Stepper Motor Control

Stepper Motor Control

AU Dec.-10, 11,12,13,14,15,18,19, May-07, 08,09, 10, 12,13, 16,17,18

A stepper motor is a digital motor. It can be driven by digital signal. Fig. 17.4.1 shows the typical 2 phase motor interfaced using 8051. Motor shown in the circuit has two phases, with center-tap winding. The center taps of these windings are connected to the 12 V supply. Due to this, motor can be excited by grounding four terminals of the two windings. Motor can be rotated in steps by giving proper excitation sequence to

these windings. The lower nibble of port 1 of the 8051 is used to generate excitation signals in the proper sequence.

The Table 17.4.1 shows typical excitation sequence. The given excitation sequence rotates the motor in clockwise direction. To rotate motor in anticlockwise direction we have to excite motor in a reverse sequence. The excitation sequence for stepper motor may change due to change in winding connections. However, it is not desirable to excite both the ends of the same winding simultaneously. This cancels the flux and motor winding may damage. To avoid this, digital locking system must be designed. Fig. 17.4.2 shows a simple digital locking system. Only one output is activated (made low) when properly excited; otherwise output is disabled (made high).

The excitation sequence given in Table 17.4.1 is called full step sequence. 

In which excitation ends of the phase are changed in one step. The excitation sequence given in Table 17.4.2 takes two steps to change the excitation ends of the phase. Such a sequence is called half step sequence and in each step the motor is rotated by 0.9°.

We know that stepper motor is stepped from one position to the next by changing the currents through the fields in the motor. The winding inductance opposes the change in current and this puts limit on the stepping rate. For higher stepping rates and more torque, it is necessary to use a higher voltage source and current limiting resistors as shown in Fig. 17.4.3. By adding series resistance, we decrease L/R time constant, which allows the current to change more rapidly in the windings. There is a power loss across series resistor, but designer has to compromise between power and speed.

Example 17.4.1 Write an 8051 assembly language program to control stepper motor using connections given in Fig. 17.4.1.

Solution:

MOV R0 # COUNT ; Initialize rotation count

AGAIN: MOV DPTR, #ETC ; Initialize pointer to excitation code table

MOV R1, #04 ; Initialize counter to excitation code sequence

BACK: MOVX A, @DPTR ; Get the excitation code

MOV P1, A ; send the excitation code

LCALL DELAY ; Wait for some time

INC DPTR ; Increment pointer

DJNZ R1, BACK ; Decrement R1; if not zero goto BACK

DJNZ RO, AGAIN ; Decrement R0; if not zero goto AGAIN

RET

ORG 3000H

ETC DB 03H,06H,09H,0CH ; code sequence for clockwise rotation

Example 17.4.2 Write assembly language program to control conveyer belt using stepper motor and 8051 controller. Belt moves continuously at rate of 1 step/sec. but stops for 5 sec. when external interrupt occurs and then continues to move.

Solution :

MAIN : MOV IE, #1000 0001B ; Enable external interrupt 0

AGAIN : MOV DPTR,#ETC ; Initialize pointer to excitation code table

MOV R1, #04 ; Initialize counter to excitation code sequence

BACK : MOVX A, @DPTR ; Get the excitation code

MOV P1, A ; send the excitation code

MOV A,#14H ; Initialize count = 20

LCALL DELAY ; Wait for 1sec

INC DPTR ; Increment pointer

DJNZ R1, BACK ; Decrement RI if not zero goto BACK

SJMP AGAIN ; Repeat

ORG 3000H

ETC DB 03H.06H, 09H, 0CH ; Code sequence for

; Clockwise rotation

Assume external interrupt INT0 is used.

ORG 0003H

MOV A,#64H ; Initialize count =100

ACALL DELAY ; Called delay routine

RETI ; Return to main program

Delay Routine

DELAY: MOV TMOD, #01 ; Time 0, model (16-bit mode)

MOV R0,A ; Read count and Initialize

BACK: MOV TL0,#B0H ; TL0=B0H, the low byte

MOV TH0,#3CH ; TH0=3CH, the high byte

SETB TR0 ; Start the timer 0

RERE: JNB TF0,REPE ; Check timer 0 flag untilit rolls over

CLR TR0 ; Stop timer 0

CLR TF0 ; clear timer 0 flag

DJNZ R0, BACK ; Decrement counter and

RET ; if not zero repeat

 Note Timer 0 gives a delay of 50 ms.

Therefore, to get delay of 1 sec = 50 ms × 20

We load 20 as a count and to get delay of 5 sec. = 50 ms  × 100

Example 17.4.3 A switch is connected to pin P2.7. Write a ALP to monitor the status of SW and perform the following.

i) If SW = 0, the stepper motor moves clockwise

ii) If SW = 1, the stepper motor moves counter clockwise

Solution :

ORG 00H ; Starting address

MAIN : SETB P2.7 ; Make it an input

MOV DPTR, #ETC ; Initialize base pointer to excitation code table

MOV R0, #00H ; Initialize pointer to excitation code table

BACK : JB P2.7, ACW ; Check switch status

CJNZ R0, #04, NEXT ; Roll over the sequence code

MOV R0, #00H

NEXT : MOV A, R0 ; Copy pointer in A

MOVC A, @A + DPTR ; Get the code to rotate clockwise

MOV P1, A ; Send code to port 1

LCALL Delay ; Wait for some time

INC R0 ; Point to next code

SJMP BACK ; Repeat

ACW : CJNZ R0, #0FFH, NEXT1 ; Roll over the sequence code

MOV R0, #03H

NEXT1: MOV A, R0 ; Copy pointer in A

MOVC A, @A + DPTR ; Get the code to rotate anticlockwise

MOV P1 A ; Send code to port 1

LCALL Delay ; Wait for some time

INC R0 ; Point to previous code

SJMP BACK ; Repeat

ACW : CJNZ R0, #0FFH, NEXT1 ; Roll over the sequence code

MOV R0, #03H

NEXT1 : MOV A, R0 ; Copy pointer in A

MOVC A, @A + DPTR ; Get the code to rotate anticlockwise

MOV P1, A ; Send code to port 1

LCALL Delay ; Wait for some time

DEC R0 ; Point to previous code

SJMP BACK ; Repeat

Delay : MOV R2, #100

AGAIN1 : MOV R3, #255

AGAIN : DJNZ R3, AGAIN

DJNZ R2, AGAIN

RET

ORG 0100H

ETC    DB 05H, 06H, 0AH, 09H

Review Questions

1. Draw the schematic for interfacing a stepper motor with 8051 microcontroller and write 8051 ALP for changing speed and direction of motor. AU : May-07, Marks 16

2. Explain the microcontroller 8051 based stepper motor control. AU : May-08,10,12, Dec.-10, Marks 16

3. Write an assembly program in 8051 to rotate the stepper motor in clock wise and anticlockwise direction. AU : May-09, Dec.-11, Marks 16

4. Write a program to generate pulses to drive and for continuous operation of a stepper motor. AU : May-12, Marks 8

5. Explain how to control the speed of the stepper motor using 8051 microcontrollers, with algorithmic AU : Dec.-12, Marks 8

6. Describe with a neat diagram the stepper motor control using microcontroller. AU : May-13, Marks 16

7. Explain how to control a stepper motor using 8051 microcontroller with a neat interfacing diagram and assembly program. AU : Dec.-15, May-17, Marks 16

8. Explain the stepper motor control using 8051 and write an assembly language program for running the stepper motor in clockwise direction. AU : May-12, Marks 16

9. Show how to interface a stepper motor to 8051 microcontroller. Also, write an assembly language program to demonstrate control of direction and speed of stepper motor rotation. AU : Dec.-19, Marks 13

10. Interface the stepper motor with 8051 and explain its operation of stepper motor with neat diagram and program to rotate in clockwise direction. AU : May.-18, Marks 13

11. Design a stepper motor control system using 8051 microcontroller. AU : Dec.-18, Marks 15