Calculating Wheel Movement for Accurate Turns
Overview:
 Two Types of Turns
 Calculating Rotations for Pivot Turns
 Equation for Calculating Pivot Turns
 Calculating Rotations for Point Turns
 Equation for Calculating Point Turns
 Relationships Between the Two Types of Turns
 Parting Thoughts
Two Types of Turns:
 Pivot Turn – robot turns about a central point located at one of the wheels
 One wheel goes forward, or reverse, while the other does not rotate
 Left wheel forward is a right turn and vice versa
 Left wheel reverse is a left turn and vice versa

 Point Turn – robot turns about a central point located midway between the wheels
 One wheel goes forward while the other goes in reverse
 Left wheel forward, right wheel reverse is a right turn
 Right wheel forward, left wheel reverse is a left turn
Calculating Rotations for Pivot Turns:
In this example we want a 180° right pivot turn
 Left wheel forward, right wheel stationary
 The left wheel will follow the blue circular path
 The diameter of this path = 2 times the width from wheel center to wheel center, or track width
 The circumference of a full circular path is: circumference = π x 2 x track width
 Since we are travelling only 180° (half) of the 360° that make up a circle, we multiply the circumference by 180°/360° (1/2) to get the length of the path the robot will travel
 The last step is to divide by the circumference of the wheel to get the rotations needed
Equation for Calculating Pivot Turns:
 For an n degree pivot turn
 Rot= n⁄360×((2×W_t×π))/C_w
 Where
 Rot = rotations
 n = degree of turn
 W_t = track width
 C_w = circumference of wheel
Calculating Rotations for Point Turns:
In this example we want a 180° right point turn:
 Left wheel forward, right wheel reverse
 The left wheel will follow the blue circular path, the right wheel will follow the red circular path
 The diameter of this path = the width from wheel center to wheel center, or track width
 The circumference of a full circular path is: circumference = π x track width
 Since we are travelling only 180° (half) of the 360° that make up a circle, we multiply the circumference by 180°/360° (1/2) to get the length of the path the robot will travel
 The last step is to divide by the circumference of the wheel to get the rotations needed
Equation for Calculating Pivot Turns:
 For an ‘n’ degree pivot turn
 Rot= n⁄360×((W_t×π))/C_w
 Where
 Rot = rotations
 n = degree of turn
 W_t = track width
 C_w = circumference of wheel
Code for Pivot Turn In Rotations:
Download Code: PivotTurn in ‘n’ Rotations.ev3s
Code for Pivot Turn In Degrees:
Download Code: PivotTurn in ‘n’ Degrees.ev3s
Relationships Between the Two Types of Turns:
Pivot Turns:
 Need twice as many rotations as a point turn
 Need more space
 More forgiving* of errors
Point Turns:
 Need half as many rotations as a pivot turn
 Need less space
 Less forgiving* of errors
*Forgiving meaning that an error in calculation is a smaller percentage of the total rotations needed.
Parting Thoughts:
 The process used for finding the path is the same process used to find the arc length of a circle in geometry, we just divide the arc length by the wheel circumference to find wheel rotations.
 The pivot turn does twice as many rotations as a point turn because only one wheel is doing the turning. In a point turn both wheels are turning so they each turn only half as many rotations as a pivot turn to turn the robot the same number of degrees.
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Post Tagged with Gyroscope, Gyro, Sensor, code, EV3G, Rotation, Calculate, Wheel, Movement, Accurate, Turns, degrees, Track Width, Point Turn, Pivot Turn
 By sparramc
 on Mar, 29, 2019
 Coding Concepts, Intermediate Coding
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