CONTINUOUS-DRIVE ACTUATORS** By: Bharath reddy manyam Naresh reddy guntaka Prabhu sampathkumar ** An actuator is a mechanical device for moving or controlling a mechanism or system. The actuator is usually a physical mechanism but also refers to an artificial intelligent agent.

What's Inside DC Motors Control of DC motors Motor Selection Criteria Induction motors Induction motor control Synchronous/Hydraulic motors Hydraulic Actuators Hydraulic Control Systems

DC MOTORS: WHAT IT DOES : Converts electrical energy (DC) into rotational mechanical energy. HOW? Fig 2: Operating principle of DC motor Fig 1: cross-section of a DC motor

Force F = B i l (B- flux density of the field DC MOTORS… Force F = B i l (B- flux density of the field i- current thru the conductor l- length of the conductor ) Back e.m.f vb =B l v (v- velocity) BACK EMF : If the conductor is free to move, the force will move it at some velocity ‘v’ in the direction of the force. As a result of this motion in the magnetic field B, a Voltage is induced. This is the ‘back’ electromagnetic force. Fig 3 : Physical Configuration of rotor

Brushless DC Motors: DC motors: Rapid wear-out Mechanical loading Heating due to brush friction Contact bounce Excessive noise Voltage ripples Mostly permanent magnets Better stator winding segments Polarity of stator is switched by electronic means Remedies : Improved brush designs Modified brush positions Fig 4: brushless DC motor system.

Fig 5a : switching seq. for max.avg. torque Fig 5b: static torque without switching

DC Motor Equations: Where va =supply voltage to armature Ra =resistance of winding La =leakage inductance in winding K and k’ =motor constants Comparison of DC motor winding types & equations:

Control of DC Motors : Armature control : Here armature voltage is used as the control input, while keeping the conditions in the field circuit constant Field control : here armature current is kept constant and the filed voltage is used as the control input. Phase- locked control: This is a modern approach using a controlled signal generator and phase is locked. Thyristor control: Here a variable resistor is connected in series with the supply source to the circuit. Fig6: Steady-state speed-torque curves for armature controlled DC motor.

MOTOR SELECTION CONSIDERATIONS : MECHANICAL CONSIDERATIONS: Mechanical time constant No-Load speed Speed at rated load No-Load acceleration Rated Torque Rated output power Frictional torque Damping Constant Dimensions and weight Armature moment of inertia

GENERAL APECIFICATIONS: ELECTRICAL SPECIFICATIONS: Electrical time constant Input power Armature resistance & inductance Field resistance & inductance Compatible drive specifications GENERAL APECIFICATIONS: Brush life and motor life Efficiency Operating temperature & environmental conditions Heat transfer characteristics Mounting configuration Coupling methods

INDUCTION MOTORS : Advantages of AC motors: Cost effectiveness Convenient power source (AC supply) No commutator/ brush mechanism needed Lower power dissipation, rotor inertia and light weight designs Virtually no electric arcing Less hazardous Constant speed operation without servo control No drift problems High reliability

DISADVANTAGES ARE: Lower starting torque AC MOTORS Contd… DISADVANTAGES ARE: Lower starting torque Auxiliary starting device need for some motors Difficulty of variable-speed control

INDUCTION MOTOR Principle Torque speed characteristics Speed control of Induction motors

An induction motor operates on principle of induction. Rotor receives power from stator due to induction rather than direct conduction of electric power.

Concept of Rotating magnetic field As per Rotating Magnetic Field Theory When windings are physically displaced by120 degree spatially and excited by currents which are displaced by 120 degrees with respect to time, it produces a magnetic field which is rotating.

In Induction motor, stator consists of windings which are physically displaced by 120 degree and excited by AC supply Fig 7

SPEED CONTROL OF INDUCTION MOTORS Excitation frequency control Supply voltage control Rotor resistance control Pole Changing

The torque developed by induction motor is given by Td = nV12R2 / S αs[(R1+R2/S)2+(X1+X2)2 Where V1 is supply voltage S is slip R1 and R2 stator and rotor resistance respectively X1 and X2 stator and rotor inductance respectively αs synchronous speed

Torque-speed characteristic curve: The induced voltage changes linearly with slip ‘S’ because it is proportional to the relative velocity of the rotating field with respect to the rotor Fig 8

Rotor Resistance Control It is a old technique prior to development of thyristor circuits and Digital signal processing chips. As torque of induction motor depends on rotor resistance, by changing rotor resistance speed of induction motor can be controlled. Fig 9

Supply voltage control As we know Torque is proportional to square of supply voltage. Induction motor can be controlled by varying supply voltage. Most common method used is Amplitude modulation of AC supply using ramp generator Appropriate for small induction motors Poor efficiency for wide speed range

Pole changing method The number of pole pairs per phase in stator windings (n) is the parameter in speed torque equation. It can be obtained by switching the supply connections in the stator winding. Fig 10

Excitation frequency control Frequency control can be accomplished using thyristor circuit. The frequency of voltage is equal to inverse of firing interval of the two thyristors. Fig 11

Fig 12

Three phase supply is rectified using Rectifier circuit. Firing of thyristors is controlled by microprocessor. Controller uses external command and feedback signals to generate required firing logic.

INDUCTION MOTOR CONTROL : Excitation frequency control Wp Supply voltage control (Vf) Rotor resistance control (Rr) Pole Changing (n) Field Feedback control

Synchronous Motors : Phase-locked servos and stepper motors can be considered synchronous motors because they run in synchronism with an external command signal under normal operating conditions. Fig 13 The rotor of the synchronous AC motor rotates in synchronism with a rotating field generated by the stator windings. These motors are widely used for constant speed applications ADVANTAGE: It can operate with larger air-gap between rotor and stator in comparison with induction motor DRAWBACK : A major drawback is that an auxiliary started is required is used to bring the rotor speed close to the synchronous speed Schematic diagram of stator-rotor configuration of synchronous motor.

HYDRAULIC CONTROL SYSTEM: Components of hydraulic control system Fig 14 A SERVOVALVE A HYDRAULIC ACTUATOR A LOAD

EQUATIONS VALVE: q = kqu-kcp HYDRAULIC ACTUATOR: q = A (dy/dt) + (V/2B)(dp/dt) LOAD : m(d2y/dt2) + b(dy/dt) = Ap - fl FEEDBACK CONTROL: A pressure feedback path and an associated hydraulic time constant Th A velocity feedback path and an associated mechanical time constant Tm Methods of feedback control Proportional control (P) Derivative control (D) Integral control (i)

References: Control sensors and actuators by Clarence W. De Silva WWW.Wikipedia.com www.google.com

Thank you Any Questions

QUESTIONS FOR ASSIGNMENT DEFINE BACK EMF, BRING OUT THE DIFFERENCES BETWEEN DC MOTOR AND BRUDHLESS DC MOTOR DISCUSS VARIOUS SPPED CONTROL METHOS OF INDUCTION MOTR DRAW THE SCHEMATIC DIAGRAM OF HRDRAULIC CONTROL SYSTEM ALSO BRIEFLY EXPLAIN ABOUT FEEDBACK CONTROL OF IT