Wavelength Electronics LFI-3500 Series, User Manual

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28
Configure the PID Constants
ERROR AMPLIFIER: The error amplifier section provides the difference
between the setpoint temperature and the actual temperature of the load
device as sensed by the temperature sensor. This difference is known as
the error term. This error term is fed to the PID processor. The range of the
setpoint temperature signal and the gain of the temperature sensor amplifier
typically determines the temperature range of the controller for a given
sensor.
The sensitivity of the error amplifier is determined by the temperature sensor
amplifier gain and the sensitivity of the temperature sensor. For example,
the error terms generated when using a 10k
Ω
thermistor and an AD590 at 15
°
C are quite different. With a thermistor current source of 100
µ
A, the
thermistor will produce a 76 mV/
°
C variation on the error term. The AD590
with a 10k
Ω
sense resistor will change the error term by 10 mV/
°
C. The
AD590 will allow you to operate over a wider temperature range than the
thermistor, but the thermistor will be more sensitive to temperature changes.
PID PROCESSOR: The PID processor section consists of a proportional
gain amplifier, an integrator, and a differentiator, all of which can be
implemented using simple op-amp circuits. In most PID controllers, the
integrator time constant (I) and differentiator time constant (D) are fixed and
only the proportional gain (P) is variable. Often, (D) is not even used, even
though it is necessary for an optimal system.
Simpler control loops utilize only the proportional gain stage. Proportional
controllers are inherently stable for low gains, but cannot produce a zero
error between the temperature setpoint and sensor feedback. A non-zero
error must be maintained to produce a finite output control signal. The
addition of the integrator function reduces the error to zero, but also reduces
the stability of the control loop. The integrator produces a finite output even
when the error term is zero because the output of the integrator is a function
Error Amplifier
PID Processor
Se
Mod. Input
Sensor
Feedback
Error Term
Switch 8
Switch 7
P
I
D
To Output
Power
Amplifier
Switch 6
Switch 9