Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 261

Figure 9.12b. Mechanical siren layout, hydraulic and control system, electro-
hydraulic and angular velocity control servomechanisms from Gardner (2002).
Control systems in actual tools would drive compact hydraulic or brushless
DC motors. They need to be robust, since mud flow rates are rarely constant
and torque characteristics change with tool erosion and random debris
impingement. Torques will also change if edges of brittle wear plates bonded to
rotors and stators are lost to the flow or if solid particles lodge within rotor-
stator gaps. Recent improvements in data rate are due mainly to incremental
improvements, e.g., advances in control, state-of-the-art microprocessors, phase
shift changes that can be carried out in two (as opposed to, say, three) wave
cycles because low-torque downstream rotors are used with powerful motors.
Our long wind tunnels are intended to evaluate game-changing telemetry
concepts. As noted, these include novel uses of signal augmentation by
constructive wave interference, the use of multiple sirens placed in series,
operating at identical or different frequencies, and so on.