## Intro: Joystick to Differential Drive (Python)

As I’m working on a Raspberry Pi Zero W based robot, doing my first tests on Python programming, I needed to port the function created previously in this article for Arduino (c++).

This time though, I added the capability to provide the limits for the input and the output on any range you need it. I’ll explain why in the next steps.

## Step 1: Joystick Input

Let’s suppose you have an input that is like a joystick, so the x, and y values provided by the joystick will vary between -1 and 1, as can be seen in the diagram.

As can be seen, the Joystick send us a 1,1 on the top right corner, a -1, -1 on the bottom, left, 0, 0 in the center, and so on.

This code will convert those x, y values to the corresponding speeds you need to apply to each wheel of your differential driven robot.

## Step 2: Differential Drive Output

As said, we need to convert the pair of input values, to something we can use to drive a Differential drive (aka tank drive) robot. You can check in the image the main idea of what we need to achieve.

## Step 3: Source Code

def joystickToDiff(x, y, minJoystick, maxJoystick, minSpeed, maxSpeed):<br> # If x and y are 0, then there is not much to calculate... if x == 0 and y == 0: return (0, 0) # First Compute the angle in deg # First hypotenuse z = math.sqrt(x * x + y * y) # angle in radians rad = math.acos(math.fabs(x) / z) # and in degrees angle = rad * 180 / math.pi # Now angle indicates the measure of turn # Along a straight line, with an angle o, the turn co-efficient is same # this applies for angles between 0-90, with angle 0 the coeff is -1 # with angle 45, the co-efficient is 0 and with angle 90, it is 1 tcoeff = -1 + (angle / 90) * 2 turn = tcoeff * math.fabs(math.fabs(y) - math.fabs(x)) turn = round(turn * 100, 0) / 100 # And max of y or x is the movement mov = max(math.fabs(y), math.fabs(x)) # First and third quadrant if (x >= 0 and y >= 0) or (x < 0 and y < 0): rawLeft = mov rawRight = turn else: rawRight = mov rawLeft = turn # Reverse polarity if y < 0: rawLeft = 0 - rawLeft rawRight = 0 - rawRight # minJoystick, maxJoystick, minSpeed, maxSpeed # Map the values onto the defined rang rightOut = map(rawRight, minJoystick, maxJoystick, minSpeed, maxSpeed) leftOut = map(rawLeft, minJoystick, maxJoystick, minSpeed, maxSpeed) return (rightOut, leftOut)

def map(v, in_min, in_max, out_min, out_max): # Check that the value is at least in_min if v < in_min: v = in_min # Check that the value is at most in_max if v > in_max: v = in_max return (v - in_min) * (out_max - out_min) // (in_max - in_min) + out_min

I added as many comments as I could, trying to make everything clear.

So, what are the differences with the Arduino version?

**The name**

Just because I think this is more accurate to the purpose of the function

**It returns a tuple**

So you just get the right and left speeds on it

**The joystick and wheel speed limits:**

This change is the most important. In reality, I´m not using a joystick to control the robot, but my *cellphone´s accelerometer*, so, thanks to this parameters, I can reconfigure the limits of the input. In case of a joystick might be -1 to 1, but in case of the accelerometer, it goes from -9.8 to 9.8 (the gravity)

Also, I wanted the output to be configurable with the minSpeed, maxSpeed parameters. This allows me to just get the range of values I need for my servo controller (in my case -4095 to 4095, but you can set whatever speed range you want there, and the output will be scaled accordingly)

### What about that map function?

If you are an Arduino programmer, you might know it already, since I port it from there. It is used to map an input value from the input range limits, to the output range limits. As I´m not aware of it existing in python, I just added to my utils functions. The only difference with the Arduino version, is that I added a constraint to the value, so it cannot be smaller or bigger than the input range specified.

I hope this makes sense, and if it doesn’t, please let me know. I promise I’ll do my best to make it more clear. You can find more about this article and the rest of the robot in my blog at savagemakers.com

## Discussions

1 year ago

Thanks for sharing :)