Abstract:
An electrical current is passed through a thermistor to raise its temperature above the temperature of oil flowing in pulses past the thermistor. A change is measured in the temperature of the thermistor occurring with respect to one or more of the pulses. A level of oil flow is determined corresponding to the measured change in temperature. A signal is issued based on the determined flow level.

Description:
[0001]    This invention relates to oil flow sensing. In a two-cycle outboard marine engine, for example, lubricating oil flows in pulses, rather than continuously. The oil may be pumped by a solenoid that is triggered by an electrical signal from an engine control module (ECM). The pulses occur at a rate that depends on engine speed and may be as high as 4 Hz or higher or as low as 0.007 Hz with a pulse duration of about 40 milliseconds. One way to make sure that oil is always reaching the parts of the engine that need lubrication is to include level switches in an oil reservoir. When the level of the oil falls unacceptably low, an alarm can be triggered or the engine can be stopped.  
         SUMMARY  
         [0002]    In general, in one aspect, the invention features a method that includes (a) passing an electrical current through a thermistor to raise its temperature above the temperature of oil flowing in pulses past the thermistor, (b) measuring a change in temperature of the thermistor occurring with respect to one or more of the pulses, (c) determining a level of oil flow corresponding to the measured change in temperature, and (d) issuing a signal based on the determined flow level.  
           [0003]    Implementations of the invention may include one or more of the following features. Measuring the change in temperature includes measuring a change in voltage across the thermistor over a period of time. The period of time corresponds to different portions of at least one of the pulses. The period of time begins at the start of one of the pulses and ends no later than the start of the next one of the pulses. The thermistor is housed in a package having an area that yields an oil flow of 10 to 20 inches per second. The area is in the range of 0.0005 to 0.002 square inches. The oil is flowing in a 2-cycle marine engine. A signal indicative of the timing of the pulses is received from an electronic control module of the engine. The signal based on the determined flow level is sent to an electronic control module of the engine. The rate of pulses is as high as 5 Hz The rate of pulses is as low as 1 pulse per day.  
           [0004]    In general, in another aspect, the invention features an apparatus that includes a coupling having (a) two open ends adapted for connection to upstream and downstream tubes of a pulsating oil circulation system of an engine and (b) a channel configured to direct the oil to flow past a thermistor connected to a sensing circuit. The sensing circuit includes elements connected to determine a change in a voltage across the thermistor and to compare the change to a threshold.  
           [0005]    Implementations of the invention may include one or more of the following features. The sensing circuit includes a sample-and-hold circuit connected to store a voltage across the thermistor. The sensing circuit includes a delay circuit connected to provide timing signals for the period over which the change in voltage is determined. In some implementations, the sensing circuit is a microcontroller that includes an analog-to-digital converter. Ports are connected to carry timing and flow-state signals between the sensing circuit and a control circuit of the engine.  
           [0006]    In general, in another aspect, the invention features a marine engine that includes (a) moving parts arranged to be lubricated by oil delivered through a supply line from a supply of oil, (b) a pump configured to pump oil from the supply to the moving parts in pulses controlled by a controller, and (c) a sensor connected to receive pulses of the oil and to detect the oil flow state of the engine using a temperature sensitive electronic element and a circuit that analyzes an electrical parameter of the temperature sensitive element at times based on the pulses of the oil.  
           [0007]    Implementations of the invention may include one or more of the following features. The temperature sensitive electronic element includes a thermistor. The circuit includes an electrical interface to a controller that controls the timing of the pulses.  
           [0008]    Among the advantages of implementations of the invention, the response time of the sensing circuit is short, only a single thermistor is required (because the flow is pulsating), there are no moving parts, and the device is insensitive to mounting orientation and vibration.  
           [0009]    Other advantages and features will become apparent from the following description and from the claims.  
       
    
    
     DESCRIPTION  
       [0010]    (FIG. 1 is a block diagram of a marine engine.  
         [0011]    [0011]FIG. 2 is a side view in section of a flow sensor.  
         [0012]    [0012]FIGS. 3 and 4 are a functional view and a schematic view of a flow circuit.  
         [0013]    [0013]FIG. 5 is a timing diagram.  
         [0014]    [0014]FIG. 6 is a three-dimensional view of a flow sensor.  
         [0015]    [0015]FIG. 7 is a diagram of a microcontroller.) 
     
    
       [0016]    As shown in the specific example of FIG. 1, a 2-cycle outboard marine engine  10  includes lubricated parts  12  that are lubricated from an oil supply  14  using a solenoid pump  16 . The solenoid is triggered in to initiate a series of pumping cycles by a pump signal  18  generated by an ECM  20 . A flow sensor  22  in the oil line  23  between the pump and the lubricated parts detects whether the flow is adequate and sends a yes-or-no flow signal  24  to the ECM to indicate whether the flow is adequate or not. The flow sensor receives the pump signal  18  for use in a manner described below.  
         [0017]    The flow sensor is housed in a coupling  30  that can be inserted into the oil line. The coupling includes an inflow tube  32 , an outflow tube  34 , and a central channel  36  that directs the flow of oil  38  so that it passes across the surface of a small chip thermistor  40  mounted on a circuit board  42 .  
         [0018]    As shown functionally in FIG. 3 and schematically in FIG. 4, a circuit arranged on the circuit board  42  includes the thermistor (sensor)  40  and other elements that use the thermistor as the core element sensing and reporting oil flow.  
         [0019]    In operation, the thermistor is self heated by a current that is driven through the thermistor. The current produces a voltage drop  53  across the thermistor that depends on the resistance of the thermistor. The flowing oil cools the sensor by an amount that depends on the mass flow rate of the oil. As the sensor is cooled, its resistance changes (increases or decreases depending on whether the thermistor has a negative or positive temperature coefficient) and in turn so does the voltage drop across the thermistor for a given driving current. The thermistor  14  may be a small (e.g., 0.04 inches by 0.02 inches in an 0402 package type) negative temperature coefficient (NTC) thermistor chip (for example, part number ERT-J0EA101J available from Panasonic). The thermal capacitance of the small thermistor is low enough (in the neighborhood of 0.2 mJ/K to 1 mJ/K [millijoules per degree Kelvin]) to permit a rapid response to changes in oil flow, rapid enough to accommodate the highest expected frequency of oil pulsation. The thermistor is designed to have an area that yields an oil flow of 10 to 20 inches per second. The area may be in the range of 0.0005 to 0.002 square inches.  
         [0020]    As shown in the timing diagram of FIG. 5, a sample-and-hold circuit  50  samples and holds the voltage across the thermistor  40  beginning at the start of each triggering of the oil pump. The timing of sampling is controlled by a timing circuit  52  driven by an open collector input  54  from the ECM.  
         [0021]    An instrumentation amplifier  62  continually monitors the voltage on line  60  and combines it with the held voltage  64  from the sample-and-hold circuit. A comparator and reference circuit continually compares the combined signal  68  with a reference value  72  (indicative of an adequate level of oil flow) and delivers the result of the comparison (“yes” or “no”) to an output latch  74 .  
         [0022]    At a timed interval  58  (FIG. 5) after the oil is pulsed (for example, 100 to 200 mSec, determined by a delay timer  68 , a delay signal  80  is sent to cause the yes-or-no flow signal  24  to be latched to a fault output  76  for use by the ECM.  
         [0023]    Turning to the details shown in FIG. 4, a 5-volt power supply  90  converts a supplied 8-36 volt dc input  92  to a 5-volt dc output  94  for the analog and digital circuitry. The input uses diodes for battery polarity protection. The resistor R 7  supplies a bias current to zener diode  96 , which regulates the 5-volt output. The NPN transistor  98  isolates the input voltage and the 5-volt output voltage. The capacitors provide noise reduction.  
         [0024]    The 50 milliamp current source  100  supplies a bias current to the thermistor using a three terminal voltage regulator  102  configured as a current source. The resistor is used to set the current level. The input to the current source is the 8-36 volts dc and is input polarity protected.  
         [0025]    The 50-milliampere current source causes the thermistor to self-heat to 65 degrees C. when the ambient temperature is 25 degrees C. The thermistor has a negative temperature coefficient, which results in a low resistance (approximately 22 ohms in air and 32 ohms in oil at 25 degrees C. ambient) when self-heated.  
         [0026]    When oil at a temperature lower than the self-heated temperature of the thermistor flows over the thermistor&#39;s surface, the thermistor cools, increasing its resistance. For a given bias current, this produces a larger voltage drop across the thermistor.  
         [0027]    The sample-and-hold circuit  60  continuously monitors the thermistor voltage  53 . The timing circuit  52  sends a signal on line  110  to the sample-and-hold circuit each time the pump is actuated to cause the sample-and-hold circuit to store the thermistor voltage.  
         [0028]    Transistor Q 1  in the sample-and-hold circuit is turned off during oil flow to hold the thermistor voltage at the level that existed when the oil pumping pulse began. This voltage is stored in capacitor C 3  until the transistor Q 1  is turned on again at the end of the measurement cycle. Resistor R 13  is used to dampen the in-rush current into capacitor C 3  at storage time. Operational amplifier U 6 -B is a unity gain follower that isolates the thermistor from the sampling circuit. Operational amplifier U 6 -C configured as a unity gain follower isolates the sampling circuit from the amplification stage.  
         [0029]    The amplifier stage  62  continually monitors the thermistor voltage P 7  and the sample-and-hold output P 6 , taking the difference of these two signals and amplifying them. The resultant output is passed on to the comparator circuit.  
         [0030]    Operational amplifier U 6 -A is configured as a unity gain follower that isolates the thermistor from the amplification stage. Operational amplifier U 6 -D is configured as a difference amplifier with a gain based on the threshold level. The resistors R 1 , R 2 , R 3 , and R 5  set up the gain for the amplification stage.  
         [0031]    The comparator/reference circuit  70  sets up the threshold level for the comparator to determine good or bad (“yes” or “no”) oil flow. The threshold level is determined by the diode voltage drop (e.g., about 0.7VDC).  
         [0032]    The comparator U 2  compares the reference level to the output of the amplifier and converts the analog signal to a digital (0 volts or 5 volt) signal which is passed on to the latch circuitry  74 . The diode D 1  sets the reference level by the 0.7 volt drop across the forward biased diode) and is biased by resistor R 11 . The output signal that is passed to the latch circuitry represents an indication of whether the oil flow is adequate or not.  
         [0033]    The latch/output circuit latches the output of the comparator circuit, and supplies signal  24  back to the ECM.  
         [0034]    D-Latch U 4 -A takes the signal from the comparator on P 4  and latches it into the output  76  when a latch pulse is presented on P 3 . The output of the latch is fed through the resistor RI  4  into the base of the NPN transistor Q 2 . The open collector of Q 2  is fed back to the engine ECM as a digital signal representing good or bad oil flow.  
         [0035]    The delay timer  52  takes the pump trigger signal  54  from the ECM and sets up a hold pulse  110 . The hold pulse causes the sample-and-hold circuit to capture and hold the thermistor voltage for the duration of the measurement cycle. The delay timer also uses the pump signal to set up a delay pulse to latch in the result of the measurement cycle.  
         [0036]    NAND gate U 5 -A &amp; U 5 -B along with resistor R 16  and Capacitor C 6  set up the trigger pulse for the latch circuit. NAND gate U 5 -C &amp; U 5 -D along with resistor R 17  and capacitor C 2  set up the hold pulse for the sample-and-hold circuit.  
         [0037]    Referring again to FIG. 3, a temperature compensation block may be provided in some contexts if needed to assure consistency of performance in the face of changes in ambient temperature.  
         [0038]    As shown in FIG. 6, the printed circuit board and thermistor are potted in a housing  120  that also includes an electrical coupling  122  that permits connection to the ECM for carrying the signals described earlier. A rubber grommet  124  provides for vibration isolation in mounting the device to an engine block.  
         [0039]    Other implementations are within the scope of the following claims. For example, the sensor is useful in applications other than 2-cycle outboard marine engines, including other applications in which oil is delivered to parts to be lubricating by a pulsating oil pump. The housing of the unit can have a variety of shapes, forms, and sizes, which enable the oil to flow past and cool the thermistor. Other circuit techniques can be used to set up the measurement cycle to synchronize with the ECM signals.  
         [0040]    The sensor may be useful with other non-conductive or high-resistance fluids in addition to oil.  
         [0041]    As shown in FIG. 7, the discrete circuitry of FIG. 4 could be replaced by a microcontroller  150  having an onboard analog-to-digital converter  152  to perform the electrical sensing and signal processing functions. For this purpose, the microcontroller would be connected to a power supply, the pump signal input, the sensor input, and the fault output.