PID control in action: Temperature control | Pressure control | Zone control | Flow control

Proportional, Integral, Derivative (PID) control for HVAC control

PID, or proportional, integral, derivative control is the most commonly used control method when precision and accuracy are critical.

It works by summing scalar multiples of the error, integral of error, and derivative of error and outputting that summation as a reference to the system, or control device.

Each fundamental part of the function is used for a different reason, and adjustments can be made to the individual components to manipulate time dependent system responses like response time, decrease oscillations, reduce overshoot, etc.

Three key components of PID control

Let’s take a closer look at the three components of PID control.

Figure 1: Theory of proportional, integral, derivative control

Proportional control

Proportional control only relies on the error between the setpoint and the feedback from the system. The larger the error, the larger the increase in output.

The proportional gain, Kp scales the error that is fed back to the system. Getting the correct amount of proportional gain is critical because if there is too much of it, the system will overshoot and oscillate.

On the other hand, if there is too little gain and the system will never get close to the setpoint and have a very slow rise time. In general, proportional gain alone will always have a small amount of steady state error.

Proportion control graph of HVAC temperature control

Figure 2: Impact of proportional gain

Integral control

The integral component integrates or sums, the error over time.

An error can be positive or negative, therefore, even a small error term will slowly add up overtime, increasing or decreasing the output to the system until there is zero steady state error.

The integral gain, Ki is a multiplier to scale the amount of error that is summed together every cycle of the function block. Increasing the integral gain will increase the amount of correction to the point where the response may be undesirable.

Therefore, the standard is to use a small amount of gain to slowly reach a zero steady state error.

Figure 3: Theory of integral gain

Increasing the integral gain will increase the amount of correction to the point where the response may be undesirable. Therefore, using just a little bit of gain is what is typically used to slowly reach zero steady state error.

Derivative control

The derivative factor is proportional to the derivative of the error, or the rate of change of the error. Increasing the derivative gain, K_d will cause the system to react more to changes in the error.

Therefore, if there is noise on the feedback signal, the system will be highly sensitive to the fluctuations. For this reason, derivative gain is not typically used.

Figure 5: Impact of derivative gain

PID control and HVAC applications

A few common HVAC applications that utilize a nano PLC’s PID control method include:

Damper position control – when pressure is controlled by adjusting the position of the damper in an HVAC system
Variable frequency drive (VFD) control – when pressure is controlled by varying the speed of a motor in an HVAC system
Valve control – when pressure is controlled by opening or closing a valve in an HVAC system.]

Explore PID control capabilities of the easyE4 Nano PLC

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PID control for HVAC control applications

PID control can be used to address a variety of control applications. These include:

Temperature control

Pressure control

Zone control

Flow control

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