Building Management System - BMS 1

8/12/2019 Building Management System - BMS 1

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Building Management System

integrates various services in a facility

with a computerized control system.

User benefits by

Optimum functionality

Energy savings

Better information systems


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Automatic controls help in

Energy consumptionOptimization of man power

Accuracy & precision

Easy maintenance and trouble shooting

Longer plant life

Hygiene & cleanliness


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Sensor- A Sensor is a device that converts a physical

property such as temperature, pressure, relative humidity,

flow etc. into an electrically or mechanically measurable

signal.

Transducer- A transducer is a device that converts oneform of energy to another form of energy such as

pneumatic to electric or vice versa.

Transmitter- Output from a sensor or a transducer may

not be strong enough to be transmitted over long distances.Transmitter is a device used to amplify the signal.

Controller- It is a device that compares the signal from a

Sensor, Transducer or Transmitter with the desired value

and generates a signal based on error.

Actuator- It is an electric, pneumatic or a mechanicaldevice which generally converts a low value signal to a

signal with sufficient force to overcome the forces acting on

control device and to actuate it.


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Controlled Device- It receives signal from controller and

varies the flow of control agent like water, air. It may be a

damper or a valve connected to an actuator.

Stat- Combination of sensor and controller functioning in

one device is often called a stat like thermostat,

humidistat, etc.

Set-Point- It is desired value of the controlled variable.

Process & Controlled Variable- In an air-conditioning

system, parameter being controlled such as temperature,RH, pressure etc. is controlled variable and the changes in

these parameters is due to influencing factors in process.

Control Medium- It is the air, steam or water the flow

regulation of which influences the process to get thedesired value of controlled variable


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TYPES OF INPUT DEVICES

Analog Input- Any input that varies with time

for e.g. Temperature, humidity , Pressure

Digital Input- Any input that works on a twostate mode ( 0 or 1)like on or off ,

clean/dirty.

Pulse input- Any input incremental in nature

and totalizes over a period of time , KWH

meter, water meter


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Control Loop

The process of controlling an HVAC

system involves three steps. These steps

include first measuring data, then

processing the data with other informationand finally causing a control action. These

three functions make up what is known as

a control loop


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Feed Back Control Loop

OutputController

Feedback

Input

Comparator


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BASIC CONTROL LOOP


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DDC CONTROL LOOP


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Controller

The controller processes data that is input fromthe sensor, applies the logic of control andcauses an output action to be generated. Thissignal may be sent directly to the controlleddevice or to other logical control functions andultimately to the controlled device. Thecontrollers function is to compare its input (fromthe sensor) with a set of instructions such as

setpoint, throttling range and action, thenproduce an appropriate output signal. This is thelogic of control.


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Controller Responses

Control responses are typically one the

following:

Two-Position

Floating

Proportional (P only)

Proportional plus Integral (PI) Proportional plus Integral plus Derivative

(PID)


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Two Position Control

Two-Position ControlTwo-position control compares the value of an analog orvariable input with instructions and generates a digital (two-

position) output. The instructions involve the definition of anupper and lower limit. The output changes its value as the

input crosses these limit values. There are no standards fordefining these limits. The most common terminology used issetpoint and differential. The setpoint indicates the point wherethe output pulls-in, energizes or is true. The outputchanges back or drops-out after the input value crosses

through the value equal to the difference between the setpointand the differential.


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COOLING

OFF

COOLING

ON

Figure shows an example of two-position control where the thermostat is set

to energize the Cooling system when the space temperature rises above 72

F and turn off when the temperature drops to 70F in the space. This is an

example of a setpoint of 70F with a two-degree differential.


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Two Position Control-- Contd

Two-position control can be used for simplecontrol loops (temperature control) or limitcontrol (e.g. Antifreeze thermostat). The analogvalue can be any measured variable includingtemperature, relative humidity, pressure, currentand liquid levels.

Time can also be the input to a two-positioncontrol response. This control responsefunctions like a time clock with pins. The outputpulls-in when the time is in the defined ontime and drops out during the defined off time.


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Floating Control

Floating control is a control response thatproduces two possible digital outputs based on achange in a variable input. One output increasesthe signal to the controlled device, while theother output decreases the signal to thecontrolled device. This control response alsoinvolves an upper and lower limit with the outputchanging as the variable input crosses these

limits. Again, there are no standards for definingthese limits, but the terms setpoint anddeadband are common. The setpoint sets amidpoint and the deadband sets the difference

between the upper and lower limits


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Proportional Control

A proportional control response produces an analog or variable outputchange in proportion to a varying input. In this control response, there is alinear relationship between the input and the output.

A setpoint, throttling range and action typically define this relationship. In aproportional control response, there is a unique value of the measured

variable that corresponds to full travel of the controlled device and a uniquevalue that corresponds to zero travel on the controlled device.

The change in the measured variable that causes the controlled device tomove from fully closed to fully open is called the throttling range. It iswithin this range that the control loop will control, assuming that thesystem has the capacity to meet the requirements.

The action dictates the slope of the control response.

In a direct acting proportional control response, the output will rise with anincrease in the measured variable.

In a reverse acting response, the output will decrease as the measuredvariable increases


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Proportional plus Integral (PI)

Control

PI control involves the measurement of the

offset or error over time. This error is integrated

and a final adjustment is made to the outputsignal from the proportional part of this model.

This type of control response will use the control

loop to reduce the offset to zero. A well set-up PI

control loop will operate in a narrow band closeto the setpoint. It will not operate over the entire

throttling range


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Proportional plus Integral

plus Derivative (PID) Control

PID control adds a predictive element to thecontrol response. In addition to the proportionaland integral calculation, the derivative or slopeof the control response will be computed. Thiscalculation will have the effect of dampening acontrol response that is returning to setpoint soquickly that it will overshoot the setpoint.

PID is a precision process control response andis not always required for HVAC applications.The routine application of PID control to everycontrol loop is labor intensive and its application

should be selective.


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Direct Digital Control (DDC

DDC control consists of microprocessor-

based controllers with the control logic

performed by software. Analog-to-Digital

(A/D) converters transform analog valuesinto digital signals that a microprocessor

can use. Analog sensors can be

resistance, voltage or current generators


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Air Handling Unit and

ChillersINPUT OUTPUT

1. RA temp sensor 3- way valve

2. RA humidity sensor Heater

3. Pressure switch across filter Fire dampers4. Smoke sensor VFD for fan motor

5. ADP sensor Dampers at RA

6. Temp Dampers at fresh air

7. Flow meter/sensor

(Pulse input)

Humidifier

8. Fan status Dampers at exhaust


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INPUT OUTPUT

1. Pressure sensors across the

chiller

VFD of pump motor

2. Pressure sensors across thecondenser Cooling tower fans

3. Inlet and outlet temperature of

chilled water

Makeup water tank

4. Inlet and outlet temperature of

cooling water

Pump on/off

5. Outlet temperature of cooling

tower

Chiller on/off

6. Flow switch at

condenser/chiller

Dampers at fresh air

7. Flow meter/sensor

(Pulse input)

8. Current transformer

9. Voltmeter ,PF, Frequency

meter


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Power Requirements

1. The DDC units require UPS 24v acGenerally the VA ratings are very low and of the order of 4-6va

2. The Analog sensors also require UPS Power of 24v ac.

Output of sensors is 0 to 10v DC or 4 to 20 ma

3. VAV boxes may also have Controllers and they will needUPS power and this will be 24v ac.

4. The ups vendor may agree to provide 230v ac UPS

power and we need to step down this voltage to 24v

5. Chillers and Precision units will also have Cards that

demand 24 v ac UPS power.

Some Chillers will have inbuilt CVTs and UPS power may

not be required for the cards in such Chillers


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COMMUNICATION

At the lowest level, bits are encoded in electrical, light or radio signals

by the Physical layer. Some examples include RS-232,RS 485

A somewhat higher Data link layer such as the point-to-point protocol

(PPP) may detect errors and configure the transmission system.

An even higher protocol may perform network functions. One verycommon protocol is the Internet protocol (IP), which implements

addressing for large set of protocols.

A common associated protocol is the Transmission control protocol

(TCP) which implements error detection and correction (byretransmission). TCP and IP are often paired, giving rise to the familiar

acronym TCP/IP.


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Twisted-Pair Cable

Twisted-pair cableis a type of cabling that is used for

telephone communications and most modern Ethernet

networks. A pair of wires forms a circuit that can transmit

data. The pairs are twisted to provide protection against

crosstalk, the noise generated by adjacent pairs. When

electrical current flows through a wire, it creates a small,

circular magnetic field around the wire. When two wires in

an electrical circuit are placed close together, theirmagnetic fields are the exact opposite of each other. Thus,

the two magnetic fields cancel each other out. They also

cancel out any outside magnetic fields. Twisting the wires

can enhance this cancellation effect. Using cancellationtogether with twisting the wires, cable designers can

effectively provide self-shielding for wire pairs within the

network media.

Two basic types of twisted-pair cable exist: unshielded

twisted pair (UTP) and shielded twisted pair (STP).


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When used as a networking medium, UTP cable has four pairs of either

22- or 24-gauge copper wire. UTP used as a networking medium has

an impedance of 100 ohms Commonly used types of UTP cabling are

as follows:

Category 1Used for telephone communications. Not suitable

for transmitting data.

Category 2Capable of transmitting data at speeds up to 4

megabits per second (Mbps).

Category 3Can transmit data at speeds up to 10 Mbps.

Category 4Can transmit data at speeds up to 16 Mbps. Category 5Can transmit data at speeds up to 100 Mbps.

Category 5e Used in networks running at speeds up to 1000

Mbps (1 gigabit per second [Gbps]).Bandwidth 100mhz

Category 6More stringent standards on noise and

crosstalk.Bandwidth 250 mhz

UTP cable often is installed using a Registered Jack 45 (RJ-45)

connector . The RJ-45 is an eight-wire connector used commonly to

connect computers onto a local-area network (LAN).

C i i C bl


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Communication Cable

ATC 2C Twisted shielded cable is generally used

The comm. Route connecting Comm ports is also called

a. C BUS

b. J BUS

c. N2 BUSd. Field BUS

Comm Cable Limitations

a. RS 232 :- 50 FT15Mb. RS 485 :- 1200 M

c. TCP/IP :- NO LIMIT


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PROTOCOLS

BACnet

Building Automation Control NetworksBacNETBACnet is anetwork communications protocol for building automation andcontrol systems. BACnet, the ASHRAE building automation and

control networking protocol, has been designed specifically to meetthe communication needs of building automation and controlsystems for applications such as heating, ventilating, and air-conditioning control, lighting control, access control, and firedetection systems and their associated equiptment. The BACnetprotocol provides mechanisms by which computerized equipment ofarbitrary function may exchange information, regardless of theparticular building service it performs

M dB


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ModBusModbusis a serial communications protocol published by Modicon in

1979 for use with its programmable logic controllers (PLCs). It had

become a de facto standard communications protocol in industry, andwas the most commonly available means of connecting industrial

electronic devices. The main reasons for the extensive use of Modbus

over other communications protocols are:

1. it is openly published and royalty-free

2. it can be implemented in days, not months

3. it moves raw bits or words without placing many restrictions onvendors

Modbus allows for communication between many devices connected to

the same network, for example a system that measures temperature

and humidity and communicates the results to a computer. Modbus is

often used to connect a supervisory computer with a remote terminalunit (RTU) in supervisory control and data acquisition (SCADA)

systems.


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LON WORKS and Zigbee

LonTalk is a protocol created by EchelonCorporation for networking devices.Also

called Lon Works

ZigBee is a short range, low-poweredwireless communication standard targeted

at Building Automation.

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Building Management System - BMS 1