Robot won't work properly, claw will only go one way and will only open or close once per cycle

Question

We're students trying to make a clawbot for a Science Seminar class. However, for some reason, whenever we try to move the arm or the claw in a certain way, it will lock up and only move that direction. Code attached. Please help.

#pragma config(Motor, port1, frWheel, tmotornormal, openLoop, reversed) //Setting up the motors
#pragma config(Motor, port5, brWheel, tmotornormal, openLoop, reversed)
#pragma config(Motor, port3, flWheel, tmotornormal, openLoop)
#pragma config(Motor, port4, blWheel, tmotornormal, openLoop)
#pragma config(Motor, port10, Arm, tmotornormal, openLoop)
#pragma config(Motor, port6, Claw, tmotornormal, openLoop)

task main() 
{
    int a = 0; //Arm integer
    int c = 0; //Claw integer
    while(true) 
    {
        motor[frWheel] = vexRT(Ch2); //Wheels
        motor[brWheel] = vexRT(Ch2);
        motor[flWheel] = vexRT(Ch3);
        motor[blWheel] = vexRT(Ch3);
        if(a >= -30 && a <= 30)  {
            if(vexRT[Btn8D] == 1) //If arm down button pressed...
            {
                motor[Arm] = --a; //then arm will go down.
            }
            else if(vexRT[Btn8U] == 1)
            {
                motor[Arm] = ++a;
            }
            else(vexRT[Btn8U] == 0 && vexRT[Btn8D] == 0);
            {
                motor[Arm] = a;
            }
        }
        else
        {

        }

        if(c <= 30 && c >= -30)
        {
            if(vexRT[Btn7U] == 1) //If claw up button pressed...
            {
                motor[Claw] = ++c; //Claw will open.
            }
            else if(vexRT[Btn7D] == 1)
            {
                motor[Claw] = --c;
            }
            else(vexRT[Btn7D] == 0 && vexRT[Btn7U] == 0);
            {
                motor[Claw] = c;
            }
        }
        else
        {

        }
    }
}

Answer 1

Your conditional logic needs to be improved. Consider the following scenario:

• a is initialized to zero.
• The operator presses Btn8d (arm down)
• a is decremented to -1, then -2, etc.
• If the button continues to be pressed, a eventually reaches -30. Depending on the loop rate, this may happen very quickly.
• When a == 30 the statement if(a >= -30 && a <= 30) is still true.
• a is decremented to -31.
• Recovery is not possible because the conditional will never be true again.

Your code needs to handle this corner condition.

Also, when is the vexRT[] array updated? If it is not continuously updated with the values of the button presses, then you will need to update it at the top of your while loop.

Answer 2

There are two reasons reason for your problem:

1. The main loop is executed very fast - you only check conditionals and assign some values to variables. Total execution time will be probably in order of milliseconds. That means that when you press the button for even as short as 0.1s the loop will execute thousands of times. I would advice on checking the button states only every 50-200 milliseconds.

2. As already mentioned by SteveO, there is also an error in your logic. When variables a and c reach their maximum values, there is no way to reset them to their original state. I would do that like:

   if(buttonIsPressed()) {
     changeVariable();
   }
   if(variable > maxValue) {
     variable = maxValue;
   }