====== Bidirectional motor control with Arduino ======
Most Arduino motor shields and the MsMot minishield from Didel use pins 4 to 7 for controlling two motors, with  PWM outputs only on pins 5 and 6. Motor's drivers inputs are connected to pins 4,5 and 6,7.

| {{:motor:arduino_motor1.png?200|Arduino with Motor Shield}} | //Examples of Arduino Boards with Motor driver shields.// | {{:motor:arduino_motor2.png?200|Freeduino with MsMot Shield}} |

===== Driving the motor =====

Let's have a motor or [[http://shop.boxtec.ch/ultrabright-redgreen-color-leds-diffused-p-40758.html|bicolor LED]] connected to pins 4 and 5. Pin 4 is set a LOW and pin 5 receive an 8-bit PWM value ( //analogWrite(value)// ). 

Now if one wish to reverse the rotation, one need to put a HIGH on pin 4 and set the PWM value on pin 5 to be the complement of what we need. 
That is, if one need a low speed, a high "analog" value is required, so the difference with the HIGH of the other pin is small. The complement,  255-value , must be used as shown in next picture.


| {{:motor:arduino_motor_diagram1.png?330|25% of PWM forward and backward}} |{{:motor:arduino_motor_diagram2.png?330|Accelerate and slow down, forward and backward}} |
| 25% of PWM forward and backward | Accelerate and slow down, forward and backward |

===== How to drive a robot with two motors =====
You have a robot with two motors? Make sure you understand how to control the motors with digitalWrite (on all pins) and then with analogWrite (only on pins 5 and 6). Wire you motors and define the pins with the following lines: \\
<code c>
  #define BackLeft   4 
  #define ForwLeft   5
  #define ForwRight  6
  #define BackRight  7
</code>
Define several functions to make it move: \\
Forward, Backward, TurnLeft, TurnRight with only one parameter. Turn left is turn on itself here. It could be turn on one wheel – test.  PWM value, 0..255 must be sent on pins 5 and 6, with the good state on pins 4 and 7, depending on direction.

<code c>
void Forward (int ss)  // 0-255
{
  analogWrite(ForwLeft, ss);
  digitalWrite(ForwLeft, LOW);
  analogWrite(ForwRight, ss);
  digitalWrite(RecD, LOW);
}

void Backward (int ss)
{
  analogWrite(ForwLeft, 255-ss);
  digitalWrite(RecG, HIGH);
  analogWrite(ForwRight,  255-ss);
  digitalWrite(RecD, HIGH); 
}

void TurnRight (int ss) 
{
  analogWrite(ForwLeft, ss);
  digitalWrite(RecG, LOW);
  analogWrite(ForwRight, 255-ss);
  digitalWrite(RecD, HIGH); 
}
</code>

This is not the best way to do it though. 

There are too many functions, and one cannot make smooth  turns. Lets define a single function,  Move(). We let the function have two parameters, the positive and negative speed for the two motors. Stop is Move (0,0), full speed backward is Move (-255,-255), etc.

| //Move(left,right)// function has two parameters of type int 16 bits signed. Valid values are between –255 and +255. \\ The function test with an if  statement if speed is positive or negative. Positive is easy. Local variable for left speed is named ls. \\ \\ For a negative speed, ls is negative, one need first to take the absolute value, -ls (-ls is positive now)  and then we need to take the complement since the other side of the motor must be set at high level. \\Value is hence 255-(-ls) = 255+ls. \\ \\ What happens if one tries  //Move(500,-400);// ? \\ This will be accepted, but a speed higher than 255 will be replaced by its modulo 256 value. A solution could be to saturate the received values at the beginnng of the procedure:<code>  if (rs>255) rs= 255;    
  if (rs<-255) rs= -255;</code> Same for ls. | <code>// pwm between –255 et +255 
void Move (int ls, int rs)  {
  if (ls > 0)
  {
    analogWrite(ForwLeft, ls);
    digitalWrite(RecG, LOW);
  }
  else 
  {
    analogWrite(ForwLeft, 255+ls);
    digitalWrite(RecG, HIGH);
  }
  if (rs > 0)
  {
    analogWrite(ForwRight, rs);
    digitalWrite(RecD, LOW);
  }
  else 
  {
    analogWrite(ForwRight, 255+rs);
    digitalWrite(RecD, HIGH);
  }
}</code> |


It is easy now to write a ballet for your robot:
<code c>
Move(200,-200); 
delay (100);      // turn right for 0.1s
Move(0,0);
delay (500);     // stop 0.5s

etc.
</code>

The loop  to accelerate, slowdown and come back is also quite simple.  Add the definitions, the set-up and the function Move() and test it. 

You can use a [[http://shop.boxtec.ch/ultrabright-redgreen-color-leds-diffused-p-40758.html|bicolor LED]] in place of a real motor for testing.

The speed of change depends on the delay. Here PWM is modified by one every 8 ms. Acceleration phase lasts 256 x 8 = 2048 ms, about 2 seconds.
<code c>
int v; 
void loop()   
{
  for (v=0;   v<255;  v++) { Move (v,v); delay (8); }
  for (v=255; v>-255; v--) { Move (v,v); delay (8); }
  for (v=-255; v<=0;  v++) { Move (v,v); delay (8); }
  delay (2000) ;
}
</code>

Have fun - JDN.

----
This Howto has been kindly provided by JDN from [[http://didel.com|Didel SA]].