This document provides an introduction to temperature control and closed-loop control systems. It describes the basic components of a control loop including controllers, sensors, control devices and the process being controlled. It explains common sensor types like thermocouples and RTDs. It also covers controller outputs, tuning processes, alarm types and typical temperature control applications.

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An Introduction to
Temperature Control

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Basic temperature control (+/- 2˚ or 3˚)
Thermostat
Output switching 18, 19, 20, 21, 22
Room temperature
measurement
Switching:
on when under temp’
And off when over temp’
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Closed-loop control: Component parts
Controller
Control
Device
Loop
Heater
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Inputs
Controllers have many types of inputs
Thermocouple DC Linear
• B, C, J, K, L, N, R, S, T • Volts
• millivolts
Remote Setpoint • Milliamps
• Volts
• millivolts RTD (Resistance Temperature Device)
• milliamps • PT100
• Potentiometer • NI120
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Thermocouples
• A thermocouple is made from two wires of different metal types,
welded at one end (the “Hot” Junction).
• The wires are connected to the controller
(the reference or “Cold” junction).
• When a difference in temperature exists between the “Hot” and
“Cold” junctions, a small electric current will flow, in proportion to
the temperature difference (this is called the thermoelectric effect).
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Thermocouple Colour Codes
• For the different types of metals
various letters and colours are used.
• These colours vary from country to
country but most now use the
international IEC584 standard
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RTD (Resistance Temperature Device)
• An RTD is made of a material that changes its resistance in a
predictable way with changes in temperature
• There are many types of RTD the most common of which is the PT100
(Platinum Resistance Thermometer )
• The PT100 has a resistance of 100Ω at 0°C
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Outputs Overview
There are 5 main output types
• Relay
• SSR Driver
• Triac
• Transmitter Power Supply
• DC Linear
> Volts
> Millivolts
> milliamps
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Closed-loop control: Terminology
• LOAD - unit to be heated
• HEATER - electrical or gas
• INPUT - thermocouple, PT100, DC
• CONTROLLER - decision making unit
• OUTPUTS - solid-state relay (SSR)
- electro-mechanical relay
- DC Volts or milliamps
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On/Off Control
Temp Output turns off – just like home thermostat
Set-point
Hysteresis
Output turns on
Time
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Proportional Only Control
Temp Offset - requires manual
adjustment to remove
0% power
Prop
Set-point band
100% power
Time
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Conventional P.I.D. Control
Temp
0% power
Prop
Set-point band
100% power
Time
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Cycle-time (time proportioning)
On/Off
e.g. 50% power
on off on off on off on
Time
Cycle time
e.g. 10 secs e.g. 75% power
on off on off on off on off
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Cycle-time & Proportional band
Temp
Prop
Set-point
band
100% power
0% power e.g. 65% power
Time
on
on on on off off
off
Cyc’ time e.g.
10s
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Integral Time (minutes)
Too short Too long
• Corrects proportional control offset error
• Too short - overshoots and oscillates
• Too long - slow warm-up and response
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Derivative Time (seconds)
Too short Too long
• Suppresses overshoots
• Effects response time to disturbances
• Too short - slow warm-up/response
• Too short - under corrects
• Too long - oscillates and over corrects
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Continuous (analogue) Control & Prop-Band
Temp
Prop
Setpoint
band
100% power
0% power e.g. 65% power
Time
20mA
4mA
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Tuning
Automatic Tuning
• Selects optimal PID values automatically
Start-Up Tuning
• Tests process when heating up
Tune at setpoint
• If process near ambient or can’t be interrupted
Continuous Tuning
• Monitors and adjusts if process moves from set-point
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Typical Start-up Auto-tuning
Temp
Auto-tune completes & enters PID values
Setpoint
Start tune (self-learning) at 50% or 75% of setpoint
Time
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Typical Tuning at Set-point
Temp
Overshoots during tuning
Setpoint
Auto-tune completes & enters PID values
Start tune (self-learning the process) @ 100% of setpoint
Time
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Self-tune
• Self-tune is a continuous improvement of the PID terms
• Self-tune monitors for oscillations in the PV
• After 5 oscillations new PID terms are calculated
Temperature
Setpoint 2
Load
Disturbance
Setpoint 1
Setpoint
Change
Time
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Process Alarm Types
• Deviation high or low alarm
• Deviation band alarm
• Full-scale high or low alarm
Deviation high or low Band Full scale high or low
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Typical Applications
Industry Applications
Plastics Extrusion, Blown-film extrusion, Hot-runners Window-frames
Packaging Bag sealers, Blister packaging, Wrapping
Food & beverage Ovens, Chillers, Packaging, Bottling, drying, Heat tunnels
Life sciences Incubators, Analytical equipment, Lab equipment, Sterilizers
Metals Foundries & furnaces, Metal treatment
Electrical, electronic Reflow ovens, Solder baths
Composite materials Autoclaves, Curing ovens
Glass & Ceramics Kilns & furnaces
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Thank you.
For more information please contact the Technical
Support Team.
Juni 12 24