Touch screen working,types,advantages and disadvantages

                         TOUCH SCREEN

• A touch screen is a computer display screen that is also an input device. The screens are sensitive to pressure; a user interacts with the computer by touching pictures or words on the screen.

Working principle:

In the construction of a touch screen there are some parameters that have to be fulfilled. They are

• ·         The recognition of the touch on the display [including multi-touch].
• ·         Mechanism to find out the location of the display and to carry on the appropriate command.

The most commonly used technologies are the resistive and capacitive sensing technologies [Both are explained below in detail]. For both these methods a touch screen should have four layers.

• ·         A conducting metal coating with a poly-ester coating on the top. The metal coating should be transparent in nature.
• ·         A spacing layer which is mostly an air-gap.
• ·         A glass layer beneath the spacing with a conducting transparent coating on its top.
• ·         An adhesive layer beneath the glass layer. This layer is mostly used for mounting purposes.

When a person presses his finger on the top of the screen, there will be a change in the electrical current in the display module. This change is measured in either ways explained below and the exact location of touch and the amount of force applied is calculated. Later, the command to be carried out is passed on to the operating system and the command is carried out.

Capacitive Touch Technology :-

• ·        Capacitive touch screens take advantage of the conductivity of the object to detect location of touch. While they are durable and last for a long time, they can malfunction if they get wet. Their performance is also compromised if a non conductor like a gloved finger presses on the screen. Most smart phones and tablets have capacitive touch screens.

Resistive Touch Technology :-

• ·        Resistive touch screens have moving parts. There is an air gap between two layers of transparent material. When the user applies pressure to the outer layer, it touches the inner layer at specific locations. An electric circuit is completed and the location can be determined. Though they are cheaper to build compared to capacitive touch screens, they are also less sensitive and can wear out quickly.

Infrared Touch Technology :-

• ·         This technology uses beams of infrared lights to detect touch events.

Surface Acoustic Wave Touch Technology :-

• ·         This type of touch screen uses ultrasonic waves to detect touch events.

Optical Touch Technology :-

• ·         This type of perimeter based technology uses optical sensors, mainly CMOS sensors to detect touch events.

ADVANTAGES:

• ·         Touch screen enable people to use computer without  any training.
• ·         This technology is simple and user friendly.
• ·         It provided sufficient security.

DISADVANTAGES:

• ·         It cannot be used to enter large amount of data.
• ·         It costs about two or thrice times of the amount of keyboard.
•  

Cyclo converters definition,types and applications

                          
                                             CYCLOCONVERTERS

• Cycloconverters are basically AC to AC power converters. They convert constant voltage AC power to adjustable voltage, adjustable frequency AC power without any intermediate DC link.

• The cycloconverter is defined as a frequency changer that converts ac power at one input frequency to output power at a different frequency with a one-stage conversion process.

Types of cycloconverters:

• Step Up cycloconverter :- These types use natural commutation and give an output at higher frequency than that of the input.

• Step Down cycloconverter :- This type uses forced commutation and results in an output with a frequency lower than that of the input.

Cycloconverters are further classified into three categories,

• Single phase to single-phase :- This type of cycloconverter has two full wave converters connected back to back. If one converter is operating the other one is disabled, no current passes through it.

• Three-phase to single-phase :- This cycloconverter operates in four quadrants that is (+V, +I) and (−V, −I) being the rectification modes and (+V, −I) and (−V, +I) being the inversion modes.

• Three-phase to three-phase :-This type of cycloconverter is majorly used in AC machine systems that are operating on three phase induction and synchronous machines.

Applications:

• HVDC transmission systems
• Static Var Generation
• Aircraft or shipboard power supplies
• Speed control of high power AC drives

Buck Boost converter operation,modes and application

  BUCK -BOOST CONVERTER

• Buck-boost converters are used both to step up voltage from a lower level to a higher level and to step down voltage from a higher level to a lower level. The magnitude of output voltage depends on the duty cycle of the switch.It is also called as step up/step down converter. The name step up/step down converter comes from the fact that analogous to step up/step down transformer the input voltage can be stepped up/down to a level greater than/less than the input voltage.

Principle of operation of Buck converter:

• The main working principle of Buck Boost converter is that the inductor in the input circuit resists sudden variations in input current. When switch is ON the inductor stores energy from the input in the form of magnetic energy and discharges it when switch is closed. The capacitor in the output circuit is assumed large enough that the time constant of RC circuit in the output stage is high. The large time constant compared to switching period ensures that in steady state a constant output voltage V_o(t) = V_o(constant) exists across load terminals.

Buck Converter Working:

• The following diagram shows the working operation of the buck converter. In the buck converter first transistor is turned ON and second transistor is switched OFF due to high square wave frequency. If the gate terminal of the first transistor is more than the current pass through the magnetic field, charging C, and it supplies the load. The D1 is the Schottky diode and it is turned OFF due to the positive voltage to the cathode.
• The inductor L is the initial source of current. If the first transistor is OFF by using the control unit then the current flow in the buck operation. The magnetic field of the inductor is collapsed and the back e.m.f is generated collapsing field turn around the polarity of the voltage across the inductor. The current flows in the diode D2, the load and the D1 diode will be turned ON.
• The discharge of the inductor L decreases with the help of the current. During the first transistor is in one state the charge of the accumulator in the capacitor. The current flows through the load and during the off period keeping Vout reasonably. Hence it keeps the minimum ripple amplitude and Vout closes to the value of Vs

Boost Converter Working:

                                     

• In this converter the first transistor is switched ON continually and for the second transistor the square wave of high frequency is applied to the gate terminal. The second transistor is in conducting when the on state and the input current flow from the inductor L through the second transistor. The negative terminal charging up the magnetic field around the inductor. The D2 diode cannot conduct because the anode is on the potential ground by highly conducting the second transistor.
• By charging the capacitor C the load is applied to the entire circuit in the ON State and it can construct earlier oscillator cycles. During the ON period the capacitor C can discharge regularly and the amount of high ripple frequency on the output voltage. The approximate potential difference is given by the equation below.

VS + VL

• During the OFF period of second transistor the inductor L is charged and the capacitor C is discharged. The inductor L can produce the back e.m.f and the values are depending up on the rate of change of current of the second transistor switch. The amount of inductance the coil can occupy. Hence the back e.m.f can produce any different voltage through a wide range and determined by the design of the circuit. Hence the polarity of voltage across the inductor L has reversed now.
• The input voltage gives the output voltage and atleast equal to or higher than the input voltage. The diode D2 is in forward biased and the current applied to the load current and it recharges the capacitors to VS + VL and it is ready for the second transistor.

Modes Of Buck Boost Converters:

There are two different types of modes in the buck boost converter. The following are the two different types of buck boost converters.

•         Continuous conduction mode.
•          Discontinuous conduction mode.

    

Continuous Conduction Mode:

• In the continuous conduction mode the current from end to end of inductor never goes to zero. Hence the inductor partially discharges earlier than the switching cycle.

Discontinuous Conduction Mode:

• In this mode the current through the inductor goes to zero. Hence the inductor will totally discharge at the end of switching cycles.

Applications of Buck boost converter:

•         It is used in the self regulating power supplies.
•         It has consumer electronics.
•         It is used in the Battery power systems.
•         Adaptive control applications.
•         Power amplifier applications.

Shaded Pole Induction Motor

Shaded-Pole induction Motors

• Like any other motors the shaded pole induction motor also consists of a stator and rotor. The stator is of salient pole type and the rotor is squirrel cage type.

• The poles of shaded pole induction motor consist of slots, which are cut across the lamination . The smaller part of the slotted pole is short-circuited with the help of a coil. The coils are made up of copper and it is highly inductive in nature. This coil is known as shading coil. The part of the pole which has the coil is called the shaded part and the other part of the pole is called un-shaded part.

 

Construction of Shaded Pole Induction Motor:

 

A shaded pole motor may be 2 pole or 4 pole. Here we are considering a 2 pole shaded pole motor. The pictures in this article also shows a 2 pole motor.

Stator:

• The stator has salient poles. Usually 2 to 4 poles are used. Each of the poles has its own exciting coil. A part of each pole is wrapped by a copper coil. The copper coil forms a closed loop across each pole. This loop is known as the shading coil.

• The poles are laminated. A slot is cut across the lamination of the pole. The slot is approximately one third distance from the edge of the pole. The short circuited copper coil described above is placed in this slot. So we can call this part as the shaded part and other part of the pole as unshaded part. 

• Selecting a 2 poled stator gives a synchronous speed of 3000 rpm while a 4 poled stator speed will be 1500rpm for 50Hz supply.

Rotor:

• The rotor of shaded pole induction motors is Squirrel Cage type rotor. The rotor bars are provided with a 60 degree skew. This is to obtain an optimum starting torque and for limiting the torque dip during run up.

WORKING:

The working operation of  shaded pole motor can be easily understood .Our main aim is to produce rotating magnetic field with single phase AC supply this is possible with shaded poles.

• Alternating flux is produced by input ac supply .We can observe waveform of the flux in below figure .The distribution of this flux in the pole area is greatly influenced by the role of copper shading band .Now assume our ac supply as sine wave as shown in 1. 

During the portion OA:

• In OA portion  the alternating-current cycle [1], the flux begins to increase and an e.m.f. is induced in the shading coil. The resulting current in the shading coil will be in such a direction (Lenz’s law) so as to oppose the change in flux. Thus the flux in the shaded portion of the pole is weakened while that in the un-shaded portion is strengthened as shown in figure 2.

During the portion AB:

• During the portion AB of the alternating-current cycle, the flux has reached almost maximum value and is not changing. Consequently, the flux distribution across the pole is uniform [Fig 3] since no current is flowing in the shading coil. 

During the portion BC:

• As the flux decreases (portion BC of the alternating current cycle), current is induced in the shading coil so as to oppose the decrease in current. Thus the flux in the shaded portion of the pole is strengthened while that in the un-shaded portion is weakened as shown in Fig 4.

• The effect of the shading coil is to cause the field flux to shift across the pole face from the un-shaded to the shaded portion. This shifting flux is like a rotating weak field moving in the direction from un-shaded portion to the shaded portion of the pole .The rotor is of the squirrel-cage type and is under the influence of this moving field. Consequently, a small starting torque is developed. As soon as this torque starts to revolve the rotor, additional torque is produced by single-phase induction-motor action. The motor accelerates to a speed slightly below the synchronous speed and runs as a single-phase induction motor.

Advantages:

• ·         Very cheap and reliable

• ·         Easy to construct

• ·         Extremely rugged in nature

Disadvantages:

• ·         Low efficiency

• ·         Low starting torque

• ·         Since the shading coil is made of copper, the copper loss is high.

Uses:

• Due to their low starting torques they are mostly employed in small instruments, toys, small fans, electric clocks, hair dryers, ventilators, circulators etc.

• Thus we have discussed in detail the working, constructional features and applications of Shaded pole Single Phase motors.

STEPPER MOTOR

STEPPER MOTOR

A Stepper Motor or a step motor is a brushless, synchronous motor which divides a full rotation into a number of steps. Unlike a brushless DC motor which rotates continuously when a fixed DC voltage is applied to it, a step motor rotates in discrete step angles. The Stepper Motors therefore are manufactured with steps per revolution of 12, 24, 72, 144, 180, and 200, resulting in stepping angles of 30, 15, 5, 2.5, 2, and 1.8 degrees per step. The stepper motor can be controlled with or without feedback.

WORKING PRINCIPLE:

• The unique feature of a stepper motor is that the shaft rotates in definite steps, one step being taken each time a command pulse is received. When a definite number of pulses are supplied, the shaft rotates through a definite known angle. The rotor of a stepper motor is gear shaped and it can be of ferromagnetic material or permanent magnet. Multiple toothed poles on which field coils are wound are arranged around the gear shaped rotor. The stator poles are magnetized in the appropriate manner by using a micro-controller or microprocessor or by other means.
• First, one pole is magnetized by supplying the corresponding field coil. This toothed pole then aligns the rotor teeth due to magnetic attraction. Rotor teeth are slightly offset from the next pole.
• At the next step, first pole is demagnetized and the second is magnetized. This causes the rotor to rotate in a fixed angle to align with the second pole and offset with the previous pole.
• This was the basic working principle of a stepper motor. The rotor can be made multiple stacked to achieve more steps. Also, different types of stepping (like full step, half step or micro step) can be used for achieving more steps.

 

Classification of Stepper Motors:

Based on the type of construction stepper motors can be classified as,

• Variable Reluctance (VR) stepper motor
• Permanent Magnet (PM) stepper motor
• Hybrid stepper motor

Variable Reluctance (VR) stepper motor:

• The variable reluctance stepper motors are those which have a rotor made of ferromagnetic substances. Hence when the stator is excited it becomes an electromagnet and the rotor feels a pull in that direction. The ferromagnetic substance always tries to align itself in the minimum reluctance path.
• By exciting the coils, a magnetic field is procured and air gap reluctance is varied. Hence it is called a variable reluctance stepper motor. In this motor, the direction of the motor is independent of the direction of the current flow in the windings.

Permanent Magnet (PM) stepper motor:

• Here the rotor is permanently magnetized. Hence, the movement of the motor is due to the attraction and repulsion between the stator and rotor magnetic poles.
• In this motor, the direction of the motor is directly dependent of the direction of the current flow in the windings as the magnetic poles are reversed my changing the direction of the current flowing through the rotor.

Hybrid stepper motor:

• The hybrid stepper motor, as the name suggest is a motor designed to provide better efficiency by combining the pros of both the permanent magnet stepper motor and variable reluctance stepper motor.
• The VR and PM stepper motors are the most common type of stepper motors. The only difference is that, in the variable reluctance stepper motor, the rotor is made of a ferromagnetic substance and in the case of permanent magnet stepper motor, the rotor is permanently magnetized.

ADVANTAGES:

• Positioning – Since steppers move in precise repeatable steps, they excel in applications requiring precise positioning such as 3D printers, CNC, Camera platforms and X ,Y Plotters. Some disk drives also use stepper motors to position the read/write head.
• Speed Control – Precise increments of movement also allow for excellent control of rotational speed for process automation and robotics.
• Low Speed Torque - Normal DC motors don't have very much torque at low speeds. A Stepper motor has maximum torque at low speeds, so they are a good choice for applications requiring low speed with high precision.

DISADVANTAGES:

• Low Efficiency – Unlike DC motors, stepper motor current consumption is independent of load. They draw the most current when they are doing no work at all. Because of this, they tend to run hot.
• Limited High Speed Torque - In general, stepper motors have less torque at high speeds than at low speeds. Some steppers are optimized for better high-speed performance, but they need to be paired with an appropriate driver to achieve that performance.
• No Feedback – Unlike servo motors, most steppers do not have integral feedback for position. Although great precision can be achieved running ‘open loop’. Limit switches or ‘home’ detectors are typically required for safety and/or to establish a reference position.

Applications:

•      Industrial Machines – Stepper motors are used in automotive gauges and machine tooling automated production equipments.
•        Security – new surveillance products for the security industry.
• .      Medical – Stepper motors are used inside medical scanners, samplers, and also found inside digital dental photography, fluid pumps, respirators and blood analysis machinery.
•     Consumer Electronics – Stepper motors in cameras for automatic digital camera focus and zoom functions.