2.5 WING DESIGN BY HARRIS MODEL
The wing details vary, but in all cases they are characterized by consisting of three hinged panels with chord wise rigid ribs and constant span wise lengths. The three panel feature, which was invented by Harris, serves to balance the time-varying lift seen by the fuselage. The center portion of the wing flap in one direction while the outer portions flapp in the opposite direction. Whereas it is not possible to completely balance the inertial and aerodynamic loads for all flight conditions, this tendency to achieve a dynamic balance would be a valuable feature for a human carrying aircraft. Moreover, for ornithopters of any scale, the three panel wing provides evening out the power required from the engine during the flapping cycle. That is, the size of engines for ornithopters is determined by the top power required during the cycle. For customary two panel designs, the power required for the down stroke may be several times greater than the power required for the upstroke. Therefore, the required engine may be much larger than average power considerations would dictate. This effects in an aircraft heavier than a fixed wing propeller driven aeroplane of similar performance. Later, C designers have pursued ways to store energy on the upstroke. For birds and bats, some storage may be provided by their tendons. It is more probable, though, that the upstroke is the period of rest required by the down stroke muscles. Such rest is not needed by mechanical systems, and energy storage has typically used elastic means, such as the bungee cords incorporated into the AeroVironment pterosaur replica. However, an ornithopter isn't required to be a scale model of an animal, and the three panel wing thus provides an attractive means for power evening as well as dynamic balance.
Fig No. 2.5 Harris model (Top view)
Fig No. 2.6 Three panel model
The wing's flapping indication is activated through the center panel's vertical oscillation & driven in simple harmonic motion by a mechanism. The middle panel is built to be rigid in bending and twisting, and includes a simple flat-bottom aerofoil (nominally, a Clark-Y). It was initially imagined that the center panel would require a direct pitch articulation in conjunction with its vertical (plunging) motion. As it was found pterosaur model. An ornithopter isn't essential to be a scale model of an animal, and the three panel wing thus provides an attractive means for power evening as well as active balance. The wing's flapping motion is triggered through the center panel's vertical oscillation. Driven in simple harmonic motion by a mechanism. The center panel is constructed to be rigid in bending and twisting, and incorporates a simple flat-bottom aero foil (nominally, a Clark-Y). It was originally envisaged that the center panel would require a direct pitch articulation in conjunction with its vertical (plunging) motion; but it was found, as described later, that it operated most effectively in plunging only. The external panels operate in a span wise- seesaw way and determined at one end by hinge connections to the center panel. There were two systems of support struts and hinges for the outer panels in the course of their evolution, from 1985 to 1987 the strut assembly was rigid, with a single hinge connection to the outer panel. The kinematic requirements were then met by means of span wise sliding hinges to the centre panel. That is, the driving hinges on the end of an outer panel had lubricated cylindrical extensions which slid, piston-like, into the center panel’s hollow spar tubes. This demonstrated to be a very consistent system, but it had the aerodynamic disadvantage of producing large, variable. Span wise gaps in the middle of the center and outer panels. Pieces of stretchable fabric were used to cover the gaps; but this, aerodynamically, was not a particularly efficient solution. In 1989, the support struts were changed to a system incorporating hinged vertical links. These allowed the sliding hinges at the centre panel to be replaced with simple non-sliding units. The gap then remained small and could be aerodynamically sealed with narrow abutting extension strips attached to the end faces of the panels. This system operated consistently during the development of the flight tests in 1989 and 1991. The external panels between 1985 and 1987 united a single-surface aerofoil and two spar assembly Attached to the ribs were short spanwise tubular pieces through which the spars accepted. The original idea was that since the single sur-face aerofoil has torsional compliance, the outer panel's pitching distribution could be driven by spanwisc rocking of the two spars. wolb JIM a WM AM;Met at Midi 4CCIIICU woe a ltddlMaLPIC (=Sign. Later, as it became obvious from flight tests and the developing analysis that the wing had extreme torsional compliance, the rear spar was extended and a portion of it changed to a 1/4 in diameter fibreglass rod. In fact, there was a flight test session where the rear spar was a 1/4 in diameter solid carbonfibre rod. Guided by the analysis, the ornithopter's performance improved steadily during this course of development. However, it was never near to achieving sustained flight. That is. The flight mode was a "powered glide", where the flapping would clearly extend the dis-tance flown compared with the non-flapping cases, but it would not climb or maintain altitude. The reason, revealed by windtunnel tests01), was that the actual aerodynamic characteristics fell short of the estimated values used as inputs in the analysis. In specific, the aerofoil had high chord-wise drag (Co = 0.063). Poor leading-edge suction en, = 0-29). It detached flow on one side or the other at all angles of outbreak. In comparison with modem double surface sections, this aerofoil was a very poor performer (typically, for a Reynolds number of 2 x 103, a modem aerofoil should give Co= 0.012 and r1, 44 0.90). It was desired, however, to retain the single-surface feature because of its torsional compliance, and to find ways to improve such an aerofoil's aerodynamic performance.
A mechanism is a device intended to transform input forces and movement into an anticipated set of output forces and movement. Mechanisms generally has moving constituents such as gears, gear trains, belt, chain drives, cam and follower mechanisms, and linkages. Friction devices such as brakes, clutches and structural components such as the frame, bearings ,bfasteners, springs, lubricants and seals.
The German scientist delivers the explanation "a machine is an arrangement of resistant bodies so arranged that by their means the mechanical forces of nature can be compelled to do work attended by certain determinate motion." In this context, his use of machine is commonly understood to mean mechanism.
The combination of force and movement describes power, and a mechanism is intended to achieve power. In order to reach a wanted set of forces and movement.
A mechanism is usually a larger process or mechanical system. Sometimes a complete machine may be mentioned to as a mechanism. Examples are the steering mechanism in a car and the winding mechanism of a wristwatch. Multiple mechanisms are machines.
Types of Mechanisms
From the period of Archimedes mechanisms were well-thought-out to be constructed from simple machines, such as the lever, pulley, screw, wedge, wheel and inclined plane. It was Reuleaux who focused on bodies, called links, and the connections between these bodies called kinematic joints or pairs.
In order to use geometry to study the drive of a mechanism, its links are modeled as inelastic bodies. This means distances in the middle of points in a link are assumed to be unaffected as the mechanism moves that is the link does not bend. Thus, the relative movement in the middle of points in two connected links is considered to result from the kinematic pair that joints them.
Kinematic pairs are measured to provide ideal constraints in the middle of two links, such as the constraint of a single point for pure rotation. The constraint of a line for pure sliding, as well as pure rolling lacking slipping and point contact with slipping. A mechanism is modeled as an assembly of inelastic links and kinematic pairs.
Reuleaux stated the ideal connections in the mid of links kinematic pairs. He distinguished in the middle of higher pairs which were said to have line contact among the two links and lower pairs that have area contact amid the links. J. Phillips shows that there are many approaches to build pairs that do not fit this simple.
A lower pair is a joint that has surface interaction amid the pair of elements. We have the following cases:
- A revolute pair requires a line in the moving body to remain co-linear with a line in the fixed body, and a plane vertical to this line in the moving body maintains contact. It imposes five constraints on the relative movement of the links, which hence has one degree of freedom.
- A prismatic joint requires a line in a moving body remain co-linear with a line in the fixed body, and a plane similar to this line in moving body sustain contact with a alike parallel plane in the immovable body. This enforces five constraints on the relative movement of the links, which consequently has one degree of freedom.
- A cylindrical joint needs a line in the moving body to remain co-linear with a line in the fixed body. It is a blend of revolute joint and sliding joint. This joint has two degrees of freedom.
- A ball joint, desires that a point in the moving body keep connection with a point in the static body. This joint has three degrees of freedom.
- A planar joint needs that a plane in the moving body maintain contact with a plane in fixed body. This joint has three degrees of freedom.
- A helical joint has one degree of freedom since the rotational indications are related to the helix angle of the thread.
Higher pairs: In general, a complex pair is a limitation that requires a line or point contact between the elemental surfaces. The contact between a cam and its follower is a higher pair called a cam joint. Likewise, the contact between the involute curves that form the enmeshing teeth of two gears are cam joints.
It is a mechanical system which is held in reserve so the paths of points in all the bodies of the method fib on planes parallel to the ground plane. The revolving axes of hinged joints that join the bodies in the system are perpendicular to this ground plane.
It is a mechanical system that the bodies change in a way that the paths of points in the system lie on concentric spheres. The rotating axes of hinged joints that associate the bodies in the system to pass through the center of these spheres.
It is a mechanical system which has at least one body that moves in a direction that its point trajectories are general space curves. The turning axes of hinged joints that connect the bodies in the system form lines in space that do not transect and have different joint normals.
2.7 Gears and gear trains
Fig No. 2.7 Spur gears
The transmission of rotation in the middle of contacting toothed wheels can be traced back to the Antikythera mechanism. Drawings by the renaissance scientist Georgius Agricola show gear trains with cylinder-shaped teeth. The presentation of the involute tooth produced a regular gear design that provides a constant speed ratio. Some main features of gears and gear trains are:
- The ratio of pitch circles of coupling gears states the speed ratio and the mechanical use of the gear set.
- A planetary gear train provides high gear reduction in a compact package.
- It is possible to design gear teeth for gears that are not-circular, but still transmit torque smoothly.
- The speed ratio of chain and speed ratio of belt drive is calculated in the same way as gear ratios. See bicycle gearing.
A gear is a revolving machine part with cut teeth that meshes with another toothed part to transmit torque. In most cases with teeth on the one gear being of same shape, and frequently also with that shape on the other gear. Two or more gears working together are called a transmission and can yield a mechanical benefit through a gear ratio and thus may be well thought-out a simple machine. Geared devices can change their speed, torque, and direction of power source. The best common situation is for a gear to mesh with another gear; and also, a gear can mesh with a non-rotating toothed part, called rack, thereby creating translation as an alternative of rotation.
The gears in a transmission are analogous to the wheels in a crossed belt pulley system. Benefit of gears is that the teeth of a gear avoid slip.
When two gears are meshed in which one gear is bigger than the other (even though the size of the teeth must match), a mechanical advantage is produced, with the rotational speeds. There is an inverse relationship between the torques
In transmissions with multiple gear ratios—like bicycles, motorcycles, and cars—the term gear, refers to a gear ratio rather than an actual physical gear. The term defines similar devices, even when the gear ratio is continuous rather than discrete, or when the device does not actually contain gears, like in a continuously variable transmission.
History of the differential gear
The earliest gears was Chinese actual examples of wheels and moulds for wheels dating from the 4th century B.C. which have been preserved. It might be remarked that these "machine" gear wheels are characterized by having a "round number" of teeth and a shank with a square hole which fits without turning on a squared shaft.
The earliest gears in Europe was circa A.D. 50 but they can be drawn back to the Greek mechanics of the Alexandrian school in the 3rd century BCE and were greatly settled by the Greek polymath Archimedes (287–212 BCE). The Antikythera mechanism is a model of a very early and complex geared device, designed to compute astronomical positions. Its time of building is projected between 150 and 100 BC.
Fig No. 2.8 Gear engage
The velocity ratio that teeth give gears offers a benefit over other drives (such as traction drives and V-belts) in exactness machines like watches that depend on an exact velocity ratio. Some cases where driver and follower are proximal, gears also have an advantage over other drives in reduced number of parts required. The gears are more expensive to manufacture and their lubrication requirements may execute a higher operating cost.
Fig No. 2.9 Gear terminology
Measured in rotation over time, such as RPM.
Measured in radians/second.
How many teeth a gear has, an integer. In the case of worms, it is the number of thread starts that the worm has.
The larger of two cooperating gears or a gear on its own.
The smaller of two interacting gears.
Path tracked by the point of contact in the middle of two meshing gear teeth.
- Line of action, pressure line:
Line along with which the force in the middle of two meshing gear teeth is focused. It has the similar direction as the force vector. The line of action varies from moment to moment during the retro of arrangement of a couple of teeth. In involute gears the tooth-tooth force is continuously directed along the same line. The line of action is constant. This suggests that for involute gears the path of contact is also a straight line, equivalent with the line of action.
Axis of spin of the gear i.e. center line of a shaft.
Point where the line of action transects a line joining the two gear axes.
- Pitch circle, pitch line:
Circle placed on and vertical to the axis, and passing through the pitch point.
A diametral position on the gear where the circular tooth thickness, helix angle and pressure angles are defined. The typical pitch diameter is an elementary measurement and cannot be measured, but is a place where other measurements are made. Its value is based on number of teeth, the helix angle and the normal module. It is calculated as:
in metric units or
in imperial units.
Since it is unreasonable to calculate circular pitch with irrational numbers, mechanical engineers generally use a scaling factor that swaps it with a regular value in its place. This is recognized as the module or modulus of the wheel and is defined as
Where, m is the module and p the circular pitch. The units of module are millimeters; Module is sometimes used with the units of inches. When the diametral pitch, DP.
in conventional metric units.
The distance between the two axis becomes
where a is the axis distance, z1
are the number of components (teeth) for each of the two wheels (gears). These numbers are often chosen among peaks to create an even contact between every cog of both wheels, and in so doing avoid unnecessary wear and damage. An identical gear wear is obtained by approving the number of teeth of the two gears meshing together are relatively key to each other. This occurs when the greatest common divisor (GCD) of each gear tooth count matches 1, e.g. GCD(16,25)=1; If a 1:1 gear ratio is sought after a relatively prime gear may be put in between the two gears; this maintains the 1:1 ratio but reverses the gear direction. A second relatively prime gear could also be inserted to return the original rotational direction while keeping uniform wear with all 4 gears in this case. Mechanical engineers at least in continental Europe use the module instead of circular pitch. The module, just like the circular pitch, can be used for all types of cogs, not just evolving based straight cog.
Cam and follower mechanisms
A cam and a follower is molded by the direct contact of two permanent shaped links. The driving link is called the cam and the link that is driven by the direct contact of the surface of both links is called the follower. The form of the mating surfaces of the cam and follower controls the motion of the mechanism. Generally a cam and follower mechanism's energy is conveyed from cam to follower. The cam shaft is rotated according to the cam profile while the follower moves up and down. Diverse types of eccentric cam followers are also available in which energy is conveyed from the follower to the cam. The main advantage of this type of cam follower mechanism is that the follower moves a tiny bit and helps to rotate the cam 6 times extra circumference span with 70% force.
A linkage is a collection of links connected by joints. The links are the structural elements and the joints let motion. The single most useful example is the planar four-bar linkage. Though, there are many more special linkages:
- Watt's linkage is a four-bar linkage that generates a rough straight line. It was serious to the operation of his design for the steam engine. This linkage acts in vehicle suspensions to prevent side movement of the body comparative to the wheels.
- The achievement of Watt's linkage leads to the design of similar straight-line linkages.
- The Peaucellier linkage produces a true straight-line output from a rotary input.
- The Sarrus linkage is a spatial linkage that generates straight-line movement from a rotary input.
- Jansen’s linkage and the Klann’s linkage are new inventions that deliver stimulating walking actions. They are individually a six-bar and an eight-bar linkage.
A flexure mechanism contains a series of inelastic bodies connected by accommodating elements that is designed to produce a geometrically well-defined motion upon claim of a force.
Have you ever wondered what kind of mechanisms causes the wind shield wiper on the front widow of car to oscillate. The mechanism, shown in Figure, transforms the rotary motion of the motor into an oscillating motion of the windshield wiper.
Fig No.2.10 Windshield wiper
The four bar linkage is the simplest and the most useful mechanism. As we mentioned before, a mechanism composed of rigid bodies and lower pair is called a linkage
. In planar mechanisms, there are two kinds of lower pair’s
prismatic and revolute pairs
The simplest closed loop linkage is the four bar linkage which has four members, 3 moving links, one fixed link and four pin joints. A linkage that has at least one fixed link is known as mechanism. The following example of a four bar linkage was formed in SimDesign
Fig No. 2.11 Four bar linkage in SimDesign
This mechanism has three moving links. Two of the links are restrained to the frame which is not shown in this image. In SimDesign, links can be nailed to the background thereby making them into the frame.
How many DOF
does this mechanism have? If we need it to have just unique, we can enact one constraint on the linkage and it will have a fixed motion. The four bar linkage is the simplest and the most useful mechanism.
A mechanism is composed of inelastic bodies and lower pairs called linkages. In planar mechanisms there are only two kinds of lower pairs
: turning pairs and prismatic pairs.
Functions of Linkages
The purpose of a link mechanism is to produce oscillating, rotating or reciprocating motion from the rotation of a crank or vice versa. Stated more specifically linkages may be used to convert:
- Continuous, with a constant or variable angular velocity ratio. rotation into continuous rotation
- Continuous rotation into oscillation or reciprocation (or the reverse), with a constant or variable velocity ratio.
- Oscillation, reciprocation with a constant or variable velocity ratio.
Linkages have many different functions, which can be classified according on the primary goal of the mechanism:
Four Link Mechanisms
- Function generation: The relative motion in the middle of the links connected to the frame.
- Path generation: The track of a tracer point.
- Motion generation: The motion of the coupler linkage.
One of the simplest examples of a constrained linkage is the four-link mechanism. A diversity of useful mechanisms can be formed from a four-link mechanism through small differences, such as altering the character of the pairs, magnitudes of links, etc. Furthermore, many complex link mechanisms are combinations of two or more such mechanisms. The popular of four-link mechanisms drop into one of the following two classes:
- The four-bar linkage mechanism, and
- The slider-crank mechanism.
Some important concepts in link mechanisms are:
- Crank: An adjacent link which revolves relative to the frame.
- Rocker: Any link which does not revolve.
- Crank-rocker mechanism: In a four bar linkage where shorter side link revolves and the other one oscillates.
- Double-crank mechanism: In a four bar linkage where both of the side links revolve.
- Double-rocker mechanism: In a four bar linkage if both side links rock.
Before categorizing four-bar linkages, we need to present some basic nomenclature.
In a four-bar linkage the line segment between hinges on a given link as a bar where:
- s = length of shortest bar
- l = length of longest bar
- p, q = lengths of intermediate bar
Grashof's theorem states that a four-bar mechanism has at least one revolving link if
s + l <= p + q
and all three mobile links will rock if
s + l > p + q
The inequality 5-1 is Grashof's criterion.
All four-bar mechanisms drop into one of the four categories:
||l + s vers. p + q
|Classification of Four-Bar Mechanisms
|Classification of Four-Bar Mechanisms
We can understand that for a mechanism to have a crank then the sum of the length of its shortest and longest links must be less than or equal to the sum of the lengths of the extra two links. However, this condition is necessary but not sufficient. Mechanisms filling this condition drop into the following three categories:
2.9 DC Motor
- When the smallest link is a side link then the mechanism is a crank-rocker mechanism. The smallest link is the crank in the mechanism.
- When the smallest link is the frame of the mechanism then the mechanism is a double-crank mechanism.
- When the shortest link is the coupler link, the mechanism is a double-rocker mechanism.
DC motors appear quite straightforward. Apply a voltage to each terminals, and it'll spins. DC motors square measure non-polarized which implies that it will reverse voltage that the motor can rotate in 2 directions, forward and backward. Typical DC motors square measure rated from regarding 6V-12V. The larger ones square measure usually 24V or a lot of except for the aim of this project, it's necessary to use 6V-12V vary motor. Voltage is directly associated with motor force.The a lot of voltage provided, the upper the force are going to be manufacture. Qualifications of all DC motors show high rebellions per minute (rpm) and low force. The DC motor is common in an exceedingly range of drive applications owing to its straightforward operation and management. By referring it's a pair of main elementsthat is rotor and mechanical device. Mechanical device is that the half wherever the permanent magnet placed and wont to generate the field and it's static. Rotor is that the motility half within the motor that contains block of core and wire loops. It additionally known as the coil.
Fig No. 2.12 DC Motor
The rotor is placed within the force field caused by 2 permanent magnets. By pertaining to things, each side of the wire loop can have a force on them. attempting to create the wire loop rotate. this is applied to the loop through the commentator, that is shown as 2 items of metal shaped into a hoop within the figure. Current is applied to the commentator by stationary C blocks, known as brushes, that rub against the commentator ring. The loop can still rotate anticlockwise till it's vertical. At now, the stationary brushes can apply current round the loop nomore as a result of they're going to be to bear with the gap between the commentator segments, however the inertia of the loop keeps it going very little additional, till the DC provide reconnects to the commentator segments, and therefore the current then goes round the loop within the wrong way. The force although remains within the same direction and therefore the loop continues to rotate.
Fig No.2.13: The Operation of DC Motor
2.9.1 DC Motor Voltage
DC motors ar non-polarized - which means that one will reverse voltage with none dangerous things happening. Typical DC motors ar rated from concerning 6V-12V. The larger ones ar typically 24V or additional. Except for the needs of this project, do keep within the 6V-12V vary. It’s expressed that voltage is directly associated with motor torsion. High voltage produces higher torsion. A DC motor is rated at the voltage wherever it's most effective at running. If only a few volts ar applied, it simply will not work. If an excessive amount of is applied, can|it'll} overheat and also the coils will soften. Therefore the general rule is to use as near the rated voltage of the motor. However don’t exceed 12V motors unless torsion is crucial badly.
2.9.2 DC Motor Current
As with all electronic equipment, one should listen to current. Too little, and it simply will not work. Too much, the motor can meltdown. Once shopping for a motor, there ar 2 current ratings one ought to listen to. The primary is working current. this is often the typical quantity of current the motor is anticipated to draw beneath a typical force. Multiply this range by the valued voltage and also the average power draw necessary to run the motor is obtained. The opposite current rating that one must listen to is that the stall current. This is often once the motor is power up, and enough force is place to force it to prevent rotating. This is often the utmost quantity of current the motor can ever draw, and therefore the utmost quantity of power too. So, one should style all management electronic equipment capable of handling this stall current. Also, if the motor is continually run, or run it over the rated voltage, it's knowing sink to stay the motor's coils from melting.
2.9.3 DC Motor Power Rating
All motors are rated at definite wattage. Wattage is energy and incompetence of energy conversion is directly connected to heat output. Too much heat, the motor coils melt. So the manufacturers of motors know how much wattage will cause motor failure, and post this on the motor specification sheets. The equation is:
Power (watts) = Voltage * Current
Increase voltage and extent current until the power is about -90% below the given power rating.
2.9.10 DC Motor Torque
Torque is well-defined as that force which inclines to harvest and maintain rotation. The purpose of torque in a DC motor is to provide the mechanical output or effort the piece of apparatus that the DC motor is committed to. There are two torque value ratings which must been pay attention to. The first is the operating torque. This is the torque the motor is calculated to give. Usually it is the listed torque value. The other rated value is stall torque. This is the torque required to stop the motor from rotating. The torque developed by the motor can be determined using Equation:
T = KQI,
T = torque
K = a constant depending on physical size of motor
Q = field flux, number of lines of force per pole
I = armature current
To buy a DC motor, there will be two torque value ratings to which attention must be paid. The first is operating torque. This is the torque the motor is considered to give. Usually it is the listed torque value. The other rated value is stall torque. This is the torque necessary to stop the motor to rotate. If one need a little more speed, going 20% above the rated motor voltage value is fairly safe. But, that this is less efficient, and the motor should be heat- sinked.
Velocity Velocity is very complex when it comes to DC motors. The general rule is motors run the most efficient when run at the highest possible speeds. Obviously however this is not possible. There are times to run the motor slowly. Just like car, it won't to keep the car constantly at high speed. The voltage and applied torque resistance clearly affects speed.
This motor using for this project PMDC motor In a dc motor, an armature rotates inside a magnetic field. The working principle of DC motor is based on the point that whenever a current carrying conductor is placed inside magnetic, there will be mechanical force experienced by that conductor. All kinds of DC work in this principle only. Therefore for constructing a dc motor it is necessary to establish a magnetic field. The magnetic field is obviously established by means of magnet. The magnet can by any types i.e. it may be electromagnet or it may be a permanent magnet. When permanent magnet is used to create magnetic field in a DC motor, the motor is referred as permanent magnet dc motor or PMDC motor. Did you ever uncover any battery operated toy, if you had uncovered, you had obviously found a battery-operated motor inside it. This battery operated motor is nothing but a permanent magnet dc motor or PMDC motor. These types of motors are basically simple in construction. These motors are commonly used as starter motor in automobiles, windshield wipers, washer, for blowers used in heaters and air conditioners, to raise and lower windows, it also extensively used in toys. As the magnetic field strength of a permanent magnet is fixed it cannot be organized externally, field control of this type of dc motor cannot be possible. Thus permanent magnet dc motor is used where there is no need of speed control of motor by means of controlling its field. Minor fractional and substitute fractional kW motors are now constructed with permanent magnet.
Fig No. 2.14 Construction of Permanent Magnet DC Motor or PMDC Motor
As it is specified in name of permanent magnet dc motor, the field poles of this motor are basically made of permanent magnet.
A PMDC motor mainly consists of two parts. A stator and an armature. Here the stator which is a steel cylinder. The magnets are mounted in the inner periphery of this cylinder. The permanent magnets are mounted in a way that the N – pole and S – pole of each magnet are alternatively faced towards armature as shown in the figure below. That means, if N – pole of one magnet is faced towards armature then S – pole of very next magnet is faced towards armature.
In addition to hold the magnet on its inner border, the steel cylindrical stator also serves as low reluctance return path for the magnetic flux. Although field coil is not required in permanent magnet dc motor but still it is sometimes found that they are used along with permanent magnet. Incase permanent magnets lose their strength, these lost magnetic strengths can be recompensed by field excitation through these field coils. Generally, rare earth hard magnetic materials are used for these permanent magnet.
Rotor: The rotor of PMDC motor is similar to other DC motor. The rotor or armature of permanent magnet dc motor also consists of core, windings and commentator. Armature core is made of many varnish insulated, slotted circular laminated steel sheets. By fixing these circular steel sheets one by one, a cylindrical shaped slotted armature core is formed. The laminated steel sheets which are varnish insulated are used to reduce eddy current loss in armature of permanent magnet dc motor. These slots on the outer periphery of the armature core are used for housing armature conductors in them. The armature conductors are connected in a manner that gives rise to armature winding. The end terminals of the winding are connected to the commentator segments placed on the motor shaft. In other dc motor, carbon and graphite brushes are put along with the spring compression on the commentator divisions to supply current to the armature.
Working Principle of Permanent Magnet DC Motor or PMDC Motor
As we said earlier the working principle of PMDC motor is just similar to the general working. That is when a carrying conductor comes inside a magnetic field, a mechanical force will be experienced by the conductor and the direction of this force is governed by Fleming’s left hand rule. Like in a permanent magnet dc motor, the armature is placed inside the magnetic field of permanent magnet; the armature is rotated in the direction of the generated force. Here each conductor of the armature experiences the mechanical force F = B.I.L Newton where B is the magnetic field strength in Tesla (weber / m2
), I is the current in Ampere flowing through that conductor and L is length of the conductor in metre comes under the magnetic field. Each conductor of the armature experiences a force and the compilation of those forces produces a torque, which tends to rotate the armature.
Fig No. 2.15 Equivalent Circuit of Permanent Magnet DC Motor or PMDC Motor
As in PMDC motor the field is produced by permanent magnet, there is no need of drawing field coils in the equivalent circuit of permanent magnet dc motor. The supply voltage to the armature may have armature resistance drop and rest of the supply voltage is opposed by back emf of the motor. Hence voltage equation of the motor is given by,
I, is armature current and R is armature resistance of the motor.
is the back emf and V is the supply voltage.
Benefits of Permanent Magnet DC Motor or PMDC Motor
PMDC motor has some advantages over other types of dc motors. They are:
- No need of field excitation arrangement.
- No input power in consumed for excitation which improves efficiency of dc motor.
- No field coil hence space for field coil is saved which reduces the overall size of the motor.
- Cheaper and economical for fractional kW rated applications.
Disadvantages of Permanent Magnet DC Motor or PMDC Motor
- In this situation, the armature reaction of DC motor may not be compensated so the magnetic strength of the field can become weak due to the demagnetizing outcome of armature reaction.
- There is also a chance of getting the poles permanently demagnetized (partial) due to excessive armature current during starting, reversal and overloading condition of the motor.
- Another key drawback of PMDC motor is that, the field in the air gap is fixed and limited and it cannot be controlled externally. Therefore, very efficient speed control of DC motor in this type of motor is difficult.
Applications of Permanent Magnet DC Motor or PMDC Motor
PMDC motor is extensively used where small dc motors are required and also very effective control is not required, such as in automobiles starter, toys, wipers, washers, hot blowers, air conditioners, computer disc drives and in many more.
Arc welding is a process which is used to join metal and metal by using electricity to create adequate heat to melt metal, and the melted metals when cool result in a binding of the metals. It is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can be used as either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The welding region is usually protected by some type of shielding gas, vapor, or slag. Arc welding processes can be manual, semi-automatic, or fully automated. First developed in the late part of the 19th century, arc welding became commercially important in shipbuilding during the Second World War. Today it remains as an vital process for the assembly of steel structures and vehicles.
To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common grouping is constant current power supplies and constant voltage power supplies. In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input. Constant current supply is broadly used in manual welding processes like gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supply keep the voltage constant and vary the current, and as a result, are mostly used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In this process arc length is kept constant, so that any fluctuation in the distance between the wire and the base material is quickly resolved by a huge change in current. In case, if the wire and the base material get very close, the current rapidly increases and in turn causes the heat to increase and the tip of the wire melts and returns to its original separation distance.
The direction of current used in arc welding plays a significant role in welding. Consumable electrode processes like shielded metal arc welding and gas metal arc welding regularly use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode has a larger heat concentration (around 60%) and, as a result, changing the polarity of the electrode affects weld properties. If the electrode is positively charged, it melts quickly, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in thin welds. Non-consumable electrode processes, like gas tungsten arc welding, can use any type of direct current (DC), as well as alternating current (AC). However with direct current the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds. Alternating current rapidly moves between these two, which results in medium-penetration welds. One disadvantage of AC, a fact that the arc requirement be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave method instead of the normal sine wave, removing low-voltage time after the zero crossings and minimizing the effects of the problem.
Fig No. 2.16 Arc welding
Duty cycle is a welding apparatus specification which explains the number of minutes, within a 10-minute period, during which a given arc welder can safely be used. For example, an 80A welder with a 60% duty cycle must be "rested" for at least 4 minutes after 6 minutes of nonstop welding. Failure to spot duty cycle confines could damage the welder. Commercial and professional-grade welders normally have a 100% duty cycle.
DESIGN AND SELECTION OF PARTS
The group reviewed through the literature to gain insight into the motion of flapping wings and how they generate lift. Previous projects involving flapping wing MAVs inspired the group.
The modeling process began with a basic idea of what the group was hoping to achieve with this project – a relatively small flapping wing vehicle capable of vertical takeoff and landing, hover, and horizontal flight. Through research, that it was decided that mimicking a two-wing bird or insect would be rather difficult and unreasonable, for these animals move in a very complex manner. Through more research and the discovery of the clap-and-fling method of flight, the group decided upon a two-wing. This is easier to model and manufacture compared to the motion required for a two-wing vehicle to take flight.
As the group continued to model the wings, the next step was to choose which components and materials would be used in the design. In the development of the body, and wings, the group researched materials that were lightweight, yet durable enough to withstand impact and rapid movement. The group also researched materials that could be manufactured to conserve the financial budget. Also involved in the design are several electronic components: a battery and a motor. These components must power the flapping the wings. Detailed descriptions of these components and their purpose, as well as why the group chose each specific item, will be included in this section.
Fig No.3.1 Isometric View of the Model
The group finally constricted the field to two different materials. The first wood and the second iron rods. The wood was used but it failed to maintain its shape due to the vibrations. Thus wood failed to maintain its perfect frame dimensions when drivetrain is fixed. Then mild steel plates are used to make the frame which maintained its shape. The shape of the frame was designed to be simple. There is another section designed to mount the drive train and hinge. The frame is made of mild steel bars. The frame shape was finalized for ease of manufacturing and for simplicity as well. The finalized drive train was used to design the front part of the frame and a very basic final design was chosen.
For a design involving two wings, it is crucial for each wing to mimic the motion of bird. For this purpose, the group decided to research thin plastic films to comprise the wing, and iron rod spars to provide support and allow for the clap and fling effect.
While researching different wing shapes, the grouptried to research different wings of various birds and tried to understand its movement and concluded that spars to bend closer to the leading edge and preventing the behind corners of the wings from being unsteady during flapping.
To finalize the size of the wings, the group used a lift equation found through texts. This equation is an assessed equation derived through testing of rectangular flapping wings.
After finalizing the size and shape of the wings, the group researched materials to form the spars and membrane. These materials need not to be as light in weight but strong enough to flap without failing.
Taking these demanding applications into account, the group researched common durable materials such as iron rods, wood and plastic. All three materials would be strong enough to support the wings, but the group chose iron rods, the strongest of the three. Iron rods can be cut and welded easily whereas the wood was not strong enough as the cracks appear due to machining operations. Iron rods are comparatively higher in price; however, after reviewing the budget, the group decided that it would be worth it is for its strength and reliability. Once we selected iron rods as the spar material, it was time to focus on a membrane that could be effectively attached to it. And the group decided to use plastic boards as the membrane.
A hinge is necessary to hold the two 0ings together and allow a flapping motion that brings the leading edges together. The group selected mild steel bars as a material. This would withstand the rapid movement without failure.
This simpler hinge was attached to the frame by welding.
3.4 Drive Train
The motion of the hinge will be created by a drive train connected to the motor. The group considered two different drive train options. The first contained gears that wouldspin parallel to the axis of forward movement (with their axels perpendicular). Although this design was successfullyemployed onto previous MAVs, it was dismissed due its complicated drive train and the effect that gyroscopic forces would have on flight.
Instead, the group chose a method involving two counter-rotating gears, spinning perpendicular to the forward movement of the MAV (with their axels parallel). Mechanically, this design is more simple and easier to construct, and due to the counter-rotation, the gyroscopic forces would be minimized.
From our motor tests we discovered that a drive train ratio of 1:1 would be desirable, and we ordered gears to match this design. This would give us an acceptable drive train assembly, and this drive train can be seen in Figure. This drive train is simplistic and secure, so we are anticipating minimal issues with its assembly.
For the design of the drive bar and some other design parameters, we used SolidWorks to create and test the kinematics of the drive train. This process can be seen in Figure
Fig No.3.2 Finalized drive train
Fig No. 3.3 Real model (Drive Train)
The group has tried to consider the lithium ion battery but due to the budget in consideration lead acid battery is used to power the motor. Specifications of battery are 12V and 1.3Ah.It is a rechargeable sealed lead acid battery.
Fig No.3.4 Lead acid battery used
The group researched different types of motors and decided that a DC motor would be most appropriate for this project. 60RPM Centre Shaft Economy Series DC Motor is of high quality and low cost DC geared motor. It has steel gears and pinions to ensure longer life and better wear and tear properties. The gears are fixed on toughened steel shafts polished to a mirror finish. The output shaft rotates in a plastic bushing. The whole assembly is covered with a plastic ring. Gearbox is sealed and oiled with lithium grease and need no maintenance. The motor is screwed to the gear box from inside. Although motor gives 60 RPM at 12V but motor runs smoothly from 4V to 12V and gives wide range of RPM, and torque. Tables below gives fairly good idea of the motor’s performance in terms of RPM and no load current as a function of voltage and stall torque, stall current as a function of voltage.
Fig No.3.5 DC motor
4.1 Wing Spars
The wing spars mounts are cut using the cutting tool in workshops. Then the mounts are welded to the frame. Wing spars of desirable lengths are cut using cutting wheel. The ends of the rods are welded with nuts on both ends. This is done for easy attachment of consecutive rods using screws.
Fig No. 4.1 Wing spars
The body was designed so that it could be easily manufactured. . A sturdy design for the body was achieved to create a very solid base. Mild steel plates are used as the frame structure. The plates are welded together by arc welding process. The base of the frame is H shape in structure.
4.3 Arc welding
The group used arc welding to weld mild steel plates for the frame. The nuts are also fixed on the both the ends of the iron rods by welding process.
4.4 Final Assembly
After manufacturing the individual parts, assembly was rather straightforward. The hinge, gears, drive bars could all be fastened to the body with small metal brads. Bolts are very effective in attaching the spars to the hinge. A photograph of this assembly is shown below in Figure 4.2.
Fig No.4.2 Photograph of the complete assembly.
In this project, we achieved control both wing mechanisms. The main task of this project make a surveillance robot. It gives adaptable operation of robot controller which does not modify the hardware. Further development of this system depends on the application we are using an area of work. The system can be added features like gas sensor, thermal image sensing, wireless control, connecting robotic arms and can be used in pick and place purposes etc… can be done. The development of this system has wide area of applications such as in Military and Law enforcement and Industrial and in Disaster management and so on. The design process detailed by this report began with an examination of the project objective. This is to build a flapping-wing. After a thorough literary review, it was decided that the wing movements of most birds and insects is very complex with twists and other motions that are not yet fully understood by experts. Because of this a two-wing design, utilizing the clap-and-fling effect, was selected. This allows for the design of a vehicle capable of flight with a simplified wing motion. These wings are attached to a symmetric hinge that is powered by a motor. A battery powers the motor, allowing for free flight with an onboard power source.
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