industrial robotics - manipulator elements

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

industrial robotics - manipulator elements

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