FormFit Watch - Core Motion Data Collector

A minimal watchOS app that records fused motion from CMDeviceMotion on Apple Watch and saves it to CSV for later analysis.

Features

• Adjustable sampling tick, default 50 ms, range 10 to 200 ms
• Rolling buffer with configurable max size (default ~5k rows)
• Live readout for accel, gyro, and attitude
• One tap Save CSV to the app Documents directory

Why

I needed a simple, reliable way to capture watch motion for prototyping form tracking and model training, without wiring the phone or server yet.

How it works

• Uses CMMotionManager deviceMotion stream
• Captures the following per sample:
  • Linear acceleration userAcceleration.{x,y,z} in m/s²
  • Rotation rate rotationRate.{x,y,z} in rad/s
  • Orientation attitude.{roll,pitch,yaw} in radians
• Keeps:
  • latest for quick UI display
  • buffer for the time series, capped at maxBuffer

CSV format

Header:

FormFit Watch - Core Motion Data Collector

A minimal watchOS app that records fused motion from CMDeviceMotion on Apple Watch and saves it to CSV for later analysis.

Features

• Adjustable sampling tick, default 50 ms, range 10 to 200 ms
• Rolling buffer with configurable max size (default ~5k rows)
• Live readout for accel, gyro, and attitude
• One tap Save CSV to the app Documents directory

Why

I needed a simple, reliable way to capture watch motion for prototyping form tracking and model training, without wiring the phone or server yet.

How it works

• Uses CMMotionManager deviceMotion stream
• Captures the following per sample:
  • Linear acceleration userAcceleration.{x,y,z} in m/s²
  • Rotation rate rotationRate.{x,y,z} in rad/s
  • Orientation attitude.{roll,pitch,yaw} in radians
• Keeps:
  • latest for quick UI display
  • buffer for the time series, capped at maxBuffer

CSV format

Header: idx,timestamp,ax,ay,az,gx,gy,gz,roll,pitch,yaw

Rows:

• idx is a zero based counter within the current buffer
• timestamp is Core Motion timestamp in seconds since the boot reference
• Other columns are doubles

File name example: formfit-YYYYMMDD-HHMMSS-ms.csv

Build and run

Requirements

• Apple Watch paired to an iPhone
• Xcode version that matches your watchOS
  • watchOS 10.6.1 pairs well with Xcode 15.4
  • watchOS 11 pairs well with Xcode 16+

Steps

1. Open FormFitWatch.xcodeproj in Xcode.
2. Select the watch app scheme.
3. Choose your real Apple Watch as the destination.
4. Run. Grant Motion permission on first launch.

The watch simulator will not produce real motion values. Use a device for data.

Configuration

In MotionManager.swift:

• tickMs: sampling interval in milliseconds
• maxBuffer: maximum number of samples to keep before trimming

You can also expose tickMs via the UI slider provided.

UI

• Start, Stop, Clear, Save CSV
• Tick control with label that shows the current ms
• Optional live metrics block for quick sanity checks

Roadmap

• Add WatchConnectivity to transfer CSV to iPhone Files
• Add session metadata and calibration markers
• Build a quick visualizer notebook for CSV files
• Move to a ring buffer for O(1) trimming on long sessions

Notes

• Ensure NSMotionUsageDescription exists in the target Info so the permission prompt is clear.
• First install can take a moment while Xcode prepares the device.