Joystick control mixing for control of 2 DC motors

[DTB Radio]

Here is the project: I'm building a life-sized RC boat using a pair of fixed-mount trolling motors. My current control scheme is like a tank-bot, IE one POT for each motor to control forward-reverse. It works just fine, but I want to move to a joystick.

My problem is that I have no clue where to start with the formulas for mixing the two control axis and deriving proper PWM and fwd/rvs control. I've spent hours on other forums and on Google, and haven't found what I need. Most of what I keep finding only refers to reading and interpreting RC pulses, not the actual mixing of the POT data. I've posted in a few other forums over the past day or so, and haven't found any help. I'm using a pair of PICAXE 08M chips, one for each motor, and I can easily write the code once I understand the formulas.

Here is a short break-down of the results I want for various joystick positions:

1. Y axis full forward, X axis centered = both motors full pseed fwd
2. Y axis full reverse, X axis centered = "       "         "      "    rvs
3. X axis full left, Y axis centered = left motor full rvs, right motor full fwd
4. X axis full right, Y axis centered = "   "        "   fwd,   "      "     "    rvs
5. X axis full right, Y axis full forward = left motor full fwd, right motor stopped
6. X axis full right, Y axis full reverse = left motor stopped, right motor full reverse

If you need any more detail, let me know.

[DTB Radio]

Here's a bit more detail of what I'm looking at:

Lets say that the joystick is kept at its maximums in the following examples. At 90 degrees, I want full ahead both motors. At 112.5 degrees, I want half speed forward on left motor, full on right. At 135 degrees, I want full stop on left motor, full speed forward on right. At 157.5 degrees, I want half speed reverse on left, full speed forward on right. At 180 degrees, I want full speed reverse on left, full speed forward on right. At 202.5 degrees, I want full speed reverse on left, and half speed forward on right. At 225 degrees, I want full speed reverse on left, full stop on right. At 247.5 degrees, I want full speed reverse on left, half speed reverse on right. At 270 degrees, I want full speed reverse on both motors. Etc, etc, etc. Keep in mind that I'm also looking for fully variable speeds at positions less than maximums.

I'm just not quite able to wrap my math-weak brain around how to come up with the formulas needed, and so far, I've not found quite what I need in my searching. If there's a simple formula or formulas that I can use, that would be great, but so far, its looking like I'm going to need to work out multiple nested if-then structures to process this. I do understand that it will boil down to more-or-less simple adding and subtracting, followed by scaling for the PWM data. If someone can simply show me the formulas I need based on a 0-255 range of POT values, I can come up with the code from there.

For now, I'm working on breaking down the control into 4 major quadrants (if-then structures) , then breaking those down into two further divisions (nested if-then structures), and working from there. I'm not sure if that's the best approach, but its the only idea I've been able to come up with so far.

[sdk32285]

 Hi
So you want some math similiar to below to determine the speeds. And then you want some if statements to determine how to assign speed1, speed2, or averageSpeed to each motor.

so:
speed1 = (axis1in - axis1ZeroPoint) * (maxSpeed / (axis1rangeOfJoystick / 2))
speed2 = (axis2in - axis2ZeroPoint) * (maxSpeed / (axis2rangeOfJoystick / 2))

averageSpeed= (speed1 + speed2) / 2

example:
if Y=forward and X=center
motor1=speed1
motor2=speed1

[DTB Radio]

That's a big help, thank you!

[DTB Radio]

Ok, I've finally managed to get a result that is satisfactory. I ended up breaking down and buying a PICAXE 28X2 chip to simplify the design, and allow PWM for two motors with a single chip.

Here is the code that worked for me. If anyone sees something that can be shortened or done better, please feel free to post the code you come up with.



    symbol throttle = b0
    symbol steering = b1
    symbol lmotor = b2
    symbol rmotor = b3
    symbol lmode = b.0
    symbol rmode = b.1

    double_speed:
    'setfreq m8

    main:
    readadc 0,throttle
    readadc 1,steering

    allstop:
    if throttle > 123 and throttle < 132 and steering > 123 and steering < 132 then
    rmotor = 0
    lmotor = 0
    low rmode
    low lmode
    goto sendpwm
    end if

    straight:
    if steering > 123 and steering < 132 then
    if throttle > 127 then
    throttle = throttle - 127
    low lmode
    low lmode
    lmotor = throttle
    rmotor = throttle
    goto sendpwm
    end if
    if throttle < 127 then
    throttle = 127 - throttle
    high lmode
    high rmode
    lmotor = throttle
    rmotor = throttle
    goto sendpwm
    end if
    end if

    rotate:
    if throttle > 123 and throttle < 132 then
    if steering > 127 then
    lmotor = steering - 127
    rmotor = steering - 127
    high rmode
    low lmode
    goto sendpwm
    end if
    if steering < 127 then
    rmotor = 127 - steering
    lmotor = 127 - steering
    high lmode
    low rmode
    end if
    end if

    rightfwd:
    if steering > 127 and throttle > 127 then
    steering = steering - 127
    throttle = throttle - 127
    lmotor = throttle + steering max 127
    low lmode
    rmotor = throttle - steering
    if rmotor > 127 then
    rmotor = rmotor - 127
    rmotor = 128 - rmotor
    high rmode
    else
    low rmode
    end if

    goto sendpwm
    end if

    leftfwd:
    if steering < 127 and throttle > 127 then
    steering = 127 - steering
    throttle = throttle - 127
    rmotor = throttle + steering
    lmotor = throttle - steering
    low rmode
    if lmotor > 127 then
    lmotor = lmotor - 127
    lmotor = 128 - lmotor
    high lmode
    else
    low lmode
    end if
    goto sendpwm
    end if

    rightrev:
    if steering > 127 and throttle < 127 then
    throttle = 127 - throttle
    lmotor = steering - throttle
    if lmotor > 127 then
    lmotor = lmotor - 127
    low lmode
    else
    high lmode
    lmotor = 127 - lmotor
    end if
    rmotor = steering - 127 + throttle max 127
    high rmode
    goto sendpwm
    end if

    leftrev:
    if steering < 127 and throttle < 127 then
    throttle = 127 - throttle
    steering = 127 - steering
    lmotor = steering + throttle max 127
    rmotor = steering - throttle
    rmotor = 127 - rmotor
    if rmotor > 127 then
    high rmode
    rmotor = rmotor - 127
    else
    low rmode
    rmotor = 127 - rmotor
    end if
    high lmode
    goto sendpwm
    end if

    sendpwm:
    pwmout c.1, 31, lmotor
    pwmout c.2, 31, rmotor
    goto main

[momomo68]

Would anything like this be useful?
http://www.robotmarketplace.com/products/RL-IMX1.html
http://www.robotmarketplace.com/products/0-SABER2X5-RC.html

[BANE]

make sure you include some deadzone.  If it is a cheap controller or the PS2 controller you might need as much as +/-10 for deadzone.  At least thats what i found to be working