Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems

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7.9 Method to Design a Microcontroller-Based System 321

' stepper.bas (PIC16F84 microcontroller)

' Design Example

' Position and Speed Control of a Stepper Motor

' Four pushbutton switches are used to index to four different

' positions (0, 45, ' 90, and 180 degrees). Another pushbutton switch

' is used to toggle in and out of speed control mode (indicated by

' an LED). When in speed control mode, a potentiometer is used to

' control the speed. When to the right of the center position,

' the motor is turned CW at a speed proportional to the pot position.

' The motor turns CCW for pot positions to the left of center. The pot

' position is read from an external A/D converter (National

' Semiconductor ADC0831). The PIC retrieves the bits from the A/D

' converter via a clock signal generated by the PIC. The stepper

' motor is controlled via an E-lab EDE1200 unipolar driver IC and

' a ULN2003A Darlington driver.

' Define I/O pin names

led Var PORTB.0 ' speed control indicator LED

AD_start Var PORTB.1 ' A/D converter conversion start bit

' (must be held low during A/D conversion)

AD_data Var PORTB.2 ' A/D converter data line

' (for serial transmission of data bits)

AD_clock Var PORTB.3 ' A/D converter clock signal (400 kHz maximum)

P1 Var PORTA.2 ' position 1 NO button (0 degrees)

P2 Var PORTA.3 ' position 2 NO button (45 degrees)

P3 Var PORTA.4 ' position 3 NO button (90 degrees)

P4 Var PORTA.1 ' position 4 NO button (180 degrees)

SPD Var PORTA.0 ' speed control NO button to toggle speed control mode

motor_dir Var PORTB.6 ' stepper motor direction bit (0:CW 1:CCW)

motor_step Var PORTB.5 ' stepper motor step driver (1 pulse = 1 step)

' Declare Variables

motor_pos Var BYTE ' current angle position of the motor (0, 45, 90, or 180)

new_motor_pos Var Byte ' desired angle position of the motor

delta Var BYTE ' required magnitude of angular motion required

num_steps Var BYTE ' number of steps required for the given angular motion

step_period Var BYTE ' millisecond width of step pulse (1/2 of period)

i Var Byte ' counter used for For loops

AD_value Var BYTE ' byte used to store the 8-bit value from the A/D converter

AD_pause Var BYTE ' clock pulse width for the A/D converter

blink_pause Var BYTE ' millisecond pause between LED blinks

bit_value Var BYTE ' power of 2 value for each bit used in the A/D conversion

' Define Constants

CW Con 0 ' clockwise motor direction

CCW Con 1 ' counterclockwise motor direction

(continued )

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322 CHAPTER 7 Microcontroller Programming and Interfacing

' Initialize I/O and variables

TRISA = $FF ' configure all PORTA pins as inputs

TRISB = %00000100 ' configure all PORTB pins as outputs except RB2

High AD_start ' disable A/D converter

Low motor_step ' start motor step signal in low state

motor_pos = 0 ' assume the current position is the 0 degree position

step_period = 10 ' initial step speed (1/100 second between steps)

AD_pause = 10 ' 10 microsecond pulsewidth for the A/D clock

blink_pause = 200 ' 1/5 second pause between LED blinks

' Blink the speed control LED to indicate start-up

Gosub blink : Gosub blink

' Wait for a button to be pressed (i.e., polling loop)

main:

If (P1 == 1) Then

' Move motor to the 0 degree position

new_motor_pos = 0

Gosub move

Else

If (P2 == 1) Then

' Move motor to the 45 degree position

new_motor_pos = 45

Gosub move

Else

If (P3 == 1) Then

' Move motor to the 90 degree position

new_motor_pos = 90

Gosub move

Else

If (P4 == 1) Then

' Move motor to the 180 degree position

new_motor_pos = 180

Gosub move

Else

If (SPD == 1) Then

' Enter speed control mode

Gosub speed

EndIf : EndIf : EndIf : EndIf : EndIf

Goto main ' continue polling buttons

End ' end of main program

' Subroutine to blink the speed control indicator LED

blink:

High led

Pause blink_pause

Low led

Pause blink_pause

Return

(continued )

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7.9 Method to Design a Microcontroller-Based System 323

' Subroutine to move the stepper motor to the position indicated by motor_pos

' (the motor step size is 7.5 degrees)

move:

' Set the correct motor direction and determine the required displacement

If (new_motor_pos > motor_pos) Then

motor_dir = CW

delta = new_motor_pos - motor_pos

Else

motor_dir = CCW

delta = motor_pos - new_motor_pos

EndIf

' Determine the required number of steps (given 7.5 degrees per step)

num_steps = 10*delta / 75

' Step the motor the appropriate number of steps

Gosub move_steps

' Update the current motor position

motor_pos = new_motor_pos

Return

' Subroutine to move the motor a given number of steps (indicated by num_steps)

move_steps:

For i = 1 to num_steps

Gosub step_motor

Return

' Subroutine to step the motor a single step (7.5 degrees) in the motor_dir

' direction

step_motor:

Pulsout motor_step, 100*step_period ' (100 * 10microsec = 1 millisec)

Pause step_period

' Equivalent code:

' High motor_step

' Pause step_period

' Low motor_step

' Pause step_period

Return

' Subrouting to poll the POT for speed control of the stepper motor

speed:

' Turn on the speed control LED indicator

High LED

' Wait for the SPEED button to be released

Gosub button_release

' Polling loop for POT speed control

pot_speed:

' Check if the SPEED button is down

(continued )

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324 CHAPTER 7 Microcontroller Programming and Interfacing

If (SPD == 1) Then

' Wait for the SPEED button to be released

Gosub button_release

' Turn off the speed control LED indicator

Low led

' Assume the new position is the new 0 position

motor_pos = 0

' Exit the subroutine

Return

EndIf

' Sample the POT voltage via the A/D converter

Gosub get_AD_value

' Adjust the motor speed and direction based on the POT value and

' step the motor a single step.

' Enforce a deadband at the center of the range

' Have the step period range from 100 (slow) to 1 (fast)

If (AD_value > 150) Then

motor_dir = CW

step_period = 100 - (AD_value - 150)*99/(255 - 150)

Gosub step_motor

Else

If (AD_value < 100) Then

motor_dir = CCW

step_period = 100 - (100 - AD_value)*99/100

Gosub step_motor

EndIf

' Continue polling

goto pot_speed

Return ' end of subroutine, but not reached (see the SPD If statement above)

' Subroutine to wait for the speed control button to be released

button_release:

Pause 50 ' wait for switch bounce to settle

While (SPD == 1) : WEND

Pause 50 ' wait for switch bounce to settle

Return

' Subroutine to sample the POT voltage from the A/D converter

' The value (0 to 255) is returned in the variable AD_value and corresponds

' to the original 0 to 5V analog voltage range.

get_AD_value:

' Initialize the A/D converter

Low AD_clock ' initialize the clock state

Low AD_start ' enable the A/D converter

Gosub pulse_clock ' send initialization pulse to A/D clock

(continued )

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7.9 Method to Design a Microcontroller-Based System 325

' Get each converted bit from the A/D converter (at 50 kHz)

bit_value = 128 ' value of the MSB

AD_value = 0

FOR I = 7 To 0 Step -1 ' for each bit from the MSB to the LSB

' Output clock pulse

Gosub pulse_clock

AD_value = AD_value + AD_data*bit_value

bit_value = bit_value / 2

Next i

' Disable the A/D converter

High AD_start

Return

' Subroutine to send a pulse to the A/D clock line

pulse_clock:

Pulsout AD_clock, 1 : PauseUS 10 ' 20 microsecond pulse

Return

THREADED DESIGN EXAMPLE

DC motor position and speed controller—Full solution with serial interface C.3

The figure below shows the functional diagram for Threaded Design Example C (see Section

1.3 and Video Demo 1.8). Presented here is the entire solution to this problem. This solution

utilizes two PIC microcontrollers. The main PIC is referred to as the “master” PIC, because

it controls most of the system functions; the secondary PIC is referred to as the “slave” PIC,

because it simply provides information to the master PIC upon command.

microcontrollers

SLAVE

PIC

MASTER

PIC

liquid crystal display

DC motor with

digital position encoder

quadrature

decoder

and counter

1 2 3

4 5 6

7 8 9

0 #

keypad

decoder

button

buzzer

H-bridge

driver

The circuit schematic and software listings are shown below. The code is commented,

so you should be able to follow the logic as it relates to the functional diagram and circuit

Video Demo

1.8DC motor

position and

speed controller

(continued )

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326 CHAPTER 7 Microcontroller Programming and Interfacing

schematic. There are two software listings. One is for the master PIC (a PIC16F88) that

monitors the keypad (see Threaded Design Example C.2), provides a menu-driven user inter-

face on the LCD (see Threaded Design Example C.2), and drives the motor (see Threaded

Design Examples C.4 and C.5). The other listing is for the slave PIC (a PIC16F84) that

monitors the digital encoder sensor on the motor shaft and transmits the position information

to the master PIC.

As with the other threaded design examples, details covering the different components

of the design can be found throughout the book. This solution is a good example of how to

communicate among multiple PICs using a serial interface. The specific code designed to

implement the communication can be found in the get_encoder subroutine in the master PIC

code and the main loop in the slave PIC code. One I/O line is simply set high or low by the

master PIC to command the slave PIC when to send data. A second I/O line then receives the

data through a standard serial communication protocol.

master PIC code:

' dc_motor.bas (PIC16F88 microcontroller)

' Design Example

' Position and Speed Control of a DC Servo Motor.

' The user interface includes a keypad for data entry and an LCD for text

' messages. The main menu offers three options: 1 - position control,

' 2 - speed control, and 3 - position control gain and motor PWM control.

' When in position control mode, pressing a button moves to indexed positions

' (1 - 0 degrees, 2 - 45 degrees, 3 - 90 degrees, and 4 - 180 degrees). When

' in speed control mode, pressing 1 decreases the speed, pressing 2 reverses

' the motor direction, pressing 3 increases the speed, and pressing 0 starts

' the motor at the indicated speed and direction. The motor is stopped with

' a separate pushbutton switch. When in gain and PWM control mode,

' pressing 1/4 increases/decreases the proportional gain factor (kP)

' and pressing 3/6 increases/decreases the number of PWM cycles sent

' to the motor during each control loop update.

' Pressing the "#" key from the position, speed, or gain menus returns control

' back to the main menu. E-Lab's EDE1144 keypad encoder is used to detect

' when a key is pressed on the keypad and transmit data (a single byte per

' keypress) to the PIC16F88. Acroname's R179-6V-ENC-MOTOR servo motor is

' used with their S17-3A-LV H-bridge for PWM control. A second PIC (16F84),

' running dc_enc.bas, is used to communicate to an Agilent HCTL-2016

' quadrature decoder/counter to track the position of the motor encoder.

' The 16F88 communicates to the 16F84 via handshake (start) and serial

' communication lines.

' Configure the internal 8MHz internal oscillator

DEFINE OSC 8

OSCCON.4 = 1 : OSCCON.5 = 1 : OSCCON.6 = 1

(continued )

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7.9 Method to Design a Microcontroller-Based System 327

' Turn off A/D converters (thereby allowing use of pins for I/O)

ANSEL = 0

' Define I/O pin names

key_serial Var PORTB.0 ' keypad serial interface input

motor_dir Var PORTB.7 ' motor H-bridge direction line

motor_pwm Var PORTB.6 ' motor H-bridge pulse-width-modulation line

stop_button Var PORTB.4 ' motor stop button

enc_start Var PORTB.2 ' signal line used to start encoder data transmission

enc_serial Var PORTA.7 ' serial line used to get encoder data from the 16F84

enc_rst Var PORTB.5 ' encoder counter reset signal (active low)

(continued )

5 V

20 ⫻ 2 LCD character display

RS R/W E

DB4 DB5 DB6 DB7

11 12

14 330 Ω

330 Ω

4.7 kΩ

330 Ω

0.1 μF

5 V

20 k

pot

5 V 1 k

PIC16F88

RA1

RA0

RA7

CLKO

RB7

RB6

RB5

RB4

RA2

RA3

RA4

RA5

RB0

RB1

RB2

RB3

456

789

keypad

EDE1144

XMIT

+5 V

GND

Beep

Valid

OSC1

OSC2

3, 4, 14

2, 5

910

5 V

buzzer

2N2222

DC motor

with

encoder

4.7 kΩ

1 kΩ

S17-3A-LV

H-Bridge

HCTL-2016

CLK

CHA

CHB

D0-7

PIC16F84

SEL

PORTBRA2

RA3

RA4

RA0

RA1

RST

5 V

1 k

stop

button

LED

5 V

6–13

1, 15–9

J2-2

J2-3

J3-1

J3-6

J2-5

J2-4

DIR

PWM

VMOTOR

GND

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328 CHAPTER 7 Microcontroller Programming and Interfacing

' Declare Variables

key_value Var BYTE ' code byte from the keypad

motor_pos Var Word ' current motor position in degrees

new_motor_pos Var Word ' desired motor position (set point) in degrees

error Var Word ' error magnitude between current and desired positions

motor_speed Var BYTE ' motor speed as percentage of maximum (0 to 100)

motion_dir Var BIT ' motor direction (1:CW/Forward 0:CCW/Reverse)

on_time Var WORD ' PWM ON pulse width

off_time Var WORD ' PWM OFF pulse width

enc_pos Var WORD ' motor encoder position (high byte and low byte)

i Var Byte ' counter variable for FOR loops

kp Var BYTE ' proportional gain factor position control

pwm_cycles Var BYTE ' # of PWM pulses sent during the position control loop

' Define constants

key_mode Con 0 ' 2400 baud mode for serial connection to keypad

key_1 Con $30 ' hex code for the 1-key on the keypad

key_2 Con $31 ' hex code for the 2-key on the keypad

key_3 Con $32 ' hex code for the 3-key on the keypad

key_4 Con $34 ' hex code for the 4-key on the keypad

key_5 Con $35 ' hex code for the 5-key on the keypad

key_6 Con $36 ' hex code for the 6-key on the keypad

key_7 Con $38 ' hex code for the 7-key on the keypad

key_8 Con $39 ' hex code for the 8-key on the keypad

key_9 Con $41 ' hex code for the 9-key on the keypad

key_star Con $43 ' hex code for the *-key on the keypad

key_0 Con $44 ' hex code for the 0-key on the keypad

key_pound Con $45 ' hex code for the #-key on the keypad

CW Con 1 ' motor clockwise (forward) direction

CCW Con 0 ' motor counterclockwise (reverse) direction

pwm_period Con 50 ' period of each motor PWM signal cycle (in microsec)

' (50 microsec corresponds to 20kHz)

enc_mode Con 2 ' 9600 baud mode for serial connection to the encoder IC

' Initialize I/O and variables

TRISB.6 = 0 ' configure H-bridge DIR pin as an output

TRISB.7 = 0 ' configure H-bridge PWM pin as an output

motion_dir = CW ' starting motor direction: CW (forward)

motor_pos = 0 ' define the starting motor position as 0 degrees

motor_speed = 50 ' starting motor speed = 50% duty cycle

kp = 50 ' starting proportional gain for position control

pwm_cycles = 20 ' starting # of PWM pulses sent during the

' position control loop

Low motor_pwm ' make sure the motor is off to begin with

Low enc_start ' disable encoder reading to begin with

Gosub reset_encoder ' reset the encoder counter

(continued )

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7.9 Method to Design a Microcontroller-Based System 329

' Wait 1/2 second for everything (e.g., LCD) to power up

Pause 500

' Receive dummy byte from the PIC16F84 to ensure proper communication

SERIN enc_serial, enc_mode, key_value

' Wait for a keypad button to be pressed (i.e., polling loop)

Gosub main_menu ' display the main menu on the LCD

main:

Serin key_serial, key_mode, key_value

If (key_value = key_1) Then

Gosub reset_encoder

Gosub position

Else

If (key_value = key_2) Then

motor_speed = 50 ' initialize to 50% duty cycle

Gosub speed

Else

If (key_value = key_3) Then

Gosub adjust_gains

Endif : Endif : Endif

Goto main ' continue polling keypad buttons

End ' end of main program

' Subroutine to display the main menu on the LCD

main_menu:

Lcdout $FE, 1, "Main Menu:"

Lcdout $FE, $C0, "1:pos. 2:speed 3:gain"

Return

' Subroutine to reset the motor encoder counter to 0

reset_encoder:

Low enc_rst ' reset the encoder counter

High enc_rst ' activiate the encoder counter

Return

' Suroutine for position control of the motor

position:

' Display the position control menu on the LCD

Lcdout $FE, 1, "Position Menu:"

Lcdout $FE, $C0, "1:0 2:45 3:90 4:180 #:<"

' Wait for a keypad button to be pressed

Serin key_serial, key_mode, key_value

' Take the appropriate action based on the key pressed

If (key_value == key_1) Then

new_motor_pos = 0

Else

(continued )

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330 CHAPTER 7 Microcontroller Programming and Interfacing

(continued )

If (key_value == key_2) Then

new_motor_pos = 45

Else

If (key_value == key_3) Then

new_motor_pos = 90

Else

If (key_value == key_4) Then

new_motor_pos = 180

Else

If (key_value == key_pound) Then

Gosub main_menu

Return

Else

Goto position

Endif : Endif : Endif : Endif : Endif

' Position control loop

While (stop_button == 0) ' until the stop button is pressed

' Get the encoder position (enc_pos)

Gosub get_encoder

' Calculate the error signal magnitude and sign and set the motor direction

' Convert encoder pulses to degrees. The encoder outputs 1230 pulses

' per 360 degrees of rotation

motor_pos = enc_pos * 36 / 123

If (new_motor_pos >= motor_pos) Then

error = new_motor_pos - motor_pos

motor_dir = CW

Else

error = motor_pos - new_motor_pos

motor_dir = CCW

Endif

' Set the PWM duty cycle based on the current error

If (error > 20) Then ' use maximum speed for large errors

motor_speed = kp

Else

' Perform proportional position control for smaller errors

motor_speed = kp * error / 20

Endif

' Output a series of PWM pulses with the speed-determined duty cycle

Gosub pwm_periods ' calculate the on and off pulse widths

For I = 1 to pwm_cycles

Gosub pwm_pulse ' output a full PWM pulse

Next I

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