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.

1. 1
The 8051 Microcontroller and
Embedded Systems
CHAPTER 16
Motor Control: Relay,
PWM, DC and Stepper
Motors

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.  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. Electromechanical Relay Symbols
4

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. 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
Driving a relay

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.  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
Solid-state relay

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. Reed switch
12

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
Optoisolator

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
Interfacing an optoisolator

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. Stepper Motor Diagram

19. Construction of Stepper Motor

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
Construction of Stepper Motor

22. Unipolar Stepper motors

27. 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

28. 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

29. 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

30. 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

31. 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

32. One Phase on
(Wave drive four step sequence)
(Normal four step sequence)

33. 8051 connection to stepper motor

34. 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

35. 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

36. 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

37. 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);
 }
 }

38. 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); }}}

39. 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

40.  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

41. 41

42.  The DC motor has rotation for clockwise
(CW) and counterclockwise (CCW) rotations
42
Unidirection Control

43. 43
Bidirectional control
 All the
switches
are
open,
which
does not
allow the
motor to
turn.

44. 44
Bidirectional control
 When
switches 1
and 4 are
closed,
current is
allowed to
pass
through
the motor.

45. 45
Bidirectional control

46. 46
Bidirectional control
 An invalid
configuration
 Current
flows directly
to ground,
creating a
short circuit

47.  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

48.  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)

49.  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)

50. 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

51. 51
DC Motor Connection using a
Darlington Transistor