James R. Stringer
(Reference: Power Engineering Third Class Edition 2.5; Part A2; Chapter 11: Control Loops and Strategies)
There are four main components of a control loop:
The Process is an action that takes material/energy and modifies it to a different form.
The Measurement (signals) is to measure the process and to convey a value to the automatic controller. It is the interface between the process and controller as well as the process and building operator. The measurement is usually a transmitter (but not always) that transmits analog or digital signals. There may be more than one device needed to measure a variable.
The Automatic Controller (the brain) compares the Measurement signal to the set point of the controlled variable. The controller will make a correction when the difference exists between the signal and set point.
The Final Control Element (FCE) is what responds to the controller, such as a valve, damper, motor, etc. The FCE may have more than one component: transducer for pneumatic signal, a positioner, the valve actuator, and then the valve itself.
Pneumatic Signals use compressed air and usually supply 20 psi and the transmitter output signal is 3-15 psi.
Electronic Signals are normally 4-20 millamps. Current is chosen over voltage for transmission as amps have better immunity to electrical noise (unwanted disturbance). The signal to the FCE is rarely used to alter a process, so a transducer is used to convert the signal from milliamps to a more powerful pneumatic signal which in turn can have a large mechanical force on a valve.
Digital Signals use microprocessors and ‘smart transmitters’ that can process information in either a digital or analog format, adjusted remotely, and can have information on themselves readily available (such as the last time of calibration).
On-Off control has no proportional control and is either fully open or fully closed. Your furnace, being forced air, may remain off until the thermostat temperature falls below set point and will turn on until the set point is reached.
Proportional control is when a process variable is controlled 0-100% open. The difference between the set point and the value of the Process is the offset (an error that cannot be eliminated by proportional control).
A manual reset is a spring that opposes the force from the negative feedback loop for the controller output. For instance, when set point is reached, the bellows will equal the spring force of the manual reset and will not open further.
Alarms are audible or visual signals to alert the building operator to abnormal conditions. They are generally to alert the operator of conditions that are outside parameters but are also used for safety measures to prevent equipment damage or potential harm. Abnormal conditions may be temporary however and they may correct automatically without operator intervention. In these cases, the operator can silence or acknowledge the alarm and monitor until the parameters normalize.
Examples of some alarms can be high or low temperatures, level, or pressures, vibration, or detection of gases (toxic or combustible).
If the parameters do not correct on their own, the equipment may deenergize and remove the source of energy from the equipment. The equipment may remain deenergized until a building operator manually resets the alarm after investigation and takes corrective action if needed.