Smart Strain Gauge Sensor
Use a Strain Gauge + HX711 Load Cell Amplifier as bed leveling sensor for any 3D Printer.
Description
This has the following benefits:
- No sensor-nozzle relative position calibration needed.
- Excellent precision and repeatability (even if the assembly moves or twists), as the tip itself is used for measuring.
- No added weight to the carriage which reduces inertia.
Contents
Analysis
Contains scripts that were used to model the behavior of the gauges under several scenarios of linear movement, acceleration, and bed touching.
Firmware
This directory contains the following subdirectories:
- examples - snippets and libs that can be used to test the parts
- fw - the functional firmware for the SSGS can be found here along with debugging scripts
- myLibs - experimental variations and tweaks of the public libraries
Hardware
Design of a small PCB containing a MCU + HX711 Load Cell Amplifier in a Full-Bridge configuration.
Detailed description
The strain gauge is able to measure extremely small variations in the size of the object it is attached to.
A simple threshold-based triggering of the sensor is not sufficient for providing a reliable method that works universally for several reasons:
- Each assembly will have different physical conditions and constants obviously can't be shared amongst different 3D Printers, requiring each one to be calibrated separately.
- Even for a single 3D Printer, many components create tension in different directions as the nozzle travels to different locations in the bed. The bowden tubes have been found to be a major source of varying forces in the nozzle holder.
Therefore, the derivative and integral components of the signal are used to trigger a touch signal. This allows for the natural deformation that the parts experience as they move to be filtered out from bed touching events.
Results
The result is a sensor that has an excellent exactness, on par with some of the best alternatives, but with interesting advantages mentioned above.
It has successfully been used in a custom-built 3D Printer which has a large, very uneven, print bed. Here is the 3D visualization of the calibration results using the SSGS as bed leveling sensor.
And the successful first layer printed afterwards.
Version History
- 0.1 2017-July-23
- First working POC
- Successfully integrated and working in Marlin
- Demo video and prints recorded
Acknowledgments
Inspiration, code snippets, etc.