Mostafa Mirshekari

Characterizing Wave Propagation to Improve Indoor Step-Level Person Localization using Floor Vibration

The objective of this paper is to characterize frequency-dependent wave propagation of footstep induced floor vibration to improve robustness of vibration-based occupant localization. Occupant localization is an essential part of many smart structure applications (e.g., energy management, patient/customer tracking, etc.). Exist- ing techniques include visual (e.g. cameras and IR sensors), acoustic, RF, and load-based approaches. These approaches have many deployment and operational requirements that limits their adaptation. To overcome these limitations, prior work has utilized footstep-induced vibrations to allow sparse sensor configuration and non-intrusive detection. However, frequency dependent propagation characteristics and low signal-to-noise ratio (SNR) of footstep-induced vibrations change the shape of the signal. Furthermore, estimating the wave propagation velocity for forming the multilateration equations and localizing the footsteps is a challenging task. They, in turn, lead to large errors of localization. In this paper, we present a structural vibration based indoor occupant localization technique using improved time-difference-of-arrival between multiple vibration sensors. In particular we overcome signal distortion by decomposing the signal into frequency components and focusing on high energy components for accurate indoor localization. Such decomposition leverages the frequency-specific propagation characteristics and reduces the effect of low SNR (by choosing the components of highest energy). Furthermore, we develop a velocity calibration method that finds the optimal velocity which minimizes the localization error. We validate our approach through field experiments in a building with human participants. We are able to achieve an average localization error of less than 0.21 meters, which corresponds to a 13X reduction in error when compared to the baseline method using raw data.

Occupant traffic estimation through structural vibration sensing

The number of people passing through different indoor areas is useful in various smart structure applications, including occupancy-based building energy/space management, marketing research, security, etc. Existing approaches to estimate occupant traffic include vision-, sound-, and radio-based (mobile) sensing methods, which have placement limitations (e.g., requirement of line-of-sight, quiet environment, carrying a device all the time). Such limitations make these direct sensing approaches difficult to deploy and maintain. An indirect approach using geophones to measure floor vibration induced by footsteps can be utilized. However, the main challenge lies in distinguishing multiple simultaneous walkers by developing features that can effectively represent the number of mixed signals and characterize the selected features under different traffic conditions. This paper presents a method to monitor multiple persons. Once the vibration signals are obtained, features are extracted to describe the overlapping vibration signals induced by multiple footsteps, which are used for occupancy traffic estimation. In particular, we focus on analysis of the efficiency and limitations of the four selected key features when used for estimating various traffic conditions. We characterize these features with signals collected from controlled impulse load tests as well as from multiple people walking through a real-world sensing area. In our experiments, the system achieves the mean estimation error of ±0.2 people for different occupant traffic conditions (from one to four) using k-nearest neighbor classifier.

Step-level person localization through sparse sensing of structural vibration

We describe a step-level indoor localization system which uses the ground vibration induced by human footsteps. Indoor localization is important for various smart building applications, including resources arrangement optimization, patient/customer tracking, etc. Geophones are used to measure the ground vibrations and time difference of arrival (TDoA) for different sensors are used to solve the multilateration localization problem. The advantages of this system include its sparsity and also its stability over time. Lesser dependency on instrument people is another upside of this system. The results of pilot tests show that this system can be successfully used for indoor localization.

Robust Occupant Detection Through Step-Induced Floor Vibration by Incorporating Structural Characteristics

The objective of this paper is to present an occupant detection method through step-induced structural vibration. Occupant detection enables various smart building applications such as space/energy management. Ambient structural vibration monitoring provides a non-intrusive sensing approach to achieve that. The main challenges for structural vibration based occupant footstep detection include that (1) the ambient structural vibration noise may overwhelm the step-induced vibration and (2) there are various other impulse-like excitations that look similar to footstep excitations in the sensing environment (e.g., door closing, chair dragging, etc.), which increase the false alarm rate for occupant detection. To overcome these challenges, a two-stage step-induced signal detection algorithm is developed to (1) incorporate the structural characteristics by selecting the dominant frequencies of the structure to increase the signal-to-noise ratio in the vibration data and thus improve the detection performance and (2) perform footstep classification on detected events to distinguish step-induced floor vibrations from other impulse excitations. The method is validated experimentally in two different buildings with distinct structural properties and noise characteristics, Carnegie Mellon University (CMU) campus building and Vincentian Nursing Home deployments in Pittsburgh, PA. The occupant footstep detection F1 score shows up to 4X reduction in detection error compared to traditional thresholding method.

FootprintID: Indoor Pedestrian Identification through Ambient Structural Vibration Sensing

We present FootprintID, an indoor pedestrian identification system that utilizes footstep-induced structural vibration to infer pedestrian identities for enabling various smart building applications. Previous studies have explored other sensing methods, including vision-, RF-, mobile-, and acoustic-based methods. They often require specific sensing conditions, including line-of-sight, high sensor density, and carrying wearable devices. Vibration-based methods, on the other hand, provide easy-to-install sparse sensing and utilize gait to distinguish different individuals. However, the challenge for these methods is that the signals are sensitive to the gait variations caused by different walking speeds and the floor variations caused by structural heterogeneity. We present FootprintID, a vibration-based approach that achieves robust pedestrian identification. The system uses vibration sensors to detect footstep-induced vibrations. It then selects vibration signals and classifiers to accommodate sensing variations, taking step location and frequency into account. We utilize the physical insight on how individual step signal changes with walking speeds and introduce an iterative transductive learning algorithm (ITSVM) to achieve robust classification with limited labeled training data. When trained only on the average walking speed and tested on different walking speeds, FootprintID achieves up to 96% accuracy and a 3X improvement in extreme speeds compared to the Support Vector Machine. Furthermore, it achieves up to 90% accuracy (1.5X improvement) in uncontrolled experiments.

Calibration-Free Footstep Frequency Estimation Using Structural Vibration

This paper introduces a calibration-free footstep frequency estimation system using footstep-induced structural vibration. Footstep frequency is an important measure for tracking health status in senior/health care and rehabilitation. Using structural vibrations for this estimation can improve intrusiveness commonly associated with long-term monitoring. Because the large number of structure types and the variety of noise they are subjected to, the main challenges of vibration-based approach are: (1) separating footsteps from other impulsive excitations (such as door shutting, cane striking, object droppings, etc.), (2) providing a system which is compatible to different structures and does not require calibration and training for every structure. To combat these challenges, we introduce an online footstep frequency estimation system which uses human walking pattern heuristics to automatically separate and tune the system to distinguish between footstep-induced vibration and other impulsive excitations in different structures. We validate our approach in two different buildings with human participants. The results show that our approach results in F1 score of 0.87, equal to 8× improvement compared to a baseline approach, which classifies the footsteps using a model trained in a different structure.

Collaboratively Adaptive Vibration Sensing System for High-fidelity Monitoring of Structural Responses Induced by Pedestrians

This paper presents a collaboratively adaptive vibration monitoring system that captures high fidelity structural vibration signals induced by pedestrians. These signals can be used for various human activity monitoring by inferring information about the impact sources, such as pedestrian footsteps, door open closing, dragging objects. Such applications often require high fidelity (high resolution and low distortion) signals. Traditionally, expensive high resolution and high dynamic range sensors are adopted to ensure sufficient resolution. However, for sensing systems that use low-cost sensing devices, the resolution and dynamic range are often limited; hence this type of sensing methods is not well explored ubiquitously. We propose a low-cost sensing system that utilizes 1) a heuristic model of the investigating excitations and 2) shared information through networked devices to adapt hardware configurations and obtain high fidelity structural vibration signals. To further explain the system, we use indoor pedestrian footstep sensing through ambient structural vibration as an example to demonstrate the system performance. We evaluate the application with three metrics that measure the signal quality from different aspects: the sufficient resolution rate to present signal resolution improvement without clipping, the clipping rate to measure the distortion of the footstep signal, and the signal magnitude to quantify the detailed resolution of the detected footstep signal. In experiments conducted in a school building, our system demonstrated up to 2X increase in the sufficient resolution rate and 2X less error rate when used to locate the pedestrians as they walk along the hallway, compared to a fixed sensing setting.

Characterizing left-right gait balance using footstep-induced structural vibrations

In this paper, we introduce a method for estimating human left/right walking gait balance using footstep-induced structural vibrations. Understanding human gait balance is an integral component of assessing gait, neurological and musculoskeletal conditions, overall health status, and risk of falls. Existing techniques utilize pressure- sensing mats, wearable devices, and human observation-based assessment by healthcare providers. These existing methods are collectively limited in their operation and deployment; often requiring dense sensor deployment or direct user interaction. To address these limitations, we utilize footstep-induced structural vibration responses. Based on the physical insight that the vibration energy is a function of the force exerted by a footstep, we calculate the vibration signal energy due to a footstep and use it to estimate the footstep force. By comparing the footstep forces while walking, we determine balance. This approach enables non-intrusive gait balance assessment using sparsely deployed sensors. The primary research challenge is that the floor vibration signal energy is also significantly affected by the distance between the footstep location and the vibration sensor; this function is unclear in real-world scenarios and is a mixed function of wave propagation and structure-dependent properties. We overcome this challenge through footstep localization and incorporating structural factors into an analytical force-energy-distance function. This function is estimated through a nonlinear least squares regression analysis. We evaluate the performance of our method with a real-world deployment in a campus building. Our approach estimates footstep forces with a RMSE of 61.0N (8% of participants body weight), representing a 1.54X improvement over the baseline.

Non-intrusive Occupant Localization Using Floor Vibrations in Dispersive Structure: Poster Abstract

We introduce a sensing system which leverages footstep-induced structural vibration for occupant localization. Such localization is important for many smart building applications, such as efficient building management, senior/health care, and security. Compared to other sensing approaches, footstep-induced vibration provides a sensing system which is sparse and non-intrusive. The main challenge of achieving high accuracy using such approach is frequency-dependent wave propagation characteristics, such as dispersion, in floor structure. These characteristics result in distortions in the shape of signal. To overcome such distortions, we decompose the vibration signals into different frequency components using wavelet transform and focus on specific components in all the sensors. In a set of experiments in a real structure, our approach results in average localization errors of 0.41 meters, a 4.4X reduction compared to a baseline approach using raw data.

Multiple Pedestrian Tracking through Ambient Structural Vibration Sensing: Poster Abstract

Tracking multiple people in an indoor environment enables various smart building applications such as HVAC energy saving, patient/child monitoring, etc. Researchers have explored various sensing methods including vision, motion, and RF, which either require specific installation requirements or high deployment density. We introduce a passive sparse sensing method based on ambient structural vibration induced by foot strikes. Our system tracks multiple people based on the premise that human foot strikes have spatio-temporal variation, and hence do not fully overlap. The system achieved less than 0.4m accuracy in both one and two persons stepping conditions.