HVAC temperature control with nano PLCs

Freezing temps in a Wisconsin laboratory cause hundreds of medical tests to be discarded 

Hank’s on the phone with Joe the maintenance guy at ABC Labs, a third-party medical laboratory that serves a small Milwaukee community. The city has been dealing with a polar-vortex so temps have bottomed out and the heating demands in the state are at an all-time high. 

Overnight, ABC’s furnace wasn’t delivering heat consistently so more than one hundred lab tests were compromised. Lab techs are contacting the local clinics and hospitals to advise that they are unable to deliver desperately needed test results to patients. Some hospitals re-send samples. Others attempt to get the tests done elsewhere. Not only has the lab lost revenue, but lives are now at risk because of the failure of the furnace to deliver a constant temp in a dynamic environment.

Hank understands the problem and begins troubleshooting a solution. 

Sound familiar? 

A furnace is down. Productivity goes with it as does business. This is one of any number of situations that can occur when the heat supplied throughout the building is unpredictably hot or cold. Not only can it be uncomfortable, but it can also impact someone’s ability to function within the space. In instances where a constant temperature is needed in a dynamic environment, nano PLCs can offer several methods for controlling said temperature. 

Temperature control methodologies 

Hank has some options for addressing this issue. He can:

• Use the On/Off method if his application requires basic course control with an acceptable window of tolerance. This is more than sufficient and is the least expensive option.

• Apply Pulse Width Modulation (PWM) where more precise control than the On/Off method is needed. This is a mid-range price option.

• Apply Proportional, Integral, Derivative (PID) control where precision and accuracy are critical. This is the most expensive, but the most commonly used option.

What Hank chooses to do is dependent upon what’s happening with the furnace. Let’s dig a bit deeper into these methods.

On/Off HVAC temperature control is one of the simplest methods for controlling temperature.

The basis of this approach is that full power is supplied to the heater until the desired setpoint is achieved. Once the feedback reaches and surpasses the setpoint, the power is completely turned off to the heater. 

When the temperature drops below a bandwidth setpoint, the power will again turn back on until the setpoint is again reached. This cycle of turning on and off repeats continuously to control the temperature within the desired bandwidth over time.

A nano PLC can be used to control the electrical power supplied to an electrical heater in a furnace by using Pulse Width Modulation (PWM). 

A variable pulse of full power is supplied to the system. The bigger the difference between the setpoint and feedback from a device, the longer the percentage of time the pulse will be on. As the pulses are added together, average power is supplied to a heater.

Proportional, Integral, Derivative (PID) control is a closed loop mathematical function. It works by:

• Applying gains to the error, derivative of the error, and integral of the error between the setpoint and feedback
• It sums those values together, and then outputs the sum to the system as a reference
• By adjusting the gains, you can adjust the response time, overshoot, and the amount and size of steady state oscillations 

An example of how a nano PLC can implement PID control is in its ability to control the position of a damper based on temperature feedback:

• The error between the setpoint and the feedback from an analog device can be fed into a PID function 
• The output of the PID can then directly control an analog output to a damper position actuator 
• The further the damper is open, the more hot or cold air flows into the room depending on whether you are heating or cooling the room 
• As the temperature fluctuates, the damper position will dynamically adjust to automatically regulate the temperature

The table below illustrates the implications of applying each methodology using a nano programmable logic controller.

The expectation is that this, plus a deeper understanding of how each works, should provide enough insight into which methodology will work for your needs. We recommend applying only what is needed to manage costs and resources.