Control

Dedicated control boards have been developed by the team in order to control artificial hands. Shared-control strategies between low-level control loops and high level control loops are currently under evaluation for prosthesis and robot-hand operation.

This page presents actual state of the art at ARTS Lab.

The action of the prosthetic hand is controlled by a control architecture composed of two main parts (picture, right side): a low-level and a high-level control. The low-level control (LLC) loop is responsible for grasp stability whereas the high-level control (HLC) has been designed both to interpret the intentions gathered from external world (i.e. user intentions from the user-prosthesis interface, UPI, or the high level robot controller), for launching appropriate action patterns, and to provide external world itself, of appropriate signals (for subject afferent stimulation or for robot control). Both control levels are crucially dependent on a sensory system. Hand operation has thus been designed to be controlled as a Finite-State-Machine where the transitions between the different states are identified and detected as crucial events by the LLC (depending on the sensory system) or by the HLC (detecting recognized commands coming from the UPI, i.e. from the user). If the transitions between the different states are identified by the LLC, we have automatic control, whereas if they are identified by the HLC we have interactive control based on external world needings. Briefly depending on the control strategy (i.e. the Finite-State-Machine) it is possible to obtain different ratios of shared control between the external world (e.g. the user) and the embedded controller of the prosthesis.

Related Publications

With the goal to control the overall system, an High Level Hand Controller (HLHC) board, able to deal with LLMCs and to act as interface with external world, has been developed. Hands are thus controlled by a hierarchical architecture consisting of several Low Level Motion Controllers (LLMCs) and one High Level Hand Controller (HLHC). Each motor is directly actuated and controlled by means of a LLMC. In such system with master/slave architecture, the HLHC directly controls each LLMC and is able to:

• control overall hand operation (up to 10 DoMs);
• deal both with all LLMC involved, and with external world (robot controller, emg interface, eng interface etc.) through two different serial BUS (standard RS232) and dedicated comunication protocols developed.
• directly acquire analogic sensor signals, after conditioning (up to 16 tensiometers, torque sensors etc.) for closed-loop force control;
• deal with multiple external SPI® systems (e.g. multiplexer for joint angle acquisition, etc.);
• drive up to 4 external simple vibrotactile feedback sistems.
  

The open architecture distributed-control approach followed in designing this system is widely accepted in motion control. Unlikely from a centralized system in which all control loops are executed on a single processor, in a distributed system, the trajectory generation and logic control are executed on the central controller, whereas the PID motor control loop is executed by intelligent drivers. A distributed approach reduces overall wiring, cost and system complexity.

The HLHC central unit is the PIC18F8722: an 80 pin high performance microcontroller with 16 inputs for A/D conversion (for torque and cable tension signal acquiring), multiple serial interfaces and multiple digital I/O ports.

The HLHC is powered by a single supply +6V source and can be mounted on LLMCs thanks to the 40 pin PC104 through-hole connector.

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