Wavelength Electronics LFI-3751, User Manual

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1. Proportional Gain – “P” and Autotune Optimization
The proportional gain, P , is measured in Amps Output Current per Volt Error
Voltage. The Error Voltage is the difference between the LFI-3751’s Setpoint
D to A and the measured sensor voltage feedback. When a difference exists
between the setpoint temperature and actual temperature, the proportional
gain directs the output power stage to supply an output current that is
proportional to the difference. Unfortunately, if the Error Voltage drops to
zero then so does the output current, which will cause the load temperature to
fall back towards ambient. The larger the
P value, the closer the proportional
gain will settle the load to the desired operating temperature. Likewise,
the larger the
P value, the greater the tendency to cause temperature
overshoot.
L and R
max
affect the proportional gain setting P . A long thermal lag, L ,
decreases the proportional gain value. P also decreases when R
max
, the
maximum temperature versus time slope, is large.
Setpoint Response Tuning results in smaller P values than Disturbance
Rejection Tuning because lower proportional gains produce less temperature
overshoot of the setpoint temperature.
2. Integrator Time Constant – “I” and Autotune
Optimization
The integrator time constant, I , is measured in seconds. The integrator
function forces the actual temperature to stabilize at the setpoint temperature,
something the proportional gain cannot do alone. This is accomplished
because the integrator “integrates” the Error Voltage over time. When the
actual temperature is far from the setpoint temperature, the integrator charges
more quickly since the Error Voltage is large at this point. As the actual
temperature approaches the setpoint temperature, the Error Voltage decreases
and the integrator charges less quickly until fi nally the Error Voltage is zero.
Once the Error Voltage is zero, the integrator stops charging and controls the
output current at the necessary level to maintain a constant load temperature
equal to the setpoint temperature.
Some thermal loads oscillate around the setpoint temperature. This oscillation
is referred to as thermal cycling. Thermal cycling can occur if the proportional
gain is greater than the system’s critical proportional gain or more commonly
too small an integrator time constant has been set to compensate for the
thermal load’s lag time,
L . A small integrator time constant charges more
quickly than a larger integrator time constant. If L is signifi cantly larger than
the integrator time constant then the integrator will force the output current to
shoot past the setpoint temperature since the temperature sensor is lagging
behind the thermal load’s actual temperature.
Theory of Autotune PID
When started,
Autotune generates
a step response
to characterize
your system.
TE Current
Time
Your system responds by
going towards the setpoint.
Temperature
Time
R
max
L
T
Chapter 2 - Front Panel Operation
Theory of Autotune PID