Opportunities exist to introduce human postural stability modelling into ship designs and operations. One of the deficiencies in this area is the lack of experimental shipboard data that focuses on human performance

In order to address the need for relevant experimental data, I designed and performed a number of postural stability experiments on board Canadian Force Auxiliar Vessel (CFAV) Quest over 8 days in November 2012. A photograph of Quest is shown below.

Participants were instructed to maintain balance without moving their feet and without holding on to a railing while performing a clipboard or tablet task.

A variety of sensing technologies were used to record all of the sensory inputs a person would use while maintaining balance. The first was a NaturalPoint Opti-track full-body motion capture system. It uses reflective markers and 8 cameras positioned around the laboratory to record body positions and orientations.

The Arena software was used to record positions of the reflective markers and map them to a human skeleton.

A second motion capture system was used that employed two Microsoft Kinect sensors. The Kinect sensor has the capability to measure the distance from itself of all objects in its view.

Shown here are images from each sensor recorded simultaneously. The different colours indicate the distance of each component from the sensor, with yellow indicating areas that are either shadowed or too far away from the sensor to measure. Due to space restrictions it was not possible to capture a full body image with the sensor shown in the top image.

The iPiStudio software was used to combine the depth data in 3D space and to map the data to a human skeleton.

Foot pressure measurements were made with instrumented insoles that were trimmed and placed inside each participant's footwear. The insoles measured the distribution of pressure over the person's feet using a grid of pressure-sensitive sensels.

In order to measure data equivalent to what the vestibular system and visual systems would measure, an inertial sensor and GoPro camera were mounted to the helmet the participants wore during the experiments. A fully-instrumented participant is shown in the image below.

The graph below shows the significant wave height measured by a wave buoy over the 8-day sea trial. Significant wave height is defined as the average of the largest 1/3 of the waves.

The time around the two peaks provided the most ideal conditions for performing the postural stability experiments. The time around the first peak was mostly spent calibrating and tuning the experimental equipment and procedures. The area around the second peaks is when most of the actual experimental trials took place.

Here is a sample of the magnitude of the ship motions.
One of the end results from the sea trial was to obtain a detailed analysis of each participant's centre of mass and centre of force positions within their base of support. The base of support is the overall outline of the person's stance. In this case the individual foot outlines were derived from the foot pressure insole data. They were positioned relative to each other using the ankle position recorded by the motion capture data. Connecting lines were then added which place an equal number of additional points to the base of support outline, regardless of which direction the person is facing.

The centre of mass position is a weighted average of the relative masses of each of a person's body segments. This is the large black dot in the image.

The centre of force is the location of the line of action of the total reaction force between the person and the ground. This is the smaller grey dot in the image.

For a person standing in a non-moving environment, the projection of both of these points on the ground plane is the same. In a moving environment such as a ship, however, these points are constantly moving relative to each other.
Here is one of the videos I created from the data:
Also of interest to me was the behaviour of the centre of mass and centre of force in advance of an event where the person would need to take a step to maintain their balance. An event of this type is called a Motion Induced Interruption (MII) and is a commonly used metric in shipboard postural stability research. Below is a video showing some of these events.
Another type of graph that I produced to analyze the results of the sea trial was a time-lapse graph showing all of the centre of mass and centre of force positions simultaneously with the average base of support points shown. The three graphs below are for a person standing at 0, 45, and 90 degrees with respect to ship centreline.

I published two papers on the sea trial experiments and the results shown above in which I was the primary author.

International Conference of Control, Dynamic Systems, and Robotics (CDSR) 2014 - Quest Q-348 Sea Trial: Human Postural Stability Studies

25th Canadian Congress on Applied Mechanics (CANCAM) 2015 - Indicators of Motion Induced Interruption Occurrences in Heavy-Weather Sea Conditions