Introduction:

A DC motor (Direct Current motor) is the most common type of motor. Interfacing DC Motor to a microcontroller forms an essential part in designing embedded robotic projects. This article shows how to interface a small DC motor with any microcontroller. Before working with DC motors we will take a look at some basic techniques for controlling them. There are several types of DC motors, but here we will use a simple brushed DC motor.

Hardware Discussion : DC Motor

Fig: DC Motor

DC motors normally have just two leads, one positive and one negative. If you connect these two leads directly to a battery, the motor will rotate. If you switch the leads, the motor will rotate in the opposite direction.

Driving electromotors needs a high current. In addition, spinning direction and speed are two important parameters to be controlled. These requirements can be handled by using a microcontroller or similar. But there is a problem; Microcontrollers cannot provide enough current to run the motor and if you connect the motor to the microcontroller directly, you may damage the microcontroller. The perfect solution is to use a motor driver circuit in between the microcontroller and the DC motor.

DC Motor Features:

• Runs on DC power or AC line voltage with a rectifier.
• No need for a drive circuit when running at constant speed.
• Able to operate at high speeds (1,000 to 5,000 rpm.)
• High-efficiency design (60-75\% Approx).
• High starting torque.
• Responsive and easy to use as speed and torque can be controlled by voltage.
• Can be divided into brushed and brushless DC motors.

Controlling a DC Motor:

In order to have a complete control over DC motor, we have to control its speed and rotation direction. The DC motor speed can be controlled by applying varying DC voltage; whereas the direction of rotation of the motor can be changed by reversing the direction of current through it. This can be achieved by combining these two common techniques.

PWM (Pulse Width Modulation) – For controlling speed

It is a technique where average value of the input voltage is adjusted by sending a series of ON-OFF pulses.

The average voltage is proportional to the width of the pulses known as Duty Cycle. The higher the duty cycle, the greater the average voltage being applied to the dc motor(High Speed) and the lower the duty cycle, the less the average voltage being applied to the dc motor(Low Speed).

H-Bridge – For controlling rotation direction

An H bridge is an electronic circuit which allows the voltage to be flown in either direction. H-bridges are used in many different applications, one of the most common being to control motors in robots. It is called an H-bridge because it uses four transistors connected in such a way that the schematic diagram looks like an "H."

Closing two particular switches at the same time reverses the polarity of the voltage applied to the motor. This causes change in spinning direction of the motor.

You can use discrete transistors to make this circuit, but for this tutorial, we will be using the L293D H-Bridge IC. The L293D can control the speed and direction of DC motors and stepper motors and can control two motors simultaneously. Its current rating is 1.2A for each motor. At these currents, however, you will need to use heat sinks.

Motor Driver L293D

L293D Features:

• Can be used to run Two DC motors with the same IC.
• Wide Supply-Voltage Range: 4.5 V to 36 V.
• Separate Input-Logic Supply.
• Internal ESD Protection.
• High-Noise-Immunity Inputs.
• Constant Current 600 mA.
• Peak Output Current 1.2 A Per Channel.
• Output Clamp Diodes for Inductive Transient. Suppression (L293D)
• Automatic Thermal shutdown is available.
• Available in 16-pin DIP, TSSOP, SOIC packages.

Circuit Diagram :

Circuit Discussion :

The circuit shown below is of an 8051 based bi directional motor whose direction can be controlled using 2 push button switches. The circuit is very similar to the previous one except for the two push button switches . These pushbutton switches are interfaced to P0rt 3 of the microcontroller. Resistors R2 and R3 are the pull down resistors for P3.0 and 3.1 respectively.

The circuit shown below is of an 8051 based bi directional motor whose direction can be controlled using 2 push button switches. The circuit is very similar to the previous one except for the two push button switches . These pushbutton switches are interfaced to P0rt 3 of the microcontroller. Resistors R2 and R3 are the pull down resistors for P3.0 and 3.1 respectively.

Assembly Program : dc_motor_8051.asm

Code Discussion :

jump if not equal instruction. In simple words the CJNE instruction compares two operands and jump to a predefined LABEL if the operands are not equal. If the two operands are equal nothing happens and the next instruction is executed. Whenever push button S2 is pressed the status of P3 will be 00000001B .This status is moved to accumulator A and compared with 00000001B using the CJNE instruction. Both operands are equal means S2 is pressed and the next instruction (MOV P1,\#00000001B) which makes the motor run clockwise is executed. If the operands are not equal that means the S2 is not pressed and the controller jumps to LABEL1 which is to check the S3. To check S3, status of P3 is moved to A again and it is compared with 00000010B using the CJNE instruction. Both operands are equal means the S3 is pressed and the next instruction (MOV P1,\#00000010B) which makes the motor run anti clockwise is executed. Both operands are not equal means S3 is not pressed and the controller goes to check S2 again and this cycle is repeated.