Day 1: Robot Controller Programming

Learn to control your robot using a wireless controller. Today you’ll implement tank-style driving with smooth movement and variable speed control.

Table of Contents

  1. Learning Objectives
  2. Setup Your Development Environment
    1. Hardware Preparation
    2. Software Setup
  3. Understanding Tank Drive
    1. Tank Drive Principles
  4. Basic Tank Drive Implementation
    1. Step 1: Basic Controller Input
  5. Adding Speed Control
    1. Step 2: Reduce Maximum Speed
  6. Smooth Movement Control
    1. Step 3: Implement Movement Smoothing
    2. Optional: Add Debug Display
  7. Variable Speed System
    1. Step 4: Multi-Speed Control
    2. Complete Control Scheme
  8. Understanding the Code
    1. Key Programming Concepts
    2. Performance Considerations
  9. Next Steps

Learning Objectives

By the end of this lesson, you will be able to:

  • Implement tank-style robot control
  • Program smooth movement using interpolation
  • Add variable speed controls
  • Understand real-time control systems

Setup Your Development Environment

Hardware Preparation

  1. Get your controller and robot brain
  2. Turn both devices on
  3. Connect controller to brain (wait for connection indicator)

Software Setup

  1. Navigate to codeexp.vex.com
  2. Click File > New Text Project > Python > EXP Brain
  3. Connect the brain (wait for the green brain icon)

Understanding Tank Drive

Think About This: Imagine a tank driving forward. It doesn’t have a steering system like a car. How does it turn left?

Write your answer in your daily tracker before continuing.

Tank Drive Principles

  • Left stick controls left side motors
  • Right stick controls right side motors
  • Forward motion: Both sticks forward
  • Turning: One side faster than the other
  • Pivot turning: One side forward, one backward

Basic Tank Drive Implementation

Step 1: Basic Controller Input

  1. In VEX Code, add a Controller device
  2. Copy this code under # Begin Project Code:
while True:
    left = controller.axis3.position()
    right = controller.axis2.position()

    drivetrain.set_drive_velocity(left, right)
    wait(20)
  1. Download and test your robot

Checkpoint: Have your instructor watch you drive and get approval in your daily tracker.

Adding Speed Control

Step 2: Reduce Maximum Speed

  1. Modify your code to reduce speed by adding * 0.5:
while True:
    left = controller.axis3.position() * 0.5
    right = controller.axis2.position() * 0.5

    drivetrain.set_drive_velocity(left, right)
    wait(20)
  1. Download and test - notice how the reduced speed affects control

Smooth Movement Control

Step 3: Implement Movement Smoothing

Sudden speed changes can make robots hard to control. Let’s add smoothing:

# Begin Project Code

left_current = 0
right_current = 0
smoothing = 0.8

while True:
    left_target = controller.axis3.position() * 0.5
    right_target = controller.axis2.position() * 0.5

    left_current = left_current * smoothing + left_target * (1-smoothing)
    right_current = right_current * smoothing + right_target * (1-smoothing)

    # Set motor speeds using the smoothed values
    drivetrain.set_drive_velocity(left_current, right_current)
    wait(20)

Your Task: Complete the right_current calculation line following the pattern from left_current.

Optional: Add Debug Display

Add this code above wait(20) to see the smoothed values:

    brain.screen.clear_screen()
    brain.screen.print("Left: {:.1f}".format(left_current))
    brain.screen.print("Right: {:.1f}".format(right_current))

Experiment: Try different smoothing values between 0.0 and 1.0. What effect does this have on robot control? Record your observations.

Variable Speed System

Step 4: Multi-Speed Control

Add speed selection using controller buttons:

# Add this above left_target = ...
if controller.buttonR1.pressing():
    speed_mult = 1.0      # Fast speed
elif controller.buttonR2.pressing():
    speed_mult = 0.25     # Slow speed  
else:
    speed_mult = 0.5      # Normal speed

# Add this before setting drive velocity
left_current *= speed_mult
right_current *= speed_mult

Complete Control Scheme

Control Function
Left Stick Left side motors
Right Stick Right side motors
R1 Button Fast speed (100%)
R2 Button Slow speed (25%)
Default Normal speed (50%)

Final Checkpoint: Demonstrate all three speed modes to your instructor for approval.

Understanding the Code

Key Programming Concepts

  1. Real-time Loop: The while True loop runs continuously
  2. Interpolation: Smoothing creates gradual transitions
  3. State Management: Variables track current vs. target values
  4. Input Processing: Controller values become motor commands

Performance Considerations

  • Update Rate: wait(20) means 50 updates per second
  • Smoothing Factor: Higher values = more smoothing, slower response
  • Speed Multipliers: Allow fine control for different situations

Next Steps

This foundational control system will be essential for:

  • Autonomous navigation - Converting sensor input to movement
  • Competition strategy - Precise maneuvering in tight spaces
  • Advanced behaviors - Building complex robot actions

Tomorrow we’ll add autonomous features to make your robot play freeze tag independently!