Time-of-flight (TOF) sonic rangefinders emit a signal composed of sound waves and measure the signal that is reflected off objects in the environment in order to measure the presence, range and/or direction to said objects. The range is determined by measuring the time of flight of the sound wave. Short duration pulses or temporally coded pulses allow the system to resolve multiple targets that are spaced in range. Phased array rangefinders incorporate multiple transmitters and/or receivers which are spatially separated in order to discern the direction (azimuth and/or elevation) to the objects.
Typical acoustic transducer designs are membrane structures which are driven into flexural vibration using piezoelectric or capacitive actuation. The vibration of the membrane creates air motion which propagates as sound away from the transducer. Returning sound waves cause the transducer to vibrate and this motion is sensed by an electronic amplifier through piezoelectric or capacitive sensing techniques. Multiple transducers are fabricated on a single substrate using wafer fabrication techniques.
By way of example, consider FIG. 1, which illustrates the prior art. An acoustic transducer array substrate 28 contains acoustic transducers 22 and 24 which are coupled through acoustic ports 25 and 26. Transducer designs may be released using a through-wafer etch which forms a trench beneath the transducer. The trench may be designed to be an acoustic resonator which has a length of approximately one quarter of a wavelength, as described in S. Shelton, O. Rozen, A. Guedes, R. Przybyla, B. Boser, and D. A. Horsley, “Improved acoustic coupling of air-coupled micromachined ultrasonic transducers,” 27th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2014), pp. 753-756, San Francisco, Calif. 2014, incorporated by reference herein in its entirety. In this way the trench serves as an acoustic waveguide. Proper design enables the transducer to have increased sensitivity and reduced mechanical response time.
In FIG. 1, the phased array acoustic transducers 22, 24 are spatially separated in order to minimize the beamwidth of the acoustic signal when the array is used as a phased array transmitter and/or receiver. The spacing is a tradeoff between beamwidth and sidelobe level. When the spacing is increased, the beamwidth is reduced, but the sidelobe level increases. In practice the array elements are preferentially spaced between one half wavelength and one wavelength apart. It is often desirable that each element in the array have a substantially isotropic acoustic radiation pattern such that the array can be electronically steered over a wide range of angles. In order to create an isotropic vibrating membrane transducer, the transducers 22, 24 must be designed to be smaller than the wavelength A of the sound wave. When the diameter of each transducer is smaller than the required spacing, there can be considerable unused area between each transducer, which increases the cost and size of the transducer array chip. A new method is required to reduce the considerable inactive space on a transducer substrate.
U.S. Pat. No. 8,199,953, describes a method to connect multiple acoustic waveguides together above a single acoustic transducer, thereby producing an acoustic horn with multiple apertures. As described in that patent, the intent of this multi-aperture horn is to take a single omnidirectional transducer and shape the acoustic output so that the single transducer produces a directional acoustic output. However, U.S. Pat. No. 8,199,953 does not teach how to use acoustic waveguides in a phased array, nor does it teach how to use an array of waveguides together with an array of transducers.
Therefore, a new method to design and operate phased array rangefinders is required to address these shortcomings in the state of the art.