Use of MEMS- based Motion Sensors in Embedded

Mobile Applications

By Arnabjyoti Kalita(10-1-5-035)

Pallav(10-1-5-023)Nilim Borah(10-1-6-001)

Monjit Nunisa(10-1-5-045)

What is MEMS ?Micro-Electro-Mechanical Systems or MEMS, is an engineering that might be outlined as scaled down

mechanical and electro- mechanical components(i.e. Gadgets and structures) that are made utilizing the methods of micro manufacture . The physical size of MEMS devices can shift

from one micron to some millimeters. The term used to describe MEMS shifts in distinctive parts of the planet.

In the United States they are overwhelmingly called MEMS, while in some different parts of the planet they are called

“Microsystems Technology” or “micro machined units”.

Components of MEMS

What is MEMS-based Motion processing?

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, application navigation and control.

Driving this revolution is the availability of consumer grade Inertial Measurement Units (IMUs) based on MEMS. These devices, when combined for six axes of motion processing, provide a simpler user interface for intuitive navigation and control of handheld Consumer Electronic(CE) products by resolving the operating complexities that

have consumed many owners of sophisticated devices.

MEMS-based Motion processing(continued)...

Since we will be dealing with MEMS motion-based sensors, we must know what its objective is. It is mainly to define position and orientation

of some object or person in a real world frame of reference. Determining position and orientation in a real world frame of reference

requires accurate measurement and tracking in 6 Degrees of Freedom(DOF) in a frame of reference. Those 6 DOF include threetranslation and three rotational (a figure is shown in the next slide).

Enabling this six axes of control using MEMS motion processing is the recent availability of smaller, lower cost and high-performance 3-axis

MEMS gyroscopes that can be used in combination with existing 3-axis MEMS accelerometers.

What is IMU or Inertial Measurement Unit?

An inertial measurement unit, or IMU, is an electronic device that measures and reports on a craft's velocity, orientation, and gravitational

forces, using a combination of accelerometers and gyroscopes, sometimes also magnetometers. IMUs are typically used to maneuver aircraft,

including unmanned aerial vehicles (UAVs), among many others, and spacecraft, including satellites and landers. Recent developments allow

for the production of IMU-enabled GPS devices.

Two Main Components Used in MEMS based sensors in Mobiles

--------Accelerometers --------Gyroscopes

MEMS based Gyroscopes

Gyroscopes measure angular rotations. While accelerometers are based on Newton's laws of motion, the gyroscopes act upon the principles of

Coriolis force and Coriolis acceleration. The Gyroscopes measure rotations about three-axes, the x-axis, the y-axis and the z-axis. It

correctly gives the angle of rotation i.e. Pitch , Roll and Yaw (pic shown in next slide)

Essentially, Pitch and Roll are the angles measured along the plane axis of the sensor package surface. Yaw is the angle measured over the out-of-

plane axis of the sensor package.

Working Principle of Gyroscope

Coriolis Force and Coriolis Acceleration

We consider the 3-axes as per usual notation. Consider a mass M moving along the x-axis with velocity v. Suppose along the y-axis, we now apply

an angular rate. After the application of the angular rate, the body experiences a force in a direction perpendicular to both x-axis and y-axis.

The direction of this force will be out-of-plane. This force is called CORIOLIS force.

As a result of the Coriolis force, the body experiences a displacement from its presumed original path. This displacement is proportional to the

angle of rotation or the angular rate. The Coriolis force is given by:

F(coriolis) = 2Mvw

Here w is the angular velocity/ angular rate applied along the y-axis. This displacement must be measured electrically/ using capacitors to

determine the actual angular rate of rotation.

A MEMS Gyroscope is essentially a Tuning Fork Gyroscope...

Performance Parameters of MEMS Gyroscope

Resolution : Resolution of the Gyro is expressed in terms of the standard deviation of equivalent rotation rate per square root of bandwidth of

detection. Unit is (degree/hour)/(hertz exp(1/2)). Angle Random Walk may be used instead of resolution. (degree/(hertz

exp(1/2))).Scale factor is defined as the amount of change in the output signal per

unit change of rotation rate and is expressed in V(degree/S).Zero rate Output : Zero Rate Output represents the output of the device

in the absence of a rotation rate.Short or Long Term Drift : It is the peak-to-peak value of a slowly

varying function which influences the output signal of a gyroscope in the absence of rotation.

Gyrometer developed by ST Microelectronics

MEMS Technology Involved In Making A Vibrating Gyroscope

1. Silicon Bulk Micromachining and Wafer Bonding.

2. Polysilicon Surface Micromachining.

3. Metal Electroforming and LIGA.

4. Combined Bulk- Surface Micromachining.

Accelerometer

● The accelerometer is a device that can measure the force of acceleration, whether caused by gravity or by movement. An accelerometer can therefore measure the speed of movement of an object it is attached to.

Basics of Accelerometers

It has two fundamental parts. A housing attached to the object whose acceleration we want to measure and a mass that, while tethered to the housing, can still move. If me move the accelerometer then the seismic mass stretches the spring. If we measure how much that spring stretches we can calculate the force of gravity.

Basics of Accelerometers

It can be easily found that using three accelerometers 3D orientation of an object can be determined.

Accelerometer

Using different fabrication techniques like photolithography and wet etching a MEMS based accelerometer is fabricated. It has a housing that is fixed to the phone and a comb like section that can move back and forth. This comb-like structure is the equivalent of the seismic ball. The spring of the accelerometer is replaced by the flexibility of silicon structure.

Accelerometer

In order to detect the changes in the orientation the motion of the central section is examined. The three fingers make up a differential capacitor i.e. If the central section moves then the current will flow. Afterwards the amount of current is correlated to the acceleartion.

Suggested Questions

● What is the basic priciples of a Gyroscope?

● What is the basic priciples of an Accelerometer?

● Describe Photolithography?

● Describe the working principle of a differential capacitor?

● Define Resolution, Zero rate Output, Scale factor and short and long-term drift (terms used in Mems Gyroscope).

● Why is gyroscope needed in addition to accelerometers for describing the accurate position of any object in 3-d space?

THANK YOU