Consulting-Specifying Engineer - February 2008 - (Page 22)

Cover Story Define the relationship between the Pv and the controlled device. Typical control relationships can be: • A control loop response • Two position control • Floating control • Proportional control • Proportional plus integral (PI) control • Proportional plus integral plus derivative (PID) control • Time schedule • Device tracking: When a particular device is energized, this device is on • Mathematical relationship. Identify all the parameters required for the relationship defined in the previous step. In many cases these parameters are the output variables of secondary control logic. For instance, the discharge air temperature setpoint might be reset by the return air temperature. This example of secondary logic is a reset control loop and requires its own measured variable (see the cooling example in Figure 1). Alternatively, this parameter may come from higher up in the control system. An example may be the resetting of setpoints based on the exceeding of an energy demand threshold. In this case, where supervisory logic is used, the network architecture and its robustness Use logic diagrams to communicate I have been teaching HVAC Controls for about 25 years. The communication method necessary for a teacher/student process is similar to what is required for an engineer/ contractor. I have found that teaching using logic diagrams works very well for communication a sequence of operation. Logic diagrams (see Figure 1) are a great visual tool in the classroom and work equally as well as a means for communicating sequences as part of a design. While teaching sequence development, it became obvious that we needed to develop an organization process for approaching the development of sequences in various HVAC applications. For those that frequently develop detailed control sequences, this process may seem second nature. The experienced control engineer may skip numerous steps or combine multiple thought process while developing this control logic for a given application. In controls language, one needs to develop a control loop for each controlled device. This control loop typically consists of a sensor, controller function and controlled device (see Figure 2). Additionally, the designer needs to consider special conditions related to the control loop and finally overall loop interaction within the system and with outside variables and parameters. Figure 2: Control logic for chilled water control valve in air handling unit. The top example shows basic control loop. The middle example shows the addition of antiwindup to the PID block, and the bottom example shows supervisory logic calculating a reset setpoint. Source: Facility Dynamics become critical to the effectiveness of the control. In other words, if you are using the network to communicate critical control parameters, speed may be an issue. This may influence specifications related to hardware, networking, and associated loading of the control system from a bandwidth perspective. Define the control logic or write the sequences to produce the parameters identified in the previous step. Identify and define the measured variables that are required for this logic. Define the relationship between the measured variable and the output of this control for each parameter. Describe this relationship in the sequence of operation or in the control logic. Determine discrete or analog conditions that apply to each process. The conditions referred to here are typically a test that needs to be proven a certain way before subsequent event can take place. This also is a useful place to emphasize the requirement that a particular condition may need to be hard-wired if the design calls for it—or if a software-based interlock is OK. Examples of discrete and analog conditions: • The end switch on an outside air damper must be proved open on a 100% OA Unit before the fan runs • The system must be in an occupied mode before the outside air damper opens • If the supply fan is not moving air, the outside air dampers will shut and the return dampers will open • If the outdoor air temperature exceeds 68 F, send the outdoor air dampers to minimum position • If the discharge air humidity exceeds 90%, shut off the humidifier. Define the logic or write the sequences that describe the discrete and analog conditions necessary. This requirement may require secondary logic, which also may require additional measured variables. It also may use information already required and collected elsewhere in the control sequence for this system or others in the building. Once this has been done for each controlled device in a system, you are ready 22 Consulting-Specifying Engineer • FEBRUARY 2008

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Consulting-Specifying Engineer - February 2008 Contents Viewpoint News M/E Roundtable How To Write Control Sequences Mentoring Engineers: Myths, Motivations, and Models Keep Young Electrical Engineers Grounded Protecting a Vulnerable Population Codes & Standards Case Study New Products Equipment Lifecycles Advertisers Index Green Space

Consulting-Specifying Engineer - February 2008

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