Patent Description:
It is an essential security feature of household appliances that a malfunction of a relay is determined, especially if the relay switches high current load. An example for a malfunction is a relay that is stuck after it has been switched off.

When working normally, as soon as the relay is switched OFF (i.e. the current coil is switched OFF), the armature moves fully back to its relaxed position (open state). This backwards movement is caused by spring power, mostly by two springs, a return spring and a contact compression spring. In addition, when working normally, the armature starts moving backwards delayed after the relay has been switched OFF. The reason for the delay is the fact that the magnetic force has to collapse below a certain level in order to release the anchor. However, sometimes the relay does not open properly, due to various reasons such that the anchor is stuck to the coil.

A commonly used mechanical relay in different positions is shown in <FIG>. The relay <NUM> consists of a static coil <NUM> and an armature having the following major parts: anchor mechanics <NUM>, anchor return spring (not shown), contact compression spring <NUM> and movable electric contact <NUM>. The relay has three basic anchor/contact positions. In <FIG> the relay <NUM> is shown in its normally open state (NO). The coil <NUM> is not energised and the anchor <NUM> has not been pulled towards the coil <NUM>. As a consequence, there is no electrical contact between the movable contact <NUM> and the static contact <NUM>. In <FIG>, the relay <NUM> is shown in its transition state. In this state, the coil <NUM> is energised and a magnetic force is built up such that the anchor <NUM> is slightly pulled towards the coil <NUM>. As a consequence, the electrical contact is created between the movable contact <NUM> and the static contact <NUM>. In <FIG>, the relay <NUM> is shown in its full closed state. In this state, the magnetic force is fully established and the anchor <NUM> is pulled fully towards the coil <NUM>. As a consequence, the electrical contact is fully closed between the movable contact <NUM> and the static contact <NUM>. Beside this shown relay, where the movable contact is firmly connected with the relay armature anchor part, several other types of relays exist. For instance, relays with a reinforced insulation may have a lever mechanism made from insulated material placed between the anchor and movable contact parts. These mechanisms can be in the form of a push-pull-mechanism, adapted to move the movable contact in both directions (ON and OFF). Alternative, only a push-mechanism can be provided, adapted to move the movable contact in push direction (ON) only. The push-pull mechanism may have some movement clearance between anchor and movable contact compression spring, or may even feature some mechanical clearance hysteresis.

It is an object of the invention to provide a reliable method for stuck relay detection, an electric circuit for a reliable stuck relay detection and an electric household appliance fulfilling high electric safety standards.

This object is solved by a method with the features of claim <NUM>, by an electric circuit with the features of claim <NUM> and by a household appliance with the features of claim <NUM>. Advantageous embodiments are disclosed in the dependent claims, the description and the figures.

According to the invention, a method for relay stuck detection, comprises the steps:.

According to the invention, the stuck relay detection works on the low voltage side by detecting if there is a change in a <NUM>rd derivative peak of the I(t) curve outside a given tolerance, which appears at the time of an expected time when the relay contact switches OFF. Preferably, the negative peak value is taken into account. The switch OFF time is the interval or period of time between a de-energizing of the relay coil to switch OFF normally open (NO) relay contact and time point when the relay contacts release their touch. This is notably the time between two large negative peaks on 3rd derivative peaks.

If there is no significant difference between the compared first peaks, a comparison between a second peak on the <NUM>rd differential curve and a second stored peak value representing a part of the switch OFF behaviour of the standard relay is taken into account for the evaluation. This can be the case if the detected first peak has an amplitude (negative amplitude) that is similar than the amplitude (negative amplitude) of the first stored peak directly after the relay has been switched OFF. At first sight, it seems that the detected relay works well, as a first <NUM>rd derivative peak having a high amplitude appears when the contacts lose their contact. However, in order to review this, a second peak value is detected and compared with stored second peak value.

In order to check if the detected relay switch-OFF time differ from a stored switch-OFF time, which can also be a signal for a failure of the relay, a calculated time difference between the detected first peak and the detected second peak of the monitored relay is compared with a stored time difference between the stored first peak and the stored second peak and taken into account for the evaluation. Thus, the switch OFF time can also indicate a possible relay failure, but preferably, the time difference is not sufficient to confirm if the relay is stuck.

In other words; the inventive method follows the following workflow during its switch OFF procedure:.

In general. an expected relay switching OFF time confirms the positive peak on <NUM>st derivative I(t) curve. Depending on particular relay type, the expected relay switch OFF time for stuck relay can be shorter, untouched or even longer than that for normally working relay. If the relay armature is not movable due to stuck contacts for instance, said derivative peaks disappear at all.

The workflow according to the invention works well for relays with relay armature firmly connected to movable contact and its compression spring and for push-pull type armature relays with low mechanical clearance and/or mechanical hysteresis. The details of electrical voltage/current values of I(t) curve, its derivatives and particular peaks respectively are relay type specific and should be adjusted and pre-memorized for relay types used to define status of normally working relay.

The proposed stuck relay detection technique can be used as standalone feature or as an add-on feature if the appliance already uses zero cross relay switching technique as exemplary described in <CIT> of the applicant. Both the techniques can form very powerful tool to switch the appliance relays, to monitor and predict their health status and to detect possible failure of relay switching contacts. All the control and detection is made solely on relay low voltage side without any measurement or controlling on its high voltage side.

According to the invention, an electronic circuit adapted to relay stuck detection, comprises a relay having a coil to be monitored and three subsequent differentiators in series each calculating one derivative of the relay coil current over the time I(t), wherein between the first differentiator and the second differentiator a low pass filter is provided and wherein the output signal of the first differentiator and the output signal of the second differentiator is amplified by a first amplifier and by a second amplified, respectively.

The differentiators reliable enable to calculate the <NUM>rd derivate according to the inventive method. The low pass filter eliminates efficiently higher interferences frequencies.

In order to eliminate inductive peaks from the relay coil and/or in order to avoid voltage peaks which can appear when a relay is switched OFF, a flyback diode can be provided in parallel to the relay coil.

In order to pick-up the relay current I(t) of the monitored relay, a resistor provided for measuring a voltage can be positioned in series with the relay coil.

Alternatively, a resistor is positioned in series with the flyback diode in order to pick-up the relay current I(t) of the monitored relay.

According to the invention, an electric household appliance comprises an inventive electric circuit. Such a household appliance fulfils a high electric safety standard as any malfunction of a relay will be detected easy and reliable.

In the following, preferred embodiments of the present invention are explained with respect to the accompanying drawings. As is to be understood, the various elements and components are depicted as examples only, may be facultative and/or combined in a manner different than that depicted. Reference signs for related elements are used comprehensively and not defined again for each figure. Shown is schematically in.

<FIG> shows a I(t) pattern <NUM> when switching OFF a relay <NUM> with normally movable NO contact. The second and third derivatives <NUM>, <NUM> are uncompensated for slight time delays coming from time constants of each differentiator circuits. Each curve peaks are relative in their measures, controllable by differentiator electronics to facilitate the detection. <FIG> shows an isolated <NUM>rd derivate curve <NUM> of the relay <NUM>.

The exact time of the relay switch OFF defines the middle of positive peak on the first derivate curve <NUM>. It should be noted that this time point does not fit with time point of a local positive peak at the I(t) curve <NUM>, it happens a bit earlier. Irregularities shown in the current I(t) curve <NUM> are based on from mechanical dynamics of the relay armature.

When comparing the I(t) curve <NUM> to a body trajectory in physics of mechanical dynamics, its first derivative dl/dt <NUM> represents speed of position change, its second derivative d<NUM>l/dt<NUM> <NUM> represents acceleration and its third derivative d<NUM>l/dt<NUM> <NUM> represents jerk. The jerk means a sudden change in body acceleration. Such mathematical representation is an acceptable view on relay behaviour, because a relay armature movement defines at any time an instant value of a relay anchor gap. This defines further an instant value of a relay coil inductance. In the figures, any peak on the jerk curve is a point of sudden strike or collision the moving body (relay armature) is exposed to. The peaks on the third derivative curve <NUM>, indicating sudden changes in armature acceleration.

In the event the relay moving contact is stuck, the relay armature is exposed to a sudden, sharp jerk, when trying to move during the switch OFF procedure. This is detectable by a peak <NUM> on the third derivative curve <NUM> and appears at the expected time of contact touch releasing. If the contact is normally movable, the expected peak <NUM> at a second breakpoint should be rather small. The reason for the peak <NUM> is a sudden mass addition due to the moving of the armature parts. If the movable contact is stuck, for instance welded together with static contact part, the moving armature is subjected to a sudden strike, when trying to peel it off, to release it from static contact part. This state can be detected by the large peak <NUM> on the d<NUM>/dt<NUM> curve appearing at a time of expected contact touch releasing.

If the movable contact of the relay were stuck for some reason, the second negative peak <NUM> on 3rd derivative curve <NUM> would be much higher due to sharp relay armature acceleration change, when jerking coming from stuck contact. This is illustrated in <FIG>.

The principle of stuck relay detection is to detect a significant change in the <NUM>rd derivative peaks <NUM>, which appears at the time of expected time t2 when the relay contact switches OFF.

Each relay has following key parameters, which may be relay type specific:.

In <FIG>, a <NUM>rd derivate of coil current I(t) of a normal working relay (<FIG>) and of a monitored relay (<FIG>) is given. A high first negative peak <NUM> and a low second negative peak <NUM> after a specific period of time (t1 - t2) are clearly detectable. The specific period of time is the switch-OFF time of the relay.

As can be seen, both curves <NUM>, <NUM> shows high first negative peaks <NUM>, <NUM> at the beginning of the switch-OFF time. In order to clarify, if the monitored relay is working abnormal, i.e. if it is stuck, second peaks <NUM>, <NUM> on both third derivate curves <NUM>, <NUM> are detected and compared regarding their (negative) amplitude and their time difference (t1 - t2) measured from the appearance of the first peaks <NUM>, <NUM> to the appearance of the second peaks <NUM>, <NUM>. In the shown case, the second negative peak value <NUM> of the monitored relay appears earlier and has a higher negative value than the stored second peak <NUM> of the normal working relay. Both the smaller time difference (t1 - t2) and the higher amplitude give a hint to a misfunction of the monitored relay.

Moreover, the movable contact can be stuck also in a position beyond its standard fully closed state. This may happen due to scattering (removing) part of relay contact material in course of relay ageing.

For a proper detection, it is important to understand if the relay armature is still movable (despite the movable contact is stuck for some reason) or not. That means, one can face two different situations:.

If the relay armature still moves, the <NUM>rd derivative curve <NUM> showing a high first peak <NUM> and a high second peak <NUM> as illustrated in <FIG>. The event that the relay magnetic force system cannot move the armature is illustrated in <FIG>.

As shown in the pattern <NUM> of <FIG>, if the relay armature does not move during the relay switching OFF, the I(t) curve <NUM> is similar to a standard exponential I(t) curve observed at an electromagnetic coil with fixed inductance. There are no significant first peaks on any of said derivative curves <NUM>, <NUM>, <NUM>. In particular, there is only a small first negative peak <NUM> on the <NUM>rd derivate curve <NUM> and no second peak <NUM> on the <NUM>rd derivative curve at the expected relay switch OFF time at all.

In <FIG>, a preferable analogue circuit diagram to generate <NUM>st, <NUM>nd, and <NUM>rd derivatives of an I(t) curve by three-step C-R differentiator <NUM> is given. The electronic circuit <NUM> is adapted to relay stuck detection, comprising a relay L1 having a coil to be monitored and three subsequent differentiators C1, R3; C3, R6 and C4, R9 in series, each calculating one derivative of the relay coil current over the time I(t). Between the first differentiator C1, R3 and the second differentiator C3, R6 a low pass filter R2, C2, R5, R4 is provided. The output signal of the first differentiator C1, R3 and the output signal of the second differentiator C3, R6 is amplified by a first amplifier U1 and by a second amplifier U2, respectively. For instance, the first C-R differentiator step includes R1, R2, C1, R3, U1, C2, R4, R5.

In detail, the elements of the of three-step C-R differentiator circuit <NUM> are as followed:.

An I(t) curve pick-up technique is to pick up the voltage from a measuring resistor which is connected in series to the relay coil, or in series to the relay flyback diode D1 (diode suppressing inductive voltage peaks when switching the relay). When the measuring resistor in series with flyback diode D1 is used, it is possible to measure only the current when the relay is switched OFF. This does not limit the proposed <NUM>rd derivative stuck relay detection.

<FIG> is a detailed view <NUM> of <FIG> and shows a basic schematic representation for picking up the I(t) curve and converting it into voltage. A resistor R1 provided for measuring a voltage in order to pick-up the relay current I(t) of the monitored relay L1 is in series with the relay coil.

In <FIG>, an alternative <NUM> to the embodiment shown in <FIG> is illustrated. The resistor R1 provided for measuring the voltage in order to pick-up the relay current I(t) of the monitored relay L1 is positioned in series with the flyback diode D1.

If the resistor R1 is connected in series with relay coil L1, the I(t) curve is picked up in both cases; when switching the relay L1 ON and OFF. If the resistor R1 is incorporated in the flyback diode circuit (flyback diode D1), picking-up is possible only for the switch OFF procedure. In this case, the resistor R1 does not lower the voltage on the relay L1 during its switching ON procedure.

Claim 1:
Method for relay stuck detection, comprising the steps:
• calculating the <NUM>rd derivative of a detected coil current I(t) of a monitored relay during its switch OFF time,
• detecting at least one peak (<NUM>, <NUM>) on a <NUM>rd differential curve (<NUM>, <NUM>) based on that <NUM>rd derivative,
• comparing the value of the at least one peak (<NUM>, <NUM>) with a stored peak value (<NUM>) representing a part of a switch OFF behaviour of a standard relay,
• evaluating at least on the basis of the peak comparison if the monitored relay is defective.