Chapter 109 - Definitions - Electronic Controls For Heating Systems

Front 1

An electronic control system consists of

Back 1

a sensor, a controller, a final control element.

Front 2

Electronic control system sensors are

Back 2

simple, low mass devices that provide stable, wide range, linear, fast response.

Front 3

Electronic controller is a

Back 3

solid state device that provides control over a discrete portion of the sensor range and generates an amplified correction signal to operate the final control element.

Front 4

Electronic controllers can provide ______ ______ ______ types of control.

Back 4

two-position, proportional, or proportional integral

Front 5

Advantages of electronic control systems

Back 5

- controllers can be located remotely from sensors and actuators

- controllers can accept a wide variety of inputs

- remote adjustments for multipls controls can be located on a single panel, even though the sensors and actuators are not.

- can be equipped with complex controls and override schemes

- can have universal type outputs which can interface with many different actuators

Front 6

Authoriy (Compensation Authority)

Back 6

a setting that indicates the relative effect a compensating sensor input has on the main setpoint.

usually expressed in %

Front 7

Compensation Control

Back 7

a process of automaticalled adjusting the control point of a given controller to compensate for changes in a second measured variable such as outdoor air temperature

Front 8

Compensation change-over

Back 8

the point at which the compensation effect is reversed in action and changes

ex: summer to winter or vice versa

Front 9

Compensation sensor

Back 9

the system element that senses a variable other than the controlled variable and resets the control point

Front 10

Controller

Back 10

a solid state device usually consisting of a power supply, a sensor amplification circuit, a process comparing circuit, an output driver section and various components that sense changes in the controlled variable and derive a control output which provides a specific control function.

Front 11

Control Point

Back 11

the actual value of a controlled variable

Front 12

Deviation (offset)

Back 12

the difference between the set point and the value of the controlled variable at any moment.

Front 13

Direct Acting

Back 13

increases its output signal on an increase in input signal.

Front 14

Final Control Element

Back 14

device, such as a valve or damper, that changes the value of the controlled variable.

Front 15

Integral Action

Back 15

an actio in which there is a continuous linear relationship between the rate of increase or decrease in the output to the final control element and the deviation of the control point

Front 16

Limit Sensor

Back 16

device that senses a variable that may be other than the controlled variable and overrids the main sensor at a preset limit

Front 17

Main Sensor

Back 17

device or component that measures the variable to be controlled

Front 18

Negative (Reverse) Compensation

Back 18

a compensation action where a decrease in the compensation variable has the same effect as an increase in the controlled variable

Front 19

Offset

Back 19

a sustained deviation between the control point and the setpoint of a proportional control system under stable operating conditions

Front 20

Proportional Band (Throttling Range)

Back 20

the control point range in a proportional controller through which the controlled variable must pass to drive the final control element through its full operating range.

Front 21

Proportional Control

Back 21

a control algorithm in which the final control element moves to a position proportional to the deviation of the value of the controlled variable from the set point.

Front 22

Proportional Integral Control

Back 22

control algorithm that combines the proportional (response) and inegral (reset response) control algorithms.

Front 23

Remote Setpoint

Back 23

a means for adjusting the controller set point from a remote location in lieu of adjusting it at the controller itself.

Front 24

Reverse Acting

Back 24

decreases it output signal on and increase in input signal and vice versa

Front 25

Setpoint

Back 25

the value on the controller scale at which the controller is set, such as the desired room temperature set on a thermostat

Front 26

Transducer

Back 26

device that converts one energy form to another.

or

a device that converts a signal in one form to a corresponding signal in a different form

Front 27

Transmitter

Back 27

device that receives information (signal) from one source (such as a sensor) and sends a corresponding output signal to a receiver (such as a controller)

Front 28

An electronic control system includes

Back 28

- sensors

- controllers

- output devices (actuators, relays)

- final control elements (valves, dampers)

- indicating, interfacing, and accessory devices

Front 29

A sensing element provides a controller with

Back 29

information concerning changing conditions

Front 30

Two types of sensors commonly used are

Back 30

analog and digital

Front 31

Analog sensors are used for

Back 31

continuously changing conditions

Front 32

Digital sensors are used for

Back 32

fixed state conditions

Front 33

Three groups of temperature sensors

Back 33

- resistance temperature devices (RTDs)

- thermistors

- thermocouples

Front 34

Resistance Temperature Devices (RTDs)

Back 34

devices which change resistance with varying temperature.

they have a positive temp coefficient, which means resistance increases with temp.

Front 35

A commonly used RTD sensor is a

Back 35

BALCO type - constructed of BALCO wire

a BALCO 500ohm resistance element provides relatively linear resistance variation from -40 to 120°C.

the sensor is small device that responds quickly to changes in temperature

Front 36

BALCO wire

Back 36

is an annealed resistance alloy with nominal composition of 70% nickel and 30% iron

Front 37

Platinum element type RTD sensor

Back 37

has a linear response

is stable over time

in some applications, a short length of wire is used to provide nominal resistance of 100ohms.

Front 38

DIsadvantages of Platinum element type RTD

Back 38

a low resistance value can cause the element to be affected by self-heating and sensor lead wire resistance

due to small amount of resistance change of the element, additional amplification must be used to increase the signal level which can cause inaccuracy in the control.

Front 39

Thermistors

Back 39

-solid state discrete sensors

-negative temperature coefficient.

(as temp increases, resistance decreases)

-typically enclosed in a very small case similar to glass diode or small transistor

-provide quick response.

Front 40

Negative Temperature Coefficient

Back 40

as temperature increases, resistance decreases

Front 41

Positive Temperature Coefficient

Back 41

resistance increases with temperature

Front 42

Thermocouples may be composed of

Back 42

Iron and Constantan

welded together at one end to form a thermocouple junction. as it is heated a millivolt voltage is generated across the open end. the amount of voltage generated is directly proportional to temp.

Front 43

Thermocouples are not often found in

Back 43

HVAC applications

because the voltage generated at room temperature is so small.

Front 44

Relative humidity sensors sensing methods include changes in

Back 44

- resistance

- capacitance

- impedance

- frequency

Front 45

Hygroscopic Salt Resistance Humidity sensor

Back 45

-uses change in resistance to determine relative humidity

- a layer of hygroscopic salt such as lithium chloride or carbon powder is deposited between two electrodes

- the material absorbs and releases moisture as a function of relative humidity causing a change in resistance of the sensor.

Front 46

common types of hygroscopic salts

Back 46

lithium chloride

carbon powder

Front 47

An electronic pressure sensor is usually a transmitter that converts pressure changes into a variable that can be used by an electronic controller such as

Back 47

- voltage

- current

- resistance

Front 48

Measuring pressure by detecting changes in resistance can be done by using

Back 48

-a small flexible diaphragm and strain gauge

- the strain gauge assembly (made of very fine serpentine wire, or thin metallic film on a non-conductive base) is stretched or compressed when measured pressure varies.

- change in the length of the strain gauge wire changes the total resistance

- the resistance can then be detected and amplified.

Front 49

The input circuits for many electronic controllers can accept a voltage range of ______ or a current range of _________.

Back 49

0-10 volts DC

4-20 milliamps

Front 50

Transmitter inputs are often classified as _________ because

Back 50

Universal

they can accept the signal from any sensor that has the correct type and range of output

Front 51

Sensors may be referred to as transmitters when

Back 51

-their outputs are conditioned and then sent as the input signal to a controller.

- they must produce an output within the required voltage or current level

ex: a signal is amplified in the sensor before being sent to the controller.

Front 52

Transmitters measure various conditions such as

Back 52

- temperature

- relative humidity

- air flow

- water flow

- air velocity

Front 53

simple transformers cannot be used to adjust current or voltage levels because

Back 53

electronic circuits require DC power supplies.

Front 54

Electronic controllers (signal process)

Back 54

receives a signal

amplifies/conditions it

compares it with a set point

derives a correction (if necessary)

output signal typically positions an actuator

Front 55

Electronic controller circuits allow a wide variety of control functions and sequences from

Back 55

very simple to multiple input circuits with several sequential outputs

Front 56

Temperature Controllers typically require a specific type or category of input sensors such as

Back 56

some use RTD sensors - BALCO or platinum elements

others use input circuits for thermistor sensors

these controllers have setpoints and throttling range scales labeled in °C

Front 57

Relative Humidity Controllers input circuits

Back 57

typically receive the sensed relative humidity signal already converted to a DC voltage signal of 0-10 volts, or to a current signal of 4-20 milliamps.

setpoints and scales for these controllers are in percentage of relative humidity.

Front 58

Enthalpy Controllers

Back 58

are specialized devices that use specific sensors for inputs.

the sensor may combine temp and humidity measurement and convert them to a single voltage representing the enthalpy of the sensed air.

or an individual dry bulb temp sensor and seperate wet bulb or relative humidity sensor provide inputs and the controller calculates enthalpy

Front 59

universal controllers input circuits

Back 59

can accept one or more standard transmitter/transducer signals.

most common input ranges are 0-10volts and 4--20 milliampts

other input ranges could be 2-10 volts DC and 0-20 milliamps

these inputs can represent a variety of sensed variables the settings and scales are often expressed as a percentage of full scale only

Front 60

Control Modes

Back 60

- can be selected to suit the application requirements.

- include two-position, proportional, and proportional integral

- also may include the addition of a compensation sensor, remote setpoint or reset capability, and override or limit control.

Front 61

Electronic controllers provide outputs to ___________________ for _______________.

Back 61

relay or actuator

final control element

Front 62

The output of an electronic controller is not dependent on

Back 62

the input type or control method

Front 63

The simplest form of electronic controller output is

Back 63

two positions, where the final control element can be in one of two states (usually on or off)

Front 64

Output devices that use the controller output signal to perform a physical function on the final control element are

Back 64

- actuators

- relays

- transducers

Front 65

Two position output devices include

Back 65

relays, motor starters, solenoid valves

damper actuators can also be designed to be two position devices.

Front 66

Two position output devices have only _________ states.

Back 66

two discrete

ex: high low, on off, open closed

Front 67

Two position output devices interface with

Back 67

the final control element

Front 68

Modulating actuators adjust the final control element using

Back 68

a varying control signal

i.e. not just on and off but a range in between.

Front 69

Transducer output controls

Back 69

in some applications a transducer converts a controller output to a signal that is useable by the actuator.

Front 70

An electronic control system can be ehanced with ___________ that _____________.

Back 70

visual displays, show system status and operation.

Front 71

Many electronic controllers have built in indicators that show

Back 71

- power

- input signal

- deviation signal

- output signal

Front 72

Indicator lights are

Back 72

a type of visual display that shows on/off status

if driven by controller circuits, the birghtness of a light can show the relative strength of a signal.

Front 73

Electronic control systems must sometimes be interfaced with? What must be determined before they can be connected?

Back 73

microprocessor based building management systems.

Compatibility of the two systems must be verified before they can be connected.

Front 74

an electric to pneumatic transducer converts a modulating 2 to 10 V DC signal from the electronic controller to a pneumatic _____________ modulating 0 to 90 kPa signal used by a _______________.

Back 74

proportional, pneumatic actuator