Project 9: Robot Arm Controller

Build a multi-servo robot arm controller with joystick input, motion smoothing, and programmable sequences.

Quick Reference

Attribute	Value
Difficulty	Level 2: Intermediate
Time Estimate	1-2 weeks
Main Programming Language	MicroPython or C
Alternative Programming Languages	C++
Coolness Level	Level 3: Robotics
Business Potential	3. The “Automation” Tier
Prerequisites	PWM basics, ADC reading, mechanics awareness
Key Topics	Servo PWM, motion smoothing, kinematics, safety limits

1. Learning Objectives

By completing this project, you will:

Generate stable PWM signals for multiple servos.
Map joystick inputs to joint angles with dead zones.
Implement motion smoothing and rate limits.
Enforce joint limits to prevent mechanical damage.
Execute scripted motion sequences.

2. All Theory Needed (Per-Concept Breakdown)

2.1 Servo PWM Timing and Calibration

Fundamentals

Servos expect a 50 Hz control signal with pulse widths typically between 1.0 ms and 2.0 ms. The pulse width maps to a target angle. Generating stable pulses is critical; jitter causes twitching.

Deep Dive into the concept

Standard hobby servos interpret pulse width as position commands. A 1.0 ms pulse might correspond to 0 degrees, 1.5 ms to 90 degrees, and 2.0 ms to 180 degrees, but the exact mapping varies by servo. You must calibrate min/max pulses to avoid stalling or hitting mechanical stops. On RP2040, PWM slices can generate the 50 Hz base with microsecond resolution. If you drive many servos, you can either assign each to a PWM slice or schedule pulses in software. Hardware PWM is more stable, but limited in number of independent slices. If using software timing, you must avoid blocking. Servo power draw is significant; use an external supply and common ground. Always test with one servo before scaling to many.

How this fits on projects

PWM timing drives §3.2 requirements and §5.10 Phase 1.

Definitions & key terms

Pulse width -> duration of high signal
Neutral position -> mid pulse (usually 1.5 ms)
PWM slice -> RP2040 hardware PWM channel

Mental model diagram (ASCII)

50 Hz frame: |--1.5ms--|_________|

How it works (step-by-step)

Set PWM frequency to 50 Hz.
Set duty cycle to desired pulse width.
Update duty cycle when target angle changes.

Minimal concrete example

pwm_set_wrap(slice, 20000); // 20ms period
pwm_set_gpio_level(pin, pulse_us);

Common misconceptions

“All servos use the same pulse range” -> many require calibration.
“PWM duty is percent” -> servos care about microseconds, not percent.

Check-your-understanding questions

Why is 50 Hz used for hobby servos?
What happens if you send 2.5 ms pulses?
Why is power supply important?

Check-your-understanding answers

Servos expect a 20 ms frame by standard.
The servo may stall, overheat, or hit mechanical limits.
Servos draw high current and can brown out the MCU.

Real-world applications

Robot arms, RC vehicles, camera gimbals.

Where you’ll apply it

In this project: §3.2, §5.10 Phase 1.
Also used in: P01-blinky-on-steroids-multi-pattern-led-controller.md for PWM basics.

References

Servo datasheets and RC PWM standards

Key insights

Servo control is precise timing plus careful calibration.

Summary

Generate stable 50 Hz pulses and calibrate each servo’s range safely.

Homework/Exercises to practice the concept

Measure pulse widths for 0/90/180 degrees on your servo.
Determine the safe min/max pulse widths experimentally.

Solutions to the homework/exercises

Typical results: 1.0 ms, 1.5 ms, 2.0 ms.
Increase/decrease until the servo just reaches limits without strain.

2.2 Motion Smoothing and Rate Limiting

Fundamentals

Abrupt changes in position cause jerky motion and stress on servos. Motion smoothing limits the rate of change so the arm moves smoothly.

Deep Dive into the concept

Motion smoothing can be implemented as a simple rate limiter: cap the maximum change in angle per update. You can also use easing curves (e.g., cubic) for more natural motion. Smoothing requires a control loop that updates at a fixed interval (e.g., 20 ms) and steps toward the target by a fixed delta. This ensures consistent speed regardless of target distance. The rate limit should consider servo speed and load; too fast leads to overshoot, too slow feels sluggish. For sequences, you can interpolate between waypoints and compute per-step angles. This is a simple form of trajectory planning and is sufficient for small hobby arms.

How this fits on projects

Motion smoothing affects §3.2 and is crucial for §5.10 Phase 2.

Definitions & key terms

Rate limiting -> cap on change per step
Easing -> non-linear interpolation for smooth motion
Waypoint -> target pose in a motion sequence

Mental model diagram (ASCII)

Target angle: 0 -> 90
Steps: 0 10 20 30 40 50 60 70 80 90

How it works (step-by-step)

Read target angle from input or sequence.
Compute delta to current angle.
Clamp delta to max step.
Update current angle and PWM.

Minimal concrete example

if (target > current + step) current += step;
else if (target < current - step) current -= step;

Common misconceptions

“Servos handle any step” -> large jumps cause strain and jitter.
“Faster updates means smoother” -> too fast can overwhelm CPU.

Check-your-understanding questions

Why use a fixed update interval?
How do you choose a max step size?
What is the advantage of easing curves?

Check-your-understanding answers

It makes motion predictable and consistent.
Based on servo speed and desired smoothness.
They reduce jerk at start/stop.

Real-world applications

Robot arms, CNC motion, camera control.

Where you’ll apply it

In this project: §5.10 Phase 2.
Also used in: P07-dual-core-weather-station-true-parallel-processing.md for timing loops.

References

Motion control primers

Key insights

Smooth motion is a scheduling problem, not a hardware problem.

Summary

Rate limiting and fixed update loops create smooth, safe movement.

Homework/Exercises to practice the concept

Implement a 5-degree step limiter and test motion.
Compare linear vs easing curve movements.

Solutions to the homework/exercises

Motion becomes smoother but slower.
Easing reduces jerk at start and stop.

2.3 Kinematics and Joint Limits

Fundamentals

Kinematics maps joint angles to end-effector position. For simple arms, you may only need joint limits to prevent mechanical damage. Understanding limits keeps motion safe.

Deep Dive into the concept

For a 2-3 DOF arm, forward kinematics can be computed with basic trigonometry. However, many projects can ignore full kinematics and focus on safe angle ranges per joint. Each joint has a physical range; exceeding it can stall the servo or damage gears. Implement hard limits in software and optionally a soft zone that slows motion near the limit. For scripted motions, validate waypoints before execution. If using a joystick, map analog inputs to allowed angle ranges and apply a dead zone around center. When multiple joints move simultaneously, consider coordination so the end effector moves smoothly, even if you are not doing full inverse kinematics.

How this fits on projects

Joint limits and mapping are part of §3.2 requirements and §5.10 Phase 2.

Definitions & key terms

DOF -> degrees of freedom
Joint limit -> min/max angle allowed
Dead zone -> input range mapped to zero movement

Mental model diagram (ASCII)

Base: [0..180], Shoulder: [20..160], Elbow: [10..170]

How it works (step-by-step)

Define min/max angles per joint.
Map input to that range.
Clamp targets to limits.
Apply smoothing and command servos.

Minimal concrete example

if (target < min) target = min;
if (target > max) target = max;

Common misconceptions

“Servos stop themselves” -> they can stall and overheat.
“Dead zones are optional” -> small input noise causes jitter.

Check-your-understanding questions

Why clamp joint angles in software?
What is a dead zone used for?
Do you need inverse kinematics for this project?

Check-your-understanding answers

To prevent mechanical damage.
To prevent jitter around neutral input.
Not necessarily; direct mapping is fine for simple control.

Real-world applications

Robotic arms, animatronics, industrial automation.

Where you’ll apply it

In this project: §3.2, §5.10 Phase 2.
Also used in: P01-blinky-on-steroids-multi-pattern-led-controller.md for input mapping.

References

Intro robotics texts (kinematics basics)

Key insights

Safety limits are a required feature, not a nice-to-have.

Summary

Clamp inputs and respect mechanical limits to keep the arm safe.

Homework/Exercises to practice the concept

Define angle limits for each joint and test them.
Implement joystick dead zone mapping.

Solutions to the homework/exercises

Use smaller ranges than servo max to avoid stalling.
Map ADC values near center to no movement.

3. Project Specification

3.1 What You Will Build

A multi-servo robot arm controller that reads joystick input, smooths motion, enforces joint limits, and supports recorded motion sequences.

3.2 Functional Requirements

PWM generation: stable 50 Hz control for at least 3 servos.
Input mapping: joystick to angles with dead zones.
Motion smoothing: rate-limited updates.
Joint limits: clamp angles to safe ranges.
Sequence playback: execute a series of poses.

3.3 Non-Functional Requirements

Performance: no jitter at idle.
Reliability: no servo brownouts or MCU resets.
Usability: clear control modes and logs.

3.4 Example Usage / Output

[SERVO] base=90 shoulder=45 elbow=120
[MODE] joystick

3.5 Data Formats / Schemas / Protocols

Pose: base,shoulder,elbow (degrees)
Sequence: list of poses + dwell time

3.6 Edge Cases

Joystick disconnected -> hold last safe pose.
Servo stalls -> log warning and stop motion.
Out-of-range input -> clamp and warn.

3.7 Real World Outcome

A smooth-moving robot arm that responds predictably to user input.

3.7.1 How to Run (Copy/Paste)

cmake .. && make -j4
picotool load -f robot_arm.uf2

3.7.2 Golden Path Demo (Deterministic)

Move joystick to center -> arm holds neutral pose.
Move joystick to full right -> base rotates smoothly to max limit.

3.7.3 Failure Demo (Bad Input)

Scenario: joystick unplugged.
Expected result: log [WARN] input lost and freeze motion.

3.7.4 If CLI: exact terminal transcript

$ minicom -b 115200 -o -D /dev/ttyACM0
[SERVO] base=90 shoulder=45 elbow=120

4. Solution Architecture

4.1 High-Level Design

Joystick -> Mapping -> Smoothing -> PWM -> Servos
             ^
          Limits

4.2 Key Components

4.3 Data Structures (No Full Code)

typedef struct {
  float base, shoulder, elbow;
} pose_t;

4.4 Algorithm Overview

Key Algorithm: Smooth Angle Update

Read target angles from input.
Clamp to limits.
Step toward target with rate limit.
Update PWM.

Complexity Analysis:

Time: O(J) for J joints
Space: O(J)

5. Implementation Guide

5.1 Development Environment Setup

# Pico SDK or MicroPython

5.2 Project Structure

robot-arm/
├── firmware/
│   ├── main.c
│   ├── pwm_servo.c
│   ├── input.c
│   └── sequences.c
└── README.md

5.3 The Core Question You’re Answering

“How do you generate precise control signals that drive real mechanical systems?”

5.4 Concepts You Must Understand First

PWM servo timing
Motion smoothing
Joint limits and mapping

5.5 Questions to Guide Your Design

How many servos will you support?
What are safe limits for each joint?
How will you switch between manual and scripted mode?

5.6 Thinking Exercise

Derive a formula mapping ADC values (0-4095) to 0-180 degrees.

5.7 The Interview Questions They’ll Ask

How do servos interpret PWM signals?
Why is motion smoothing important?
How do you enforce joint limits?

5.8 Hints in Layers

Hint 1: Drive one servo at 50 Hz and sweep. Hint 2: Add joystick input. Hint 3: Add rate limiting and dead zone. Hint 4: Add sequences and safety limits.

5.9 Books That Will Help

5.10 Implementation Phases

Phase 1: Foundation (2-3 days)

Single servo control and calibration. Checkpoint: servo sweeps smoothly.

Phase 2: Core Functionality (3-5 days)

Multi-servo control + joystick mapping. Checkpoint: arm follows joystick input.

Phase 3: Motion & Sequences (2-4 days)

Add smoothing, limits, and sequences. Checkpoint: scripted sequence runs safely.

5.11 Key Implementation Decisions

6. Testing Strategy

6.1 Test Categories

6.2 Critical Test Cases

Neutral pose: joystick center -> neutral angles.
Limit clamp: input beyond range -> clamp to safe angle.
Sequence timing: dwell time accurate within 5%.

6.3 Test Data

ADC=0 -> angle=0
ADC=4095 -> angle=180

7. Common Pitfalls & Debugging

7.1 Frequent Mistakes

7.2 Debugging Strategies

Use a servo tester to verify hardware.
Log target vs actual angles.

7.3 Performance Traps

Too many serial prints can slow update loop.

8. Extensions & Challenges

8.1 Beginner Extensions

Add a gripper with open/close control.
Add a “home” button.

8.2 Intermediate Extensions

Record and playback motion sequences.
Add accelerometer input for gesture control.

8.3 Advanced Extensions

Implement inverse kinematics for end-effector control.
Add torque sensing for collision detection.

9. Real-World Connections

9.1 Industry Applications

Automation in small pick-and-place setups.
Education in robotics labs.

OpenManipulator references

9.3 Interview Relevance

PWM control and motion planning are common robotics topics.

10. Resources

10.1 Essential Reading

Servo control guides
RP2040 PWM documentation

10.2 Video Resources

Servo calibration tutorials

10.3 Tools & Documentation

Logic analyzer or oscilloscope for PWM verification

P01-blinky-on-steroids-multi-pattern-led-controller.md for PWM basics
P07-dual-core-weather-station-true-parallel-processing.md for timing loops

11. Self-Assessment Checklist

11.1 Understanding

I can explain PWM servo timing.
I can implement motion smoothing.
I can enforce joint limits.

11.2 Implementation

Arm moves smoothly without jitter.
Joystick mapping is stable.
Limits prevent mechanical strain.

11.3 Growth

I can add new joints and sequences.
I can explain trade-offs in motion planning.

12. Submission / Completion Criteria

Minimum Viable Completion:

Control at least 3 servos from joystick.

Full Completion:

Smooth motion with safety limits and sequences.

Excellence (Going Above & Beyond):

Inverse kinematics and collision detection.

13. Additional Content Rules

13.1 Determinism

Use fixed update interval (e.g., 20 ms). Log target and current angles for reproducible tests.

13.2 Outcome Completeness

Success demo: §3.7.2
Failure demo: §3.7.3
CLI exit codes: host joystick calibration tool returns 0 success, 2 device not found, 3 invalid calibration.

13.3 Cross-Linking

Concept references in §2.x and related projects in §10.4.

13.4 No Placeholder Text

All sections are fully specified for this project.