122-28176 Parallax Inc, 122-28176 Datasheet - Page 292

• 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 #3: PROPORTIONAL CONTROL AT BIAS TEMPERATURE
Parts required:
Same as Activity #1
The next evaluation in the PID equation is proportional control of the system. The
amount of drive from proportional control is a direct relationship to how much error
exists in the system. The greater the error, the greater the proportional drive. Error is the
difference between the value we want the system to be and its measured value.
Note that when the temperature is greater than the setpoint, it produces a negative error.
This may seem arbitrary, but it's an important fact. As temperature increases, the drive
must decrease to bring the system back to the setpoint. A temperature above the setpoint
will produce a negative error. This is opposite of how the light sensor testing was
performed. This is an example of the drive being inversely proportional. The output
drive is proportional to the error, but in an opposite direction.
As mentioned, we will work in percentages, so let's review the math needed. First, we
need to define a band of temperature control. How about 100 °F +/− 0.5 °F? This
defines an allowable temperature band of 1.0 °F.
For a temperature error of +0.3 °F:
For -0.8 °F?
Quite simply, the %Drive due to proportional control is the proportional gain (K
the %Error. A gain of 1 with a percent error of 30 yields a drive change of 30%!
Co
%Drive
Error = Setpoint −Actual
%Error = E / 1.0 °F x 100% = 0.3 °F/1 °F x 100% = 30%
%Error = E / 1.0 °F x 100% = −0.8 °F/1 °F x 100% = -80%
PID
= B + (K
TOTAL
= %Drive
P
E)
BIAS
+% Drive
PROP
P
) times