Joystick deadband calibration centering work-around

[jgraber]

Re-quoting this from tail of the 269 vs 393 discussion, to put in its own thread.
This code is a work-around for the common problem of Vexnet Joysticks -
The imperfect centering of analog joysticks.
Common symptoms of this problem:

• Motors hum when they should be stopped and quiet.
• Motors slowly turn at startup when they should be stopped.
• Jiggling the stick is required to stop the motors.
• Large deadband settings required to stop the motors without jiggling the stick.

Overview:
Record current position of joystick as “center” when some button is pressed.
When reading joystick values, subtract off the recorded “center” value.

These example EasyCv4 code chunks also demonstrate a coding style that uses predefined Constants like ‘Joy1=1’ to make the code more self-documenting.
EG: GetJoystickAnalog(1,2); does that mean joystick1, stick2, or joy2, stick1?
New: GetJoystickAnalog( JOY1, STK_2); // much more clear

Also demonstrated is RCScaling code, which is easier to understand visually if you copy the math to a spread sheet and plot the shape of the output vs input, for different parameters.

@jgraber

This part detects a centering request, and saves the current position.

 Bcentered = GetJoystickDigital( JOY1, CHN_5, UP ); // request for recenter
 if ( Bcentered > 0 ) { // any shoulder buttons mean recenter joystick 
    StickCenter[STK_1] = GetJoystickAnalog( JOY1, STK_1 ) ; // current is Center 
    StickCenter[STK_2] = GetJoystickAnalog( JOY1, STK_2 ) ;  
    StickCenter[STK_3] = GetJoystickAnalog( JOY1, STK_3 ) ; 
    StickCenter[STK_4] = GetJoystickAnalog( JOY1, STK_4 ) ;  } 

This part is ScaleRC, to get a parabolic or other power function scaled value from joystick, including recentering feature, and deadband.

 int Scale_RC1 ( unsigned char StickNum, int Deadband, int MinPower, double Exponent )
{ //  input^exponent if greater than deadband( 1, 15, 20, 3) 
int StickIn; float SIa; int MaxPower; 
MaxPower = 127 - MinPower ; 
StickIn  = GetJoystickAnalog( JOY1 , StickNum ) ; 
StickIn = StickIn - StickCenter[StickNum] ; // recenter!!! 
if ( Abs(StickIn) <= Deadband ) {    return 0 ; }
SIa = Pow ( Fabs(StickIn)/MaxPower , Exponent ) ; 
SIa = SIa * MaxPower + MinPower ;
if ( SIa > 127 ) { SIa = 127 ; }  // limit check,  only positive values at this point 
if ( StickIn > 0 ) {    return SIa ;  } else {      return -SIa ; }
} // end Scale_RC

I’m using these input parameters to get scaled values to pass to usercoded arcade drive function; arm motor is set directly from ArmSpeed;

// var              stick,   dead, min, scale
Stick1   = Scale_RC( STK_1,  5,    9, 1.3 ) ; // direction 
Stick2   = Scale_RC( STK_2,  2,   15, 1.5 ) ; // speed 
ArmSpeed = Scale_RC( STK_3,  5,   30, 1.7 ) ; // 

Here is an example of Macro/constant Main.h showing use of STK_1=1, to make it easier to understand a long list of numeric parameters.
Also the arraydef for StickCenter; It took me too long to figure out that definition should include [5] to get subscripts [0…4]; This is a typical C rookie error.

#define UP  2   //JoystickDigitaltoMotor UP = 2
#define STK_1   1   //  
#define STK_2   2   // these values are used as subscripts to StickCenter]
#define STK_3   3   
#define JOY1    1  
extern int StickCenter[5] ; // note subscripts are [0]..[4], [0] is unused

There you have it.

Potential Bugs/pitfalls:

• Doesn’t work for EasyC provided JoysticktoMotor or Arcade/Tank/Holo functions, so you’ll have to write your own; (thats a feature!)
• Didn’t write it as a wrapper eg: myGetJoystickAnalog(), just provided this example
• Doesn’t include limit check, so center-corrected values can be out of (-127,+127) range. (that would be good to put into a wrapper)
• Not as good as “real” joystick calibration method, which also sets min,max limits correctly, (but this is simpler to trigger in the middle of a game)
• Pressing “center” button when joysticks are deliberately NOT centered may have unexpected consequences when the joysticks become centered again.

Let me know if you have questions on the code.