A Portable System for In-Situ Re-Calibration of Force Platforms: Experimental Validation (original) (raw)
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Medical & biological engineering & computing, 2004
The paper provides a new technique based on a least-squares approach for the accurate estimation of a force platform calibration matrix using simple manual procedures, when the direction of the applied loads cannot be perfectly aligned with the axes of the platform. This new procedure can be applied to all force platforms and allows the combined application of vertical and horizontal forces, both static and time-varying. The robust calibration method includes the angular errors in the least-squares parameter vector, thus reducing the bias in the estimated calibration matrix parameters. The performance of the robust method was compared with the conventional one, using a numerical simulation approach starting from a known calibration matrix. With the conventional approach, in noiseless conditions, the maximum error due to load misalignment (SD = 3 degrees) was 6% for the direct terms and over 10% for the cross-talk terms. With the robust method, these errors reduced to zero and were always below 0.4%, even when realistic noise was superimposed on the measures. With perfectly aligned loads and realistic output noise, the confidence intervals of the calibration matrix parameters were very similar for the two methods, demonstrating that the increased number of parameters did not affect the reliability of the estimate.
Dynamic Calibration of Force Platforms by Means of a Parallel Robot
Proceedings of the International Conference on Biomedical Electronics and Devices, 2013
Force platforms are the basic equipment to measure ground reaction forces and moments in biomechanical studies. So, accurate in situ calibration of force platforms is critical for ensuring the accuracy and precision of the results of experimental studies. Although there are different avaliable approaches for in situ calibration, most of the existing methods do not use realistic and repeteable force patterns to calibrate platforms. In this paper, a new technique based on the use of a 3PRS parallel robot for applying a predefined dynamical load is proposed, where force patterns can be reproduced in a similar way as the used during actual experimental measures. This robot can be programmed to apply force patterns simulating the conditions of human gait, running or jumping. Calibration is performed by comparing the forces measured by the platform and the ones measured by a calibrated load cell. A new algorithm was developed for correcting the sensitivity coefficients, including an estimation of errors in the orientation of the load cell. This method has been validated by means of an specific experiment.
A force plate based method for the calibration of force/torque sensors
Journal of Biomechanics, 2012
This study describes a novel calibration method for six-degrees-of-freedom force/torque sensors (FTsensors) using a pre-calibrated force plate (FP) as a reference measuring device. In this calibration method, the FTsensor is rigidly connected to a FP and force/torque data are synchronously recorded while a dynamic functional loading procedure is applied by the researcher. Based on these data an accurate calibration matrix for the FTsensor can easily be obtained via least-squares optimization.
Investigation on calibration methods for multi-axis, linear and redundant force sensors
Measurement Science and Technology, 2007
A new method for the calibration of multi-axis, linear and redundant force sensors is presented. This new procedure, named device hyperplane characterization method, is inspired by the shape from motion method for it reduces the burden represented by the need for a huge number of force measurements, typical using least-squares methods, in order to reject errors during the calibration procedure. The proposed technique is an application of the rank theorem and achieves good noise rejection without much time consumption focusing on sensor output measurements, and reducing the effect of disturbances operating the projection of raw output data on the hyperplane to which measurements are ideally compelled to belong in the case of redundant sensors.
Optimised procedure for the calibration of the force platform location
Gait & Posture, 2003
An innovative optimised method, including an experiment and a mathematical model, for the calibration of the force platform location in the optoelectronic reference frame is proposed. The calibration experiment adopts a bearing-marker testing object contacting the platform and does not directly measure the platform location. The experiment is designed in order to avoid the main drawbacks possibly occurring in commonly adopted methods. The mathematical model of the experiment estimates the expected ground reaction. An optimisation algorithm identifies the optimal platform location as the one that best matches the measured outcome of the calibration experiment with the corresponding model estimate. The innovative calibration procedure has been assessed in terms of inter-tester reliability and compared with commonly used calibration procedures of platform location. These results evidenced how the introduction of such optimised procedure could improve the reliability of the calibrated platform location and, consequently, of the kinetic variables considered in posture and gait analysis. # (M. Rabuffetti).
Spot check of the calibrated force platform location
Medical & Biological Engineering & Computing, 2001
In a movement analysis laboratory, stereophotogrammetric motion capture systems and force platforms must share one absolute reference frame that allows the computation of joint moments and powers. The correct calibration of the platform location identifies the transformation between force plate and absolute reference systems, which determines the spatial coherence among the equipments' measurements. The aim of this study was to develop and test a spot check for the assessment of platform location calibration. Platform location calibration was assessed by comparing the measured outcome of an experiment performed with a pointed rigid rod bearing a set of markers with the corresponding expected results, computed with a model. A set of indices was then proposed to define a confidence volume in which the true ground reaction force is expected to be. The spot check was applied to a real laboratory setup and the effects of simulated platform mislocations were analysed. It was verified that the hip joint moment may be equally affected by a single marker misplacement of about 20 mm during platform location calibration, an occurrence that was clearly identified by the spot check, and by a hip centre location inaccuracy of 30 mm.
Development of a low cost force platform for biomechanical parameters analysis
Research on Biomedical Engineering
The maintenance of balance and body orientation during standing is essential to perform different activities. One of the devices used to measure balance them is the force platform. This device measures the ground reaction force (GRF) and displacement of the center of pressure (COP), both biomechanical parameters involved in human motion. This article proposes a new design for non-commercial low-cost force platforms for scientific research purposes. Methods for calibration and validation are also described. Methods: A force platform, developed according to International Standards of Measurement and dedicated to measuring feet contact forces was built for approximately one tenth of the cost of commercial platforms. Calibration was performed by loading known masses, centralized or distributed, on the platform. An experimental study was conducted with four volunteers in different conditions to validate and verify the practical applicability of the device. Results: The platform calibration showed an adequate connectivity, linearity and reliable measurement of the variables proposed in this research, being suitable for studies of human postural behavior. Conclusion: Based on the validation results, we believe the low-cost platform can be used as stabilometric device to measure postural control and balance in clinical or sports experiments. However future studies will be required to provide a final validation and compare its performance with other force platforms.