Back to BAS Basics Actuation

Paul Ehrlich, Ira Goldschmidt & Angela Lewis Building Intelligence Group As published   April Issue - Column

This month we are going to go “back to basics” and discuss some of the fundamental concepts involved with the design, maintenance and upgrades for building automation systems. 

In any control loop we have a number of fundamental elements. The key elements are identified in figure 1:

• Sensed Variable: for example temperature, pressure, or humidity
• Controller:  In this case a Direct Digital Controller or DDC.  Within the controller we are executing a control loop, which may include proportional as well as integral and control strategies.
• Control Output: The control output can be either binary (on / off) or analog.  For now we are going to focus on controlling an analog output – in this example a control valve.
  

        
Let's focus on the details of how the control output is actuated.  The controller is going to produce some form of analog signal that needs to be converted into a physical action, which will in turn position the control valve. 

Signaling:  The analog output from the controller is typically a 0 – 10 Volt DC or 4 – 20 mA.  Other alternatives include digitally communicated values, or the use of set of binary outputs (one to drive open and the other closed).  

[an error occurred while processing this directive] Positioning and Feedback:  The challenge with controlling a mechanical device is that the positioning of the valve may not react smoothly and linearly with the control signal.  Much of this is due to friction, flow and hysteresis.  As a result changes in control output will not always result in a linear change to the output. The impacts of hysteresis will always exist but can be minimized by providing feedback on position within the controlled device.  Without position feedback control loops will tend to be more unstable.

Actuation:  The actuator needs position the control output (in this case a valve) in responses to the output signal from the controller.  There are several challenges with actuating a mechanical device.

The most common options for actuation include:

• Electronic Actuators:  These devices use small motors to position the valve stem.  The electronic actuators typically have the ability to do positive positioning built into the actuator.
• Pneumatic Actuators:  These are often used for older systems that are retrofitted with DDC controls as well as on large valves and special applications for new installation.  Pneumatic actuators require both a way to convert the signal from the controller into a pneumatic output, and also typically require a pilot positioner to compensate for hysteresis.
  

A good control design and deployment will have accurate and stable control.  While it is possible to do this with pneumatic actuation, it is more complicated and prone to error.  This is why we prefer to see systems deployed with electronic actuation.

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