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Noob PLTW teacher with programming question 
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Joined: Mon Nov 12, 2012 1:17 pm
Posts: 2
Post Noob PLTW teacher with programming question
Let me apologize immediately if I placed this is the wrong forum. I didn't intend to.

I am a new PLTW teacher, and we do not have the VEX in the curriculum yet. However, I am involved in a robot competition, and I need to help with programming a space elevator winch. :-)

I want to have the winch climb with the following commands and then brake. If I delete the else statement, the buttons work as described, although the motor is on until I give a different command. I would like the motor to stop when no command is given. Later, I might have to add a percent of motor to compensate for added weight and free fall.

When I give a 0 or a 10 in the else statement, the motor chudders during any of the other commands. I realize this is a simple issue to many of you, but if you could help me with my limited knowledge, I would greatly appreciate it.

#pragma config(Motor, port2, winch, tmotorVex393, openLoop)
#pragma config(Motor, port3, rotatearm, tmotorVex269, openLoop)
#pragma config(Motor, port4, armextend, tmotorVex269, openLoop)
#pragma config(Motor, port5, claw, tmotorServoStandard, openLoop)
#pragma config(Motor, port6, clawtilt, tmotorServoStandard, openLoop)
//*!!Code automatically generated by 'ROBOTC' configuration wizard !!*//

task main()
{
while (1==1)
{
{
if (vexRT[Btn5U]==1) // button 5U pressed
motor[winch]=127; // climb high speed
if (vexRT[Btn5D]==1) // button 5D pressed
motor[winch]=-40; // decend high speed
if (vexRT[Btn7U]==1) // button 7U pressed
motor[winch]=63; // climb low speed
if (vexRT[Btn7D]==1) // button 7D pressed
motor[winch]=-10; // decend low speed
else (motor[winch]=0);} // this turns the motor off

{motor[armextend]=vexRT[Ch3]/3;} //extends/retracts arm using joystick

{if (vexRT[Btn6U]==1) //button 6U pressed
motor[claw]=127; //claw clamp
if (vexRT[Btn6D]==1) //button 6D pressed
motor[claw]=-127;} //claw unclamp


{if (vexRT[Btn8U]==1) //button 8U pressed
motor[clawtilt]=127; //claw tiltup
if (vexRT(Btn8D)==1) //button 8D pressed
motor[clawtilt]=-127;} //claw tiltdown
}

}


Mon Nov 12, 2012 1:36 pm
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Posts: 496
Post Re: Noob PLTW teacher with programming question
OK, lets take a look at some pseudo code, and I think you'll realize the mistake right away. If my program says:

Code:
while(true){
    if (up button is pressed) { go up }
    else { stop }
    if (down button is pressed) { go down }
    else { stop }
}


Lets walk though what will happen when I press the up button line by line:

up button is pressed, so motor goes up;
skips else since up button is pressed;
down button is not pressed, so does not go down;
down button is not pressed, so the else statement stops the motor;
repeat;

Do you see what is happening here? you're giving the motor conflicting commands for stop and go rapidly, so it starts stuttering. One way to implement the behavior you want is as follows:

Code:
while(true){
    if (up button is pressed) { go up }
    else if (down button is pressed) { go down }
    else { stop }
}


Does that clear things up?

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Mon Nov 12, 2012 2:10 pm
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Joined: Mon Nov 12, 2012 1:17 pm
Posts: 2
Post Re: Noob PLTW teacher with programming question
Thanks

It helped a lot.

Also, is there a way to use the analog stick on the controller but set a lower range? We have delicate work to do in areas, we need less of a range, but the control of the stick.


Mon Nov 12, 2012 7:15 pm
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Post Re: Noob PLTW teacher with programming question
There are a few ways to go about this. One of the simplest is to divide the input. For example, using vexRT[Ch2]/2 will halve your input range.

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Mon Nov 12, 2012 9:29 pm
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Joined: Sun Oct 21, 2012 10:01 pm
Posts: 76
Post Re: Noob PLTW teacher with programming question
We don't use VEX at school (I'm used to programming for the NXT in RobotC), but we set up a system such that when a button is pressed on the controller, we divide input by a number before feeding it to a motor. Here's a pseudocode example.

Code:
float scaleJoystick = 1.0;
float joystickValue = 0;
while (true)
{
   if (buttonPressed() == kPressed)
      scaleJoystick = 0.5;  // When button is pressed, divide motor power by two. 
   else
      scaleJoystick = 1.0;

   joystickValue = scaleJoystick * readJoystick();
   motor = joystickValue;
}


Or something along those lines.

EDIT: You can actually write it in fewer lines using RobotC's ternary operator...
Code:
motor = readJoystick() * (buttonPressed() == kPressed ? 0.5 : 1.0);


Wed Nov 14, 2012 2:13 am
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