I currently detect movement on the y axis. How does one calculate the direction it moved on the axis?
I get the same values when moving up or down.
Is the Gyro needed for this?
Do remember that the accelerometer will reflect the force of gravity. So movement up and down will generally be reflected as 9.81 m/s2 plus or minus the actual acceleration of the device relative to the Earth.
Related
I did some research on OpenPose and the output is x and y coordinates with confidence point. x and y coordinates are good for detecting up, down, left, and right movements. I was wondering is it possible to detect turning movements. Turning usually happens on the z axis. Is there a way to tell if body part has rotated 180 degrees with x and y coordinate.
I had few ideas like calculating the slope of the hand line. The slope tells us if the hand is tilted or not. If the slope is high or very low then the hand has rotated. Same concept for all other body parts. But I don't think that will work in all cases. Please check 2nd figure here https://cmu-perceptual-computing-lab.github.io/openpose/web/html/doc/md_doc_02_output.html to understand output of OpenPose.
I wrote an app that writes gravity, userAcceleration, and attitude quaternion to CSV while driving in a vehicle. The intent is to capture the dynamics of the vehicle (e.g. braking, accelerating, cornering) in the earth frame using an iPhone.
Then, I sum gravity and userAcceleration, and rotate the resulting raw acceleration vector by the quaternion provided by CMAttitude to get the acceleration in the earth frame. In about 60% of recording sessions, the average z values are not +9.81m/s^2 and jump to varying magnitudes besides +9.81m/s^2. For example (each tick mark in the y-axis represents 5m/s^2):
But, I expect a plot with a consistent average value for acceleration in the z axis like the following:
When I start device motion updates, I use the xMagneticNorthZVertical attitude reference frame, like so:
motionManager.startDeviceMotionUpdates(using: .xMagneticNorthZVertical, to: OperationQueue(), withHandler: didUpdateDeviceMotion)
The following computes raw acceleration in the global frame using the attitude quaternion:
let accel = CMAcceleration(x: (motion.userAcceleration.x+motion.gravity.x), y: (motion.userAcceleration.y+motion.gravity.y), z: (motion.userAcceleration.z+motion.gravity.z))
let a = SIMD3<Double>(accel.x, accel.y, accel.z)
let a_vehicle = simd_act(attitude.quaternion, a)
I also have written the equivalent in MATLAB resulting with the same problem.
xMagneticNorthZVertical should result in an attitude that computes the direction of gravity. The direction of X or Y does not matter to me.
I do not have any magnets in the vicinity to skew the computed attitude.
In contrast: Android's rotationVector consistently rotates the accelerometer readings to the earth frame. Surely the quality of iPhone is better than Android.
What might be the cause of the attitude quaternion to not always rotate the device frame to the earth frame such that Z is in the direction of gravity?
In the project I am using IMU sensors to track real player's hand and apply the transformation values onto 3D hand inside Unity.
As IMU sets the Y axis orientation relative to magnetic north of the earth, in the game, hand initializes on specific direction.
What I want is to calculate the offset of IMU's given Y values and 3D hand's original Y rotation, so that I can subtract that value to the 3D model's Y rotation (that will seem like player's initial Y Rotation is the same as 3D hand's). Code would be: transform.Rotate(Vector3.up, offset, Space.World);
IMU sends Euler angles (does it well, as I wasn't able to get Gimbal lock)
As I understand, I need to find out angle difference between 3D Hand's initial rotation and IMU's given initial rotation on XZ plane (or through Y Axis)
How do I calculate the offset?
You can use Quaternion.FromToRotation to calculate offset, something like:
var offset = Quaternion.FromToRotation(Vector3.up, imuUp);
transform.rotation *= offset;
I want to use the iPhones's accelerometer to detect motions while driving. I'm a bit confused what the accelerometer actually measures, especially when driving a curve.
As you can see in the picture, a car driving a curve causes two forces. One is the centripetal force and one is the velocity. Imagine the iPhone is placed on the dashboard with +y-axis is pointing to the front, +x-axis to the right and +z-axis to the top.
My Question is now what acceleration will be measured when the car drives this curve. Will it measure g-force on the -x-axis or will the g-force appear on the +y axis?
Thanks for helping!
UPDATE!
For thoses interested, as one of the answers suggested it measures both. The accelerometer is effected by centrifugal force and velocity resulting in an acceleration vector that is a combination of these two.
I think it will measure both. But don't forget that the sensor will measure gravity as well. So when your car is not moving, you will still get accelerometer readings. A nice talk on sensors in smartphones http://www.youtube.com/watch?v=C7JQ7Rpwn2k&feature=results_main&playnext=1&list=PL29AD66D8C4372129 (it's on android, but the same type of sensors are used in iphone).
Accelerometer measures acceleration of resultant force applied to it (velocity is not a force by the way). In this case force is F = g + w + c i.e. vector sum of gravity, centrifugal force (reaction to steering centripetal force, points from the center of the turn) and car acceleration force (a force changing absolute value of instantaneous velocity, points along the velocity vector). Providing Z axis of accelerometer always points along the gravity vector (which is rare case for actual car) values of g, w and c accelerations can be accessed in Z, X and Y coordinates respectively.
Unless you are in free fall the g-force (gravity) is always measured. If I understand your setup correctly, the g-force will appear on the z axis, the axis that is vertical in the Earth frame of reference. I cannot tell whether it will be +z or -z, it is partly convention so you will have to check it for yourself.
UPDATE: If the car is also going up/downhill then you have to take the rotation into account. In other words, there are two frames of reference: the iPhone's frame of reference and the Earth frame of reference. If you would like to deal with this situation, then please ask a new question.
How can i use the yaw/pitch/roll to estimate the tilt angle of a motorbike, assuming the iphone is in the pocket. I'm thinking along the lines of storing the baseline yaw/pitch/roll when the user begins riding in the upright position then any deviation from that will give me rotations about all 3 axis, but how do i use this to figure out the lateral tilt angle of a motorbike? Thanks.