122-28176 Parallax Inc, 122-28176 Datasheet - Page 300

• Manufacturer Part Number: 122-28176
• Description: GUIDE STUDENT PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 122-28176

• Accessory Type: Manual
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Propeller Education (PE) Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

ACTIVITY #5: PROPORTIONAL-INTEGRAL CONTROL
Parts required:
Same as Activity #1
So far we’ve looked at what occurs when quick disturbances occur to our system in
equilibrium. Proportional control may be used to drive the temperature back to the
desired setpoint. But what happens when the disturbance affects the equilibrium of our
system over a long period of time? At the end of the last experiment, we saw what occurs
when the bias drive is not sufficient to make-up for average losses. Because some error
must exist for proportional drive, the setpoint temperature cannot be maintained.
Integral control can be used to drive-away any error remaining due to long lasting
disturbances or imbalances in the system. These errors may be from additional losses or
gains of energy that remain for a long period of time. Consider our incubator. We found a
bias temperature at which a 50% bias drive was sufficient to make up for the losses in the
system, maintaining it in equilibrium.
But what would happen if the room temperature were 10 degrees cooler? Continuous
system losses would be higher. The 50% bias drive will be insufficient to maintain the
temperature, and proportional drive will respond to the error in an attempt to drive the
system back toward to the setpoint with a steady-state error remaining. The system will
stabilize at a temperature below the desired setpoint. Over time, integral control can be
used to drive away this error, allowing the temperature to reach the setpoint.
Integral drive is also used when a slow approach with long stabilization times are needed
to ensure no overshoot. Consider the example of cooking soup. After cooking a bit, you
taste, add an amount of salt you feel appropriate for what you would like the final taste to
be. Do you taste immediately and add more? No, you wait a while to allow the salt to
blend in, then taste, and add a bit more until you finally reach your desired taste. What if
too much salt is added? Cutting back is a bit more difficult!
Co
%Drive
PID
= B + (K
TOTAL
= %Drive
P
E) + (K
BIAS
I
ΣE∆t)
+% Drive
PROP
+ %Drive
INT