Abstract:
A system for detecting motion and proximity by determining capacitance between a sensor and an object. The sensor includes sensing surfaces made of a thin film of electrically conductive material mounted on a non-conductive surface. In another embodiment, the sensor is a human body. The sensor senses the capacitance between a sensor&#39;s surface and an object in its vicinity and provides the capacitance to a control system that directs machine movement. Because the sensor does not require direct contact or line-of-sight with the object, a machine can be controlled before harm occurs to the object.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to devices used to provide safety for humans in proximity with moving equipment, and more specifically to motion and proximity sensors employed as part of a control system to orient equipment based on capacitance. 
     Safety is important when people are close to moving machines. One such example is locally controlled machines or robotic equipment where people are in close proximity to moving mechanical components. Another example is in the medical imaging equipment industry. 
     In known systems, conventional safety mechanisms such as mechanical switches and fluid-filled bladders connected to pressure switches are typically mounted directly to the moving mechanical components, or in proximity of the hazardous area. These conventional safety mechanisms require direct contact between the person or inanimate object and the safety mechanism to operate. For example, the fluid-filled bladder mounted to a moving mechanical component uses a pressure sensor or a pressure switch inside the bladder to detect increased pressure as the bladder makes contact with an object. The sensed pressure increase typically is an input to a control system which stops the moving mechanical component. 
     In other known systems, plates, levers, cables, and rings are connected to mechanical switches and mounted on the moving mechanical component. The switches are activated when the plate, lever, cables, or ring contacts the person or object, and the machine is stopped before any harm occurs. 
     Disadvantages of the above described systems include expense (fluidfilled bladders) and the fact that the sensing area is highly localized (mechanical switches). Such devices are typically ON or OFF and therefore provide no information to the control system regarding relative distance between the subject and the sensor. systems. The drawback to those systems is that an unobstructed line-of-sight between the detector and the subject is required. As applied to medical imaging equipment, required sterile covers and drapes preclude use of line-of-sight proximity detector systems. Depending on the implementation specifics, these sensors are also highly directional and impacted by object properties such as reflectivity and specularity, which further limits their applicability. 
     In the listed examples, safety cannot be enhanced, nor injury prevented simply by increasing the distances between man and machine because each example requires close proximity between man and machine. It would therefore be desirable to provide a system whereby proximity and relative distance to a person or an object can be sensed and the information regarding proximity and distance used to control movement and prevent contact with the person or object and thereby increase the safety of such a system. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the present invention relates to a method and system for detecting motion using a capacitance between a sensor and an object. Alternatively, the invention detects proximity of an object using the capacitance between a sensor and the object. In an exemplary embodiment, a capacitance based proximity sensor is used as a detector. The sensor includes sensing surfaces made of a thin film of electrically conductive material mounted on a non-conductive surface. The nonconductive surface can take any shape and form. The sensor senses the capacitance between a conductive surface and an object placed in its vicinity, and the sensor provides a capacitance value to a control system. The control system is programmed to use the capacitance data to control the movement of a machine or piece of equipment. In one embodiment, the piece of equipment is a medical imaging system. 
     In another embodiment, the sensing surface is a human body. A relative capacitance between the human body and surrounding objects is determined. The control system uses the capacitance information to determine a position of the body and proximity of objects near the body to control movement of a machine or piece of equipment. 
     Accordingly, because the sensor can take any size and shape, and does not require direct contact or line-of-sight with the object to determine if an object has moved, a machine or piece of equipment can be controlled before harm occurs to the object. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a system for detecting capacitance change based upon movement; 
     FIG. 2 is a diagram showing an alternative system for detecting capacitance based upon movement; 
     FIG. 3 is a diagram showing one embodiment of a capacitance based proximity sensor; 
     FIG. 4 is a diagram showing an alternative embodiment of a capacitance based proximity sensor; 
     FIG. 5 is a diagram of a third embodiment of a capacitance based proximity sensor; 
     FIG. 6 is a diagram of a fourth embodiment of a capacitance based proximity sensor; 
     FIG. 7 is a diagram of a sensing field for a capacitance based proximity sensor; 
     FIG. 8 is a diagram of a sensing field for a capacitance based proximity sensor shaped by sensor surface geometry; 
     FIG. 9 is a diagram of a medical imaging system using capacitive based proximity sensors; and 
     FIG. 10 is an illustration of an irregularly shaped apparatus with an outer surface covered with sensing material. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a diagram showing a system  10  for detecting capacitance change based upon movement of, or proximity to, an object  12 . Object  12  is connected to a capacitance sensing circuit  14  via a conductive strap  16 . Sensing circuit  14  senses capacitance and supplies data relating to the measured capacitance to control system  18 . Object  12  is on a surface  20  which includes a non-conductive surface  22  such as a film or mat placed upon a conductive surface  24 . Control system  18  is programmed to use the measured capacitance data to control movement of component  26  in both horizontal and vertical axes via a motor  28 . Component  26  in one embodiment is a radiation source. Component  26  in an alternative embodiment is a detector. Component  26  in a further alternate embodiment is a sensor. Component  26  in a still further embodiment is a nuclear medicine imaging source. Component  26  in another embodiment is a laser source. Component  26  in yet another embodiment is a component of a medical system, e.g., computer aided tomography (CAT), magnetic resonance imaging (MRI), computed tomography (CT), digital fluoroscopy, positron emission tomography (PET), positron emission transaxial tomography (PETT), and mammography. 
     The amount of capacitance sensed by circuit  14  changes as object  12  moves. In another embodiment, the capacitance sensed also changes as a proximity of object  12  changes with respect to component  26 . The change in capacitance is received by control system  18  which in turn causes changes in predetermined movement of component  26  such that the trajectory of object  26  is optimized for a procedure being performed. An ability to detect unexpected motion of object  12  provides control system  18  or an operator of control system  18  with a signal to slow or stop movement of component  26  to prevent injury to object  12  or damage to the above described equipment. Capacitance sensing circuit  14  is capable of measuring small changes (15-30 femtoFarads) in capacitance. Since an object  12  changes capacitance as object  12  moves, raising of arms, crossing of legs, finger wiggling, toe wiggling, and torso motion are all detectable. 
     Capacitance sensing circuit  14  uses charge transfer technology to measure the capacitance of object  12  connected to circuit  14 . Conductive strap  16  is used, along with circuit  14  to measure an effective nominal capacitance of object  12 . Capacitance sensing circuit  14  is manually or automatically re-calibrated for new nominal capacitive loads, such as, for example a different object  12 . The re-calibration process changes the nominal capacitance about which small changes, such as the movement of object  12  described above, are detected. Re-calibration allows system  10  to accommodate objects  12  of different sizes, shapes, clothing, and body hair, for example. Re-calibration also can take into account the environment object  12  is placed, such as temperature and relative humidity. 
     FIG. 2 shows an alternative embodiment to the system shown in FIG. 1 employing an alternative method for the measurement of capacitance. Non-conductive surface  22  and conductive surface  24  are as described above, however, a conductive mat  40  is placed on top of non-conductive surface  22  and is electrically connected to capacitive sensing circuit  14 . The measured capacitance is based upon an amount of object  12  actually touching conductive mat  40 , as well as movement of object  12 , such as raising of arms, crossing of legs, finger wiggling, toe wiggling, and torso motion. For instance, the measured capacitance of a human body laying supine on mat  40  will be greater than the measured capacitance of a human body laying supine on mat  40  with both legs bent and the soles of the feet resting flat on mat  40 . As object  12  moves and less of the body is touching mat  40 , the measured capacitance will decrease. The larger the surface area touching mat  40 , the higher the capacitance. 
     The embodiments shown in FIGS. 1 and 2 demonstrate, for example, how a human body, can be used as a detector for a capacitive sensing circuit. Measured capacitance depends on the location of objects relative to the body, and other objects or persons near the subject being used as a detector can be detected. Using a subject as a detector may be ideal when there is the potential for a number of moving components to make contact with the subject, a sensor being installed on every moving component being unfeasible. 
     FIG. 3 is a diagram showing one embodiment of a capacitance based proximity sensor  50  used in systems where the subject is not used as the detector. Sensor  50  includes a sensing surface  52  which is made of a thin film of conducting material mounted on a front side  54  of non-conductive backing material  56 . A backing surface  58  of electrically grounded thin film conducting material mounted on a back side  60  of non-conductive backing material  56  completes the sensor. As stated above, backing surface  58  is connected to an electrical ground  62 . Sensing surface  52  is electrically connected to a capacitive sensing circuit  64  and as shown in FIG. 3, may be configured to be of a size smaller in surface area than that of backing material  56 . 
     FIG. 4 is a diagram showing an alternative embodiment of a capacitance based proximity sensor  70 . Sensor  70  is cylindrically shaped and consists of sensing surfaces  72 ,  74  and  76  of the thin film electrically conductive material. Sensing surface  72  covers an outer surface of sensor  70  and sensing surfaces  74  and  76  cover end surfaces of the cylinder, a top surface and a bottom surface respectively. 
     Non-conductive backing material  78  is the “body” of the cylinder, giving sensor  70  strength and a surface for the mounting of surfaces  72 ,  74  and  76  which are electrically connected to a capacitive sensing circuit. A backing surface (not shown) is electrically connected to ground. In another embodiment, the backing surface is not utilized by sensor  70 . 
     FIG. 5 is a diagram of one embodiment of a proximity sensor  90  configured to shape the sensing field. Sensor  90 , in the embodiment shown in FIG. 5 consists of an outer sensing surface  92 , a central sensing surface  94 , and an inner sensing surface  96 . Outer sensing surface  92 , central sensing surface  94  and inner sensing surface  96  are electrically connected with conductive strips  98  to form an electrically continuous circuit and are mounted on non-conductive backing material  100 . In one exemplary embodiment, sensor  90  has sensing surface dimensions where inner sensing surface  96  has a dimension of 3 cm×3 cm, a space of 3 cm separates inner sensing surface  96  from an inner circumference  102  of central sensing surface  94  which is 3 cm wide. Another 3 cm gap in sensing material separates an outer circumference  104  of central sensing surface  94  from an inner circumference  106  of outer sensing surface  92 . Outer sensing surface  92  is 3 cm in width. In the exemplary embodiment, backing material  100  is fabricated from mylar, which is virtually invisible to x-ray radiation. The thickness of the mylar backing depends on mechanical strength requirements of an application. The sensing surfaces of sensor  90  are variable in size and in number in order to shape the sensing field of sensor  90  and are connected to a capacitive sensing circuit  108 . In the exemplary embodiment, sensing surfaces  92 ,  94  and  96  of sensor  90  are fabricated from 3 um thick aluminum foil and bonded to the mylar. In another embodiment, aluminum plates are bonded to the mylar. To be invisible to a vascular spectrum, surfaces  92 ,  94  and  96  are fabricated from aluminum foil/plates less than 5 um in thickness. In a further embodiment, sensing surfaces  92 ,  94  and  96  are fabricated from copper. In a still further embodiment, sensing surfaces  92 ,  94  and  96  are fabricated from tin. 
     FIG. 6 is a diagram of an alternative circular embodiment of a proximity sensor configured to shape the sensing field. Sensor  110 , in the embodiment shown in FIG. 6 consists of an outer sensing surface  112 , a central sensing surface  114 , and an inner sensing surface  116 . Outer sensing surface  112  and central sensing surface  114  are ring shaped. Inner sensing surface  116  is circularly shaped. Outer sensing surface  112 , central sensing surface  114 , and inner sensing surface  116  are electrically connected with conductive strips  118  to form an electrically continuous circuit and are mounted on non-conductive backing material  120 . In one exemplary embodiment, sensor  110  has sensing surface dimensions where inner sensing surface  116  has a diameter of 3 cm. A ring shaped space  122  that is 3 cm wide separates inner circular sensing surface  116  from central sensing surface  114 . Central circular sensing surface  114  has a inner ring  124  and an outer ring  126 . Inner ring  124  has a diameter of 9 cm and outer ring  126  has a diameter of 12 cm, such that central sensing surface  114  has a sensor ring area of 3 cm in diameter. Another 3 cm wide ring shaped space  128  in sensing material separates central sensing surface  114  from outer sensing surface  112 . Outer sensing surface  112  has an inner ring  130  and an outer ring  132 . Inner ring  130  has a diameter of 21 cm and outer ring  132  has a diameter of 27 cm, such that outer sensing surface  112  has a sensor ring area of 3 cm in diameter. In the exemplary embodiment, backing material  120  is fabricated from mylar, which is virtually invisible to x-ray radiation. The thickness of the mylar backing depends on mechanical strength requirements of an application. The sensing surfaces of sensor  110  are variable in size and in number in order to shape the sensing field of sensor  110  and are connected to a capacitive sensing circuit  133 . In the exemplary embodiment, sensing surfaces  112 ,  114  and  116  of sensor  110  are fabricated from 3 um thick aluminum foil and bonded to the mylar. In another embodiment, aluminum plates are bonded to the mylar. To be invisible to a vascular spectrum, surfaces  112 ,  114  and  116  are fabricated from aluminum foil/plates less than 5 um in thickness. In a further embodiment, sensing surfaces  112 ,  114  and  116  are fabricated from copper. In a still further embodiment, sensing surfaces  112 ,  114  and  116  are fabricated from tin. 
     FIG. 7 is a diagram  134  of a sensing field for a capacitance based proximity sensor where no shaping has been employed, for example, where the sensing surface is a solid rectangular or square thin-film conductor. Such a sensor is able to detect capacitive changes omni-directionally. The sensor which produces the type of field shown in FIG. 7 is more sensitive to objects which approach the sensor along an x=0 axis  136  and less sensitive to objects approaching along a y=0 axis  138 . Objects moving along axis  136  are detected more quickly and from a farther distance. This non-uniform sensitivity is not particularly desirable. 
     FIG. 8 is a diagram of a sensing field  140  where the sensing surface has been shaped using sensor  90 , as described above and shown in FIG.  5 . In one embodiment, annular surfaces  92 ,  94 , and  96  are optimized to flatten the field at a 5 cm distance. The field shown is more uniform compared to the field shown in FIG.  8 , and is illustrative of an ability to customize field shaping by using segmented sensing surfaces. The circular construction used for field shaping can be extended to circular and cylindrical geometries (shown in FIG.  6 ). 
     FIG. 9 is a diagram of a medical imaging system  160  using capacitive based proximity sensors  162  electrically connected to capacitive sensing circuits  164 . Capacitive sensors  162  also provide a non-contact method of measuring the relative capacitance of a human body covered in paper, plastic and clothing. Circuits  164  provide data to a control system  166  regarding position and orientation of component  168  relative to  170 . Component  168  in one embodiment is a radiation source. Component  168  in an alternative embodiment is a detector. Component  168  in a further alternate embodiment is a sensor. Component  168  in a still further embodiment is a nuclear medicine imaging source. Component  168  in another embodiment is a laser source. Component  168  in yet another embodiment is a component of a medical system, e.g., computer aided tomography (CAT), magnetic resonance imaging (MRI), computed tomography (CT), digital fluoroscopy, positron emission tomography (PET), positron emission transaxial tomography (PETT), and mammography. Although not shown in the figure, system  166  controls elevation, longitudinal movement and horizontal orientation of component  168 . By using a capacitive based proximity approach, system  166  is configurable to follow the contours of an object, such as a body  170 . System  166  can then be programmed to optimize the trajectory of component  168  relative to the object  170 . In an exemplary embodiment, system  166  reduces exposures to radiation to body  170  compared to known systems, which either do not change the radiation source, detector elevations, or employ sensing devices, and which require a touching of body  170  before a control system adjusts movement of component  168 . 
     FIG. 10 is an illustration of an irregularly shaped apparatus  180  with an outer surface  182  covered with sensing material  184 . Sensing material  184  is fabricated from a thin-film conducting material, e.g., aluminum, copper or tin. In an exemplary embodiment, thin-film sheets of copper foil are joined together with conductive epoxy  186 . In one embodiment, the copper foil is 25 um in thickness. In an alternative embodiment, the thin-film sheets are fabricated by “spray depositing” a film of conductive material, e.g., tin, to a backing surface. Sensing material  184  is bonded to a backing surface (not shown). In an alternative embodiment, apparatus  180  is configured to take any form and shape and is not limited to a certain size range. In addition, sensing material  184  is electrically coupled to a capacitive sensing circuit (not shown). Apparatus  180  has one sensing zone  188 . In an alternative embodiment, sensing material  184  has a plurality of sensing zones  188 . Sensing zone  188  is capable of measuring changes in capacitance, e.g., 15-30 femtoFarads. In one embodiment, sensing zone  188  is optimized for detecting predetermined objects at a specified distance. In an alternative embodiment, apparatus  180  includes a plurality of sensing zones, each sensing zone optimized to detect a predetermined object at a specified distance. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.