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Monday, March 3, 2008

Sensors Based Robot Control

Hey this post u will find about the sensors used in ROBOTICS and their classification

Robotics has matured as a system integration engineering field defined by M.
Bradley as “the intelligent connection of the perception to action”. Programmable
robot manipulators provide the “action” component. A variety of sensors and
sensing techniques are available to provide the “perception”.


Since the “action” capability is physically interacting with the environment, two
types of sensors have to be used in any robotic system:
1)proprioceptors: -> for the measurement of the robot’s (internal) parameters;

2)exteroceptors: -> for the measurement of its environmental (external, from the robot point of view) parameters.

Data from multiple sensors may be further fused into a common representational
format (world model). Finally, at the perception level, the world model is
analyzed to infer the system and environment state, and to assess the
consequences of the robotic system’s actions.


From a mechanical point of view a robot appears as an articulated structure
consisting of a series of links interconnected by joints. Each joint is driven by an
actuator which can change the relative position of the two links connected by that
joint. Proprioceptors are sensors measuring both kinematic and dynamic
parameters of the robot. Based on these measurements the control system
activates the actuators to exert torques so that the articulated mechanical
structure performs the desired motion.

The usual kinematics parameters are the joint positions, velocities, and
accelerations. Dynamic parameters as forces, torques and inertia are also
important to monitor for the proper control of the robotic manipulators.
CEG 4392 Computer Systems Design Project.

The most common joint (rotary) position transducersare: potentiometers,
synchros and resolvers, encoders, RVDT (rotary variable differential transformer)
and INDUCTOSYN. The most accurate transducers are INDUCTOSYNs (+ 1 arc
second), followed by synchros and resolvers and encoders, with potentionmeters
as the least accurate.

Acceleration sensors are based on Newton’s second law. They are actually measuring the force which produces the acceleration of a known mass. Different types of acceleration transducers are known: stress-strain gage, piezoelectric,capacitive, inductive. Micromechanical accelerometers have been developed. In this case the force is measured by measuring the strain in elastic cantilever beams formed from silicon dioxide by an integrated circuit fabrication technology.


Exteroceptors are sensors that measure the positional or force-type interaction of
the robot with its environment.

Exteroceptors can be classified according to their range as follows:

1)contact sensors

2)proximity (“near to”) sensors

3)far away sensors

1)Contact Sensors:

Contact sensors are used to detect the positive contact between two mating
parts and/or to measure the interaction forces and torques which appear while
the robot manipulator conducts part mating operations. Another type of contact
sensors are the tactile sensors which measure a multitude of parameters of the
touched object surface.

2)Proximity Sensors:

Proximity sensors detect objects which are near but without touching them.
These sensors are used for near-field (object approaching or avoidance) robotic
operations. Proximity sensors are classified according to their operating
principle; inductive, hall effect, capacitive, ultrasonic and optical.
Inductive sensors are based on the change of inductance due to the presence of
metallic objects. Hall effect sensors are based on the relation which exists
between the voltage in a semiconductor material and the magnetic field across
that material. Inductive and Hall effect sensors detect only the proximity of
ferromagnetic objects. Capacitive sensors are potentially capable of detecting
the proximity of any type of solid or liquid materials. Ultrasonic and optical
sensors are based on the modification of an emitted signal by objects that are in
their proximity.

3)Far Away Sensing:

Two types of far away sensors are used in robotics:

1)Range sensors:

Range sensors measure the distance to objects in their operation area. They are
used for
robot navigation,obstacle avoidance,to recover the third dimension
for monocular vision

Range sensors are based on one of the two principles:


Time-of-flight sensors estimate the range by measuring the time elapsed
between the transmission and return of a pulse. Laser range finders and sonar
are the best known sensors of this type.


Triangulation sensors measure range by detecting a given point on the object
surface from two different points of view at a known distance from each other.
Knowing this distance and the two view angles from the respective points to the
aimed surface point, a simple geometrical operation yields the range.

2)vision Range Sensors:

Robot vision is a complex sensing process. It involves extracting, characterizing
and interpreting information from images in order to identify or describe objects in

A vision sensor (camera) converts the visual information to electrical signals
which are then sampled and quantized by a special computer interface
electronics yielding a digital image. Solid state CCD image sensors have many
advantages over conventional tube-type sensors as: small size, light weight,
more robust, better electrical parameters, which recommends them for robotic

The digital image produced by a vision sensor is a mere numerical array which
has to be further processed till an explicit and meaningful description of the
visualized objects finally results. Digital image processing comprises more steps:
preprocessing, segmentation, description, recognition and interpretation.
Preprocessing techniques usually deal with noise reduction and detail
enhancement. Segmentation algorithms, like edge detection or region growing,
are used to extract the objects from the scene. These objects are then described
by measuring some (preferably invariant) features of interest. Recognition is an
operation which classifies the objects in the feature space. Interpretation is the
operation that assigns a meaning to the ensemble of recognized objects.

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