Fatahul Arifin Student Bridging Course National Kaohsiung University Of Applied Science Year 2012/2013
7.6 Design a pneumatic valve circuit to give sequ equence A+, followed by B+ and then simu simult ltan aneou eousl sly y follo followe wed d by A- and and B-. B-. Answ Answer er : To solve this problem problem we can use the diagram movement movement of cylinder is like table below: below:
Cylinder A
+ -
Cylinder B
+ -
After this table of movement we can draw draw the figure below. below.
The equipments equi pments are are need needed ed:: 1. 1 unit com compress essor 2. 1 unit air-ser -serv vice 3. 2 unit unit doub ouble-a le-act ctin ingg-cy cyli lind nder( er( A and and B) Homework Chapter 7,8,9
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4. 2 unit 5/2-way-control-valvewith pneumatic actuated 5. 4 unit 3/2-way-control-valvewith roller actuated
8.9 Design a mechanical system which can be used to: a. Operate a sequence of micro switches in a timed sequence. b. Move a tool at a steady rate in one direction and then quickly move it back to the beginning of the path. c. Transform a rotation into a linear back-and-forth movement with simple harmonic motion. d. Transform a rotation through some angle into a linear displacement. e. Transform a rotation of a shaft into rotation of another, parallel shaft some distance away. f.
Transform a rotation of one shaft into rotation of another close shaft which is at right angles to it.
Anwer a. Operate a sequence of micro switches in a timed sequence. The mechanical design for to do this work is camshaft. Camshaft is a shaft that a cam is fixed firmly, or a cam forms an integral part.
b. Move a tool at a steady rate in one direction and then quickly move it back to the beginning of the path. A quick return mechanism such as the one seen below is used where there is a need to convert rotary motion into reciprocating motion. As the disc rotates the black slide moves forwards and backwards. Many machines have this type of mechanism and in the school workshop the best example is the shaping machine.
Homework Chapter 7,8,9
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c. Transform a rotation into a linear back-and-forth movement with simple harmonic motion. The equipment is an eccentric cam to do this work. It is perfectly circular, but the rotating shaft is off-centre, which affects how it turns. This type of cam produces a smooth, symmetrical rise and fall motion in the follower, which never pauses to dwell.
d. Transform a rotation through some angle into a linear displacement. Rack and pinion is a kind of linear actuator that comprises a pair of gears, which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby translating the rotational motion of the pinion into the linear motion of the rack.
e. Transform a rotation of a shaft into rotation of another, parallel shaft some distance away. Belt is a loop of flexible material used to link two or more rotating shaft mechanically. Belts may be used as a source of motion, to spread power efficiently, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the same direction, or the belt can be crossed, so the movement of the shafts is reverse .
Homework Chapter 7,8,9
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f.
Transform a rotation of one shaft into rotation of another close shaft which is at right angles to it. Bevel gears are used to do this work. Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The pitch surface of bevel gears is a cone.
9.4 Suggest possible motors, d.c. or a.c., which can be considered for applications where (a) cheap, constant torque operation is required, (b) high controlled speeds are required, (c) low speed are required, (d) maintenance requirements have to be minimised. Answer: a. D.C. shunt wound A shunt wound motor has a high-resistance field winding connected in parallel with the armature. It responds to increased load by trying to maintain its speed and this leads to an increase in armature current. This makes it unsuitable for widely-varying loads, which may lead to overheating.
b. Induction motor with inverter An induction motor is an asynchronous AC motor where power is transferred to the rotor by electromagnetic induction, much like transformer action. An induction motor resembles a rotating transformer, because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side.
c. d.c. motor Homework Chapter 7,8,9
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A
DC motor is
a mechanically commutated electric motor powered from direct current (DC).
The stator is stationary in space by definition and therefore so is its current. The current in the rotor is switched by the commutator to also be stationary in space. This is how the relative angle between the stator and rotor magnetic flux is maintained near 90 degrees, which generates the maximum torque.
d. a.c. motor An AC motor is an electric motor driven by an alternating current (AC). It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.
9.5 Explain the principle of the brushless d.c. permanent magnet motor Answer : The brushless DC motor is the combination of a permanent excited synchronous motor and a frequency inverter. The inverter has to replace the commutator of a conventional DC motor. Its armature winding corresponds to a three phase winding in delta connection. The commutator acts like a three phase frequency converter. Stator (excitation) and rotor (armature) change places. The commutation of a brushless DC motor depends on the position of the rotor. The angle o
between the magneto-motive forces of stator and rotor is fixed to 90 (el.), so the motor produces maximum torque and needs low reactive current - it might be useful to advance commutation by few degrees to compensate the effects of the stray inductance and minimize reactive current. Speed can only be controlled by the the motor voltage. The motor behaves like a DC motor. Unlike the synchronous motor there are no problems with instability at any speed. Because of the PWM frequency inverter, variation of the motor voltage can be achieved easily by changing the duty cycle of the pulse width modulation. Suitable PWM techniques allow regenerative breaking, which increases dynamic and efficiency of the drive.
9.6 Explain the principles of operation of the variable reluctance stepper motor. Answer: Variable reluctance stepper is kind of the stepper motor. With this form the rotor is made of soft steel and is cylindrical with four poles, i.e. fewer poles than on the stator. When an opposite pair of windings has current switched to them, a magnetic field is produced with lines of force which pass from the stator poles through the nearest set of poles on the rotor. Since lines of force can be considered to be rather like elastic thread and always trying to shorten themselves, the rotor will move until the rotor and stator poles line up. This is termed the position of minimum reluctance. o
o
This form of stepper generally gives step angles of 7.5 or 15 . Homework Chapter 7,8,9
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