130-28176 Parallax Inc, 130-28176 Datasheet - Page 191

• Manufacturer Part Number: 130-28176
• Description: KIT PARTS PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 130-28176

• Accessory Type: Education Kit
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Board of Education Full Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

Chapter 6: Open Loop Continuous Process Control · Page 181
Chapter 6: Open Loop Continuous Process
Control
Continuous process control involves maintaining desired process conditions. Heating or
cooling objects to a certain temperature, rolling a thickness of aluminum foil, or setting a
flow rate of material into a vat in to order maintain a constant liquid level are all
examples of continuous process control. The condition we want to control is termed the
“process variable.” Temperature, thickness, flow rate, and liquid level are the process
variables in these examples. Industrial output devices are the control elements. Motors,
valves, heaters, pumps, and solenoids are examples of devices used to control the energy,
determining the outcome of the processes.
The control action taken is based on the dynamic relationship between the output
device’s setting and its effect on the process. Generally speaking, process control can be
classified into two types: open loop and closed-loop. Closed-loop control involves
determining the output device’s setting based on measurement and evaluation during the
process. In open-loop control, no automatic check is made to see whether corrective
action is necessary.
For a simple example of open-loop control, consider cooling your bedroom on a hot
summer evening. Your choices are using a window fan or an air conditioner. The window
fan is a device that you set – low, medium, or high speed – based on your evaluation of
what the situation needs for control. This evaluation involves an understanding of what
the cause-and-effect relationship is of your speed setting vs. the room conditions. There
is also an element of prediction involved. Once you make the setting decision, you are in
for the night. You are setting up an open-loop control system. If your evaluations are
correct, you will have a great night’s sleep. If they are not, you may wake up shivering
and cold or sweaty and hot! On the other hand, a room air conditioner allows you to set a
certain desired temperature. A thermostat continuously compares the desired temperature
with a measurement of actual room temperature. When room temperature is over the
desired setpoint, the air conditioner is turned on. As the room cools below the setpoint,
the air conditioner is turned off. As the night goes on and the outside temperature cools
down, this closed-loop system will automatically spend less time on than off. This is an
example of closed-loop feedback control, because the action is taken based on
measurement of room temperature.