Total control

3 min read

When developing its Boston Digital Arm upper limb prosthesis, Liberating Technologies identified the inflexibility of the control system as the primary limiting factor in upper limb prosthetic performance.

Until recently, upper limb prostheses were based on analog controls, meaning that a user relied solely on upper-arm muscle movements to control the prosthetic device.

If an amputee had limited use of the upper arm muscles, however, he or she may have been unable to use prosthesis or may only have been able to benefit from a fraction of its capabilities.

The power that the gripper exerted was controlled by a single predefined limit, meaning that the same amount of force used for lifting a heavy object would also be applied for holding an egg or a child’s hand.  Also, traditional artificial limbs were limited to controlling only three joints one at a time – the elbow, wrist and hand.

When developing its Boston Digital Arm upper limb prosthesis, Liberating Technologies identified the inflexibility of the control system as the primary limiting factor in upper limb prosthetic performance.

“When we developed our system we considered both microcontrollers (MCUs) and digital signal controllers,” said Bill Hanson, president, Liberating Technology.

Eventually, the company settled on TI’s C2000 controllers because they provided the ability to generate pulse width modulated (PWM) signals for driving the DC motors that are used in the prostheses. One TI digital signal controller drives five motors, expandable to nine with an add-on module.

Because of that, the Boston Digital Arm upper limb prostheses allows users to control movement of five joints or axes, while competitive solutions only permit control of up to three joints. Users can also apply variable amounts of force to a gripping task.

The Boston Digital Arm System was developed using TI’s Code Composer Studio Integrated Development Environment (IDE) and is controlled by signals generated from one or more of the user’s non-injured upper limb muscles.

TLV2432 operational amplifiers and INA121 BB instrumentation amplifiers from TI detect, condition and amplify the signals generated by the muscles. The C2000 controller then examines the strength of the signals, comparing them to signals from other sensors, and determines how much voltage to send to motors in the elbow, wrist and hand.

The device uses five pulse-width modulation (PWM) outputs to drive each motor, making it possible to go beyond the traditional arm, wrist and hand motion to, for example, provide shoulder movement for amputees without working shoulder muscles.

The device also uses the controller’s input/out (I/O) options, such as a serial port interface (SPI) digital to analog converter (DAC), to control up to four additional motors on an independent prosthetic controller. This enhanced capability allows the prosthetic arm to swing instead of hanging stiffly while a person walks, providing a more natural, comfortable motion. The controllers’ additional processing power also makes it possible for users to move their joints simultaneously, making it easier for amputees to accomplish everyday tasks like reaching and grabbing an object at the same time.

C2000 controllers integrate up to 265 Kilobytes (KB) of flash memory for simple reprogramming during development and in-field software updates. Optimized control peripherals include PWM generators, programmable general-purpose timers, capture modules for time stamping and glueless quadrature encoder interfaces.

The C2000 platform also features up to 12-bit analog-to-digital converters (ADC) that provide fast conversions – up to 12.5 MSPS – for tight control loops. Up to five different on-chip standard communication ports, including CAN, provide simple communication interfaces to hosts, test equipment, displays and other components or networks.

The controllers’ embedded flash memory and in-field re-programmability allows Liberating Technologies to update and customize the Boston Digital Arm with new software.

Traditional prostheses are typically limited to control from electrical signals generated when a wearer flexes the biceps and triceps. Liberating Technologies, however, has developed over 30 unique ways to control the limb in response to wearers’ needs.

For example, if an amputee’s bicep is injured, a prosthetist can program the control to use only the tricep; if the tricep is injured, the prosthetist can program the arm to use signals generated from the bicep.

If an amputee gains strength over time in a previously useless muscle, Liberating Technologies can reprogram the Boston Digital Arm so that it can also be controlled by the newly strengthened muscle, in addition to being controlled by the muscles it operated from previously.