CONTROLLERS

  CONTROLLERS

In this new Farm automation system, we are using different controllers to control the parameters such as temperature, light, soil moisture and atmospheric humidity. The detailed descriptions of these controllers are given below.

5.1 SOIL MOISTURE CONTROLLER

A water system needs to move the water produced from the source to its customers. In almost all cases, the source is at a lower elevation than the user so the water must be raised to a higher level. Some type of pumping equipment must be used to generate the pressure for raising the water to the higher elevation. Since centrifugal pump delivers a constant flow of water at a constant pressure for any given set of conditions, it is ideal for delivering water to customers. Most well pumps are centrifugal pumps. They are ideal for use in the distribution system since they do not produce pulsating surges of flow and pressure. This pump operates on the theory of centrifugal force. As the impeller rotates in the pump case, it tends to push water away from the centre of the rotation. As the water is pushed away from the centre of the impeller, additional water is pulled into the eye, or centre, of the impeller. The water that has been pushed to the outside of the impeller is removed from the pump through the discharge piping. This water will have a pressure that is determined by the pitch of the impeller and the speed at which the impeller is turning. There are many types of centrifugal pumps, but they all have major parts in common

1. Pump Case

    The pump case or volute is designed to allow the liquid being pumped to move to the centre of the impeller as well as to allow the water to be removed from the pump through the discharge. The case, which fits closely around the impeller on all but the discharge side, is made of cast iron or brass. If the liquid is abrasive or corrosive, other materials, such as a rubber lining, may be used.

2. Impellers

    The impeller generates the centrifugal force that moves the liquid. Variations in the impeller are based on whether a particular application calls for large quantities of water, high pressure, or both. The design of the impeller is important to the development of pressure and flow.

The submersible pump is especially suited to deep well and booster service for industrial, commercial, and municipal water systems. The pump utilizes a submersible motor coupled directly to the bowl assembly and is designed to operate completely submerged in the fluid being pumped. Power is supplied to the motor by waterproof electrical cable. In deep well applications the pump motor and cable are suspended in the well by the riser pipe. Booster applications involve installing the unit in a steel suction barrel or horizontally in a pipe line. Since the entire unit is either enclosed or below the surface of the ground, there are several applications where the submersible pump has many advantages.

1.     Extremely deep wells where problems with shafting are likely to be encountered.

2.     In installations where flooding would damage standard above ground motors.

3.     Applications such as boosters pumps which require quiet operation.

4.     Installations where there is little or no floor space.

5.     Horizontal pipeline booster pumps placed directly in the pipeline where conditions require a minimum amount of excavation or use of land surface.

Submersible pumps may be operated and controlled in the same manner as any other type of turbine pump in similar applications. No special consideration peculiar to the submersible is generally necessary, with the exception of the motor starting equipment.

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Fig.5.1.1 submersible pump

5.2  HUMIDITY CONTROLLER

A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air. A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing objects from contacting the fan blades. Most fans are powered by electric motors, but other sources of power may be used, including hydraulic motors and internal combustion engines and solar power. Fans produce air flows with high volume and low pressure, as opposed to compressors which produce high pressures at a comparatively low volume. A fan blade will often rotate when exposed to an air stream, and devices that take advantage of this, such as anemometers and wind turbines, often have designs similar to that of a fan. Typical applications include climate control and personal thermal comfort (e.g., an electric table or floor fan), vehicle and machinery cooling systems, ventilation, fume extraction, winnowing, removing dust (e.g. in a vacuum cleaner), drying (usually in combination with heat) and to provide draft for a fire.

While fans are often used to cool people, they do not actually cool air (if anything, electric fans warm it slightly due to the warming of their motors), but work by evaporative cooling of sweat and increased heat conduction into the surrounding air due to the airflow from the fans. Thus, fans may become ineffective at cooling the body if the surrounding air is near body temperature and contains high humidity.

Fig.5.2.1 axial fans

The axial-flow fans have blades that force air to move parallel to the shaft about which the blades rotate. Axial fans blow air along the axis of the fan, linearly, hence their name. This type of fan is used in a wide variety of applications, ranging from small cooling fans for electronics to the giant fans used in wind tunnels. Axial flow fans are applied for air conditioning and industrial process applications. Standard axial flow fans have diameters from 300-400 mm or 1800 to 2000 mm and work under pressures up to 800 Pa.

Examples of axial fans are:

• Table fan: Basic elements of a typical table fan include the fan blade, base, armature and lead wires, motor, blade guard, motor housing, oscillator gearbox, and oscillator shaft. The oscillator is a mechanism that moves the fan from side to side. The axle comes out on both ends of the motor, one end of the axle is attached to the blade and the other is attached to the oscillator gearbox. The motor case joins to the gearbox to contain the rotor and stator. The oscillator shaft combines to the weighted base and the gearbox. A motor housing covers the oscillator mechanism. The blade guard joins to the motor case for safety.
• Ceiling fan: A fan suspended from the ceiling of a room is a ceiling fan. Ceiling fans can be found in both residential and industrial/commercial settings.
• In automobiles, a mechanical fan provides engine cooling and prevents the engine from overheating by blowing or sucking air through a coolant-filled radiator. It can be driven with a belt and pulley off the engine's crankshaft or an electric fan switched on or off by a thermostatic switch.
• Computer cooling fan.

5.3 LIGHT INTENSITY CONTROLLER

The incandescent light bulb produces light by heating a filament wire to a high temperature until it glows. The hot filament is protected from oxidation in the air with a glass enclosure that is filled with inert gas or evacuated. In a halogen lamp, filament evaporation is prevented by a chemical process that redeposit’s metal vapour onto the filament, extending its life. The light bulb is supplied with electrical current by feed-through terminals or wires embedded in the glass. Most bulbs are used in a socket which provides mechanical support and electrical connections.

Incandescent bulbs are manufactured in a wide range of sizes, light output, and voltage ratings, from 1.5 volts to about 300 volts. They require no external regulating equipment, have low manufacturing costs, and work equally well on either alternating current or direct current. As a result, the incandescent lamp is widely used in household and commercial lighting, for portable lighting such as table lamps, car headlamps, and flashlights, and for decorative and advertising lighting. Some applications of the incandescent bulb deliberately use the heat generated by the filament. Such applications include incubators, brooding boxes for poultry, heat lights for reptile tanks infrared heating for industrial heating and drying processes, and the Easy-Bake Oven toy. But waste heat can also significantly increase the energy required by a building's air conditioning system. Incandescent light bulbs consist of an air-tight glass enclosure (the envelope, or bulb) with a filament of tungsten wire inside the bulb, through which an electric current is passed. Small wires embedded in the stem in turn support the filament and its lead wires. The bulb is filled with an inert gas such as argon (93%) and nitrogen (7%) to reduce evaporation of the filament and prevent its oxidation. Early lamps and some small modern lamps used only a vacuum to protect the filament from oxygen. Filament temperatures depend on the filament type, shape, size, and amount of current drawn. The heated filament emits light that approximates a continuous spectrum.

Figure 5.3.1 incandescent bulb

CHAPTER 6

RELAYS

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.

Fig. 6.1 Relay switch

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC main circuit. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical.

The relay’s switch connections are usually labelled COM, NC and NO:

·       COM=Common, always connect to this, it is the moving part of the switch.

·       NC=Normally Closed, COM is connected to this when the relay coil is off.

·       NO=Normally Open, COM is connected to this when the relay coil is on.

Fig. 6.2: Normally open/close connections

6.1 RELAY SELECTION

We need to consider several features when choosing a relay:

·       Physical size and pin arrangement

If we are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. We should find this information in thee supplier’s catalogue.

·       Coil voltage

The relay’s coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.

·       Coil current

The circuit must be able to supply the current required by the relay coil. You can use Ohm’s law to calculate the current:

                        Relay coil current = supply voltage / Coil resistance

·       Switch ratings (voltage and current)

The relay’s switch contacts must be suitable for the circuit they are to control.

·       Switch contact arrangement (SPDT, DPDY etc)

Most relays are SPDT or DPDT which are often described as “single pole changeover” (SPCO) or “double pole changeover” (DPCO).

6.2  PROTECTION DIODES FOR RELAYS

       Transistors and ICs (chips) must be protected from the brief high voltage ‘spike’ produced when the relay the coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected across the relay coil to provide this protection.

       Note that the diode is connected ‘backwards’ so that it will normally not conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage ‘spike’ in its attempt to keep the current flowing.

Fig 6.2.1: Protection circuitry for Relays