Back to Basics - Control Loops

Ira Goldschmidt, P.E., LEED®AP Engineering Consultant, Goldschmidt Engineering Solutions ira.goldschmidt@comcast.net Contributing Editor As published   March Issue - BAS Column

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Good building mechanical design requires a firm grasp of the mechanical engineering fundamentals (e.g., thermodynamics, psychrometrics, the fan laws, etc.).  The same can be said about the BAS design needed to control these mechanical systems.  However, engineers are not normally taught control theory and its application to commercial buildings.  Therefore BAS designs often lack the same engineering foundation as that for the associated mechanical systems.  This month I will revisit an important aspect control fundamentals.

Control Loops are the foundation of how a BAS performs temperature control.  There are often thousands of control loops in even a moderately sized BAS, and there are usually three or more control loops in a single VAV box controller.  There are two types of control loops: open and closed.  Open loop control bases its action on input, but that action does not have an effect on the input.  Examples include time clock control, reset schedules based on outside air temperature, etc. Open loops are important to temperature control, but they should never be used when closed loop is the right approach.

Closed Loops are the key element of temperature control.  They differ from open loop control by the fact that the input (i.e., the “Sensed Variable” aka “Controlled Variable” – see Figure 1) provides feedback to the action of the loop.  Sensors (i.e., temperature, pressure, humidity…) provide the “Sensed Variable” value and feedback.  There are two types of closed loops depending on the device/equipment controlled: on/off and modulated (where “modulated” means that the device’s position or equipment’s capacity can be set to any one of a large number of positions between open/closed, low/high speed, etc.).

• On/off Control is used when the controlled device/equipment cannot be modulated (e.g., a single speed fan, solenoid valve, DX compressor, simple boilers/furnaces, etc.).  The control algorithm used is essentially a mathematical version of an on/off thermostat.  On/off temperature control does not provide very accurate control but it may be the only choice given the type or class of controlled device/equipment.  Therefore it still is an important ingredient to temperature control.
  

• Modulated Control is generally the most-used type of temperature control in a commercial building.  A “PID” algorithm is typically used by a BAS to implement closed-loop control.   In short, a PID algorithm (which is part of the “Controller” programming) compares the “Sensed Variable” to the setpoint, the value to which the “Sensed Variable” is expected to be maintained.  Based on the error between the “Sensed Variable” and setpoint the PID algorithm continuously determines a “Control Output” value (typically 0-100% which is then converted by the BAS hardware to 0-10vDC, 4-20maDC, etc.) with the goal of maintaining the “Sensed Variable” at the setpoint.

What’s the Challenge?

PID control loops require tuning of gain parameters to ensure that they can provide a response that reaches setpoint in a reasonable time and remains stable thereafter.  These gain parameters determine how much effect a change in the error between the “Sensed Variable” and setpoint has on a change in the “Control Output” value.  If the gains are too sensitive, then the loop will continuously cycle (aka “hunt”) above and below the setpoint, and if not sensitive enough then the loop will be slow to respond and may never reach setpoint.  There are no universal “gains” that can be used for all or even a subset of HVAC applications.  What’s more, the correct gain parameters can change over time as the controlled device/equipment performance changes over time.  Therefore getting and keeping PID loops properly functioning is one of the more challenging (and perhaps the most overlooked) aspects of operating/maintaining a BAS.