PID Control in HVAC Systems with Modbus Actuators and ESP32

Fundamentals of PID Control in Climate Systems

In control engineering, maintaining a stable room temperature requires much more than simply turning equipment on and off when things get warm. This is where the PID algorithm comes in, standing for Proportional, Integral, and Derivative. In practice, this method works like an experienced driver: looking at the distance to a turn, calculating how long they have been off-speed, and anticipating curves to adjust the gas pedal smoothly. In climate systems, this translates to avoiding energy waste and ensuring thermal comfort without sudden temperature swings.

The Proportional term acts in the present, applying a correction directly proportional to the difference between current and target temperatures. The Integral term looks at the past, summing accumulated errors over time to eliminate persistent offsets caused by external factors like sunlight hitting a window. Finally, the Derivative term anticipates the future, evaluating the rate of change in temperature to brake the actuation before the system overshoots the setpoint. Combined, these three calculations transform an oscillating system into a precise and stable mechanism.

The Role of the ESP32 in Processing and Networking

The ESP32 is a low-cost microcontroller widely used in Internet of Things projects due to its versatility and dual-core processing power. In this project, it acts as the brain of the HVAC system, executing the PID loop in real time and handling decision logic. While one core manages network connectivity for data transmission and remote monitoring via web servers or MQTT, the other is dedicated exclusively to sensor reading and control command calculations.

Beyond computational power, pin flexibility and hardware serial ports facilitate integration with other devices. Instead of relying solely on simple relays that only switch a compressor on or off, the ESP32 can send precise numerical commands. This allows it to command the exact opening of a chilled water valve or the rotation speed of a fan, drastically increasing the energy efficiency of the air conditioning system.

Industrial Communication with Modbus and RS-485

To connect the microcontroller to heavy actuators in the climate system, such as compressor frequency drives or flow meters, the Modbus RTU protocol over the RS-485 physical layer is used. Modbus is a standardized, robust industrial language that enables message exchange between different manufacturers simply and directly. In practice, the ESP32 acts as the network master, periodically polling device states and sending new operational parameters.

Choosing the RS-485 interface is crucial in this scenario due to its immunity to electromagnetic interference. Climate control environments contain powerful electric motors, compressors, and long cables capable of corrupting data in common connections like USB or standard serial. The RS-485 standard uses differential signaling over twisted pairs, meaning the signal is interpreted by the voltage difference between two wires, neutralizing picked-up noise and guaranteeing flawless communication.

Putting the control loop into code requires attention to sampling times, as the PID algorithm must run at strictly constant intervals so mathematical derivative calculations do not get distorted. In the ESP32 development environment, we use the Arduino library to structure the logic for reading temperature sensors, processing errors, and sending the Modbus packet. Below is an essential snippet demonstrating serial communication initialization and reading an external Modbus register.

#include <ModbusMaster.h>

ModbusMaster node;
const int RX_PIN = 16;
const int TX_PIN = 17;

void setup() {
  Serial2.begin(9600, SERIAL_8N1, RX_PIN, TX_PIN);
  node.begin(1, Serial2);
  pinMode(2, OUTPUT);
}

void loop() {
  uint8_t result;
  result = node.readHoldingRegisters(0x0000, 1);
  if (result == node.ku8MBSuccess) {
    uint16_t temperaturaLida = node.getResponseBuffer(0);
    // Processing read value through PID
  }
  delay(1000);
}

In this code snippet, we configure the secondary serial port of the ESP32 (Serial2) to talk to the RS-485 network using dedicated pins. The ModbusMaster library simplifies sending requests like 'readHoldingRegisters', allowing the microcontroller to capture temperature readings from remote industrial instruments. This raw data is then converted and fed into the PID formula to determine the next corrective action on the actuator.

Installation Challenges and Loop Tuning

Bringing a PID system to life in practice reveals challenges that theory books often ignore, such as thermal transport delay. Cooled air takes a few seconds to circulate through the duct, sweep the environment, and return to the sensor, creating a lag between actuator action and measured response. If the designer does not account for this delay, the system tends to oscillate violently, opening and closing valves excessively and causing discomfort.

Fine-tuning the proportional, integral, and derivative gains must be done directly in the field, observing plant behavior under real load conditions. A common method is to zero out the integral and derivative parts, increase the proportional gain until the system begins sustained oscillation, and then apply empirical formulas to recalculate ideal values. This procedure ensures the response is fast enough to address new occupancy demands, but damped enough to prevent mechanical stress.

Final Considerations

Combining the ESP32 with the Modbus protocol and PID control proves that accessible prototyping technologies can be successfully applied in demanding automation environments. Replacing traditional on-off controls with intelligent modulation results in significant electrical energy savings and extends the lifespan of mechanical equipment. With proper hardware planning and resilient code, it is possible to build highly efficient, stable climate systems ready for integration with modern supervision platforms.