The above-seen design allows for easy implementation of gravity compensation and very low motor torque requirements. Furthermore, it interrupts the torque propagation from the shoulder to the forearm segment. 

With this new design, the required torque for a specific task could be reduced from the original 14 Nm to only 1 Nm. Further validation proved the advantages of the low motor inertia. Smaller motors are faster when it comes to changing directions, therefore the system becomes more resistant to human disturbances. 

So this exoskeleton got advantages over traditional exoskeleton designs (One big motor per human joint axis) by changing the mechanical structure and by adjusting the motor size. 

Next up Mr. Calanca spoke about how to optimize the spring which is offloading the motor. Again the goal is to reduce motor size by adding a spring in parallel with the motor. The motor specifications were presented in a radar chart. By introducing the spring certain characteristics (max. required torque, average squared torque, max power, etc.) could be optimized. So one should optimize a spring that reduces one of the motor specifications. One possible expansion of this exokseleton architecture is to make the elastic element adjustable. One way of doing it is to make it slide up and down on the shoulder part to change it´s angle attachment. You can see that working in the video on Agadexo shoulder´s website.

Inertia Transparency

Mr. Calanca showed a video in his presentation to us where the exoskeleton tried to move out of the users way to maintain a constant force at its force sensor. This force sensor might be located at one of the exos cuffs in the final version. In the video we saw that the system was very sensible, the exoskeleton moved out of the way even when the force sensor was only touched by a paper towel. 

This “constantly moving out of the way” of the exo is described as inertia masking or inertia transparency. So by using a force sensor, actuators and a control system the mechanical system friction and inertia can be virtually removed. This allows the user to move with the exoskeleton without feeling any kind of resistance even when the exo is heavy.   

Transparency, open issues

Non-collocation describes when the sensor and the motor are not mounted on the same rigid body. Any industrial robot has its torque sensor directly on the motor, this is crucial for the stability of the control system. Otherwise, the system usually starts to oscillate. Mr. Calanca showed us a video with the same inertia transparency control but this time instead of using a paper towel, he used a piece of steel to make the exoskeleton react. The exoskeleton started to vibrate instead of smoothly lifting as before. 

“As force control gains are raised to improve performance and transparency vibrations occurred(due to non-collocation)” (from one of the slides in his presentation)

So this high gain system, which in this context translates to high sensibility in combination with a non-collocated sensor is prone to vibration when in contact with a stiff material. 

A solution to this problem is to add an accelerometer, which detects when oscillations occur. This way vibrations can be compensated for. Mr. Calanca showed us in the next video that this approach indeed works and the actuators movements remained smooth even when the force sensor was interfaced with something very stfff. 

Mr Calanca mentioned that there he will be organizing an industrial exoskeleton workshop at CASE 2024.