Acoustic mapping technology

Agenda Surface mapping with a wide angle beam Measuring distance in a 3D environment Advance Radar Signal Processing Different perspective: Multi frequency, Beam tilting Directional filtering of false echo

Level measurement with narrow beam Goal: Measure the distance to a single, well defined point on the top surface Distance measurement based on the time of flight Point measurement requires narrow beam Narrow beam =>Ultrasonic => Poor dust penetration

Surface mapping - wide angle beam Goal: Multipoint (X,Y,Z) measurement of the surface Large coverage => Transmit a Wide angle pulse Wide beam=>Low freq.=> Effective dust penetration Multiple reflected echo from the entire surface Emanating from Large surfaces and Extreme points Material / Silo Wall interface Silo Walls – False echo to be filtered Separated by Time of Arrival Require estimating both Range and Direction

The advantage of angle estimation R1,θ1 Multiple antennas device R2,θ2 Distance and direction R3,θ3 Single antenna device R1 R3 R2 Distance measurement

Direction of arrival (DOA) - Array processing Plane wave impinging a Microphone array Simplified example - 1D θ Direction of the plane wave C Speed of sound d Array spacing The Time Of Arrival (TOA) is different Wave Path difference between Microphones ΔP=d*Cos θ The time of arrival determines the angle ΔT =ΔP/C= d*Cosθ/C Path difference => TOA difference => θ

3 Dimensional surface reconstruction 3D Surface mapping requires a 2D array 2 Dimensional Array of 3 Antenna Estimating per echo: Distance (R), Azimuth (Ф) and Elevation (Θ) angles Convert R, Ф, Θ to Silo X,Y,Z coordinates Final surface reconstruction, based on Extreme and boundary points The Material angle of repose The silo geometry Acoustic principles

Measuring distance in a 3D environment May 2011 Measuring distance in a 3D environment Distance (R) = velocity times the time an acoustic wave travels R = Velocity x (time/2) A stop watch starts at beginning of transmission and stops when echo is received Challenge: to find the right echo as well as the direction from which it arrives. z z1 y R = radial distance x Speed of sound in the air = 1,236 Km/H (343 m/sec)

Measuring distance in a 3D environment z1 xi, yi, zi z Calculating distance in a 3D environment: x - y - z R Distance: absolute radial (diagonal) distance measured from the top of the transducer case to any point on the surface Z Distance (x, y, z coordinates): straight down distance from a given point on the surface to the top of the transducer case at a given point x,y offset on the roof R y y1 xi, yi 0.0.0 K x1 x      

Measuring distance in a 3D environment Each APM scanner uses 3 antennas Each antenna transmits acoustic waves and receives echoes coming from all directions. Echoes coming from the walls of the silo are faulty and must be filtered out. Reference Point 0,0,0 R1 R2 R3

3 antennas – 3 different measurements of a single point May 2011 3 antennas – 3 different measurements of a single point Guarantees knowing the precise x-y-z location of each point Since scanner is located in a specified position (xi, yi, zi) on the silo roof, all measurements must be normalized for the true, central reference point, 0.0.0.

Example from a large dome in the US 60M Dome, 6 Scanners, 130 Mapping points

Advanced Radar Signal Processing Range resolution: Key figure - The minimal distance between two echo sources that are still separable Signal to Noise Ratio (SNR): Required for the detection of small echoes. Long pulse => higher energy => better SNR It is a challenge to create pulses that excel in both range resolution and SNR. A very active research topic for military radars

Advanced Radar Signal Processing (2) Long pulse (7M ) - modulated sine wave, rectangular envelope After match filter -Very poor range resolution - limited to 7M!

Advanced Radar Signal Processing (3) State of the art - Chen and Cantrell 2002 After match filter - 50cm resolution, 1/13 side lobes

Advanced Radar Signal Processing (4) APM Proprietary – US Patent 8,391,336 After detection: Significantly improved resolution, virtually no side lobes!

Advanced Radar Signal Processing (5) State of the art APM 3 targets: The importance of resolution and low side lobs More points => Better mapping

Advanced Radar Signal Processing (6) The resolution in detection of a short pulse The energy of a long pulse Using relatively small bandwidth, optimized for the application Virtually no side lobes Applicable for the entire scanning range Maintain constant resolution across the scanning range The longer the distance – the higher the pulse energy Very efficient numerical implementation

Beam steering with the array Tilting the main lobe with relative phase shifts in the antennas Significantly improves false echo separation and the investigation of complex surfaces by Looking at the same point through several beam tilts to solve symmetry problems (i.e. dual echo at +10 and -10 degrees) Allowing high SNR at a much larger sphere

Multi-frequency technology Gather information separately with different frequencies Beam angle - Interaction with silo walls Interaction with material, specular vs. scattering reflections Different multipath trajectories Different dust penetration capabilities Improved robustness to acoustic noise sources

Directional False echo filtering Automatic mapping of False echoes Silo does not need to be empty when the scanner is installed Automatic Identification of static reflection during the emptying process Takes advantage of the 3D information for robust estimation Multipath echoes: Due to multiple reflections between the material and the structure of the silo Analysis based on Distance and Direction measurement Information from different frequencies Information from different beams Echo tracking over time Silo geometry Fuzzy logic based decision system