Feedback and Control Systems

The U.S. National Science Foundation Laser Interferometer Gravitational-wave Observatory (NSF LIGO) is one of the most sophisticated, complex, and precise scientific instruments ever built. The interferometers contain the purest, most stable lasers on Earth, precisely machined and polished mirrors, the second-largest ultra-pure vacuum system in the world, complex, multi-stage pendulum systems connected by glass fibers a mere 0.4 millimeters thick, and countless other parts, each of which is essential to NSF LIGO's success. The immensity, stability, and yet fragility of LIGO cannot be overstated.

All of LIGO’s components must be kept in perfect stillness and alignment in order to function optimally and detect gravitational waves. Despite the sophistication of LIGO’s construction and design, the biggest challenge and threat to LIGO’s success comes from environmental factors that human beings simply cannot control. Earthquakes, weather, tides, even traffic on nearby roads can all affect LIGO’s ability to detect a gravitational wave.

While we can’t stop the world and its inhabitants from causing vibrations, what we can control is LIGO’s responses to these environmental disturbances. We do this through the use of hundreds of levels of feedback and control systems, which maintain all of LIGO’s parts in near-perfect stillness in the quietest man-made environment on Earth.

What is Feedback?

We are surrounded by feedback and control systems. Thermostats and cruise-control systems in cars are but two common examples. Each of these devices takes in a signal (current temperature, current speed), compares that signal with a set value (preferred room temperature, preferred speed), determines if there is a difference between those numbers, and if there is a difference, feeds that "error" back to a control mechanism, which then acts on the device to eliminate the difference. If the room becomes too hot by 3 degrees, the air conditioner reduces the temperature by 3 degrees to eliminate the error in temperature and bring the room back to expected levels. Every so often, it takes a reading and adjusts the temperature to maintain the desired level. In cruise control systems, if the vehicle speed drops below a set value, the system detects and measures the difference in speed (the 'error') and then applies the throttle to eliminate the difference.

LIGO’s feedback and control systems operate under the same basic principles, but on thousands more levels. While a typical cruise control system reports current speed to the car’s computer about ten times per second (or 10 Hertz), the feedback and control system that keeps just LIGO’s arms in perfect alignment reports changes 16,384 times per second, or 16 kilohertz (kHz)! That’s once every 0.000061 seconds, or 3500 times faster than the fastest human reaction time of 0.215 seconds.