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Loop gain is the gain in a parameter when a portion of the output is fed back as input. Negative and positive loop gain systems are used in control techniques for various systems. A combined feedback/feed-forward control system is used to maintain a constant output value. Open-loop gain does not use process information to control system behavior. Closed-loop gain feedback systems are used in production and statistical process control, while biological systems use gain cycle controls.
Loop gain is the term applied to the gain in a parameter when a portion of the output of a device in the circuit or system is fed back as an input to the device. There are negative and positive loop gain systems. This feedback mechanism is important as a control technique in chemical process control, production lines, electronic circuits, and biological systems.
Screal from a speaker when a microphone is aimed at it illustrates out-of-control loop gain. The microphone picks up the sound from the speaker and sends it back to the amplifier, which magnifies the sound and sends it back to the speaker. As the cycle continues, the signal is greater than the speaker’s operational input range and annoying sound issues.
In negative feedback, information is moving backwards. Input streams arrive at a processing stage, the process or change occurs, and the output parameter is measured and compared to the desired setpoint or value. An upstream adjustment is made and the cycle repeats. This is a closed loop gain feedback system.
Positive feedback, also called feed forward, occurs when input stream parameters are measured before the process and the controller is adjusted before the process occurs. Both systems have merit and are typically combined using a feedback controller to provide information to the feed-forward controller. The feed-forward controller processes this data with the input stream parameter data to effect the adjustment.
Heating a stream of liquid to a set temperature by passing the liquid through a heat exchanger demonstrates this combined control system. The temperature and flow rate of the incoming liquid are sent to the feed-forward controller. The downstream temperature is sent to the feedback controller, which compares it with the setpoint. The feed forward controller modifies the change requested by the feedback controller with the current information it is receiving. Using this combined closed-loop gain control system, swings in the temperature of the heated liquid are avoided and the remaining process runs more smoothly.
The combined feed-forward/feedback control system could be called a buffer because it works to maintain a constant output value. An operational amplifier is an electronic device that does the same thing. It accepts two inputs and has a single output that goes high or low, depending on how the inputs differ. The design of some op amps causes one of the input values to invert or change sign, and then the output is fed back as one of the inputs. The goal is to balance both inputs to the same value.
Open-loop gain refers to a control mechanism that does not use any process information to control system behavior. An op amp can be characterized as an open-loop gain device, but is placed in a larger feedback control system, which is closed. It serves as a buffering device in the circuit.
Production uses closed-loop gain feedback systems, although often through manual processes. The basis of statistical process control is another expression of a feedback system. Biological systems use gain cycle controls, from controlling cellular respiration to balancing entire ecosystems.
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