Process and apparatus for drop-off recognition in a magnetically operated device

A method and apparatus for the recognition of an armature drop off in an electromagnetically actuated device supplies an on/off controlled switching current to the solenoid coil of the device. A high level of current is caused to flow through the coil for attracting the armature, and reduced level of current is applied to the coil for holding the solenoid armature in its on position. If the armature should be caused to drop off from its on position by a mechanical shock, a transitory expansion in the period of the switching current occurs, due to the sudden decrease in magnetic circuit inductance. This increased switching current period is detected and compared to a predetermined threshold value. If the detected period exceeds the threshold value, it is recognized as an armature drop off, and corrective action can be taken immediately. An alternate embodiment detects a sudden increase in the holding current amplitude as an indicator of armature drop off.

FIELD OF THE INVENTION
 The present invention relates to a process and apparatus for the
 recognition of the state of a magnetically operated device.
 More specifically, the present invention relates to a sensing technique for
 armature drop off in a device such as a solenoid valve.
 BACKGROUND OF THE INVENTION
 It is well known in the solenoid valve art that solenoid coil power is
 supplied by either a longitudinal regulator or a switching regulator. In
 either case, the function of the regulator is to adjust and control the
 supply voltage at an appropriate level for the magnetizing solenoid coil.
 Since a switching regulator is generally more efficient than a longitudinal
 regulator, it is usually preferred in solenoid valve applications. A
 switching regulator alternately switches the supply voltage on and off to
 the solenoid coil. When the supply is switched on, the resultant current
 flow in the solenoid coil rises exponentially. When the current level
 reaches an upper limit value, the regulator shuts the supply off. As a
 result, the current then drops exponentially. When the current reaches a
 lower limit value, the supply voltage is again switched on by the action
 of the regulator. Thus, a mean supply voltage is established, which is
 regulated at a suitable value for a particular magnetizing solenoid coil.
 This type of process is disclosed in the German patent application DE 38
 17 770.
 To be able to switch on a solenoid valve rapidly and reliably, a relatively
 strong current is initially required for the magnetizing coil. The
 resulting magnetic force causes the armature of the solenoid to be moved
 against the force of a return spring. When the armature has reached its
 switch on position, the magnetizing current is lowered to a holding value,
 which is sufficient to hold the armature in place. This holding current is
 preferably adjusted to the lowest possible value, in order to save energy
 during continuous operation. There is, of course, a nominal holding
 current limit for reliably preventing the armature from dropping off. This
 nominal current limit must be set conservatively, i.e., at a higher than
 minimal level, if the solenoid valve is located in an area exposed to
 mechanical shock and vibration. This is generally the case, for example,
 if the solenoid valve is installed in a motor vehicle, especially in the
 proximity of the engine. This type of environment may cause a holding
 armature to drop off accidentally, causing the solenoid valve to shut off
 (or on) a pressure medium. Such an erroneous actuation should be avoided
 in any situation, but especially when the solenoid valve is used in an
 application that is relevant for safety, e.g., in the anti-lock braking
 system of a vehicle.
 In the prior art, as disclosed in German patent applications DE 27 28 666
 and DE 38 17 770, there are known techniques for recognizing the type of
 solenoid failure caused by a jammed armature. In these configurations, the
 switch on current waveform is monitored to detect a sudden deflection,
 which typically occurs at the time of armature movement.
 However, the present invention has a different objective; namely, to detect
 armature drop off from a holding position as a result of a mechanical
 disturbance, and to take immediate corrective actions, including automatic
 restart and transmittal of an error message.
 It is a further object of the present invention to detect armature drop off
 without the addition of special sensors.
 It is yet a further object of the present invention to enable a significant
 reduction in the holding current safety margin, to such an extent that the
 power loss of a solenoid valve in continuous operation can be reduced by
 half. As a result, the structural volume of the solenoid valve can also be
 reduced.
 SUMMARY OF THE INVENTION
 In accordance with an illustrative embodiment of the present invention, a
 method for the recognition of an armature drop off within a solenoid valve
 is as follows:
 a) supplying a large magnitude regulated switching current to the solenoid
 valve coil to achieve switch on of the solenoid valve armature,
 b) reducing the switching current magnitude to a lower level which is
 sufficient to hold the armature in the switch on position,
 c) monitoring the period of the lower level switching current with a time
 period measuring device,
 d) comparing the measured lower level switching current period with a
 predetermined threshold value, which represents a transitory expanded
 lower level switching current period attributable to a drop off of the
 armature,
 e) generating an error signal when the measured lower level switching
 current period exceeds the predetermined threshold value, and
 f) restarting the initial turn on cycle in order to return the armature to
 its switch on position.
 An alternative embodiment of the present invention monitors the amplitude
 of the coil current, in order to detect a sudden increase in the lower
 level switching current, which is also indicative of an armature drop off.
 The aforementioned illustrative embodiments of the present invention are
 more fully described below in conjunction with the following drawings.

DETAILED DESCRIPTION OF THE INVENTION
 As shown in FIG. 1, a solenoid valve MV is powered by a switching regulator
 SR in an on/off operation. Switching regulator SR receives its power from
 supply voltage U.sub.B, and provides an alternating (switching) coil
 current i through the coil winding of solenoid valve MV. An electronic
 system E serves to control switching regulator SR. Electronic system E can
 also receive signals, such as the regulating frequency, from switching
 regulator SR.
 FIG. 2 shows a variant of the actuating circuitry for solenoid valve MV. In
 this configuration, the coil of solenoid valve MV is driven by alternating
 pulses, which are generated by a micro-controller .mu.C, via an amplifier
 V. The circuit is powered by a supply voltage U.sub.B. The resultant
 current i through the solenoid coil is sensed by means of a measuring
 resistance R.sub.M, which is connected in series with the coil of solenoid
 valve MV. Resistance R.sub.M is approximately 0.1 ohm, and provides
 feedback of a coil current waveform signal to micro-controller .mu.C. A
 recovery diode D is also provided to facilitate the switching off of the
 coil current i, and is connected in parallel with the coil of solenoid
 valve MV.
 FIG. 3 shows a graph of coil current i versus time t. At time t.sub.0,
 solenoid valve MV is switched on, by switching regulator SR or by
 micro-controller .mu.C, connecting the full supply voltage U.sub.B across
 the coil of solenoid valve MV. Current i rises exponentially until it
 reaches the upper limit value i.sub.3 at time is t.sub.2. The limit value
 i.sub.3 is sensed by switching regulator SR, or by micro-controller .mu.C,
 causing supply voltage U.sub.B to be switched off.
 Prior to reaching upper limit value i.sub.3, the switch-on current waveform
 shows a small deflection at t.sub.1, which typically occurs at the time of
 armature movement.
 Since the initial voltage U.sub.B impressed across the solenoid coil is
 considerably higher than its nominal (holding) voltage, solenoid valve MV
 is switched on very rapidly. Upper limit current i.sub.3, however, is not
 suitable for continuous operation of solenoid valve MV. Therefore, when
 limit value i.sub.3 is reached at time t.sub.2 switching regulator SR, or
 micro-controller .mu.C, switches supply voltage U.sub.B off. Current i
 then drops exponentially via the recovery diode in the driver circuitry
 (within switching regulator SR in FIG. 1, or through diode D in FIG. 2).
 When current i reaches a lower limit value i.sub.1 at time t.sub.3, an
 on/off regulation cycle begins. That is, supply voltage U.sub.B is
 switched on again until current i reaches holding limit value i.sub.2 at
 time t.sub.4. At this time, supply voltage U.sub.B is again switched off.
 Consequently, coil current i varies within the limits i.sub.1 and i.sub.2,
 in a holding operational state. The resultant mean current level is such
 that the armature of solenoid valve MV is held securely in the switched on
 state. This mean (holding) current level, however, would not be sufficient
 for switching on solenoid valve MV initially.
 At time t.sub.5, it is assumed that a mechanical shock impacts solenoid
 valve (MV) with sufficient magnitude to overcome the holding current
 force. This causes the solenoid armature to drop off from its switch on
 position. Depending on the construction of solenoid valve MV, the number
 of windings in the magnetizing coil, and the current level I.sub.1 flowing
 at time t.sub.5, a characteristic perturbation of the current flow occurs,
 as shown in FIG. 3, between t.sub.5 and t.sub.6.
 This current flow perturbation is due to the fact that when the armature
 drops off, the air gap in the magnetic circuit of solenoid valve MV is
 increased, which causes the magnetic circuit inductance to decrease, in
 accordance with the following equation:
 L=N.sup.2 /(R.sub.Fe +R.sub.air) Equation (1)
 where
 L=inductance of the magnetic circuit
 N=number of windings in the coil
 R.sub.Fe =magnetic resistance of the solenoid core
 R.sub.air =magnetic resistance of the solenoid air gap
 The electromagnetic energy W (=1/2 Li.sup.2), which was previously stored
 in the magnetic circuit, is discharged at t.sub.5, and causes a brief rise
 of coil current i, from value I.sub.1 to value I.sub.2. Current i then
 drops again to value i.sub.1 at time t.sub.6.
 From t.sub.6 on, the normal on/off switching cycle continues, except that
 the switching frequency of holding current i increases, as shown by the
 shorter time intervals after t.sub.6, in FIG. 3. This increase in
 switching frequency is due to the decreased inductance of the magnetic
 circuit, as a result of the increased air gap.
 The inventive method and apparatus is designed to evaluate the distinctive
 expansion of the current i waveform from time t.sub.5 to time t.sub.6,
 which occurs as a result of the armature drop off. This type of evaluation
 can be readily implemented by suitable electronic systems or conventional
 micro-controllers.
 Importantly, the expansion of the period duration of coil current i from
 t.sub.5 to t.sub.6 can be evaluated for error recognition, i.e., armature
 drop off. This expanded period duration is approximately double that of a
 normal period duration. Therefore, either electronic system E (FIG. 1) or
 micro-controller .mu.C (FIG. 2) can transmit an error message when a
 current i period duration exceeds a fixed limit value, as in the case of
 the time period t.sub.5 to t.sub.6, in FIG. 3.
 It is especially advantageous if a new switch on impulse cycle (t.sub.0
 -t.sub.3) can be restarted automatically upon recognition of an armature
 drop off. When this is done, the failure is corrected immediately.
 In accordance with the inventive method, electronic system E, connected to
 the switch regulator SR in FIG. 1, or micro-controller .mu.C in FIG. 2,
 can be provided with a device which senses the period duration of coil
 current i. This period duration can be determined by measuring the time
 intervals between the minimal value levels (i.sub.1) of coil current i, as
 shown in FIG. 3.
 In an alternative embodiment, the increase of coil current i to a maximum
 value I.sub.2 (after t.sub.5 in FIG. 3), which occurs during the drop off
 of the armature during a holding period, can also be evaluated for error
 recognition. Preferably, this is implemented by electronic system E or
 micro-controller .mu.C transmitting an error message when coil current i
 exceeds a predetermined limit value. As in the previous embodiment, a new
 switch-on impulse cycle (t.sub.0 -t.sub.3) can be restarted automatically
 upon recognition of an armature drop off.
 In order to evaluate the level of current i, electronic system E or
 micro-controller .mu.C can be equipped with a device for measuring the
 magnitude of coil current i. This can be done conveniently by measuring
 the voltage drop across measuring resistance R.sub.M, in FIG. 2.
 As the micro-controller .mu.C, the readily available model C167 of the
 Siemens Corporation may be used to advantage. As the switch regulator SR
 with associated electronic system E, model TCA965B of the Siemens
 Corporation can be used in the present invention. These devices
 incorporate all of the components and integrated circuits required for
 carrying out the functions described above, e.g., generating the error
 signal, measuring the time periods and amplitudes, various integrations,
 etc.
 FIG. 4 is a flow chart illustrating the operations of a first embodiment of
 the invention. These operations are carried out by the electronic system E
 of FIG. 1 or the micro-controller .mu.C in FIG. 2. For purposes of
 brevity, the following description will refer to the electronic system E
 and the micro-controller .mu.C as the "controller."
 In decision block 102, the controller determines whether the solenoid valve
 MV is to be switched on or not. Upon receipt of an appropriate signal, the
 controller switches on the solenoid valve MV as previously described so
 that a resultant current of a first magnitude flows through the coil of
 the solenoid valve MV. This is illustrated in block 104. In decision block
 106, the controller determines whether the solenoid valve is to remain in
 the switch on position or holding state. If the solenoid valve is to
 remain in the holding state, the controller measures the length of the
 time periods of the cycles illustrated in FIG. 3, as illustrated in block
 108. In decision block 110, the controller determines whether the length
 of a time period of such cycles exceeds a predetermined threshold. If such
 threshold is not exceeded, steps 106, 108, and 110 are repeated. However,
 if a time period exceeds such predetermined threshold, an error signal is
 generated as illustrated in block 112. In the preferred embodiment of the
 invention, the controller automatically goes back to step 104 in order to
 bring the solenoid valve back to the switch on position.
 FIG. 5 is a flow chart illustrating a second embodiment of the invention.
 Like blocks in FIG. 5 have been given the same numbers as in FIG. 4.
 However, in FIG. 5, instead of measuring the time periods of the cycles,
 in block 114 the controller measures the amplitude of the current flowing
 through the coil during the holding state of the solenoid valve. In block
 116, if the amplitude exceeds a predetermined threshold, the controller
 determines that there has been armature drop off and an error signal is
 generated in block 112. As in the case of FIG. 4, the controller
 automatically recycles back to step 104.
 In short, a method and apparatus is disclosed which enables the rapid
 detection and recovery of an armature drop off in a solenoid valve.
 Moreover, the disclosed invention can also be implemented with any type of
 electromagnetically actuated armature device.
 The above described embodiments of the invention are intended to be
 illustrative only. Numerous alternative embodiments may be devised by
 those skilled in the art without departing from the spirit and scope of
 the following claims.