Defense Research and Development Canada - Atlantic Research Centre (DRDC Atlantic) is developing a towed underwater projector called the Towed Multichannel Acoustic Transmitter (TOMCAT). The projector is directional and can steer acoustic signals electronically. The acoustic signals are produced by a two dimensional phased transducer array mounted in an internal frame.
The transducers have been constructed using piezoelectric materials. When the transducer receives alternating voltage it expands and contracts. The motion creates pressure waves in water which can be heard at audible frequencies. The transducers are arranged in eight columns of four and the transducers in each column are electrically connected in parallel. The arrangement of transducers in the internal frame can be seen in Figure 1. The column groups are called staves and each stave is controlled by a dedicated signal channel. Using the propagation speed of sound travelling in water the staves can be activated in sequence allowing the sonar operator to direct acoustic signals in a narrow pattern at a specified bearing.
Figure 1 - Transducer Array Frame
The TOMCAT tow body lacks active stabilization and is subject to hydrodynamic forces that can cause the tow body and the internal transducer array to experience pitch and roll when towed. The current design allows the transducer array frame to be locked in place up to five degrees above or below the tow body’s horizontal plane. This allows for compensation of the natural pitch associated a specific tow profile at a specific speed. Many factors determine the orientation of the tow body while being towed. If the tow body is tilted in pitch and roll the projected sound waves from one side of the tow body will be directed deeper into the water while the other side will be projected towards the surface. This may result in interference from reflections from thermal layers below or from the surface above.
TOMCAT utilizes an acoustic projector capable of directing narrow patterned acoustic signals at specific bearings. The TOMCAT tow body will react to the hydrodynamic forces while being towed causing the tow body and the transducer array to rotate in pitch and roll. The alignment of the transducer array is unknown causing error in the expected direction of the acoustic signals. Rough sea conditions will also contribute to unwanted rotation of the transducer array.
The purpose of this project is to create a system capable of stabilizing the transducer array in pitch and roll while keeping the array perpendicular to the horizon as the tow body pitches and rolls. By maintaining the position of the transducer array in a known state, the sonar operator will have increased confidence in the direction of the projected acoustic signals. This system will also allow the tow body to operate in rougher sea states.
The Tow Body