Abstract:
A method for monitoring the state of health (SOH) of a solenoid powered by a battery includes measuring a voltage and a current supplied to the solenoid by the battery, using a processor to determine each of an equivalent resistance and inductance of the solenoid using the voltage and the current, comparing the equivalent resistance and the equivalent inductance to a corresponding calibrated threshold, and recording deviations from the corresponding calibrated thresholds as a pair of SOH values. A trend of the SOH values is continuously monitored, and an appropriate control action is taken when either SOH value drops below a calibrated lower limit. A solenoid monitoring system includes a solenoid, voltage and current sensors, and a controller having an algorithm for continuously monitoring a state of health (SOH) of the solenoid as set forth above.

Description:
TECHNICAL FIELD 
     The present invention relates generally to electro-mechanical solenoid devices, and more particularly to a method and an apparatus for continuously monitoring the ongoing state of health of a solenoid device. 
     BACKGROUND OF THE INVENTION 
     Solenoid devices or solenoids are linear actuator devices having a host of useful applications. For example, solenoids typically serve as a reliable type of on/off switch for precise operational control of various devices, e.g., electrical motors, valves, assembly robots, etc. Solenoids are ordinarily configured as electromechanical devices, although hydraulic and pneumatic variants exist that provide similar utility using different motive forces. A typical solenoid of the electromagnetic variety includes one or more coils of conductive wire surrounding a moveable piston portion or plunger, all of which is positioned within a solid ferromagnetic core. That is, a single-coil solenoid can be used in conjunction with a return spring, while a dual-coil solenoid can include each of a pull-in coil and a hold-in coil, with each coil dedicated to the specified function. 
     As with an electric motor, the passage of an electrical current though the solenoid coil induces a magnetic field around the coil. Selective application of the magnetic field thus moves the plunger in a particular and controllable manner, either by pushing or pulling the coil in a desired direction. That is, the induced magnetic field either attracts or repels the plunger, which is ordinarily constructed of iron or steel to facilitate this response. When the electrical current supplied to the solenoid is terminated, the induced magnetic field likewise terminates, thus allowing a return spring to move the plunger back to its original or de-energized position. 
     While the operational diagnosis of a solenoid can be provided using various means, such as by detecting the rate of a rise and fall in a measured solenoid current to determine if the solenoid is presently operating within specification, conventional methods can require the commitment of substantial computational resources, and/or the use of complex waveform or pattern recognition techniques. Additionally, such methods can be relatively expensive to implement due to the need for analog circuitry dedicated to the detection of electrical current transitions. Moreover, a determination of whether or not a particular solenoid is presently performing to specification does not provide a prognostic or predictive capability, and therefore can be less than optimal when used to predict remaining life of a solenoid. Such predictive value can be particularly useful when used in certain applications, such as but not limited to the monitoring of a solenoid used for controlling a vehicular starter motor. 
     SUMMARY OF THE INVENTION 
     Accordingly, a method is provided for monitoring the ongoing state of health or SOH of an electromechanical solenoid device or solenoid, i.e., a solenoid powered via electrical current supplied by a battery or other suitable supply of electrical energy. Execution of the method, which can be embodied as a computer-executable algorithm as explained hereinbelow, thus enables at least some degree of estimation of remaining life of the solenoid, thus predicting a failure point well in advance of the actual occurrence of such an event. For example, in an exemplary solenoid used in conjunction with a vehicular starter motor, the predictive value provided by the method can enhance the perceived reliability of the vehicle by minimizing instances of walk-home situations in which an unexpected solenoid failure is the root cause. 
     In particular, the method includes measuring, sensing, or otherwise determining a voltage and current supplied to the solenoid, determining by calculation or estimation a total or equivalent resistance and inductance of the solenoid, as that term will be understood by those of ordinary skill in the art, using the voltage and current, and recording deviations of the equivalent resistance and inductance from a calibrated value or threshold as a pair of SOH values, i.e., an SOH resistance value and an SOH inductance value. The method continuously monitors the trend in the SOH values, and executes a suitable control action when either SOH value drops below a calibrated lower limit. Optionally, an SOH value for an opposing or back electromotive force (EMF) of the solenoid can also be calculated and used in a similar manner, as explained below. 
     Additionally, a solenoid monitoring system includes the solenoid described above, a current sensor, a voltage sensor, and a computational device or computer, referred to herein as a controller, that is in communication with the sensors. The controller includes an algorithm for continuously monitoring the SOH of the solenoid device by determining the equivalent resistance and inductance, calculating SOH factors for the resistance and inductance, and optionally the back-EMF, and executing a control action in a particular manner when the values drop below a minimum threshold. The system can also estimate the back-EMF of a plunger of the solenoid device using a predetermined parameter estimation technique as set forth herein. 
     Within the scope of the invention, the total equivalent resistance and inductance of the solenoid device can be determined via calculation or estimation depending on the particular design or configuration of the solenoid device. That is, in a solenoid device that does not move until an exponential solenoid current reaches a peak or a maximum before the plunger begins to move, a first method can be used to calculate the equivalent resistance and inductance, while a second method involving parameter estimation can be used in a solenoid that begins to move before the solenoid current reaches such a peak. The back-EMF of the solenoid device can be estimated via a predetermined parameter estimation technique, e.g., regression analysis, least squares, maximum likelihood, etc., and compared to a back-EMF threshold in order to determine any potential performance issues or degradation of the plunger. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a solenoid monitoring system operable for monitoring the state of health (SOH) of an electro-mechanical solenoid device; 
         FIG. 2  is a graphical illustration of a set of performance curves for an exemplary solenoid usable within the monitoring system of  FIG. 1 ; 
         FIG. 3A  is a schematic electrical circuit diagram for a starting circuit having an exemplary dual-coil solenoid; 
         FIG. 3B  is an equivalent circuit diagram for the starting circuit of  FIG. 3A  in a first solenoid operating region; 
         FIG. 3C  is an equivalent circuit diagram for the starting circuit of  FIG. 3A  in a second solenoid operating region; and 
         FIG. 4  is a graphical flow chart describing an algorithm suitable for executing the method of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, and beginning with  FIG. 1 , a solenoid monitoring system  10  includes a solenoid device or a solenoid (S)  18  connected to an electric motor (M)  22  or other solenoid-controllable device, e.g., a vehicular starter motor, a motorized valve, a robot, etc. having an output shaft or member  23 . Motion of the member  23  can be harnessed as needed to perform any of a variety of useful work. The solenoid  18  is of the electro-mechanical type, and therefore includes one or more wire coils  20  surrounding a moveable piston or plunger  28 , with the motion of plunger  28  indicated in  FIG. 1  by the arrows A and B. The solenoid  18  can be configured as either a single coil or a dual-coil solenoid as described above without departing from the intended scope of the invention. 
     The system  10  includes an electronic control unit or controller (C)  50  and a pair of sensors  16 A,  16 B, with the sensors  16 A,  16 B being adapted for measuring, sensing, detecting, or otherwise determining or collecting a predetermined set of electrical values, and for relaying the values to the controller  50 . The solenoid  18  can be electrically connected to a battery (B)  12  or other suitable electrical energy supply, and is in wireless or hardwired communication with the sensors  16 A,  16 B to receive values corresponding to the battery voltage (V B ) and the solenoid current (I S ), respectively. A temperature sensor  16 C can also be provided for use in adjusting certain measurements as set forth below. 
     Depending on the particular system in which the system  10  is used, the battery  12  can also be electrically connected to one or more accessories (A)  21 . For example, if the system  10  is used in conjunction with a vehicle (not shown), the accessories  21  can include a radio, interior or exterior lights, seat warmers or positioning devices, etc. The solenoid  18  can also serve as a starter solenoid or electromagnetic switch for control of a starter motor. If one or more accessories  21  are used, the electrical current (I S ) supplied to the solenoid  18  can be determined or calculated by the controller  50 , such as by subtracting the known or estimated accessory current (I A ) from the known or measured battery current (I B ). Likewise, if no such accessories  21  are used, the solenoid current (I S ) is equal to the battery current (I B ). 
     Still referring to  FIG. 1 , the controller  50  includes one or more microprocessors or central processing units (CPU)  60  and sufficient computer-accessible memory  70 . Such memory  70  can include, for example, read only memory (ROM), random access memory (RAM), electrically-programmable read only memory (EPROM), etc., of a size and speed sufficient for executing the method or algorithm  100  as set forth below with reference to  FIG. 4 . The controller  50  can also be provided with other required hardware, such as a high speed clock, requisite analog to digital (A/D) and digital to analog (D/A) circuitry, any necessary input/output circuitry and devices (I/O), as well as appropriate signal conditioning and/or buffer circuitry. Any algorithms resident in the controller  50  or accessible thereby, including the algorithm  100  of the invention as described below, can be stored in memory  70  and automatically executed to provide the respective functionality. 
     The controller  50  can be electrically connected to an audio/visual indicator  80  and/or a display  84 , with the display  84  being adapted for displaying information or a text message  82 . For example, the indicator  80  can be a warning lamp, the activation of which can sound an audible tone or alarm alone or in conjunction with illumination of the indicator  80 . Likewise, the display  84  can be a display portion of a control panel or a marquee when used, for example, on a manufacturing floor, or as a portion of an instrument panel, center console, rear view mirror assembly, etc. (not shown) of a vehicle when used aboard a vehicle, such as in the example of a solenoid  18  used for controlling a motor  22  configured as a vehicular starter motor. 
     Referring to  FIG. 2 , a performance curve  30  for the solenoid  18  shown in  FIG. 1  includes three solenoid operating regions I, II, and III, with the Y-axis denoting the solenoid current (I S ), and with the X-axis denoting time (t). With respect to region I, the curve segment  32  in this region describes the exponential rise in solenoid current (I S ) from t=0 until a peak or maximum solenoid current (I S, MAX ) is reached at point D, or at t 1 . Within the segment  32 , the point C corresponds to the exponential time constant (τ), as that term will be understood in the art. That is, in region I the solenoid current (I S ) increases exponentially, and is dictated by the equivalent resistance (R EQ ) and the equivalent inductance (L EQ ) of the solenoid  18 . Therefore, the solenoid current (I S ) at point C is approximately 63% of the maximum, i.e., I S, MAX /e. 
     With respect to region II, this region begins at point D and describes the duration or interval during which the plunger  28  of the solenoid  18  (see  FIG. 1 ) begins to move in response to an induced magnetic field, as set forth above. In region II, the segment  34  between points D and E represents a region in which solenoid inductance increases, thus resulting in a decrease in solenoid current (I S ). In other words, an opposing electro-motive force, referred to hereinafter as the back-EMF, is created as the plunger  28  moves in response to the induced magnetic field. 
     Finally, region III or segment  36  commences at point E, i.e., the point in time at which the motor  22  starts. For example, at point E the solenoid  18  configured as a switch can be fully actuated, thus allowing the battery  12  of  FIG. 1  to energize the motor  22 . 
     Referring to  FIG. 3A , an exemplary electrical circuit  25  is shown for an exemplary starting circuit using the solenoid  18  of  FIG. 1 . The circuit  25  represents the battery  12  via the battery voltage (V B ) and the battery resistance (R B ). The solenoid  18  in this example is a dual-coil solenoid as described above, and therefore includes a hold-in coil  20 A and a pull-in coil  20 B, as those terms are understood in the art. The hold-in coil  20 A and the pull-in coil  20 B can be represented by each of an inductance (L H , L P , respectively) and a resistance (R H  and R P , respectively). Likewise, the motor  22  of FIG.  1 , or more precisely the windings thereof, can be represented by an inductance (L A ) and a resistance (R A ). 
     Referring to  FIG. 3B , the circuit  25  of  FIG. 3A  can be reduced in region I of  FIG. 2  to the equivalent circuit  25 A. That is, within region I, the battery voltage (V B ) can be determined using the equation V B =(I S )(R EQ )+(L EQ )(ΔI S /Δt). The rate of change of the solenoid current (I S ) can be modeled and plotted as: I S (t)=(V B /R EQ )(1−e −t/τ ). The time constant (τ) for the solenoid current to rise to the level of (1/e) of the maximum or peak current (I S, MAX ) can thus be measured, and the equivalent inductance (L EQ ) can be calculated as L EQ =τ*R EQ . 
     As will be understood by those of ordinary skill in the art, the total or equivalent resistance and inductance of any electrical device can be calculated using basic circuit analysis equations. For example, the respective resistance and inductance values of resistors and inductors in series can be added to determine the equivalent resistance and inductance, while (N) resistors or inductors in parallel can be calculated via the equation 1/R EQ =1/R 1 +1/R 2 + . . . 1/R N  and 1/L EQ =1/L 1 +1/L 2 + . . . 1/L N . 
     Also as will be understood by those of ordinary skill in the art, solenoid designs can vary, with some types of solenoids reaching a maximum current or steady peak before the plunger  28  begins to move, and other types having a plunger  28  that moves well in advance of reaching such a peak. In the first type of solenoid, i.e., a type reaching a peak or maximum current (I S, MAX ) before motion of the plunger  28 , the rate of change of the solenoid current (I S ) is zero at the peak of point D, and at this point D the above equation reduces to: V B =(I S )(R EQ ,) i.e., with R EQ =V B /I S  at the peak of point D. 
     In the second type of solenoid, the equivalent resistance (R EQ ), and the equivalent inductance (L EQ ) at point D can be estimated using a predetermined parameter estimation technique, for example regression analysis, linear least squares, polynomial least squares, recursive least squares, etc. That is, using the linear form y=ax+b, with y=ΔI S /Δt, x=I S , a=−R EQ /L EQ , and b=1/L EQ , it follows that ΔI S /Δt=(−R EQ /L EQ )(I S )+1/L EQ . Thus, using regressive least squares (RLS) techniques, R EQ =−a/b and L EQ =1/b. Regardless of the particular type of solenoid  18  used in the system  10  of  FIG. 1 , the performance of the solenoid  18  in region I can be modeled. 
     Referring to  FIG. 3C , an equivalent circuit  25 B is shown for region II of  FIG. 2 . In region II, the battery voltage (V B ) can be determined by the equation: V B =(I S )(R EQ )+(L EQ )(dI S /dt)+E(x g ), with the variable E(x g ) being equal to the back-EMF of the solenoid  18  as a function of travel of the plunger  28 . The equivalent resistance (R EQ ) from region I can be used, and E(x g ) and L EQ  can be estimated using a predetermined parameter estimation technique as set forth above. 
     Referring to  FIG. 4 , and with particular reference to the various elements of the system  10  shown in  FIG. 1 , the method of the invention is executable by the controller  50  using the algorithm  100 . Beginning at step  102 , the values of the battery voltage (V B ) and the solenoid current (I S ) are measured, detected, or otherwise acquired, such as by using the sensors  16 A,  16 B as explained above. If an optional accessory  21  draws power from the battery  12  along with the solenoid  18 , this value is first considered when determining the solenoid current (I S ), and prior to energizing the solenoid  18 . For example, the actual or estimated current draw of the accessory  21  or multiple accessories  21  is first subtracted from a measured or known battery current (I B ) to determine the solenoid current (I S ) before proceeding to step  104 . 
     At step  104 , the algorithm  100  can check a set of predetermined conditions and, using these conditions, can determine whether it is appropriate to proceed. For example, the algorithm  100  can sense or detect a “power on” or start signal for starting the motor  22  or other connected device, or can determine if the motor  22  or other device connected to the solenoid  18  is operating as expected, such as by referencing diagnostic codes (not shown) in the controller  50 , by conducting a test of the motor  22  or other such components, etc. If the predetermined conditions are met at step  104 , the algorithm  100  proceeds to step  106 . Otherwise, the algorithm  100  is finished. 
     At step  106 , the equivalent resistance (R EQ ) and the equivalent inductance (L EQ ) of the solenoid  18  are determined using any appropriate means. For a solenoid of the first type explained above, i.e., a solenoid reaching a peak or maximum solenoid current before motion of the plunger begins, the algorithm  100  can calculate the equivalent resistance (R EQ ) and the equivalent inductance (L EQ ) using the peak current (I S, MAX ) and the time constant τ. For a solenoid of the second type, i.e., a solenoid that does not reach such a peak or maximum solenoid current before motion of the plunger begins, the algorithm  100  can use the parameter estimation techniques set forth previously hereinabove in order to estimate the values of the equivalent resistance (R EQ ) and the equivalent inductance (L EQ ). However these values are ultimately determined, the algorithm  100  proceeds to step  108  once such a determination is made. 
     At step  108 , the algorithm  100  can obtain nominal or calibrated values for resistance (R CAL ) and the inductance (L CAL ), i.e., associated resistance and inductance values previously determined and recorded as reference values for a known “good” solenoid. These calibrated values can be adjusted as needed for the present temperature of the solenoid  18 , which can be readily determined using a temperature sensor  16 C as shown in  FIG. 1 . Likewise, the values for R EQ  and L EQ  can be adjusted for temperature to match the temperature at which the calibrated values were originally determined. The calibrated values for the resistance (R CAL ) and the inductance (L CAL ) can be stored temporarily in a memory location resident in or accessible by the controller  50 , after which the algorithm  100  proceeds to step  110 . 
     At step  110 , using the calibrated values from step  108 , i.e., R CAL  and L CAL , a state of health of the solenoid  18 , hereinafter referred to as the SOH factor, is calculated for the solenoid  18  for each of the resistance and the inductance values. In particular, an SOH factor for the resistance, or SOH R , can be calculated using the equation:
 
SOH R =1−(Δ R/R   EQ )
 
with ΔR defining the absolute value of the deviation of the equivalent value of the resistance, i.e., R EQ , determined at step  106  above, from the calibrated or threshold resistance value (R CAL ), i.e., ΔR=|R EQ −R CAL |. The same calculation is performed to determine an SOH factor for the inductance, or SOH L .
 
     The comparative values can be used to further isolate the root cause of failure within the solenoid  18 . For example, when the equivalent resistance value, or R EQ , exceeds that of the corresponding calibrated threshold (R CAL ) by a predetermined margin, determined during calibration and therefore usable as a threshold value, the result can indicate an open circuit in the solenoid  18 . Likewise, if the equivalent resistance value (R EQ ) is less than that of the corresponding calibrated threshold (R CAL ) by a predetermined margin, also determined during calibration and therefore usable as another threshold or lower limit value, the result can indicate a shorted winding in the solenoid  18 . If a dual-coil solenoid is used, similar comparisons can be used to determine whether a hold-in coil or a pull-in coil is open. The values of each SOH factor, i.e., SOH R  and SOH L , can be plotted or otherwise recorded in an accessible manner within memory of the controller  50 , after which the algorithm  100  proceeds to step  112 . 
     At step  112 , the values of the SOH factors are compared, and the minimum value is selected. If the minimum of the two SOH factors SOH R  and SOH L  is less than a predetermined or calibrated threshold, the algorithm  100  proceeds to step  114 . Otherwise, the algorithm  100  proceeds to step  116 . 
     At step  114 , the controller  50  determines that the winding  20  of the solenoid  18  is likely faulty, and executes a control action, such as by setting an appropriate flag or recording a suitable value indicating such a prognosis, or alternately by activating one or both of the indicator  80  and display  84 . The algorithm  100  is then finished. 
     At step  116 , and during region II, the algorithm  100  uses the equivalent resistance value (R EQ ) determined at step  106 , and estimates the back-EMF or E(x g ) using a suitable parameter estimation technique as set forth above, e.g., regressive least squares (RLS) in an exemplary embodiment. The algorithm  100  then proceeds to step  118 . 
     At step  118 , the algorithm  100  determines if the back-EMF, or E(x g ), exceeds a calibrated threshold. Alternately or concurrently, step  118  can also include executing a similar state of health (SOH) determination as explained above at steps  110  and  112 . That is, after the back-EMF is estimated or otherwise determined at step  116 , a calibrated back-EMF value, or E CAL , can be referenced in memory, and a deviation or ΔE value can be determined as ΔE=|E(x g )−E CAL |. If so, the algorithm  100  proceeds to step  120 . Otherwise, the algorithm  100  is finished. 
     At step  120 , the controller  50  determines that the plunger  28  is likely faulty, and sets an appropriate flag or records a suitable value indicating such a prognosis. The controller  50  can alternately or concurrently activate the indicator  80  and/or display  84  as set forth above. The algorithm  100  is then finished. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.