Controller makes prosthetic arms more flexible

The Boston Digital Arm provides amputees with unique, upper limb prostheses using the control optimised performance and integration offered by TI's TMS320C2000 digital signal controllers.

The Boston Digital Arm from Liberating Technologies provides amputees with unique, upper limb prostheses that are dramatically more flexible and capable, due to the control optimised performance and integration offered by TI's TMS320C2000 digital signal controllers.

The Boston Digital Arm upper limb prostheses allows users to control movement of five joints or axes, whereas competing solutions only permit control of up to three joints, allowing a greater range of flexibility and programmability - providing patients with an optimal, customised solution, enabling more lifelike movement and sensitivity and the ability to apply variable amounts of force to a gripping task.

Until recently, upper limb prostheses were based on analogue 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 are limited to controlling only three joints one at a time - the elbow, wrist and hand.

Liberating Technologies identified control system inflexibility as the primary limiting factor in upper limb prosthetic performance and was determined to leverage the latest advancements in control technology when developing the Boston Digital Arm.

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

"We selected TI's C2000 controllers because they provide vastly superior abilities to generate pulsewidth modulated (PWM) signals for the most efficient method of driving the DC motors that are used in prostheses".

"One TI digital signal controller gives us the ability to drive five motors, expandable to nine with an addon module".

"In contrast, some competing solutions require two microcontrollers to drive only three motors".

The Boston Digital Arm System, which was developed using TI's Code Composer Studio integrated development environment (IDE), is controlled by signals generated from one or more of the user's noninjured 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 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 analogue convertor (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 much easier for amputees to accomplish everyday tasks like reaching and grabbing an object at the same time.

C2000 controllers integrate up to 265Kbyte of Flash memory for simple reprogramming during development and in-field software updates.

Optimised 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 12bit analogue-to-digital convertors (ADCs) that provide fast conversions - up to 12.5Msample/s - 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 reprogrammability allows Liberating Technologies to update and customise the Boston Digital Arm with new software, so the prosthesis can interface with the wearer in a nearly infinite number of ways.

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 off of previously.

C2000 digital signal controllers are designed for applications that require the high performance of TI's leading digital signal processing (DSP) technology in combination with the peripheral integration and ease-of-use typically found in an MCU, while enabling capabilities far beyond those of MCU-driven competitive prostheses.

For example, a traditional prosthesis turns off the motor when power consumption reaches a certain limit in order to avoid gripping an object with excess force, meaning a user must reverse the motor's direction to resume activity.

The C2000 controller continuously monitors the motor's power consumption to provide a closed loop system that offers a much higher level of control.

Consequently, the prosthetic arm is "smart" enough to allow the wearer to control his or her gripping force and adapt power limits to the task at hand - from picking up a heavy item to handling an object as delicate as an egg.

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