Selection and integration of MEMS-based motion processing devices

Motion processing, which measures and intelligently processes the movements of devices in three dimensional spaces, is the next major disruptive technology that will drive innovation in handheld consumer electronic (CE) product design, human interface design, and application navigation and control.

Driving this revolution is the availability of consumer-grade inertial measurement units (IMUs) based on micro-electromechanical systems, or MEMS. These devices, when combined for six axes of motion processing, provide a simpler user interface for intuitive navigation and control of handheld CE products by resolving the operating complexities that have confused many owners of sophisticated devices.

Enabling this six axes of control using MEMS motion processing is the recent availability of smaller, lower cost, and high performance three-axis MEMS gyroscopes that can be used in combination with existing three-axis MEMS accelerometers.

This paper will define a six-axis motion processing solution and examine critical factors involved in the selection and integration of this technology into everyday consumer electronic systems. Assuring compliance of the four critical factors outlined in this article will lead to higher integration efficiency when implementing new designs with six-axis motion processing, resulting in excellent performance in the end user device.

Motion-processing applications
At the Electronic Entertainment Exposition (E3) all three major game console brands demonstrated some form of motion-actuated human interface for their current or next-generation systems, with Nintendo being the first to announce the commercial availability of a six-axis motion processing solution in the Wii MotionPlus accessory. Console game software developers quickly introduced new game titles to take advantage of six-axis motion processing functionality: Nintendo is set to release the sequel to the popular Wii Sports title in July 2009 with Wii Sports Resort. Early product reviews acknowledge the higher precision and 1:1 tracking of controller movements with on-screen game play enabled by motion-processing.

Mobile handsets are the next frontier for motion processing as consumers have already embraced the novel features provided by three-axis accelerometers. The Apple iPhone is a case in point and Apple continues to develop unique motion-sensing applications, including the addition of a shake-to-undo motion gesture for its iPhone 3.0 during copy and paste functions, as well as App Store applications which turns the iPhone into a light saber. The addition of six-axis motion processing to future mobile handsets, and other handheld CE systems, delivers console gaming performance to these products, while giving software developers measurements of the handset's absolute position in 3D space with higher precision, accuracy and responsiveness.

Where once being first to market with traditional buttons and wheels might have been key to a design's success, in this fast-changing environment, that success will depend on who can create the most compelling user experience where complex control and navigation commands may now be executed with common hand motions enabled by this six-axis motion processing.

Motion processing solution
The key enabling technology for delivering motion processing capability is the gyroscope, which is traditionally used for measuring absolute rate of rotation. Vibratory mass gyros use energy transfer between the two resonating modes of a structure due to Coriolis acceleration, which arises in a rotating reference frame and is proportional to the rate of rotation as shown below in Figure 1. Gyroscopes measure angular velocity (Ω) by sensing Coriolis acceleration.

Vibratory tuning fork mass gyroscope implementations typically contain a pair of vibrating masses that are driven to oscillation with equal magnitude and in opposite directions. When the gyro device is rotated, the Coriolis force creates an orthogonal vibration force proportional to the rate of rotation, which is typically measured using capacitive sensing techniques between comb "fingers" along the perimeter of the oscillating proof mass structure and the comb of the stationary frame surrounding the proof mass. Good gyroscope designs have high Coriolis acceleration and low mechanical noise. Achieving high Coriolis acceleration requires a higher proof mass velocity, which is actuated by electrostatic forces, and achieving high sensitivity requires less integrated circuit (IC) amplification, which leads to lower noise.

While accelerometers provide basic motion sensing for simple orientation and tilt applications, there are limitations that affect accelerometer operation and performance in more complex applications, such as optical image stabilization (OIS). Accelerometers can only deliver the sum of linear and centripetal acceleration, gravity and vibration. Extracting a single element of the accelerometer's linear motion information is not feasible without the addition of a gyroscope. A gyroscope is required to accurately measure angular rate of rotational movement within the motion processing solution.

To provide corrections for rotation error in the accelerometers, some vendors use magnetometer sensors as a solution to fill the sensing need traditionally met by gyroscopes. These devices determine the rotational movement of a handheld device relative to magnetic north and are typically used to reorient a displayed map to correspond to the general direction a user is facing. Magnetometers are inadequate for fast rotational measurements (greater than 5 Hz) and are prone to data corruption in the presence of external magnetic fields, such as in the presence of a speaker or audio headset, or simply in the presence of ferrous materials within the device or its surroundings. Gyroscopes are the only inertial sensors that provide accurate, latency-free measurement of rotations without being affected by any external forces, including magnetic, gravitational or other environmental factors.

The introduction of silicon MEMS-based technology has enabled new MEMS gyroscopes that are no longer cost-prohibitive for consumer electronics and are suitable for achieving a challenging industry cost target of less than $1.00 USD per axis, while meeting the package size and the appropriate level of rotational sensing accuracy to become suitable for mobile phones, game controllers, remote controls and portable navigation devices. Motion processing solutions have been enabled by smaller, high performance, low cost MEMS gyroscopes and their companion MEMS accelerometers.