Controlling Fan Speed with Temperature-Based Logic in Embedded Systems

Mar 24, 2025 - ⧖ 4 min

In embedded systems, managing thermal performance is often critical for maintaining reliability and efficiency. One common strategy involves controlling the speed of cooling fans based on real-time temperature readings. This post explores a Rust module built by me, fan-speed-control, designed to handle this task effectively.

Overview

The fan-speed-control module is a lightweight, no_std library written in Rust. It provides a configurable controller for managing fan speed based on temperature thresholds. It is a Implementation of a linear fan speed controller.

Key Features

  1. Configurable Parameters: The module allows users to define minimum and maximum fan speeds, along with temperature thresholds for turning the fan on, off, and reaching full speed.
  2. Temperature-Based Control: Fan speed is adjusted linearly based on temperature readings between defined thresholds.
  3. Spin-Up Logic: The controller includes a spin-up phase where the fan runs at maximum speed for a specified duration to overcome initial inertia.

fan-speed-graph

Module Structure

The module consists of two main components: FanSpeedControlConfig and FanSpeedControl.

FanSpeedControlConfig

This struct holds the configuration parameters for the fan speed controller:

#[derive(Debug)]
pub struct FanSpeedControlConfig {
    pub min_speed: u32,
    pub max_speed: u32,
    pub temp_on: f32,
    pub temp_off: f32,
    pub temp_full: f32,
    pub spin_up_duration: embedded_time::duration::Seconds,
}
  • min_speed: Minimum fan speed.
  • max_speed: Maximum fan speed.
  • temp_on: Temperature to turn on the fan.
  • temp_off: Temperature to turn off the fan.
  • temp_full: Temperature for full fan speed.
  • spin_up_duration: Duration to spin up the fan at maximum fan speed in seconds.

Pro Tip: You can use pwm duty cycles for min_speed and max_speed

FanSpeedControl

This struct represents the fan speed controller and includes methods for stepping the controller state machine with temperature readings and current time.

Operation Modes

  1. Off Mode: The fan is stopped. When the temperature rises above temp_on, the controller transitions to SpinUp mode.
  2. SpinUp Mode: The fan runs at maximum speed for spin_up_duration seconds to overcome initial inertia. After this period, it transitions to Ramp mode.
  3. Ramp Mode: The fan speed varies linearly with temperature between min_speed at temp_on and max_speed at temp_full. If the temperature falls below temp_off, it returns to Off mode.
---
title: Fan controller states
---
stateDiagram-v2
    [*] --> Off

    Off --> SpinUp : Temperature is above on threshold
    SpinUp --> Ramp : Spin up time has passed
    Ramp --> Off : Temperature is below off threshold

The Math of Ramp mode

The linear speed algorithm employed in the fan-speed-control module is a straightforward yet effective method for adjusting fan speed based on temperature readings. This algorithm operates in the Ramp mode, where the fan speed increases linearly from a minimum speed at the temperature threshold for turning the fan on (temp_on) to a maximum speed at the temperature threshold for full speed (temp_full).

The algorithm calculates the fan speed using a linear equation of the form speed = m * temperature + b, where m is the slope and b is the intercept. Both m and b are calculated by the specified FanSpeedControlConfig:

let m = (max_speed - min_speed) / (temp_full - temp_on);
let b = min_speed - (m * temp_on);

This approach ensures a smooth and proportional response to temperature changes.

This linear approach provides a predictable and gradual adjustment of fan speed, helping to maintain stable thermal conditions without abrupt changes.

Example Usage

Here’s a simple example of how to use the FanSpeedControl:

use fan_speed_control::{FanSpeedControlConfig, FanSpeedControl};
use embedded_time::{duration::Seconds, Instant, Clock};
use std_embedded_time::StandardClock;

let config = FanSpeedControlConfig {
    min_speed: 20,
    max_speed: 100,
    temp_on: 40.0,
    temp_off: 35.0,
    temp_full: 60.0,
    spin_up_duration: Seconds(2),
};
let mut controller = FanSpeedControl::new(config);
let clock = StandardClock::default();
let speed = controller.step(45.0, clock.try_now().unwrap());

Conclusion

The fan-speed-control module provides a robust and flexible solution for controlling fan speed in embedded systems. Whether you're working on a microcontroller project or a larger embedded system, this module can help you maintain optimal cooling performance.

For more details, you can explore the full source code and contribute to its development.