Armature movement detection circuit

Abstract: A solenoid valve includes a monitoring circuit adapted to sense the current drop that occurs through the solenoid coil (1) when the solenoid armature moves correctly, sensing of the drop being used to cause a neon (11) to glow continuously so as to indicate correct operation of the solenoid. If the drop is not sensed, the neon (11) is caused to blink so as to indicate incorrect operation of the solenoid. The circuit may in addition or alternatively be arranged to generate a voltage signal (12), upon correct operation of the solenoid, for transmission to a remotely located programmable logic controller used to control operation of the valve and like valves that form part of a pneumatically controlled machine.

Description: This invention relates to a circuit for detecting the movement of a solenoid armature associated, for example, with a solenoid valve, whereby correct or incorrect operation of the armature, and therefore of the valve, may be monitored, especially from a remote location. Pneumatically operated systems, for example, typically comprise a large number of solenoid valves which function in a predetermined, desired sequence and control the operation of pneumatic devices such as cylinders. Usually, operation of the system is monitored from a remote, central console and it would be desirable to provide, as part of the console, means to indicate that each solenoid armature is operating correctly, or not as the case may be. It is an object of the present invention to provide such means, which relies on the fact that, when a solenoid armature moves in response to energisation of the solenoid coil, the inductance of the coil changes which causes the current flowing through the coil to drop momentarily. US patent specification No 4321946, WO patent specification No 89/02648 and German OLS (=DE-A-) 2251472 describe circuits for monitoring operation of a solenoid armature which rely, for their operation, on that momentary change in current flowing through the solenoid coil when it is energised. However, the circuits described are relatively complex in that they require means to monitor the rate of change of, i.e. to differentiate, the current value or require means to determine directly the change in inductance of the solenoid coil using a secondary coil magnetically coupled to the solenoid coil . The present invention,on the other hand, proposes a novel approach requiring simpler circuitry that may readily be incorporated into the type of plug conventionally used to connect the solenoid to its power source. According to one aspect of the present invention, therefore, there is provided a circuit for combination with a solenoid comprising a solenoid coil and an armature that moves in response to initial energisation of the coil, said circuit serving to monitor movement of the armature upon initial energisation of the coil by monitoring the current flowing through a resistor connected in series with the coil and therefore through the coil during initial energisation thereof, characterised in that the circuit comprises: a) a resistor across which a voltage drop proportional to said current is estabished; b) an RC circuit responsive to said voltage drop for producing a steady output voltage signal corresponding substantially to the peak voltage drop across the resistor during initial energisation of the coil; and c) a comparator for comparing the steady output voltage signal of the RC circuit with the voltage drop across the resistor during initial energisation of the coil and adapted to produce differing output signals depending on whether of not a reduction in said voltage drop relative to said peak voltage drop occurs, said differing output signals therefore being indicative of whether or not the armature has moved correctly. The resistor typically has a much lower resistance than, for example about 1% of, the resistance of the solenoid coil. The output signal produced by the comparator and which is indicative of the presence or absence of a reduction in the voltage drop, and therefore of correct or incorrect operation of the solenoid, may be utilised in a number of ways. For example, the signal may serve to permit or prevent, as the case may be, continuing operation of the system of which the solenoid forms a part, i.e. it may be an interlock signal. In addition or alternatively, it may serve, in conjunction with a visual or audible indicating means, to provide, possibly at a remote location, visual or audible indication of correct or incorrect operation of the solenoid. The indicating means may, for example, comprise a light-emitting diode adapted, on the one hand, to glow continuously in the event that a reduction in the voltage drop has occurred (thereby indicating that the armature has moved properly) or, on the other hand, to remain unlit or to "blink" in the event that said reduction has not occurred (thereby indicating a fault). The present invention also provides, in a second aspect, a solenoid comprising, in combination, a solenoid coil, an armature that moves in response to initial energisation of the coil and a circuit as defined above. Preferably, the circuit is embodied in a plug to which a power source is connectable and which itself is connectable to the solenoid, the power source serving to drive the solenoid and the circuit. Conveniently, the plug may be connectable, by a simple push-on connection, to the usual connecting tabs of the solenoid. The present invention also provides, in a third aspect, a solenoid-operated fluid flow control valve including the combination as defined by the second aspect of the invention. The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is a graph showing the typical relationship between coil current and time during energisation of a solenoid coil; FIGURE 2 is a schematic circuit diagram of one form of armature movement detection circuit according to the invention for monitoring operation of a solenoid valve; and FIGURE 3 is a detailed diagram of the circuit shown schematically in Figure 2. Referring firstly to Figure 1 of the drawings, this shows a typical relationship of coil current (I) to time (t) when a conventional solenoid is energised. When the current is first switched on (point A on the graph) it gradually rises until the strength of the magnetic field generated by the coil becomes sufficient to draw the armature (eg of soft iron) to a position within it. At this point in time, and provided that the armature moves fully, a significant change in the coil's inductance occurs, whereby the current I flowing through the coil momentarily reduces (point B on the graph). The current I then reverts to a value at, or slightly above, the armature-actuating value (point C on the graph). In accordance with the present invention, that momentary reduction (or otherwise) in the current I flowing through the solenoid coil is utilised to monitor correct (or incorrect) operation of the solenoid. Referring to Figure 2, the solenoid coil 1 associated with the valve (not shown) is connected to a DC control voltage source 2 (typically 12 to 24 volts generated, for example, from a remotely located programmable logic controller, not shown, used to control the system of which the valve forms a part) via a series resistor 3. The voltage drop across the resistor 3, which is proportional to the current flowing through the coil 1, is amplified by an amplifier 4 and the amplified signal is conditioned by an RC circuit 5 comprising a capacitor 6 and resistor 7. The voltage waveform at D in the circuit obviously corresponds to that shown in Figure 1. Thus, it will be seen that, when the current I through the coil 1 momentarily reduces (point B in the graph of Figure 1) the voltage V at E (see Figure 2) remains constant. The difference between the voltage at E and the amplified voltage at D is sensed by a comparator circuit 8 which, via a latching circuit 9 and an oscillator 10 operating at high frequency, causes a light-emitting diode 11 to glow continuously, thereby indicating correct operation of the solenoid, until the circuit has reset after de-energisation of the coil 1. The circuit 8 also produces, in response to correct operation of the solenoid, a voltage signal 12 (see Figure 3) that may be sent to, for example, the remotely located programmable logic controller referred to above. In the event that the armature of the solenoid coil 1 does not move fully, or at all, into the coil 1 upon energisation thereof, the aforesaid reduction in the coil current I will not occur and the voltage at E will be the same as the amplified signal at D and so the diode 11 will, by virtue of the oscillator 10 operating at a much lower frequency, flash on and off thereby indicating a fault condition, eg a jammed armature. Figure 3 sets out, in detail, the circuit shown in Figure 2. The various components are denoted by their conventional symbols and their operation and function will be readily apparent to those skilled in the art. The light-emitting diode 11 or other indicating device may, in particular, as will be appreciated, be located remotely from the coil, for example on a remote control/monitoring console which normally will be linked up to a large number of solenoid-operated devices (which will be connected to respective detection circuits of the invention) such as pneumatic solenoid valves used to control the operation of a machine. Alternatively, for example, the light-emitting diode 11 may be located, in a visible position, on the plug referred to earlier.

Claims: A circuit for combination with a solenoid comprising a solenoid coil (1) and an armature that moves in response to initial energisation of the coil (1), said circuit serving to monitor movement of the armature upon initial energisation of the coil (1) by monitoring the current flowing through a resistor (3) connected in series with the coil (1) and therefore through the coil (1) during initial energisation thereof, characterised in that the circuit comprises: a) a resistor (3) across which a voltage drop proportional to said current is established; b) an RC circuit (5,6,7) responsive to said voltage drop for producing a steady output voltage signal corresponding substantially to the peak voltage drop across the resistor (3) during initial energisation of the coil (1); and c) a comparator (8) for comparing the steady output voltage signal of the RC circuit (5,6,7) with the voltage drop across the resistor (3) during initial energisation of the coil (1) and adapted to produce differing output signals depending on whether or not a reduction in said voltage drop relative to said peak voltage drop occurs, said differing output signals therefore being indicative of whether or not the armature has moved correctly. A circuit according to claim 1 further comprising a visual or audible indicating device (11) for indicating, in response to the output signal of the comparator (8), the presence or absence of said reduction in the voltage drop across the resistor (3) relative to said peak voltage. A circuit according to claim 2 wherein said indicating device (11) comprises a light emitting diode. A circuit according to any one of claims 1 to 3 which is embodied in a plug to which a power source (2) is connectable, the plug being connectable to a solenoid coil (1), and the power source serving to energise both the coil (1) and the circuit. A solenoid comprising, in combination, a solenoid coil (1), an armature that moves in response to initial energisation of the coil (1) and a circuit as claimed in any one of claims 1 to 4. A solenoid-operated fluid flow control valve including a solenoid as claimed in claim 5.

Inventors: PAGE MICHAEL, PAGE, MICHAEL
Applicants: IMI NORGREN LTD, IMI NORGREN LIMITED
Application year: 1992
Main classification: H01F7
IPC codes: H01F7, G05B19