Babel Buster Network Gateways: Big Features. Small Price.
Having the control network installed early in the construction phase allows the building automation system to do as much of the heavy lifting as possible including centralized data collection, analysis and reporting.
Building systems commissioning is an area that continues to grow in interest around the world. While offering many obvious benefits to the owner, it requires an investment of both time and money. This investment can become quite significant depending on the criticality and complexity of the systems involved (e.g., FDA validation). In fact, many buildings do not go through a formal commissioning process unless it is required by a detailed specification or by a code body. This needs to change and technology can play a critical role in reducing both the time and cost of commissioning building systems. Smart control devices, easy to use commissioning tools and advanced building automation system capabilities can all be used to help automate the commissioning process as well as to collect, analyze and report system performance data. The time has come to let technology do its share of the heavy lifting associated with building commissioning, and provide a means for verifying and improving system performance at the lowest cost.
BUILDING COMMISSIONING DEFINED
The industry has many different working definitions of commissioning. Practical considerations dictate that the initial tasks tend to be more component-related while later tasks focus on verifying the performance of systems at the facility level. While this article describes these tasks from a building automation perspective, there are similar tasks performed during the start-up phase for the other major building systems. Most of the technologies presented in this article could save time and money across the entire range of building mechanical and electrical systems.
Often, installation checkout sheets are used as a checklist to help the installer (or later the commissioner) perform and document the completion of necessary tests.
The first step of the systems commissioning process involves the verification of installed devices including sensors, actuators and controllers. This includes verification of power to the devices, proper wiring and polarity, controller communications and addressing, input/output (I/O) jumper settings as well as proper actuator travel and direction. Often, installation checkout sheets are used as a checklist to help the installer (or later the commissioner) perform and document the completion of necessary tests. These forms are useful but require manual verification of installed devices and are difficult to archive for documentation purposes. A guiding principle of any process improvement activity is to avoid rework and do it right the first time. To support the installer in being able to verify proper operation during installation, a handheld tool based on a popular personal digital assistant (PDA) has been developed that automates and streamlines common verification tests, and records the results of the tests in a computer-readable file. This file can be transferred to a PC as documented evidence of task completion and verification of test results.
The next step of the process usually includes I/O device calibration including sensors and actuators. This involves manually calibrating devices using reference instrumentation, then adjusting device settings or software configuration parameters to match the reference values. In some cases, it is possible to automate the calibration of sensing devices that operate under known conditions such as zero flow or pressure. Many VAV terminal unit controllers include a capability that automatically zeros the air pressure or velocity sensor when the damper is closed or the flow pick-ups are otherwise closed off. Similarly, some recently introduced electric actuators include self-calibration capabilities that automatically determine their own zero and span adjustments via a self-test procedure initiated during device power-up or on demand. When these actuators are configured to provide a feedback signal to the controller, the feedback signal is automatically adjusted to match the range of the controlled device (i.e., 0-100 percent). Improper or failed attempts to auto-calibrate the actuator can then be easily detected at an operator workstation. Because the auto-calibration capability is built into the actuator, the operation of the device can be verified any time after installation by performing another self-test. Some actuators even track long-term changes in calibration and continuously make small adjustments to compensate for valve seat wear and other effects.
Once the mechanical systems are operational, the individual control loops for each process are typically tuned on an exception basis. This relates to the traditional practice of selecting reasonable default tuning parameters for use in DDC programs and then manually tuning only the loops that turn out to be excessively sluggish or unstable at the job site. One problem is that buildings often perform much differently during the start-up phase than during normal occupancy and operation. This presents a sort of catch-22 situation in that you need stable control to balance the mechanical systems, but you need balanced systems to tune the controllers. Also, a controller tuned under one set of operating conditions can often become unstable during other conditions. Luckily, controllers are starting to be offered that include self-tuning capabilities. The most technologically advanced of these controllers continuously adjust their tuning parameters to provide optimum control over the entire range of operating conditions. This capability solves once and for all the start-up issues associated with loop tuning, as the controllers provide the best performance possible under all operating conditions.
Up to this point, the majority of the commissioning tasks have focused on individual components or processes. In the next step, sequence verification, we start to verify installed performance at a systems level. Sequence verification typically involves downloading the DDC controller application programs and then verifying the proper sequence of operations. This is usually a manual process of stepping through the specified control sequences and verifying proper operation of the control devices. In many cases, the proper sequence can be verified from an operator workstation if appropriate instrumentation is included in the project. An example is adding position feedback to actuators to verify proper travel and position. A new technology that can help with sequence verification is the use of finite state machine logic to program control sequences. In traditional control sequences, control devices are often hardware sequenced using the output of a single controller (i.e., 0-50 percent output is heating, 50-100 percent output is cooling). Finite state machines define specific states, or modes of operation, for the process (i.e., heating, cooling, satisfied) and contain specific control logic for each state. This eliminates some of the most common problems associated with control sequences including simultaneous heating and cooling and rapid cycling between heating and cooling modes. A program written using finite state machines is also easier to troubleshoot and verify both at the job site and remotely. Sophisticated applications of this technology include special states (e.g., commissioning) that eliminate the normal state transition delays in order to accelerate sequence verification at the job-site.
TEST & BALANCE
Today, testing and balancing is often performed as a team with a balancing contractor making measurements with a flow hood, and a controls technician changing the appropriate control software parameters using a laptop computer. Ideally, this process should be completed quickly and easily by one person with permanent changes made immediately to the controller software. Technology offers two viable solutions to this challenge. The first approach is to use a special intelligent flow hood that communicates to the VAV terminal unit controller via a network connection through the thermostat cover. The technician can use the intelligent flow hood to view the current controller parameters, override the position of the controlled devices, calculate any necessary calibration parameters (e.g., K-factors) and then download these parameters to the controller. The second approach is to use a balancing tool based on a popular handheld PDA allowing control network access via the thermostat. This tool automates the balancing process, including a visual indication of flow performance during testing as well as uploads and downloads of all required controller parameters.
Test sequences can also be automated and can be executed on demand. Data collected during these special sequences can be used to produce a summary report documenting the test results and providing valuable diagnostic information.
At the heart of commissioning, functional performance testing is arguably the most critical and time-consuming step in the start-up process. This is the point at which all of the components and subsystems are tested together to verify that the overall facility meets design objectives from a systems perspective. This step takes time because it generally requires coordinated participation by all the suppliers and the simulation of a wide range of normal and abnormal operating conditions. Because of the large scope and thus expense required, a practice known as statistical commissioning has developed. In this method, a percentage of systems (e.g., air-handling units, terminal units) are randomly selected and verified using a rigorous performance testing process. Systematic problems that are found in a large number of systems are then addressed in the remaining non-verified systems. While this is admittedly better than not commissioning at all, why can't technology be applied in ways to allow 100 percent performance testing? There are two ways technology can help. First, the building automation systems can be used as effective tools to aid in performance testing given proper instrumentation and data analysis capabilities. A good example of the proper use of instrumentation is the practice of adding discharge air temperature sensors in VAV terminal units with re-heat. With this instrumentation, the proper operation of the terminal unit can be verified from the operator workstation instead of by physical inspection. Test sequences can also be automated and can be executed on demand. Data collected during these special sequences can be used to produce a summary report documenting the test results and providing valuable diagnostic information. In most cases, the typical functional performance test would call for recording flow at the closed, mid-point and maximum positions. The typical test would not have detected the erratic behavior of the terminal unit as discovered by the automated test. The second way technology can help is by continuously monitoring the performance of installed systems. This technology relies on statistical methods to determine metrics of the average operating performance as well as the normal variance in performance of monitored systems. For VAV terminal units, these statistics include measures of average temperature and flow as well as actuator duty cycles. Using statistics, each terminal unit can be compared to the normal distribution of the entire population of terminal units with exceptions flagged for further investigation. Using this type of diagnostic tool, an operator can easily answer the question "what are the 10 worst performing terminal units in my facility?" Poor performance can then be repaired or replaced to improve comfort and save energy. From a commissioning perspective, a diagnostic tool like this could be used to select the subset of systems requiring detailed functional performance testing instead of relying on random sampling or guesswork. Technology such as these two examples would allow 100 percent commissioning to be completed in an automated and cost effective manner.
Technology will play an increasing role in building systems commissioning by providing owners with higher performance buildings at a lower cost. It is important to consider what role commissioning technologies will play up-front during the design phase so that proper instrumentation and end-devices can be specified that will enable the use of the advanced tools and technologies. It is also important to start the verification tasks early during the installation phase and make it easy for installers to do it right the first time through better processes, training and tools. Having the control network installed early in the construction phase allows the building automation system to do as much of the heavy lifting as possible including centralized data collection, analysis and reporting. Finally, plan for continuous improvement. Many of the technologies and tools described in this article can be used throughout the life cycle of the building, not just during start-up. This can lead to buildings that work well initially and actually improve over time.
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