HOW DO METAL DETECTOR WORKS?

How do metal detectors work?

                    Airport security gates, treasures sweepers for hunting lost change or jewellery on the beach, traffic lights and coin slots in vending machines have one thing common. They all exploit the fact that a piece magnetic force has one or more of its physical properties changed in away that can be picked up by a sensor. Each type of metal detector contains a transmission elements for generating a magnetic field and receiving element for translating a secondary physical change in to an electronic signal that registers on a dial or triggers a sonic alarm.

                   The simplest of these devices generates an electric current that runs through a coil of wire, producing a magnetic field. When the coil passes over a piece of metal, the metal induces a second magnetic field, which interferes with first one and reverses the flow of electricity in the coil. This reversal is picked up by electronic circuitry. More sophisticated detectors that screen for concealed weapons, bombs or bits of metal contamination food, paper pulp or raw plastic may contain additional coils and computer chips that increases their sensitivity.

Measuring magnetic fields:

                 When metallic objects pass through a security gate, the flux or strength and extend of the magnetic field, increases. The rise in flux is accompanied by a voltage increase in the current flowing through the receiver coils, which triggers an alarm.

Control box:

                 To make sure that a security alarm sounds only if there is an unusual amount of metal guards adjust the sensitivity on a control box.

Two magnetic responses:

                Nickel, iron and other easily magnetized metals enhances a magnetic field, while copper, aluminium and other less easily magnetized ones do not.

Nickel or iron attracts magnetic lines of forces.

Copper repels the magnetic lines of forces.

Security checkpoint:

              To ferret out hidden guns or other weapons, airports and office buildings rely on security gates like the one at right. A series of coils scan a person’s body and alert guards when sizable amount of metals – possibly caused by the presence of weapons – are detected.

Detecting ferromagnetic materials:

              The ferromagnetic materials nickel, iron and cobalt become easily magnetized when exposed to as magnetic field. The special metal detector locates buried metals of this sort. The magnetic field in its two coils cancels each other out until exposed to a metal object. Then, the objects induced magnetism trips an electrical gate, sending a low current from the receiver coil to the detector’s electronic circuitry.

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Biography of Michael Faraday

Michael faraday was a leading physicists and pioneer in the field of Electrochemistry. His life is an example for all the dreamers of science who believe in himself and science. James Faraday, his father, was a blacksmith while his mother Margaret hastwell was daughter of a farmer. Michael was born on 22 september, 1791. His school teaching was, as would have expected, of the most elementary kind and ended at the age of 12.
Faraday worked as book binder because it was necessary for the poor family and he had to help his parents. But these days were passed among books and many of those books dealt with science, particularly with chemistry and Electricity, fields to which to which he found drawn himself. Michael was very much interested in attending public meetings and lectures, the one among the famous who impressed him is Sir Humphry Davy. Michael made careful notes of these lectures, wrote them out, illustrated them with his own drawings, and bounded them together, and the whole making was a beautifully bound book of 386 pages. He sent them with a letter to davy, asking for a job in his lab. Then he was appointed as a lab assistant and his chief job was to clean lab and repair apparatus.
In Oct 1813, davy set out on a scientific tour to europe and he took Faraday with him as his assistant and secretary. He got a chance to met Ampere and other scientists in Paris and enriched his knowledge. After his arrival from London in following years he made many investigations. First, he wrote a history of progress of Electromagnetism and then introduced new kinds of glasses and the alloys of steels. In 1823, prepared the liquified chlorine for the first time. In 1825, he wrote a paper on new compounds of carbon and hydrogen and he announced the discovery of Benzol. He was then worked to induce electric current and thus he was able to convert mechanical energy into electrical energy and discover the first dynamo.

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Windmill Power Station in Poolavadi near Udumalpett, Tamilnadu

Windmill Power Station in Poolavadi near Udumalpett, Coimbatore, Tamil Nadu

It is one of top most energy producing windmill power station in india producing more than 10MW. The name of the power station is Cherranjji wind energy limited. It consist of nearly 140 wind mill systems with 20-25 service men. The cost of individual system is about Rs 1.25 crores and it is serviced every month which costs approximately Rs 10,000. They had made an agreement for 20 years with the local electricity board to share the power generated in the windmill power station.

The main parts of the wind-mill:
   1. Anemometer
   2. Rotor
   3. Generator
   4. Tower
   5. Gear box
   6. Brake
   7. Wind wane
   8. Motors
   9. Blade
  10. Nacelle
  11. Yaw drive
  12. Hub
  13. Transformer
  14. Cable
  15. Microprocessor
  16. Power panel

Anemometer: The instrument used to measure the wind speed is anemometer.

Wind wane:  Its is a small instrument used to measure the wind direction and it communicates with the yaw drive. There are two types of turbines namely (a)Upwind turbine (b) Down wind turbine. Upwind turbine is one which is facing into the wind whereas the downwind turbine is one which is facing away from the wind.

Rotor: Usually rotor consists of two parts namely hub and the rotating blades or wings.

Hub is used to connect the blades/wings to the gear box through high speed shaft. The actual size of the hub is about 1.5m.

Blade: The blades used are made up of fibre materials which weighs about 750Kg and are imported from Germany. Each blades is about 11.5m length. In some system the aerodynamic brakes are used which are about nearly 1m length. It makes 40 to 50 rotation per minute (rpm).

Gear box: The purpose of the gear box is to convert the low rotating speed of the blades nearly 40-50rpm  into high speed nearly about 960-1000rpm. The one end of the gear box is connect to the low speed shaft whose extension is connected to the hub and the another is connected to the high speed shaft which is connected to the generator. the gear box used in Poolavadi windmill power station is about 1:24 parts which means that 40rpmX24parts = 960rpm at high speed shaft. There is another gearbox of 1:38.35 parts.

Tower: The tower used in power stations are nearly ranging from 30m to 80m high. Its because to to have the  undistrubed wind blow.

Generator:The power generated in poolavadi is about 250KW which is the minimum power usually generated using this system. By changing the gear box or using 4 or 6 poles generator, we can vary the power generated to 1.25MW, 1.65MW also. During peak wind 3500 units can be generated.

Motor: Usually three types of motors are used namely air motor, gear motor, hydraulic motor.

1.  Air motor: It is usually used for yawing or adapting the blades towards the wind direction.
2.  Gear motor: It used for oil pulling. Mostly the synthetic oil is used which is of more thickness namely 1710/320 gear oil.
3.  Hydraulic motor: It is used to operate the hydraulic brake. In hydraulic motor, the less lubricant oil is used namely DTE11M.

Rotating union:  Oil is supplied to the dividing piece in hub for opening and closing of the disc brake.

Brake: There are two types of brake in the wnd mill system namely:

(1) hydraulic brake (2) tip brake.

Hydraulic brake: it is controlled by hydraulic motor.

Tip brake/Aerodynamic brake: It is used reduce to the speed of the blade to 300 rpm from 960 rpm by extending the tip or it will damage the whole system if sudden brake is applied.

Yaw drive: It  can be operated by yaw motor / air motor. The yaw drive is connected to the communication cable to microprocessor and its operated at its entrance.

Interdistance positionizing windmill system:

for column-wise arrangement: Maximum blade’s diameter X 7

for row-wise arrangement: Maximum blade’s diameter X 5

EB Meter: It is used to kow the export and import of the current. There are two parts of settings: (1) EB line (2) consumer line.

Power panel: 

1. MCCB   
2. Junction contact   
3. Thyristor

MCCB (Main current circuit breaker): The voltage obtained from the EB (Electricity Board) is about 22,000V is step-down to 110KV and step down to 415V. It is opearated at 415V.

Thyristor: During cut-in position, the generator is in operating or generating mode whereas during cut-off position , the generator will be in free wheeling mode.

Varistor : It is a over voltage controller which used to ground the high voltages generated.

By-pass contact: During cut-off mode, the generator recieves power from  EB through thyristor whereas during cut-in mode, the generator supplies power to the EB.

Earth fault relay: The leakage current will dripped or grounded in this earth fault relay.

Consumption unit: During April – October, the power consumption is about 4 unit whereas during July –August  ie peak wind months the power capacity generation is about 250KW. Nearly 4000 units (app.) during grid-off whereas no units is consumed while grid-on.

Multipilication factor: M.F =  current  transformerpotential  transformer
Multification factor can defined as the ration of current tranformer to the potential transformer.

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