Robot-assisted, source-camera-coupled multi-view broadband imagers for ubiquitous sensing platform

The work addresses a growing demand in industry and IoT for non-destructive, environment-independent sensing of arbitrarily shaped and located targets using electromagnetic waves. Prior imaging systems span visible, IR, THz/sub-THz and MMW, but comprehensive multi-view, broadband monitoring that is flexible, portable, and uncooled remains underdeveloped. The research aims to realize: (i) flexible multi-view stereoscopic sensing with switchable reflective/transmissive modes via a freely attachable uncooled broadband photo-absorber; (ii) built-in miniature photo-sources within flexible imagers; and (iii) target-location-independent, high-speed, omni-directional monitoring through robot assistance. The authors propose CNT-film photo-thermoelectric (PTE) imagers as a core technology enabling broadband, uncooled, flexible operation, and demonstrate integrated modules and robotic implementations for ubiquitous sensing.

The paper situates its contribution among advances in EM-based non-destructive imaging across visible, IR, THz/sub-THz, and MMW. Flexible, wearable imagers have enabled conformal scanning with reflective/transmissive modes, yet integration with sources and robotic platforms is limited. Materials explored for uncooled broadband photodetection include graphene, CNTs, MoS2, SnSe, NbS2, and PEDOT:PSS, with CNT films offering strong mechanical flexibility, broadband absorption, and sizable Seebeck coefficients suitable for PTE operation. Prior CNT devices (e.g., PN-junction types) had NEP ~30 pW Hz^-1/2, lagging state-of-the-art uncooled broadband detectors (sub-10 pW Hz^-1/2). Related developments include thermo-phototronic effects, portable photo-sources (QCLs, RTDs, Gunn diodes), flexible THz imagers, and robotics for unmanned inspections. However, comprehensive integration of flexible broadband imagers with embedded sources and robotic multi-view operation had not been adequately demonstrated.

Device and materials: Randomly stacked SWCNT films (semiconducting/metallic mixed, ~30 µm thick used in some demonstrations) serve as PTE channels, chosen for broadband absorption, mechanical strength, flexibility, and Seebeck coefficients of tens to hundreds of µV/K. A liquid-coating chemical-carrier N-type doping method (NaOH + 15-crown-5 ether) is applied locally to originally P-type films to form PN-like thermocouple structures for PTE response. Doping is done at room temperature and atmospheric pressure and is immediately operable post-drying.

Thermal/device design: Steady-state thermal simulations (ANSYS) modeled heat distribution and temperature gradients under illumination using CNT film properties (k ~10 W/mK, emissivity ~0.9) at 295 K to optimize channel geometry and substrates for temperature rise ΔT and sensitivity. Channel geometries typically: length 5 mm, width 1 mm, thickness ~1 µm, pitch 1 mm.

Fabrication: Selective suction filtration through a membrane filter (70 µm thick, 200 nm pores) with a UV-laser patterned (355 nm, 10 µm resolution) polyimide mask (5 µm) defines CNT channels. Stretchable silver-nanowire conductive paste electrodes are patterned and annealed (100 °C, 10 min), providing >50% omni-directional stretchability. Substrates and housings are 3D-printed (PLA/ABS) with 50 µm XY and 100 µm Z resolution: hemicylindrical capsule (14 mm radius, 3 mm thickness, 30 mm height, 2 mm square window), column supporter (12 mm radius, 100 mm height), and compact cylindrical substrate (15 mm radius, 3 mm thickness, 30 mm height, six 2 mm square windows).

Electrical/optical characterization: DC I–V measured with a current amplifier and programmable source; PTE DC voltages recorded with a 100 nV-resolution multimeter. NEP evaluated using NEP = sqrt(4kBTR/S^2Δf)*(VNoise/VSensitivity); noise spectra (1 Hz–100 kHz) measured via lock-in. TE performance assessed with ZT = S^2σT/κ; dependencies of Seebeck, resistance, thermal conductivity, and THz absorbance on dopant concentration were measured. Seebeck was measured using a micro-ceramic heater to induce +5 °C gradient and a K-type thermocouple; thermal conductivity via Xenon laser flash (100–500 °C, diffusivity 0.01–2000 mm^2/s, conductivity 0.1–4000 W/mK); THz absorbance via THz-TDS (0.5–7 THz, 2 fs time resolution, 3.8 GHz frequency resolution).

Photo-sources: Frequency multipliers (sub-THz/THz; λ=1.15 mm; λ=577 µm; λ=300 µm), CO2 gas laser (λ=10.3 µm; collimated or fiber-delivered), broadband FIR radiator (600 W), and NIR LED (λ=870 nm). Appropriate horns/collimation optics used.

Signal readout and scanning: Multiplexer data logger (up to 500 ch/s; used at 50 ch/s, 100 nV resolution) reads multi-pixel arrays, enabling >20× faster 2D imaging versus single-pixel scans. Stepping motors (500 nm step, 10 Hz scan) control sample positions; scan interval typically 100 µm (longer for longer wavelengths). Multi-pixel arrays used: 20 pixels (reflective capsule), 18 (transmissive endoscope), 40 (self-driving endoscope), 24 (portable 360° imager).

Module operation protocols: Reflective capsule—target column inserted within a hemicylindrical imager; ~60° effective monitoring region; ~1/6 of target circumference illuminated; calibration by background subtraction; 3D reconstruction from unwrapped 2D scans. Transmissive endoscope—columnar supporter wrapped with CNT array, inserted into cylinders; ~80° effective region; ~2/9 illumination; 3D reconstruction analogous. Portable 360° imager—six NIR LEDs embedded in windows of cylindrical substrate; six 4-pixel CNT arrays rolled into 360° sheet; background subtraction; 3D reconstruction of around-view images.

Robotic implementations: A self-driving unit coated with the flexible imager performed autonomous endoscopy in a miniature L-shaped tunnel under FIR illumination. A commercial hydraulic 5-axis arm integrated NIR LEDs and 8-pixel CNT arrays on upper/lower grips for omni-directional inspection, enabling grasping, swing/rotation, and precise positioning along targets.

• A physically and chemically optimized CNT-film flexible PTE imager achieved uncooled, non-vacuum minimum NEP of 8.57 pW Hz^-1/2 at λ = 10.3 µm, comparable to state-of-the-art uncooled broadband detectors (sub-10 pW Hz^-1/2).
• Substrate choice and device geometry boosted photo-responses up to 30×; device maintained PTE conversion under full folding; demonstrated broadband detection from sub-THz to NIR; noise suppression and imaging feasibility confirmed; device time constant supports potential 200 Hz imaging with appropriate readout.
• Reflective multi-view stereoscopic capsule imager: Non-destructive inspections of beverage bottles (non-metallic/metallic) revealed concealed defects (metal impurities, breakages, scratches) via local reflection changes. Multi-frequency sensing (sub-THz λ=1.15 mm and NIR λ=870 nm) enabled hierarchical image extraction of a multilayer column: NIR captured outer shell features (oil droplets on plastic), sub-THz probed inner metallic core with minimal surface interference (oil droplet transparency: NIR 44% vs sub-THz 95%). Simple reconstruction combined inner/outer images.
• Transmissive multi-view stereoscopic endoscope: Non-destructive endoscopy of plastic water pipes (sub-THz) detected concealed light-absorbing impurities via attenuated transmission; gas pipe inspection (FIR λ=10.3 µm) visualized distributed breakages. Effective viewing angles: ~60° (reflective), ~80° (transmissive). Scanning conducted at 10 Hz.
• Robot-assisted self-driving endoscopy: A self-driving unit with the imager navigated a confined L-shaped tunnel at 10 mm/s under FIR illumination, mapping breakages in 3D.
• Source-integrated portable 360° imager: Six embedded NIR LEDs (λ=870 nm) and a cylindrical CNT-array enabled omni-directional reflective imaging of columns at 10 Hz without rotational scanning. Demonstrations captured discrete metallic impurities and spiral metallic wire patterns. Multi-pixel coverage reduced acquisition time 24× relative to single-pixel scanning for the same spatial resolution (600 s → 25 s at 100 µm steps).
• Robot-assisted multi-axis movable PTE monitor arm: A hydraulic 5-axis arm with built-in NIR LEDs and 8-pixel CNT arrays in both grips performed non-destructive, unmanned, omni-directional inspection of a miniature aerial winding road-bridge at 15 mm/s, detecting front metallic impurities and rear light-absorbing defects via reflection changes.
• Fabrication enablers: 3D printing (50 µm XY; 100 µm Z) and UV laser micromachining (10 µm) supported custom windows and precise integration of miniature photo-sources, suggesting extensibility to other compact sources (QCL, RTD, Gunn, IMPATT) for mid-IR to MMW/sub-THz operations.

The integrated flexible CNT-film PTE imagers, with switchable reflective/transmissive multi-view geometries and embedded photo-sources, address the need for environment-independent, non-destructive, and target-agnostic sensing. Broadband operation (sub-THz to NIR) allows selecting spectral bands that emphasize specific layers or materials, enabling hierarchical imaging of multilayer structures. The free-form, conformal nature eliminates blind spots on curved targets and supports both external reflection and internal transmission modalities. Robot-assisted implementations extend reach, speed, and safety by enabling unmanned inspections in constrained or hazardous locations, demonstrating omni-directional, high-manoeuvrability monitoring. Open challenges remain in optimizing the trade-off between scan speed and image quality under different robotic motion profiles, scaling pixel arrays (e.g., 2D matrices) for motionless high-speed imaging up to the device’s intrinsic response (~200 Hz), and building spectral databases for materials like concrete and polymers to improve defect classification and depth inference, especially for inner defects where visible inspection is insufficient. Environmental robustness (dust, moisture, corrosion) via conformal coatings would further support long-term outdoor deployments.

The study introduces a unified, robot-assisted ubiquitous sensing platform that combines free-form CNT-film PTE imagers with built-in miniature photo-sources to realize multi-view stereoscopic broadband monitoring in both reflective and transmissive modes. The optimized CNT PTE devices reach an uncooled NEP of 8.57 pW Hz^-1/2 at 10.3 µm. Two module types were validated: a reflective capsule for columnar surfaces and a transmissive endoscope for cylindrical interiors. Demonstrations included non-destructive inspection of beverage bottles, water and gas pipes, hierarchical imaging of multilayer structures using sub-THz and NIR, a source-integrated portable 360° imager (10 Hz, 24× faster than single-pixel scanning), and a robot-assisted 5-axis arm achieving unmanned omni-directional inspection of a miniature aerial winding road-bridge at 15 mm/s. These results outline a roadmap toward universal safety and quality assurance networks. Future directions include larger-area high-density pixel matrices, roll-to-roll fabrication, AI-based image processing, integration of diverse compact sources (mid-IR to MMW/THz), optimization of speed–quality trade-offs, development of broadband spectral databases for industrial materials, and robust environmental coatings for sustained field operation.

• The speed–image quality trade-off for the scanning and readout setup remains unquantified; current imaging speed (10 Hz) is limited by stepping motors and data logging, though device response suggests up to ~200 Hz is feasible with motionless large-area arrays.
• Material spectral databases for targets such as concrete and polymers are needed to enhance accuracy, particularly for non-visible inner defects (e.g., internal breakages, chloride ingress, corrosion, liquid impurities) across MMW/sub-THz/THz.
• Environmental robustness (dust, moisture, corrosion) requires further development of conformal protective coatings for long-term outdoor deployment.
• Demonstrations are on miniature models and selected industrial components; broader validation across real-scale infrastructures and diverse environmental conditions is still needed.