Whenever a robotics hobbyist talk about making a robot, the first thing comes to his mind is making the robot move on the ground. And there are always two options in front of the designer whether to use a DC motor or a stepper motor. When it comes to speed, weight, size, cost... DC motors are always preferred to stepper motors. There are many things which you can do with your DC motor when interfaced with a microcontroller. For example you can control the speed of motor, you can control the direction of rotation, you can also do encoding of the rotation made by DC motor i.e. keeping track of how many turns are made by your motors etc. So you can see DC motors are no less than a stepper motor.
In this part of tutorial we will learn to interfacing a DC motor with a microcontroller. Usually H-bridge is preferred way of interfacing a DC motor. These days many IC manufacturers have H-bridge motor drivers available in the market like L293D is most used H-Bridge driver IC. H-bridge can also be made with the help of transistors and MOSFETs etc. rather of being cheap, they only increase the size of the design board, which is sometimes not required so using a small 16 pin IC is preferred for this purpose.
Working Theory of H-Bridge
The name "H-Bridge" is derived from the actual shape of the switching circuit which control the motion of the motor. It is also known as "Full Bridge". Basically there are four switching elements in the H-Bridge as shown in the figure below.
- basic-bridge.gif (11.63 KiB) Viewed 6392 times
- current-fwd.gif (18.88 KiB) Viewed 6392 times
- ttdcm.JPG (17.01 KiB) Viewed 6392 times
Now you might be thinking why i did not discuss the cases like High side left on and Low side left on or high side right on and low side right on. Clearly seen in the diagram, you don't want to burn your power supply by shorting them. So that is why those combinations are not discussed in the truth table.
So we have seen that using simple switching elements we can make our own H-Bridge, or other option we have is using an IC based H-bridge driver. Both of them are discussed in the next section of the tutorial.
BJT H-Bridge
A simple H-bridge can be made with the help of Power BJTs like TIP31 and TIP32. An example and a working demo of this circuit is shown in the figure below.
- bjt-hbridge.gif (11.42 KiB) Viewed 6392 times
- bjtdemo.gif (48.13 KiB) Viewed 6392 times
L293D is a dual H-Bridge motor driver, So with one IC we can interface two DC motors which can be controlled in both clockwise and counter clockwise direction and if you have motor with fix direction of motion the you can make use of all the four I/Os to connect up to four DC motors. L293D has output current of 600mA and peak output current of 1.2A per channel. Moreover for protection of circuit from back EMF ouput diodes are included within the IC. The output supply (VCC2) has a wide range from 4.5V to 36V, which has made L293D a best choice for DC motor driver.
A simple schematic for interfacing a DC motor using L293D is shown below.
- l293d.gif (9.48 KiB) Viewed 6391 times
As per the truth mentioned in the image above its fairly simple to program the microcontroller. Its also clear from the truth table of BJT circuit and L293D the programming will be same for both of them, just keeping in mind the allowed combinations of A and B. We will discuss about programming in C as well as assembly for running motor with the help of a microcontroller.
Programming Microcontroller
ASM Code
Code: Select all
L293D_A equ P2.0 ;L293D A - Positive of Motor
L293D_B equ P2.1 ;L293D B - Negative of Motor
L293D_E equ P2.2 ;L293D E - Enable pin of IC
org 0H
Main:
acall rotate_f ;Rotate motor forward
acall delay ;Let the motor rotate
acall break ;Stop the motor
acall delay ;Wait for some time
acall rotate_b ;Rotate motor backward
acall delay ;Let the motor rotate
acall break ;Stop the motor
acall delay ;Wait for some time
sjmp Main ;Do this in loop
rotate_f:
setb L293D_A ;Make Positive of motor 1
clr L293D_B ;Make negative of motor 0
setb L293D_E ;Enable to run the motor
ret ;Return from routine
rotate_b:
clr L293D_A ;Make positive of motor 0
setb L293D_B ;Make negative of motor 1
setb L293D_E ;Enable to run the motor
ret ;Return from routine
break:
clr L293D_A ;Make Positive of motor 0
clr L293D_B ;Make negative of motor 0
clr L293D_E ;Disable the o/p
ret ;Return from routine
delay: ;Some Delay
mov r7,#20H
back: mov r6,#FFH
back1: mov r5,#FFH
here: djnz r5, here
djnz r6, back1
djnz r7, back
retCode: Select all
#include <AT89X51.H>#define L293D_A P2_0 //Positive of motor
#define L293D_B P2_1 //Negative of motor
#define L293D_E P2_2 //Enable of L293D
// Function Prototypes
void rotate_f(void); //Forward run funtion
void rotate_b(void); //Backward run function
void breaks(void); //Motor stop function
void delay(void); //Some delay
void main(){ //Our main function
while(1){ //Infinite loop
rotate_f(); //Run forward
delay(); //Some delay
breaks(); //Stop
delay(); //Some delay
rotate_b(); //Run Backwards
delay(); //Some delay
breaks(); //Stop
delay(); //Some delay
} //Do this infinitely
}
void rotate_f(){
L293D_A = 1; //Make positive of motor 1
L293D_B = 0; //Make negative of motor 0
L293D_E = 1; //Enable L293D
}
void rotate_b(){
L293D_A = 0; //Make positive of motor 0
L293D_B = 1; //Make negative of motor 1
L293D_E = 1; //Enable L293D
}
void breaks(){
L293D_A = 0; //Make positive of motor 0
L293D_B = 0; //Make negative of motor 0
L293D_E = 0; //Disable L293D
}
void delay(){ //Some delay...
unsigned char i,j,k;
for(i=0;i<0x20;i++)
for(j=0;j<255;j++)
for(k=0;k<255;k++);
}