1. Acoustic mapping technology

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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)

9. Measuring distance in a 3D environmentz1
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

10. 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

11. 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,

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

13. 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

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

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

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

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

18. 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

19. 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

20. 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

21. 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