A new approach to relay-based autotuning PID controllers and their evaluation in pH control of industrial photobioreactors

This paper presents a novel adaptive autotuning strategy for controlling the pH in microalgae raceway reactors, a critical variable in optimizing biomass productivity under dynamic environmental conditions. The method expands on existing relay-based autotuning principles, tailored to suit the seasonally variable dynamics of microalgae cultivation systems influenced by CO 2 injection and photosynthetic activity. The novelty of the proposed approach relies on the use of a relay-based autotuning technique that applies the relay signal to the setpoint and not to the control signal. This method allows for seamless integration into existing control loops without manual recalibration across seasons. Moreover, the method is coupled with a classification algorithm to assess weather conditions, enhancing the reliability of autotuning under fluctuating solar radiation. Using the developed autotuning method, low-order models are identified, and the autotuner is coupled with a Proportional–Integral controller plus a static feedforward compensation to counteract solar radiation disturbances. The designed algorithm has been successfully evaluated for five consecutive days in a real semi-industrial-scale raceway reactor and compared with a classical PI with a feedforward strategy. The results demonstrate reliable performance across clear and cloudy conditions, offering a scalable and automated solution for adaptive pH regulation in microalgae production systems.