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7/30/2019 Industrial Robotics - Manipulator Elements
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Manipulator
Elements
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Manipulator Elementso Manipulator
oActuatorso Sensors
o End Effectors
o Controller
o Software
o Power supply
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Actuatoro An actuator is a type of motor formoving or controlling a
mechanism or system.
o It is operated by a source of energy.
o It converts energy into some useful mechanical work.
o There are three kinds of energy sources
1. Electric Energy
2. Pneumatic Energy
3. Hydraulic Energy
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Electric Actuatorso Electric actuators are powered by electric energy.
o Mechanical output can be either rotational or linear
o Motors provide rotational motion
o Electromagnets and motor-gear assemblies provide linearmotion.
ElectricalEnergy
ActuatorMechanical
Work
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Electric Actuator Types
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DC Motorso As the name suggests, a DC
motor uses DC power to
operate.o Can run in both directions
o Controlling speed Is easy.
o Advantages
Cheap Easy to control
Motors exist in a widepower range (1W
100kW) High torque at zero
speed.
Can achieve higherspeeds
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DC Motorso Disadvantages
Brushes wear.
Limited overloading onhigher speeds.
Higher maintenance.
Expensive compared toAC motors
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DC Motors Componentso Stator : Stator is a
stationary winding or a
permanent magnet.
o Permanent magnets arefound in low powerapplications where as high
power applications useswound stators.
o Armature : Armature is a
rotational winding.
o The rotating shaft of themotor is connected to thearmature
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DC Motors Components
o Commutator : Rotationalelectrical switch
o Windings of the armatureis connected to thecommutator.
o Brushes : Carbon/softmetallic contacts whichcompletes the circuit
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Types of DC motorso Permanent Magnet DC Motors
o Wound Rotor DC Motors
Shunt DC Motors
Series DC Motors
Compound DC Motors
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DC Motors Operation
A simple DC electricmotor. When the coil ispowered, a magnetic fieldis generated around the
armature. The left side ofthe armature is pushedaway from the left magnetand drawn toward theright, causing rotation.
The armature continues torotate due to its inertia.
When the armaturebecomes horizontallyaligned, the commutatorreverses the direction of
current through the coil,reversing the magneticfield.
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DC Motor Characteristicso Torque of a motor is produced by the principle of Lorentz
Force.
o Lorentz force is the force exerted on a current carryingconductor placed within an external magnetic field
o Force exerted on the conductor is given by
o Direction of the force can be found
using the Flemings Left Hand Rule.
o In a permanent magnet DC motor the torque isproportional to the armature current. And back emf(ElectroMotive Force)
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DC Motor CharacteristicsGoverning Equations of DC motor
o Electrical torque e
o Back EMF Eb
o DC motor equivalent circuit
o DC motor characteristic equation
+
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DC Motor Characteristics
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H-bridgeo H-bridge is a current
amplifier.
o PWM controller
o Advantages
Easy to control
Higher efficiency Small component
Can drive DC motors inboth directions
Can be driven with amicrocontroller
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H-bridge Operationo The direction of the motor can be changed without changing
the supply polarity
o
PWM signal form the microcontroller is applied to the relevantgates of the H-bridge
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DC Motor Controlo PWM (Pulse Width Modulation)
Duration of the on time in a pulse is varied.
!"#
!$%$&' (
D = duty cycle =!"#
!$%$&'
0 * ( * 1
Voltage supplied to the motor can be changed bychanging the duty cycle of the pulse.
Microcontroller or a microprocessor is used to change the
duty cycle of the pulse
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DC Motor Controlo In a permanent magnet
DC motor armature
resistance and fieldstrength is constant.
o Since
o
According to this equationspeed varies linearly withthe supply voltage.
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DC Motor Control
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Stepper (Reluctance) Motoro Stepper motor is a
brushless DC motor.
o Full rotation is divided in toa number of equal steps
o Can be positioned withoutany feedback (open-loop
control)
o Armature is a roundtoothed permanentmagnet disk.
o Stator is a wire woundstator with several numberof poles.
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Stepper (Reluctance) Motoro Applications
Precise positioning systems
Open loop positioning
systems Floppy disk drives
Computer printers
3D plotters (RepRap)
o Advantages Can have precision control
Open loop controller
o Disadvantages
Jerky movements
Higher power consumption
Higher cost
Low torque to weight ratio
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Stepper Motor Operationo Stepper motors effectively have multiple "toothed"
electromagnets arranged around a central gear-shaped
piece of iron/magnet.o A pair of electro magnets is energized by an external control
circuit.
o This makes the one tooth of the armature toothed wheelattracted to the electromagnets teeth.
o When one pair of teeth of the armature isaligned to the stator energized pair of teeth,the other teeth of the armature are slightlyoffset with the next electromagnet.
o When the next electromagnet is turned onand the first is turned off, the gear rotatesslightly to align with the next one, and fromthere the process is repeated.
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Stepper Motor Operationo Frame 1: The top electromagnet (1) isturned on, attracting the nearest teethof the gear-shaped iron rotor. With theteeth aligned to electromagnet 1, they
will be slightly offset from rightelectromagnet (2).
o Frame 2: The top electromagnet (1) isturned off, and the right electromagnet(2) is energized, pulling the teeth into
alignment with it. This results in a rotationof 3.6 in this example.
o Frame 3: The bottom electromagnet (3)is energized; another 3.6 rotationoccurs.
o Frame 4: The left electromagnet (4) isenergized, rotating again by 3.6. Whenthe top electromagnet (1) is againenabled, the rotor will have rotated byone tooth position; since there are 25teeth, it will take 100 steps to make a full
rotation in this example.
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Stepper Motor Typeso There are two basic winding
arrangements for theelectromagnetic coils stepper motors
Unipolar MotorsA unipolar stepper motor has one winding
with center tap per phase. Each section of
windings is switched on for each direction
of magnetic field. Since in this
arrangement a magnetic pole can bereversed without switching the direction of
current.
Bipolar MotorsBipolar motors have a single winding
per phase. The current in a winding
needs to be reversed in order to reverse
a magnetic pole.
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Stepper Motor Controlo Driving a stepper motor requires
a 4 step switching sequence for
full-step mode
o Stepper motors can also bedriven in 8 step switchingsequence for half-step mode
(higher resolution)
o A new stepping mechanism iscalled micro-stepping whichresults in higher resolution butneed a special drive.
o Step sequence can be very fast,the resulting motion appears to
be very smooth
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Stepper Motor ControlFull Step Mode
o There are two modes ofexcitation
Single phase full stepexcitation
Dual phase full stepexcitation
o In single phase excitationonly one phase is excited.
o In dual phase excitation twophases are excited.
o Dual phase excitationproduce 40% more torque
and need higher power.
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Stepper Motor ControlHalf Step Mode
o First one phase is excitedo Then the excited phase and
the next phase are bothexcited.
o Finally the previous phase isde-energized.
o This sequence continues.
o This produce higher
resolution.
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Brushless DC Motors(Permanent Magnet Synchronous Motors)
o Construction of BLDC motor is similar toan AC induction motor.
o The stator is made out of laminatedsteel stacked up to carry the windings.
o There are three phases wound in thestator and connected in delta pattern
and star pattern.o The armature is made of permanent
magnets and there can be 4-poles, 8-poles or more.
o
The underlying principles for theworking of a BLDC motor are the sameas for a brushed DC motor.
o The feedback is taken from Hall sensorsor optical encoders.
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Brushless DC Motors(Permanent Magnet Synchronous Motors)o Advantages
o No brushes and therefore no wearing parts
o Low inertia
o Can achieve higher speeds and higher speed range
o Excellent power to weight ratio
o Simple constructiono Virtually maintenance free
o Excellent thermal characteristics
o Disadvantages
o More complicated controller (ESC)
o Needs both speed and angle feedback
o For small to medium power (10W - 50kW)
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Asynchronous AC motor(Induction Motor)o Stator is a wire wound stator. The stator can have
two poles, four poles or higher number of poles.
o Two types of rotors available
Wire wound rotors
Squirrel cage rotor
o
AC power is supplied to the stator.Current in the stator produce arotating magnetic field in thestator.
o The rotating magnetic fieldinduces current in the rotor.
o Mechanical force is exertedon the current carryingconductor in the magnetic field.
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Asynchronous AC motor(Induction Motor)
o Advantages
No brushes hence no wearing parts
Virtually no maintenance
Constant torque over wide range of speeds
o Disadvantages
Speed control is difficult
Need complex controllers