feedback systems in hvac - ashrae madison · feedback systems in hvac ashrae distinguished lecture...

Handout 1

Feedback Systems in HVACASHRAE Distinguished Lecture Series

Jim CooganSiemens Building Technologies

ASHRAE, Madison ChapterOctober, 2014

Page 2 Copyright © 2014. All rights reserved.

Agenda

Definitions: feedback and closed-loop controlTypes of feedback algorithmsApplication: Supply Air Temperature Control

DX cooling – staged controlChilled water valve – PI and PID control

Application: Mixed Air ControlAdaptive control

Application: Duct pressure resetSpecifying loop performance

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What is a loop?

Closed loop controltemperature affects controller outputvalve curve is not crucialuses a temperature sensordelivers temperature data

Open loop controltemperature does not affect controllerdepends on known valve curvedoesn’t need a sensordelivers no data

A closed loop controller responds to the results of it’s own actions

PlantController


Examples of closed loop control

Many feedback technologiesFloat valveBi-metal pneumatic statDDC with sensor, controller, actuator

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Characterizing response

Mathematical measures of:SpeedAccuracyStability

Apply feedback theory to get the behavior we want

Diagram from: Control TechnologySiemens Switzerland, 2013


Some things stay true,1958 ASHRAE Handbook

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AHU Supply Air Temperature Control

Consider the cooling loop in 2 similar air handlers4-stage DX unit

Diagrams from:Sequences of Operation for Common HVAC SystemsASHRAE, 2005

Chilled water coil and modulating valve


Staged control of DX machine

Equipment cycles on and offTemperature rises and fallsCan reduce the size of the swing by cycling equipment faster

Handout 5


Staged control of DX machine

Fixed relationship between cycle rate and size of temperature swing

Slower cycle = Bigger swing

Depends on mechanical and thermal sizing,NOT on control calculations

Diagram adapted from: Control TechnologySiemens Switzerland, 2013


Modulating control of a Chw valve

Refinement of the on/off approachDon’t just turn it on and off, adjust for steady operationExecuted with many different technologiesToday we use DDCToday we (usually) use PID

ProportionalIntegralDerivative

We need to tune itDiagram from: Control TechnologySiemens Switzerland, 2013

Handout 6


What is tuning?

Adjust feedback parameters for closed loop performanceDetermines speed, stability and accuracyTuning is matching controller to the mechanical system

Match the size of responseMatch the timing or response


What is tuning

Matching values of the feedback parameters to the characteristics of the system under control

For PIDProportional bandIntegral timeDerivative timeSample time

What are “characteristics”Physical sensitivityResponse time

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PID Operation and Tuning

Mathematical model simulates the cooling control process

Easy to study effects of tuning and other conditions

Schematic diagram from:Sequences of Operation for Common HVAC SystemsASHRAE, 2005



P: 50 deg

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PID Operation and TuningP: 5 deg



P: 0.5 deg

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P: 5 degI: 100 sec

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P: 5 degI: 400 sec



P: 5 degI: 20 sec

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PI vs PID?

Tuning presents range of acceptable performancetrade-off of speed vs. stability

PID (rather than PI) can extend the range allowing a better trade-off

Occasionally valuable

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PI+ bias

total

D

Handout 13


Second Application: Mixed air loop

Q: What is the sensitivity of the process? A: Difference between temperature inside and outWeather changes, loop behavior changes tooOptions:

Tune for stability in cold, accept slow response in warm weatherRecalculate gains based on OATApply adaptive control

`Diagram from: Sequences of Operation for Common HVAC SystemsASHRAE, 2005


Adaptive Control

Approaches to the varying physical system

Calculate varying gain for PID from known conditionsApply generic PID gain adjusting algorithmApply alternate, non-linear adaptive technology

Adaptive controllers are available in the market today

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Handout 14


Pink – VFD Blue – Supply AirGreen – Return Air

PID Control Adaptive Control

Return Air Tracking Loopwith Adaptive Control


Duct Pressure Control

FlowController

FanController

VSD

F F F F

FlowController

FlowController

FlowController

FlowController

FanController

VSD

F F F F

FlowController

FlowController

FlowController

FanController

VSD

F F F F

Handout 15


Duct Pressure Control,Duct Pressure Reset

Duct pressure loop is usually not very difficultSystem can oscillateDe-coupling is the key

Duct pressure reset can be trickyPressure is “reset until one zone damper

is nearly wide open”


ASHRAE Energy Standard

Setpoint Reset. For systems with direct digital control of individual zone boxes reporting to the central control panel, static pressure setpoint shall be resetbased on the zone requiring the most pressure; i.e., the setpoint is reset lower until one zone damper is nearly wide open.

Handout 16


Duct Pressure Reset

“reset until one zone damper is nearly wide open”

What control algorithm?Some use open loopSome use PID

What is the controlled variable?What is the setpoint?

Some use ad hoc feedback (“trim and respond”)


Specifying loop performance

Already discussed loop behaviorAlready characterized responsesShould be easy to define performance requirementsDifficult because it’s hard to nail down test conditionBehavior defined in math worldTest occurs in real worldStandard 195 defines flow control performance testIntended as a product qualification test, not a

commissioning procedure

Handout 17


Summary

Closed-loop control concepts2-state controlModulating control and PIDTuning and performanceAdvanced approaches


Questions?

Jim Coogan, [email protected]

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feedback systems in hvac - ashrae madison · feedback systems in hvac ashrae distinguished lecture...

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