Use of MEMS- based Motion
Sensors in Embedded Mobile
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
What is MEMS-based Motion
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.
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 three
translation 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
What is IMU or Inertial Measurement
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
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
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
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
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.
MEMS Technology Involved In Making A
1. Silicon Bulk Micromachining and Wafer Bonding.
2. Polysilicon Surface Micromachining.
3. Metal Electroforming and LIGA.
4. Combined Bulk- Surface Micromachining.
● The accelerometer is a
device that can measure the
caused by gravity or by
accelerometer can therefore
measure the speed of
movement of an object it is
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
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.
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.
● 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
● Define Resolution, Zero rate Output, Scale factor
and short and long-term drift (terms used in Mems
● Why is gyroscope needed in addition to
accelerometers for describing the accurate position
of any object in 3-d space?