Force control for an underactuated prosthetic hand
A prosthetic hand that only opens and closes does not know how hard it is squeezing. This project gave that information to the lab's underactuated hand. A load cell was developed from an optical sensor and a printed part, at a cost of about 5000 Colombian pesos, along with a controller that follows a force command with a delay under 0.5 s.
Context
In an underactuated hand, grasp force is proportional to the tension in the actuation tendon. Measuring that tension at a single point makes it possible to regulate the whole grasp. Commercial load cells are expensive and bulky for use inside a low-cost prosthesis.
The plant: a stiffness that changes
When the tendon is pulled, the relation between force and displacement of the prosthesis is not linear. First the finger springs are overcome, then the fingers contact the object, and finally the structure deforms.
Force against tendon displacement, with three stiffness regions
The plant was modeled as three springs of different stiffness. A single controller does not behave well in all three regions.
Load cell
Three measurement principles were tried: a spring with a potentiometer, and two elastic parts with a slotted optical sensor that measures how much an infrared beam is interrupted as the part deforms. The chosen version uses U-shaped beams.
Deformable part with U-shaped beams
Ambient light disturbed the reading, which was fixed with a black housing.
Load cell with a housing that shields the sensor from ambient light
Sensor signal as a function of applied force
The cell measures from 0 to 5 kgf with an uncertainty of 0.1 kgf.
Controller
A controller was tuned for each stiffness region, by two routes: simulation with a plant model and experimental tuning. The constants obtained differ, but both controllers respond well to a step.
Bench with motor, load cell, spring and electronics
Step response of an experimentally tuned proportional controller and PID
The final controller is hybrid: it switches constants according to the stiffness region the prosthesis is in.
Results
Desired and measured force for a time-varying command
With a varying command, the controller reaches the desired force with a delay under 0.5 s, overshoots below 0.1 kgf and no oscillation. The hybrid controller was tested on the prosthesis and was stable.
What is missing
The force command was generated artificially. It does not come from a muscle signal, and the author warns that the noise in such a signal can cause problems with derivative action. The cell is large to fit inside the hand and its plastic part may fatigue; making it smaller and in aluminum is recommended. Force feedback to the user was not implemented.
How it fits in Robiolab
This is the third project in the prosthetic hand line. Its load cell and controller were incorporated, with a redesigned deformable part, into the integrated version tested with an amputee user. The sensor illustrates a way of working that is common in the lab: turning a well-designed flexible structure into a measuring instrument.