Here we are talking about sensors used in the Robotics. As we know that robotics uses the sensors to its maximum limits, various types of sensors are used for various purposes in various application in various locations.

Classification of Sensors Used In Robotics Based On Physical Input Quantity:

1. Tactile sensor.
2. Proximity Sensors.
3. Range sensors.
4. Navigation sensors.
5. Vision sensors.
6. Miscellaneous sensors.

1. Tactile Sensors:

Tactile sensors are devices which indicate contact between themselves and some other solid object-in this case, an obstacle. This kind of sensors can be subdivided into two more sutypes:

• Touch Sensors:

They are used to indicate that contact has been made between two objects without regard to the magnitude of the contraction force. A binary output signal indicates whether or not a contact has been made with the object.

Ex:- limit switches, micro switches etc.

• Force Sensors:

They are also called as stress sensors. They not only indicate that the contact has been made but also the magnitude of the contact force between the two objects.

The capacity to measure forces permits the robot to perform a number of tasks. These include the capability to grasp parts of different sizes in material handling, machine loading and assembly work, applying the appropriate level of force for the given part. In assembly applications, force sensing could be used to determine if screws have become cross-headed or if parts are jammed.

2. Proximity Sensors:

Proximity sensors are devices that indicate when an object is close to other. How close the object must be in order to activate the sensor depends upon the particular device. The distances can be anywhere from few millimeters to several feet.

Proximity sensors can be used to detect the presence or absence of a work part or object. Another important application is sensing human beings in the robot work cell. Range sensors will determine the location of an object (work part) in relation to the robot.

Ex:- optical proximity sensors, acoustical sensors, magnetic field proximity sensors.

3. Range Sensors:

This is type of sensors which can be considered very near to proximity sensors. Range sensors measure the distance between the object and the sensor. These sensors are located on the wrist or end-effecter as these are the moving parts of the robot.

4. Navigation Sensors:

As name navigation indicates, this is kind of sensors which can be used for estimate positioning of objects. This kind of sensors can be classified in two main subclasses.

• Relative Positioning Measurement:

They measure the signal within the robot itself. They can be again subdivided into:

• Proprioaptive sensor.
• Inertial navigation systems – Gyroscopes and Accelerometers.
• Odeometry.

• Absolute Positioning Measurement:

As name absolute, this sensors take measurements from some kind of fixed or universally fixed datum. Types Under This kind of sensors is:

• Magnetic Compasses.
• Global Positioning Systems (GPS).
• Active Beacons – triangulation.
• Landmark Navigation.
• Model Matching – Map matching.

5. Vision Sensors:

Machine vision is concerned with sensing of machine vision and its interpretation by a computer. The typical vision system consists of the C.C.D. camera and digitizing hardware, a digital computer and software necessary to interface them. This interface hardware and software is after referred to as a preprocesser. The operation of the vision system consists of 3 functions :

a) Sensing and digitizing data.

b) Image processing and analysis

c) Application

For object recognition, along with processing, robot programming is also necessary.

6. Miscellaneous Sensors:

This category covers the remaining types of sensors and transducers that might be used for interlocks and other purpose.

This category includes devices with the capability to sense variables such as temperature, pressure, fluid flow, velocity and electrical properties. Hence this category includes sensors and transducers like tachometers. LVDT, Pr gauges etc.

Related Topics and Articles:

1. Autonomous Mobile Robots Overview
2. Ultrasonic Level Sensor
3. Linear Variable Differential Transformer-LVDT

Seminar on Ultrasonic Level Sensor:

Ultrasonic Level Sensor is good seminar topic for Electronics Engineering:

1. What is Sensor?
2. Ultrasonic Level Sensor
3. Construction.
4. Working.
5. Advantages.
6. Limitations.
7. Applications.

What is Sensor or Transducer:

Transducer is device which converts the mechanical signal to the electrical signal. Transducer includes various devices like LVDT, tachogenerator, RVDT etc.

Ultrasonic Level Sensor:

Ultrasonic Level Sensor is the device which is used to measure the level of matter in tank or similar. As name suggest, it uses the ultrasonic waves for this measurement.

Construction and Working of Ultrasonic Level Sensor :

Ultrasonic Level Sensor is consist of two main parts viz. transmitter and receiver. The assembly is placed on the tank in which level is to be measured. The transmitter directs the sound waves to the matter and those waves gets reflected back from its surface. Receiver receives the waves and counts down the time delay between the transmission and back receive. From this data and some standard data like speed of ultrasonic sound in the air it finds the distance of level of matter from the sensor.

This sensor provides very fast and reliable data output. Whole answer is mainly dependent upon time delay or lag. The ultrasonic sound is created by the Piezoelectric crystal inside the sensor. It converts electrical energy to ultrasonic sound energy.

Types of Ultrasonic Level Sensor:

1. Continuous Level Sensor:

Continuous Level Sensor can monitor continuously for change in level. In this the phase of emission of waves, their reception and calculation of level is ongoing process.

2. Point Level Sensor

Apart from continuous level sensor, Point level sensor is fire up type. It emits the sound, receives it and calculates the depth for one time. Its name can explain it.

Advantages:

1. Non disturbing technique.
2. Can be used for wide range of things like solid and liquids with wide range of viscosity.
3. Low price.
4. High functionality.
5. Very accurate and low error.
6. Long life instrument.
7. Can measure the level of different liquids having different densities in same tank.

Limitations:

1. Sensor mounting must be properly done.
2. Tank should be free from obstacles otherwise it may cause error in measurement.
3. Result may get affected due to change in temperature or pressure inside the tank or container.

Applications:

1. Automatic pump control.
2. Flow measurement.
3. open channel flow measurement.
4. Used in water/waste applications.

Related Topics and Articles:

1. Ultrasonic Devices
2. Ultrasonic Distance Sensing
3. Ultrasonic Flowmeters

Principle of RVDT:

RVDT is used where precise angular movement is to be measured. RVDT uses the same principle as the LVDT except that, LVDT is for linear measurement while RVDT is for Angular one.

Construction and Working of RVDT:

RVDT is almost identical in the construction with LVDT. The ferrite core in LVDT is replaced by heart or cardiode shaped core. This core is mounted on the shaft or plunger which is capable of rotating. This rotational movement of the shaft creates the imbalance in the external circuit and this imbalance signal is used to measure the angular displacement of the shaft. RVDT give linear output up to displacement of +-40 degree. Then output starts becoming nonlinear.

Applications:

1. RVDT in the signal conditioning as RVDT Conditioner.
2. RVDT as the position sensor.

Examples:

1. Quarter Turn Valve.
2. Mail Sorters.
3. Fractional Shaft measurements.
4. Actuator position sensing in the Aircraft.
5. Marine, Constructional and Agricultural vehicles.

Advantages of RVDT:

1. Low cost.
2. Solid and Robust construction makes it to work in different environments.
3. No frictional resistance, so high accuracy over long time duration.
4. Negligible hysteresis.
5. Excellent reproducibility.

Disadvantages of RVDT:

1. Contact between plunger and measuring surface is always not possible.
2. RVDT provide linear output for about +-40 degree which limits its usability.

Related Topics and Articles:

1. Linear Variable Differential Transformer-LVDT
2. Ultrasonic Level Sensor
3. Ultrasonic Distance Sensing

What is LVDT?

The major use of LVDT is to convert the Rectilinear Physical Motion into Equivalent electrical signal. So LVDT is the sort of transducer which, by means of change in the electrical field of charged coil, changes the output and this change in the output is used to measure the displacement of the object.

Construction of LVDT:

LVDT is provided with two windings

1. Primary Winding.
2. Secondary Winding.

The plunger is provided in the centre which is capable of having linear motion. The ferrite core is mounted on this core which remains in the centre and equidistant form all windings at zero displacement. When this plunger moves towards left or right the imbalance is created in the external circuit connected to measure and this signal is used to measure the displacement of the plunger.

Application of LVDT:

1. LVDTs are used for position feedback in servomechanisms.
2. For measurement in automated machine tools.
3. For many other industrial and scientific applications.

Practical Examples of LVDT:

1. Soil Strength Testing Machine.
2. Weighing Machine.
3. Robot Manipulator.
4. Robot Cleaner.
5. Pill Making Machine.

Limitation:

Major restriction of LVDT is that it can not measure the rotary displacement. But this restriction of LVDT can be overcome by making some change in its construction. i.e. by RVDT(Rotary Variable Displacement Transformer).

Related Topics and Articles:

1. Rotary Variable Differential Transformer-RVDT
2. Ultrasonic Level Sensor
3. Sensors In Robotics