DESCRIPTION

[0001] The present invention concerns a diagnostic device for solenoid valves.

[0002] As shown in the example in figure 1, a solenoid valve 1 or electromagnetic valve, is typically composed of an electromagnet 10 and a valve body 12, wherein one or more orifices are made.

[0003] The electromagnet 10 comprises a solenoid 102 wherein is situated and slidable a movable core 104 made of ferromagnetic material. The flow through an orifice of a fluid intercepted by the solenoid valve, e.g. compressed air, is blocked or allowed by the movement of the movable core 104 when the solenoid is electrically powered or is deactivated.

[0004] When the solenoid 102 is powered, the movable core 104 directly opens the orifice of a normally closed (NC) valve or closes the orifice of a normally open (NO) valve. When the solenoid is deactivated, a spring 106 returns the core to its original position.

[0005] The valve operates in a pressure range from 0 bar to the maximum pressure rating. The force required to open the valve is proportional to the section of the orifice and the pressure of the fluid. [0006] In the field of pneumatics for industrial automation, the performance of pneumatic components is increasingly high and must be guaranteed over time. In some applications, pneumatic components need to be replaced as soon as performance declines.

[0007] In particular, there is an increasing need to monitor the behavior of solenoid valves to detect possible malfunctions or a significant reduction in the performance thereof.

[0008] The object of the present invention is to meet such a need by providing a diagnostic device capable of monitoring the performance of a solenoid valve to detect a deviation from the expected values thereof, for example, due to a malfunction or degradation due to wear.

[0009] Another object of the invention is to propose a diagnostic device suitable to be implemented directly on a solenoid valve, also - when possible - as an accessory to be applied to an existing solenoid valve.

[0010] These and other objects and advantages of the invention are achieved with a diagnostic method according to claim 1, with a diagnostic device according to claim 6, and with a solenoid valve according to claim 13. The dependent claims describe preferred embodiments of the invention.

[0011] Further features and advantages of the device and of the diagnostic method according to the invention will nevertheless become evident from the description of the preferred embodiments thereof provided hereinafter by way of indicative and non-limiting example, with reference to the accompanying figures, wherein :

- Figure 1 is a sectional view of a solenoid valve according to the state of the art;

- Figure 2 is a sectional view of a solenoid valve equipped with a diagnostic device according to the invention;

- Figure 3 is a graph representing a typical waveform of the solenoid current;

- Figures 4 and 4a represent two graphs of the waveform of the solenoid current in the presence of friction between the movable core and the solenoid;

- Figure 5 is a flow diagram of a diagnostic algorithm, in one embodiment;

- Figure 6 is a circuit diagram of a detection circuit of the peak current of the waveform of the solenoid current ;

[0012] Figure 2 shows a solenoid valve 1 equipped with a diagnostic device according to the invention.

[0013] The solenoid valve 1 comprises an electromagnet 10 and a valve body 12 wherein an inlet orifice 14, an outlet orifice 16 and a discharge passage 18 are made.

[0014] The inlet orifice 14 forms a valve seat 20 to which is associated a sealing valve element 22 that is movable between a closed position and an open position of the valve seat 20.

[0015] The electromagnet 10 comprises a solenoid 102 wound on a coil 108 and a magnetic circuit formed of a fixed plunger 110, having for example a portion that partially penetrates the axial sleeve of the coil 108, and a movable core 104, which also partially penetrates in the sleeve.

[0016] The valve element 22 may be constructed as an integral part of the movable core 104 or as a separate element. In some embodiments, the sealing valve element 22 has a cylindrical base with comb teeth facing the movable core.

[0017] If the valve element is made as a separate element

- as in the example in figure 2 - a spring 22', placed under its cylindrical base, has the task of pushing the valve element towards the movable core 104 as the position of the latter changes. [0018] The solenoid 102 has a pair of electrical terminals connected to a power supply and control circuit board 120 housed in a protective casing 122 integral with the electromagnet 10. In turn, the power supply and control circuit board 120 is provided with power supply terminals 124 suitable to be connected to a power supply connector.

[0019] In accordance with one aspect of the invention, the diagnostic device comprises an electrical diagnostic circuit 30 implemented on the power supply and control circuit board 120.

[0020] In a direct current valve of the type described above, powered with a constant power supply voltage, for example 24V, 12V, or 5V, as soon as the solenoid 102 is energized, the current in the solenoid increases, causing an expansion of the magnetic field, until it becomes sufficiently elevated to move the movable core 104. The movement of the movable core 104 increases the concentration of the magnetic field, as the magnetic mass of the core moves increasingly to the inside of the magnetic field.

[0021] A magnetic field, which changes in the same direction as the current that generates it, induces a voltage of the opposite sign in the solenoid coils. Since the magnetic field expands rapidly as the movable core moves to the end-of-stroke position, such a field causes a brief reduction in the current flowing through the winding of the solenoid. After the movable core has reached the end of its stroke, the current begins to increase again until it reaches its maximum level. A typical waveform of the solenoid current IS is shown in Figure 3.

[0022] As long as no power supply voltage is applied to the solenoid, the solenoid current is zero.

[0023] When the solenoid is powered, there is a first increase of the solenoid current IS, until reaching, at the time Tl, a peak value I _PEAK , which, as stated, corresponds to the engagement of the movable core, i.e., the beginning of its stroke towards the fixed plunger. The solenoid current then decreases until it reaches, at the moment T2, a minimum value I _VALLEY , which corresponds to the abutment of the movable core against the fixed plunger. At this point, the solenoid current begins to increase again until it reaches its maximum value .

[0024] The simplified equation of the solenoid of a solenoid valve could be represented as follows : 1 where V is the voltage applied to the solenoid, R is the resistance of the solenoid and F expresses the variation in magnetic flux as a function of the current and the position (x) of the movable mechanical parts (movable core and fixed core, or plunger) .

[0025] The first term of the equation is relative to the change in resistance and the second term expresses the induced voltage .

[0026] The solenoid inductance depends on the position of the movable parts, as the magnetic reluctance of the solenoid depends on the position of the electromagnetic parts and therefore on the reclosing of the magnetic field itself.

[0027] The flux F in the coil depends on the current passing through the coil and the distance of the ferrous parts, according to the formula:

Being F = Li, the preceding relationship becomes [0028] The third addend represents the counter electromotor force induced by the movement of the ferrous parts .

[0029] This relationship clearly shows that the magnetic flux and thus the attractive force of the solenoid valve depend on the values of the resistance R and the inductance L of the solenoid.

[0030] It is known, moreover, that the resistance R of a solenoid is given by the relationship

R= p*l/S

where p is the average resistivity of the material, 1 is the length of the wire and S is the section.

[0031] p and S may be assumed as constants, while the length 1 is subject to thermal expansion, which may be expressed thusly:

it -f s(T - fS)J where TO is the temperature at 20° and s is the material-dependent thermal coefficient.

[0032] Returning now to the waveform of the solenoid current, it may be observed that such waveform is substantially independent of the temperature.

[0033] On the other hand, it has been found that the waveform of the solenoid current, and in particular the time interval T1 between the solenoid excitation and the peak current IPE _A K preceding the temporary drop, depends on the pressure of the fluid in the inlet orifice and/or on the wear of the solenoid valve, which is reflected, for example, in deformations of the sealing valve element, loss of spring force, friction.

[0034] In one embodiment, the solenoid current values are obtained by sampling the waveform at predetermined time intervals using a current sampling circuit.

[0035] In one advantageous embodiment the diagnostic circuit 30 being implemented in the same power supply and control circuit board 120 that is already provided on the solenoid valve, this diagnostic circuit must be optimized in terms of bulk and therefore size and number of electronic components. For example, the diagnostic circuit 30 comprises a microcontroller 32, the computing power and memory of which are selected so that they may be mounted on the power supply and control circuit board 120 without affecting the dimensions of the latter.

[0036] Therefore, even an existing solenoid valve may be equipped with a diagnostic device by simply replacing the traditional power supply and control circuit board with a power supply, control and diagnostic circuit board having the same dimensions as the former. [0037] For this purpose, in one embodiment, the microcontroller 32 compares each value of the sampled current received by the sampling circuit with the previous value of the sampled current and stores the sampled value in the memory only if the outcome of the comparison reveals the attainment of the peak current or of the minimum current.

[0038] To detect the moment of time Tl, in one embodiment the diagnostic device comprises an analog peak detection circuit 40, as shown in figure 6. In this circuit, the solenoid current detected by a shunt resistor RSHUNT (inverting input) and the solenoid current to which is applied a delay given by an RC network (non-inverting input) are fed to the input terminals of an operational amplifier 42 with comparator function. In this way, the circuit is able to detect the moment Tl wherein the solenoid current reaches the peak value IPE _A K.

[0039] From the analysis of the solenoid current waveforms over time, it was noted that some evident malfunctions or failures of solenoid valves were preceded by the appearance of a flexion or even relative minimum and maximum points in the descending segment of the current waveform, between the peak value (I _PEAK ) and the absolute minimum value (IV _ALLEY ) . Since this segment of the curve of the solenoid current waveform corresponds to the phase of movement of the movable core, such variations in the slope of this curve segment may be due to deterioration of the solenoid valve due in particular to friction between the movable core and the solenoid.

[0040] In accordance with one aspect of the invention, the diagnostic method is therefore based on an observation of the slope of this segment of the current waveform curve between the current peak (moment Tl) and the minimum current value (moment T2) .

[0041] As said and with reference to Figures 4 and 4a, slope variation refers in particular to the appearance of an inflection F (Figure 4) or further relative minimum (m) and maximum (M) points (Figure 4a) , as in the waveform portions circled in the drawings.

[0042] An example of a diagnostic protocol for this embodiment is shown in the flow diagram in figure 5.

[0043] With reference to the flow diagram in Figure 5, the current peak I _{PEAK (} step 400) is detected by means of the peak current detection circuit, for example by means of the comparator circuit.

[0044] After such detection, for example by sending a peak detect interrupt, the diagnostic circuit, for example by means of a microcontroller that receives the interrupt and by means of a voltage sampling circuit on the shunt, samples the current circulating in the solenoid, and in particular its rate of change Kl' over time (di/dt) (step 402).

[0045] In one embodiment, the value of such gradient Kl' is compared with a predetermined value Kl .

[0046] If the gradient of the current in the solenoid is negative, it means that the movable core continues its movement with a dynamic proportional to the value Kl' of the gradient itself.

[0047] The derivative of the current over time (di/dt) is calculated continuously, sampling the current in a time range contiguous to the one wherein the waveform peak occurred .

[0048] In one embodiment, in the event of a null or positive value of the derivative of the current (di/dt) (step 404) the time value Ta corresponding to this value (step 406) is stored, for example by the microcontroller.

[0049] The calculation of the derivative of the current over time (di/dt) continues in order to establish if the gradient remains negative (step 408) . [0050] In one embodiment, the average value of such derivative, in this observation phase, may be stored in the parameter K2' and compared with a second reference value K2.

[0051] If the gradient K2' always remains positive, it means that the point of the waveform at the moment Ta previously detected was an absolute minimum point, i.e., the point IV _ALLEY , and therefore the moment Ta coincides with the moment T2 (step 410) .

[0052] In one embodiment, in the event of a second null or positive value of the current derivative (di/dt) (step 412), the time value Tb corresponding to such value (step 414) is stored, for example by the microcontroller.

[0053] Note that the values for the moments in time Ta,

Tb and the gradients Kl' and K2' are initialized to zero for each solenoid activation.

[0054] The simultaneous presence, after the detection of the current peak, of two non-null values Ta and Tb and/or of at least one of the lower values of the detected gradients Kl', K2', in absolute value, with respect to the predetermined value Kl, K2 , indicates that during the movement of the movable core a discontinuity in the displacement dynamics has occurred, due to friction.

[0055] As regards the gradients Kl' and K2', in particular, a negative but lower value, in absolute value, than the predetermined values Kl, K2 indicates that the slope of the descending segment of the curve is lower than a predetermined slope due to the presence of friction in the stroke of the movable core.

[0056] The diagnostic circuit thus generates an alarm signal (step 416) .

[0057] As mentioned above, in one embodiment the diagnostic device is implemented on the power supply and control circuit board 120 of the solenoid valve and is suitable to communicate with an external control unit, such as a PLC, using a communication protocol based on a single diagnostic cable 125, which becomes the third cable of the solenoid valve, in addition to the two power cables 124. Thus, the alarm signal relative to a deterioration of the solenoid valve may be sent to this external control unit.

[0058] It should be noted that the aforementioned diagnostic cable may be replaced by a wireless communication system, according to the needs and available technology.

[0059] If, on the other hand, the diagnostic device is not connected to an external control unit, a visual indication of the anomalous operating status may be given, for example, by flashing an LED at a certain frequency .

[0060] In one embodiment, the values Kl', K2' of the derivatives of the waveform of the solenoid current are compared with corresponding values Kl, K2 of the derivatives calculated in a test phase of the solenoid valve during or after its release from the factory, or at the beginning of its life cycle, so as to detect any significant variations caused by the wear of the solenoid valve, as explained above.

[0061] In a variant embodiment, the derivatives of the waveform detected are compared with the values of the derivatives calculated in the previous excitation of the solenoid valve (or with average values relative to a certain number of previous excitations) .

[0062] To the embodiments of the solenoid valve diagnostic device method according to the invention, one skilled in the art, to satisfy contingent needs, may make modifications, adaptations and replacements of some elements with others that are functionally equivalent, without departing from the scope of the following claims. Each of the features described as belonging to a possible embodiment may be implemented independently from the other described embodiments.