Motion Capture Sensor Systems

By Kal Kaur

Introduction
Non-optical Motion Capture Systems
•    Inertial Motion Sensors
•    Mechanical Motion Capture Sensors
•    Magnetic Motion Sensors
References

Introduction

If any of you have seen The Lord of the Rings film trilogy (2001 to 2003) then you’ll certainly remember the gaunt fictional character Gollum. In the trilogy, Gollum is introduced as a Hobbit, once known as Sméagol, but later corrupted by the One Ring and then renamed as Gollum after the unpleasant swallowing noise as he speaks. This character was probably one of the most difficult ones to bring to life for the trilogy, but was made possible using motion capture sensor technology. Let’s look at Gollum in action to demonstrate exactly how effective motion capture sensor systems can be in animating a fictional character on screen...


Motion capture or motion-tracking systems record the movement of an object or person. The system is highly sophisticated and requires a certain number of capture sessions during which the movement of an object is recorded multiple times per second. The data on movement of an object is mapped out into a 3D model that mirrors the exact movement positions as the object that is wired to the motion sensor network.

The following flowchart illustrates the different types of motion sensors.


Optical motion sensors capture the image data and triangulate the 3D positioning of an object using multiple cameras at varying angles to the object. The markers that are attached to the object make this positioning possible. The markers (i.e., the motion sensors) are attached directly to the surface of the object being captured. Passive markers are coated with a retro-reflective material that can reflect the light source. This technique is important for making sure that only movement is tracked. Software is integrated to help model the 3D image after receiving data on movement of the object. In relation to the fictional character Gollum, the actor creating the movement for this character was dressed in a suit attached to reflective markers. The video cameras recording the actor’s movements for a particular scene in the film ‘The Lord of the Rings’, captured the motion data from several different angles allowing the software to monitor the movement of the markers on the actor and to use this data to program sliders (camera movement system) to help animate the character.

With active marker systems, the sensor (marker) light source, often an LED, emits an electromagnetic field that is detected in real-time. The one main problem with the active system is the large amount of wiring involved which could stop the subject from performing intricate movement.

Non-optical Motion Capture Systems

Non-optical systems for motion capture are split into three different types of sensors:

  • Inertial sensors
  • Mechanical motion sensors
  • Magnetic sensors
Inertial Motion Sensors

Measurement sensors such as accelerometers and gyroscopes are commonly applied for motion tracking. The motion data from these sensors is detected wirelessly by a computer software system and recorded. Tracking movement using inertial sensors can be difficult as the data recorded can be ambiguous and this is why it is better to create models of human motion as a prerequisite for being able to obtain the most accurate readings on human movement. The combination of both accelerometers and gyroscopes to measure movement is controlled by applying an algorithm. The gyroscope in this motion sensor network measures the orientation of the sensor, data that reflects gravitational acceleration. By subtracting the gravitational acceleration from the sensor frame, the accelerometer can calculate initial position.

Mechanical Motion Capture Sensors

A mechanical motion capture system is a structure that is attached to the subject to act out a sequence of movements. Typically, a mechanical motion system consists of electrogoniometers - a sensor system made up of potentiometers or transducer technology designed to estimate joint angles when positioned close to a joint on the subject’s body. In comparison to inertial sensors or optical-based motion sensors, the mechanical motion capture system allows for direct measurement of movement, which means that the subject can move around more freely in a large environment without any movement being out of view by a central camera system, nor is the capture system affected by reflective light. A wireless mechanical motion sensor system can enhance the capture volume.

A good example of a wireless mechanical motion capture sensory system is the Gypsy 5 engineered by Meta Motion. This motion capture technology has a half-a-mile range. The mechanical motion system may give the subject more freedom to move around in a larger space, but this system limits the subject’s degrees of freedom and the positioning of sensors on the attached mechanical system. The following video by Inition, a 3D technology and production company, present a GypsyGyro18 demo of a mechanical motion capture system in use. The system does not require extensive calibration yet still delivers clean data on movement of a subject.


Magnetic Motion Sensors

This type of motion capture sensor calculates a low-frequency magnetic field created by a transmitter. The magnetic current is detected by the orthogonal coils in the transmitter. There are two types of a magnetic current flow: the direct current (DC) technique and the alternating current (AC) system. With the DC system, the magnetic current flows in the form of square pulse waves; whereas, sine pulse waves are generated with an AC current.

A magnetic motion system involves using 6–11 sensors around the joint to a subject’s body where each sensor works to produce measurements on the position and rotation of the corresponding joint. Multiple actor magnetic systems all performing in one space reduces the accuracy of position measurements due to interfering sensors from the opposing magnetic capture systems resulting in distortion of results. This capture system can react to magnetic fields in the surrounding environment which interferes with the magnetic current flow generated by the transmitters and receivers on the subject, hence why this type of capture system is better used in controlled environments. As mentioned previously, the mechanical capture system can only offer a limited degree of freedom for the subject to move around in, but the magnetic motion capture setup is built with transmitters that can allow for up to six degrees of freedom (including roll orientation angle measurements), which offers the subject to be slightly more creative with movements.

The military are typically known for using magnetic motion capture systems for head tracking. In this context, the system is used to calculate the pilot’s line of sight so that weapons are released with maximum accuracy when hitting the target. Further application of magnetic motion sensors is in the medical industry where such systems have been engineered to help measure the size of organs inside the human body, particularly useful during medical procedures to allow for better accuracy and outcome of surgery.

References

Date Added: Aug 10, 2012 | Updated: Aug 10, 2012
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