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Power System Overview

Introduction

The power system is the "circulatory system" of a robot, responsible for reliably distributing stored electrical energy to every subsystem. A well-designed power architecture must simultaneously meet power demands, efficiency targets, safety constraints, and size/weight limitations.

Power Architecture Overview

graph TD
    BAT[Battery Pack] --> BMS[Battery Management System<br/>BMS]
    BMS --> MAIN[Main Power Bus<br/>24V/48V]

    MAIN --> DCDC1[DC-DC Buck<br/>24V → 12V]
    MAIN --> DCDC2[DC-DC Buck<br/>24V → 5V]
    MAIN --> DCDC3[DC-DC Buck<br/>24V → 3.3V]
    MAIN --> DRIVER[Motor Drivers]

    DCDC1 --> |12V Rail| MOTOR[Small Motors/Servos]
    DCDC1 --> |12V Rail| LIDAR[LiDAR]
    DCDC2 --> |5V Rail| SBC[Compute Board<br/>Jetson/RPi]
    DCDC2 --> |5V Rail| SENSOR[Sensors]
    DCDC2 --> |5V Rail| USB[USB Devices]
    DCDC3 --> |3.3V Rail| MCU[Microcontroller]
    DCDC3 --> |3.3V Rail| LOGIC[Logic Circuits]
    DRIVER --> |High Power| BLDC[Brushless Motors]

    style BAT fill:#f9a825
    style BMS fill:#ef5350
    style MAIN fill:#42a5f5

Voltage Rail Design

A robot system typically requires multiple voltage rails, each serving different types of loads:

Voltage Rail Typical Voltage Served Components Current Range Requirements
Logic level 3.3V MCU, sensor ICs, communication modules 0.1–2A Low noise, high stability
Sensor/USB 5V Cameras, IMU, USB devices, SBC 1–10A Moderate power, stable
Low-voltage actuators 12V Servos, small DC motors, LEDs 2–20A Moderate power
High-voltage actuators 24V/48V Large BLDC motors, hydraulic pumps 5–100A+ High power, high efficiency

Voltage Rail Selection Principles

  • 3.3V: Operating voltage for modern MCUs (STM32, ESP32) and most digital sensors
  • 5V: Standard voltage for Raspberry Pi and USB devices; many analog sensors also use 5V
  • 12V: Common voltage for RC servos and small-to-medium DC motors; industrial sensors (like LiDAR) often use 12V
  • 24V: Industrial standard voltage; BLDC motor drives for quadruped/humanoid robots commonly use 24V or higher

Power Distribution Topologies

Centralized Distribution

All DC-DC converters reside on a single centralized power board, with wire harnesses distributing power to subsystems:

  • Advantages: Simple design, easy debugging, centralized heat dissipation
  • Disadvantages: Long harnesses cause voltage drop, single point of failure risk
  • Suitable for: Small robots (desktop-scale, educational robots)

Distributed Distribution

Each subsystem module has its own local voltage regulation, drawing power only from the main bus:

  • Advantages: Reduced voltage drop, modular design, fault isolation
  • Disadvantages: Higher cost, more PCB area needed
  • Suitable for: Medium-to-large robots, industrial robots

Hybrid Distribution

Centralized provision of main voltage rails (e.g., 12V and 5V), with subsystems using LDOs for local regulation (e.g., 5V → 3.3V):

  • Advantages: Balances complexity and performance
  • Suitable for: The preferred approach for most robot projects

Power Budget Methodology

The power budget is the first step in power system design, determining battery capacity, DC-DC specifications, and thermal solutions.

Budget Process

graph LR
    A[List all subsystems] --> B[Determine rated<br/>power for each]
    B --> C[Calculate peak and<br/>average power]
    C --> D[Add safety margin<br/>20-30%]
    D --> E[Select battery capacity]
    E --> F[Verify runtime]
    F --> G[Iterate and optimize]
    G --> A

Power Calculation Basics

Total power is the sum of all subsystem power consumption:

\[P_{total} = \sum_{i=1}^{n} P_i = \sum_{i=1}^{n} V_i \times I_i\]

Accounting for DC-DC efficiency, the battery-side power consumption is:

\[P_{battery} = \sum_{i=1}^{n} \frac{P_i}{\eta_i}\]

Where \(\eta_i\) is the efficiency of the corresponding DC-DC converter (typically 85%–95%).

Typical Robot Power Consumption

Robot Type Typical Power Battery Capacity Runtime
Small wheeled (educational) 5–15W 2000–5000 mAh 1–3 h
Medium wheeled (ROS) 20–60W 10–20 Ah 1–3 h
Quadruped robot 100–500W 500–2000 Wh 0.5–2 h
Humanoid robot 200–2000W 1–10 kWh 0.5–2 h
Drone 100–1000W 1000–10000 mAh 10–40 min

Power Protection Strategy

Protection Layers

  1. Battery-level protection: BMS overcharge/overdischarge/overcurrent protection
  2. Bus-level protection: Main fuse / eFuse
  3. Branch-level protection: Per-branch current limiting / fuses
  4. Chip-level protection: LDO/DC-DC built-in OCP (overcurrent protection)

Reverse Polarity Protection

  • Diode method: Series Schottky diode (simple but has voltage drop \(V_d \approx 0.3V\))
  • P-MOSFET method: Low on-resistance (\(R_{DS(on)} < 50m\Omega\)), recommended approach
  • Ideal diode IC: Such as LTC4357, combines low voltage drop with protection

Surge Protection

  • TVS diode: Transient voltage suppression
  • Soft-start circuit: Limits inrush current at power-on
  • Input filter: LC filter to suppress EMI

Key Design Metrics

Metric Description Typical Requirement
Efficiency Overall efficiency from battery to load >80%
Ripple DC-DC output voltage ripple <50 mV (logic), <100 mV (motors)
Transient response Voltage recovery under load transients <5% droop, <100 us recovery
EMI Electromagnetic interference level Meet CE/FCC standards
Temperature range Power system operating temperature -20°C to 60°C

Chapter Structure

This chapter provides an in-depth discussion of the various aspects of robot power systems:

  • Battery Technology: Characteristics and selection of various battery types
  • Power Management Circuits: BMS and DC-DC converter design
  • Power Budget and Runtime: System power analysis and optimization
  • Charging and Safety: Charging solutions and safety design

References

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