Models 2510 and 2510-AT User’s Manual PID Control Concepts 4-9
Autotune limitations
The Model 2510-AT autotune algorithm assumes that the system response to a step func-
tion in the TEC voltage is an exponential temperature rise of the form:
(for t < t
Lag
)
(for t ≥ t
Lag
)
This relationship is depicted graphically in Figure 4-6.
Figure 4-6
System response to step function
In this case T
step
is positive, but it could also be negative. Temperature responses of a dif-
ferent mathematical model will cause the autotune function to fail (or give unpredictable
results). Once the Lag and Tau times are extracted from the sampled data taken from the
voltage step response, the tuning constants can be calculated. Inherent limitations that will
cause the tuning function to fail include:
• A system Tau time of less than 1 second or greater than 470 seconds
• A system Lag time greater than 0.6 × Tau
• Ambient temperature outside the range protection limits (programmed by the user)
• Ambient temperature movements during the autotune sequence of greater than ±1°C
• Noisy temperature measurements
• Reaching any temperature, current, or voltage limits during the final autotune volt-
age step test
T
system
T
initial
=
T
system
T
initial
T
step
1e
tt
lag
–()
τ
-------------------
–
–
+=
Tlag0
Time
Temp
Tfinal
Tinitial
Lag Tau
Tstep
63%
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