Abstract:
A linear capacitance displacement transducer ( 1 ) comprising first ( 2 ) and second ( 3 ) fixed capacitor plate and a dielectric structure ( 5 ) moveable longitudinally within a space ( 4 ) between the first ( 2 ) and second ( 3 ) capacitor plates, the dielectric structure ( 5 ) being operatively coupled to a moveable element ( 8 ). The capacitor plates and the dielectric material may be cylindrical and disposed coaxially and concentrically. The transducer ( 1 ) enables a displacement sensor that is capable of monitoring liquid levels in a syringe type drug reservoir ( 101 ) with sufficient sensitivity as to allow detection of erroneous drug delivery. The sensor is inexpensive to manufacture and provides reliable performance through robust design.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation of International Application No. PCT/GB2009/002353, which designated the United States and was filed on Oct. 2, 2009, which claims priority under 35 U.S.C. §119 or 365 to United Kingdom Application No. 0818077.0, filed on Oct. 2, 2008. The entire teachings of the above applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    In devices for the programmed delivery of therapeutic products into the human or animal body, there is generally provided a pressurised reservoir of therapeutic product working in cooperation with a pumping chamber and valve means. The therapeutic product is typically pumped by the device through a tube to a cannula that pierces the patient&#39;s skin. The device can be capable of providing a variable rate of infusion of the therapeutic product to the patient over several days. This invention is directed to an improved displacement sensor for the pressurised reservoir. 
       BACKGROUND TO THE INVENTION 
       [0003]    Many different measurement techniques have been used previously as the basis for displacement sensors. 
         [0004]    In one type of displacement sensor the action of linearly or rotationally displacing a wiper of a potentiometer is converted to a voltage and/or current signal. Such potentiometric sensors often suffer from the problems of mechanical wear, frictional resistance in the wiper action, limited resolution in wire-wound units, and high electronic noise. 
         [0005]    Linear Variable Displacement Transducers (LVDT) are commonly available. An LVDT typically includes three coils of wire wound on a hollow form. A core of permeable material can slide freely through the centre of the form. The inner, primary coil is excited by an ac source. Flux formed by the primary coil is linked to two outer, secondary coils, inducing an ac voltage in each coil depending on the position of the core. If the two secondary coils are wired in series opposition then the two voltages will subtract; that is, a differential voltage is formed. When the core is centrally located, the net voltage is zero. When the core is moved to one side, the net voltage amplitude will increase. In addition, there is a change in phase with respect to the source when the core is moved to one side or the other. Additionally, these devices require separate coils at either end of the measurement coils to provide electrical shielding to create a low noise transducer. These manufacturing requirements make these transducers expensive to manufacture and have a length dimension at least twice the distance they can measure. 
         [0006]    A number of devices have also been described based on optical measurement systems such as optical encoders. Devices based on ultrasonic techniques have also been described. These devices tend to be expensive to manufacture and are restricted in the type of application in which they can be employed. 
         [0007]    A variety of capacitance based displacement sensors have been described for measuring or detecting linear displacements. One type of capacitance displacement sensor is based on the principle of two opposing plates, where measurement of displacement either alters the overlapping area of the two plates or changes the dielectric properties of the gap between the plates. Examples of this type of displacement sensor are offered below. 
         [0008]    GB 1275060 A discloses a displacement sensor comprising of guided rod forming a first plate of the capacitor and a receptor tube in which the rod moves in and out forming a second pate of the capacitor. 
         [0009]    U.S. Pat. No. 4,961,055 discloses a displacement sensor similar to that of GB 1275060 A and further discloses a third tube that acts to shield the sensing plate of the capacitor from electrostatic charges, which can cause signal noise. 
         [0010]    A number of other moving plate capacitor sensors have also been described that utilise patterns of electrodes on either flat or tubular plates. Examples of these are given by JP 8-159704 and GB 2273567 A. The construction of these devices also presents considerable challenges in manufacturing inexpensive devices. 
         [0011]    The use of capacitance displacement sensors has been described for a variety of applications including monitoring fluid levels in reservoirs, as disclosed in EP 0520201 A. 
         [0012]    U.S. Pat. No. 5,135,485 discloses a capacitance measurement employed in a drug reservoir to either detect when the reservoir is empty or provide a measure of the level of liquid in the reservoir. The sensor described for monitoring the level of liquid in the reservoir comprises two plates of a capacitor with the liquid forming the dielectric between them. The greater the quantity of liquid present in the reservoir the more the gap between the plates becomes filled with the liquid and this is reflected in the capacitance measured by the sensor. 
         [0013]    U.S. Pat. No. 6,210,368 discloses a capacitor based sensor that monitors liquid levels in a reservoir. In one embodiment an amount of overlap between two plates of a capacitor changes as the reservoir volume changes. In another embodiment an amount of liquid phase propellant absorbed in a dielectric material of a capacitor changes according to the reservoir volume, causing a change in the dielectric properties of the capacitor. 
         [0014]    U.S. Pat. No. 6,352,523 discloses a method for measuring the amount of insulin remaining in a syringe after an administration based on a barrel and plunger of a syringe being adapted as the two plates of a coaxial capacitor. The device additionally requires that the syringe is placed into a reader to generate the displacement information. 
         [0015]    Alternative techniques for monitoring levels of a drug in a reservoir include the use of optical encoders. U.S. Pat. No. 4,498,843 and WO2004/009163 both describe a linear displacement measurement system based on an optical encoder that is used to monitor the position of a syringe barrel as part of an infusion system. 
         [0016]    There is a need in the art for a displacement sensor capable of monitoring the level of liquid in a syringe type drug reservoir with sufficient sensitivity as to allow detection of erroneous drug delivery. It is also required that the sensor is inexpensive to manufacture and provides reliable performance through robust design. 
       SUMMARY OF THE INVENTION 
       [0017]    According to a first aspect of the present invention there is provided a linear capacitance displacement transducer comprising a first cylindrical capacitor plate, a second cylindrical capacitor plate disposed around the first cylindrical capacitor plate so as to form a space between the first and second cylindrical capacitor plates, the first and second cylindrical capacitor plates being substantially spatially fixed relative to one another, and a third cylinder composed of dielectric material moveable longitudinally within the space such that a proportion of said space filled with the dielectric material can be altered relative to a fixed electric field created, in use, between the first and second cylindrical capacitor plates. 
         [0018]    According to a second aspect of the present invention there is provided a reservoir for containing a volume of fluid bound in part by an moveable element, in combination with a linear capacitance displacement transducer for measuring the volume of fluid contained in the reservoir, the displacement transducer comprising a fixed structure including first and second capacitor plates, and a dielectric structure moveable longitudinally within a space between the first and second capacitor plates, wherein the moveable dielectric structure is operatively coupled to the moveable element. 
         [0019]    According to a third aspect of the present invention there is provided an infusion system for infusion of liquid therapeutic product, including a linear capacitance displacement transducer according to the first aspect, or a linear capacitance displacement transducer in combination with a reservoir according to the second aspect. 
         [0020]    In the linear capacitance displacement transducer the electric field created, in use, between the first and second capacitor plates remains stationary whilst the proportion of the space between these plates that is filled with high dielectric material is altered by movement of the dielectric structure. This construction is advantageous in that since the first and second capacitor plates are substantially spatially fixed relative to one another, electrical connections for connecting thereto do not need to move thereby simplifying and making more robust the transducer and improving the reliability of a signal output by the transducer. Additionally, since the second capacitor plate is substantially spatially fixed relative to the electric field created, in use, the second plate acts as an effective shield against adverse external electrical influences. 
         [0021]    In a preferred embodiment of the invention, the fixed structure of the linear capacitance displacement transducer is a durable portion, whereas the reservoir and the moveable structure of the linear displacement transducer are a disposable portion, of an infusion system. In this manner, a disposable reservoir initially containing liquid therapeutic product may be formed integrally with the dielectric portion of the transducer to be fitted onto the durable portion of the infusion system having the electrical portion of the transducer. This allows a highly accurate transducer measurement to be made whilst keeping the manufacturing cost of the disposable portion of the infusion system low. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which: 
           [0023]      FIG. 1  is a schematic view of an embodiment of the linear capacitance displacement transducer in accordance with the invention, shown in an extended position; 
           [0024]      FIG. 2  is a schematic view of the transducer of  FIG. 1 , shown in a retracted position; and 
           [0025]      FIG. 3  is a schematic view of an infusion system comprising a reservoir and a transducer in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Turning firstly to  FIG. 1 , the linear capacitance displacement transducer  1  includes a first capacitor plate  2  and a second capacitor plate  3  defining a space  4  between the first and second capacitor plates  2 , 3 . The displacement transducer  1  further includes a dielectric structure  5  movable longitudinally within the space  4  in the direction of arrows X. The first and second capacitor plates  2 , 3  are electrically and physically connected to a printed circuit board or similar layer  6  having a semiconductor integrated circuit  7 , such as AD7746, mounted thereon. The dielectric structure  5  is operatively coupled to a movable element  8 . The movable element  8  is connected to guide means  9  cooperating with guide means  10  extending from the printed circuit board  6 . 
         [0027]    In the preferred embodiment shown in  FIG. 1  the first capacitor plate  2  is a solid right circular cylinder disposed coaxially and concentrically with a hollow right circular cylinder of the second capacitor plate  3 . It will be appreciated by those skilled in the art that the first and second capacitor plates  2 , 3  need not necessarily be cylinders but may instead be flat plates, for example. Cylindrical capacitor plates are preferred as the outer, second cylindrical capacitor plate  3  effectively shields the fixed electric field created, in use, between the first and second cylindrical capacitor plates  2 , 3 . It will also be appreciated by those skilled in the art that the first and second capacitor plates  2 , 3  need not be right circular cylinders and may instead take solid or hollow hexagonal, octagonal or other polygonal or irregular forms. The first and second cylinders  2 , 3  need not be-disposed concentrically but it is preferred that they are so such that the electric field created is substantially uniform within a cross-section of the transducer  1 . 
         [0028]    In the preferred embodiment shown in  FIG. 1 , the first and second capacitor plates  2 , 3  are of substantially the same length. However, it is envisaged that in alternative embodiments, the first and second capacitor plates  2 , 3  may be of different lengths and in particular the second cylinder  3  may be longer than the first cylinder  1  to provide more effective shielding against electrostatic interference. 
         [0029]    The guide means  10  may have a further function as a fourth cylinder disposed around the second cylindrical capacitor plate  3  for providing additional shielding against electrostatic interference to further improve the signal quality of the transducer  1 . 
         [0030]    The first and second capacitor plates  2 , 3  are physically connected to the printed circuit board  6  which acts as a support structure for supporting adjacent ends of the first and second capacitor plates  2 , 3 . It will be appreciate by those skilled in the art that a support structure other than the printed circuit board  6  may be provided as the physical connection at those ends of the first and second capacitor plates  2 , 3 , and wiring may be provided to a separate printed circuit board. However, to achieve space saving and drive down manufacturing costs the printed circuit board  6  acts as the physical support structure for the first and second capacitor plates  2 , 3 . 
         [0031]    The first and second capacitor plates  2 , 3  are electrically isolated and connected to the printed circuit board having the integrated circuit  7  for performing conversion of a capacitance signal output by the first and second capacitor plates  2 , 3 . Integrated circuit  7  also performs analog to digital conversion of the raw capacitance signal output by the first and second capacitor plates  2 , 3 . The AD7746 integrated circuit is provided as a purely exemplary integrated circuit and it will be appreciated by those skilled in the art that other circuits may be used in the alternative. 
         [0032]    To increase the effective surface area of the capacitor plates  2 , 3 , these may have a fluted surface. To ensure that the dielectric structure  5  is reliably retained between the first and second capacitor plates  2 , 3  the dielectric structure  5  may also be provided with a fluted surface. It is intended that the dielectric structure is slidably movable within the space  4  by a clearance fit with the capacitor plates  2 , 3  but leaving little, if any, play. 
         [0033]    In some preferred applications, the dielectric structure may be biased into or from the space  4  in the longitudinal direction. The bias may be provided by a spring or other such means and is particularly suitable where the movable member  8  connected to the distal end of the dielectric structure  5  constitutes a part of a reservoir or the like, a plunger of which is displaced and the transducer  1  measures that displacement. 
         [0034]    The linear capacitance displacement transducer  1  is shown in a retracted position in  FIG. 2  in which the dielectric structure  5  occupies substantially all of the space  4  between the first and second capacitor plates  2 , 3 . As the dielectric structure  5  moves between the fully extended and retracted positions of  FIGS. 1 and 2 , respectively, the proportion of the space  4  between the first and second capacitor plates  2 , 3  that is filled with the dielectric material  5  changes between a minimum and a maximum. The capacitance signal output by these first and second capacitor plates  2 , 3  can be calibrated to the linear displacement of the movable member  8  according to the change in capacitance as the movable element  8  moves between the fully extended and fully retracted positions. In this manner, the position and relative displacement of the movable element  8  can be measured by interrogating the capacitance signal. 
         [0035]    The linear capacitance displacement transducer  1  of the invention has broad application to a variety of devices. This may include displacement of a piston within its cylinder, displacement of a bowden cable, displacement of a linear switch, and the like. Its application is almost boundless and many other uses will be readily appreciated by those skilled in the art. 
         [0036]    However, a particular application of the linear capacitance displacement transducer in accordance with the invention is in combination with a reservoir for containing a volume of fluid bound in part by a movable element, such as a plunger. The plunger may be, or may be attached to, the movable element  8  described with reference to the linear capacitor displacement transducer  1  of  FIGS. 1 and 2 . 
         [0037]    The reservoir and transducer  1  in combination may form a fixed structure including the first and second capacitor plates  2 , 3 , and the dielectric structure  5  movable longitudinally within the space  4  between the first and second capacitor plates  2 , 3 . The movable dielectric structure  5  is operatively coupled to the movable element  8  as described previously. The fixed structure of the transducer may form a durable part, and the reservoir and the movable structure of the transducer may form a disposable part, with the movable element of the reservoir being integrally formed with the movable structure. Such a combination is particularly suitable for use in an infusion system for infusion of liquid therapeutic product. 
         [0038]    The infusion system shown in  FIG. 3  includes a pressurised reservoir  101  of therapeutic product  102 . The therapeutic product  102  is pressurised within the reservoir by application of a force, indicated by  103 , on a plunger  104  movable within the reservoir cavity. An outlet  105  of the reservoir is connected to an inlet of a micropump  106 . Means for fluidically coupling the micropump  106  to a human or animal body to which the therapeutic product is to be delivered is connected at one end to a patient, and at the other end to an outlet  107  of the micropump  106 . This means may be a cannular or other similar device. 
         [0039]    In the micropump  106 , the fluid inlet  105  leads to an inlet valve  108 . Operation of an actuator  109  having a gearing assembly causes a change in volume of a pumping chamber  110 . Upon increasing the volume of the pumping chamber  110  by operation of the geared actuator  109  the inlet valve  105  opens and fluid flows from the inlet  105  through the inlet valve  108  to fill the pumping chamber  110 . Once the pumping chamber  110  is full, operation of the geared actuator  109  to produce the volume of the pumping chamber  110  forces the fluid along a conduit  111  to an outlet valve  112 . Since the fluid passing through the conduit  111  is under pressure from the geared actuator  109 , the outlet valve  112  opens and fluid exits the pump  106  via outlet  107 . 
         [0040]    The inlet and outlet valves  108 , 112  are one way valves such as described in the applicant&#39;s co-pending UK patent application GB 0621343.3, the contents of which are incorporated herein by reference. The actuator, which may be a geared actuator  109 , such as described in applicant&#39;s co-pending UK patent application GB0621344.1, the contents of which is incorporated herein by reference. 
         [0041]    The one way valves  108 , 112  are such that upon a decrease in the volume of the pumping chamber  110  fluid therein does not pass through the inlet valve  108  to the inlet  105  and only passes along the conduit  111 . Also, the outlet valve  112  closes when the pressure in the fluid in the conduit  111  decreases below a predetermined value. Repeated operation of the geared actuator  109  causes fluid to be pumped from the inlet  105  to the outlet  107 . 
         [0042]    The actuator  109  is preferably controlled by an electronics module (not shown) that works in cooperation with at least one flow rate indicator to ensure programmed delivery of the therapeutic product with a high degree of accuracy. The at least one flow rate indicator may be derived from an output of the linear capacitance displacement transducer  1  of the present invention. 
         [0043]    Various modifications of the invention are envisaged as will be appreciate by the skilled person without departing from the scope of the invention, which is defined by the appended claims.