Slew Constants

note

Most robots will work perfectly fine without slew.  You can visit this page if you start to see the undesired behavior described below.  

What is it?

Slew is when your motors start at a lower speed and build up to a faster speed.  This can be useful if your robot is prone to tipping, can wheelie easily, or is prone to wheel slip at the start of motions.  

EZ-Template treats the output of slew as a maximum speed the PID can output. The output of slew does not get set to the motors. If slew is enabled for an extremely small motion, it is very likely that slew will not do anything and your motion will still be as fast as possible.

Tuning Slew

There are 2 parameters to tune with slew in EZ-Template; the distance to slew for and starting speed.  The robot will accelerate very quickly with a low distance to slew for, regardless of the starting speed.  

There are 2 parameters to tune in EZ-Template.

• Distance to slew for
  • The larger this number is, the longer it will take to reach your maximum speed. Making this number smaller will cause the robot to accelerate quickly.
• Starting speed
  • The smaller this number is, the slower the robot will start motions. Making this number smaller will cause the robot to accelerate slowly.

There is a push and pull balance between these two variables and you have to find the middle ground.

I will tune slew by running normal motions but having the robot go slower.  The goal is to find the fastest the robot can go without any of the undesirable behaviors described above.  To enable slew you need the true parameter at the end of your x_pid_set().  

void tuning_slew() {
  chassis.drive_pid_set(24_in, 80, true);
  chassis.pid_wait();
}

After you've found this number, you can ensure this line is in your default_constants() function in src/autons.cpp.  This should be near the top.  You can set the second number, speed, to the number you've found in the test above.  

void default_constants() {
  // . . .
  chassis.slew_drive_constants_set(7_in, 80);
  // . . .
}

Now you can lower the distance until the robot is able to successfully reach DRIVE_SPEED without undesirable behavior!  

void tuning_slew() {
  chassis.drive_pid_set(24_in, DRIVE_SPEED, true);
  chassis.pid_wait();
}

Driving

Sometimes robots aren't symmetrical and can be "tippier" in one direction than the other.  This can be solved by choosing one of the following functions to place in default_constants().  

chassis.slew_drive_constants_set(7_in, 80);  // This sets forwards and backwards constants
chassis.slew_drive_constants_forward_set(7_in, 80);
chassis.slew_drive_constants_backward_set(7_in, 80);

Turning

This is the simplest one.  Turning should be symmetrical on your robot, so this is the only function to tune with.  

chassis.slew_turn_constants_set(5_deg, 50);

Swinging

Swinging is the most complex.  Because of the introduction of arcs, EZ-Template now supports either inches or degrees for tuning slew.  You can also have independent forward and backward constants.  it doesn't matter if you decide to use inches or degrees for tuning, you can change it later.  

Using degrees, these are your options for setting slew constants.

chassis.slew_swing_constants_set(5_deg, 50);  // This sets forwards and backwards constants
chassis.slew_swing_constants_forward_set(5_deg, 50);
chassis.slew_swing_constants_backward_set(5_deg, 50);

Using inches, these are your options for setting slew constants.

chassis.slew_drive_constants_set(7_in, 80);  // This sets forwards and backwards constants
chassis.slew_drive_constants_forward_set(7_in, 80);
chassis.slew_drive_constants_backward_set(7_in, 80);