25
Fig. 7.3 Heat/Cool Control – Principle of Operation
7.3
Introduction to Heat/Cool Control
7
CONTROL LEVEL…
Information.
•
P.I.D. Output –
is the output value calculated by the controller. The output is divided
into two different control elements, one for raising the product temperature (heat
output) and one for lowering the product temperature (cool output).
•
Transition Bandwidth –
used to transfer smoothly from one set of control terms to
the other.
•
Crossover Value –
defines the changeover point between heat output active and
cool output active. The crossover value is also the centre of the transition and off
hysteresis bands.
•
Output Off Hysteresis Band –
for the majority of applications the heat and cool
outputs have opposing control actions, i.e. one is direct acting and the other is
reverse. In this configuration both outputs are at 0% within the off hysteresis band.
The band setting is used to prevent oscillation of control changes.
•
Heat/Cool Outputs –
refer to
P.I.D. Output,
above.
Note. Refer to Sections 7.3.2 and 7.3.3 for Crossover Value and Transition
Bandwidth Value examples.
0
Heat
Output
(%)
Cool
Output
(%)
100
0
10
20
30
40
50
60
70
80
90
10
20
30
40
50
60
70
80
90
100
Max. Cool Output
= 75%*
Max. Heat
Output = 88%*
0
100
Q Heater = 1.5kW
Q Cooler = 0.75kW
Output Off
Hysteresis Band
=15%*
*User definable limits
Transition
Bandwidth
20%
Proportional
Band (Cool)
25.0%
Integral Action
Value (Cool)
Derivative Action Value
Proportional
Band (Heat)
20.0%
Integral Action
Value (Heat)
Derivative Action
Value
100%
0%
Crossover
Value
(33%)
P.I.D. Output (%)
