Abstract:
A gait modulation system including: (a) a sensor device including a sensor adapted for associating with at least one lower limb of the patient, the sensor for transducing at least one parameter related to a gait of the patient, so as to obtain gait data related to the gait, and (b) a muscle stimulator including: (i) an electrical stimulation circuit, the circuit adapted to supply an electrical stimulation output to an electrode array for performing functional electrical stimulation of at least one muscle of the lower limb, and (ii) a microprocessor, operatively connected to the at least one sensor, the microprocessor adapted for: receiving a stream of gait information based on the gait data; processing the gait information, and controlling the stimulation output based on the processing of the gait information, and wherein the microprocessor is further adapted to identify a failure in the stream of gait information, and to consequently control the electrical stimulation circuit to deliver a fail-safe stimulation output over a portion of a duration of the failure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/333,184, entitled “Functional Electrical Stimulation Systems”, filed on Jul. 16, 2014, which is a divisional of U.S. application Ser. No. 12/299,043, entitled “Functional Electrical Stimulation Systems”, now U.S. Pat. No. 8,788,049, which is the U.S. national phase application of International Application No. PCT/IL2007/000531, filed May 1, 2007, entitled “Improved Functional Electrical Stimulation Systems,” which claims the benefit of priority from U.S. Provisional Application Ser. No. 60/746,060, entitled “Foot Sensor-Dynamic Gait Tracking Algorithm,” filed May 1, 2006, U.S. Provisional Patent Application Ser. No. 60/805,359, entitled “Foot Sensor Envelope,” filed Jun. 21, 2006, and International Patent Application Serial No. PCT/IL2006/001326, entitled “Gait Modulation System and Method,” filed Nov. 16, 2006. 
         [0002]    International Application No. PCT/IL2006/001326 claims the benefit of priority from U.S. Provisional Application Ser. No. 60/736,858, entitled “Hybrid Orthosis; Foot Sensor; Electrode,” filed Nov. 16, 2005, U.S. Non-Provisional patent application Ser. No. 11/380,430, entitled “Orthosis for a Gait Modulation System,” filed Apr. 27, 2006, and U.S. Non-Provisional patent application Ser. No. 11/552,997, entitled “Sensor Device for Gait Enhancement,” filed Oct. 26, 2006. U.S. Non-Provisional patent application Ser. No. 11/552,997 claims priority to U.S. Provisional Application Ser. No. 60/736,858, entitled “Hybrid Orthosis; Foot Sensor; Electrode,” filed Nov. 16, 2005, U.S. Provisional Application Ser. No. 60/746,060, entitled “Foot Sensor-Dynamic Gait Tracking Algorithm,” filed May 1, 2006, and U.S. Provisional Application Ser. No. 60/805,359, entitled “Foot Sensor Envelope,” filed Jun. 21, 2006. 
     
    
     FIELD AND BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to functional electrical stimulation (FES) devices and systems and, more particularly, to an improved envelope for force-sensitive resistors of such devices, and to FES devices and systems having improved monitoring, analysis, control, safety, energy conservation, and communication features. 
         [0004]    It is known that various pathologies of the neuromuscular system due to disease or trauma to the central nervous system, such as stroke, spinal cord injury, head injury, cerebral palsy and multiple sclerosis, can impede proper limb functioning of the legs. Gait, the biomechanical description of walking, can suffer static and dynamic parameter variations due to neuromuscular impairments that cause non-symmetrical walking and reduced walking speed and stability, and often require increased energy consumption. 
         [0005]    Drop foot describes the gait attributable to weak or uncoordinated activation of the ankle dorsi-flexors due to disease or trauma to the central nervous system. A patient suffering from drop foot tends to drag the foot during the swing phase of walking and usually try to compensate for this dragging by hiking the hip or swinging the affected leg in a circular motion. These patients tend to have impaired stability, are prone to frequent falls, and have walking movements that are unaesthetic and energy consuming. 
         [0006]    It is known, however, that functional electrical stimulation (FES) can generally be used to activate the leg muscles of such patients. Precisely timed bursts of short electrical pulses are applied to motor nerves to generate muscle contractions, which are synchronized with the gait of the patient, so as to improve the leg function and enhance the gait. The timing of these pulses is critical, and must be synchronized with the gait. This is advantageously achieved by sensing gait events such as a foot-floor force reaction, using a force-sensitive resistor (FSR) disposed beneath the heel region of the patient, and transmitting the information to the stimulator unit. 
         [0007]    The FSR sensor must be protected against water, humidity, dirt, and mechanical stress by means of a casing or envelope. 
         [0008]    U.S. Pat. No. 6,507,757 to Swain, et al., discloses one typical foot sensor device of the prior art, in which a foot pressure switch, or sensor, is permanently disposed in the shoe of the affected leg. An electrical circuit is interrupted during the stance phase, when a significant weight is placed on the heel, and reconnects when the heel is lifted during the swing phase. Wires disposed under the clothing connect the sensor with an external stimulator unit that can be attached to the belt or kept in a pocket of the user. The stimulator unit is connected to the electrodes by additional electrical wires. 
         [0009]    The cumbersome wires may be obviated by using a radio frequency (RF) system in which the foot sensor device and other components of the FES orthotic system communicate in a wireless fashion. However, the use of such an RF system necessitates integrating an RF transmitting unit, or head, within the foot sensor device. The RF communication with other components of the FES orthotic system must be robust and reliable, even in areas in which various types of wireless signals are prevalent, such as local area networks (LANs). The FES orthotic system must also be robust and reliable in areas in FES clinics and the like, in which one or more additional wireless FES systems may be operating simultaneously. 
         [0010]    There is therefore a recognized need for, and it would be highly advantageous to have, an FES orthotic system for neuroprosthetic gait enhancement that overcomes the various deficiencies of the known systems. It would be of particular advantage for such a system that is robust and reliable, avoids the discomfort associated with various prior art stimulation devices, and is secured so as to operate in a safe and robust fashion. 
       SUMMARY OF THE INVENTION 
       [0011]    According to the teachings of the present invention there is provided a gait modulation system utilizing functional electrical stimulation for improving lower limb function of a patient having neuromuscular impairment of a lower limb, the gait modulation system including: (a) a sensor device including at least one sensor adapted for associating with at least one lower limb of the patient, the sensor for transducing at least one parameter related to a gait of the patient, so as to obtain gait data related to the gait, and (b) a muscle stimulator including: (i) an electrical stimulation circuit, the circuit adapted to supply an electrical stimulation output to an electrode array for performing functional electrical stimulation of at least one muscle of the lower limb, and (ii) a microprocessor, operatively connected to the at least one sensor˜the microprocessor adapted for: receiving a stream of gait information based on the gait data; processing the gait information, and controlling the stimulation output based on the processing of the gait information, and wherein the microprocessor is further adapted to identify a failure in the stream of gait information, and to consequently control the electrical stimulation circuit to deliver a fail-safe stimulation output over at least a portion of a duration of the failure. 
         [0012]    According to further features in the described preferred embodiments, the microprocessor is adapted to control the electrical stimulation circuit to provide the fail-safe stimulation output so as to reduce a falling risk of the patient. 
         [0013]    According to still further features in the described preferred embodiments, associated with the microprocessor is a timing mechanism for timing the stimulation output based on the stream of gait information. 
         [0014]    According to still further features in the described preferred embodiments, the microcontroller is adapted to make a prediction of a gait event of the patient based on the stream of gait information. 
         [0015]    According to still further features in the described preferred embodiments, the microcontroller is adapted to control the electrical stimulation circuit to deliver the fail-safe stimulation output at a time based on the prediction of the gait event. 
         [0016]    According to still further features in the described preferred embodiments, the prediction of the gait event is related to a prediction of a heel-contact event. 
         [0017]    According to still further features in the described preferred embodiments, the prediction of the gait event is related to a prediction of a heel-off event. 
         [0018]    According to still further features in the described preferred embodiments, the prediction of the gait event is related to a prediction of a SWING phase of the gait. 
         [0019]    According to still further features in the described preferred embodiments, the prediction of the gait event is related to a prediction of a STANCE phase of the gait. 
         [0020]    According to still further features in the described preferred embodiments, the failure includes a communication failure from a transmitting unit of the sensor device. 
         [0021]    According to still further features in the described preferred embodiments, the communication failure is a radio frequency communication failure. 
         [0022]    According to still further features in the described preferred embodiments, the sensor device further includes a microprocessor, electrically associated with the sensor, for receiving a signal pertaining to the parameter, and a transmitting unit for transmitting, in a wireless fashion, the gait information to a unit of the gait modulation system external to the sensor device. 
         [0023]    According to another aspect of the present invention there is provided a gait modulation system utilizing functional electrical stimulation for improving lower limb function of a patient having neuromuscular impairment of a lower limb, the gait modulation system including: (a) at least one sensor adapted for associating with at least one lower limb of the patient, the sensor for transducing at least one parameter related to a gait of the patient, so as to obtain gait data related to the gait; (b) a muscle stimulator including: (i) an electrical stimulation circuit, the circuit adapted to supply an electrical stimulation output to an electrode array for performing functional electrical stimulation of at least one muscle of the lower limb, and (c) a microprocessor, operatively connected to the at least one sensor, the microprocessor adapted for: receiving a signal containing gait information based on the gait data; processing the signal, and controlling the stimulation output based on the processing of the signal, wherein the sensor is a pressure sensor, and wherein the processing the signal includes: (i) calculating a dynamic range between maximal pressure values, and minimal pressure values on the pressure sensor, and (ii) calculating a high threshold and a low threshold based on the dynamic range, the low threshold for triggering on the electrical stimulation output, the high threshold for triggering off the electrical stimulation output. 
         [0024]    According to still further features in the described preferred embodiments, the microprocessor is further adapted to detect a deviation from an ambulating mode. 
         [0025]    According to still further features in the described preferred embodiments, the ambulating mode is a SWING state. 
         [0026]    According to still further features in the described preferred embodiments, the ambulating mode is a STANCE state. 
         [0027]    According to still further features in the described preferred embodiments, the microprocessor is further adapted to identify invalid peaks or valleys. 
         [0028]    According to still further features in the described preferred embodiments, the microprocessor is further adapted to determine whether the patient is in a SWING, STANCE, SITTING, or STANDING state. 
         [0029]    According to still further features in the described preferred embodiments, the microprocessor is further adapted to make a determination of an ambulating state of the patient, and to identify invalid peaks or valleys based on the determination. 
         [0030]    According to still further features in the described preferred embodiments, the microprocessor is further adapted to utilize the dynamic range in identifying the invalid peaks or valleys. 
         [0031]    According to still further features in the described preferred embodiments, the microprocessor has a plurality of different thresholds for determining peak validity or valley validity, the plurality of different thresholds based, at least in part, on an ambulating state of the patient. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. Throughout the drawings, like-referenced characters are used to designate like elements. 
           [0033]    In the drawings: 
           [0034]      FIG. 1  is a perspective view of the inventive sensor assembly; 
           [0035]      FIG. 2  is a schematic, exploded view of the inventive sensor assembly, including an envelope cover, an envelope base, an FSR sensor, an electrical connection unit, and an absorbent protective layer for disposing on the FSR sensor; 
           [0036]      FIG. 3A  is a cross-sectional view of inventive envelope cover; 
           [0037]      FIG. 3B  is a magnified view of a portion of  FIG. 3A ; 
           [0038]      FIG. 3C  is a cross-sectional view of the inventive envelope showing the relative disposition of the envelope cover, envelope base, FSR sensor, and absorbent layer; 
           [0039]      FIG. 3D  is a schematic illustration of a preferred embodiment of the inventive envelope in which the envelope has a mechanism for advantageously securing FSR sensor to external wires; 
           [0040]      FIG. 3E  is a schematic illustration of the inventive sensor assembly disposed within a conventional shoe; 
           [0041]      FIG. 4  is a schematic electronic diagram of the inventive foot sensor device; 
           [0042]      FIG. 5  is a schematic electronic diagram of one embodiment of the inventive functional electrical stimulation (FES) system, showing the internal workings of the foot sensor device, stimulator unit, and control unit, along with the communication between the components; 
           [0043]      FIG. 6  is a schematic plot showing the pressure exerted on a pressure transducer as a function of time, during gait assisted by one embodiment of the system of the present invention; 
           [0044]      FIG. 7  is an exemplary block diagram showing the logical sequence of analysis and control performed by a microcontroller unit of the present invention, based on data received from the pressure transducer; 
           [0045]      FIG. 8  is a schematic, simplified plot showing the pressure exerted on the pressure sensor as a function of time, during gait assisted by a system of the present invention; 
           [0046]      FIG. 9  is a schematic plot of current as a function of time for a bipolar stimulation pulse of the prior art; 
           [0047]      FIG. 10  is a schematic plot of current as a function of time for successive bipolar stimulation pulses, showing exemplary sampling points; 
           [0048]      FIG. 11  is a block diagram showing an exemplary embodiment of the inventive logical sequence of sampling, analysis and control performed by a microcontroller unit of the present invention; 
           [0049]      FIG. 12  is a schematic plot showing one embodiment of charge balancing-reduced phase amplitude of a negative current phase; 
           [0050]      FIG. 13  is a schematic plot showing another embodiment of charge balancing—reduced phase width (duration) of a negative current phase, and 
           [0051]      FIG. 14  is a schematic plot showing yet another embodiment of charge balancing—increased current to a greater than nominal level during a low impedance section of the positive current phase. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0052]    The principles and operation of the present invention may be better understood with reference to the drawings and the accompanying description. 
         [0053]    Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0054]    Various prior art sensor envelopes have appreciable deficiencies. One particular disadvantage is the lack of sufficient protection of the sensor by the sensor envelope. This lack of protection may cause an uncontrolled or uneven force distribution over the surface of the sensor, resulting in a relatively short life span for the sensor. 
         [0055]    The FSR sensor assembly and envelope of the present invention is designed, preferably, for inserting under the inner sole (insole) of the shoe, typically beneath the heel. The protective casing is made of a cover and a base, with the sensor fitting therebetween. An additional piece of absorbent material is disposed between the cover and the FSR sensor. Typically, the absorbent material is adhered to the cover. The cover and base of the sensor casing can be connected to each other by ultrasonic welding, gluing, heat welding, RF welding or by pins. Various commercially available force-sensitive resistor (FSR) sensors are suitable for use in conjunction with the inventive casing, including some FSRs manufactured by Interlink®, CUI®, Tekscan®, and Peratech®. The inventive casing can also be used with other types of sensors such as membrane switches, capacitance-based sensors and piezo-electric foils. 
         [0056]    The envelope is preferably made of acetal [also known as polyacetal, polyoxymethylene (POM), or polyformaldehyde] or polypropylene, but other materials may be engineered to provide the requisite physical and mechanical properties, e.g., polyethylene terephthalate (PET). 
         [0057]      FIG. 1  is a perspective view of one embodiment of a sensor assembly  25  of the present invention. 
         [0058]      FIG. 2  is a schematic, exploded view of sensor assembly  25 , including an envelope  5  having an envelope cover  10  and an envelope base  20 ; a force-sensitive resistor (FSR) sensor  30 ; an electrical connection unit  40 ; and an absorbent protective layer  50  for disposing on FSR sensor  30 . 
         [0059]    Base  20  forms sockets for FSR sensor  30  and for electrical connection unit  40 . The sockets are preferably contoured to match the topographical features of the underside of the sensor and electrical connection unit. Base  20  has a circumferential rim for closely bounding FSR sensor  30 , thereby determining the position of the sensor. Thus, the sockets enable precise, repeatable location of the sensor on the base. 
         [0060]    Preferably, envelope base  20  is harder/less flexible than cover  10 . This mechanical property reinforces the FSR sensor against bending forces, which can cause deviations in the sensor readings and can also cause excessive wear and damage to the sensor. 
         [0061]      FIG. 3A  is a cross-sectional view of envelope cover  10 ;  FIG. 3B  is a magnified view of a portion of envelope cover  10  shown in  FIG. 3A ;  FIG. 3C  is a cross-sectional view of sensor assembly  25  showing the relative disposition of envelope cover  10 , envelope base  20 , FSR sensor  30 , and absorbent layer  50 . 
         [0062]    It is evident from  FIGS. 3A-3C  that envelope cover  10  is supported around the circumference and largely unsupported towards the center. It is further evident from  FIG. 3C  that envelope cover  10 , envelope base  20 , and absorbent layer  50  are disposed such that a first void space  11  is situated between envelope cover  10  and absorbent layer  50 , and such that a second void space  13  is situated between envelope cover  10  and envelope base  20 . The flexibility of cover  10 , along with the maneuverability provided by void spaces  11 ,  13 , enables the cover to act like a membrane that collapses (bends) towards the center of the top face of FSR sensor  30 , and transmits the pressure (force), via absorbent protective layer  50 , thereto. 
         [0063]    Preferably, the radius of cover  10  near the perimeter thereof is about 2-5 mm and more preferably, 3-4 mm. 
         [0064]    The rims of cover  10  and base  20  are preferably contoured in complementary fashion. The closure of these rims is preferably made by ultrasonic welding. The bonding of the rims, coupled with the curved structure near the perimeter and the elevated rim thereunder, provide the requisite rigidity to the envelope. Consequently, routine forces exerted by the foot on the sensor will not collapse cover  10  near the envelope perimeter, and the collapsing is confined within the center area of the cover. The bonding of the rims actually generates a surface tension that allows the cover to collapse solely within that center area. This also eliminates distortion of the rims. 
         [0065]    Absorbent protective layer  50 , for disposing on FSR sensor  30 , is preferably made of Poron®, or another flexible, high density, microcellular material that exhibits, over long-term use, good resistance to compression set (collapse), high resiliency, and good impact absorption. 
         [0066]    The above-described features of the envelope and closure thereof allow more accurate, repeatable and reproducible collapse of cover  10  upon sensor  30 . This permits repeatable readings of the sensor for a specific pressure (force). Perhaps more importantly, the above-described shape and structure eliminate or drastically reduce shear forces on sensor  30 , and greatly contribute to the longevity of FSR sensor  30 . The structure of the rims also improves the structural stability and durability of the envelope. 
         [0067]    The sensor is anchored to the base of the envelope within a specific socket structure in base  20 . In one embodiment, the wires are tightened by a metal crimp, which is positionally locked into the socket, thereby inhibiting movement of the sensor, as well as undesirable tension in the area of the wires (and especially to the welding points thereof) of electrical connection unit  40  as result of accidental pulling of the external wire. 
         [0068]    Preferably, the sensor is attached to the shoe inner surface by loop and hook fasteners such as Velcro®. One fastening element is attached to the bottom of sensor base cover, and the complementary fastening element is attached to the shoe insole. 
         [0069]    A graphical symbol of a foot is preferably provided on cover  10 , so as to direct the user to properly align the FSR sensor device within the shoe. 
         [0070]    The inventive envelope is easy and inexpensive to manufacture, and enables facile and reproducible assembly of the FSR sensor device. 
         [0071]      FIG. 3D  schematically illustrates a preferred embodiment of the present invention having an inventive mechanism for advantageously securing FSR sensor  30  to external wires  58 . Wires  58  typically connect FSR sensor  30  with the head of the sensor device containing, inter alia, the microprocessor and radio frequency (RF) transceiver. 
         [0072]    External wires  58  are anchored around protrusions such as protrusion  56 , which juts out of a base  54  of FSR sensor  30 . External wires  58  are wrapped around these protrusions in such a way that undesirable tension in the area of the wires (especially at the welding points  59 ) of the electrical connection is avoided. This anchoring mechanism enables the user to pull the envelope out of the shoe without inadvertently causing damage to the welding points in the area of the electrical connection. 
         [0073]    Preferably, silicon is poured over the ends of wires  58  after wires  58  have been positioned, so as to maintain the positioning of the wires during assembly, as well as to further protect the welding area and to seal out water and dirt from the opening around the wire. 
         [0074]      FIG. 3E  is a schematic illustration of inventive sensor assembly  25  disposed within a conventional shoe or footwear  15 . Sensor assembly  25  can be situated in various positions, e.g., under the foot/above the insole, between the insole and sole, and within the sole. 
         [0075]    As used herein in the specification and in the claims section that follows, the term “footwear” refers to any kind of foot covering that a foot being covered presses down upon during gait, including, but not limited to, shoes, boots, sandals, socks, and stockings. 
         [0076]      FIG. 4  is a schematic electronic diagram of inventive foot sensor device  100 . Sensor element  16  is connected to, and preferably powered by, electronics or communication unit  31  by means of wiring  21 . Communication unit  31  includes a digital circuit and microcontroller unit  80 , a radio frequency (RF) transceiver  82 , and an antenna unit  83  having a matching network for converting the signal from the wired medium to a wireless medium, and from the wireless medium to the wired medium. 
         [0077]    The resistance of sensor element  16  changes with the force applied thereon. According to one embodiment of the present invention, foot sensor device  100  is equipped with a voltage divider consisting of sensor element  16  and a bias resistor  81  (preferably disposed in unit  30 ), in order to measure the resistance of sensor element  16 . When a voltage is applied to the voltage divider, the voltage is divided according to the resistance ratio between sensor element  16  and bias resistor  81 . This voltage is measured in order to assess the resistance of sensor element  16 . 
         [0078]    One skilled in the art will appreciate that there are numerous ways of measuring the resistance of sensor element  16 . 
         [0079]    Communication unit  31  is also equipped with a small coin battery  84  that provides power to microcontroller unit  80 , RF transceiver  82 , and sensor element  16 . 
         [0080]    Digital circuit and microcontroller unit  80  controls and monitors the operation of foot sensor device  100  and executes the various algorithms (e.g., gait detection, RF control, and power management algorithms) thereof. Preferably, microcontroller unit  80  communicates with RF transceiver  82  via a Serial Peripheral Interface (SPI). 
         [0081]      FIG. 5  is a schematic electronic diagram of one embodiment of the inventive functional electrical stimulation (FES) system  500 , showing the internal workings of foot sensor device  100 , stimulator unit  150 , and control unit  250 , and the communication therebetween. 
         [0082]    As above, foot sensor device  100  includes small coin battery  84  that provides power to microcontroller unit  80 , RF transceiver  82 , and sensor element  16 . Coin battery  84  may also power an analog circuit  78  having sensor signal conditioning (such as amplification, filtering, and division) and an analog-to-digital signal converter. 
         [0083]    Stimulator unit  150  typically includes an RF transceiver  182  having an antenna  183  having a matching network, a digital circuit and microcontroller unit  180 , and a stimulation circuit  195 , all powered by a power supply  184   b.  Stimulation circuit  195  typically receives power from power supply  184   b  via high voltage circuit  190 . 
         [0084]    Power supply  184   b  may be powered by a battery such as rechargeable battery  184   a.  A charging and battery monitor  184   c  is advantageously associated with rechargeable battery  184   a,  and interfaces with an external power supply, such as a regulated, preferably medical-grade, wall adapter. 
         [0085]    By means of antenna  83  of foot sensor device  100  and antenna  183  of stimulator unit  150 , RF transceiver  82  communicates with RF transceiver  182  of stimulator unit  150 . RF transceiver  182  transmits digital information to and receives digital information from digital circuit and microcontroller unit  180 . Similarly, microcontroller unit  180  and stimulation circuit  195  exchange digital information. Stimulation circuit  195 , based on digital information from microcontroller unit  180 , and powered by high voltage circuit  190 , is configured to deliver electrical stimulation pulses to the patient by means of electrodes  196   a,    196   b  disposed in the orthosis unit. 
         [0086]    Control unit  250  typically includes an RF transceiver  282  having an antenna  283  having a matching network, a digital circuit and microcontroller unit  280 , and a user interface circuit  192 , all powered by a power supply  284   b.    
         [0087]    Power supply  284   b  may be powered by a battery such as rechargeable battery  284   a.  A charging and battery monitor  284   c  is advantageously associated with rechargeable battery  284   a,  and interfaces with an external power supply, such as a regulated, preferably medical-grade, wall adapter. 
         [0088]    By means of antenna  183  of stimulator unit  150  and antenna  283  of control unit  250 , RF transceiver  182  communicates with RF transceiver  282  of control unit  250 . RF transceiver  282  transmits digital information to and receives digital information from digital circuit and microcontroller unit  280 . Similarly, microcontroller unit  280  and user interface circuit  192  exchange digital information. For example, user preferences for various operating parameters can be communicated from user interface circuit  192  to microcontroller unit  280 . Microcontroller unit  280  may be adapted to provide user interface circuit  192  with display information, including pertaining to stimulation parameters. 
         [0089]    As is known in the art, PDAs such as PDA  450  are small, hand-held portable computers having a Central Processing Unit (CPU) and electronic memory, and are generally used for storing and organizing information and for providing tools for everyday tasks. The PDA may advantageously be operated by the Windows Mobile 5 software of Microsoft®. PDA  450  preferably has a database containing a gait log and various personal parameters of the patient, and is programmed to configure the stimulation parameters of the electrical stimulation system. 
         [0090]    PDA  450  and control unit  250  are preferably in digital and electrical communication, such that the orthosis system can be configured on-line by the clinician during actual usage of the orthosis by the patient. In this arrangement, control unit  250  actually serves as the transmitter of PDA  450 , enabling PDA  450 , via control unit  250 , to communicate with and command the other components of the electrical stimulation system. 
       RF Protocol-Fast Wireless Link Failure Identification (FLFI) Algorithm and Response 
       [0091]    A microprocessor within the inventive system, by means of the RF protocol software, implements a method for a Fast wireless Link Failure Identification (FLFI). If failure is identified, the system provides a fail-safe stimulation to promote gait stability. 
         [0092]    As used herein in the specification and in the claims section that follows, the term “stance time” refers to the time differential between a heel-off event and the previous heel-contact event. 
         [0093]    As used herein in the specification and in the claims section that follows, the term “swing time” refers to the time differential between a heel-contact event and the previous heel-off event. 
         [0094]    When, for whatever reason, a ‘heel-off’ event is not identified immediately after receiving or identifying a ‘heel-contact’ event, the situation of the user may be precarious: the stimulator resumes its ‘heel-contact’ activity and does not deliver stimulation, which may cause the patient to lose balance, to stumble, or even to fall. 
         [0095]    In order to reduce this risk, the system (e.g., microcontroller unit  80  of foot sensor device  100  or in other possible embodiments, microcontroller unit  180  of stimulator unit  150 ) frequently or substantially constantly calculates, and/or monitors, the last or average stance time of the patient. From the average stance time, microcontroller unit  80  calculates a ‘keep-alive’ duration, which is longer than the stance time. Preferably, the ‘keep-alive’ duration is at least one hundredth of a second, more preferably, at least one tenth of a second, most preferably, at least 0.8 seconds. As a function of stance time, preferably, the ‘keep-alive’ duration is at least 0.01 times the stance time, preferably, at least 0.1 times the stance time, and most preferably, at least slightly longer than the stance time. 
         [0096]    Microcontroller unit  80  transmits this ‘keep-alive’ duration along with any heel event, to stimulator unit  150 . 
         [0097]    If, after detecting a heel-contact event, microcontroller unit  80  does not detect a heel-off condition, microcontroller unit  80  transmits a ‘keep-alive’ message after the ‘keep-alive’ duration, so that stimulator unit  150  is aware that the link with foot sensor device  100  is functional, but that there are no events to report. 
         [0098]    If, on the other hand, the RF link is blocked right after transmitting the last heel-contact event (and the ‘keep-alive’ duration thereof), microcontroller unit  180  recognizes that the link with foot sensor device  100  is not functional (no event message, nor ‘keep-alive’ message), and in the absence of gait event information, commands stimulation circuit  195  to apply a fail-safe stimulation for a pre-defined period of time. The fail-safe stimulation is delivered to the tissue slightly after the heel-off event should have been received, had no RF blocking occurred, since the ‘keep-alive’ duration is calculated based on the stance duration. This fail-safe stimulation helps the patient with dorsiflexion and reduces the risk of falling by substantially imitating the function of a mechanical orthosis (ankle-foot orthosis). 
       RF Protocol-Range-Dependent Registration 
       [0099]    Referring again to  FIG. 5 , FES system  500  employs a registration mechanism that enables several such systems to simultaneously operate in the same frequency channel. The registration is based on a unique identifier, preferably incorporated into the hardware of control unit  250 , which serves as a digital ‘family name’ for all of the components of FES system  500 : foot sensor device  100 , stimulator unit  150 , and control unit  250 . 
         [0100]    Each transmission of each system component  100 ,  150 ,  250  preferably carries this identifier as a part of the payload. When one of transceivers  82 ,  182 ,  282  receives the transmitted message, the transceiver first verifies that the transmitter belongs (is registered) to the same family, and only after verification proceeds to handle the transmitted data. 
         [0101]    The registration process also defines how the new component is introduced into an existing system, for example, as a replacement part. In this case, the end user moves the system to ‘registration mode’ by pressing a pre-defined key sequence on control unit  250 . Preferably, this key sequence is the same, regardless of the new component that is being introduced (registered) to FES system  500 . 
       Foot Sensor-Dynamic Gait Tracking Algorithm 
       [0102]    A microcontroller unit such as microcontroller unit  80  of foot sensor device  100  (or another microcontroller unit within the system, such as microcontroller unit  180  of stimulator unit  150 ) is preferably configured to implement a ‘Dynamic Gait Tracking’ algorithm. This algorithm is designed to handle variable sensor response arising from various sources, including:
       variations between sensors;   variations in signal level and pattern due to variable patient weight;   variations in signal level and pattern due to differences in weight bearing form over the sensor;   variations in signal level and pattern due to changes in sensor characteristics caused by the operation environment (sensor heats up within a shoe);   variations in signal level and pattern due to changes in sensor characteristics caused by prolonged use;   variations of forces over the sensor due to differences between individual shoes and differences between individual insoles.       
 
         [0109]      FIG. 6  is a schematic plot  400  showing, on the Y -axis, a magnitude or amplitude of pressure (or force) exerted on a pressure transducer (such as pressure transducer  16  shown in  FIG. 5 ) as a function of time, during gait assisted by an FES system of the present invention. The plot has a calculated dynamic range  402 , which is a smoothed and or averaged differential between maximal or peak pressure values, and adjacent minimal or valley pressure values on pressure transducer  16 . From the dynamic range are calculated a high threshold  404  and a low threshold  406 , which serve as references for determining heel-contact events and heel-off events, respectively. 
         [0110]      FIG. 6  will be more readily understood after describing  FIG. 7 , which is an exemplary block diagram showing the logical sequence of analysis and control performed by microcontroller unit  80  of foot sensor device  100 , based on data received from pressure transducer  16 . 
         [0111]    In step  1 , microcontroller unit  80  samples the signal of pressure transducer  16 . If a peak or valley is detected (step  2 }, microcontroller unit  80  determines whether the peak or valley is a valid peak or valley, or an invalid peak or valley {step  3 ). If the peak or valley is found to be valid, the relevant trendline is updated (step  4 ), and the new dynamic range is calculated (step  5 ). As described hereinabove, high threshold  404  and low threshold  406  are recalculated based on the new dynamic range (step  6 ). 
         [0112]    Next, the signal sampled in step  1  is compared with high threshold  404  and low threshold  406  (step  7 ), and microcontroller unit  80  determines (using signal data from at least one previous sampling) whether high threshold  404  or low threshold  406  has been crossed (step  8 ). If either threshold has been crossed, microcontroller unit  80  effects a change in the state of the system (step  9 ), from a STANCE state to a SWING state, triggering electrical stimulation, or from a SWING state to a STANCE state, triggering a cutting off of the stimulation. The logical sequence of analysis and control returns to step  1 , in which microcontroller unit  80  again samples the signal of pressure transducer  16 . 
         [0113]    In the routine event that a peak or valley is not detected (step  2 ), or that the peak or valley detected is not valid (step  3 ), the logical sequence preferably proceeds directly to step  7 , in which the sampled signal is compared with high threshold  404  and low threshold  406 . 
         [0114]    If microcontroller unit  80  determines, in step  8 , that high threshold  404  or low threshold  406  has not been crossed, the time elapsed within the current system state (STANCE or SWING) is evaluated (step  10 ). If the time elapsed exceeds a particular value, e.g., a calculated value based on the average stance/swing period, microcontroller unit  80  determines (step  11 ) that the user of the FES system is now in a STANDING state or in a SITTING state. The particular value may be an absolute value, a calculated value based on the average stance/swing period, or based on a previous stance/swing period or periods, a function of the elapsed time of the previous peak or peaks, and/or a function of another gait parameter. 
         [0115]    The logical sequence of analysis and control returns to step  1 , in which microcontroller unit  80  again samples the signal of pressure transducer  16 . 
         [0116]    Referring back to  FIG. 6 , each of points  407  represents a crossing of high threshold  404 ; each of points  409  represents a crossing of low threshold  406 . After determining that high threshold  404  has been crossed, microcontroller unit  80  effects a change in the state of the system from a SWING state  416  to a STANCE state  418 . Similarly, upon determining that low threshold  406  has been crossed, microcontroller unit  80  effects a change in the state of the system from a STANCE state to a SWING state. Typically, stimulation circuit  195  is commanded to provide stimulation current during the course of SWING state  416 . 
         [0117]    Peak  430  is characteristically long with respect to typical STANCE peaks during gait. If the time elapsed since crossing a high threshold point  429  exceeds a particular value (without crossing low threshold  406 ), microcontroller unit  80  determines that the state of the user of the FES system has changed from a STANCE state to a STANDING state. As in the parallel case described hereinabove, the particular value may be an absolute value, a calculated value based on the average stance/swing period or based on a previous stance/swing period or periods, a function of the elapsed time of the previous peak or peaks, and/or a function of another gait parameter. 
         [0118]    Similarly, if the time elapsed for a particular valley exceeds a pre-determined value, microcontroller unit  80  determines that the state of the user has changed from a SWING state to a SITTING state. 
         [0119]    As described briefly hereinabove, microcontroller unit  80  determines whether a peak or valley is valid or invalid. Peak  414  is an example of a valid peak; valley  416  is an example of a valid valley. 
         [0120]    An invalid peak, such as invalid peak  420 , has an amplitude that is less than a particular level. This pre-determined level is, at least in part, a function of the dynamic range. Thus, by way of example, a peak may be considered invalid if the peak amplitude is less than a pre-determined percentage of the dynamic range. Similarly, a valley may be an invalid valley such as invalid valley  420 , if the amplitude of the valley (i.e., the drop in pressure from the previous peak to the valley is less than a pre-determined percentage of the dynamic range. 
         [0121]    Since invalid peaks and valleys are not entered into the calculation of the trendlines, the dynamic range remains substantially unchanged. Consequently, these invalid peaks and valleys do not influence the determination of high threshold  404  and low threshold  406 . 
         [0122]    With reference now to  FIG. 8 ,  FIG. 8  is a schematic, simplified plot showing the pressure exerted on the pressure transducer as a function of time, during gait assisted by a system of the present invention. The time elapsed for valley  442  greatly exceeds the time elapsed for typical valleys such as valleys  444 . Accordingly, microcontroller unit  80  determines that the state of the user has changed from a SWING state to a SITTING state. 
         [0123]    Similarly, if the time elapsed from the start of a peak exceeds the time elapsed for typical peaks (such as peak  430  in  FIG. 6 ) by a pre-calculated or predicted value, microcontroller unit  80  determines that the state of the user has changed from STANCE to STANDING. 
         [0124]    In a preferred embodiment of the present invention, the determination of peak and valley validity is additionally and preferably dependent on the gait state. Each gait state preferably has an individual, dynamic threshold—typically a percentage or other function of the dynamic range—for determining peak and valley validity. This threshold should not to be confused with the heel-off and heel-contact thresholds described hereinabove. 
         [0125]    By way of example, the inventors have discovered that while in a SITTING state, a relatively high threshold reduces the occurrence of false stimulation. By means of such a high threshold, the system is largely impervious to the effects of weight shifting while sitting, because the relatively low peaks generated by such weight shifting are considered invalid, and are not ‘entered’ into the trendline calculation. Consequently, these false gait peaks do not “pull” downward the peak trendline, do not decrease the dynamic range, and do not falsely sensitize the stimulation threshold (low threshold). As a result, the user enjoys a more quiet sitting, in which false stimulation while sitting is appreciably reduced. 
         [0126]    Similarly, during standing, the system is largely impervious to the effects of weight shifting, because the relatively low amplitude of the valleys generated by such weight shifting are considered invalid, and are not ‘entered’ into the trendline calculation. Consequently, these false gait valleys do not “pull” upward the valley trendline, do not decrease the dynamic range, and do not falsely sensitize the stimulation threshold (low threshold). As a result, a standing user who shifts his weight from time to time is less inconvenienced by false stimulation, which can be appreciably reduced. 
         [0127]    Typical validity conditions for each of the four states—STANCE; STANDING; SWING, and SITTING—are provided below: 
         [0128]    STANCE state: valid peak amplitude ≧25%·dynamic range 
         [0129]    STANDING state: valid peak amplitude ≧62.5%·dynamic range 
         [0130]    SWING state: valid valley amplitude ≧25%·dynamic range 
         [0131]    SITTING state: valid valley amplitude ≧50%·dynamic range 
         [0132]    Thus, it is observed in  FIG. 8  that while peak  446  and peak  448  are of substantially equal amplitude, peak  446  is considered to be a valid peak, while peak  448  is considered to be an invalid peak. Peak  446  belongs to the SWING state, whereas peak  448  belongs to the SITTING state. 
       Foot Sensor-Dynamic Gait Tracking Algorithm 
       [0133]    The software preferably samples the signals before and during each of the stimulation pulses. The monitored parameters and conditions may include:
       Body leakage current (hazard)   Pulse balance monitoring and correction (hazard)   Tissue impedance estimation and electrode disconnection identification   Pulse over current (hazard)   Pulse over duration (hazard)       
 
         [0139]    With reference now to  FIG. 9 ,  FIG. 9  is a schematic plot of current as a function of time, for a bipolar stimulation pulse  450  of the prior art. Stimulation pulse  450  is substantially a square wave having a positive current phase  452  and a negative current phase  454 . 
         [0140]    It is known that over the course of applying a large plurality of stimulation signals to the tissue of the user, an imbalance between the charge delivered in the positive current phases and the charge delivered in the negative current phases can cause irritation to the tissue and discomfort to the user. It is also known that delivering current to the tissue so as to effectively cause FES typically leads to such a disadvantageous imbalance. 
         [0141]    Without wishing to be limited by theory, the inventors believe that this phenomenon is related to the dynamic impedance behavior of the tissue. Initially, the impedance of the tissue is relatively low, such that the requisite current can be delivered at an acceptably low voltage. With time, however, the impedance of the tissue may increase substantially, and to deliver constant current (so as to obtain a square wave), the voltage must be increased. According to Ohm&#39;s Law: 
         [0000]    
       
      
       V=I·Z  
      
     
         [0000]    where V is the potential difference between two points in the tissue that include an impedance Z, and I is the current flowing through the impedance. Thus, the voltage is increased substantially proportionally to the impedance or resistance. 
         [0142]    However, the voltage applied to the human body generally cannot be raised above a certain level, e.g., 120 Volts, consequently, as the impedance builds up, the current delivered may be limited—even severely limited—by the ceiling voltage. 
         [0143]    Referring again to  FIG. 9 , stimulator devices of the prior art are often constant voltage devices. Thus, at the beginning of the signal (point A), when the impedance of the tissue is relatively low, positive current phase  452  is substantially a square wave. At point B, the impedance of the tissue has increased, but the source voltage still exceeds the multiplication product I·Z. At point C, however, the impedance of the tissue has increased to the point that the source voltage exactly equals the multiplication product I·Z. Thus, a further build-up in the impedance of the tissue forces the current delivered to drop (point D), monotonically, until positive current phase  452  is completed (point E). 
         [0144]    Positive current phase  452  is not, therefore, a perfect square wave, and the total charge delivered is substantially less than the calculated total current based on the square wave model. Consequently, the total charge delivered in negative current phase  454  tends to exceed the total charge delivered in positive current phase  452 , which often results in skin irritation in the area through which the current is passed. 
         [0145]    Such stimulator devices of the prior art are of further disadvantage in that the use of constant voltage near the beginning of positive current phase  452  can be wasteful from an energy standpoint. 
         [0146]    The method and system of the present invention perform digital pulse balancing, in real time, on the bipolar stimulation signal, so as to greatly improve current balance. Referring collectively to  FIGS. 5 and 10  along with  FIG. 11 ,  FIG. 11  is a block diagram showing an exemplary embodiment of the inventive logical sequence of sampling, analysis and control performed by a microcontroller unit of the present invention. The sequence is designed to adjust or balance a bipolar digital stimulation current pulse  550  delivered by stimulation circuit  195 . 
         [0147]    In step  1 , a positive current phase  552   a  of bipolar current pulse  550  is sampled/monitored over n preferably evenly-spaced sample points. Preferably, the voltage is also sampled/monitored, and the impedance is calculated. The sampling/monitoring is preferably conducted at least 3 times, and more preferably, at least 5 times, over the duration of positive current phase  552   a.  In terms of timing, sampling is preferably conducted at least once every 10 microseconds over the duration of positive current phase  552   a.    
         [0148]    In step  2 , a negative current phase  554   a  of bipolar current pulse  550  is sampled/monitored over m preferably evenly-spaced sample points. Preferably, the voltage is also sampled/monitored. 
         [0149]    The charge in positive phase  552   a  and the charge in negative phase  554   a  are calculated based on the sampling points, and in some cases, the sampling times (steps  3  and  4 ), and these charges are then compared (step  5 ) to see if they are substantially equal, or that the charge differential is relatively small. If so, no balancing action is required, and the system waits for the next stimulation pulse. 
         [0150]    If the charge differential is significant, pulse balancing is performed (step  6 ), preferably on at least one of positive current phase  552   b  and negative current phase  554   b  of the next current pulse. The pulse balancing is performed by controlling at least one pulse parameter so as to improve charge balance between positive current phase  552   a  and a negative current phase such as negative current phase  554   b.    
         [0151]    Various pulse parameters may be controlled to improve the charge balancing, including at least one of the following: current (positive phase or negative phase), positive current phase width, and negative current phase width. Preferably, charge balancing is performed by controlling a pulse parameter of the negative phase. 
         [0152]    Some exemplary embodiments of the charge balancing are provided in  FIG. 12 —reduced phase amplitude of a negative current phase;  FIG. 13 —reduced phase width (duration) of a negative current phase; and  FIG. 14 : increased current to a greater than nominal level, at least during a portion of the positive current phase. 
         [0153]    Preferably, at low impedance levels, the voltage is adjusted to achieve substantially the minimum voltage satisfying Ohm&#39;s Law, so as to conserve energy/battery power. 
         [0154]    Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to elnbrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.