Patent Publication Number: US-2019169018-A1

Title: Stress isolation frame for a sensor

Description:
RELATED APPLICATIONS 
     This application claims priority to and the benefit of co-pending U.S. Patent Provisional Patent Application 62/595,015, filed on Dec. 5, 2017, entitled “STRESS ISOLATION FRAME FOR MEMS DEVICE,” by Senkal et al., having Attorney Docket No. IVS-769.PR, and assigned to the assignee of the present application, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     A sensor is a device, module, or subsystem whose purpose is to detect events or changes in its environment and send the information to other electronics, frequently a computer processor. There are many types of sensors, including magnetometers, clocks, accelerometers, gyroscopes, microphones, and pressure sensors. Of interest herein are micro-electro-mechanical systems (MEMS), which are based on the technology of microscopic devices, particularly those with moving parts. Examples of MEMS sensors include clocks, gyroscopes, accelerometers, Lorentz force magnetometers, and membrane sensors such as microphones and pressure sensors. 
     Micro-electro-mechanical systems (MEMS) technology has been under steady development for some time, and as a result, various MEMS sensors (e.g., accelerometers for measuring linear acceleration and gyroscopes for measuring angular velocity) have been implemented within several applications. For example, individual accelerometer and gyroscope sensors are currently being used in vehicle air bag controls, gaming consoles, digital cameras, video cameras, and mobile phones. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the Description of Embodiments, illustrate various embodiments of the subject matter and, together with the Description of Embodiments, serve to explain principles of the subject matter discussed below. Unless specifically noted, the drawings referred to in this Brief Description of Drawings should be understood as not being drawn to scale. Herein, like items are labeled with like item numbers. 
         FIG. 1  is a block diagram of an example mobile electronic device that includes a MEMS sensor. 
         FIGS. 2A-C  are diagrams illustrating schematic top plan views of a device for reducing package stress sensitivity of a sensor, according to some embodiments. 
         FIGS. 3A through 3D  are diagrams illustrating examples of different shapes of a rigid frame structure, according to some embodiments.  FIG. 3B  is an enlargement of a portion of  FIG. 3A . 
         FIGS. 4A-C  are each a diagram illustrating examples of different crab-leg compliant structures employed in the devices shown in  FIGS. 2A-C , according to some embodiments. 
         FIGS. 5A-F  are each a diagram illustrating examples of folded spring compliant structures employed in the devices shown in  FIGS. 2A-C , according to some embodiments. 
         FIG. 6  is a flow chart illustrating one embodiment of a method for reducing package stress sensitivity of the sensor. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following Description of Embodiments is merely provided by way of example and not of limitation. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding background or in the following Description of Embodiments. 
     Reference will now be made in detail to various embodiments of the subject matter, examples of which are illustrated in the accompanying drawings. While various embodiments are discussed herein, it will be understood that they are not intended to limit to these embodiments. On the contrary, the presented embodiments are intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope the various embodiments as defined by the appended claims. Furthermore, in this Description of Embodiments, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present subject matter. However, embodiments may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the described embodiments. 
     Notation and Nomenclature 
     Some portions of the detailed descriptions which follow are presented in terms of procedures, logic blocks, processing and other symbolic representations of operations on data within an electrical device. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. In the present application, a procedure, logic block, process, or the like, is conceived to be one or more self-consistent procedures or instructions leading to a desired result. The procedures are those requiring physical manipulations of physical quantities. Usually, although not necessarily, these quantities take the form of sensed linear acceleration, angular velocity magnetic fields, and pressure, for example. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the description of embodiments, discussions utilizing terms such as “providing,” “capturing,” “combining,” “receiving,” “sensing,” or the like, refer to the actions and processes of an electronic device such as a sensor. 
     In the figures, a single block may be described as performing a function or functions; however, in actual practice, the function or functions performed by that block may be performed in a single component or across multiple components, and/or may be performed using hardware. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, logic, circuits, and steps have been described generally in terms of their functionality. Whether such functionality is implemented as hardware depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. Also, the example systems described herein may include components other than those shown, including well-known components. 
     As used herein, a gyroscope is a sensor used for measuring or maintaining orientation and angular velocity. A MEMS-based gyroscope is a miniaturized gyroscope found in electronic devices. It takes the idea of the Foucault pendulum and uses a vibrating element. 
     The terms “rigid” and “compliant” are used in the context of their customary definitions. That is to say, “rigid” as applied to a structure means unable to bend or be forced out of shape; not flexible, while “compliant” as applied to a structure means the ability of that structure to yield elastically when a force is applied. 
     It is to be noted that, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Also, any reference herein to “top”, “bottom”, “upper”, “lower”, “up”, “down”, “front”, “back”, “first”, “second”, “left” or “right” is not intended to be a limitation herein. Herein, the term “about” when applied to a value generally means within the tolerance range of the equipment used to produce the value, or may mean plus or minus 10%, or plus or minus 5%, or plus or minus 1%, unless otherwise expressly specified. Further, the terms “substantially” and “about”, as used herein, mean a majority, or almost all, or all, or an amount within a range of about 51% to about 100%. It is appreciated that, in the following description, numerous specific details are set forth to provide a thorough understanding of the examples. However, it is appreciated that the examples may be practiced without limitation to these specific details. In other instances, well-known methods and structures may not be described in detail to avoid unnecessarily obscuring the description of the examples. Also, the examples may be used in combination with each other. 
     Overview of Discussion 
     Discussion begins with a description of an example mobile electronic device with which or upon which various embodiments described herein may be implemented. In particular, the mobile electronic device includes a MEMS sensor, such as a gyroscope. A description of an improved stress isolation frame for MEMS sensors and other sensors is then provided. 
     Example Mobile Electronic Device 
     Turning now to the figures,  FIG. 1  is a block diagram of an example mobile electronic device  100  that includes a MEMS sensor, such as a gyroscope. As will be appreciated, mobile electronic device  100  may be implemented as a device or apparatus, such as a handheld mobile electronic device. For example, such a mobile electronic device may be, without limitation, a mobile telephone phone (e.g., smartphone, cellular phone, a cordless phone running on a local network, or any other cordless telephone handset), a wired telephone (e.g., a phone attached by a wire), a personal digital assistant (PDA), a video game player, video game controller, a navigation device, an activity or fitness tracker device (e.g., bracelet, clip, band, or pendant), a smart watch or other wearable device, a mobile Internet device (MID), a personal navigation device (PND), a digital still camera, a digital video camera, a portable music player, a portable video player, a portable multi-media player, a remote control, or a combination of one or more of these devices. 
     As depicted in  FIG. 1 , mobile electronic device  100  may include a host processor  110 , a host bus  120 , a host memory  130 , a display  140 , and a sensor processing unit (SPU)  170 . Some embodiments of mobile electronic device  100  may further include one or more of an interface  150 , a transceiver  160  (all depicted in dashed lines) and/or other components. In various embodiments, electrical power for mobile electronic device  100  is provided by a mobile power source such as a battery (not shown), when not being actively charged. 
     Host processor  110  can be one or more microprocessors, central processing units (CPUs), DSPs, general purpose microprocessors, ASICs, ASIPs, FPGAs or other processors which run software programs or applications, which may be stored in host memory  130 , associated with the functions and capabilities of mobile electronic device  100 . 
     Host bus  120  may be any suitable bus or interface to include, without limitation, a peripheral component interconnect express (PCIe) bus, a universal serial bus (USB), a universal asynchronous receiver/transmitter (UART) serial bus, a suitable advanced microcontroller bus architecture (AMBA) interface, an Inter-Integrated Circuit (I2C) bus, a serial digital input output (SDIO) bus, a serial peripheral interface (SPI) or other equivalent. In the embodiment shown, host processor  110 , host memory  130 , display  140 , interface  150 , transceiver  160 , sensor processing unit  170 , and other components of mobile electronic device  100  may be coupled communicatively through host bus  120  in order to exchange commands and data. Depending on the architecture, different bus configurations may be employed as desired. For example, additional buses may be used to couple the various components of mobile electronic device  100 , such as by using a dedicated bus between host processor  110  and host memory  130 . 
     Host memory  130  can be any suitable type of memory, including but not limited to electronic memory (e.g., read only memory (ROM), random access memory, or other electronic memory), hard disk, optical disk, or some combination thereof. Multiple layers of software can be stored in host memory  130  for use with/operation upon host processor  110 . For example, an operating system layer can be provided for mobile electronic device  100  to control and manage system resources in real time, enable functions of application software and other layers, and interface application programs with other software and functions of mobile electronic device  100 . Similarly, a user experience system layer may operate upon or be facilitated by the operating system. The user experience system may comprise one or more software application programs such as menu navigation software, games, device function control, gesture recognition, image processing or adjusting, voice recognition, navigation software, communications software (such as telephony or wireless local area network (WLAN) software), and/or any of a wide variety of other software and functional interfaces for interaction with the user can be provided. In some embodiments, multiple different applications can be provided on a single mobile electronic device  100 , and in some of those embodiments, multiple applications can run simultaneously as part of the user experience system. In some embodiments, the user experience system, operating system, and/or the host processor  110  may operate in a low-power mode (e.g., a sleep mode) where very few instructions are processed. Such a low-power mode may utilize only a small fraction of the processing power of a full-power mode (e.g., an awake mode) of the host processor  110 . 
     Display  140  may be a liquid crystal device, (organic) light emitting diode device, or other display device suitable for creating and visibly depicting graphic images and/or alphanumeric characters recognizable to a user. Display  140  may be configured to output images viewable by the user and may additionally or alternatively function as a viewfinder for camera. 
     Interface  150 , when included, can be any of a variety of different devices providing input and/or output to a user, such as audio speakers, touch screen, real or virtual buttons, joystick, slider, knob, printer, scanner, computer network I/O device, other connected peripherals and the like. 
     Transceiver  160 , when included, may be one or more of a wired or wireless transceiver which facilitates receipt of data at mobile electronic device  100  from an external transmission source and transmission of data from mobile electronic device  100  to an external recipient. By way of example, and not of limitation, in various embodiments, transceiver  160  comprises one or more of: a cellular transceiver, a wireless local area network transceiver (e.g., a transceiver compliant with one or more Institute of Electrical and Electronics Engineers (IEEE) 802.11 specifications for wireless local area network communication), a wireless personal area network transceiver (e.g., a transceiver compliant with one or more IEEE 802.15 specifications for wireless personal area network communication), and a wired a serial transceiver (e.g., a universal serial bus for wired communication). 
     Mobile electronic device  100  also includes a general purpose sensor assembly in the form of integrated SPU  170  which includes sensor processor  172 , memory  176 , a sensor  178 , and a bus  174  for facilitating communication between these and other components of SPU  170 . In some embodiments, SPU  170  may include at least one additional sensor  180  (shown as sensor  180 - 1 ,  180 - 2 , . . . ,  180 - n ) communicatively coupled to bus  174 . In an embodiment, one of the sensors, for example, sensor  180 - 1 , may be a MEMS sensor, such as a gyroscope. In some embodiments, all of the components illustrated in SPU  170  may be embodied on a single integrated circuit. It should be appreciated that SPU  170  may be manufactured as a stand-alone unit (e.g., an integrated circuit), that may exist separately from a larger electronic device. 
     Sensor processor  172  can be one or more microprocessors, CPUs, DSPs, general purpose microprocessors, ASICs. ASIPs, FPGAs or other processors which run software programs, which may be stored in memory  176 , associated with the functions of SPU  170 . 
     Bus  174  may be any suitable bus or interface to include, without limitation, a peripheral component interconnect express (PCIe) bus, a universal serial bus (USB), a universal asynchronous receiver/transmitter (DART) serial bus, a suitable advanced microcontroller bus architecture (AMBA) interface, an Inter-Integrated Circuit (I2C) bus, a serial digital input output (SDIO) bus, a serial peripheral interface (SPI) or other equivalent. Depending on the architecture, different bus configurations may be employed as desired. In the embodiment shown, sensor processor  172 , memory  176 , sensor  178 , and other components of SPU  170  may be communicatively coupled through bus  174  in order to exchange data. 
     Memory  176  can be any suitable type of memory, including but not limited to electronic memory (e.g., read only memory (ROM), random access memory, or other electronic memory). Memory  176  may store algorithms or routines or other instructions for processing data received from sensor  178 , which may be an ultrasonic sensor, for example, and/or one or more sensors  180 , as well as the received data either in its raw form or after some processing. Such algorithms and routines may be implemented by sensor processor  172  and/or by logic or processing capabilities included in sensor  178  and/or sensor  180 . 
     A sensor  180  may comprise, without imitation; a temperature sensor, a humidity sensor, an atmospheric pressure sensor, an infrared sensor, a radio frequency sensor, a navigation satellite system sensor (such as a global positioning system receiver), an acoustic sensor (e.g., a microphone), an inertial or motion sensor (e.g., a gyroscope, accelerometer, or magnetometer) for measuring the orientation or motion of the sensor in space, or other type of sensor for measuring other physical or environmental quantities. In one example, sensor  180 - 1  may comprise a gyroscope, sensor  180 - 2  may comprise an acoustic sensor, and sensor  180 - n  may comprise a motion sensor. 
     In some embodiments, sensor  178  and/or one or more sensors  180  may be implemented using a micro-electro-mechanical system (MEMS) that is integrated with sensor processor  172  and one or more other components of SPU  170  in a single chip or package. Although depicted as being included within SPU  170 , one, some, or all of sensor  178  and/or one or more sensors  180  may be disposed externally to SPU  170  in various embodiments. 
     Example Stress Isolation Frame 
     Many sensors, such as MEMS sensors, are sensitive to external forces that adversely affect the sensing and lead to inaccurate results. Package stresses are one of the primary sources of offset shift in MEMS gyroscopes. For example, in gyroscopes that include a stress isolation frame and a mechanical element suspended in the frame, tension/compression and bend can cause the stress isolation frame and the mechanical element to deform. Package stresses can also adversely affect the sensing capabilities of other MEMS sensors and other sensors in general. The drive for thinner and more compact mobile devices necessitates components with thinner/smaller packages, resulting in an increased sensitivity to package stresses. This trend is likely to continue in the upcoming years, creating a demand for methods and devices for reducing sensitivity of the MEMS sensors and other sensors to package stresses. Such package stresses also exist for other MEMS sensors, such as accelerometers, Lorentz force magnetometers, membrane sensors, and other MEMS transducers, as well as for non-MEMS sensors, such as magnetometers, clocks, and pressure sensors. 
     Embodiments described herein provide for the reduction of package sensitivity of MEMS sensors (e.g., gyroscopes, accelerometers, oscillators, etc.), as well as non-MEMS sensors. Embodiments described herein provide improved mechanical isolation of the MEMS sensor or sensor from the package, allowing for improved rejection of the effect of package/PCB stresses on the MEMS sensor&#39;s/non-MEMS sensor&#39;s mechanical element. 
     Embodiments of the present invention include a rigid stress isolation frame to keep the mechanical element of the MEMS sensor or other sensor from deforming, and a compliant suspension, such as a crab-leg suspension or folded spring suspension, between an anchor and the stress isolation frame, or rigid frame structure, to prevent package strain from propagating onto the MEMS sensor. 
       FIG. 2A  illustrates a schematic top plan view of a device  200  for reducing package stress sensitivity of a sensor  210 . The device  200  comprises one or more anchor points  220  for attaching to a portion of a substrate  230 . The device  200  further comprises a rigid frame structure  240  configured to at least partially support the sensor  210 . Finally, the device  200  includes a compliant element  250  between each anchor point  220  and the rigid frame structure  240 . In the device  200  depicted in  FIG. 2A , four anchor points  220  are depicted and four compliant elements  250 , each disposed between an anchor point and the rigid frame structure  240 , are depicted. However, it should be appreciated that there can be any number of compliant elements  250  not less than one, of which the illustrated embodiment is one example. 
     The sensor  210  may be one of a micro-electro-mechanical system (MEMS) sensor or a non-MEMS sensor, such as a magnetometer, a dock, or a pressure sensor. The MEMS sensor may be one of a gyroscope, an accelerometer, and a membrane sensor. Other examples of MEMS and non-MEMS sensors are listed in the Background section above. 
     The sensor  210  may be partially or fully suspended from the rigid frame structure  240 . In the device  200  depicted in  FIG. 2A , the sensor  210  is shown fully suspended from the rigid frame structure  240 . 
     Each anchor point  220  may be a rectilinear-shaped member embedded in the substrate  230 . Each compliant element  250  may comprise one connection  252  (or in some embodiments, two connections  252 ) to the anchor point  220  and a plurality of connections  254  to the rigid frame structure  240 . At least one “leg”  252  of the compliant element  250  may be fixedly attached to the anchor point  220  and at least one “leg”  254  may be fixedly attached to the rigid frame structure  240  to provide a rigid-compliant-rigid connection from anchor point  220  to compliant element  250  to rigid frame structure  240 . 
     The rigid frame structure  240  may be of any shape that supports and protects the sensor  210 . Rigid frame structure  240  fully surrounds sensor  210  on all sides. However, it should be appreciated that the rigid frame structure may have a different shape for supporting sensor  210 , e.g., as illustrated in  FIGS. 3C and 3D . 
     The device  200  comprises four anchor points  220  for attaching to a substrate  230 . The device  200  further comprises a rigid frame structure  240  configured to support the sensor  210 . Finally, the device  200  includes four cab-leg suspension elements  250 , one between each anchor point  220  and the rigid frame structure  240 . The crab-leg suspension element is compliant. 
       FIG. 2B  illustrates a schematic top plan view of a device  202  for reducing package stress sensitivity of a sensor  210 . The device  202  comprises one or more anchor points  220  for attaching to a portion of a substrate  230 , a rigid frame structure  242  configured to at least partially support the sensor  210 , and a compliant element  250  between each anchor point  220  and the rigid frame structure  242 . Rigid frame structure  242  surrounds sensor  210  on three sides. 
     In the device  202  depicted in  FIG. 2B , four anchor points  220  are depicted and four compliant elements  250 , each disposed between an anchor point and the rigid frame structure  242 , are depicted. However, it should be appreciated that there can be any number of compliant elements  250  not less than two, of which the illustrated embodiment is one example. 
       FIG. 2C  illustrates a schematic top plan view of a device  204  for reducing package stress sensitivity of a sensor  210 . The device  204  comprises one or more anchor points  220  for attaching to a portion of a substrate  230 , a rigid frame structure  244  configured to at least partially support the sensor  210 , and a compliant element  250  between each anchor point  220  and the rigid frame structure  244 . Rigid frame structure  244  is L-shaped and surrounds sensor  210  on two sides. 
     In the device  204  depicted in  FIG. 2C , four anchor points  220  are depicted and four compliant elements  250 , each disposed between an anchor point and the rigid frame structure  244 , are depicted. However, it should be appreciated that there can be any number of compliant elements  250  not less than two, of which the illustrated embodiment is one example. 
       FIGS. 3A-D  illustrate examples of different shapes of the rigid frame structure  240  and includes an enlarged portion  300  that depicts an anchor point  220 , attached to a portion of the substrate  230 , and a compliant element  250 , attached to the rigid frame structure  240 . In some embodiments, the rigid frame structure  240 ,  242 ,  244  may be a full frame ( FIGS. 3A and 33, 240 ), in other embodiments, a half frame ( FIG. 3C, 242 ), and in still other embodiments, an L-shaped frame ( FIG. 3D, 244 ). In some embodiments, the rigid frame structure  240  may be T-shaped. In other embodiments, a straight edge on at least one side of the sensor element may be used to form the rigid frame structure  240 , such as the bottom edge  246  of the frame in  FIG. 3A . Also, the rigid frame structure  240  can comprised a few straight edges on each side of the sensor  210  that can be connected together through some connections. In other words, a few straight edges may be used that each cause some isolations but are not necessary form a frame for an L-shape, for example. The rigid frame structure  240 ,  242 ,  244  may comprise a material selected from the group consisting of silicon, silicon nitride, silicon oxide (glass), metals and alloys such as aluminum, titanium, steel, copper, gold, and plastics. 
     The compliant element  250  is more compliant than the rigid frame structure  240 . Examples of the compliant element material may be selected from the same group of materials listed for the rigid frame structure  240  above. In some embodiments, both the rigid frame structure  240  and the compliant element  250  may be of the same material, such as silicon. The difference in compliance may be achieved, for example, by a change in the physical dimensions, such as by making the compliant element  250  thinner than the rigid frame structure  240 . In some embodiments, the sensor  210 , the rigid frame structure  240 , and the compliant element  250  may be fabricated in the same process step out of the same layer/material. 
     The compliant element  250  is a suspension element and may be one of a crab-leg structure, straight beam, and a folded spring. The crab-leg suspension element  250  is depicted in  FIGS. 2A-C  and  3 A. 
     Examples of crab-leg compliant structures  250  are shown in  FIGS. 4A-C , but the claims of the present disclosure are not limited to the particular structures shown in  FIGS. 4A-C . Rather, the structures  250  are merely exemplary of the various crab-leg structures that may be employed in the practice of the embodiments disclosed herein. An “H” crab-leg structure  250  is shown in  FIG. 4A , while inverted “Y” crab-leg structures  250  are shown in  FIGS. 4B-C . The structure  250  shown in  FIG. 4B  is the same as depicted in  FIGS. 2A-C  and  3 A, but the present claims are not to be construed as limited to this particular structure. 
     Examples of folded spring compliant structures  250  are shown in  FIGS. 5A-F , but the claims of the present disclosure are not limited to the particular structures shown in  FIGS. 5A-F . Rather, the structures are merely exemplary of the various folded springs that may be employed in the practice of the embodiments disclosed herein. 
       FIG. 6  depicts a method  600  for reducing package stress sensitivity of a sensor  210 . The method  600  comprises providing  605  a substrate  230 . The method  600  further comprises providing  610  one or more anchor points  220  for attaching to the substrate  230 . The method  600  additionally comprises providing  615  a rigid frame structure  240  at least partially supporting the sensor  210 . The method still further comprises attaching  620  the rigid frame structure  240  to the anchor points  220  through corresponding compliant elements  250 . 
     Examples of the material for the compliant element  250  may be selected from the same group of materials listed for the rigid frame structure  240  above. Attachment of the rigid frame structure  240  to the sensor  210  or the compliant element  250  to the substrate  230  may be achieved by any of fusion bonding, eutectic bonding, plasma bonding, welding, and adhesive bonding, for example. In addition, the rigid frame structure  240 , the compliant element  250 , and the sensor  210  may be monolithically fabricated out of same material/layer. Such a monolithic process requires no attachment or bonding. 
     Fabrication of the rigid frame structure  240  and the compliant element  250  may be achieved by any of etching, patterning, embossing, and machining as a way to fabricate the frame and compliant elements, for example. 
     In summary, it should be appreciated that the MEMS sensor or non-MEMS sensor can be fully or partially attached onto a stress isolation structure. In various embodiments, the sensor can be a gyroscope, accelerometer, Lorentz force magnetometer or some other MEMS transducer or non-MEMS sensor. It should be appreciated that there can be one or more anchor points to the stress isolation structure. It should be appreciated that the compliant (e.g., suspension) element can be a crab-leg suspension or some other compliant structure. 
     Embodiments of the present invention use a rigid stress isolation frame and a compliant suspension built into the stress isolation frame, as opposed to attempting to build the compliance into the stress isolation frame itself (rigid stress isolation frame+compliant suspension vs compliant isolation frame+rigid suspension). 
     It is appreciated that, in the foregoing description, numerous specific details are set forth to provide a thorough understanding of the examples. However, it is appreciated that the examples may be practiced without limitation to these specific details. In other instances, well-known methods and structures may not be described in detail to avoid unnecessarily obscuring the description of the examples. Also; the examples may be used in combination with each other. 
     While a limited number of examples have been disclosed, it should be understood that there are numerous modifications and variations therefrom. Similar or equal elements in the Figures may be indicated using the same numeral.