Motion sensing- Entering into the world of Gyroscopes and Accelerometers

August 10, 2012

Well, you must be familiar with the word sensors…..what comes to your mind when you hear the word sensor? The text book definition of a sensor is “a detector/converter that measures any physical quantity and converts it into a signal which can be read by an observer or by an instrument.”

There are sensors which are used to guide us in many applications that deal with self-correction or motion based… among these are the two very versatile sensors known as the gyroscopes and accelerometers

An accelerometer is a sensor that measures acceleration relative to a free-falling frame of reference. They can measure the magnitude and direction of the acceleration, and can be used to sense the orientation of the device. Always keep in mind: the accelerometer is measuring just the linear acceleration of the device. It doesn’t take into account the orientation. At rest, it’ll give you the force of gravity acting on the device, and correspondent to the roll and pitch of the device (in atleast x-y directions). When the device is in motion, it gives us the acceleration due to gravity, plus the acceleration of the device relative to its rest frame. A two axis accelerometer will give us the direction of gravity on our balancing instrument. Accelerometer measures vibrations. It has extensive applications in engineering, machinery monitoring, building and structural monitoring, navigation, transport and consumer electronics. These used in smart phones and your PS-3 and wii remotes. Also now, they have been incorporated in new generation laptops and notebooks.

 

Hmm…..so basically what is pitch, roll and yaw?

For the aero-modeling guys, this would sound pretty familiar…..for others to understand these terms, imagine 3 lines running through an airplane, all at right angles to each other, meeting at the centre of gravity of the plane…

*The rotation around the front-to-back axis is called Roll. Known as the dipping of the plane’s wing. The movement is done about a horizontal axis stretching from the nose to the tail of the aircraft. Something known as Ailerons (located on the trailing edge of both wings) control the roll.

*Rotation around side-to-side axis is called pitch. It is a measure of the plane’s movement either up or down. The movement is done about the horizontal axis stretching from one wing tip to the other. It is controlled by the elevator located on the rear of the aircraft on the tail, along with the rudder.

*And the rotation around the vertical axis is called yaw. Yaw allows the plane to move left or right while in flight. The movement is done about a vertical axis. The yaw is controlled by the rudder which is located at the rear of the aircraft on the tail.

 

 

A gyroscope is another sensor that can be used to either measure, or maintain, the orientation of a device. Unlike an accelerometer, which measures the linear acceleration of the device, a gyroscope measures the orientation directly. Mechanically speaking, a gyro is a spinning wheel/disk in which the axle is free to take any orientation. The orientation will change in response to the external torque in a different direction than it would without the large angular momentum associated with the disk’s high rate of spin and moment of inertia. Now, the external torque can be minimized by mounting the device in gimbals, so its orientation remains nearly fixed, regardless of any motion of the platform on which it is mounted.

Gyros based on other operating principles also exist, like the electronic, microchip-packaged MEMS gyroscope devices found in consumer electronic devices, solid-state ring lasers, fibre-optic gyroscopes and the extremely sensitive quantum-gyroscope.

Typically a gyro is used to measure angular position using the principle of rigidity of space of the gyroscope. It has many practical applications. It can be used for navigation, on unmanned aerial vehicles, and radio controlled helicopters and has very large scope in consumer electronic devices too.

Gyroscope and Accelerometer together

When combining a 3 axis gyroscope and a 3 axis accelerometer, it will provide a six-axis (3+3) interpretation of movement through space. This is especially useful in robotics and other hand applications, because it can now easily filter out the unintended ambient movement and vibration of a user’s hand or any other external noise, thereby allowing a more accurate measurement of movements. When a gyro and accelerometer are combined, it becomes possible to simultaneously measure the acc and gravitational placements in the x, y and z axis. This integration gives us a total of six orientation measurements. Hence it’s possible to better balance a robot/bot which usually is a very unstable set-up in itself. Both of them are used together to create a more accurate measurement of the overall movement and location through space. The purpose of this sensor fusion is to take each sensor measurement data as inputs and then apply digital filtering algorithms to compensate each other and give out the accurate and responsive dynamic altitude (pitch/roll/yaw) results.

 

A very small application


Whenever you hear the word gyroscope, your mind tends to think of mechanical gyroscopes, the ones with a spinning disc that is mounted inside multiple gimbals allowing it to take any orientation. However, the gyros inside the iPhone 4 are a MEMS-based gyro (more like the three independent MEMS gyroscopes). A MEMS gyroscope takes the idea of the Foucault pendulum and uses a vibrating element, known as a Micro Electro-Mechanical System.

The original iPhone, and first generation iPod Touch, use the LIS302DL 3-axis MEMS based accelerometer produced by STMicroelectronics. Later iPhone and iPod Touch models use a similar LIS331DL chip, also manufactured by STMicroelectronics.

 The iPhone's accelerometer measures the linear acceleration of the device so that it can report its roll and pitch, but not its yaw. If you are dealing with an iPhone 3GS, which has a digital compass, you can combine the accelerometer and magnetometer readings to have roll, pitch, and yaw measurements.

When the iPhone's accelerometer measurements are combined with the gyroscope measurements, developers can create applications that can sense motion on six-axes: up and down, left and right, forward and backwards, as well as the roll, pitch and yaw rotations. So now we know how all our motion sensing games actually work….

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