Abstract:
The invention provides a sensor assembly which can sense human tissue in a protected area of a machine. An emitter plate and a plurality of sensor plates are disposed co-planar to each other and adjacent to the protected area of the machine. An electric field is generated between the emitter plate and the sensor plates and directed towards the protected area. The sensor plates produce electrical signals corresponding to the electric field received at the sensor plate. The electrical signals are amplified, measures, and analyzed to determine if human tissue is within the protected area. If human tissue is detected, a machine interface circuit will shut down the machine to prevent injury.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     A sensor assembly for sensing human tissue in a protected area of a machine. 
     2. Description of the Prior Art 
     Automated manufacturing machines can pose a hazard to their human operators. These machines typically have moving components that may produce enormous forces and cause an operator injury. They are often outfitted with various forms of safety devices in order to reduce the danger to their human operators. These devices include emergency mechanisms that include a sensor and/or switch that sense operator presence in a protected area and produce as signal that is used to deactivate the machine, these include stop pushbuttons, safety latches, light beams, optical sensors, weight sensors, kick bars, belly bars, and pull bars, to name a few. However, these devices are often not optional in the applications in which they are implemented. Therefore, these devices are frequently used in various combinations in order to enhance the sensing of various hazardous conditions. Thus, these devices sometimes fail to give optimum protection for the operators. For example, they may afford protection for some hazards and not others. These problems are evident many applications, including a typical rubber mill. 
     A rubber mill utilizes at least one pair of rollers. The rollers of each pair are driven in mutually opposite directions. In normal operation, rubber is inserted (by the operator) into the rollers for flattening. This operation causes a potentially dangerous situation in which a worker can inadvertently get his or her fingers, hand or arm caught in the rollers. The roller construction, dimensions and spacing are sufficient to cause severe injury should this occur. 
     One approach to protecting an operator is disclosed in U.S. Pat. No. 6,376,939 (the &#39;939 patent) to Suzuki et al. The &#39;939 patent discloses a sensor apparatus having two electrodes disposed on opposite sides of a protected area of a machine. Material being fed into the machine must pass between the two plates. A circuit detects capacitance change between the electrodes as objects pass between the two plates. A monitor then attempts to determine whether the object includes human tissue. 
     Unfortunately, due to the planar separation of the electrodes, space restrictions make the sensor apparatus of the &#39;939 patent can be difficult to implement in many machines. Furthermore, the apparatus may not be able to detect human tissue that is embedded within a dielectric, such as the rubber sheets that are being fed into the rollers of the rubber mill by the operator. 
     There remains an opportunity for a sensor apparatus which can be implemented in machines having two or three dimensional protected areas or spaces where it is desirable to detect human or other animal tissue within these areas or spaces. Still further, there remains an opportunity for an apparatus which can detect the presence of such tissues in the presence of other materials, such as dielectric materials, and particularly in the presence of rubber. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     The invention provides a sensor assembly for sensing human or other animal tissue, and is particularly suited for sensing human tissue in a protected area of a machine. The sensor assembly includes a power source. An emitter plate, formed of an electrically conductive material, is disposed adjacent to the protected area of the machine. The emitter plate is electrically connected to the power source for generating an electric field directed towards the protected area. At least one sensor plate, also formed of an electrically conductive material, is disposed adjacent to the protected area. The at least one sensor plate receives the electric field and produces an electrical signal corresponding to the electric field. The at least one sensor plate is disposed adjacent to and substantially coplanar with the emitter plate. A device such as a meter, electrically connected to the at least one sensor plate and measures the properties or characteristics of the electrical signal produced by the at least one sensor plate, such as the capacitance. An analyzer is electrically connected to the meter for analyzing the properties of the electrical signal and determining if the properties are indicative of human tissue disposed within the protected area. 
     The invention also provides a sensor assembly including a power source electrically connected to at least one emitter plate. The emitter plate is formed of an electrically conductive material and disposed adjacent to a protected area of a machine. The emitter plate operates to generate a dynamic electric field within the protected area in response to application of the dynamic electric waveform from the power source to the emitter plate. At least one sensor plate formed of an electrically conductive material is disposed adjacent to the protected area. The sensor plate receives the dynamic electric field and produces an electrical signal which is operative to vary in response to changes in the electric field. At least one meter is electrically connected to the at least one sensor plate for measuring at least one property of the electrical signal produced by the at least one sensor plate. A signal analyzer is electrically connected to the meter for analyzing the at least one property of the electrical signal and is operative to determine if the property is indicative of human tissue disposed within the protected area. 
     Due to the co-planar configuration of the emitter plate in relation to the at least one sensor plate, the sensor assembly of the present invention may be implemented in machines having various spacings between the plates, and thus is suitable for use in a wide variety of applications. The application of a dynamic electric field allows the sensor assembly to detect human tissue embedded with sheets of rubber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein like elements in the figures are given like notations, and wherein: 
         FIG. 1  is top view of an emitter plate and a plurality of sensor plates; 
         FIG. 2  is an exploded perspective view of a sensing bar showing the plates integrated with insulation layers, a support member, and amplifiers; 
         FIG. 3  is an assembled perspective view of the sensing bar; 
         FIG. 4  is a cross-sectional end view of the sensing bar along the line  4 - 4  in  FIG. 3 ; 
         FIG. 5  is a perspective view of a machine showing preferred mounting locations of a plurality of sensing bars; 
         FIG. 6  is a side view of the machine; 
         FIG. 7  is a cross-sectional view of the machine along the line  7 - 7  in  FIG. 6 ; and 
         FIG. 8  is a schematic block diagram showing electrical connections between a power source, the emitter plate, the sensor plates, the amplifiers, meters, an analyzer, and the machine. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, the subject invention provides a sensor assembly  20  for sensing human tissue in a protected area  22  of a machine  24 . 
     Referring to  FIG. 1 , the sensor assembly  20  includes an emitter plate  26 . The emitter plate  26  is formed of an electrically conductive material. The electrically conductive material is preferably copper, aluminum or steel. However, other suitable conductive materials are known to those skilled in the art. 
     The sensor assembly  20  further includes at least one sensor plate  28 . The at least one sensor plate  28  is preferably disposed adjacent to the emitter plate  26 . Furthermore, it is preferred that the at least one sensor plate  28  is substantially coplanar with the emitter plate  26 . However, the at least one sensor plate  28  need not be coplanar with the emitter plate  26  to function properly. The at least one sensor plate  28  is formed of an electrically conductive material. It is preferred that at least one sensor plate  28  and the emitter plate  26  utilize the same type of electrically conductive material. The at least one sensor plate  28  and the emitter plate  26  are separated by a nonconductive dielectric, such as, but not limited to, air. 
     In a preferred embodiment, the at least one sensor plate  28  is implemented as a plurality of sensor plates  28 . The quantity of sensor plates  28 , shape and dimensions of the sensor plates  28 , shape and dimensions of the emitter plate  26 , and placement of the sensor plates  28  with respect to the emitter plate  26  are necessitated by the dimensions of the protected area  22  of the machine  24 , the characteristics of the body part (human tissue) that is likely to enter the protected area  22 , and other factors that are obvious to one skilled in the art. For example, where the body part likely to enter the protected area  22  is a hand, the sensor plates  28  are preferably sized no larger than a typical human hand. For convenience, the at least one sensor plate  28  will be referred to hereafter as the sensor plates  28 , but this usage should not be construed as limiting. 
     In the preferred embodiment, as shown in  FIG. 1 , the emitter plate  26  is substantially rectangular in shape with exemplary dimensions of a length of about 54 inches and a width of about 2 inches. The sensor assembly  20  includes fourteen (14) sensor plates  28 . Each sensor plate  28  is substantially rectangular with exemplary dimensions of a length of about 3.65 inches and a width of about 2 inches. The sensor plates  28  are spaced apart from each other and disposed lengthwise such that they are substantially parallel to the emitter plate  26 . As such, the combined length of the sensor plates  28 , along with inter-plate spacing, is generally equivalent to the length of the emitter plate  26  (i.e., in this example about 54 inches). The sensor plates  28  are separated from the emitter plate  26  by about 1.5 inches. In the preferred embodiment, the sensor plates  28  are disposed on one side of the emitter plate  26 . In a first alternative embodiment shown in  FIG. 3 , the sensor plates  28  may be disposed on both sides of the emitter plate  26 . In a second alternative embodiment shown in  FIG. 4 , the sensor plates  28  may be disposed around the emitter plate  26 . Furthermore, as mentioned above, the plates  26 ,  28  may have any shape and are not limited to rectangles (e.g., circular, triangular, elliptical and other regular and irregular curvilinear shapes). Moreover, multiple emitter plates  26  may be implemented. In addition, plates  28  need not be of the same shape or size or have the same spacing either for one another or emitter plate  26 . Further, in cases when emitter plates  26  are used, plates  26  need not have the same shape or size or have the same spacing for sensor plates  28 . By varying these characteristics, as well as by varying the magnitude of the applied potential, the magnitude and shape and size of the electric field my be varied to extend the sensed area and volume to cover protected areas  22  having many different two and three dimensional shapes and sizes. 
     The shape, size and strength of the electric field associated with the embodiments of  FIGS. 1-4  are different and may be determined using known instrumentation or methods for measuring or calculating electrical field strength respectively. The arrangements of sensor plates  28  and emitter plate or plates  26  may be selected to obtain an electric field which is particularly adapted to detect the presence of human or other tissue in a certain area or volume of a machine or other item for which such detection is desired. 
     Referring now to  FIGS. 2-4 , the emitter plates  26  and sensor plates  28  are sandwiched between a pair of insulation layers  30  for electrically insulating the plates  26 ,  28 . An adhesive layer  32  is also sandwiched between the insulation layers  30 . The adhesive layer  32  serves to adhere the insulation layers  30  and the plates  26 ,  28  together and provide a dielectric medium between each plate  26 ,  28 . The area of the insulation layers  30  and the adhesive layers  32  is preferably larger than the area formed by the layout of the plates  26 ,  28  to completely encase the plates  26 ,  28  within the insulation layers  30 . 
     The sensor assembly  20  further includes a support member  34 . The support member  34  supports the insulation layers  30  and the plates  26 ,  28 . The support member  34  is formed of an electrically conductive material. Therefore, the support member  34  also provides a ground plane for the plates  26 ,  28 , as will be discussed additionally later. The support member  34  preferably defines a channel  36 . The channel  36  provides a convenient area for housing electrical connections and other components as will be described later. 
     For convenience and clarity purposes, the structure formed by the support member  34 , insulation layers  30 , and plates  26 ,  28  may be referred to hereafter as a sensing bar  38 . Those skilled in the art realize that this usage is in no way limiting and the invention may be practiced without the plates  26 ,  28  being encased in the insulation layers  30  or attached to the support member  34 . 
     Referring now to  FIGS. 5 through 7 , in the preferred embodiment, the sensing bar  38  is mounted to the machine  24  such that the emitter plate  26  and the sensor plates  28  are disposed adjacent to the protected area  22  of the machine  24 . The plates  26 ,  28  are disposed between the support member  34  and the protected area  22  of the machine  24 . Of course, a plurality of sensing bars  38 , each with their own set of emitter and sensor plates  26 ,  28 , may be mounted on the machine  24 , depending on the quantity of protected area(s)  22 , the size of the protected area(s)  22 , etc. 
     The machine  24  of the preferred embodiment is a rubber mill as described in the “Background” section above. The protected areas  22  of the rubber mill are obviously the points where rollers create “pinch points”, as well as the entry passages to these points. 
     Referring to  FIG. 8 , sensor assembly  20  may also include a power source  40 . Power source  40  is electrically connected to the emitter plate  26 . Preferably, the power source  40  is a function generator to provide a 1 Vp-p, 250 kHz electrical waveform to the emitter plate  26 . Other voltages and frequencies of the electrical waveform may also be utilized. 
     Referring to  FIG. 8 , when energized by the power source  40 , the emitter plate  26  generates a dynamic electric field which may be varied in either the frequency domain (i.e., frequency-varying) or time domain (i.e., time-varying) and which is received by the sensor plates  28 . Referring to  FIGS. 5-7  the electric field is directed towards the protected area  22  of the machine  24 . In all embodiments of sensor assembly  20 , a capacitance is produced between the emitter plate  26  and the sensor plates  28 . When an object enters the electric field (i.e., passes near the plates  26 ,  28 ), the capacitance between the plates  26 ,  28  is changed. 
     In the case of dynamic electric field, each sensor plate  28  produces an electrical signal having a displacement current. Each electrical signal corresponds to the electric field received at each sensor plate  28 . Human tissue is conductive due to the large percentage of water that makes up the human body. Therefore, when human or other animal tissue, such as a hand or other body part, enters the electric field, some of the field is shunted to the body&#39;s stray capacitance. This results in a significant reduction (i.e., lowering) of the displacement current of the electrical signal generated by the sensor plate(s)  28  near the human tissue. However, rubber is an insulator (i.e., non-conductive). Therefore, when rubber (or another insulator) enters the electric field, no significant reduction of the displacement current occurs. Said another way, the change in displacement current is different in sign for human tissue than for rubber. Therefore, the sensor assembly  20  is operative to discriminate between human tissue and the insulating material being processed in the machine, such as rubber. 
     Sensor assembly  20  also may include an amplifier  42  for amplifying the electric signal produced by sensor plates  28 . Amplifier  42  may be electrically connected to each sensor plate  28  for amplifying the electrical signal. Each amplifier  42  is preferably physically as close as possible to the sensor plate  28 , to avoid contamination of the electrical signal from sources of electrical interference. In the preferred embodiment, the amplifiers  42  are disposed in channel  36  of the support member  34 , as best seen in  FIG. 4 . A sensor bridge piece  44  electrically connects the sensor plate  28  and the amplifier  42  through a hole  46  defined in the support member  34 . However, the amplifiers  42  may be located more distant from the sensor plates  28  and electrically connected with a shielded cable which helps to reduce contamination by electrical interference. In an alternate embodiment (not shown) electric signals could be multiplexed and an amplifier  42  could be electrically connected to the multiplexed signal. 
     The amplifier  42  is preferably a current-to-voltage amplifier  42  for converting the displacement current to a voltage. The current-to-voltage amplifier  42  may include a feedback loop (not shown) having a T bridge circuit (not shown). The T bridge circuit increases the bandwidth by lowering the effective impedance seen by the amplifier  42 . Suitable techniques and components for T bridge circuits are well known to those skilled in the art. Preferably, the current-to-voltage amplifier  42  has a bandwidth up to about 1 MHz. 
     Referring again to  FIG. 8 , sensor assembly  20  may also include a meter  48 . Meter  48  is electrically connected to each sensor plate  28  via the amplifier  42 . The meter  48  measures properties of the electrical signal produced by the sensor plate  28 . Preferably, the meter  48  is a voltage meter for measuring the voltage produced by the amplifier  42 . Experimentation with the sensor assembly  20  has shown that a human hand located 12 inches from one of the sensor plates  28  and embedded under 6 inches of rubber results in a voltage reduction of 2.5 mV measured at the meter  48 . The rubber alone does not provide a voltage reduction on a similar scale. 
     Those skilled in the art realize that the meter  48  can be implemented as a current meter for measuring current, a capacitance meter for measuring capacitance, etc. The construction and operation of meter  48  may be in accordance with conventional meter construction and operational methods. Those skilled in the art realize that the amplifier  42  and meter  48  could be combined into an integrated unit. Also, as shown in  FIG. 8 , if the signal is multiplexed and demultiplexed, a single meter  48  may be used to measure the properties of all sensor plates  28 . 
     Referring again to  FIG. 8 , sensor assembly  20  may also include a signal analyzer  50 . Signal analyzer  50  is electrically connected to the meters  48  for analyzing the properties of the electrical signals. These electrical signals correspond to the displacement currents received at the sensor plates  28 . This analyzer  50  determines if the properties are indicative of human tissue disposed within the protected area  22  (e.g., a reduction in voltage as described above). Specifically, in the preferred embodiment, the signal analyzer  50  monitors the electrical signal for rapid temporal changes (i.e., changes occurring within approximately one second) from an established average. The signal analyzer  50  also monitors the electrical signal for spatial changes, by comparing the electrical signals from multiple meters  48  to deternining if human tissue is disposed within the protected area  22 . Additionally, the signal analyzer  50  may determine if a malfunction has occurred in one or more of the sensor plates  28 , amplifiers  42 , or meters  48 . As shown in  FIG. 14 , sensor assembly  20  may use a single signal analyzer  50  to analyze signals from all sensor plates  28 . The construction and operation of signal analyzer  50  may be in accordance with conventional signal analyzer construction and operational methods. 
     As shown in  FIG. 8 , a machine interface circuit  52  is preferably electrically connected to the analyzer  50  and the machine  24 . The machine interface circuit  52  is operative to provide a stop signal to stop the machine  24  in response to the analyzer  50  determining that human tissue is present in the protected area  22 . The stop signal may also be activated in response to a malfunction in the sensor apparatus being detected by the analyzer  50 . 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.