Tutorial 4.1: Tank Drive¶
Time: ~10 minutes Prerequisites: Tutorial 3: Python Basics
What is Tank Drive?¶
Tank drive is the simplest way to control a robot: each joystick controls one side of the drivetrain, just like driving a tank!
flowchart LR
subgraph Controller
LS1["Left Stick ↑"]
RS1["Right Stick ↑"]
LS2["Left Stick ↑"]
RS2["Right Stick ↓"]
LS3["Left Stick ↓"]
RS3["Right Stick ↑"]
end
subgraph Response1["Both Forward"]
LW1["Left Wheels ↑ Forward"]
RW1["Right Wheels ↑ Forward"]
end
subgraph Response2["Spin Left"]
LW2["Left Wheels ↑ Forward"]
RW2["Right Wheels ↓ Backward"]
end
subgraph Response3["Spin Right"]
LW3["Left Wheels ↓ Backward"]
RW3["Right Wheels ↑ Forward"]
end
LS1 --> LW1
RS1 --> RW1
LS2 --> LW2
RS2 --> RW2
LS3 --> LW3
RS3 --> RW3Understanding axis3 and axis2¶
These are the NAMES that VEX gave to joystick directions:
LEFT JOYSTICK RIGHT JOYSTICK
axis3 axis2
↑ ↑
| |
axis4 ←●→ axis4 axis1 ←●→ axis1
| |
↓ ↓
axis3 axis2
QUICK REFERENCE:
┌──────────────────────────────────────────────────┐
│ axis3 = Left stick, up-down movement (Y-axis) │
│ axis2 = Right stick, up-down movement (Y-axis) │
└──────────────────────────────────────────────────┘
Think of the axis numbers as addresses: - axis3 = "Left stick, up-down movement" - axis2 = "Right stick, up-down movement"
Memory Trick: "3-2" reads left-to-right, just like the sticks on the controller!
The Tank Drive Logic¶
The code is surprisingly simple:
while True:
# Read left joystick Y-axis (axis3)
left_speed = controller.axis3.position()
# Read right joystick Y-axis (axis2)
right_speed = controller.axis2.position()
# Send to motors
left_motors.spin(FORWARD, left_speed, PERCENT)
right_motors.spin(FORWARD, right_speed, PERCENT)
wait(20, MSEC)
That's it! The joystick value (-100 to +100) directly becomes the motor speed.
Wait, What Does FORWARD Mean Here?¶
Here's what might confuse you:
left_motors.spin(FORWARD, left_speed, PERCENT)
↑ ↑ ↑
| | |
| | Unit of speed (out of 100)
| The actual speed number
Direction reference point
FORWARD doesn't mean "go forward"! It means: - "Spin in the positive direction when speed is positive" - "Spin in the negative direction when speed is negative"
HOW IT WORKS:
┌──────────────────────────────────────────────────┐
│ If left_speed = +50: │
│ Motors spin FORWARD at 50% speed │
│ │
│ If left_speed = -50: │
│ Motors spin BACKWARD at 50% speed │
│ │
│ FORWARD just means "follow the number's sign" │
└──────────────────────────────────────────────────┘
PERCENT tells the motor what unit the speed is in: - 100 PERCENT = maximum speed - 50 PERCENT = half speed - 0 PERCENT = stopped
Code Walkthrough: driver_control.py¶
Let's look at the full implementation in src/driver_control.py:
def driver_control_loop():
"""
Main driver control loop using tank drive.
"""
brain.screen.clear_screen()
brain.screen.set_cursor(1, 1)
brain.screen.print("Driver Control Active")
while True:
# Get joystick positions (-100 to 100)
left_speed = controller.axis3.position() # Left joystick Y
right_speed = controller.axis2.position() # Right joystick Y
# Apply deadband to prevent motor drift
left_speed = deadband(left_speed, threshold=5)
right_speed = deadband(right_speed, threshold=5)
# Set motor velocities and spin
left_motors.set_velocity(left_speed, PERCENT)
right_motors.set_velocity(right_speed, PERCENT)
left_motors.spin(FORWARD)
right_motors.spin(FORWARD)
wait(20, MSEC)
Why Deadband?¶
Joysticks are never perfectly centered. There's always a tiny bit of drift:
Joystick at rest ideally returns 0, but in reality returns small values like 2, 3, or -1. Without deadband, motors creep slowly! With deadband, values under the threshold become 0.
Deadband Step-by-Step¶
Let's trace what happens with threshold=5:
flowchart TD
subgraph Example1["Example 1: Joystick returns 3"]
Input1["Input: 3"] --> Check1{"|3| < 5?"}
Check1 -->|"YES (3 < 5)"| Output1["Output: 0<br/>Motor stays still"]
endflowchart TD
subgraph Example2["Example 2: Joystick returns -50"]
Input2["Input: -50"] --> Check2{"|-50| < 5?"}
Check2 -->|"NO (50 >= 5)"| Output2["Output: -50<br/>Motor runs at 50% reverse"]
endflowchart TD
subgraph Example3["Example 3: Joystick returns 5"]
Input3["Input: 5"] --> Check3{"|5| < 5?"}
Check3 -->|"NO (5 = 5, not less than)"| Output3["Output: 5<br/>Motor runs at 5% forward"]
endGeneral Deadband Decision Flow:
flowchart TD
Start["Joystick Value"] --> AbsCheck{"|value| < threshold?"}
AbsCheck -->|YES| Zero["Return 0<br/>(Ignore drift)"]
AbsCheck -->|NO| Original["Return original value<br/>(Real movement)"]
Zero --> Motor1["Motor stays still"]
Original --> Motor2["Motor spins at given speed"]Movement Patterns¶
Forward¶
Backward¶
Turn Left (Pivot)¶
Left Stick: ↓ (-100) Left Motors: ↓ Backward
Right Stick: ↑ (+100) Right Motors: ↑ Forward
Robot pivots counter-clockwise!
Turn Right (Pivot)¶
Left Stick: ↑ (+100) Left Motors: ↑ Forward
Right Stick: ↓ (-100) Right Motors: ↓ Backward
Robot pivots clockwise!
Arc Turn (Gradual)¶
Left Stick: ↑ (+100) Left Motors: ↑ Forward (100%)
Right Stick: ↑ (+50) Right Motors: ↑ Forward (50%)
Robot arcs to the right!
Practice Pattern Dimensions¶
When practicing movement patterns, use these recommended sizes:
STRAIGHT LINE TEST:
●═══════════════════════════●
START END
Distance: 6 feet (~1.8 meters)
Goal: Drive without veering left or right
────────────────────────────────────────
PIVOT TEST:
↺
╱ ╲
│ ● │ Spin 360° in place
╲ ╱
↻
Goal: End facing the same direction you started
────────────────────────────────────────
ARC TURN TEST:
●═══════════════╗
START ║ Width: 3 feet (~90 cm)
╚═══════════════★ END
Goal: Smooth curve, no jerky corrections
Tank Drive Diagram¶
CONTROLLER ROBOT (top view)
+-----+ +--------+
| ↑ | axis3.position() → [LF][LB] (left side)
+-----+ | |
| |
+-----+ | |
| ↑ | axis2.position() → [RF][RB] (right side)
+-----+ +--------+
Each stick controls one side independently!
Pros and Cons of Tank Drive¶
Advantages¶
- Simple to understand - one stick = one side
- Easy to code - just read two axes
- Precise turning - independent control of each side
- Good for beginners - intuitive mapping
Disadvantages¶
- Requires both hands - can't drive one-handed
- Harder for curves - need to coordinate both sticks
- Can be jerky - small stick differences cause wobble
Exercise: Experiment with Deadband¶
Goal: Change the deadband threshold and observe the effect.
Step 1: Open src/driver_control.py
Step 2: Find these lines:
Step 3: Try different values:
- threshold=0 - No deadband (any small movement = motor movement)
- threshold=10 - Larger deadband (need to push stick further)
- threshold=20 - Very large deadband (significant push required)
Questions:
1. What happens with threshold=0? Does the robot drift?
2. What happens with threshold=20? Is it hard to make small movements?
3. What's the best balance for your driving style?