Abstract:
A detection method and apparatus that includes a controller and a plurality of remote sensor units, each containing sensor elements, connected to the controller to achieve custom detection profiles and resolutions that are optimized for a given application by alteration of scanning sequences used by the controller, variation of scanning frequencies, adjustment in response times, and utilization of multi-modal sensing methods.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/639,768, filed Aug. 11, 2003, now U.S. Pat. No. 7,251,587, which application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 60/402,628, filed Aug. 12, 2002, these applications are incorporated herein by reference in their entireties. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention pertains to a detection system and method, and more particularly to a flexible platform for scanning and sensing and related method for identification of a target within a detection field. 
   2. Description of the Related Art 
   Photoelectric scanners and sensors are widely used in industrial applications for detection, measurement, identification and differentiation of objects. Sensors might for example incorporate a photoelectric transmitter and receiver that detects the presence or absence of varying amounts of light transmitted from the transmitter and received by the receiver as an indication of the presence or absence, type, or position of an object. Scanners might for example incorporate multiple transmitter and receiver channels, which are scanned one after another to determine the presence or absence of varying amounts of light transmitted from an individual transmitter and received by a corresponding receiver. 
   Industrial scanners are most commonly made in the form of two bars, with one bar containing receivers and another bar containing transmitters. In a typical application the bars are positioned in such a manner as to form a light curtain, consisting of a multitude of light beams between individual emitters and receivers. When an object to be detected blocks all or a portion of the light curtain, the scanner indicates the presence of such an object by asserting its output. Another common implementation of the scanner is in the form of a single bar where both the receiver and transmitter are integrated adjacent to each other within the same housing. In this case, the light curtain is formed with a retro-reflective tape mounted opposite of the scanner. Another common implementation of an industrial scanner is in the form of a fork or rectangle, with transmitters and receivers mounted opposite of each other to form a light curtain. One of the limitations of present scanner architectures is that the distance between the transducer elements within each bar is fixed, and consequently so is the dimension of the detection profile. To meet the demands of high resolution (close beams) and height of coverage (long scanners), a multitude of models must be manufactured. Furthermore, the user is not able to customize beam location and is therefor unable to achieve a desired mix of resolutions and detection profiles. These restrictions dramatically increase cost and drastically reduce overall penetration of the scanner technology into industrial sensing applications. 
   In many applications, scanners are required to perform an operation or suspend execution of an operation in response to external input. An example of such an operation is a measuring scanner, whose measurement timing is determined by the stand-alone sensor used as an event trigger. Practical implementation of this relatively simple application requires the use of a stand-alone scanner, a stand-alone controller, such as a Programmable Logic Controller, and a stand-alone sensor. The complexity and cost, as well as multiple potential failure points of such an installation, limits the use of such present scanners to areas where no alternatives are practical. Mechanical installation requirements of present scanners, which tend to be large and bulky, are incompatible with relatively small areas allocated to sensing equipment and therefor further limit penetration of the scanners into applications traditionally dominated by sensors. The performance of present scanners is highly limited by the sequential nature of their architecture, where each individual element is activated one after another until a complete scan is performed. Such architecture neither allows for any part of the scanner to function at a response time that is different from the remainder of the scanner nor does it allow for flexible assignment of scanning sequences. Although flexible scanning capability is highly desirable in more traditional scanning applications, especially as it relates to the combination of scanning and sensing, it is essential for sensing equipment that incorporates multiple mechanisms of sensing, such as photosensors, IR, RF ultrasound etc, where the difference in response times of the transducers is substantial. 
   BRIEF SUMMARY OF THE INVENTION 
   The disclosed embodiments of the present invention overcome many of the foregoing problems by providing a low cost Flexible Scanner and Sensor (FLEXI) architecture as will be described hereinafter. In accordance with one embodiment of the present invention, a flexible scanning and sensing platform is provided that includes a controller and a plurality of remote sensor units, each containing sensor elements, connected to the controller by means of cable, such as a twisted pair. The remote units can be operated in opposed or proximity mode, in either synchronous or asynchronous manner, and can be adapted to perform any type of sensing including photoelectric, inductive, capacitive, magnetic, IR, ultrasound, RF, temperature, pressure, PIR, etc. The remote units can be positioned to achieve custom detection profiles and resolutions that are optimized for a given application. Furthermore, custom profiles and resolutions can be achieved by alteration of scanning sequences used by the controller. Control of scan frequencies allows remote units in one area of the detection profile to operate at a different effective response time than remote units in other areas of the detection profile, thus providing for a degree of control required by multi-mode sensor and scanner applications. These and other advantages allow FLEXI to provide cost effective, enhanced performance solutions to the areas of industrial sensing and scanning that have not been achieved by present art sensors and scanners. 
   In accordance with one embodiment of the invention, a detection apparatus is provided that includes a plurality of remote sensing units, and a single controller coupled to the plurality of remote sensing units, the controller configured to activate each remote sensing unit in one of either a simultaneous mode and a predetermined activation sequence mode. 
   In accordance with another embodiment of the invention, a detection method is provided that includes providing multiple remote sensing units; and providing a controller coupled to the multiple remote sensing units and configuring the controller to independently control each remote sensing unit in accordance with multiple modalities and to process outputs independently from each of the remote sensing units. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing features and advantages of the invention will be more readily appreciated as the same become better understood from the following detailed description when taken in conjunction with accompanying drawings, wherein: 
       FIG. 1  is an overall block diagram of an embodiment of the flexible scanner and sensor platform of the present invention; 
       FIG. 2  is an exploded isometric projection of a mechanical implementation of the embodiment of the flexi scanner and sensor platform of  FIG. 1 ; 
       FIG. 3  is detailed diagram of the embodiment of a flexible scanner and sensor platform of  FIG. 1 ; 
       FIGS. 4A-4B  are schematics of embodiment of the remote receiver application specific integrated circuit of the present invention; 
       FIG. 5  is a detailed diagram of an alternative embodiment of the flexible scanner and sensor platform of the present invention; 
       FIG. 6  is detailed top-level schematic of an embodiment of the flexible scanner and sensor platform of the present invention; 
       FIG. 7  is detailed schematic of receive multiplexer of an embodiment of the flexible scanner and sensor platform of the present invention; 
       FIG. 8  is detailed top level schematic of transmit multiplexer and driver section of an embodiment of the flexible scanner and sensor platform of the present invention; 
       FIG. 9  is detailed schematic of an optical remote receiver unit and optical remote transmitter unit of an embodiment of the flexible scanner and sensor platform of the present invention; 
       FIG. 10  is detailed schematic of an output multiplexer and driver section of an embodiment of the flexible scanner and sensor platform of the present invention; 
       FIG. 11  is detailed top-level schematic of a microcontroller, A/D, power supply and RS232 interface of an embodiment of the flexible scanner and sensor platform of the present invention; 
       FIG. 12  is an isometric projection of the FLEXI configured as a scanner with adjustable detection height and adjustable resolution; 
       FIG. 13  is an isometric projection of the FLEXI configured as a scanner with custom scanning sequence and resulting crossbeam profile; 
       FIG. 14  is an isometric projection of the FLEXI configured as a combination of scanner and sensors; 
       FIG. 15  is an isometric projection of the FLEXI configured as a combination of scanner, stand-alone sensor and configuration of conveyer sensors; 
       FIGS. 16A-16F  are schematics illustrating examples of implementation and timing for direct visual identification of the target location within a beam pattern; 
       FIGS. 17A-17D  are timing diagrams and corresponding schematics of examples of sensitivity control implementation by means of transmit signal adjustment; 
       FIG. 18  is an overall block diagram of an embodiment of the flexible scanner and sensor platform of the present invention implementing RF remote units; 
       FIG. 19  is an overall block diagram of an embodiment of the flexible scanner and sensor platform of the present invention implementing ultrasonic remote units; 
       FIG. 20  is an overall block diagram of an embodiment of the flexible scanner and sensor platform of the present invention utilizing PIR remote units; 
       FIG. 21  is an overall block diagram of the embodiment of the flexible scanner and sensor platform of the present invention implementing capacitive remote sensing units; 
       FIG. 22  is an overall block diagram of the embodiment of the flexible scanner and sensor platform of the present invention implementing inductive remote sensing units; 
       FIG. 23  is a table of the sequence of operation of one embodiment of the present invention; and 
       FIG. 24  is a table defining a serial interface in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   While the disclosed embodiments of the invention are susceptible of implementation in many different forms, there is shown on the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. 
   Referring now to  FIGS. 1 and 2 , a Flexible Scanning and Sensing Platform (FLEXI) in accordance with one embodiment of the present invention is generally shown as  100 . The platform  100  includes remote receiver units  110  and remote transmitter units  120  in cooperation with a controller  130 . The remote receiver and transmitter units  110 ,  120  can, for example, be housed in separate packages, as would be the case for an opposed optical system, or they can be combined in the same housing, as would be the case for proximity inductive or proximity optical systems. The remote receiver units  110  are individually connected to the controller by means of a 3-wire cable  131 . The remote transmitter units  120  are individually connected to the controller  130  by means of a 2-wire cable  133 . The controller  130  accepts programming signals and provides status information via a serial interface  134  using the well-known RS232 protocol format. It should be apparent that although the RS232 format is used in this particular embodiment of the invention, any protocol used for serial or parallel communication would comfortably accomplish the required tasks. 
   The controller  130  can also receive inputs via a discrete input interface  135 , and such inputs can, for example, be used for the purpose of test, external interrupt, configuration selection, etc. Discrete outputs  140  are used by FLEXI  100  to control external devices in response to the commands from the controller. The function of the discrete outputs  140  can be defined via a serial interface, discrete input, or via controller default setting. The definitions of discrete outputs and inputs are determined by the programming of the controller  130 . 
   In this exemplary embodiment of the invention, RJ-45 interface connectors have been used to terminate two cables  131  and  133  into the controller  130 , and the RJ-11 interface connector has been used to terminate at the serial interface  134 . The choice of connectors should not be considered limiting since FLEXI&#39;s architecture contemplates operation with any of the standard industrial connectors, including direct terminal wiring. Referring to  FIG. 2 , FLEXI has been designed for effortless integration into an existing industrial environment, and the remote units  110 ,  120  can be positioned using standard off the shelf brackets or threaded directly into the target equipment. The controller  130  can be fitted with a standard DIN rail bracket for easy, trouble-free installation into any industrial facility. 
   Referring now to  FIG. 3 , the controller  130  is implemented by a receive multiplexer  137 , a transmit multiplexer  138 , transmit drivers  136 , an A/D converter  139 , an input interface  135 , and an output multiplexer and interface  140  in cooperation with a microcontroller  141 . The microcontroller  141  controls the sequence and timing of operation by means of a common control bus  160 . The microcontroller  141  may be a commercially available microcontroller, such as the PIC16F870. This microcontroller is RISC based microcontroller with on-board EEPROM from Microchip Technology, Inc. Other suitable microcontrollers can of course be used as will be known to those skilled in the art. The multiplexer  137  may be the commercially-available unit 74HC4051, and the multiplexer  138  may be the commercially-available unit 74HC259. The transmit driver  136  may be a plurality of commercially available transistors, such as the unit BCV47 and current setting resistors. The discrete input section may be implemented by commercially available transistors, such as the BCX70. The serial interface may be the commercially-available unit MAX221E RS232 controller from Maxim Integrated Products, Inc., and the discrete output section may be implemented by the commercially-available unit BCW 66h. The A/D converter may be the commercially-available LTC1196 converter from Linear Technology, Inc. Since the frequency of operation required by state-of-the-art photoelectric sensors is approaching 1 MHz, a fast A/D converter should be used to properly process a received signal. Referring to  FIG. 3 , an operating sequence is initiated by the microcontroller  141 , enabling single or multiple channels on the transmit multiplexer  138  as well as single or multiple channels on the receive multiplexer  137 . The drive transistors  136  associated with selected channels of the transmit multiplexer  138 , alone with input power, are connected to a remote unit  120   a  by a 2-wire transmit bus  133 , causing excitation current to flow through the transducer device in the remote unit  120   a , implemented as a Light Emitting Diode (LED)  170 . In some embodiments of present invention it might be advantageous to move the drive transistor  136  within the remote units  120 , and in that case the connection cable  133  would need to expand from 2 to 3 conductors to provide current return path for the drive circuit. The resulting LED light signal is received at the receiver  110   a , converted by the receiving transducer, implemented as a photodiode  180 , into a voltage that is conditioned by an application specific IC  181 , and coupled to the controller  130  via a 3-wire cable  132 , containing power, ground and the conditioned receive signal. The receiver  110   a  is implemented by photodiode  180 , and an application specific IC  181 , which provides the level of signal conditioning and miniaturization needed to meet dimensional requirements of the remote unit and noise requirements of industrial installations. 
   Referring now to  FIG. 4A , an application specific IC  181  contains a voltage reference AVREF for a DC level shift  182 , a transconductance amplifier combined with a bandpass filter stage  183 , and a voltage gain stage for additional amplification  184 . Referring again to  FIG. 3 , the conditioned signal from the 3-wire cable  132  is routed via a receive multiplexer  137  to an A/D converter  139 . The resulting digital signal is evaluated by the microcontroller  141  to determine the status of the channel. The A/D converter  139  under the control of the microcontroller  141  samples the received signal at optimum times as determined by the technology of the transducer and surrounding noise environment. 
   For example, in photoelectric applications the microcontroller  141  activates the remote receive unit  110  just prior to the remote unit  120  in order to allow the A/D converter  139  to sample the operating noise environment before an excitation signal is produced by the transmitter  120  and received by the receiver  110 . If the received signal is below a detection threshold, the microcontroller  141  may resolve that the channel is blocked and communicate such information via the RS232 interface  135  or assert an appropriate output  140  or both. The microcontroller  141  may process single or multiple activation events before concluding a detect or no-detect status of the channel, and such multiple events may be integrated or counted to avoid false detections. The operating sequence is repeated for all sixteen units. Alternatively, a single transmitter  120   a  can be activated for reception by multiple receivers, for example receivers  110   a - d , and the controller  130  selectively actives the receivers to establish a detection profile. More than one transmitter may be activated, sequentially or simultaneously to establish unique detection profiles as desired by the user or dictated by the application. 
     FIGS. 6 through 11  are detailed schematic drawings of the exemplary embodiment of this invention. The sequence of operation is detailed in Table  1  set forth in  FIG. 25 , and a definition of serial interface is provided in Table  2  set forth in  FIG. 24 . In another embodiment of this invention, as shown in  FIG. 5 , the A/D converter  139  can be eliminated if the output of the individual received remote units  110  is provided in the digital form. 
   Referring to  FIG. 4B , which shows a modified version of an application specific IC  181 , a voltage gain stage is followed by a comparator  185 , which compares the resulting amplified signal to a known threshold and generates a digital output. The remote receiver units  110  can of course be implemented in a discrete form, utilizing commercially available amplifier ICs and passive components, although such implementation would sacrifice miniaturization and noise immunity aspects of the remote units as presented in this exemplary embodiment. 
   Referring now to  FIG. 12 , FLEXI is arranged to form a light curtain scanner with adjustable detection zone height H 1  and resolution R 1  for remote units  110 / 120   a,b,c  and resolution R 2  for remote units  110 / 120   c,d,e,f . As illustrated in  FIG. 3 , an operating sequence is initiated by the microcontroller  141 , enabling a single channel on the transmit multiplexer  138 , corresponding to drive the transistor  136   a , and the remote transmit unit  120   a , as well as a single channel on the receive multiplexer  137  corresponding to the remote unit  110   a . Absent an object to be detected, the radiated signal produced by the remote unit  120   a  is received by the remote receive unit  110   a , and routed via a selected receive channel of the multiplexer  137  to the A/D converter  139 . The resulting digital signal is evaluated by the microcontroller  141  to determine the status of the channel. When the received signal magnitude is above a detection threshold, the detection zone between the receiver unit  110   a  and the remote unit  120   a  is considered to be clear. When the received signal magnitude is below a detection threshold, the detection zone between the remote units  110   a  and  120   a  is considered to be obstructed. The sequence is repeated for remaining remote units  110 / 120   b,c,d,e.    
   The response of the controller  130  to an obstruction is program dependent. For example, obstruction of any part of the scanner can result in assertion of a single output  140   a . The outputs can be programmed to indicate not only presence but also the size of the obstruction. For example, the output  140   b  could be asserted for the channel  110 / 120   e  obstruction, the output  140   c  for obstruction of channels  110 / 120   d,e , the output  140   d  for obstruction of channels  110 / 120   c,d,e  the output  140   e , for obstruction of channels  110 / 120   b,c,d,e  and the output  140   f  for obstruction of channels  110 / 120   a,b,c,d,e , and the output  140   g  for obstruction of channels  110 / 120   b,c,d,e,f . The outputs could, for example, then drive relays, which will route different sized packages to secondary conveyers. 
   Status of the remote units and outputs can also be made available via a serial interface. 
   FLEXI&#39;s ability to perform non-sequential application specific scanning sequences enables implementation of different detection profiles and resolutions without adjustment to the position of the remote units. 
   Referring now to  FIG. 13 , the scan sequence of the controller for the remote units  110 / 120   a,b  has been maintained as shown in  FIG. 12 , but the scan sequence for remote units  110 / 120   c,d,e,f  has been changed in such a manner as to effectively increase the resolution in the central area of the detection profile allowing for detection of thin wide objects. The sequence of the scan is as follows:
         enable remote transmit unit  120   f  and remote receive unit  110   e:      enable remote transmit unit  120   e  and remote receive unit  110   f , followed by  110   d;      enable remote transmit unit  120   d  and remote receive unit  110   e , followed by  110   c;      enable remote transmit unit  120   c  and remote receive unit enable remote channel  110 / 120   b ; and   enable remote channel  110 / 120   a.          

   The response of the controller  130  to obstruction is program dependent. For example, obstruction of the crossbeam detection area could result in the output  140   a ; whereas obstruction in all areas could result in the output  140   b.    
   Referring now to  FIG. 14 , the remote receiver unit  110   g  and the remote transmitter unit  120   g  are positioned in such a manner as to form an optical proximity sensor, which could for example be responsible for interrupting movement of the conveyer when the cart  201  is removed. Remote units that form the light curtain  110 / 120   a,b,c,d,e,f  are scanned by the controller  130  in the same manner as indicated in  FIG. 12  and described in the respective detailed description. Activation of the remote sensor channel  110 / 120   g  can be assigned by the programming of the controller  130  to any place in the scanning sequence. For example, the controller  130  can enable remote units  110 / 120   g  after remote units  110 / 120   f  and before remote units  110 / 120   a  are activated. When the cart  201  is present, the emitted LED signal produced by the  120   g  is reflected to the remote unit  110   g  and the resulting signal is routed via a selected receive channel of the multiplexer  137  to an A/D converter  139 . The resulting digital signal is evaluated by the microcontroller  141  to determine the status of the channel. When the received signal magnitude is above the detection threshold, the sensing channel  110 / 120   g  is considered to be obstructed, therefor cart  201  is considered present. When the resulting receive signal magnitude is below the detection threshold, the cart  201  is considered to be absent and the controller  130  could then assert the output  140   h , which can for example be used to interrupt movement of the conveyer. 
   Of course, remote receiver and transmitter units can also be located in the same housing to perform proximity or reflex type of sensing. Furthermore although the remote sensor channel  110 / 120   g  is described in terms of an optical sensor, ultrasound, inductive, capacitive, IR, PIR, RF, etc sensing methods can be employed instead. 
   The remote sensor channel formed by units  110 / 120   g  can be scanned by the controller at the same or at a different frequency than remote units  110 / 120   a,b,c,d,e,f . Since the movement of the cart is much slower than the movement of the packages on the conveyer, the controller  130  can, for example, be programmed to scan units  110 / 120   g  every 20 scans, thus maintaining a very fast response time required by the measuring light curtain generated by units  110 / 120   a,b,c,d,e,f  and yet providing a sufficiently fast response time to the sensor pair  110 / 120   g  to assure compliant operation. From the perspective of this exemplary application, the FLEXI architecture therefor provides for independent operation of a light curtain and a sensor. Of course, since a multitude of remote units can be integrated, much more complex applications can be addressed with a single FLEXI platform, as is demonstrated on  FIG. 15 . From the perspective of this application, the light curtain implemented by the units  110 / 120   a,b,c,d,e,f  and a cart sensor  110 / 120   g , and four-sensor conveyer  110 / 120   h,i,k,l  are independent entities, performing their functions independently of each other. Mutual interference, which is inherent within any sensor systems functioning in proximity of each other is completely avoided by the synchronous nature of FLEXI&#39;s operation. 
   Implementation of the application of  FIG. 15  using present devices would require 5 discrete sensors, a light curtain, and a programmable logic controller (PLC). The cost of such a solution would be significantly higher than that offered by the FLEXI. Furthermore, a PLC and light curtain would require a different set of programming tools and instructions. Sensors would have to be positioned sufficiently far away from each other and the light curtain to avoid mutual interference, thus eliminating a large number of potential installations. 
   Many variations for the use of the present platform are possible, for example. Consider  FIGS. 18-22  which briefly demonstrate a way in which Flexible scanner and sensor platform can be deployed. 
     FIG. 18  is an overall block diagram of an embodiment of the flexible scanner and sensor platform of the present invention implementing RF remote units. In this embodiment, a RF horn  320  or other antenna is driven at an RF frequency to provide a signal that is picked up by the RF detector  310  and measured under control of the controller  130 . 
     FIG. 19  is an overall block diagram of an embodiment of the flexible scanner and sensor platform of the present invention implementing ultrasonic remote units. In this embodiment, a ultrasonic horn  420  or other transducer is driven by the drive section  136 , and ultrasonic signals are received at the horn or other transducer the by remote unit  410  and measured under control of the controller  130 . 
     FIG. 20  is an overall block diagram of the embodiment of an flexible scanner and sensor platform of the present invention implementing PIR remote units. In this embodiment, IR radiation is picked up by the PIR detector  510  and measured under control of the controller  130 . 
     FIG. 21  is an overall block diagram of an embodiment of the flexible scanner and sensor platform of the present invention implementing a capacitive remote sensing unit. In this embodiment, the capacitor  620  is driven by a drive section  136 , while the receiver  610  measures a change in the capacitance under control of the controller  130 . 
     FIG. 22  is an overall block diagram of an embodiment of the flexible scanner and sensor platform of the present invention implementing an inductive remote unit. In this embodiment, the LC tank consists of an inductor  730  and a capacitor  720  and is driven by the drive section  136 . The receiver  710  measures the response of the LC tank to the drive signal from the drive section  136 . More particularly, the inductor  730  is driven by the drive section  136 , while the receiver  710  measures a change in the inductance in response to control signals from the controller  130 . 
   To facilitate direct visual identification of the target location within a beam pattern, the FLEXI scanner and sensor platform integrates a unique approach to target identification that can be extended to standard stand-alone sensors as well as fixed light curtains. In the context of the photoelectric scanner and sensor environment, dedicated LEDs are used to provide a visual indication of an obstructed channel. A method for direct visualization of the target location within a beam pattern will now be described. Referring now to  FIG. 3  and  FIG. 16 , in one embodiment of this invention the controller maintains two different brightness levels for the LED in the remote transmitter unit. The brightness level “Bright” will correspond to the unobstructed status of the channel, and a “Dim” level will correspond to an obstructed status of the channel. In one exemplary embodiment, the pulse width of the LED corresponding to the Bright state can be made wider than the pulse width of the LED corresponding to the Dim state. In another exemplary embodiment, the current passing through the LED in the Bright state can be set higher than the current passing through the LED in the Dim state. In another exemplary embodiment, the period of the LED can be set lower, resulting in higher effective LED current for the Bright state; and the period can be set higher to give a dimmer appearance to indicate Dim state. When remote units are placed adjacent to each other, as in the case of light curtain scanners, the actual location of the target within the LED beam pattern is visible as a shadow, providing an intuitive mechanism for visualizing the location of the object in a two-dimensional as well as a three dimensional detection field. 
     FIGS. 16A-16F  are schematics illustrating examples of implementation and timing for direct visual identification of the target location within a beam pattern. 
   Referring to  FIGS. 17A-17B , and  FIGS. 17C-17D , a timing diagram and a schematic of a photoelectric version, respectively, of FLEXI&#39;s sensitivity control is demonstrated. System sensitivity control is accomplished by means of a change in the transmitter pulse width and filter characteristic of the receiver. The transmitter pulse width T 2  is chosen in such a way as to correspond to the peak magnitude signal level X 2  at the receiver output. When the pulse width is reduced to T 1 , the low pass characteristic of the filter produces signal X 1  at the output of the receiver. The pulse width of the transmitter is controlled by the controller and can be varied with great accuracy, providing an effective and novel mechanism for sensitivity and hysteresis control at the receiver, by controlling time domain behavior of the transmitter. 
   The sensitivity control provided by the combination of the pulse width adjustment and the threshold setting for the A/D output provides FLEXI with dynamic range necessary to perform not only digital detect or no-detect measurements, but also relative analog measurements required by applications where the contrast between detect and no-detect is so low that absolute measurement is not possible, such as in the case of transparent material detection. 
   While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims and the equivalents thereof.