28176 Parallax Inc, 28176 Datasheet - Page 292
![KIT PARTS PROCESS CONTROL W/TEXT](/photos/8/99/89989/mfg_28176_sml.jpg)
28176
Manufacturer Part Number
28176
Description
KIT PARTS PROCESS CONTROL W/TEXT
Manufacturer
Parallax Inc
Datasheet
1.122-28176.pdf
(330 pages)
Specifications of 28176
Accessory Type
Process Control
Product
Microcontroller Accessories
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
For Use With/related Products
BASIC Stamp® 2 and Board of Education
Lead Free Status / RoHS Status
Lead free / RoHS Compliant, Contains lead / RoHS non-compliant
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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
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