IMU Motion Switch

With some disabilities an individual can still move parts of their body but only with a very low level of control. They could for example make their arm move but not to a specific place let alone with a specific speed or force. Also sometimes their arm may move involuntarily. For a person in this situation it can be extremely hard to interact with their environment. Even trying to hit a large accessibility switch may not be possible.

I partnered with Yusuke Namiki at Roosevelt Children’s Center to design and build an affordable, customizable and wireless switch for children with these types of disabilities. Finding a switch that works for these children not only gives them some control of a given toy or game. It shows them that their actions can have a real effect in their environment.

Below is a video of a 3 1/2 year old boy with Sturge Weber Syndrome using our switch. SWS affects his central nervous system and severely limits volitional movement of his extremities. He has some spastic movement of his arms, but not with enough control and accuracy for him to hit targets or cross mid line. In this video, he is able to listen to a sample of each instrument using his motion switch to activate the program (Boardmaker) and turn the page to the next instrument.

There are many different approaches to making a motion activated switch including PIR, mercury switches, rolling ball tilt switches, spring vibration switches, depth cameras (like Microsoft Kinect), or regular cameras doing color blob tracking. We chose to use an inertial measurement unit (IMU) as used in the Nintendo Wii Remote and smartphones. An IMU can measure the magnitude of linear acceleration and angular velocity.  By comparing these values to customized thresholds an MCU can trigger a switch closure when a volitional movement is detected while ignoring involuntary movements.

This current version is 2 parts. The transmitter is worn by the user on a part of their body they have at least some slight control over. When the transmitter detects enough acceleration or rotation it triggers the receiver which can act as a switch, mouse or keyboard. This enables the wearer to wirelessly control various switch adapted devices and toys, and also to control a computer.

These are the components and boards used:

Transmitter

  • Adafruit Feather M0 RFM69HCW: MCU board (w/ built-in RF tranceiver and LiPo battery port)
  • MPU-6050: IMU (w/ built-in DMP to do sensor fusion)
  • 350 mAh LiPo Battery
  • power switch

Receiver

  • Adafruit Feather M0 RFM69HCW: MCU board (w/ built-in RF tranceiver and LiPo battery port)
  • VO14642AT and 1K resistor: solid state relay to control switch output jack
  • mono audio jack: switch output jack for using an audio cable to plug into switch-enabled devices to control them
  • LED and 1K resistor: indicator LED
  • 3-way switch: for quick sensitivity selection (low, medium, high)
  • USB cable: for power, configuration, and mouse/keyboard emulation

Here are pictures of building the transmitter:

Here are pictures of building the receiver:

Here’s a picture of 3 completed pairs and a picture of one of the transmitters being worn in a pouch on the wrist:

You can grab the code for both the transmitter and receiver here: https://github.com/bobparadiso/IMU_switch

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