There are many words used to describe motion in space.
We use our very small 3DWear IMUs as a tool to objectively track the epicenter of motion.
The epicenter of an IMU refers to the point within the device where the sensors such as the accelerometers, gyroscopes, and sometimes magnetometers are ideally placed to measure the movements of the device on a human landmark of interest. This point is often referred to as the center of rotation or center of mass, and it is crucial for accurate motion detection.
However, there is no universally fixed epicenter in the context of an IMU. The term epicenter is not a standard term used for describing IMUs in general. More commonly, people will refer to the origin or reference point within the IMU or its mounting on a system (such as a human, robot or vehicle). This point determines the reference for all motion measurements, especially for translating accelerations and rotations to real-world movements.
For example: An epicenter refers to the point on the Earths surface directly above the focus (or hypocenter) of an earthquake. The focus is the location within the Earth where the earthquake originates, while the epicenter is the point on the surface where the effects of the earthquake are usually felt most strongly.
In simpler terms, the epicenter is the surface location of an earthquakes (displacements/shaking) origin.
Key points:
Epicenter: The point on the Earths surface directly above the earthquakes origin.
Focus (or Hypocenter) is the actual location within the Earth where the earthquake starts, usually at a certain depth below the surface.
The epicenter is often where the most severe shaking is experienced and is typically used to describe the location of an earthquakes displacement.
Its important to note that the epicenter is not the place where the earthquake is necessarily strongest (shaking can be felt differently depending on factors like distance from the epicenter, local geology, and the depth of the earthquake), but it is often where the earthquakes effects are first noticed.
In practice:
For accurate motion tracking, the 3DWear IMU is often mounted in such a way that its center of mass coincides with the desired point of measurement. For instance, on a drone, the IMU might be located near the center of the drones frame (e.g. pelvis ring) as this is often the point where rotational and translational movements are most accurately represented.
The term epicenter refers to the point on the Earths surface directly above the focus (or hypocenter) of an earthquake. The focus is the location within the Earth where the earthquake originates, while the epicenter is the point on the surface where the “effects” of the earthquake are usually felt most strongly.
In simpler terms, the epicenter is the surface location of an earthquakes origin.
Key points:
Epicenter: The point on the Earths surface directly above the earthquake origin.
Focus (or Hypocenter): The actual location within the Earth where the earthquake starts, usually at a certain depth below the surface.
The epicenter is often where the most severe shaking is experienced and is typically used to describe the location of an earthquake.
Its important to note that the epicenter is not the place where the earthquake is necessarily strongest (shaking can be felt differently depending on factors like distance from the epicenter, local geology, and the depth of the earthquake), but it is often where the earthquakes effects are first noticed.
If an IMU is not positioned at the center of mass, the sensor readings might need to be corrected for any offset between the IMU position and the true center of rotation or mass.
In short, while term epicenter is not a standard term used for IMUs, the key concept is that the IMU sensors should ideally be positioned at or near the center of rotation or the center of mass to provide the most accurate measurements of motion and orientation.
When using our 3DWear Inertial Measurement Unit (IMU), the x, y, and z motions in space refer to movements along three perpendicular axes that are typically used to describe the orientation and movement of an object (e.g. pelvis ring on the saddle) in 3D space. These axes are usually defined as follows:
1. X-axis (forward/backward or roll axis):
This axis generally represents the movement along the forward-backward direction. For example, when an object center moves forward, it would show positive displacement along the x-axis, and moving backward would show negative displacement.
In rotational terms, roll is the rotation around the x-axis. When an object tilts to the left or right, it is rolling about the x-axis (e.g. airplane wings).
2. Y-axis (left/right or pitch axis):
This axis typically represents the left-right or lateral movement. Moving to the left is usually considered negative displacement along the y-axis, while moving to the right is positive.
In terms of rotation, pitch refers to rotation around the y-axis. This involves tilting the object up or down, like the nose of an airplane moving up or down.
3. Z-axis (up/down or yaw axis):
This axis generally represents vertical movement. Moving upward along the z-axis is positive, while downward movement is negative.
Yaw is the rotation around the z-axis, which refers to turning left or right, like the turning of the steering wheel in a car.
Translational vs. Rotational Motion:
Translational motion (movement along the axes) is detected by accelerometers in the IMU. For instance, if you move your device left-right, it will show a change in the y-axis value.
Rotational motion (rotation around the axes) is detected by the gyroscopes in the IMU. For example, tilting your device forward or backward will show changes in pitch (rotation around the y-axis), turning left or right shows changes in yaw (around the z-axis), and rolling left or right shows changes in roll (around the x-axis).
By tracking the accelerations (x, y, z) and rotational movements (roll, pitch, yaw) over time, our 3DWear IMU can give a detailed description static or dynamic of the object motion in 3D space.
Some systems attempt to make the language more simply. So they use lateral travel when riding a bike refers to the side-to-side movement of the riders body and the bike during pedaling and handling. Lateral travel doesn’t describe the value of rolling. The amount of lateral travel varies based on several factors:
1. Rider Pedaling Technique: As the rider pedals, there is a natural, small amount of lateral movement of the hips (e.g. pelvic ring) to generate force, especially when pushing harder on the pedals, as that flexes the bike frame. This is typically around 1-2 centimeters (about 0.5 to 1 inch) in each direction. Excessive lateral movement often indicates poor saddle positioning or a lack of pelvis ring stability.
2. Bike Geometry: The design of the bike (such as the frame, bottom bracket height, and crank length) also influences lateral travel. For example, a bike with a more aggressive geometry may have less lateral movement compared to a more relaxed design.
3. Riding Surface: On smooth, tracks, even roads, lateral travel is minimal. However, when riding over uneven surfaces or during turns, the rider may shift slightly side-to-side to maintain balance.
4. Saddle and Bike Fit: Poor saddle fit or improper bike sizing can increase unnecessary lateral movement. For example, if the saddle is too high or low, or the pedals are misaligned, or your cleats aren’t precise, it can cause more side-to-side rocking.
In general, some lateral movement is normal, as your pelvic ring flexes more than people think, excessive movement may suggest a need for adjustments in bike fit or technique. Ideally, the riders motion should remain smooth and controlled and efficient, minimizing unnecessary side-to-side travel both upon the saddle surface and travel of of the bike frame.
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