Principle of single chip microcomputer controlled stepper motor driver

Principle of single chip microcomputer controlled stepper motor driver

Stepper motors are widely used in control systems. They can convert pulse signals into angular displacements and can be used as electromagnetic brake wheels, electromagnetic differentiators, or angular displacement generators.

Sometimes, if you want to use a stepper motor removed from some old equipment (this type of motor is generally not damaged) for other purposes, you usually need to design your own driver. This article introduces a driver designed for a stepper motor removed from an old Japanese printer.

This article first introduces the working principle of the stepper motor, and then introduces the software and hardware design of its driver.

1. Working principle of stepper motor

The stepper motor is a four-phase stepper motor, which is powered by a unipolar DC power supply. As long as the phase windings of the stepper motor are energized in a suitable timing, the stepper motor can be made to rotate step by step. Figure 1 is a schematic diagram of the working principle of the four-phase reactive stepper motor.

Figure 1 Schematic diagram of four-phase stepper motor stepping

At the beginning, switch SB is connected to the power supply, SA, SC, and SD are disconnected, and the B-phase magnetic pole is aligned with the rotor teeth 0 and 3. At the same time, the rotor teeth 1 and 4 are staggered with the C and D-phase winding magnetic poles, and the teeth 2 and 5 are staggered with the D and A-phase winding magnetic poles.

When the switch SC is connected to the power supply and SB, SA, and SD are disconnected, the rotor rotates due to the magnetic lines of force between the C-phase winding and the 1st and 4th teeth, and the magnetic poles of the 1st and 4th teeth and the C-phase winding are aligned. However, the 0th and 3th teeth are staggered with the A- and B-phase windings, and the 2nd and 5th teeth are staggered with the A- and D-phase windings. By analogy, when the A, B, C, and D-phase windings are powered in turn, the rotor will rotate in the A, B, C, and D directions.

Four-phase stepper motors can be divided into three working modes according to the different power-on sequences: single four-beat, double four-beat, and eight-beat. The step angles of single four-beat and double four-beat are equal, but the torque of single four-beat is small. The step angle of eight-beat working mode is half of that of single four-beat and double four-beat. Therefore, eight-beat working mode can maintain a higher torque and improve control accuracy.

The power-on timing and waveform of the single four-beat, double four-beat and eight-beat working modes are shown in Figure 2.a, b, and c respectively:

a. Single 4 beats b. Double 4 beats c. Eight beats

Figure 2. Stepper motor working timing waveform

2. Circuit principle of stepper motor driver system based on AT89C2051

The circuit principle of the stepper motor driver system is shown in Figure 3:

Figure 3 Stepper motor driver system circuit diagram

AT89C2051 outputs the control pulse from P1.4~P1.7 of P1 port, and enters 9014 after being inverted by 74LS14. After being amplified by 9014, it controls the photoelectric switch. After photoelectric isolation, the power tube TIP122 amplifies the pulse signal in voltage and current to drive each phase winding of the stepper motor. The stepper motor can perform forward rotation, reverse rotation, acceleration, deceleration and stop according to different pulse signals. In the figure, L1 is a phase winding of the stepper motor. AT89C2051 uses a crystal oscillator with a frequency of 22MHz. The purpose of using a higher crystal oscillator is to minimize the influence of AT89C2051 on the pulse signal cycle of the host computer under mode 2.

In Figure 3, RL1~RL4 are the internal resistance of the winding, and the 50Ω resistor is an external resistor that acts as a current limiter and is also a component that improves the loop time constant. D1~D4 are freewheeling diodes, which attenuate the back electromotive force generated by the motor winding through the freewheeling diodes (D1~D4), thereby protecting the power tube TIP122 from damage.

Connecting a 200μF capacitor in parallel with a 50Ω external resistor can improve the leading edge of the current pulse injected into the stepper motor winding and improve the high-frequency performance of the stepper motor. The 200Ω resistor in series with the freewheeling diode can reduce the discharge time constant of the loop, making the trailing edge of the current pulse in the winding steeper and the current falling time shorter, which also plays a role in improving the high-frequency working performance.

3. Software Design

The driver has three working modes to choose from according to the different combinations of the DIP switches KX and KY:

Mode 1 is the interrupt mode: P3.5 (INT1) is the step pulse input terminal, and P3.7 is the forward and reverse pulse input terminal. The host computer (PC or single-chip microcomputer) is connected to the driver with only two lines.

Mode 2 is serial communication mode: the host computer (PC or single-chip microcomputer) sends the control command to the driver, and the driver completes the relevant control process according to the control command.

Mode 3 is the DIP switch control mode: directly control the stepper motor through different combinations of K1~K5.

When the power is turned on or the reset button KR is pressed, AT89C2051 first detects the status of the DIP switches KX and KY, and enters different working modes according to the different combinations of KX and KY. The following is the program flow chart and source program of mode 1.

When compiling the program, special attention should be paid to the handling of the stepper motor during commutation. In order to make the stepper motor transition smoothly during commutation and avoid step errors, a flag should be set in each step. The 20H unit is the forward flag of the stepper motor; the 21H unit is the reverse flag. When rotating forward, not only the forward flag is assigned a value, but also the reverse flag is assigned a value; the same is true when reversing. In this way, when the stepper motor commutates, the previous position can be used as the starting point for reverse movement, avoiding step errors when the motor commutates.

Figure 4: Method 1 program flowchart

Method 1 source program:

MOV 20H,#00H; 20H unit is reset to initial value, motor forward position pointer

MOV 21H,#00H; 21H unit is reset to initial value, motor reverse position pointer

MOV P1,#0C0H; Initialize the P1 port to prevent the motor from short circuiting when powered on

MOV TMOD,#60H; Initialize T1 counter and enable interrupt

MOV TL1,#0FFH

MOV TH1,#0FFH

SETB ET1

SETB EA

SETB TR1

SJMP$

;***********Counter 1 interrupt procedure************

IT1P: JB P3.7, FAN; Motor forward and reverse pointer

;****************Motor rotates forward*****************

JB 00H,LOOP0

JB 01H,LOOP1

JB 02H,LOOP2

JB 03H,LOOP3

JB 04H,LOOP4

JB 05H,LOOP5

JB 06H,LOOP6

JB 07H,LOOP7

LOOP0: MOV P1,#0D0H

MOV 20H,#02H

MOV 21H,#40H

AJMP QUIT

LOOP1: MOV P1,#090H

MOV 20H,#04H

MOV 21H,#20H

AJMP QUIT

LOOP2: MOV P1,#0B0H

MOV 20H,#08H

MOV 21H,#10H

AJMP QUIT

LOOP3: MOV P1,#030H

MOV 20H,#10H

MOV 21H,#08H

AJMP QUIT

LOOP4: MOV P1,#070H

MOV 20H,#20H

MOV 21H,#04H

AJMP QUIT

LOOP5: MOV P1,#060H

MOV 20H,#40H

MOV 21H,#02H

AJMP QUIT

LOOP6: MOV P1,#0E0H

MOV 20H,#80H

MOV 21H,#01H

AJMP QUIT

LOOP7: MOV P1,#0C0H

MOV ; 20H,#01H

MOV 21H,#80H

AJMP QUIT

;***************Motor reverses*****************

FAN: JB 08H,LOOQ0

JB 09H,LOOQ1

JB 0AH,LOOQ2

JB 0BH,LOOQ3

JB 0CH,LOOQ4

JB 0DH,LOOQ5

JB 0EH,LOOQ6

JB 0FH,LOOQ7

LOOQ0: MOV P1,#0A0H

MOV 21H,#02H

MOV 20H,#40H

AJMP QUIT

LOOQ1: MOV P1,#0E0H

MOV 21H,#04H

MOV 20H,#20H

AJMP QUIT

LOOQ2: MOV P1,#0C0H

MOV 21H,#08H

MOV 20H,#10H

AJMP QUIT

LOOQ3: MOV P1,#0D0H

MOV 21H,#10H

MOV 20H,#08H

AJMP QUIT

LOOQ4: MOV P1,#050H

MOV 21H,#20H

MOV 20H,#04H

AJMP QUIT

LOOQ5: MOV P1,#070H

MOV 21H,#40H

MOV 20H,#02H

AJMP QUIT

LOOQ6: MOV P1,#030H

MOV 21H,#80H

MOV 20H,#01H

AJMP QUIT

LOOQ7: MOV P1,#0B0H

MOV 21H,#01H

MOV 20H,#80H

QUIT: RETI

END

4. Conclusion

The driver has been verified by experiments to be able to drive a 0.5Nm stepper motor. By adjusting the parameters of the resistance, capacitance and freewheeling diode of the driver part, it can drive a 1.2Nm stepper motor. The driver circuit is simple and reliable, with a compact structure, and is particularly suitable for systems with limited I/O port lines and microcontroller resources.