By Stephen Trier, Cleveland FES Center and Case Western Reserve University Department of Biomedical Engineering, Tina Vrabec, Cleveland FES Center and Case Western Reserve University Department of Biomedical Engineering, and Jeff Weisgarber, Cleveland FES Center and Case Western Reserve University Department of Biomedical Engineering
For individuals with neurological impairments, functional electrical stimulation (FES) can help make real what was once only imagined: the restoration of movement to paralyzed arms and legs. Depending on the location and severity of the disability, FES can significantly improve quality of life by enabling the individual to regain capabilities such as walking, grasping objects, or maintaining bladder control.
FES devices send electrical impulses to electrodes—implanted in the body, worn on the skin, or operating through the skin—to produce and control movement. While a great deal is known about how electrical stimulation affects paralyzed nerves and muscles, until now a challenge has been the need to customize each device for the individual patient’s abilities and disabilities.
At Case Western Reserve University we developed the Universal External Control Unit (UECU), a flexible, configurable system technology platform for FES applications. With UECU, engineers in the clinic can change an FES controller and immediately see the results, enabling them to make improvements up to ten times faster than they could do before.
In developing the UECU, we had three primary goals. First, we wanted the UECU hardware and software to be modular and flexible enough to be used for a wide variety of FES applications. Second, the design environment had to enable researchers and rehabilitation engineers—not expert programmers—to develop rapid prototypes and quickly translate basic research into actual clinical systems. Third, the UECU had to work effectively in the laboratory, in the clinic, and in the patient’s home.
Researchers at the Cleveland FES Center have developed FES devices for a diverse range of applications, including:
Each application uses a combination of one or more implanted stimulators, percutaneous electrodes, or surface electrodes to produce movement.
We used Simulink from the very start of the project. As a proof of concept, we connected a Freescale microcontroller evaluation board to a prototype implant control module and wrote S-functions to control this hardware. We created a simple FES controller in Simulink and generate code from the model with Real-Time Workshop. The proof-of-concept system demonstrated that we could get real-time control of stimulation pulses from a Simulink model.
From there, we developed and refined the software. While the hardware engineers designed the electronics, the software developers divided the UECU’s capabilities into building blocks that researchers can use to assemble FES applications. We included these blocks in a custom Simulink blockset. The UECU Blockset provides more than 75 blocks, including individual blocks for accessing input from analog devices, joint angle transducers, myoelectric signals, and simple buttons, as well as blocks for generating simulation pulses and producing sounds, lights, and other feedback from the control unit.
Along with the blockset, we developed a custom Real-Time Workshop target for the UECU. An adaptation of the generic real-time target provided in the package, our target configures the C compiler and other tools to correctly compile for the UECU. With the target, a clinical engineer can use a single
Build command to turn a Simulink model into a compiled program downloaded to a UECU and ready to run.
Because applications are assembled from Simulink blocks, our researchers can build FES control software up to ten times faster than before.
FES applications used to be developed in C by dedicated programmers. The development group supported multiple clinical groups, each of which specialized in a different disability. When a clinical group needed a new feature or capability, they sent a request to the programmers, waited for a new version of the software, and then tested it in the clinic. While coding in C was very flexible, the overall process was slow, limiting how quickly we could move from idea to reality. Worse, because the programmers might not understand what was needed, it often took several iterations before the clinical group had a change that met their needs.
The Simulink based UECU has enabled our central development group to focus on building and maintaining a single system. The system’s flexibility and ease of use make it possible for each clinical group to rapidly develop and refine their own FES applications (Figure 1).
Once a design is completed, researchers use Real-Time Workshop to generate code from their Simulink model. Often, researchers test new ideas by building a rapid prototype with xPC Target. This approach enables them to use simpler floating-point designs to validate ideas on PC-compatible hardware before committing the design to portable, fixed-point target hardware.
Once a basic controller design has been developed, we must still fine-tune the stimulus parameters that it uses to control the patient’s muscles (Figure 2).
We designed the UECU with non-volatile memory in which we store a table of stimulus data. In an arm control application, for example, this table is used to translate a scalar control signal into a vector of 8 to 16 numbers that control the pulses sent to nerves and muscles in the patient’s arm. To optimize the stimulus patterns, we set the signal levels at an initial starting point, see how they perform with the patient’s arm, then adjust the levels (Figure 3). Because muscles grow stronger or weaker over time, we have to repeat this process every so often to keep the FES system performing at its best.
A case involving an early version of a myoelectric control highlights the speed with which clinicians can now implement new ideas. A patient from out of town would be the first to use a control in which the electrical activity of a fully functional muscle is used to control paralyzed muscles. At that time the controller did not work as well as it does today, and we were unable to get the system running to our satisfaction.
We all wanted to send the patient home with a system that he could use to control his hands. On the last day of the patient’s visit, one of the rehabilitation engineers had an idea: Why not use the sip-and-puff control, a straw that the patient blows or sucks to control his wheelchair. In one day, the engineer developed an algorithm that incorporated this unique feature into the control software. Using Simulink and the UECU Blockset, he had the algorithm tested and working by the time the patient left the clinic. Previously, this kind of effort would have taken a week or more.
At present, the UECU is used primarily to restore limb movement, but research groups also use it to develop other applications. In one initiative, a biomedical engineering graduate student used Simulink and the UECU to develop an experimental application for bladder control. Because only a limited amount of hardware was available for testing, he modeled, simulated, and tested the control logic entirely in Simulink. When hardware was available, we needed less than an hour to get his model working on the actual UECU.
Another group of graduate students used the UECU to develop a slip detector that can sense when an object is slipping from someone’s hand. Using a piezoelectric sensor and signal processing developed in Simulink, a slip detector like this may eventually provide valuable feedback for those who have no sense of touch in their hands.
With UECU, our clinical teams can build increasingly sophisticated applications that give patients more control than before. We can, for example, control two implants with a single external controller. Using our rapid prototyping environment, clinical engineers have built neuro-prosthetic applications for many different types of injury. More than 30 UECUs are currently in use by patients.
We are also planning a DSP module that will significantly increase the computing power of the UECU, allowing the system to handle more computationally intense applications.
In the next five to ten years, we aim to replace the UECU and its companion implants with a fully internal system. Having an FES system completely inside the body will improve independence in places UECU cannot go today, such as in the shower or the pool, and will provide a more natural experience in any setting. Lab prototypes of the new system are already running programs created with Simulink and Real-Time Workshop.
Published 2008 - 91625v00