Wyatt Page

Dynamic accuracy of inertial measurement units during simple pendulum motion

A motion measurement system based on inertial measurement units (IMUs) has been suggested as an alternative to contemporary video motion capture. This paper reports an investigation into the accuracy of IMUs in estimating 3D orientation during simple pendulum motion. The IMU vendors (XSens Technologies) accuracy claim of 3° root mean squared (RMS) error is tested. IMUs are integrated electronic devices that contain accelerometers, magnetometers and gyroscopes. The motion of a pendulum swing was measured using both IMUs and video motion capture as a reference. The IMU raw data were processed by the Kalman filter algorithm supplied by the vendor and a custom fusion algorithm developed by the authors. The IMU measurement of pendulum motion using the vendors Kalman filter algorithm did not compare well with the video motion capture with a RMS error of between 8.5° and 11.7° depending on the length and type of pendulum swing. The maximum orientation error was greater than 30°, occurring approximately eight seconds into the motion. The custom fusion algorithm estimation of orientation compared well with the video motion capture with a RMS error of between 0.8° and 1.3°. Future research should concentrate on developing a general purpose fusion algorithm and vendors of IMUs should provide details about the errors to be expected in different measurement situations, not just those in a ‘best case’ scenario.

The static accuracy and calibration of inertial measurement units for 3D orientation

Inertial measurement units (IMUs) are integrated electronic devices that contain accelerometers, magnetometers and gyroscopes. Wearable motion capture systems based on IMUs have been advertised as alternatives to optical motion capture. In this paper, the accuracy of five different IMUs of the same type in measuring 3D orientation in static situations, as well as the calibration of the accelerometers and magnetometers within the IMUs, has been investigated. The maximum absolute static orientation error was 5.2°, higher than the 1° claimed by the vendor. If the IMUs are re-calibrated at the time of measurement with the re-calibration procedure described in this paper, it is possible to obtain an error of less than 1°, in agreement with the vendors specifications (XSens Technologies B.V. 2005. Motion tracker technical documentation Mtx-B. Version 1.03. Available from: www.xsens.com). The new calibration appears to be valid for at least 22 days providing the sensor is not exposed to high impacts. However, if several sensors are ‘daisy chained’ together changes to the magnetometer bias can cause heading errors of up to 15°. The results demonstrate the non-linear relationship between the vendors orthogonality claim of < 0.1° and the accuracy of 3D orientation obtained from factory calibrated IMUs in static situations. The authors hypothesise that the high magnetic dip (64°) in our laboratory may have exacerbated the errors reported. For biomechanical research, small relative movements of a body segment from a calibrated position are likely to be more accurate than large scale global motion that may have an error of up to 9.8°.

Sensor fusion: Towards a fully expressive 3D music control interface

This paper describes a study into the realisation of a new method for capturing 3D sound control data. It expands on our established practice with the radiodrum 3D input device by incorporating a computer vision platform that we have developed using the Xbox Kinect motion sensing input device. It also extends our previous work in systemizing a low-cost, open-source 3D gesture sensor system, as a novel method for musically oriented human computer interaction. We discuss in detail the development process and the system performance in different scenarios and outline the future directions of this development.

Influence of extremely low frequency magnetic fields on chromosomes and the mitotic cycle in Vicia faba L., the broad bean.

Vicia faba seedlings were subjected to one of the following magnetic fields continuously for 3 days: 0 Hz (DC) at 5 mT, 50 Hz at 1.5 mT, 60 Hz at 1.5 mT, and 75 Hz at 1.5 mT. The lengths of all the phases of mitosis differed from the controls in all treatments using alternating magnetic fields and for prophase and metaphase in the DC condition. In particular, all treatments increased the length of prophase significantly in meristematic root-tip cells compared with the controls. The implications of these results for chromosome coiling are discussed. The length of prophase, however, did not vary significantly between any of the treatments. Furthermore, none of the exposed seedlings had a greater frequency of chromosome breakages above that of the control plants.

Automatic event detection for noisy hydrophone data using relevance features

In this paper, a new context-aware method for detecting events in noisy hydrophone data is proposed. The method transforms first the 1D hydrophone data into a 2D relevance map. A dynamic context-aware relevance features set is then proposed extracted from the normalized relevancy map. Feature classification is finally performed using a least-squares support vector machine (LS-SVM). The method shows event detection sensitivity in excess of 97% for rare events such as whale calls from original noisy hydrophone recordings from the NEPTUNE Canada project, with more than 94% specificity and 95% overall accuracy. With relatively less parameters to adjust and high accuracy, the proposed method is useful for automated long-term monitoring of a wide variety of marine mammals and human related activities from hydrophone data.

Noise Producing Toys and the Efficacy of Product Standard Criteria to Protect Health and Education Outcomes

An evaluation of 28 commercially available toys imported into New Zealand revealed that 21% of these toys do not meet the acoustic criteria in the ISO standard, ISO 8124-1:2009 Safety of Toys, adopted by Australia and New Zealand as AS/NZS ISO 8124.1:2010. While overall the 2010 standard provided a greater level of protection than the earlier 2002 standard, there was one high risk toy category where the 2002 standard provided greater protection. A secondary set of toys from the personal collections of children known to display atypical methods of play with toys, such as those with autism spectrum disorders (ASD), was part of the evaluation. Only one of these toys cleanly passed the 2010 standard, with the remainder failing or showing a marginal-pass. As there is no tolerance level stated in the standards to account for interpretation of data and experimental error, a value of +2 dB was used. The findings of the study indicate that the current standard is inadequate in providing protection against excessive noise exposure. Amendments to the criteria have been recommended that apply to the recently adopted 2013 standard. These include the integration of the new approaches published in the recently amended European standard (EN 71) on safety of toys.

Automatic event detection for long-term monitoring of hydrophone data

In this paper, we propose an efficient method for long-term monitoring of a wide variety of marine mammals and human related activities using hydrophone data. The proposed method uses a combination of a two-stage denoising process followed by a new event detection function that estimates temporal predictability. The detection function utilizes long-term and short-term predictions in order to detect various acoustic events from the background noise. The first stage of the denoising process uses temporal decomposition via Empirical Mode Decomposition to improve the correct detection rate, while the second stage uses Wavelet Packet spectral decomposition to reduce the false detection rate. Applied to event detection in NEPTUNE hydrophone recordings, the method demonstrates an accuracy of 95% and an F-measure of 94%.

A new method for classification of events in noisy hydrophone data

In this paper a new method for classifying events in noisy hydrophone data is developed. The method takes an image processing approach to the 1D hydrophone data by first converting it into a log-frequency spectrogram image (cepstrum). This image is then filtered by reconstructing it based on mutual information (MI) criteria of the dominant orientation map. The features of the reconstructed cepstrum are then enhanced using a combination of edge-tracking and noise smoothing. Feature classification on the processed cepstrum is performed using a least-squares support vector machine (LS-SVM). The method showed event detection sensitivity in excess of 99% for rare events such as whale calls from noisy hydrophone recordings from the NEPTUNE Canada project, with in excess of 97% specificity and 98% overall accuracy. With relatively low computational cost and high accuracy, the proposed method is useful for automated long-term monitoring of a wide variety of marine mammals and human related activities from hydrophone data.

Wavelet median denoising of ultrasound images

Ultrasound images are contaminated with both additive and multiplicative noise, which is modeled by Gaussian and speckle noise respectively. Distinguishing small features such as fallopian tubes in the female genital tract in the noisy environment is problematic. A new method for noise reduction, Wavelet Median Denoising, is presented. Wavelet Median Denoising consists of performing a standard noise reduction technique, median filtering, in the wavelet domain. The new method is tested on 126 images, comprised of 9 original images each with 14 levels of Gaussian or speckle noise. Results for both separable and non-separable wavelets are evaluated, relative to soft-thresholding in the wavelet domain, using the signal-to-noise ratio and subjective assessment. The performance of Wavelet Median Denoising is comparable to that of soft-thresholding. Both methods are more successful in removing Gaussian noise than speckle noise. Wavelet Median Denoising outperforms soft-thresholding for a larger number of cases of speckle noise reduction than of Gaussian noise reduction. Noise reduction is more successful using non-separable wavelets than separable wavelets. When both methods are applied to ultrasound images obtained from a phantom of the female genital tract a small improvement is seen; however, a substantial improvement is required prior to clinical use.

Calculating rhythmicity of infant breathing using wavelets

Breathing signals are one set of physiological data that may provide information regarding the mechanisms that cause SIDS. Isolated breathing pauses have been implicated in fatal events. Other features of interest include slow amplitude modulation of the breathing signal, a phenomenon whose origin is unclear, and periodic breathing. The latter describes a repetitive series of apnea, and may be considered an extreme manifestation of amplitude modulation with successive cessations of breathing. Rhythmicity is defined to assess the impact of amplitude modulation on breathing signals and describes the extent to which frequency components remain constant for the duration of the signal. The wavelet transform was used to identify sections of constant frequency components within signals. Rhythmicity can be evaluated for all the frequency components in a signal, for individual frequencies. The rhythmicity of eight breathing epochs from sleeping infants at high and low risk for SIDS was calculated. Initial results show breathing from infants at high risk for SIDS exhibits greater rhythmicity of modulating frequencies than breathing from low risk infants.