Mechatronics

Mechatronics

The Hall Effect Sensor

We could not end this section on Magnetism without a mention about magnetic sensors and especially the Hall Effect Sensor. Magnetic sensors convert magnetic or magnetically encoded information into electrical signals for processing by electronic circuits, and in the Sensors and Transducers tutorials we looked at inductive proximity sensors and the LDVT as well as solenoid and relay output actuators.

Magnetic sensors are solid state devices that are becoming more and more popular because they can be used in many different types of application such as sensing position, velocity or directional movement. They are also a popular choice of sensor for the electronics designer due to their non-contact wear free operation, their low maintenance, robust design and as sealed hall effect devices are immune to vibration, dust and water.

One of the main uses of Magnetic Sensors is in automotive systems for the sensing of position, distance and speed. For example, the angular position of the crank shaft for the firing angle of the spark plugs, the position of the car seats and seat belts for air-bag control or wheel speed detection for the anti-lock braking system, (ABS).

Magnetic sensors are designed to respond to a wide range of positive and negative magnetic fields in a variety of different applications and one type of magnet sensor whose output signal is a function of magnetic field density around it is called the Hall Effect Sensor.

Hall Effect Sensors are devices which are activated by an external magnetic field. We know that a magnetic field has two important characteristics flux density, (B) and polarity (North and South Poles). The output signal from a Hall effect sensor is the function of magnetic field density around the device. When the magnetic flux density around the sensor exceeds a certain pre-set threshold, the sensor detects it and generates an output voltage called the Hall Voltage, VH. Consider the diagram below.
Hall Effect Sensor Principals

hall effect sensor principals


Hall Effect Sensors consist basically of a thin piece of rectangular p-type semiconductor material such as gallium arsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs) passing a continuous current through itself. When the device is placed within a magnetic field, the magnetic flux lines exert a force on the semiconductor material which deflects the charge carriers, electrons and holes, to either side of the semiconductor slab. This movement of charge carriers is a result of the magnetic force they experience passing through the semiconductor material.

As these electrons and holes move side wards a potential difference is produced between the two sides of the semiconductor material by the build-up of these charge carriers. Then the movement of electrons through the semiconductor material is affected by the presence of an external magnetic field which is at right angles to it and this effect is greater in a flat rectangular shaped material.

The effect of generating a measurable voltage by using a magnetic field is called the Hall Effect after Edwin Hall who discovered it back in the 1870’s with the basic physical principle underlying the Hall effect being Lorentz force. To generate a potential difference across the device the magnetic flux lines must be perpendicular, (90o) to the flow of current and be of the correct polarity, generally a south pole.

The Hall effect provides information regarding the type of magnetic pole and magnitude of the magnetic field. For example, a south pole would cause the device to produce a voltage output while a north pole would have no effect. Generally, Hall Effect sensors and switches are designed to be in the “OFF”, (open circuit condition) when there is no magnetic field present. They only turn “ON”, (closed circuit condition) when subjected to a magnetic field of sufficient strength and polarity.

Hall Effect Magnetic Sensor

The output voltage, called the Hall voltage, (VH) of the basic Hall Element is directly proportional to the strength of the magnetic field passing through the semiconductor material (output ∝ H). This output voltage can be quite small, only a few microvolts even when subjected to strong magnetic fields so most commercially available Hall effect devices are manufactured with built-in DC amplifiers, logic switching circuits and voltage regulators to improve the sensors sensitivity, hysteresis and output voltage. This also allows the Hall effect sensor to operate over a wider range of power supplies and magnetic field conditions.

automobile braking mguniversity

Braking System

Brakes are employed to stop or slow down the speed of vehicle.
When brake applied to wheel braking force is created that force oppose the speed of wheel or rotation of force.

Braking requirement:

1)The vehicle must stop in smallest distance.
2) It must act suddenly in emergency.
3)It must have strong braking force.
4)It must neither slip nor skid the vehicle.
And less heat production.
5)It must operate on least effort.

Types of brakes:

Breaks are divided into seven types as per there uses, functionality, locations etc.
1) On the basis of purpose saved.
a) Main brake.
b) Parking brake.

2) On the basis of location.

a) Wheel mounted.
b) Transmission mounted.

3) On the basis of drivers ergonomics.

a) Foot brake.
b) Hand brake.
4) On the basis of actuating.

a) Mechanical brake.

b)Hydraulic brake.
c) Air brake.
d)Electric brake.

5) On the basis of construction.

a) Drum brake.
b)Disc brake.

6) On the basis of application of brake efforts.

a)Manual brake.
b)Power brake.
c)Power assisted.

7) On the basis of action of brake shoes.

a)Internal expanding brake.
b)External contracting brake.

Brake Drum:

Construction of Brake Drum:
The brake drum is mounted on axle hub and whole assembly is hold in wheel to brake shoes are handed on the back plate by mines of pin expander is fitted in between shoes.
 The friction material is pasted or biretta on brake shoes the expanded (cam) expander.
The brake shoe and press on drum. Due to friction action brake will get applied.
Brake drum is shown in figure.

Brake Drum

Following parts used in break drum
1) Brake drum.
2) Back plate.
3) Brake shoe.
4) Brake lining.
5) Expander.
6) Anchor.
7) Returning spring.
8) Adjuster.

Disc brake:

Construction of disc break:
1) Caliper or cylinder casing.
2) Rooter disc.
3) Piston.
4) Friction pad.
5) Pad supporting plate.
6) Bleeder plug.
Working of disc break:
Disc brake is shown in figure.

Disc brake

The disc brake are appeaser hydraulically when the vehicle is to brake the brake fluid pumped by the master cylinder it them flows and pushes the piston and pad which makes friction with  rooter disc and due to frictional action brake will get applied.

Mechanical brake:

This brake system is operated by mechanical linkage.
This brake system applied light vehicle two wheeler and some three wheeler this system required more efforts from driver and applied less force to the brake system to that system doesn’t used on four wheeler or heavy vehicle.

Hydraulic brake system :-

In this system the brake is operated with the help of oil pressure in this system master cylinder and wheel cylinder is most important part.
This system is very popular on four wheeler vehicle basically light and medium vehicle.

Working of hydraulic break system :-

The diagram of hydraulic brake system is shown in figure.

Hydraulic brake

The brake system is fitted with brake oil when driver presses the brake pedal the master cylinder creates oil pressure and sent to wheel cylinder.
Piston in wheel cylinder get expands due to oil pressure and the brake shoes also get expand and rub on brake shoes.
Also get expand and rub on brake drum so that brake will get apply.

Master cylinder:

Master cylinder it is having following part:
1) Cylinder body.
2)  Piston.
3)  Primary cup.
4)  Secondary cup.
5)  Check valve.
6)  Return spring.
7)  Push rod.
8) Dust cover.
Working of master cylinder:
The diagram of master cylinder is shown in figure.

Master cylinder

When driver depress the brake pedal the push rod pushes the piston forward and oil get compresses due to pressure the brake fluid open the hole of check valve and flows to wheel cylinder when driver release the brake pedal brake flied come back and pushes the check valve return spring and piston back ward and oil stores in reverie.

Wheel cylinder:

The diagram of wheel cylinder is shown in figure.

Wheel cylinder
As driver pressure the brake pedal and increases the pressure of oil that oil inters in the wheel cylinder it expand the piston towards outside they expand brake shoe and applies the brake.
When release the spring pushes the piston tension and flows the oil return to the master cylinder like this brake get release.

Pneumatic or air brake system :-

In this brake system air pressure is used to apply brake.
Parts or components used in air brake system :-
1) Air compressor.
2) Reservoir air tank.
3) Water and oil separator.
4)  Brake valve.
5)  Air pressure gauge.
6)  Safety valve.
7)  Brake chamber.
8)  Pipes.
Working of air brake system :-
The diagram of pneumatic or air brake system is shown in figure.

Pneumatic or Air Brake System
Mostly it is used heavy vehicle the compressor run by engine sucked air from atmosphere and the piston of compressor compress the air and supplied to air tank.
 In between compressor and tank water separator is used which remove the water partial from air the air tank stores the air with pressure up to 8kg/cm2.the safety valve is used in the tank to manifold constant air pressure.
 The pipe is connected from tank to brake valve and from brake valve to brake pedal the brake valve get open and pressure air flows from tank to brake chamber as soon as the air pressure inter in brake chamber the diaphragm get deflect which pushes the push rod for tarring the cam as the cam get turned it expand the brake shoe and brake will get applied.
When driver release the brake pedal the brake valve gets closed and air pressure cannot supplied to the brake chamber so that brake will get not applied.

Brake lining:

The brake lining are high friction material beings used rub again the rotating brake drums and to stop them will the brake are applied for on efficient braking and longer lifer they are expect to have high standard of quality they are required to full fill various requirement such as given below:-
1) No water swells.
2) Low heat swells.
3) Low wear rate.
4) The high coefficient of normal and hot friction.
5) High strange and physically properties.

Brake lining material:

The brake lining are made of asbestos, rubber, metallic particles, resin, minerals, and coefficient of friction modifies among these the asbestos is most important there for brake.
Lining are commonly known as asbestos brake linings.
1) Chrysalides asbestos fibers are comparatively cheap posses the hot coefficient of friction around 0.33 to 0.40 in the temperature range of 200 to 300’.
2) The metallic partial of aluminum brass and zinc are used for high heat dissipation and better antiwar characteristics.
3) The asbestos can resist temperature up to 350’ without folding.

Types of brake linings:

On the basis of manufacturing the brake lining are group in to following types.
1) Rigid molded.
2) Flexible molded.
3) With metallic partial.
4) The wealth out metallic partial.
1)  Non asbestos brake lining.
i)  Flexible.
ii)  Roll lining.

Rigid molded asbestos brake lining :-

Rigid molded asbestos brake lining has a random asbestos filigrees friction material with metallic inclusion in the form of steel wool brass chips.
 The coefficient of friction is well maintained even at higher temperature it is suitable for used under dry operating condition only it offer exceptional resistance to wear wealth out any penalty in the form of drum damage it finds application in light the medium and heavy commercial drum brake such a lining are two types:-
With Metallic Partial
Without Metallic Partial

Flexible molded roll brake lining :-

The flexible molded non metallic roll brake lining are of a dark gray in color having a random asbestos they have a random asbestos they have a high level of friction with a good anti squall properties and excellent temperature and wear resistance they are produce in roll and liner form and are suitable used under a dry operating condition only its material is manufactured with a good finish on both the surface suit may be bounded on inter surface.

Wire meshed roll lining:

The molded rubberized and flexible wire meshed backed and with medium coefficient of partition these brake lining are suitable car and light commercial vehicle running in cities and also for agricultural tractors.

Woven brake lining:

These are solid non magically using women the configuration of abbots fabric this lining do not feed up to 250 to 300 wire inclusion is also done for better heat transfer the turnery since copper or this alloys are preferred there coefficient of friction morally lies in the vicinity of 0.4.

The wear properties of brake lining:

The wear resistance is a very important properties aliening should have the backing efficient will be have only of the timing are capable of sustaining a large number of braking stroke at a elevated temperature a test report conducted on lining at a 400 if show that the lining are stable or a long period of time and there is no reduction in brake effective even a after 100 cycle of inter brake operation a good brake lining should work well over about 800 km.

HEAT AND MASS TRANSFER

Differential equation of heat transfer

In order to find out the rate of heat transfer ,let consider an element from the bulk

  

Heat flowing in x direction  

During the time dt                          =-k­­­­­­­­­­­­­­­­­­­­­­_x dz dy ∂T/∂x dt

Heat flowing in x direction in time dt = Q_xdt

 By expansion of the equation

(f_(x+h)­) =f_x +h f’_x+h²f’’_{x+………..}

Then Q_(x+dx) = Q_x+(∂ Heat flowing in x direction is Qx = -k­­­­­­­­­­­­­­­­­­­­­­_x dz dy ∂T/∂x

Qx/∂x) dx

Then heat accumulation = Q_x+dx– Q_x

Q_X +  ∂Q_X/∂_X - Q_x

(∂Qx/∂x)             = (∂(-kdzdy)/∂x) × ∂T/∂x × dt dx

                            =  -k_x ∂²T/∂x² dx dy dz dt

Similarly

Heat accumulated due to heat flow in y direction

K_y ∂²T/∂y²dxdydzdt

Total heat accumulated = sum of heat accumulation in all direction

=(k_x∂²T/∂x² + k_y∂²T/∂y^{2 +}∂²T/∂z²)dxdydzdt

If q^. is the heat generated in differential element /volume

Then heat generated in time dt is=  q^.dxdydzdt

Net heat accumulation =

(k_x∂²T/∂x²+k_y∂²T/∂y²+k_z∂²T/∂z²+q^.)dxdydzdt                                            6

Due to heat accumulation body temperature changes by dT

Q= mC_pdT                                                                                          

=D×dxdydz×C_pdT                                                                                          7

Equating  6 and 7

D C_pdT = (k_x∂²T/∂x²+k_y∂²T/∂y²+k_z∂²T/∂z²+q^.)dt 

Dividing by dt

(kx∂²T/∂x²+k_y∂²T/∂y²+k_z∂²T/∂z²+q^. )= DC_p∂T/∂t

For isotropic material k= constant

K(∂²T/∂x²+∂²T/∂y²+∂²T/∂z²+q^./k)    = DC_p∂T/∂t

(∂²T/∂x²+∂²T/∂y²+∂²T/∂z²+q^./k)    = 1/(k/DC_p) ×∂T/∂t

1/(k/DC_p) = thermal diffusibility  =1/α

HEAT AND MASS TRANSFER

HEAT TRANSFER BY CONDUCTION

Fouriers law of conduction

Q is proportional to area perpendicular to heat flow
and is directly proportional to temperature differential dT
and inversely proportional to thickness dx

there fore fouriers law of heat conduction is
Q =kAdT / dx
k= thermal conductivity of material


                  conduction by.....

   free electrons   : contribute to thermal conduction

   lattice vibration produce lattice wave

     variation of thermal conductivity of material with temperature 

For solids in general the variation is shown in graph

For liquids and gases  the variation in general is as follows

In liquids and gases conduction occur due to molecular collisions