Model Predictive Control vs. Proportional, Integral, Derivative Control

January 18, 2022

Controller Systems

Two different control methods are practiced in the Heating, Ventilation, and Air-Conditioning (HVAC) system.

  1. Model Predictive Control (MPC) controller
  2. Proportional, Integral, Derivative (PID) controller

MPC Controller vs. PID Controller

The MPC controller uses algorithms in adjusting the control points with a consideration of various operation constraints and prediction of disturbances. It further aids in energy storage systems, renewable energy sources, and energy efficiency enhancement.

On the other hand, the PID controller automatically adjusts the control output based on the input data. If the difference between current point and set point occurs, then the adjustment takes place.

PID Controller

For today's commercial buildings, PID controller is most commonly used as a conventional thermostat in a buildings' Heating, Ventilation, and Air-Conditioning (HVAC) system. It allows the occupants to adjust to the desirable temperatures through the use of set temperature. Basically, the thermostat sensors the current temperature, and only reacts in case of difference between the current and set temperature with further alerts to the occupants.

The use of conventional thermostat seems to result in energy and cost savings as it controls the temperature through the use of set temperature. However, the major drawback revolves around the reaction time interval in adjusting current temperature to set point. It usually incurs delays for preventing the short cycle, where sudden replacement occurs in heating and cooling equipment. Moreover, despite the fact that it seems to save energy, a study finds that the energy saving depends on the operating pattern or cycle of the system as well as energy efficiency of dwelling and climate conditions. It is further mentioned that the energy savings seem low, very low, or non-existent. Even though the cost-effectiveness may exist due to low initial costs, the operation costs might exceed the initial costs.

MPC Controller

On the contrary, the MPC controller exhibits remarkable benefits regarding application in HVAC system, with the key advantage of ability to deal with the constraints. In comparison with the PID controller, the MPC controller's one notable distinction is that it is capable of managing diverse constraints. Furthermore, the MPC controller demands a process model, while the PID controller does not.

Benefits of MPC Controller

There are list of additional benefits which offered by MPC controller as follows:

  1. System model for anticipatory control actions.
  2. Integration of a disturbance model for disturbance rejection.
  3. Ability to handle constraints and uncertainties.
  4. Ability to handle time-varying dynamics.
  5. Ability to cope with slow-moving processes with time delay.
  6. Integration of energy conservation strategies in the controller formulation.
  7. Use of a cost function for achievement of multiple objectives.
  8. Use of advanced optimization algorithms for computation of control vectors.
  9. Ability to control the system at both the supervisory and local loop levels.

Additionally, according to one research comparing the MPC and PID controllers, the Heating, Ventilation, and Air-Conditioning (HVAC) simulation test results in superior performance of the MPC controller. The MPC takes less time to reach the set point under steady state conditions, and the offsets are smaller compared to the PID controller. Overall, the result of the MPC controller exhibits outstanding performance than PID controller results.

Regarding MPC vs. PID controllers, it appears that MPC controller has superior characteristics than the PID controller, particularly in terms of cost effectiveness and energy efficiency.

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