CONTINOUS-DRIVE ACTUATORS

1
CONTINOUS-DRIVE ACTUATORS

• EEEN 5324
• System Synthesis
• Dr. Rajab Challoo
• Mid Term- Paper
  Presentation
• Group 1
• Date April 03, 2006
• 
  
• 
  BY
• 
  Falguni Gandhi
• Feroza Salam
• Deepa Deshmukh
• Preethi Krishnan

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INTRODUCTION

• Digital actuators causes the actuators to move by
  a predetermined, fixed increment of displacement.
• Direct current servomotors and servovalve driven
  hydraulic and pneumatic actuators were the most
  widely used .
• Presently AC motors are widely used as
  servomotors.

3

Contd..

• Pulse Width modulated signals (PWM)
• signals are increasingly being used to drive
  continuous (analog) actuators.
• Proper design and selection of actuator have a
  significant economical impact in many
  applications of industrial control.

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DC MOTORS

• DC motor converts direct current into rotational
  mechanical energy.
• Magnetic force,
• F bil
• Electromotive Force or EMF,
• Vb blv

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Principle of DC Motors
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Brushless DC motors

• The main disadvantages of slip ring and brush
  mechanism were rapid wear out, mechanical
  loading, excessive noise, contact bounce and
  electrical arcing.
• Brushless DC motors were used to overcome the
  shortcomings of slip ring and brush mechanism.

7

Contd..

• In brushless DC motor , the polarity of the rotor
  (permanent magnet )is fixed relative to the rotor
  itself, and the polarity of the stator is
  switched by electronics means.

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Brushless DC Motors
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Static Torque Curve With No Switching
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Switching Sequence For Maximum Average Torque
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Control of DC motors

• DC motors are operated over a wide range of
  speeds and torques.
• Control of a DC motor is achieved by either
  controlling the stator field flux or the armature
  flux.
• There are two methods of control
• Armature Control
• Field Control

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Contd..

• In armature control the field current in the
  stator circuit is kept constant and the input
  voltage is varied to achieve the desired
  performance.
• In field control the armature voltage is kept
  constant and the input voltage to the field
  current is varied.

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DC Servomotors
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Phase-Locked Control

• Phase locked loop is a modern approach to
  controlling DC motors.
• The position command is generated to the desired
  motion of the motor using a control signal
  generator.

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Block Diagram
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Thyristor Control

• A solid state switch frequently used in motor
  control circuits is the thyristor.
• The thyristor is also known as semi conductor
  controlled rectifier (SCR).
• It is made of four layers of semiconductor
  material and has three terminals-anode, cathode,
  and the gate.

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Basic Thyristor Circuit
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Servo Amplifiers

• It is used to drive DC motors in servo control
  applications.
• Servo amplifiers use PWM to drive servomotors
  efficiently under variable speed conditions, with
  less power losses.
• A typical servo amplifier card consists of a
  differential amplifier, servo preamplifier, and
  PWM amplifier.

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Torque Motors

• Torque motors are high torque DC motors.
• These actuators posses a linear torque speed
  relationship.
• The output torque can be increased by using a
  gear system with high gear reduction.
• Torque motors are suitable for high precision
  applications.

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Motor Selection Considerations

• To select a particular motor the two primary
  considerations are torque and speed.
• The specifications that are considered are the
  Mechanical, electrical and general
  specifications.

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Contd

• The operating conditions of a motor can be
  determined by using its torque-speed curve.

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INDUCTION MOTORS
ADVANTAGES 1.Convienient power supply. 2.No commutator and brush mechanism. 3.Constant speed operation. 4.No electric arching. DISADVANTAGES 1.Lower starting torque. 2.Auxillary sharing device required for few motors. 3.Difficulty of varied speed control.
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TORQUE SPEED CHARCTERISTICS
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INDUCTION MOTOR CONTROL

• 1.EXCITATION FREQUENCY CONTROL
• 2.SUPPLY VOLTAGE CONTROL.
• 3.ROTOR RESISTANCE CONTROL.
• 4.POLE CHANGING.

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EXCITATION FREQUENCY CONTROL
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ROTOR RESISTANCE CONTROL
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SUPPLY VOLTAGE CONTROL
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POLE CHANGING METHOD
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SYNCHRONOUS MOTORS

• The rotor of the synchronous AC motor rotates in
  synchronism of the rotating field generated by
  stator windings.
• These motors are particularly suited for constant
  speed applications under variable load
  conditions.
• The drawback of this motor is that an auxiliary
  starter is required to bring the rotor speed
  close to synchronous speed.
• The staring torque is always zero.

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RECTILINEAR MOTORS.

• Flat stators and rectilinearity elements are
  employed.
• These motors employ auxiliary kinematic
  transducer,such as cam follower mechanism ,a
  rack-pinion mechanism .
• Solenoid is a common rectilinear actuator that
  consists of a coil and a plunger.These are
  push-pull actuators and can be classified as
  digital actuators
• Common Applications include valve
  actuators,switches ,relays and other two position
  systems.

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Hydraulic System

• Components of Hydraulic system
• Hydraulic actuator
• Hydraulic pumps and motors
• Hydraulic servovalves
• Hydraulic accumulators

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Hydraulic control system

Power supply
(T, w)
Fluid return
Reference i/p
Pressurized fluid supply
Torque motor
Feedback signals
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Hydraulic Actuators

• Hydraulic Actuators uses the hydraulic power of a
  pressurized liquid
• They are capable of providing large forces at
  very high power levels
• Control gains required in a high power hydraulic
  control system is less than the comparable
  electromagnetic control system

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Hydraulic pumps

• Hydraulic pump is used to provide pressurized
  liquid to actuators
• The Vane pump
• The gear pump
• The axial piston pump

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Contd
Vane Pump
Hydraulic pump efficiency ?p PQ/?T
where P pressure increase in the fluid
Q fluid flow rate
? rotating speed of the pump T
drive torque of the pump
Gear Pump
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Hydraulic servovalves

• Valves can be categorized as
• Flow control valves
• Pressure control valves
• Functions of valves are as follows
• Change flow direction
• Change flow rate
• Change fluid pressure

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Valve control systems

• Constant pressure systems
• The supply pressure to servovalve is maintained
  constant
• Constant flow systems
• The supply flow is kept constant at varying
  pressure levels

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Hydraulic control system

• Block diagram

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Hydraulic accumulators

• An accumulator is a tank to hold excessive fluid
  during pressure surges so that pressure
  fluctuations can be filtered

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Conclusion

• DC motors
• AC motors
• Hydraulic systems