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Servos and Joysticks 
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Joined: Thu Nov 19, 2009 1:21 pm
Posts: 11
Post Servos and Joysticks
Hello Folks!
We are a new team and trying to get the hang of RobotC. We are doing okay with the motor work, but having trouble getting going with the servo controls.
Our problem is this: we can get the servors to move when we push a button, but only a full range trip.

if(joy1Btn(3) ==1)
servoTarget[Arm] = 180;
servoChangeRate[Arm] = 1;
if(joy1Btn(4) == 1)
servo[Claw] = 198;
if(joy1Btn(2) == 1)
servo[Claw] = 140;

We want to control our servos with a change rate, so instead of the arm going all the way down, it goes down 10 every time we push it.
Does that make sense? I have found lots of autonomous programming info online, but is there a good FTC teleop resource for programming a controller?

Thu Nov 19, 2009 2:43 pm
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Post Re: Servos and Joysticks
Moved to more appropriate forum.


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Thu Nov 19, 2009 2:47 pm
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Joined: Mon Oct 27, 2008 9:59 pm
Posts: 139
Post Re: Servos and Joysticks
Based on your post, I assume that you want to move the servo a small amount on each press of a button which would not require you to interact with the changeRate which controls how quickly the servo moves to a new position.

The basic approach I've seen to this is to set the servo target to the current value plus an incremental step.
For example:

//max value we want the servo to move
const int ARM_MAX_RANGE = 198;
//min value we want the servo to move
const int ARM_MIN_RANGE = 0;
//how many degrees to move at a time
const int ARM_STEP_SIZE = 10;

if(joy1Btn(3)) //button 3 to move up
   if(servoValue[Arm] < (ARM_MAX_RANGE - ARM_STEP_SIZE))
      //move arm up by one step
      servoTarget[Arm] = servoValue[Arm] + ARM_STEP_SIZE;
      //if step would exceed max; set to max
      servoTarget[Arm] =ARM_MAX_RANGE;   
else if(joy1Btn(4)) //button 4 to move down
   if(servoValue[Arm] > (ARM_MIN_RANGE + ARM_STEP_SIZE))
      //move arm up by one step
      servoTarget[Arm] = servoValue[Arm] - ARM_STEP_SIZE;
      //if step would exceed min; set to min
      servoTarget[Arm] =ARM_MIN_RANGE;

Please note that I have not tested the code above.
Also note that care is taken to avoid moving the servo beyond the intended range. Another thing you will have to contend with is holding the button down may shoot through the loop and end up instantly setting you to your min or max targets. You may have to either use a variable to control moving in and out of the if statements or move the code into another task with a delay.

Fri Nov 20, 2009 1:02 pm
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