Vibrations
 

Preface Introduction
Mass, Acceleration and Force
Gravitational Forces
Continua
Mechanical Vibrations & their Characteristic Modes
Friction
Vibration of Drill Rigs
Vortex-Induced Vibrations
Feedback
Stable & Unstable Motion induced by Forced Vibrations
Aerodynamics
Human-induced Vibrations
Electromagnetism
Probability Waves & Quantum Mechanics 

  

Feedback
  We have seen that the vibrations of a drill string can lead to dramatic and damaging motions unless these effects are controlled. One attempt to prevent such effects building up involves placing mechanical sensors on the side of the string that respond by generating small electric currents to strains in the material. These electrical signals can be analysed and amplified signals sent to the power source driving the rotary motion. If the sensors are place at the top of the drill string the amplified signals are electrical in nature.
Some sensors detect the motion of the end of the dill string using gyroscopes cradled within the BHA. Information near the cutting action is sent to the surface as sound pulses through the "mud" used to lubricate and effect the removal of rock cuttings. Such "mud telemetry" is more reliable in a mechanically hostile environment. By varying the rate of torque production a compensating torsional wave can be sent down the drill-string to prevent the build up of slip-stick vibrations at the drill-bit. This is an example of "feedback" and is widely used to control unwanted torsional vibrations. The mechanism is analogous to the way an experienced car driver can control his brakes to escape from a skid.
Instead of continuously applying a pressure to the break pedal he alternatively applies pressure on and off the pedal rapidly. The result is that the effective frictional adhesion between the road and the car tyres is dynamically modified and traction restored. In this case the feedback sensor is the driver's sensory apparatus. Active feedback is achieved via the driver's brain and the steering and break pedal. In modern cars the ABS servo-control replaces this feedback loop sometimes to advantage.