Working principle of the optical foot contact switch
We designed simple, low cost, lightweight and robust optical foot contact switch for our quadruped robot.
The contact switch consists of a spring loaded aperture that is located between a led light source and a light sensor.
External loads displace the aperture and change the light intensity measured by the sensor.
For centering the foot we use 12 elastic silicone elements (6 on each side).
The foot contact switch is symmetric and can detect forces from a wide range of different angles (270 degrees).
For visual feedback and monitoring there are bi-color leds mounted on each side of the foot.
Left side: Foot Contact Switch loaded / Right side: Foot Contact Switch unloaded
Lower Leg and Foot Contact Switch Component Overview - the weigth of the lower leg assembly is 35g
Lower Leg and Foot Contact Switch assembled
The weight of the foot, the contact switch electronics and the silicone elements is about 10g.
To prevent the foot from rotating we use a steel pin that is guided by the lower leg structure.
Under load the foot deflects up to 2mm from its neutral position.
The electronics are supplied with 3.3V and generate an analog output signal between 3V and 0V.
The signal is converted by the microcontroller ADC and transmitted over CAN at 1kHz.
We measured the delay of the foot contact switch with respect to a ATI Mini40 force sensor for a drop of the 2dof leg from a height of 8cm.
The measured delay between the ATI force sensor signal and the foot contact signal was 2ms.
The data was logged at 1kHz using a Realtime Linux PC.
Foot Contact Switch Electronics
Elastic elements cut from silicone tube
- Silicone tube - outer diameter 6mm / inner diameter 4mm / length 3mm
- 12 elements / 6 on each side
- Steel Pin 1,5mm x 12mm
- Bicycle tire "Schwalbe Lugano 28" 23-622"
- cut to 65mm x 10mm pieces
- bonded to the foot with Loctite 404
The files for 3d printing the parts are available in the STL folder.
The foot contact switch consists of two boards with traces and components on all four sides.
The diameter of the electronics boards is 10mm.
Foot Contact Switch Main Board - Left: Outer side / Right: Inner Side
Foot Contact Switch Satellite Board - Left: Outer side / Right: Inner Side
Special thanks to Joshi Walzog for designing and manufacturing the electronics boards!
- Bill Of Materials Foot Contact Switch - PDF Document
- Bill Of Materials - Excel Document
- Eagle Files - Zipped Folder
To view the Eagle files you need to download the free Eagle PCB Design Software for Windows, Mac and Linux.
You can also use a simplified version of the lower leg if you don't need the foot contact information.
This lower leg has the exact same dimensions as the original lower leg with foot contact switch.
We print it on our FDM printer using sparse infill to match the weight of the original lower leg.
Foot Contact Switch Connectors and Wiring
- x-axis -> vertical force [N] measured with ATI Mini40 Sensor
- y-axis -> foot contact sensor ouput signal inverted and normalized
- 0-1 on the y-axis corresponds to 3V - 0V range
The sensitivity of the switch can be adjusted depending on the application by changing:
- the number of elastic elements
- the diameter of elastic elements
- the wall thickness, length or material of the elastic elements
- the diameter of the aperture
- the distance between emitter and receiver
- the emitter led brightness
- 3d print elastic elements
- Reduce weight of traction element on the foot
- Reduce sensor signal noise
- To get an better overview of the assembly you can view this pdf file
- I recommend downloading the free eDrawings viewer for Windows, Mac, iOS and Android here.
- The eDrawing Viewer Software allows you to open and explore this easm cad assembly file
Felix Grimminger
Joshi Walzog
BSD 3-Clause License
Copyright (c) 2019, Max Planck Gesellschaft, New York University
Open Dynamic Robot Initiative - Webpage
Open Dynamic Robot Initiative - YouTube Channel
Hardware Overview
Software Overview