Accelerometer

1. Introduction

The Accelerometer API extends the Generic Sensor API [GENERIC-SENSOR] interface to provide information about acceleration applied to device’s X, Y and Z axis in local coordinate system defined by device.

2. Examples

let sensor = new Accelerometer({includeGravity: false, frequency: 60});
sensor.start();

sensor.onchange = event => {
    console.log("Linear acceleration for an X-axis: " + event.reading.x);
    console.log("Linear acceleration for an Y-axis: " + event.reading.y);
    console.log("Linear acceleration for an Z-axis: " + event.reading.z);
}

sensor.onerror = event => console.log(event.error.name, event.error.message);

3. Security and Privacy Considerations

There are no specific security and privacy considerations beyond those described in the Generic Sensor API [GENERIC-SENSOR].

4. Model

The Accelerometer’s associated Sensor subclass is the Accelerometer class.

The Accelerometer’s associated SensorReading subclass is the AccelerometerReading class.

The Accelerometer has a default sensor, which is the device’s main accelerometer sensor.

The Accelerometer has a single supported reporting mode which is "periodic".

The Accelerometer’s permission name is "accelerometer". It has no associated PermissionDescriptor.

The Accelerometer has an associated abstract operation to retrieve the sensor permission which must simply return a permission whose name is "accelerometer".

The Accelerometer has an associated abstract operation to construct a SensorReading object which creates a new AccelerometerReading object and sets its x, y and z attributes to zero.

The linear acceleration is an acceleration that is applied to the device that hosts the sensor, without the contribution of a gravity force.

The AccelerometerReading's attribute values must be in [SI] units for acceleration, metre per second squared (m/s^2), expressed in a three-dimentional Cartesian local coordinate system defined by the device.

The frame of reference for the acceleration measurement must be inertial, such as device in free fall would provide 0 (m/s^2) acceleration value for each axis.

The sign of the acceleration values must be according to the right-hand convention in a local coordinate system defined by the device.

Note: The local coordinate system of a mobile device is usually defined relative to the device’s screen when the device in its default orientation (see figure below).

5. API

5.1. The Accelerometer Interface

[Constructor(optional AccelerometerOptions accelerometerOptions)]
interface Accelerometer : Sensor {
  readonly attribute AccelerometerReading? reading;
  readonly attribute boolean includesGravity;
};

To Construct an Accelerometer Object the user agent must invoke the construct a Sensor object abstract operation.

5.2. The AccelerometerOptions Dictionary

dictionary AccelerometerOptions :  SensorOptions  {
  boolean includeGravity = true;
};

By default, Accelerometer would provide acceleration information including the effect of the gravity force. In cases, when linear acceleration information is required, AccelerometerOptions dictionary with and dictionary member includeGravity that is set to false, must be provided to Accelerometer constructor.

5.3. The AccelerometerReading Interface

[Constructor(AccelerometerReadingInit AccelerometerReadingInit)]
interface AccelerometerReading : SensorReading {
    readonly attribute double x;
    readonly attribute double y;
    readonly attribute double z;
};

dictionary AccelerometerReadingInit {
    double x = 0;
    double y = 0;
    double z = 0;
};

5.3.1. The Accelerometer attributes

The includesGravity attribute of the Accelerometer interface represents whether the acceleration information provided by the sensor includes effect of the gravity force. In case when includesGravity equals to false, Accelerometer will provide linear acceleration information.

5.3.2. The AccelerometerReading constructor

The AccelerometerReading constructor accepts AccelerometerReadingInit dictionary that is used for initialization of AccelerometerReading attributes.

5.3.3. The AccelerometerReading attributes

The x attribute of the AccelerometerReading interface represents the acceleration along X-axis.

The y attribute of the AccelerometerReading interface represents the acceleration along Y-axis.

The z attribute of the AccelerometerReading interface represents the acceleration along Z-axis.

6. Acknowledgements

Tobie Langel for the work on Generic Sensor API.

7. Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

A conformant user agent must implement all the requirements listed in this specification that are applicable to user agents.

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]