Advanced Signal Processing Dramatically Improves Capability of Artificial Limbs

Just a few years ago, all 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 a prosthesis, or may only have been able to benefit from a fraction of its capabilities.



The power that the prosthetic 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.



Today, however, a new generation of prostheses uses an advanced signal processing-based motor control strategy to dramatically improve these capabilities. The Boston Digital Arm from Liberating Technologies provides amputees with unique, upper limb prostheses that are dramatically more flexible and capable, in large part due to the control optimized performance and integration offered by TMS320C2000 digital signal controllers from Texas Instruments.



By leveraging digital signal processor (DSP) based control technology, the Boston Digital Arm upper limb prosthesis allows users to control movement of five joints or axes. The prostheses also offers the flexibility and programmability to provide patients with an optimal, customized device, enabling more life-like movement and sensitivity and the ability to apply variable amounts of force to a gripping task.



Selecting a Core Processor for the Prosthesis

When Liberating Technologies developed the Boston Digital Arm system, they considered using both microcontrollers (MCUs) and digital signal controllers. The problem was that traditional analog systems generally provide three axes of motion, and it seemed fairly obvious that microprocessors would have difficulty surpassing this number.



“We wanted to control as many axes as motion as possible because we were sure that as soon as we proved this type of capability was possible, wearers would demand it,” said Bill Hanson, president, Liberating Technology. “We realized that we would need advanced signal processing to accomplish what we planned, so we went with digital signal controllers.”







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 non-injured upper limb muscles. Operational amplifiers and instrumentation amplifiers from TI detect, condition and amplify the signals generated by the muscles. The C2000 digital signal 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.



Digital Signal Controller Provides Optimized Motor Control

Liberating Technologies engineers used the controller’s DSP processing power and motor control peripherals to create multiple pulse width modulated (PWM) outputs for an efficient way to drive the three-phase brushless direct-drive DC motors used to power prosthetics. While MCU-based solutions require two MCUs to drive three motors, a single digital signal controller can drive five motors. With an add-on module, the number of motors can even be increased to nine.







Increasing the number of controlled motors means that amputees can control more different joints as opposed to previous control strategies, which only allowed manipulation of the elbow, wrist and hand. Liberating Technologies can now provide mechanical shoulder joints for upper arm amputees, making it possible for wearers to swing their arms naturally while they walk, rather than having their arms hang awkwardly at their sides.



DSP processing power allowed Liberating Technologies to switch to three-phase motors that provide ten-foot pounds of torque and three to four times the lifting power of competitive, MCUbased designs. The voltage delivered to the motor is controlled by changing the duty cycle of a PWM block, and the duty cycle is controlled by a PID block. This approach continually monitors and adjusts the stator currents to provide the correct operating speed while avoiding surges, which can cause positioning problems for the user.



Controlling the Force Applied by the Hand

The controller’s programmability is a key component in the Boston Digital Arm system. The digital signal controller is programmed to monitor the power consumed by the motor, and this signal is fed back to the controller for a closed-loop control strategy. When the motor reaches a pre-defined power limit, its speed is reduced to avoid using up battery power too quickly or becoming too hot.



Limitations are also applied to the power delivered to the arm to prevent the device from applying too much pressure for tasks such as shaking hands or handling eggs. Unlike many prostheses, the Boston Digital Arm system allows wearers to control and adjust gripping pressure to levels that are appropriate for very different tasks. Additionally, some wearers of these devices use life-like coverings that typically cost between $5,000 and $8,000. Limiting the force applied by the prostheses can substantially increase the life of these coverings.



The company is also using the controller’s flexibility to set different power limits for different devices such as hands, hooks, and grippers that may be attached to the prosthesis. The appropriate strength limits for each device are stored in a lookup table on the digital signal controller. Liberating Technologies has also been testing several different methods for giving the wearer feedback on how much force is being applied by their prosthesis and giving them the ability to control that force. Today, current is limited on a pulse-by-pulse basis, controlled by a comparator with a programmable input level. One possible future method is providing a sound that varies in volume to indicate the amount of force.



The myoelectric signals that are generated during muscle contractions are captured by tiny sensors placed on the surface of the wearer’s skin. The low magnitude of these signals means that they are sensitive to interference from ambient noise such as that generated by fluorescent lighting. The ability of the DSP to perform digital filtering made it easy for Liberating Technologies to program a selectable notch filter that operates at 60 Hz for prostheses being used in the United States, and at 50 Hz for those being used in Europe. Integrating this feature helped Liberating Technologies to avoid adding external, discrete components that would have been required if they had used analog electronics and microprocessors instead of DSP technology.





Providing a Wide Range of Control Strategies

Earlier generations of upper arm prostheses are limited to a single control strategy. The user moved the arm by flexing the biceps and triceps. While this strategy is appropriate for many wearers, patients whose upper arm muscles are damaged cannot use this type of prosthesis. One revolutionary aspect of the Boston Digital Arm system is that the controller’s flexibility and input-output (I/O) capacity make it possible to program the device for multiple control strategies, meaning that this type of upper-limb prosthesis is extremely adaptable to amputees’ varying needs.



As an example, a Liberating Technologies patient who was a power company worker lost both of his arms when he inadvertently touched a highvoltage line. His doctor rerouted the severed nerves for his shoulder, elbow, wrist and hand into his chest and connected them to bands of pectoral muscle. As a result, when his brain sends a “flex elbow” or “rotate wrist” command, his chest responds. Liberating Technologies then programmed the Boston Digital arm to respond to the chest muscle’s motion to produce motion of the appropriate motor. This is only one of the 36 control strategies the company has developed since the introduction of the arm, and the company is working continually to expand the number of strategies available.



The digital signal controllers’ embedded flash memory and in-field reprogrammability allows Liberating Technologies to constantly update and customize the Boston Digital Arm with new software. Today a patient can buy a pre-programmed generic system that can be remotely reprogrammed to use any of Liberating Technologies’ control strategies.



“A patient can keep trying different strategies until they find the one that works best for them,” Hanson said. “We can sit here in our office, look at the signals generated by their arm, and make adjustments on the fly to provide the optimal control strategy. Recently, we set up a system with a voice command for a patient with shoulder level amputations on both sides. This kind of flexibility simply did not exist in the past.”





About the author

Chris Clearman is with TMS320C2000 Business Development, Texas Instruments. For more information, please call (972) 644-5580 or email controlanswers@list.ti.com; www.dspvillage.ti.com