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Motor Control Relay, Pwm, DC and Stepper Motors

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In this presentation, a brief introduction of relay, optoisolaters, interfacing and working of stepper motor and DC motor is given.
The contents are referred from the book of mazidi.

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Motor Control Relay, Pwm, DC and Stepper Motors

  1. 1. 1 The 8051 Microcontroller and Embedded Systems CHAPTER 16 Motor Control: Relay, PWM, DC and Stepper Motors
  2. 2. RELAYS AND OPTOISOLATORS  A relay is an electrically controllable switch widely used in industrial controls, automobiles, and appliances  It allows the isolation of two separate sections of a system with two different voltage sources  For example, a +5V system can be isolated from a 120V system by placing a relay between them 2
  3. 3.  One such relay is called an electromechanical (or electromagnetic) relay  The EMRs have three components: the coil, spring, and contacts  When current flows through the coil, a magnetic field is created around the coil (the coil is energized), which causes the armature to be attracted to the coil 3 RELAYS AND OPTOISOLATORS
  4. 4. Electromechanical Relay Symbols 4
  5. 5. Criteria for Choosing a Relay  The contacts can be normally open (NO) or normally closed (NC).  There can one or more contacts. For example, we can have SPST (single pole,single throw), SPDT (single pole, double throw), and DPDT (double pole, double throw) relays  The voltage and current needed to energize the coil  The maximum DC/AC voltage and current that can be handled by the contacts 5
  6. 6. Driving a relay  Digital systems and microcontroller pins lack sufficient current to drive the relay  While the relay’s coil needs around 10 mA to be energized, the microcontroller’s pin can provide a maximum of 1-2 mA current  For this reason, a driver is placed, such as the ULN2803, or a power transistor between the microcontroller and the relay 6
  7. 7. 7 Driving a relay
  8. 8. Solid-state relay  In this relay, there is no coil, spring, or mechanical contact switch  The entire relay is made out of semiconductor materials  These relays have switching response time much faster than that of electromechanical relays  The life cycle for the electromechanical relay can vary from a few hundred thousands to few million operations 8
  9. 9.  Wear and tear on the contact points can cause the relay to malfunction after a while  Solid-state relays have no such limitations  Extremely low input current and small packaging make solid-state relays ideal for microprocessor and logic control switching  They are widely used in controlling pumps, solenoids, alarms, and other power applications 9 Solid-state relay
  10. 10. 10 Solid-state relay
  11. 11. Reed switch  When the reed switch is placed in a magnetic field, the contact is closed  When the magnetic field is removed, the contact is forced open by its spring  The reed switch is ideal for moist and marine environments where it can be submerged in fuel or water  They are also widely used in dirty and dusty atmospheres since they are tightly sealed 11
  12. 12. Reed switch 12
  13. 13. Optoisolator  Optoisolator (also called optocoupler) are used to isolate two parts of a system  An optoisolator has an LED (light-emitting diode) transmitter and a photosensor receiver, separated from each other by a gap  When current flows through the diode, it transmits a signal light across the gap and the receiver produces the same signal with the same phase but a different current and 13 amplitude
  14. 14. 14 Optoisolator
  15. 15. Interfacing an optoisolator  The optoisolator comes in a small IC package with four or more pins  When placing an optoisolator between two circuits, we must use two separate voltage sources, one for each side  Unlike relays, no drivers need to be placed between the microcontroller/digital output and the optoisolators 15
  16. 16. 16 Interfacing an optoisolator
  17. 17. Introduction to Stepper Motor 17  Stepper motor is a widely used device that translates electrical pulses into mechanical movement  Stepper motor is used in applications such as  disk drives  dot matrix printer  robotics etc.  Stepper motors commonly have a permanent magnet rotor (shaft) surrounded by a stator
  18. 18. Stepper Motor Diagram
  19. 19. Construction of Stepper Motor
  20. 20. Construction of Stepper Motor  Commonly used stepper motors have four stator windings that are paired with a center – tapped common. Such motors are called as four-phase or unipolar stepper motor.  It has a permanent magnet rotor which is surrounded by a stator.  A practical PM stepper motor will have 1.8 degrees step angle and 50 tooth on its rotor.  There are 8 main poles on the stator, each having 5 tooth in the pole face
  21. 21. 21 Construction of Stepper Motor
  22. 22. Unipolar Stepper motors
  23. 23. Stepper Motor Selection  Permanent Magnet / Variable Reluctance  Unipolar vs. Bipolar  Number of Stacks  Number of Phases  Degrees Per Step  Microstepping  Pull-In/Pull-Out Torque  Detent Torque
  24. 24. Stepper Motor Selection  Most common stepper motors have 4 stator windings that are paired with a center-tapped common as shown in the fig  This type of stepper motor is commonly referred to as a four phase or unipolar stepper motor  The center tap allows a change of current direction in each of two coils when a winding is grounded, there by resulting in a polarity change of the stator
  25. 25. Working of Stepper Motor  The stator is a magnet over which the electric coil is wound  One end of the coil is connected commonly either to ground or +5V  The other end is provided with a fixed sequence such that the motor rotates in a particular direction  Stepper motor shaft moves in a fixed repeatable increment, which allows one to move it to a precise position
  26. 26. Working of Stepper Motor  Direction of the rotation is dictated by the stator poles  Stator poles are determined by the current sent through the wire coils 30
  27. 27. Step Angle  Step angle is defined as the minimum degree of rotation with a single step.  No of steps per revolution = 360° / step angle  Steps per second = (rpm x steps per revolution) / 60  Example: step angle = 2°  No of steps per revolution = 180
  28. 28. One Phase on (Wave drive four step sequence) (Normal four step sequence)
  29. 29. 8051 connection to stepper motor
  30. 30. Program:  Write an ALP to rotate the stepper motor clockwise / anticlockwise continuously with full step sequence.  MOV A,#66H  BACK: MOV P1,A  RR A  ACALL DELAY  SJMP BACK  DELAY: MOV R1,#100  UP1: MOV R2,#50  UP: DJNZ R2,UP  DJNZ R1,UP1  RET  Note: motor to rotate in anticlockwise use instruction RL A instead of RR A
  31. 31. Program:  : A switch is connected to pin P2.7. Write an ALP to monitor the status of the SW. If SW = 0, motor moves clockwise and if SW = 1, motor moves anticlockwise.  ORG 0000H  SETB P2.7  MOV A, #66H  MOV P1,A  TURN: JNB P2.7, CW  RL A  ACALL DELAY  MOV P1,A  SJMP TURN  CW: RR A  ACALL DELAY  MOV P1,A  SJMP TURN
  32. 32. Program:  Write an ALP to rotate a motor 90° clockwise. Step angle of motor is 2°.  Step angle = 2°  Steps per revolution = 180  For 90° rotation the no of steps is 45  ORG 0000H  MOV A, #66H  MOV R0, #45  BACK: RR A  MOV P1, A  ACALL DELAY  DJNZ R0, BACK  END
  33. 33. Programming stepper motor in ‘c’  #include <reg51.h>  void main ()  { while (1)  {  P1=0x66;  MSDELAY (200);  P1=0x33;  MSDELAY (200);  P1=0x99;  MSDELAY (200);  P1=0xCC;  MSDELAY (200);  }  }
  34. 34. Programming stepper motor in ‘c’  #include <REG51xD2.H>  void delay(unsigned int x) /* Delay Routine */  { for(;x>0;x--);}  main(){  unsigned char Val,i;  while(1) {  Val = 0x88;  for(i=0;i<4;i++) {  P0 = Val;  Val = Val>>1;  delay(575); }}}
  35. 35. DC MOTOR INTERFACING AND PWM  A direct current (DC) motor is another widely used device that translates electrical pulses into mechanical movement  The DC motor has only + and – leads  Connecting them to a DC voltage source moves the motor in one direction  By reversing the polarity, the DC motor will move in the opposite direction 39
  36. 36.  Small fans used in many motherboards to cool the CPU are run by DC motors  While a stepper motor moves in steps of 1 to 15 degrees, the DC motor moves continuously  The DC motor has two rpms: no-load and loaded  The manufacturer’s data sheet gives the no-load rpm 40 DC MOTOR INTERFACING AND PWM
  37. 37. 41
  38. 38.  The DC motor has rotation for clockwise (CW) and counterclockwise (CCW) rotations 42 Unidirection Control
  39. 39. 43 Bidirectional control  All the switches are open, which does not allow the motor to turn.
  40. 40. 44 Bidirectional control  When switches 1 and 4 are closed, current is allowed to pass through the motor.
  41. 41. 45 Bidirectional control
  42. 42. 46 Bidirectional control  An invalid configuration  Current flows directly to ground, creating a short circuit
  43. 43.  H-Bridge control can be created using relays, transistors, or a single IC solution such as the L293  When using relays and transistors, it must be ensured that invalid configurations do not occur 47 Bidirectional control
  44. 44.  The speed of the motor depends on three factors: – (a) load – (b) voltage – (c) current  For a given fixed load we can maintain a steady speed by using a method called pulse width modulation (PWM) 48 Pulse width modulation (PWM)
  45. 45.  By changing (modulating) the width of the pulse applied to the DC motor we can increase or decrease the amount of power provided to the motor, thereby increasing or decreasing the motor speed  PWM is so widely used in DC motor control that some microcontrollers come with the PWM circuitry embedded in the chip 49 Pulse width modulation (PWM)
  46. 46. 50 Pulse width modulation (PWM)  Although the voltage has a fixed amplitude, it has a variable duty cycle  That means the wider the pulse, the higher the speed
  47. 47. 51 DC Motor Connection using a Darlington Transistor

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