Continuing last month’s fundamentals-based theme, I’d like to address the devices that make closed-loop control work. As discussed, control loop algorithms are implemented by software within a BAS controller. However, the input and output signals associated with the loops come from the sensors and and go to the actuators that are mounted on/near the controlled equipment. These are the most important parts of what are typically referred to as the “field devices” portion of a BAS.
Sensors provide the “controlled variable” signal input to the algorithm. These devices always provide a varying signal (i.e., they are not on/off devices) and are mostly for temperature or pressure measurement. Airflow and CO2 sensors are becoming more prevalent, and humidity, level, etc., are also used in some applications.
Of the input field devices, temperature sensor selection involves the least challenge. Thermistors or RTDs are essentially all that are used, both of which have a varying resistance vs. temperature and can be readily read by BAS inputs without the need for “transmitter” electronics to amplify or convert the sensor’s low-level signal. Most BAS are designed with a resistance-to-temperature lookup table so that one of these (along with one of the myriad of versions within each type) are most easily accommodated. Therefore, it is best to just specify a reasonable accuracy for these sensor types (i.e., +/-0.5°F) and let the contractor use the sensor technology/type designed for the BAS. More importantly is to specify where/when the various types of sensor configurations are to be used — i.e., surface wall-mount, flush wall mount, pipe insertion (a strap-on should be rarely allowed), single-point duct, extended duct averaging, and outside air with sunshield.
The other sensor types involve a much greater variety of sensing technologies, accuracies, and costs for the engineer to choose from. They also include a transmitter to output (most typically) a 0-5vDC signal readily used by BAS input, so a BAS’s sensor signal “preference” is not the issue. Instead, a cost/benefit approach for selecting the best sensors for each client/project/application is needed.
Actuators are the field devices (along with the associated dampers and valves) that receive the output signal determined by a control algorithm. VFD speed control signaling has also become a common output for modulating control; two-position control more often than not is used for turning on/off fan motors, DX compressors, etc. However, these latter items are not BAS field devices per se, and are not normally specified with the BAS. So actuator specification/selection is important to the success of closed-loop control.
Two-position actuators (i.e., that either fully open or close the damper/valve) typically actuate based on the absence or presence of a 24 or 120vac signal, whereby the energized motor (or other positioning technology) is operating against a spring (or closes from the force of the spring when de-energized). Solenoid valves are a good example of a common combination two-position actuator/valve. There are also actuators with bi-directional motors that must be controlled by two BAS outputs (one to open the actuator and the other to close it). These are sometimes called “floating” or “three-wire” actuators and are rarely used except for VAV box dampers.
Modulating actuators use a variety of motor and position feedback technologies such that the technology used often is considered proprietary to each actuator manufacturer. Therefore, it is probably more important to stick with a known manufacturer rather than to delve into the technological specifics. However, there are a few issues that are important. First, the actuator should use a 0-10vdc input signal. More importantly is the ability of the actuator to provide many steps of positioning (aka “resolution” for which 100 steps is common). There are other specs to consider when the associated damper/valve behavior is also considered, but this will be covered in another column.
What About Pneumatic Actuators?
Many assume that “DDC” means electronic actuation, but that’s simply not true. Pneumatic actuation was the standard approach for DDC control until electronic actuator technology became reliable and cost-effective around the turn of the century. Even today, pneumatic actuation is typically less expensive than electronic, but an additional (and costly) device is needed to convert the BAS’s electronic output to a pneumatic signal. However, applications that require fast actuation (e.g., lab pressure control) may still benefit from pneumatic actuation.