Abstract:
A fiberoptic transceiver system for use in an industrial paint spraying apparatus where analog information from flow sensors in the flow meter on the spray head is converted to a single channel digital signal for transmission on a fiberoptic cable to a receiver for decoding into an analog signal representing flow rate and a signal indicating flow direction. The encoding of two signals onto one channel is accomplished by encoding the flow rate as the frequency of the digital signal and the direction as the pulse width. The direction of the flow is determined by comparing the two analog signals from the flow meter and determining which signal is lagging the other.

Description:
FIELD OF THE INVENTION 
     The invention described herein relates to the field of flow meters, and more particularly to flow meters used in industrial paint spraying apparatus. More particularly, it relates to a method and apparatus for communicating two channels of flow meter information from the spray head along a single optical fiber cable from the metering apparatus mounted on the spray head to a unit located remote from the spray head, which may then provide two channels of output for interfacing with a system controller for controlling the industrial spraying apparatus. 
     BACKGROUND OF THE INVENTION 
     In industrial spraying apparatus, it is desirous to have sensors mounted in the spray head as opposed to elsewhere in the dispensing circuit, in order to accurately measure the rate of flow of the fluid being dispensed from the spray head. 
     The known industrial painting systems use electrostatic means to charge the paint to reduce waste and improve coverage during the painting operation. The historical use of solvent based paints, which are non-conductive, allowed the use of a conventional wired transmitter on the flow meter in the spray head. Due to environmental and other concerns, the paint industry is moving toward the use of water based paints, which are inherently conductive. With water based paints the flow meter can no longer be grounded as in the prior art spray apparatus, as this would short-circuit the electrostatic charging voltage. The use of a fiber-optic interface between the flow meter and the other electronics of the dispensing system allows the meter to be electrically “floated” while the fiber optic cable isolates the receiver and control electronics from any voltage that may be present on the flow meter due to its location on the spray head. This system although initially designed for industrial spray painting systems could have other applications where dielectric isolation of the spray head and meter and intrinsic safety are important. 
     Presently, there are two systems on the market that use a fiber optic cable to transmit flow rate information from a flow meter on a spray head to a system controller. Among other differences, neither system allows for transmission of quadrature signals or two channels of information on one fiber optic channel and neither provides flow direction information, which allows for detection of a leaking or malfunctioning valve in the industrial spraying system. 
     SUMMARY OF THE INVENTION 
     Broady described, the invention herein is a method and apparatus for sensing the rate and direction of a moving body by generating two signals, converting rate and direction information into a single signal capable of being transmitted along a single communications channel, and reconstructing said rate and direction information after transmission. 
     In the particular application described in the preferred embodiment, the invention described herein is a method and apparatus for transmitting flow rate and direction information from a flow meter to a system controller by converting analog signals into a digital signal transmitted by pulses of light through a single optical fiber to a receiver, where the information is regenerated into conventional two channel quadrature signals suitable for input into a system controller. The apparatus, having both a transmitter section and a receiver section, may be referred to as a transceiver. The apparatus is specifically designed for industrial spray painting systems, but may be applied to other applications. 
     In general, the spraying apparatus employs a spray head integrating a flow meter or with a flow meter attached thereto. The flow meter includes at least two sensors that generate quadrature signals which are fed to a transmitter for transmission along a single communication channel to a receiver unit which receives and processes the signal for use by a system controller that controls the amount of fluid sprayed from the spray head. The flow meter is associated with a transmitter to encode the signals received from the sensors into light pulses. The communication channel used herein is a fiber optic cable which transmits the light pulses to the receiving unit which then converts the digital light pulses into electromagnetic signals usable by the controller electronics. The pulses of light encode the quadrature signals from the sensors so that the light pulse frequency represents the rotation rate of the gears as sensed by the sensors, and the pulse width represents the direction of rotation of the meter gears or flow through the meter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the main components of the fiber optic transceiver system. 
     FIG. 2 is a perspective view of the electronics module. 
     FIG. 3 is a perspective view of the electronics module mounted on the flow meter. 
     FIG. 4 is a schematic showing the sensors, transmitter, power supply and associated circuitry. 
     FIG. 5 is a block diagram showing the fiberoptic transceiver in functional blocks. 
     FIG. 6 is a schematic diagram showing the receiver, microcontroller and associated circuitry. 
     FIG. 7 is a representation of signals available at the pseudo quadrature outputs when the outputs are configured for 5 volt logic output. 
     FIG. 8 is a representation of signals available at the pseudo quadrature outputs when the outputs are configured for +Vin logic. 
     FIG. 9 is a representation of signals available at the pseudo quadrature outputs when the outputs are configured for an open collector output. 
     FIG. 10 is a representation of signals available at the pseudo quadrature outputs when the outputs are configured for an isolated output. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, flow meter  10  includes an input port  8 , by which fluid may flow into the flow meter  10  and flow thru the flow meter  10  in a flow path  9 . The fluid moving along flow path  9  in the flow meter  10  at the spray head turns metallic spur gears  12   a  and  12   b  of the flow meter  10  before being expelled at a spray nozzle  13 . One familiar with flow meters will recognize that the diagram does not show the components in detail, and will further recognize that many types of flow meters may be used, such as piston meters, oval gear meters, and other types where speed and direction of rotation is sensed by electromagnetic sensors. A first sensor  16  and a second sensor  17  are positioned in proximity to the teeth of the spur gear  12   b , and as the teeth rotate, the sensors  16  and  17  experience a changing magnetic field. The sensors  16  and  17  convert this information into an analog electrical signal. Two sensors are used in order to determine the direction of the flow of the fluid. The sensors  16  and  17  are positioned about the spur gear  12   b  so that the signal generated by one sensor is 90 degrees out of phase with respect to the signal generated by the other. Thus, while the first sensor  16  generates a peak signal, the second sensor  17  generates a rising signal. The situation is reversed if the flow is reversed, thus allowing determination of direction. It is not necessary that the sensors read the same spur gear, so long as the signals generated are out of phase with each other, preferably by 90 degrees. Thus, the first sensor  16 , may be placed to read spur gear  12   a  and the second sensor  17  placed to read spur gear  12   b.    
     The signals from the sensors  16  and  17  are conveyed along wires  18  and  19  to an electronics module  20  to produce digital signals for transmission to a receiver unit  30 . The electronics module  20  includes circuits to encode flow rate and direction information into a digital signal and includes a transmitter section to pulse an LED, whereby the signal is transmitted along a single fiber optic cable  22 . The information is encoded so that the frequency of the light pulses are proportional to the flow rate through flow meter  10 , and the width of the individual pulses indicates direction of flow through the flow meter  10 . It is preferred that the fiber optic cable be of a plastic fiber as opposed to glass, as a plastic fiber allows for a smaller bending radius and does not break as easily. A fiber optic cable  22  having a bend radius of at least 6 inches is preferred. One such cable is a {fraction (980/1000)} um plastic fiber encased in ½ inch outside diameter polyethylene pneumatic tubing. A glass cable may be used, particularly when the signal is in the infrared band. As fiber optic cables carry both visible light and infrared, the LED may be one that produces visible light or infrared. Therefore, as “light pulse”  13  used here, it should be understood to refer to both visible light and infrared portions of the electromagnetic spectrum. It is preferred that the LED emit light having a wave length of 660 nanometers. Additionally, with the teachings for the transceiver described herein, one skilled in the art would recognize that a transmitter and receiver pair, properly constructed, could use any portion of the electromagnetic spectrum, provided an appropriate transmitter, channel, and receiver, could be provided. For instance if a transmitter and receiver pair constructed to transmit and receive in the radio band were provided, a shielded cable could be used to carry the digital signal between the transmitter and receiver. This however, is not the preferred method, as radio interference from other machines in the industrial setting, could affect the quality of the signal being transmitted despite the present of a shielded cable. 
     As shown in FIGS. 2 and 3, the electronics module  20  which transmits the flow rate signal may be located on the flow meter  10  and can be removably attached to thereto, allowing for easy replacement of the electronics module. The electronic module  20  can include the sensors  16  and  17 . As shown in FIG. 2, the sensors can be mounted on members which distend from the main housing of the electronics module  20 . The distending members fit into recesses in the flow meter  10  and allow the sensors  16  and  17  to be in close proximity to the spur gears. 
     Because the electronics module  20  is potted and electrically isolated, it utilizes a non-replaceable battery as a power source, creating a concern regarding power consumption. To reduce power consumption, the pulses of the LED are very narrow. A typical duty cycle is 1:10 4  to 10 6 . Pulse width is used to encode direction, and the more narrow pulse width encodes a forward direction, which is more common, whereas a wider pulse width encodes the reverse direction. It is preferred that the narrow pulse width is approximately 0.5 ms, and the wider pulse width is approximately 5.0 ms. Since direction and rate are encoded by one LED, a more power consuming embodiment using two LED&#39;s, one each for a direction signal and a rate signal, is avoided. 
     With reference to FIG. 1, the pulse signal carried by the fiber optic cable  22  is then received at a receiver unit  30  which decodes the information into a pseudo quadrature output to interface to the system controller  31  or other control equipment. The words “pseudo quadrature” are used here to describe the signal because of its short duty cycle. The receiver unit  30  preferably uses a microcontroller to accomplish the conversion, as set forth herein. The pseudo quadrature output signals are carried to the system controller  31  by a shielded bundle of wires  32 . 
     With reference to FIG. 4, the electronics housed in the electronics module  20  receive signals from the first sensor  16  and second sensor  17 , which detect the movement of the flow meter spur gears  12   a  or  12   b . The reluctance sensors  16  and  17  output a sine wave voltage that is proportional to the velocity of the target metal spur gear as it passes the respective sensor. In the preferred embodiment, the frequency of the signal generated by the sensors  16  and  17  is in the range of 1.2 Hz to 475 Hz. The specific range will vary depending upon the flow meter and sensors used. The voltage is typically in the range of millivolts to tens of volts, but can vary with the type of sensor chosen. One skilled in the art will recognize that other types of sensors, generating other types of signals, such as digital signals, may be used, and the down stream electronics adjusted accordingly to use such signals, consistent with the teachings of the invention. 
     As shown in FIG. 4 the analog electromagnetic signals produced are conveyed electronically to inputs of a transmitter  40  in the electronics module  20 . For the input from each sensor, diodes  51   a-d  and  52   a-d  form a redundant voltage clamp to limit the output voltage from the respective sensor. This is advantageous as it makes the design intrinsically safe and protects the input of op-amps  55  and  56  from damage. One skilled in the art will recognize that the components need not be physically located as described here. For instance, the diodes  51   a-d  and  52   a-d  forming the voltage clamp may be located near the sensors  16  and  17 , and not on a circuit board holding the transmitter  40  or encoding circuitry. In other words, including the voltage clamping electronics as part of a transmitter is a matter of convenience and not a requirement. The voltage clamping electronics could just as easily be included as part of the sensors  16  and  17  and located there upon or in close proximity thereto. 
     A signal conditioning section of the transmitter  40  is present for each sensor  16  and  17 . With respect to each sensor  16  and  17 , the signal conditioning section includes op-apms  55  and  56  and NAND gates  61  and  62  and associated components. Op-amps  55  and  56  and associated components form a comparator with hysteresis. The arrangement of components shown in FIG. 4 provides symmetrical hysteresis while employing only a single supply voltage. With respect to op-amp  55 , resistors  81  and  87  bias one side of the sensor network and op-amp input at the midpoint of the DC voltage supply. The sinusoidal input signal from the respective sensor  16  then causes the other op-amp input to cross the threshold set by the bias network. Resistors  82  and  86  and the intrinsic resistance of the sensor control the amount of positive feedback and hence the amount of hysteresis. Resistor  82  is purposely chosen to be large to prevent its current from significantly influencing the  81  and  87  biasing network voltage. With respect to op-amp  56 , resistors  91  and  97  perform the biasing, and resistors  92  and  96  provide the control of the positive feedback. 
     The op-amps  55  and  56  are preferably micro power devices such as model OP281 made by PMI and as such are low frequency devices with weak output stages compared to a traditional type of op-amp. This causes the output to exhibit relatively long rise and fall times that are unsuitable for driving further logic. NAND gates  61  and  62  are preferably Schmitt trigger devices that transform the slew rate limited op-amp outputs into sharp signals that can drive further logic. One skilled in the art will recognize that there are other components that can achieve the same or similar results. 
     NAND gates  101  and  102  and associated components form a monostable multivibrator that sets the output pulse width which will cause LED  110  to light. The input to this monostable vibrator comes from the signal conditioning circuit associated with one of the sensors  16  or  17 . As shown in FIG. 4, the input comes from NAND gate  61  associated with the signal conditioning circuit of the first sensor  16 . The output pulse width is determined by the delay caused by the network including resistor  115  and the capacitance from NAND gate  61  to ground. This capacitance includes capacitor  117  which is always in the circuit and capacitor  119  which is switched by the direction flip-flop  120 . Resistor  122  prevents leakage through MOSFET  125  from inadvertently charging capacitor  119  when MOSFET  125  is off. 
     The outputs of both signal conditioning sections are fed into flip-flop  120  which acts as direction detector. When the spur gears  12   a  and  12   b  of the flow meter are rotating in one direction, the D input of the flip-flop  120  is always low at the rising edge of the signal delivered to the clock input of the flip-flop  120  from the signal conditioning circuit of the second sensor  17 . This causes the flip-flop  120  to output a logic low signal at its output Q. When the spur gears  12   a  and  12   b  are rotating the other direction, the D input of flip-flop  120  is always high on the rising edge of the signal delivered to the clock input, causing the flip-flop  120  to output a logic high. The flip-flop  120  inverted output is used to drive MOSFET  125 , which switches the pulse width generator between different time constants, thereby generating different pulse widths used to communicate the direction information. 
     The LED  110  is driven by PNP transistor  130  from the output of the monostable pulse width generator. While the LED  110  may emit in any part of the spectrum, it is preferred that the LED  110  emit in the visible band. Resistor  132  limits the current through the LED  110 . The LED  110  is placed in proximity to the fiber optic cable so that the light pulses generated by the LED  110  may be transmitted along the fiber optic cable to a receiver unit. 
     The components of the transmitter  40  are powered by a power supply isolated from the receiver unit  30 . It is preferable that the power supply is a non-replaceable battery  41 , as the electronics of the transmitter  40  are potted. 
     With reference to FIG. 5, the receiver unit  30  has two sub-assemblies. The first is a shielded receiver module  290  that connects directly to the plastic fiber optic cable  22  and receives the signal transmitted along the fiber optic cable  22 . The second sub-assembly is the microcontroller module  295 . The micro controller subassembly  295  includes circuitry for processing the received signal into pseudo quadrature signals and includes outputs for sending pseudo quadrature digital signals  297  to the system controller. 
     As shown in FIG. 6, the circuitry inside the receiver module  290  includes a photosensor  150 , such as a model PDF120F2, electrically connected to the signal input of a receiver chip  155 , such as an LT1328, designed as an off-the-shelf solution for infrared data communication between computers and peripherals at up to 4 Mbps. In the present application, this high band width is advantageous in permitting the reception and detection of the narrow pulses transmitted over the fiber optic cable. A capacitor  157  sets the frequency of the high pass filter within the receiver chip  155 . The frequency is chosen to minimize any low frequency interference, either electrical or optical, while minimizing pulse width distortion of the intended signal. An inductor  158  and capacitors  159  and  160  form a filter to prevent any noise on the power supply  170  from coupling into the receiver chip  155  and generating a spurious output pulse. 
     The microcontroller sub-assembly takes the signal generated by the receiver module and processes it to generate pseudo quadrature output. A microcontroller chip  180  and associated components comprise the microcontroller sub-assembly that provides the function of converting the pulse signal of the receiver chip  155 . It is preferred that the microcontroller chip  180  is model MSP430P112 manufactured by Texas Instrument, although other microcontrollers may also be used. The pulse conversion functionality desired is contained within the program that continually executes within the microcontroller chip  180 . The microprocessor chip  180  provides 4 outputs  190   a-d . Output  190   a  drives a green LED  195  to indicate forward flow at the flow meter, and output  190   b  drives a red LED  196  to indicate reverse flow. Outputs  190   c-d  provide the quadrature phase information which is sent to additional circuitry to produce final output signals at quadrature phase outputs  220   a-b  and  221   a-b.    
     Optoisolators  200  and  201  provide isolation of the microcontroller chip  180  from the quadrature phase outputs  220   a-b  and  221   a-b . The phase outputs  220   a-b  and  221   a-b  are configurable in 4 different output configurations as set by jumpers  226   a-c  and  227   a-c . The microcontroller sub assembly can provide signals on the quadrature phase outputs  220   a-b  and  221   a-b  in any one of four different formats, to accommodate a wide variety of system controllers which may be connected to the quadrature phase outputs  220   a-b  and  221   a-b.    
     In a 5 volt logic output configuration, the output is a 5 volt logic signal regardless of the input supply voltage to the receiver unit  30 . Representative output signals on the pseudo quadrature outputs  220   a-b  and  221   a-b  for a 5 volt logic signal showing a forward flow at the flow meter are as shown in FIG.  7 . Reference ground for the signals is provided by the power supply  170  ground. 
     In a +Vin logic configuration the output is a logic signal that tracks the input supply voltage to the receiver unit  30 . Representative output signals on the pseudo quadrature outputs  220   a-b  and  221   a-b  for a Vin logic signal showing a reverse flow at the flow meter are shown in FIG.  8 . Reference ground for the signals is provided by the power supply  170  ground. 
     In an open collector output configuration, the receiver unit provides access to the collector of the output transistors  228  and  229 . The controller system must provide a current limited voltage source (+Voc) to the quadrature phase outputs  220   a-b . Representative output signals on the pseudo quadrature outputs  220   a-b  and  221   a-b  for the open collector configuration showing a reverse flow at the flow meter are shown in FIG.  9 . In this configuration, outputs  220   b  and  221   b  are grounded at the receiver ground and outputs  220   a  and  221   a  output the signal. 
     In an isolated output configuration, the quadrature phase outputs  220   a-b  and  221   a-b  are galvanically isolated from the receiver unit  30  power supply and the rest of the receiver circuitry. In this configuration the quadrature phase outputs behave like switches except that the current will only flow one way when the switch is closed. The source of the current must be current limited and supplied to the receiver unit  30  from the system controller. Representative output signals on the pseudo quadrature outputs  220   a-b  and  221   a-b  for forward flow at the flow meter are shown in FIG.  10 . Outputs  220   a  and  221   a  should be pulled to a higher voltage than  220   b  and  221   b  in order for current to flow. 
     With reference to FIG. 6, resistors  230  and  231  are provided to improve the output waveforms by speeding up the turn off of the transistors  228  and  229  in the optoisolators  200  and  201 . Capacitors  234  and  235  are provided to reduce the effect of crosstalk in the output cable between the two phases of the quadrature signal. 
     The power supply  170  for the receiver module consists of a voltage regulator  240 , such as model MC78L05ACD and associated components. Capacitors  242  and  243  provide the recommended decoupling for regulator stability. Capacitors  244  and  245  act as bypass capacitors for the receiver chip  155  and the micro-controller  180 . Diode  245  provides reverse polarity protection. 
     The general operation of the fiber optic pulse transceiver may be described in block diagram form with reference to FIG.  5 . The first sensor  16  and second sensor  17  generate out of phase electromagnetic signals. The signals are fed into an electronics module  20  which contains circuitry for signal conditioning  270 , circuitry for a monostable vibrator  280 , circuitry for direction detection  285 , and a LED  110  for generating a light pulse. The electromagnetic signals generated by each of the first sensor  16  and second sensor  17  are each fed into a signal conditioner  270   a  or  270   b  containing appropriate circuitry. The output of a first signal conditioner  270   a  is fed to the monostable vibrator  280 , which includes circuitry for pulse generation. The signal from the first signal conditioner  270   a  is also first fed to a direction detector  275  containing appropriate circuitry. The signal from the second signal conditioner  270   b  is also fed to the direction detector  285 . An output signal from the direction detector  285  is fed to the monostable vibrator  280 , whose output is then fed to the LED  110  to produce the light pulses of appropriate frequency and width to correspond to the rate and direction of rotation sensed by the first and second sensor  16  and  17 . The light pulse travels down a fiber optic cable  22  where it is received by a receiver unit  30 . As shown in FIG. 5, the receiver unit  30  contains, in general, two sub-assemblies a receiver module  290 , and a microcontroller sub-assembly  295 . The receiver module  290  contains appropriate circuitry to receive the light pulse and converts the light signal back into an electric signal. The microcontroller sub-assembly  295  contains appropriate electronics to decode the electric signal fed from the receiver module  290  and output two quadrature signals  297 . Those signals may then be fed to further control electronics such as a system controller which may regulate and control the flow of liquid through the spray head. 
     The specific components used and described herein are meant only as examples, and are not meant to limit the scope of the claimed invention. One skilled in the art will recognize that other components may be used to construct the apparatus and carry out the method consistent with the teachings herein.