Introduction

Stepper motor as the name depicts takes steps to complete a full 360 degree circular rotation. We can rotate stepper in clock and anti clock wise direction. Stepper motor speed can also be varied. Stepper motors are widely used in industrial, medical, consumer electronics application. In simple words, anywhere precision rotation or positioning of an object is needed. This article is about interfacing stepper motor to 8051 microcontroller.

Hardware Discussion: Stepper Motor

Stepper Motor Features :

• small step angle.
• high positioning accuracy.
• high torque to inertia ratio.
• stepping rate and accuracy.
• Available in Unipolar and Bipolar configuration.
• Used in Computing, Robotics, Camera, Printers,Process automation and pack-aging machinery, Positioning of valve pilot stages for fluid control systems.

Stepper motor is brushless DC motor, which can be rotated in small angles, these angles are called steps. Generally stepper motor use 200 steps to complete 360degree rotation, means its rotate 1.8 degree per step. Stepper motor used in manydevices which needs precise rotational movement like robots, antennas, hard drivesetc. We can rotate stepper motor to any particular angle by giving it proper in-structions. Stepper motor consumes high current during step rotation with load.20.To handle the extra power needed for stepper motor we will use high current drivingICs like ULN2003, L293D etc.

How it works?

Stepper motor is a brush less motor which converts electrical pulses into mechanical rotation. As the name indicates it rotates in steps according to the inputpulses. A stepper motor usually have a number of field coils (phases) and a too the rotor. The step size of the motor is determined by the number of phases and the number of teeth on the rotor. Step size is the angular displacement of the rotor inone step.

If a stepper motor has 4 phases and 50 teeth, it takes 50×4=200 steps tomake one complete rotation. So step angle will be 360/200=1.8°.The stepper motor we are using has 4 poles and a 1/64 reduction gear mechanism for increasing torque. The step angle of the motor is 5.64°. But when considering the reduction gear, the step angle of the output shaft is 5.64/64°. The internal schematic of the stepper motor is given below.

The stepper motor is rotated by switching individual phases ON for a given timeone by one. The sequence is given in the graph below.

Circuit Diagram :

Circuit Discussion :

The circuit diagram for interfacing stepper motor to 8051 is shown above. P1.0,P1.1, P1.2 and P1.3 pins are used for controlling the phases A1, A2, A3 and A4 ofthe stepper motor respectively. ULN2003 is used for driving the individual phasesof the stepper motor. ULN2003 is a darlington transistor array used for driving highcurrent loads such as relays and motors. ULN2003 has 8 individual channels eachwith 1A capacity. The channels can be paralleled to increase the current capacity.Each channels are fitted with individual freewheeling diodes. The ULN2003 is op-erated in current sinking mode. Each channel is activated by giving a logic LOWat the corresponding input. For example if we make pin 1 of ULN2003 LOW, phaseA1 of the stepper motor gets switched ON.

Assembly Program : stepper_8051.asm

Code Discussion:

The program first clears P1.0 for activating phase 1 (A1) of the stepper motor. This condition is maintained for 65mS and then P1.0 is set for deactivating phase 1 of the motor. Then the same process is repeated for the port pins P1.1 to P1.3 and the entire cycle is repeated over and over to make the motor rotate in the clockwise direction. Timer 0 of the microcontroller is configured in Mode1 for producing the 65mS delay. 65mS is the width of each control pulse.