122-28176 Parallax Inc, 122-28176 Datasheet - Page 290

• 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

Page 280 · Process Control
Analyzing our run in Figure 8-13, we see that it required about 5 minutes to stabilize at
the 50% bias temperature of around 98 °F.
Changing the %Bias to 60% and then to
55%, the temperature slowly drifted and stabilized closer to the setpoint.
Is this system acting as a closed-loop? Not really. Just as in Chapter 6, we are manually
manipulating the drive to control the system. As seen from the bottom graphs, the only
time that the %Dt (Drive Total) changes is when we adjust the bias. Following a
disturbance at 13 minutes, the system required approximately 7 minutes to recover and
stabilize once again at the setpoint, though slightly higher - the room's winter heater may
have raised temperature in the room, slightly changing conditions.
At 55% drive, the energy added by the heater equals the amount of energy lost at the
setpoint to maintain the system stable. But, if the air around the incubator becomes
cooler, greater energy escapes resulting in a lower stabilized temperature.
Program Discussion
The program IncubatorPID-SP.bs2 and macro sic_pc_pid_sp_calc.spm work together to
control the drive to the heater. Upon BASIC Stamp reset, the temperaure limits for
monitoring and the drive time are read. With each loop, the raw data is sent to StampPlot
and the PID calculations and plotting are performed. The BASIC Stamp reads the total
drive percent and drives the heater based upon that value. StampPlot can perform
floating point math and handle very large numbers.
All integral and derivative
calculations are based on the real time that they arrived. One drawback is that it takes
time to send data from the BASIC Stamp, allow StampPlot to perform calculations, and
accept the returning data. With a drive time of 1 second, typical times of ∆t were 1.75 to
2.10 seconds, and may be slower depending on the speed of your computer. Also, the
incubator will not work unless StampPlot is running.