I am currently developing an App which should calculate the speed by the accelerations of the device. To achieve this I am using Sensing Kit. The app records data of the users motion and saves it to the local storage. When the user stops the recording it is possible draw the collected accelerations in velocity by time (Three graphs for every axis of the accelerometer).
Sensing Kit uses startAccelerometerUpdates of the CMMotionManager to get the accelerations. To calculate the velocity I do some signal processing and an integration of the acceleration (and multiply it by 9.81 because apple measures the acceleration in increments of gravity). The result is the velocity over the recorded time (Its not very precise but that doesn't matter in my case).
I tested my app by sliding the phone over a table with the screen up and the upper screen side in moving direction. The movement later shown up in the resulting graph of Y-Axis, but it has a negative velocity (The accelerations are negative, too). I expect that the velocity and the acceleration should be positive, because I moved the device in the positive direction of the Y-Axis.
The same happens with the X-Axis wehen I move the phone on the table in the direction of this axis.
I tested it today without Sensing Kit and get same results.
The gravity is always as I expect a negative velocity on the z-Axis, because its an acceleration to the ground.
Can somebody explain to me why the acceleration of the sensor has the wrong sign?
Thank you.
https://developer.apple.com/documentation/foundation/unitacceleration https://developer.apple.com/documentation/foundation/units_and_measurement
The
Foundationlibrary has a function for this and you could use this to convert betweenUnitAccelerationandUnitSpeed.