WO2001006162A2

WO2001006162A2 - Method and apparatus for measuring the position of a solenoid valve stem

Info

Publication number: WO2001006162A2
Application number: PCT/US2000/040367
Authority: WO
Inventors: Michael L. Rhodes; Arthur Noel John Pearman; James E. Lenz; William A. Harris
Original assignee: Parker Hannifin Corporation
Priority date: 1999-07-16
Filing date: 2000-07-12
Publication date: 2001-01-25
Also published as: WO2001006162A3; AU7134400A

Abstract

A solenoid valve includes a solenoid coil and a valve stem positionable with respect to the solenoid coil. A transient response produced by the solenoid coil upon varying an energization state of the coil is measured to analyze various characteristics of the solenoid valve. In one embodiment, an excitation pulse is applied to the solenoid coil and is preferably of a duration sufficiently short so as to not actuate movement of a solenoid valve stem. A transient response of the solenoid coil is monitored following application of the excitation pulse to measure a rise time, decay time and/or transition time of the current of voltage across the solenoid coil. The rise time, decay time, and/or transition time of the current or voltage across the solenoid coil varies as a function of an inductance of the solenoid coil. The inductance of the solenoid coil, in turn, varies as a function of a position of the valve stem with respect to the solenoid coil. Accordingly, a position of the valve stem is determined based on measuring a transient response of the solenoid coil to the excitation pulse.

Description

Title: METHOD AND APPARATUS FOR MEASURING THE POSITION OF A SOLENOID VALVE STEM

Technical Field The present invention relates generally to a solenoid valve, and more particularly to a diagnostic circuit for analyzing characteristics of the solenoid valve including a position of the valve stem and an energization state of the solenoid coil.

Background of the Invention

Solenoid valves are widely used in a variety of fields including aerospace, automobile, industrial controls, automation, power plants, and the like. A typical solenoid valve includes a moveable valve stem surrounded by a coil. The valve stem is also referred to as an armature or plunger. Upon energization of the coil, the valve stem is caused to either extend from, or retract into, the solenoid valve. Upon de-energization of the coil, the valve stem is caused to move in a direction opposite to the direction of movement during the energization state. A solenoid driver coupled to the coil controls the energization state of the coil and thereby controls the movement of the valve stem.

In certain applications, it may be beneficial to know the position of the valve stem at a given time. For example, real time information related to the solenoid valve stem position can be useful in situations involving precise flow control needs such as in fuel injection systems for aerospace applications. While the solenoid driver is effective to cause the valve stem to move between an open and closed position, it is oftentimes beneficial to have an independent measure of the valve stem position. Such information can help to detect valve failure, provide feedback control, etc. Known methods for determining the position of a solenoid valve stem include the use of devices which are directly coupled to the valve stem. For example, it is known to couple devices such as position sensors, micro switches and/or optical sensors onto the valve stem and use such devices to monitor physical movement in order to determine positioning. While such devices are useful in determining the position of a solenoid valve stem, there are significant disadvantages to their use. For example, embedding such sensing devices onto the valve stem involves redesigning the valve stem in order to accommodate the additional components. Additionally, difficulties are encountered with respect to solenoid valve stems that are designed to be used with corrosive materials, since devices attached to such valve stems would also then need to be resistant to corrosion. Accordingly, there has been a trend toward finding alternative methods for determining solenoid valve stem position which do not involve direct coupling of devices to the valve stems.

U.S. Pat. No. 3,789,876 ("the '876 patent") provides a valve stem position detection system which does not involve the coupling of devices to the valve stem itself. Rather, the '876 patent utilizes a power source to excite an inductance bridge formed from opposing ends of the solenoid coil. A phase comparator evaluates a difference in phase between the bridge output and a reference to determine the position of the valve stem. While providing accurate measurements, the '876 patent involves the inclusion of an extra power source to determine valve stem position.

U.S. Pat. No. 4,950,985 ("the '985 patent") also provides a method of determining a position of a solenoid valve stem which does not involve the coupling of devices to the valve stem itself. According to the '985 patent, a resonant circuit is formed in parallel with a supply line feeding a solenoid coil. As the valve stem moves, an inductance of the coil varies which in turn varies the resonant frequency of the parallel resonant circuit. In order to measure a position of the valve stem, an exciting frequency is introduced to the resonant circuit and the resonant frequency of the parallel resonant circuit is then measured. Unfortunately, the inclusion of the parallel resonant circuit in the '985 patent adds a layer of complexity to the overall solenoid design.

U.S. Pat. No. 4,809,742 ("the '742 patent") provides a valve stem position sensor which utilizes an electrical coil having reactance that varies with incident magnetic flux. The incident magnetic flux, in turn, varies with the position of the valve stem. Unfortunately, the electric coil used to sense the valve stem position is in addition to the solenoid coil already made part of the solenoid valve design and thus involves added hardware complexities to the solenoid valve design.

U.S. Pat. No. 5,583,434 ("the '434 patent") provides a method of determining a position of a solenoid valve stem by exciting a solenoid coil with a continuous A.C. signal. An impedance and resistance of the solenoid coil can be measured to determine information indicative of valve stem position. Accordingly, the '434 patent necessitates the use of an A.C. source and requires calculating impedance based on measured voltage and current.

Accordingly, there exists a strong need in the art for a simple, cost effective manner in which to measure a position of a solenoid valve stem which overcomes the above referenced drawbacks and others.

Summary of the Invention

A solenoid valve includes a solenoid coil and a valve stem positionable with respect to the solenoid coil. An electronic circuit coupled to the solenoid valve analyzes characteristics of the solenoid valve including a position of the valve stem and whether the solenoid coil is properly energized.

In order to determine a position of the valve stem, an energization state of the solenoid coil is varied in order to obtain a transient response.

Preferably, the energization state is varied by application of an excitation pulse thereto, the excitation pulse having a pulse width sufficiently short so as to not initiate actual movement of a solenoid valve stem itself. The transient electrical response of the solenoid coil is monitored following application of the excitation pulse by measuring a growth time and/or decay time of the current flowing through the solenoid coil. The growth and/or decay time of the current through the solenoid coil varies as a function of a position of the valve stem with respect to the solenoid coil.

For example, in a first state in which the valve stem is retracted into the solenoid coil, the inductance of the solenoid coil is at its highest level thereby resulting in the longest rise and/or decay time. Comparatively, when the valve stem is in a second such that the valve stem is fully extended from the solenoid coil, the inductance of the solenoid coil is at its lowest level thereby resulting in the shortest rise and/or decay time. Accordingly, by measuring the transient response of the solenoid coil in response to a change in energization state, the present invention is able to accurately calculate a position of the valve stem at any given time in a simple and reliable manner.

The present invention also uses the transient response of the solenoid coil to determine whether the solenoid coil is properly energized. For example, upon varying the energization state of the solenoid valve, the transient response of the solenoid coil prior to reaching a steady state is measured. Next, an expected transient response of the solenoid coil for a given position of the valve stem is compared with the actual transient response measured following the varying of states. Based on this comparison, a determination can be made as to whether the solenoid coil is properly energized.

According to one aspect of the present invention, a solenoid valve is provided. The solenoid valve includes a solenoid coil, a valve stem at least partially disposed within the solenoid coil, and a diagnostic circuit coupled to the solenoid coil. The diagnostic circuit includes circuitry for changing an energization level of the solenoid coil, circuitry for monitoring a transient response of the solenoid coil following a change in the energization level, and circuitry for determining a position of the valve stem based on a characteristic of the transient response.

In accordance with another aspect of the present invention, a diagnostic circuit for a solenoid valve including a solenoid coil and a valve stem at least partially disposed within the solenoid coil is provided. The diagnostic circuit includes a pulse generator for applying an excitation pulse to the solenoid coil, a comparator coupled to the solenoid coil for comparing a transient response produced by the solenoid coil upon application of the excitation pulse to a reference value, and a microcontroller coupled to the comparator for measuring at least one of a rise time, a decay time, and transition time associated with the transient response reaching the reference value.

In accordance with yet another aspect of the present invention, a solenoid valve is provided. The solenoid valve includes a solenoid coil, a valve stem at least partially disposed within the solenoid coil, and a diagnostic circuit coupled to the solenoid coil. The diagnostic circuit includes a means for changing an energization level of the solenoid coil, a means for monitoring a transient response produced by the solenoid coil upon changing the energization level, and a means for determining a position of the valve stem based on a characteristic of the transient response.

In accordance with still another aspect of the present invention, a diagnostic method for use with a solenoid valve comprising a valve stem and a solenoid coil is provided. The method includes the steps of changing an energization level of the solenoid coil, monitoring a transient response produced by the solenoid coil upon changing the energization level, and determining a position of the valve stem based on a characteristic of the transient response.

To the accomplishment of the foregoing and related ends, the invention then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such embodiments and their equivalents. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

Brief Description of the Drawings

In the annexed drawings:

Fig. 1 is a diagrammatic view of a solenoid valve in accordance with the present invention;

Fig. 2 is a schematic view of a diagnostic circuit in accordance with the present invention; Figs. 3(a)-3(c) represent in relevant part the equivalent circuit of the diagnostic circuit of Fig. 2 during a de-energized steady state condition of the solenoid coil, and corresponding graphs of the control voltage V_cont and coil current l_C0l| in accordance with the present invention;

Figs. 4(a)-4(c) represent in relevant part the equivalent circuit of the diagnostic circuit of Fig. 2 during an energized steady state condition of the solenoid coil, and corresponding graphs of the control voltage V_cont and coil current l_C0l| in accordance with the present invention;

Figs. 5(a)-5(d) represent in relevant part the equivalent circuit of the diagnostic circuit of Fig. 2 immediately following a transition from an energized state to a de-energized state of the solenoid coil, and corresponding graphs of the control voltage V_cont, coil current l_C0l, and voltage V_ιn in accordance with the present invention;

Figs. 6(a)-6(d) represent in relevant part the equivalent circuit of the diagnostic circuit of Fig. 2 immediately following a transition from a de- energized state to an energized state of the solenoid coil, and corresponding graphs of the control voltage V_cont, coil current I_C01, and voltage V_bιas in accordance with the present invention;

Fig. 7(a) graphically represents a control pulse used to excite a solenoid coil in accordance with the present invention;

Fig. 7(b) graphically represents a voltage V_bιas indicative of the transient response curves corresponding to a solenoid coil having a valve stem in an open and closed position, respectively, in accordance with the present invention;

Fig. 7(c) graphically represents an output waveform V_out used to calculate a transient response rise and decay time in accordance with the present invention; Fig. 8 is a lookup table depicting a correlation between a characteristic of the transient response and a position of the valve stem in accordance with the present invention; and

Fig. 9 is a flowchart depicting steps involved with analyzing characteristic of the solenoid valve in accordance with the present invention.

Detailed Description of the Invention

The present invention will now be described with respect to the accompanying drawings in which like numbered elements represent like parts.

Referring initially to Fig. 1 , an exemplary solenoid valve and control system 10 suitable for use in accordance with the present invention is depicted. The system 1 0 includes a DC solenoid valve 1 1 , an operating circuit 1 2, a remote valve control unit 1 3, and a DC power source 1 4. As will be described in more detail below, the operating circuit 1 2 is directly coupled with the solenoid valve 1 1 to control the solenoid valve 1 1 operation and to perform diagnostic testing thereof. The DC power source 14 is coupled to the operating circuit 1 2 and provides one or more DC output voltages for powering the operating circuit 1 2.

The remote valve control unit 1 3 is coupled to the operating circuit 1 2 and serves as a master control for the solenoid valve 1 1 . For example, the remote valve control unit 1 3 controls the energization state of the solenoid valve 1 1 and initiates and monitors the results of diagnostic testing performed by the operating circuit 1 2. In the present embodiment, warning indicator lights 1 3a and 1 3b are included on the remote valve control unit 1 3 to indicate visually when various fault conditions are detected with respect to the solenoid valve 1 1 as discussed in more detail below. The warning indicator lights 1 3a, 1 3b may be, for example, light emitting diodes which are illuminated when a fault is detected. It will be appreciated, however, that other visual and/or audio outputs could alternatively be provided,

The solenoid valve 1 1 includes a housing 1 6, a solenoid coil 1 8 and a valve stem 20 moveably disposed within the solenoid coil 1 8. The valve stem 20 is moveable with respect to the solenoid coil 1 8 between a closed position P1 wherein the valve stem 20 is retracted into the solenoid coil 1 8 as shown in Fig. 1 , and an open position P2 wherein the valve stem 20 is fully extended from the solenoid coil 1 8 as represented in phantom in Fig. 1 . In the present embodiment, a compression spring 1 7 is coupled to the valve stem 20 for extending the valve stem 20 to its open position P2 during a de- energization state of the solenoid coil 1 8. During an energized state of the solenoid coil 1 8, the valve stem 20 is caused to retract to its closed position P1 in accordance with conventional techniques known in the art. The energization state of the solenoid valve 1 1 is controlled by application of an appropriate voltage and current across terminals "a" and "b" of the solenoid coil 1 8 by the operating circuit 1 2 in accordance with command instructions received from the remote valve control unit 1 3.

Although the present embodiment depicts a solenoid valve 1 1 having a spring biased valve stem 20, it will be appreciated that the present invention is suitable for use with solenoid valves of various configurations including those not utilizing spring biased movement. A typical solenoid valve 1 1 of the type which may be used in conjunction with the present embodiment of the invention is a Parker Skinner Valve, model number 71 31 5SN2EN00, or Lucifer Valve, Model Number 491 51 4C2, commercially available from Parker

Hannifin Corporation of Cleveland, Ohio.

Depending on the position of the valve stem 20, an inductance of the solenoid coil 1 8 as seen across terminals "a" and "b" varies. In particular, the valve stem 20 is made of metal such as iron and serves as a core within the solenoid coil 1 8. Since the inductance of a coil varies as a function of its core, the position of the valve stem 1 8 has a direct effect on solenoid coil 1 8 inductance. For example, when the valve stem 20 shown in Fig. 1 is in the closed position P1 the inductance of the solenoid coil 18 is at its highest value whereas in the open position P2, the inductance of the solenoid coil 1 8 is at its lowest value.

Variations in the inductance of the solenoid coil 1 8, in turn, cause variations in the transient response of the solenoid coil 1 8 to changes in energization levels. Accordingly, the present invention provides for monitoring features of the transient response of the solenoid coil 1 8 in order to analyze characteristics of the solenoid valve 1 1 . For example, by monitoring the transient response, it is possible to determine a position of the valve stem 20 and verify whether the solenoid coil 1 8 is properly energized. As it is possible to measure the transient response in an inexpensive and nonintrusive manner using the operating circuit 1 2 described in detail below, the present invention provides significant advantages over known techniques for determining valve stem position.

More particularly, in order to determine a position of the valve stem 20 at any given time, an energization state of the solenoid coil 1 8 is varied and the transient response of the solenoid coil 18 is measured. In one embodiment, a pulse is applied to the solenoid coil to vary its energization state. Preferably, the pulse has a sufficiently short duration such that application of the pulse to the solenoid coil 1 8 does not cause movement of the valve stem 20. In the event the valve stem 20 is in the closed position

P1 , the inductance of the solenoid coil 1 8 is high and a rate at which the current through the solenoid coil 1 8 rises or decays to a predetermined level in response to receiving the pulse is long as compared to a rate at which the current through solenoid coil 1 8 rises or decays to the predetermined level when the valve stem 20 is in the open position P2. Accordingly, by applying an excitation pulse to the solenoid coil 1 8 and measuring its transient response it is possible to determine whether the valve stem 20 is in the closed position P1 or open position P2.

Further, as also described in more detail below, a determination as to whether the solenoid coil 1 8 is properly energized can also be made based on the transient response of the solenoid coil 1 8 following a change in energization states. In order to determine whether the solenoid coil 1 8 is properly energized, a comparison is made between an expected transient response of the solenoid coil 1 8 and the actual transient response measured.

Referring now to Fig. 2, the operating circuit 1 2 which controls the operations of the solenoid valve 1 1 is depicted in accordance with one embodiment of the present invention. The operating circuit 1 2 performs three primary functions including: 1 ) controlling the energization or de- energization state of the solenoid coil 1 8 for the purpose of positioning the valve stem 20; 2) performing a test to determine proper coil energization state; and 3) determining the position of the valve stem 20. As shown in Fig. 2, a microcontroller 30 is included within the operating circuit 1 2 and serves as a central processor for controlling the various operations described herein. An oscillator 32 coupled to the microcontroller 30 provides the microcontroller 30 with a continuous timing signal to allow, for example, timing measurements to be taken. In the present embodiment, the microcontroller 30 is an 8-bit microcontroller model number 1 6C56 commercially available from Microchip Technologies. It will be appreciated, however, that other suitable controllers could alternatively be used.

To command the microcontroller 30 to keep the valve stem 20 in an open or closed position, the remote valve control unit 1 3 provides a valve state command signal V_command which is input to the microcontroller 30 via line 31 . The microcontroller 30 provides to the remote valve control unit 1 3 a valve state indicator signal (state_valve) via line 36, and a coil state indicator signal (state_coil) via line 37. Additional information regarding these signals is provided in detail below.

In response to the valve state command signal V_command, the microcontroller 30 controls the current through the solenoid coil 1 8. More specifically, the microcontroller 30 controls the on/off state of a switch 35 by appropriately inserting control voltage V_cont either high or low via line 38. The on/off state of switch 35, in turn, directly controls the energization level of the solenoid coil 1 8. In the present embodiment, the switch 35 is a field effect transistor having its gate coupled to the microcontroller 30 via line 38, its source coupled to ground and its drain coupled to terminal a of the solenoid coil 1 8. Terminal b of solenoid coil 18 is connected to a positive DC voltage supply (e.g. + 24 V) through a current monitoring resistor R1 . As described in more detail below, when V_cont is low, the switch 35 is off, and the solenoid coil 1 8 is placed in an de-energized state in which no current flows through the coil 1 8. When V_cont is high, the switch 35 is on, and the solenoid coil 1 8 is placed in an energized state in which the rated current flows through the coil 1 8. Accordingly, by appropriately inserting V_cont, the microcontroller 30 is able to control the energization state of the solenoid coil 1 8 which in turn allows the microcontroller 30 to control the position of the valve stem 20.

As discussed above, in addition to controlling the position of the valve stem 20, the operating circuit 1 2 of the present invention further includes diagnostic circuitry for measuring various characteristics of the solenoid valve 1 1 . For example, the present invention provides for measuring the transient response of the solenoid coil 1 8 in relation to changes in energization level in order to determine characteristics such as the valve stem 20 position and whether the solenoid coil 1 8 is properly energized.

In particular, a voltage V_r1 across the current monitoring resistor R1 varies in proportion to variations in the current flowing through the solenoid coil 1 8. Thus, as will be described in more detail below, the transient response of the solenoid coil 1 8 is tracked by monitoring the voltage V_rl across the current monitoring resistor R1 . A comparator 50 is provided in the operating circuit 1 2 for comparing variations in the voltage V_r1 to a predetermined level. Based on the output of the comparator 50, the microcontroller 30 determines an amount of time it takes for V_r1 to rise and/or decay to a predetermined level. By measuring such rise and/or decay time, which corresponds to the transient response of the solenoid coil 1 8, the present embodiment of the invention is able to determine characteristics of the solenoid valve 1 1 such as the position of the valve stem 20 and whether or not the solenoid coil 1 8 is properly energized.

More particularly, continuing to refer to Fig. 2, in order to measure characteristics of the transient response of the solenoid coil 1 8, the voltage V_r1 is coupled to an inverting input of the comparator 50 through a blocking capacitor C1 . A voltage divider circuit consisting of resistors R2 and R3 is provided at the inverting input and serves to provide a bias voltage V_bias thereat. Resistor R2 of the voltage divider circuit is coupled between the positive DC voltage supply (e.g., + 24V) and the inverting input, and resistor R3 is coupled between the inverting input and ground. The voltage level input at the inverting input of the comparator 50 will hereinafter be referred to as voltage V_in.

A predetermined reference voltage V_ref, which is slightly greater than V_bias, is applied to the non-inverting input of the comparator 50 and is compared by the comparator 50 with the voltage V_in. The reference voltage V_ref is established by a voltage divider circuit formed by resistors R4 and R5 coupled to the positive DC voltage supply and ground, respectively. As will be described in more detail below, by comparing the reference voltage V_ref with the voltage V_ιn, a rise time or decay time of a transient response signal produced by the solenoid coil 1 8 can be measured.

An inverted output of the comparator 50 is coupled to a base of npn- type transistor 75 included in the operating circuit 1 2. Because the comparator 50 has a inverted output, when V_ref is greater than V_ιn, the output of the comparator 50 is low and thus the transistor 75 is off. Conversely, when V_ιn is greater than V_ref, the output of the comparator 50 is high and the transistor 75 is turned on. The collector of the transistor 75 is coupled to a timing input pin 51 of the microcontroller 30 while the emitter of the transistor 75 is coupled to ground. During times when the transistor 75 is off, the timing input pin 51 of the microcontroller 30 is maintained at a logic "high" voltage level due to a pull-up resistor R6 being tied between the collector of the transistor 75 and a + 5 voltage source. During times when the transistor 75 is turned on, the timing input pin 51 of the microcontroller 30 is pulled to ground and the voltage level at the timing input pin 51 of the microcontroller 30 is at a logic "low" voltage level. The voltage level applied to the timing input pin 51 of the microcontroller will hereinafter be referred to as V_out.

In order to control a direction of flow of a relaxation current produced by the solenoid coil 1 8 following toggling of the switch 35 from an on state to an off state, a diode 55 and a zener diode 60 are coupled in series between the terminal a of the solenoid coil 1 8 and the positive DC voltage supply as shown. The zener diode 60 is reversed biased whereas the diode 55 is forward biased. The zener diode 60 has a reverse breakdown voltage which in the present embodiment is + 24 volts. Adjustments to the breakdown voltage can vary the transient response, allowing optimization of time intervals.

As will be better appreciated based on the description herein, the combination of the zener diode 60 and the current measuring resistor R1 serves a primary role of controlling the discharge rate of the resultant inductance resistance (LR) circuit through a constant voltage load. Generally speaking, the higher the voltage of the zener diode 60, the faster will be the discharge rate of the energy stored in the windings of the coil 1 8. By simply changing the values of the resistor R1 and the zener diode 60, a wide variety of coil impedances can be accommodated while maximizing the sensitivity.

The diode 55 serves to control the direction of current flow by virtue of blocking the 24 volt power supply from the low side of the coil 1 8 when the switch 35 is on.

As also shown in Fig. 2, a voltage regulator 87 supplies the operating circuit 1 2 with the appropriate digital logic voltages (e.g. + 5 V) needed for providing power to the appropriate elements. The voltage regulator 87 is supplied with power from the DC power source 1 4 shown in Fig. 1 which, in the present embodiment, provides the + 24 volt DC supply power.

Referring now to Figs. 3(a) - 6(d), a portion of the operating circuit 1 2 is shown in its equivalent form during various energization states of the solenoid coil 1 8. The states of the solenoid coil 1 8 include fully de-energized Figs. 3(a) - 3(c), fully energized Figs. 4(a) - 4(c), switching from energized to de-energized Figs. 5(a) - 5(d), and switching from de-energized to energized Figs. 6(a) - 6(d) . Additionally, corresponding graphs comparing the control voltage V_cont applied to switch 35 and the current flow through the solenoid coil l_coi, and are also shown for each state. Referring initially to Figs. 3(a)-3(c), a portion of the operating circuit 1 2 is depicted during a time period in which the switch 35 is off and the solenoid coil 1 8 is in a fully de-energized steady state. The switch 35 is kept in an off state by virtue of the microcontroller 30 maintaining V_cont at a low logic level (e.g. 0 volts) as shown in Fig. 3(b) . When the switch 35 is off, the connection between terminal a of the solenoid coil 1 8 and ground is open circuited. Consequently, in the steady state condition no current will flow through the coil 1 8 (e.g. I_coi| = 0). As a result, the voltage V_ιn applied to the inverting input of the comparator 50 is substantially equal to the bias voltage V_bias created by the voltage divider circuit (R2 and R3). The valve stem 20, in such case, is in its deenergized position assuming proper operation.

Referring now to Figs. 4(a)-4(c), a portion of the operating circuit 1 2 is depicted during a time period in which the switch 35 (Fig. 2) is on and the solenoid coil 1 8 is in a fully energized steady state. The switch 35 is kept in an on state by virtue of the microcontroller 30 maintaining control voltage

V_cont at a logic high level (e.g. + 5 volts) as shown in Fig. 4(b). When the switch 35 is on, terminal a of the solenoid coil 1 8 is coupled to ground. Thus, in this state, current flows through the coil 1 8 from the + 24 volts supply to ground via steady state current path l_ss. Since the solenoid coil 1 8 has reached a steady state in Fig. 3(c), the voltage V_in applied to the non- inverting input of the comparator 50 is substantially equal to the bias voltage V_bias created by the voltage divider circuit consisting of resistors R2 and R3 by virtue of the coupling capacitor C1 . In this state, a current flowing through the solenoid coil l_coi| is a constant value as depicted by Fig. 4(c). Again assuming proper operation, the valve stem 20 in such case is in its energized position.

Referring now to Figs. 5(a)-(d), a portion of the operating circuit 1 2 is depicted during a time period immediately following toggling the switch 35 (Fig. 2) from an on state to an off state thereby causing the solenoid coil 1 8 to change from an energized state to a de-energized state. The switch 35 is toggled from an on state to an off state by virtue of the microcontroller 30 toggling V_cont from a high voltage (e.g. + 5 volts) to a low voltage (e.g. zero volts) as shown in Fig. 5(b). It will be appreciated that since Fig. 5(a) relates to a state in which the switch 35 is off, the equivalent circuit depicted in Fig. 5(a) is the same as that shown above with respect to Fig. 3(a) . However, unlike the case in Fig. 3(a), the circuit of Fig. 5(a) is not in a steady state.

Rather, during the time period in which the switch 35 was on, current flowing through the coil 1 8 caused energy to be stored in a magnetic field associated with the coil 1 8. Upon the microcontroller 30 toggling the switch 35 from an on state to an off state, a relaxation current l_re,_ax is caused to flow from the solenoid coil 1 8 and a voltage V_a._b across the solenoid coil 1 8 is caused to increase to a peak level and then decay to zero. As the peak voltage for V_a._b is above the zener diode 60 breakdown voltage level, the relaxation current l_re,_ax passes through the diodes 55 and 60 to the positive DC voltage supply. The rate at which the current l_re(ax decays is dependant on the electrical time constant, which depends upon the inductance value of the solenoid coil 1 8, which in turn varies in accordance with a change in position of the valve stem 20. An exemplary decay curve for the current l_re|_ax is shown in Fig. 5(c). The voltage change across V_a-b which varies over time as a function of l_re)ax, is coupled through the capacitor C1 thereby causing V,_n to respond in the manner shown in Fig. 5(d) . More particularly, V_ιn is shown to peak to its high value during toggling of V_cont and then decay back to its stead state value of V_bιas. Once the circuit of Fig. 5(a) reaches a steady state

(e.g. the solenoid coil 1 8 is fully de-energized), the circuit functions in the manner described above with respect to Fig. 3(a). Referring now to Figs. 6(a)-(d), a portion of the operating circuit 1 2 is depicted during a time period immediately following toggling the switch 35 (Fig. 2) from an off state to an on state thereby causing the solenoid coil 1 8 to change from a de-energized state to an energized state. The switch 35 is toggled from an off state to an on state by virtue of the microcontroller 30 toggling V_cont from a logic low voltage (e.g. 0 volts) to a logic high voltage (e.g. + 5 volts) as shown in Fig. 6(b) . It will be appreciated that since Fig. 6(a) relates to a state in which the switch 35 is on, the equivalent circuit depicted in Fig. 6(a) is the same as that shown above with respect to Fig. 4(a). However, unlike the case in Fig. 4(a), the circuit of Fig. 6(a) is not in a steady state.

Rather, upon the microcontroller 30 toggling the switch 35 from an off state to an on state, an energization current l_e is caused to flow from the positive DC voltage supply, through the solenoid coil 1 8 and to ground at a varying rate and the voltage V_a._b across the solenoid coil 18 is caused to vary in a manner substantially opposite to that described above with respect to Figs 5(a)-5(d) . The energization current l_e increases gradually from zero to a steady state value as a result of the inductive characteristics of the solenoid coil 1 8. The rate at which the current increases is dependent on the electrical time constant, which depends upon the inductance of the solenoid coil 18 which, in turn, varies in accordance with a change in position of the valve stem 20. The resultant variation in current flow rate through the solenoid coil l_cor, with respect to time following the change in energization levels is shown in Fig. 6(c). The voltage change across V_a._b, which varies over time as a function of l_C01|, is coupled through the capacitor C1 to cause

V_ιn to respond in the manner shown in Fig. 6(d). Once the circuit of Fig. 6(a) returns to a steady state (e.g. the solenoid coil 1 8 is fully energized), the circuit functions in the manner described above with respect to Fig. 4(a) . Referring now to Figs. 7(a)-7(c), there is illustrated in more detail a manner in which a transient response of the solenoid coil 1 8 may be measured upon application of an excitation pulse. Referring initially to Fig. 7(a), in the present embodiment, the excitation pulse is applied to the solenoid coil 1 8 by virtue of the control voltage V_cont toggling between a high and low state within a predetermined time limit. It is possible to provide a positive excitation pulse by virtue of V_cont initially transitioning from a low state to high state as shown in with respect to pulse 1 00a, or a negative excitation pulse by virtue of V_cont initially transitioning from a high state to a low state as shown with respect to pulse 1 00b (pulses 1 00a and 1 00b will collectively be referred to as pulse 100). The pulse 1 00 is a single square wave pulse having a pulse width W1 . The pulse width W1 is preferably of sufficiently short duration relative to the dynamic response of the valve stem 20 such that application of the pulse 1 00 to the switch 35 does not cause movement of the valve stem 20 within the solenoid coil 1 8. For example, in the present embodiment, it is preferable that the pulse width W1 remain less than or equal to 10% of the minimum valve operate time for the applied DC power 14 (e.g., 1 .0 milliseconds). In this manner, a transient response of the solenoid coil 1 8 can be analyzed for determining valve stem 20 position without modifying or repositioning the actual location of the valve stem 20 itself. As depicted in Fig. 4(a) the positive pulse 1 00a is applied by the microcontroller 30 for diagnostic purposes at a time during which V_cont is initially low (i.e. during a deenergized state). The negative pulse 100b is applied by the microcontroller 100 for diagnostic purposes at a time during which V_cont is initially high (i.e. during an energized state) .

Referring now to Fig. 7(b) there are depicted two corresponding curves 1 10, 1 1 5 representing a voltage V_ιn which is created by application of the pulse 1 00 to the solenoid coil 1 8 and measuring the solenoid coil 1 8 transient response. The curve 1 1 0 corresponds to V,_n formed from a transient response of a solenoid coil 1 8 in a case where the solenoid coil 1 8 is deenergized and the valve stem 20 is fully retracted into the solenoid coil 1 8 (e.g. position P1 of Fig. 1 ) . The curve 1 1 5 corresponds to V_ιn formed from a transient response of a solenoid coil 1 8 in a case where the solenoid coil is energized and the valve stem 20 is fully extended out of the solenoid coil 1 8 (e.g. position P2 of Fig. 1 ).

It will be appreciated that in the present embodiment, regardless of whether the pulse 1 00 is a positive pulse 1 00a or negative pulse 1 00b, the V,_n input to the comparator 50 (Fig. 2) instantaneously decreases in conjunction with a rising edge 1 02a, 1 02b, of the pulses 100a, 1 00b, respectively and instantaneously increases in conjunction with a falling edge 1 03a, 1 03b of the pulses 1 00a, 100b, respectively. In particular, during periods when the solenoid coil 1 8 is not in a steady state, the blocking capacitor C1 serves to couple the change in voltage seen at V_r1 to the inverting input of the comparator 50. Accordingly, when the voltage at V_r1 increases there is a corresponding rise in V_ιn. However, when the change in voltage at V_r1 decreases there is a corresponding fall in V_ιn. As discussed above, the rate of increase and/or decrease of V_ιrι is proportional to the inductance of the solenoid coil 1 8 and thus can be used to determine the valve stem 20 position and whether the solenoid coil 1 8 is properly energized.

Referring now to Fig. 7(c) it is possible to measure one or more characteristics of the curves 1 1 0, 1 1 5 in order to determine the position of the valve stem 20 and/or determine whether the solenoid coil 1 8 is properly energized. For example, by monitoring V_out, the microcontroller 30 is able to measure a decay time which refers to an amount of time it takes V_ιn to decay to V_ref following a falling edge 1 03a of the pulse 1 00. The decay time is measured by the microcontroller 30 using the oscillator 32. It will be appreciated that since the decay time varies in accordance with the electrical time constant which depends upon the inductance of the coil 1 8, which in turn varies in accordance with a position of the valve stem 20, by monitoring the decay time it is possible to determine whether the valve stem 20 is in an open or closed position.

More particularly, as shown in Fig. 7(c), a decay time t_d1 for the curve 1 10 is measured beginning at a time t, corresponding to the falling edge 103a of the pulse 1 00a and ends when the transient curve 1 10 reaches the predetermined threshold value V_ref. As the decay time t_d1 corresponds directly to the time during which V_out is low, the microcontroller 30 it able to measure the decay time t_d1 by measuring the time between a falling edge 1 17 of V_out and a rising edge 1 1 8 of V_out.

While the present embodiment depicts measuring decay time of the solenoid coil 1 8 to determine the position of the valve stem 20, it will be readily appreciated by those in the art that this is but one of a variety of measurable characteristics and the present invention is not limited to measuring decay time as exemplified herein.

Turning now to Fig. 8, a lookup table 1 50 having a time column 1 52 and a position column 1 54 may be used to determine a position of a valve stem 20 for decay times and/or other measurable attributes associated with a transient response of the solenoid coil 1 8. More particularly, for each entry in the time column 1 52, a corresponding entry in the position column 1 54 is stored in the lookup 1 50. The data for the position column 1 54 may be obtained through known computer simulation programs, experimentation, or the like. Based on such information, a direct correlation between the decay, time and position of the valve stem 20 is obtainable for output to a user or further use by the operating circuit 1 2 in control of the solenoid valve 1 0. The lookup table 1 50 may be stored in a memory which is locally or remotely accessible by the microcontroller 30 such as, for example, in the remote valve control unit 1 3.

In addition to determining a position of the valve stem 20, it will be appreciated that decay time calculations may be used to determine whether the solenoid coil 1 8 is properly energized at any given time. More particularly, for a given position of the valve stem 20, there is a corresponding known and expected transient response by the solenoid coil 1 8. Accordingly, by pre-storing the expected decay time associated with the transient responses for various valve stem 20 positions in a lookup table

(similar to the lookup table 1 50 in Fig. 8), it is possible to compare the measured decay time with the expected decay time. For example, a deviation could be calculated between a measured decay time and an expected decay time for a given valve stem position (e.g. Δ_td = t_{d actua|}- t_{d eXpected})- Based on the results, a determination can be made as to whether the energization level of the solenoid coil 1 8 is proper for the given valve stem 20 position.

In an embodiment of the invention where it is desirable simply to know if the valve stem 20 is in an "open" or "closed" position, the lookup table 1 50 can be simplified to storing a single value half way between the expected decay time for a valve stem in the open position and the expected decay time for the valve stem in the closed position. If the measured decay time is greater/less than the stored value, the valve stem is determined to be either open or closed.

Referring now to Fig. 9 a flowchart depicting the operations of the present invention is provided in conjunction with an exemplary embodiment. Hardware components referred to in the exemplary embodiment are generally shown in Fig. 2. In particular, beginning at step 200, an initial calibration step takes place during which the information regarding the expected response of the valve 1 8 to an excitation pulse 1 00 is measured and stored for various positions of the valve stem 20 as shown, for example in Fig. 8. Next, in step 203, a transient response of the solenoid coil 1 8 is obtained by modifying an energization level applied thereto. For example, the energization level may be modified by having the microcontroller 30 apply the pulse 1 00 to the switch 35 thereby causing an excitation pulse to be applied to the solenoid coil 1 8. The microcontroller 30 preferably is configured to initiate the pulse 1 00 at predefined intervals thereby providing ongoing feedback concerning sensed valve stem 20 positioning. For example, the microcontroller 30 may be configured to initiate the pulse 1 00 in one second intervals. Depending on the current energization state of the solenoid coil 1 8 as governed by the V_command signal supplied to the microcontroller 30, either a positive or negative pulse 1 00a, 100b, respectively, is applied. In order to apply the pulse 100 to the solenoid coil 1 8, the microcontroller 30 is configured to assert V_cont high or low (depending on whether a positive or negative pulse is to be applied) for a duration of time corresponding to the pulse width W1 (Fig. 7(a)) of the pulse to be applied.

Next in step 205, the microcontroller 30 measures an amount of time it takes V_ιn to decay to a predetermined level following the falling edge 1 03a,

103b of V_cont. For example, according to one embodiment of the present invention, the transient signal produced by the solenoid coil 1 8 is monitored to determine an amount of time it takes the current across the solenoid coil 1 8 to grow such that one volt is produced across the current monitoring resistor R1 at V_r1. Accordingly, the reference voltage V_ref is set so that the comparator 50 triggers when the voltage V_r1 reaches one volt. Of course, as discussed above, the reference voltage V_ref could alternatively be set to other desired values and the present invention is not limited to any particular values described with respect to the exemplary embodiment.

The time associated with the solenoid coil 1 8 reaching the reference voltage V_ref is calculated by the microcontroller 30 in accordance with the characteristic being measured which in the preferred embodiment is the decay time. For example, the decay time is measured by beginning an internal counter at time t, corresponding to a time of application of the falling edge 103a, 103b of the pulse 1 00. In response to the falling edge 1 03a, 1 03b of the pulse 1 00, the solenoid coil 1 8 produces a transient response as described above in detail with respect to Figs. 5-8. In particular, upon application of the falling edge 1 03a, 1 03b of the pulse 1 00, V_ιn rises above V_ref and the inverted output of comparator 50 switches from a low state to a high state thereby causing switch 75 to turn on and V_out to go low. Upon sensing the change from a high to low state at the timing input pin 51 , the microcontroller 30 begins its count at time t_r The microcontroller 30 then continues to count until it senses a change from the low state to a high state at the timing input pin 51 and utilizes the elapsed time beginning from time t, as the calculated decay time for the particular solenoid valve 1 0.

Once measured, the microcontroller 30 in step 210 performs a correlation between the measured time and an associated characteristic of the solenoid valve 1 0. For example, as discussed above, the decay time may be used to determine a position of the valve stem using the correlation table 1 50 depicted in Fig. 8 or may be used to access whether the solenoid coil 1 8 is properly energized. In the event the solenoid valve 20 is not properly positioned or the energization state of the solenoid coil 1 8 is not correct, the microcontroller 30 informs the remote valve control unit 1 3 via the valve state indicator line 36 or coil state indicator line 37, respectively. Upon receiving an indication that either the valve stem 20 is improperly positioned and/or that the solenoid coil 1 8 is not properly energized, the remote valve control unit 1 3, in turn, illuminates the appropriate warning light 1 3a, 1 3b (Fig. 1 ), respectively, to warn a user of the malfunction so that corrective steps can be taken.

The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. For example, while the exemplary embodiment depicts an analog comparator for comparing a reference voltage with a voltage indicative of a current flowing through the solenoid coil 1 8 to determine valve stem position, it will be appreciated that digital circuitry could alternatively be used. Further, while the present embodiment depicts the excitation signal to be controlled and produced by the microcontroller, it will be appreciated that the excitation signal could be introduced through a dedicated pulse generation circuitry or through other devices or circuitry independent of the microcontroller.

Furthermore, it will be appreciated that although the present invention has been described primarily in the context of measuring the position of a solenoid valve stem, the invention further extends to other solenoid driven devices such as actuators, etc. For example, the valve stem 20 described herein may instead represent more generally an actuator stem.

It is intended that the invention be construed as including all such modifications alterations, and equivalents thereof and is limited only by the scope of the following claims.

Claims

What is claimed is:

1 . A solenoid valve comprising: a solenoid coil; a valve stem at least partially disposed within the solenoid coil; and a diagnostic circuit coupled to the solenoid coil, the diagnostic circuit including: circuitry for changing an energization level of the solenoid coil; circuitry for monitoring a transient response of the solenoid coil following a change in the energization level; and circuitry for determining a position of the valve stem based on a characteristic of the transient response.

2. The solenoid valve of claim 1 , wherein the circuitry for changing the energization level causes an excitation pulse to be applied to the solenoid coil.

3. The solenoid valve of claim 2, wherein the excitation pulse has a pulse width of a duration such that application of the excitation pulse to the solenoid coil does not reposition the valve stem.

4. The solenoid valve of claim 3, wherein the pulse width is less than or equal to 1 0% of the minimum valve operate time for an applied DC power.

5. The solenoid valve of claim 1 , wherein the circuitry for monitoring the transient response includes a comparator for comparing a signal representative of the transient response to a predetermined reference value.

6. The solenoid valve of claim 5, wherein the circuitry for monitoring the transient response includes a current monitoring resistor for monitoring an amount of current flowing through the solenoid coil and the signal representative of the transient response is proportional to a voltage level across the current monitoring resistor.

7. The solenoid valve of claim 1 , wherein the characteristic of the transient response used to determine the position of the valve stem is decay time.

8. A diagnostic circuit for a solenoid valve including a solenoid coil and a valve stem at least partially disposed within the solenoid coil, the diagnostic circuit comprising: a pulse generator for applying an excitation pulse to the solenoid coil; a comparator coupled to the solenoid coil for comparing a transient response produced by the solenoid coil upon application of the excitation pulse to a reference value; and a microcontroller coupled to the comparator for measuring a decay time-associated with the transient response reaching the reference value.

9. The diagnostic circuit of claim 8, wherein the excitation pulse is of a duration which is sufficiently short so as to not cause the valve stem to move.

1 0. The diagnostic circuit of claim 8, further comprising circuitry for determining a position of the valve stem based on the decay time measured by the microcontroller.

1 1 . The diagnostic circuit of claim 9, further comprising a transistor coupled between an output of the comparator and the microcontroller.

1 2. A solenoid valve comprising: a solenoid coil; a valve stem at least partially disposed within the solenoid coil; and a diagnostic circuit coupled to the solenoid coil, the diagnostic circuit including: means for changing an energization level of the solenoid coil; means for monitoring a transient response produced by the solenoid coil upon changing the energization level; and means for determining a position of the valve stem based on a characteristic of the transient response.

1 3. The solenoid valve of claim 1 2, wherein the means for changing the energization level causes an excitation pulse to be applied to the solenoid coil.

14. The solenoid valve of claim 1 3, wherein the excitation pulse has a pulse width of a duration such that application of the excitation pulse to the solenoid coil does not reposition the valve stem.

1 5. A diagnostic method for use with a solenoid valve comprising a valve stem and a solenoid coil, the method comprising the steps of: changing an energization level of the solenoid coil; monitoring a transient response produced by the solenoid coil upon changing the energization level; and determining a position of the valve stem based on a characteristic of the transient response.

1 6. The method of claim 1 5, wherein the step of changing the energization level causes the energization level to change for a predetermined period of time.

1 7. The method of claim 1 6, wherein the predetermined period of time is an amount of time which does not cause the valve stem to move.

1 8. The method of claim 17, wherein the predetermined period of time is less than or equal to 1 0% of the minimum valve operate time for an applied DC power.

1 9. The method of claim 1 5, wherein the characteristic of the transient response used to determine the position of the valve stem is a decay time associated with the transient response reaching a predetermined reference value.

20. A solenoid actuator comprising: a solenoid coil; an actuator stem at least partially disposed within the solenoid coil; and a diagnostic circuit coupled to the solenoid coil, the ^"diagnostic circuit including: circuitry for changing an energization level of the solenoid coil; circuitry for monitoring a transient response of the solenoid coil following a change in the energization level; and circuitry for determining a position of the actuator stem based on a characteristic of the transient response.