The objective of this project was to create a new sensor to enable the Roombots modules to collect information about the direction of a contact and the intesity of the applied force. This information is useful for the two main tasks the Roombots are designed for. For the self-assembly and reconfiguration of structures, it can detect collisions between modules and facilitate the docking process between the modules. For the locomotion, it can detect the contact of the legs with the ground and be used in a control loop.
We decided to design a 4-axis force/moment sensor based on strain gauges (load cell). Several models were tested in simulation using SolidWorks simulation toolkit, and the most promising model was implemented and tested for real. An experiment setup was created to characterize the load cell.
Final model in SolidWorks
How it fits in the Roombots modules
Strain gauges glued on beams
Axis definition and location of the strain gauge pairs:
Measurements for the different force/moment components at different intensities. Values predicted by simulation are in full lines, the crosses are the measured points.
Mz measurements are inacurate because of the experiment setup.
Results are coherent but do not fit the simulation prediction closely. Some of the results are not as linear as expected. Those differences could be due to several reasons: mechanical model not accurate enough, problems in the strain gauge bonding process…
However, the load cell can still be fully characterized and could be used in the Roombots modules. But each load cell should be individualy characterized by measurements.