MjBots r4 and eagle power PID inner loop

Owned by Mazen Mohsen
Last updated: Oct 17, 2024
3 min read

Overview

This document presents a comprehensive analysis of the performance results obtained from testing the Moteus MJBots R4 actuator. The evaluation focuses on the motor's behavior under various load conditions, PID controller tuning, thermal management, and overall system stability. Detailed numerical data and observations are provided to support the conclusions drawn.

Introduction

The objective of this analysis is to evaluate the performance of the Moteus MJBots R4 actuator in various operational scenarios. The assessment includes motor smoothness, stiffness, torque handling, thermal behavior, and overall precision. The findings aim to optimize actuator settings for enhanced performance and reliability in high-torque applications.

System Configuration

Hardware Components

  • Actuator: Moteus MJBots R4

  • Load Requirements:

    • Continuous Torque: 1.5 Nm

    • Torque Spikes: 2.5 - 3 Nm

Control Parameters

  • PID Controller Settings:

    • Proportional Gain (Kp): 4- 150

    • Derivative Gain (Kd): 0.5 - 10

    • Integral Gain (Ki): Not utilized

Performance Results

No-Load Condition

Under no-load conditions, the actuator demonstrated exceptional smoothness and stiffness with the initial PID settings (Kp = 80, Kd = 0.5). The motor operated without noticeable vibrations or inconsistencies, indicating a stable control environment.

Loaded Condition

When subjected to a load requiring continuous torque of 1.5 Nm and experiencing torque spikes between 2.5 Nm and 3 Nm, adjustments to the derivative gain (Kd) were necessary. The initial Kd value of 0.5 resulted in significant overshoot, compromising system stability. Additionally, high kp value was needed to reach the maximum torque withing the defined value which is 4 NM. However, anything beyond 80 doesn’t really affect torque

PID Controller Tuning

Initial Settings

  • Kp: 80

  • Kd: 0.5

  • Ki: 0 (Not used)

These settings provided a baseline for assessing motor performance under no-load conditions.

Adjustments for Load Conditions

To address the high overshoot observed under load, the derivative gain (Kd) was increased to 7. This adjustment improved the system's response to torque spikes but introduced a new issue: excessive rattling of the motor in no-load scenarios.

Implementation of Kd Scaling

To balance the need for higher Kd under load without compromising no-load stability as using any value above 1 in Kd causes the motor to rattle extremly, a variable termed Kd Scale was discovered. The approach involves maintaining Kp at 80 and Kd at 0.5 during no-load conditions. When executing a trajectory that requires additional damping, the Kd Scale dynamically increases Kd to a predetermined value (7) , ensuring optimal performance across varying load conditions.

Feedforward Torque Application

In cases where extra stiffness is required, a Feedforward Torque variable is utilized. This method increases the overall torque without interfering with the PID loop. Notably, increasing Kp beyond 80 led to excessive overshoot; hence, feedforward torque serves as an effective alternative for augmenting torque while preserving system stability. However, this forces the motor to exert more torque than needed by the torque which might cause some mechanical instability

Exclusion of Ki

The integral gain (Ki) was deliberately omitted from the controller settings. Inclusion of Ki resulted in unpredictable system behavior and motor rattling. Additionally, documentation from MJBots recommends against using Ki in high-torque applications due to potential instability.

Thermal Management

Heat Generation Observations

Under continuous operation, the motor remained smooth, with heat generation primarily localized to the controller chip. Excessive temperature rise was identified as a secondary factor contributing to motor slip when Temperature derate value was reached.

Cooling Solutions

To mitigate thermal buildup and ensure prolonged chip operation, the following cooling solutions are proposed:

  • MJBots Heat Spreader Plate: An investment of €20 for the heat spreader plate will aid in distributing heat more evenly across the chip.

  • Heatsink Installation: Adding a heatsink will further enhance cooling efficiency, allowing for sustained high-torque operations without thermal-induced slip.

Conclusions

The Moteus MJBots R4 actuator exhibits superior performance characterized by precision, smoothness, and strength under optimized settings. Key findings include:

  • Smooth and Stiff Operation: Achieved with Kp = 80 and Kd = 0.5 under no-load conditions.

  • Enhanced Load Handling: By implementing Kd scaling and feedforward torque, the system effectively manages torque spikes up to 4 Nm without sacrificing stability. This happens by increasing Kd to 7 and using feedforward value of 0.5Nm if needed.

  • Thermal Efficiency: With appropriate cooling measures, the actuator can sustain prolonged high-torque operations without overheating. However, during normal testing, we should increase temperature derate value and use a fan directed at the motor.

Additionally, empirical evidence supports the theory that voltage does not significantly impact torque. The motor maintains consistent torque levels even at very low speeds, further validating the robustness of the control strategy.

Appendices

Video Documentation

  • Upcoming Videos: Videos demonstrating the actuator's performance and reprinted parts will be added to the Confluence page once available.