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Home » Applications & Technologies » Industrial Application - Two-Stage AHU Controller
Applications & Technologies

Industrial Application - Two-Stage AHU Controller

An air handling unit (AHU) conditions and circulates air for the purpose of maintaining suitable indoor air quality for people and equipment. “Two-stage” refers to simple binary (ON/OFF) heating or cooling in the form of resistive electric elements or direct expansion (DX) cooling with a closed-system refrigerant. An AHU controller regulates all the different aspects of the AHU system, with the goal of reducing energy expenses while maintaining a safe and comfortable living or working environment. The main functions of an AHU include: heating/cooling the air to a desired temperature, mixing and filtering air to ensure proper air quality, providing enough air pressure for adequate circulation, dehumidification, and sometimes humidification (not to be discussed here).

Pressure
Sensor
CO2
Sensor
Air Flow
Sensor
Temperature
Sensor
RH&T
Sensor
Hall Effect
Sensor
Amp   
Amp   
Amp   
   ADC
VREF
MUX
Processor
ZigBee
Module
USB
Transceiver
RS-232
Transceiver
Ethernet
Transceiver
Driver
Driver
Digital
Pot
Driver
Interface
ESD
Interface
ESD
Ethernet
ESD
Relay
Relay
Relay
USB
Receptacles
RJ-45
D-SUB
Stage 1
Stage 2
Motor

This design is for reference only. The design, as well as the products suggested, has not been tested for compatibility or interoperability.

Pressure Sensors for Two-Stage AHU Controllers

A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor transducer generates an electronic signal as a function of the pressure imposed. Pressure sensors are used for control and monitoring in thousands of everyday applications. Pressure sensors can also be used to indirectly measure other variables such as fluid/gas flow, speed, fluid level, and altitude. Pressure sensors can alternatively be called pressure transducers, pressure transmitters, pressure senders, pressure indicators, piezometers, and manometers, among other names. Pressure sensors can vary drastically in technology, design, performance, application suitability and cost. A special category of pressure sensors can dynamically measure very high speed changes in pressure. Example applications are combustion pressure in an engine cylinder or in a gas turbine. These sensors are commonly manufactured out of piezoelectric materials such as quartz .

A conservative estimate is that there are over 50 technologies and at least 300 companies making pressure sensors worldwide.

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CO2 Sensors for Two-Stage AHU Controllers

A carbon dioxide (CO²) sensor measures the concentration of CO² in the surrounding air. CO² is an asphyxiant, and as such it is often monitored carefully in high-density indoor environments. The U.S. Occupational Safety and Health Administration (OSHA), among other agencies and organizations, have set an upper limit of 5000 ppm for an 8-hour work day, although adverse health effects may be noticed at as little as half this value.

The two most common technologies are Nondispersive Infrared (NDIR) sensors, which measure CO² spectroscopically, and chemical based sensors. The latter has greater problems with sensor drift and relatively low service life, but can often be smaller, lower cost, and more power efficient.

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Air Flow Sensors for Two-Stage AHU Controllers

Air flow sensors are devices that measure the volume or mass of air passing through a given area per unit time. One of the most common of these sensors measure air flow using thermal-based sensing. Greater air flow causes faster cooling of a thin wire or film which is heated above the ambient air temperature. As air flow increases, more current must be passed through the heated object to maintain a constant temperature. Together with a bridge circuit and thermistor (to compensate for variations in air temperature), the sensor is capable of measuring air flow with high precision and fast response time.

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Temperature Sensors for Two-Stage AHU Controllers

In electronics, silicon bandgap sensors are among the most common for highly accurate temperature measurements over a relatively small range. The technology is based on the principle that as the temperature of a silicon diode increases, the forward voltage of the diode increases proportionally. This method is generally very low cost, allowing the sensor to be built into an integrated circuit. Because air temperature regulation is major function of most HVAC systems, accurate and reliable temperature sensors are often essential to HVAC design.

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Relative Humidity & Temperature Sensors for Two-Stage AHU Controllers

A Relative Temperature and Humidity (RT&H) sensor is a combination of both a thermometer and a hygrometer. Relative humidity sensors (hygrometers) measure the amount of moisture present in the air. Capacitive humidity sensors are some of the most common; the dielectric polymer of a sensing capacitor absorbs and releases water as relative humidity rises and falls, effectively changing the measured capacitance. These sensors can be important for an Air Handling Unit (AHU), as an AHU may include a humidifier and/or de-humidifier for ensuring an appropriate indoor humidity.

Because relative humidity is a function of both temperature and absolute humidity, these devices must either include a temperature sensor or implement some form of temperature compensation. In electronics, silicon bandgap sensors are among the most common for highly accurate temperature measurements over a relatively small range. The technology is based on the principle that as the temperature of a silicon diode increases, the forward voltage of the diode increases proportionally. This method is generally very low cost, allowing the sensor to be built into an integrated circuit. Because air temperature regulation is a major function of most HVAC systems, accurate and reliable temperature sensors are often essential to HVAC design.

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Hall Effect Sensors for Two-Stage AHU Controllers

Hall Effect sensors are magnetically biased transducers that vary output voltage or current in response to changes in a magnetic field. Hall Effect sensors can be designed to sense rotary movement of a motor shaft. The rotation of the motor shaft changes the IC's position with respect to the magnets, and thus detects the change in flux density. The output of the IC is converted to a linear output over 90 degrees of shaft travel. The Hall Effect sensor gets its name from Edwin Hall, who, in 1879 discovered that a voltage difference can be produced across an electrical conductor where the magnetic field is perpendicular to the direction of current flow.

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Amplifiers for Two-Stage AHU Controllers

Amplifiers have enormous voltage gain, use feedback to operate, and can be classified in different ways. They can be identified by the device they are intended to drive (e.g., headphone amplifier, speaker amplifier), the input that they are to amplify (e.g. guitar amplifier), the frequency range of the signal (e.g., RF, Audio), and by the function that they perform (e.g. inverting amplifier, power amplifier.)

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Analog-to-Digital Converters for Two-Stage AHU Controllers

An Analog-to-Digital Converter (ADC or A/D converter) measures the magnitude of an input analog signal and converts it to a digital number that is proportional to the magnitude of the voltage or current. An ADC often converts signals collected from the real-world to digital signals for processing. One of the more important specifications of an ADC is the resolution that it offers, which is the number of discrete values (represented in bits) that the ADC produces in relation to the analog signal it is converting. The more bits, the higher the resolution. A higher resolution yields a more accurate approximation of the analog input.

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Reference Voltage ICs for Two-Stage AHU Controllers

A voltage reference produces a constant level of voltage over time regardless of load, changes in power supply, or temperature. Voltage references are used in power supplies, analog-to-digital converters, digital-to-analog converters, and many other applications where voltage levels must be maintained at a steady level. Without a voltage reference, precision is greatly affected and may render the device inoperable. Voltage references can vary greatly in performance. A voltage reference for a power supply might hold its output to within only a few percentage points off of its nominal or stated value; however, a voltage reference to instrumentation-level standards are measured in parts per million regarding stability and precision to the nominal or specified value.

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Multiplexers for Two-Stage AHU Controllers

A multiplexer (or MUX) is an electronic device with multiple inputs (analog or digital) from which it can select one to forward as an output. In other words, a MUX allows a circuit to switch between inputs for a signal line. A MUX can reduce cost and part count in many applications by enabling multiple signals to “time-share” a single resource, such as an Analog-to-Digital Converter (ADC) or a microprocessor.

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Processors for Two-Stage AHU Controllers

The term "processor" refers to an electronic device that performs computational functions and carries out the instructions of a stored program. Other terms for processor are microprocessor, central processing unit, and digital signal processor. Essentially, the processor refers to "the brains of a computer."

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ZigBee Wireless Modules for Two-Stage AHU Controllers

Zigbee is a specification for communication in wireless personal area networks (WPANs) and is designed for low-power, cost-efficient, and low duty cycle applications. Built upon the IEEE 802.15.4 standard, Zigbee adds both routing and multi-hop functionality. Star networks as well as peer-to-peer (e.g., mesh and cluster tree) are supported, making Zigbee networks dynamic, scalable, and decentralized. These qualities and more make Zigbee an excellent technology for applications like home/industrial automation, Industrial Applications monitoring, and HVAC control.

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USB Transceivers for Two-Stage AHU Controllers

USB is a standard connection interface between computers and digital devices. A USB transceiver is a physical layer device that prepares data for transmission and then sends to, and receives data from, another transceiver. The transceiver detects connection and provides the low level USB protocol and signaling. The term "transceiver" indicates an implementation of both transmit and receive functions. It transmits and receives, encodes and decodes data, provides error indication, implements buffers to stage data until it can be managed, and adjusts for the clock rate from the serial stream on the USB SuperSpeed bus to match that of the “link layer” higher up on the communication stack.

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RS-232 Transceivers for Two-Stage AHU Controllers

RS-232 refers to a set of standards for single-ended serial communication, and covers such things as logic voltage levels, timing, signal rate, and physical characteristics of the connectors. An RS-232 port was once the standard interface for connecting a personal computer with peripherals such as keyboards and modems, but has since been largely replaced by USB. It continues to be used in other areas however; much of today’s industrial automation equipment incorporates RS-232.

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Ethernet Transceivers for Two-Stage AHU Controllers

Ethernet is the most commonly used technology for non-wireless local area networks (LANs). Ethernet controllers perform the function of interfacing computers and other electronic devices in a network. Ethernet itself only defines the physical (PHY) and datalink (MAC) layers of the OSI Model; however, processors with integrated Ethernet controllers can provide additional functions such as a TCP/IP protocol stack.

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Relay Drivers for Two-Stage AHU Controllers

Designers of power electronic circuits must often drive power switches that feed DC, AC, or power signals to a variety of workloads. Logic-level electronic circuits provide the driving signals. In general, however, the power sources and their loads have reference levels different from that of the control circuitry (ground). MOSFET selection begins by choosing devices that can handle the required current, then giving careful consideration to thermal dissipation in high current applications.

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Digital Potentiometers for Two-Stage AHU Controllers

A potentiometer can be understood as an adjustable voltage divider or a variable resistor. A digital potentiometer is a potentiometer that can be controlled with a digital signal. This allows the device to be electronically adjusted by an MCU or processor and can provide a simple, cost-effective way to implement digital-to-analog conversion. Many digital potentiometers are made using a resistor ladder IC, such as the common R-2R ladder. There are a number of options when choosing a digital potentiometer, such as resolution, number of taps, linear or log taper, non-volatile memory options, and I/O interfaces.

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Interface ESD for Two-Stage AHU Controllers

If you have ever been zapped by a socks-wearing kid who has just discovered static charge build up, you have experienced ESD first hand. ESD is like a miniature, localized lightning bolt caused by an electrical discharge. ESD can have seriously damaging effects on an integrated chip or system, or can cause poor performance or failure later on by merely weakening the circuits.

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Ethernet ESD for Two-Stage AHU Controllers

If you have ever been zapped by a socks-wearing kid who has just discovered static charge build up, you have experienced ESD first hand. ESD is like a miniature, localized lightning bolt caused by an electrical discharge. ESD can have seriously damaging effects on an integrated chip or system, or can cause poor performance or failure later on by merely weakening the circuits.

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Electromechanical Relays for Two-Stage AHU Controllers

Simply put, a relay is an electrical switch that is controlled electrically. Relays are often necessary to control high power/current/voltage circuits with a low-power signal while maintaining complete electrical isolation between them.

Many relays are electromechanical in nature; the control signal energizes an electromagnetic coil creating a magnetic field. This field exerts a force on a movable armature of ferromagnetic material (such as iron) which makes or breaks the electrical connection.

Solid State Relays (SSRs) are another common type, and unlike their electromechanical equivalent, they have no moving parts. Instead, one or more transistors, such as a MOSFET, are used to provide a similar function. Electrical isolation must be added in this case, often by using an on-chip optocoupler.

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USB Receptacles for Two-Stage AHU Controllers

USB plugs and receptacles are designed to reduce human error by their unique shape; they fit together in only one way. USB plugs and receptacles are Type A (connecting to hosts or hubs) or Type B (connecting to devices) and are available 3 sizes: standard, mini, and micro. Type A plugs always face upstream, Type B faces downstream. USB is used in many applications covering all areas of electronics that require communication, but more commonly with devices that need fast or easy connections for interaction with computers. Since USB provides a small charging current as well, it is becoming a de facto standard for charging portable devices.

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D-SUB Connectors for Two-Stage AHU Controllers

A D-sub or "D-Subminiature Connector"contains two or more parallel rows of pins or sockets, usually surrounded by a D-shaped metal shield that provides mechanical support, ensures correct orientation, and may screen against electromagnetic interference. The part containing pin contacts is called the male connector or plug, while that containing socket contacts is called the female connector or socket.

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RJ-45 Connectors for Two-Stage AHU Controllers

RJ-45 connectors are a type of electrical connector commonly used for Ethernet jacks. The term "RJ-45" is something of a misnomer, but it almost always refers to an 8P8C (eight position, eight contact) modular connector with wiring pin-outs compatible with standard Ethernet.

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