Help Hold Arm in up position - RobotMesh Python Cortex

Programming Support
#1

We are using the old cortex electronics in my 6th grade robotics class. With v5, we can easily add a hold to the arm motor to have it hold in the position when not being moved up or down.

With the old cortexes, I can’t figure out the command to get it to work.

In short, everything works fine but when I change the last command, everything stops working.

Only the arm commands:

if joystick.b5up():
  arm.run(80)
elif joystick.b5down():
  arm.run(-80)
else:
  arm.off()

That last line I think goes like this:
else:
arm.stop(vex.BrakeType.HOLD)

When I add that line, the entire program fails to move the robot.
Help!

#2

cortex motors did not have that capability

4 Likes
#3

Oh, that explains everything then. My kids are going to be frustrated when they find that out because my example was a v5 robot and it does have that capability.

#4

For an arm you can try blindly commanding something like 15 power. If you have a sensor on the arm you can do something smarter but people have done the “dumb” fixed value to hold the arm.

2 Likes
#5

Cortex motors are much, much simpler than V5 motors. V5 motors can sophisticated electronics, encoders, and voltage regulation inside of them that allow them to put placed into a brake mode and hold their own position. Cortex motors on the other hand do not have these (aside from the optional IME module for an encoder), and are simply just a set of wires directly connected to a motor which spins the output. You can either use PID control to maintain the arm at a particular position (complicated), or you can take the simpler route that many VRC teams of the era did for gravity-affected mechanisms such as arms - just apply a small upward power. For example, for lift arms with 393 motors, I would usually apply a small power of about 8-15 control value (out of [-127,127]) to counteract gravity and that worked fine enough for driver control. You should also consider using rubber bands or other mechanical methods to counteract gravity and give the arm a more neutral buoyancy.

4 Likes
#6

With Cortex you can use Optical Shaft Encoders (or a Potentiometer) to create a hold control.

I had to dig through some old files but I think the following is pretty close.

void max(int value, int maxValue){
	return maxValue > value ? maxValue : value;
}
void min(int value, int minValue){
	return minValue < value ? minValue : value;
}

int armTarget = 0;
long armPos = 0;

while(true)
{
	wait1Msec(10);

	if(vexRT[Btn6U]){
	  //Move Up
	  motor[leftArm] = motor[rightArm] = 60;
	  armTarget = SensorValue[armEncoder];
	}
	else if(vexRT[Btn6D]){
	  //Move Down
	  motor[leftArm] = motor[rightArm] = -60;
	  armTarget = SensorValue[armEncoder];
	} else 
	{
	//Hold Position
	  armPos = SensorValue[armEncoder];
	  if(abs(armTarget-armPos)> 10)
	  {
		//Apply power to motor to move back to position  
		motor[leftArm] = motor[rightArm] = max(min(armTarget-armPos, 20),120);
		armMoving = true;
	  }
	  else
	  {
		motor[leftArm] = motor[rightArm]= 0;
		armMoving = false;
	  }
	}
}
3 Likes
#7

Thank you both for great suggestions!

Very well thought out responses that help me immensely. Thank you thank you thank you!