Feedback in engineering

Feedback is ubiquitous in engineering. Its application has led to device features and machines which would not otherwise function. Here are few examples:

Climate control: A sensor measures the temperature and humidity in a room and then heats or cools and humidifies or dehumidifies accordingly.

Automobile cruise control: The car measures its speed and then applies the accelerator or not depending on whether the speed must be increased or decreased to maintain the target speed.

Highly maneuverable fighter jets: The F-16 Falcon fighter jet is an inherently unstable aircraft (i.e. the airframe will not glide on its own). The F-16 does fly because 5 onboard computers constantly measure the aircraft’s flight characteristics and then apply corrections to the control surfaces (i.e. rudder, flaps, ailerons, etc…) to keep it from tumbling out of control. The advantage of this technique is that the aircraft has the very rapid response and maneuverability of a naturally unstable airframe, while also being able to fly.

Feedback in electronics:
Op-amps use feedback to achieve very high linearity and predictability for their closed-loop gain by sacrificing some of their extremely high open-loop gain. Another common application of feedback in electronics is in precision, fast- response power supplies. Constant current and constant voltage power supplies which have a high degree of stability use feedback to regulate their current or their voltage, by measuring the current and voltage across a precision shunt resistors and then using feedback to automatically correct for any deviations from the desired output. Feedback also allows the power supply to adjust its voltage or current very quickly and controllably in response to a change in load.

Feedback in physics
Feedback has become a familiar tool for experimental physicists to improve the stability of their instruments. In particular, physicists use feedback for precise control of temperature, for stabilizing and cooling particle beams in accelerators, for improving the performance of atomic force microscopes, for locking the optical frequency of lasers to atomic transitions, and referencing quartz oscillators to ground state atomic hyperfine microwave transitions in atomic clocks, to name just a few example.
Temperature control: Many delicate physics devices, such as crystals, lasers, RF oscillators, and amplifiers, require their temperature to very stable in order to guarantee their performance. For example, the wavelength of diode lasers generally has a temperature dependence on the order of 0.2 nm/°C.
Stochastic cooling: In a particle accelerator, the transverse momentum spread of particles must be kept to a minimum. This increases the particle density, or beam luminosity, and consequently the probability of collisions with a similar counter-propagating particle beam in the detector area. Stochastic cooling works by measuring the transverse positions and momenta of the particles as they pass through a section of the accelerator, and then applying appropriate momentum kicks to some of the particles at other points in the accelerator ring to reduce the overall transverse momentum spread. The process is repeated until the momentum spread is sufficiently reduced. The 1984 Nobel Prize in Physics was awarded in part to Simon van der Meer for his invention of stochastic cooling which contributed to the discovery of the W and Z bosons (weak force mediators) at CERN.
Atomic force microscope: An atomic force microscope uses a very sharp tip (just a few nanometers in size at the very tip) which is scanned back and forth just a few nanometers above the surface to be imaged. Instead of scanning the tip at a constant height above the surface, which could lead to the tip actually running into a bump on the surface, the microscope uses feedback to adjust the tip height such that the force (from the surface atoms) on the tip is constant.
Laser locking: Many experiments in atomic and optical physics require lasers which have a very stable optical frequency. The optical frequency of the laser is locked by measuring the optical frequency difference the laser and an atomic transition and using feedback to set this difference to a constant value. Lasers can be routinely stabilized with feedback to better than 1 MHz out of 3×1014 Hz (about 1 part per billion), though stabilities close to 1 Hz have been reported after heroic efforts.
Atomic clocks: In an atomic clock, the frequency of an RF oscillator (a quartz crystal for example) is compared to that of a ground state atomic hyperfine microwave transition (6.8 or 9.2 GHz). The frequency difference is measured and the frequency of the RF oscillator is corrected by feedback. The process is constantly repeated to eliminate any drift in the frequency of the RF oscillator. Atomic fountain clocks can achieve accuracies in the range of 1 part in 1015, and plans are underway to construct optical atomic clocks with accuracies and stabilities of about 1 part in 1018.