.

This makes using piezoelectric materials quite feasible for a wide variety of electronic devices. Other applications of piezoelectric technology, apart from the Nike + iPod Sport Kit sensor, also involve motion sensing. The Apple iPhone, Nintendo Wii, and Sony Playstation 3 are popular electronic devices that use accelerometers to detect the motion of their respective controllers.

The accelerometer inside the Nike + iPod Sport Kit sensor serves to detect whenever the runner’s foot is in contact with the ground. Because of the piezoelectric nature of the material in the sensor, it produces an electrical current whenever it is compressed by the runner’s weight on the ground. It is this current that can be detected by other sensors.

The sensor also contains, apart from the accelerometer itself, a processor to deal with the information from the accelerometer. The accelerometer reports whenever the runner’s foot is on the ground. When a person is just walking, we can observe that his feet spend more time on the ground than in the air. As he increases his pace, he takes faster steps, and his feet spend more time in the air, not in contact with the ground. This simple observation is the basis of the processor’s method to determine the runner’s speed, based on the data from the accelerometer. Once this information has been processed, it can then be relayed wirelessly to the receiver attached to the iPod Nano. The sensor contains a small wireless transmitter and an antenna just for this purpose.

The accelerometer has another important function. The sensor’s built-in battery has a finite lifetime, and it cannot be replaced. Thus, it would be necessary to conserve this energy as much as possible. The accelerometer is also designed to act as the sensor’s on-off switch. When the sensor is not in use (that is, when it is at rest), the accelerometer would not report any foot steps to the sensor’s processor. If the sensor observes that it has not been in use for a certain period of time, it would turn itself off to conserve battery life. Conversely, once the accelerometer begins reporting foot steps, the sensor would turn itself on to be ready for use. The accelerometer thus ensures that the sensor is only turned on when needed.

The other part of the Nike + iPod system is of course the receiver. This receiver is attached to the iPod Nano, and draws its power, storage, and software from the iPod. This means that the receiver is much simpler than the sensor. The receiver is connected to the iPod via a multi pin connector. Through this connection, the two devices can exchange both power and data.

The sensor transmits at a frequency of 2.4 gigaHertz, and has a unique transmission code that links the sensor with its appropriate receiver. This prevents any interference from occurring between two or more Nike + iPod Sport Kits, even if the runners run side by side. That is, one person’s sensor will not report to the other person’s receiver, because of this unique transmission code. The receiver learns to recognize this code the first time that the Nike + iPod Sport Kit is used.

Apart from the Sport Kit’s sensor and receiver, of course, an iPod Nano is required to complete the system. The receiver will only work with an iPod Nano (either the older version or the newer one will do), and no other type of iPod or digital music player. An iPod Nano costs $150 - $250 dollars – and of course it is also a great music player by itself, apart from being used with the Sport Kit.

Nike has a line called Nike+ that are specially designed to have a space in the insole for the sensor to be placed in. Although other manufacturers have come up with pouches that can be used with other shoes, the accuracy of the sensor would still be best with Nike+ shoes. These shoes start at $70.

The Nike + iPod Sport Kit itself costs only $30, completing the ensemble.

As the Sport Kit becomes more popular, some exciting possibilities have arisen, especially when it comes to RUNNER+. Each runner’s information can be uploaded to a central server, and so comparison and tracking is now possible. Aside from personally keeping track of your performance, you can compete and race with your friends and other people from all over the world! The Nike + iPod Sport Kit is indeed a great new addition to any runner’s gear.

>> Buy the Nike+ iPod Sport Kit

>> View All Nike+ iPod Dissection Photos