Performance Tuning Guide

This guide covers advanced performance tuning techniques for the RCR Common Robotics Platform, focusing on motor calibration for straight driving and PID tuning for optimal dynamic response.

Table of Contents

  1. Motor Calibration for Straight Driving

  2. PID Tuning for Dynamic Response

  3. Serial Commands for Real-time Tuning

  4. Troubleshooting Common Issues

Motor Calibration for Straight Driving

Overview

Motor calibration ensures that both wheels rotate at the same speed when given identical commands, enabling the robot to drive straight. This is critical for accurate navigation and odometry.

Prerequisites

  • Robot hardware assembled and functional

  • closed_loop.ino firmware from the add_tuning_and_odom branch uploaded to the microcontroller (see Firmware Updates for upload procedure)

  • Serial monitor access (115200 baud) (see Common Issues for debug output setup)

  • Level surface for testing

  • Measuring tape or ruler

Calibration Process

1. Initial Setup

See Keyboard Teleoperation Setup.

2. Basic Motor Test

# Enable motor drive (You may need to send the M character multiple times to enable the motors)
M

# Test individual motors
A0.5    # Left motor at 0.5 m/s
Z0.5    # Right motor at 0.5 m/s

# Stop motors
M

3. Straight Line Test

  1. Mark Starting Position

    • Place robot on level surface

    • Mark starting position with tape

    • Ensure robot is facing straight ahead

  2. Run Straight Test

    # Reset odometry
X

# Drive forward at moderate speed
↑ (Up Arrow)

  3. Measure Deviation

    • Measure how far the robot deviated from straight line

    • Record the deviation distance and direction

4. Coarse-tuning Parameters

Motor Scale Adjustment (Recommended First)

# If robot drifts left, increase right motor scale
N1.1     # Increase right motor scale to 1.1

# If robot drifts right, increase left motor scale
B1.1     # Increase left motor scale to 1.1

PID Tuning for Dynamic Response

Overview

PID (Proportional-Integral-Derivative) controllers regulate motor speed to match target velocities. Proper tuning ensures:

  • Fast response to speed changes

  • Minimal overshoot

  • Steady-state accuracy

  • Stability under load

PID Parameters

Proportional (P) Gain

  • Effect: Immediate response to error

  • Too High: Oscillations, instability

  • Too Low: Slow response, steady-state error

  • Default: 8.0

Integral (I) Gain

  • Effect: Eliminates steady-state error

  • Too High: Oscillations, windup

  • Too Low: Persistent error

  • Default: 0.8

Derivative (D) Gain

  • Effect: Damping, reduces overshoot

  • Too High: Noise sensitivity, instability

  • Too Low: Overshoot, oscillations

  • Default: 0.0 (disabled)

Tuning Methodology

1. Start with P-Only Control

# Reset PID parameters
X

# Set P gain only
Q5.0     # Left motor P=5.0
U5.0     # Right motor P=5.0
W0.0     # Disable left integral
O0.0     # Disable right integral
E0.0     # Disable left derivative
T0.0     # Disable right derivative

2. Test Response

# Test step response
S0.5     # Test both motors at 0.5 m/s

Observe:

  • Rise time (time to reach target)

  • Overshoot (maximum error)

  • Steady-state error

  • Oscillations

3. Adjust P Gain

If too slow/steady-state error:

Q6.0     # Increase left P gain
U6.0     # Increase right P gain

If oscillating:

Q4.0     # Decrease left P gain
U4.0     # Decrease right P gain

4. Add Integral Control

Once P gain is stable:

W0.5     # Add small left I gain
O0.5     # Add small right I gain

Test for:

  • Elimination of steady-state error

  • No oscillations

  • Smooth response

5. Add Derivative Control (Optional)

For systems with overshoot:

E0.1     # Small left D gain
T0.1     # Small right D gain

Monitor for:

  • Reduced overshoot

  • No noise amplification

  • Stable response

Advanced Tuning Techniques

1. Ziegler-Nichols Method

  1. Find Ultimate Gain (Ku)

    • Start with P=1.0, I=0, D=0

    • Increase P until sustained oscillations

    • Record Ku and oscillation period Tu

  2. Calculate Parameters

    • P = 0.6 × Ku

    • I = 2 × P / Tu

    • D = P × Tu / 8

2. Load-Dependent Tuning

Test under different load conditions:

# Light load (no payload)
F1.0

# Heavy load (with payload)
F1.0

Adjust gains for consistent performance across load ranges.

3. Speed-Dependent Tuning

Test at different speeds:

# Low speed
F0.2

# Medium speed
F0.5

# High speed
F1.0

Performance Metrics

1. Response Time

  • Target: < 0.5 seconds to reach 95% of target speed

  • Measurement: Monitor speed feedback via serial output

2. Steady-State Error

  • Target: < 2% of target speed

  • Measurement: Compare target vs actual speed

3. Overshoot

  • Target: < 10% of target speed

  • Measurement: Peak speed during step response

4. Settling Time

  • Target: < 1 second to settle within 5% of target

  • Measurement: Time to reach and stay within error band

Serial Commands for Real-time Tuning

The closed_loop.ino firmware provides serial commands for real-time parameter adjustment:

Basic Commands

Command

Description

Example

M

Toggle motor drive on/off

M

X

Reset all state

X

H

Show help with all commands

H

Motor Testing Commands

Command

Description

Example

S<speed>

Test both motors at same speed

S0.5

A<speed>

Test left motor only

A0.5

Z<speed>

Test right motor only

Z0.5

Motor Scale Commands

Command

Description

Example

B<scale>

Set left motor scale (0.5-2.0)

B1.1

N<scale>

Set right motor scale (0.5-2.0)

N0.9

PID Tuning Commands

Command

Description

Example

Q<value>

Set left motor PID P

Q8.0

W<value>

Set left motor PID I

W0.8

E<value>

Set left motor PID D

E0.1

U<value>

Set right motor PID P

U8.0

O<value>

Set right motor PID I

O0.8

T<value>

Set right motor PID D

T0.1

Parameter Management Commands

Command

Description

Example

Y

Save tuning to EEPROM

Y

G

Load tuning from EEPROM

G

C

Reset tuning to defaults

C

Monitoring Commands

Enable debug output to monitor performance:

// In the setup.h header for closed_loop.ino, set debug levels:
#define PRINT_MOVES 2    // Enable status updates
#define PRINT_PID 1      // Enable PID debug output

Adjust the maximum allowed speed

// In the motor control header MotorControl.h, set the maximum PWM allowed:
static constexpr int MAX_PWM = 120; // Limit PWM magnitude (0-255) on or around line 46

Recompile the firmware and upload after changing PRINT_MOVES or MAX_PWM (see Firmware Updates for procedure).

Troubleshooting Common Issues

1. Robot Won’t Drive Straight

Symptoms:

  • Consistent drift in one direction

  • Uneven wheel speeds

Solutions:

  • Verify encoder connections

  • Calibrate individual motor PWM scaling factors

  • Check for mechanical binding

2. Oscillations/Instability

Symptoms:

  • Motor speed oscillates around target

  • Robot shakes or vibrates

Solutions:

  • Reduce P gain

  • Add derivative control

  • Check for mechanical issues

  • Verify encoder noise

3. Slow Response

Symptoms:

  • Takes too long to reach target speed

  • Sluggish acceleration

Solutions:

  • Increase P gain

  • Add integral control

  • Check for mechanical friction

  • Verify power supply

4. Steady-State Error

Symptoms:

  • Robot doesn’t reach target speed

  • Persistent speed error

Solutions:

  • Increase I gain

  • Check for load variations

  • Verify target speed calculations

  • Check for mechanical binding

5. Noise Sensitivity

Symptoms:

  • Erratic behavior with small changes

  • Sensitive to disturbances

Solutions:

  • Reduce D gain

  • Add filtering

  • Check encoder connections

  • Improve mechanical isolation

6. Motor Stall

Symptoms:

  • High-pitched whining or squealing sound from motors

  • Motors not responding to commands

  • Can cause motors to heat up due to current not producing useful work

Solutions:

  • P gain too high causing oscillations, reduce P gain

  • Mechanical binding or obstruction, inspect wheels

  • Insufficient power supply, check battery voltage, charge if necessary

  • Inadequate PWM scaling, adjust calibration