GB2257524A - Device for detecting defects in sensors - Google Patents

Description

2257524 1 - Device for detectina defects in sensors

Prior art

The invention relates to a device for detecting defects in sensors, preferably differential inductive sensors of the generic type of the main claim.

Such sensors, including an associated ev-aluatior. cir.,-,uit, are usually used to sense adjusting paths of components in the case of rotational or 'Linear moveruents. Customary inductive differential sensors are, for example. half -differential short-circukting ring sensors or differential-reactor sensors.

In the case of such sensors, owing to the s- try there is generally the difficulty that. in the ...-..e event of a broken supply lead there is generated at the centre tap in the evaluation circuit a constant direct voltage, the level of which does not leave the usual measurIng range, so that a fault cannot be detected simply with the aid of a so-called signal-range check. In order to detect such a fault, therefore. a further fault detection has to be carried out.

A dif f erential -reactor path sensor which has a series connection of two inductances, the value of which is equal in the central position, is known from German Offenlegungsschrift 31927j833. If the part to be measured moves, one of the inductances becomes less, the other greater to the same extent.

The two inductances are excited by high-frequency alternating voltage, at the connection of the two inductances the measuring signal is picked up. which is processed in the following evaluation circuit for position detection. Although the known sensor arrangement is very accurate, it has the disadvantage that an interruption in one of the supply leads can result in erroneous measured values.

A device for path measurement with the aid of an inductive sensor, in which it is detected in a monitoring circuit arrangement whether a fault occurs in the sensor itself or in one of the supply leads, is known f rom German Of fenlegungsschrift 2,805,876. In the case of this known device, it is checked in particular whether the signal voltage leaves a predeterminable range, for example in the event of interruption of a supply lead or in the event of a short-circuit occurring. This known device cannot, however, detect a fault if, given the existence of special circumstances, the output voltage does not leave the range allowed.

Advantages of the invention The device according to the invention for detecting defects in sensors, having the characterising features of the main claim, has the advantage that, by logically combining two conditions, the existence of which is not possible simultaneously during a relatively long time, the fault or defect is detected. Consequently, a simple fault detection is possible.

Further advantages can be achieved by the measures specified in the subclaims, for example it is possible in the event of a detected fault to trigger an indicator and to perform measures for influencing the controller, so that no inadmissible operating states can occur.

Furthermore., it is advantageous to monitor the actual value additionally for whether it lies in a range 3 - possible under the given circumstances (signal-range check).

- The device according to the invention can be used particularly advantageously in conjunction with an inductive exhaust displacement check-back indicator.

Drawing An exemplary embodiment of the invention is represented in the drawing and explained in further detail in the following description. In this, the single figure shows in block symbols a monitoring device according to the invention for a differential inductive sensor.

Description of the exemplary embodiment

A differential inductive sensor 10, for example an exhaust displacement check-back indicator, which comprises the inductances Ll and L2 as well as a movable part 11,, serves as actual-value pick-up for the following controlled system. The two inductances L1 and L2 are connected in series and are under an oscillating voltage having the peak values +U and -U.

The connection point of the two inductances L1 and L2 is connected to an evaluation circuit 12. at the output of which the actual value is produced. The evaluation circuit 12 is shown in the exemplary embodiment as separate from the sensor, but it may also be a component part of the sensor 10.

The output 13 of the evaluation circuit 12 is connected to an input of the controller 14. The set value is fed to the controller 14 via a further input.

A f irst output of the controller 14 leads to a final control element 15, which acts indirectly or directly on the movable part 11 of the sensor 10. A second output of the controller 14 leads to a logic circuit 16, from this logic circuit 16 a further connection leads back to the controller 14.

Furthermore, the evaluation circuit 12 is conriected via its output 13, at which the actual value is produced, to the logic circuit 16, the second output of which leads to a fault indicator 17. The differential inductive sensor 10 is fed with two voltages in phase opposition, preferably square-wave voltages, of constant frequency and amplitude, in the figure this is represented by +U and -U. In the position of the movable part 11 represented in the figure, the two inductances Ll and L2 are equal, upon a shift of the movable part 11 in the direction of the arrow, the inductance L2 becomes less, whereas the induction Ll becomes greater to the same extent.

The voltage between the two inductances Ll and L2 changes as a function of the position of the movable part 11, the amplitude of this voltage being approximately proportional to the difference between the two inductances Ll and L2.

This voltage is further processed in a suitable way by the evaluation circuit 12, so that an evaluable actual value is produced at the output 13 of this evaluation circuit 12.

Similarly to a set value, this actual value is fed to the controller 14, in which the customary comparison operations between set value and actual value proceed and which ultimately acts via the final control element 15 on the movable part 11, so that its position is controlled in a suitable way.

The position of the movable part 11, which generates a corresponding actual value at the output of the evaluation circuit 12, is now, according to the invention, to be checked together with the output of the controller 14 for plausibility and used for fault detection in the controlled system or in the sensor itself. For this purpose, both the actual value produced at the output 13 of the evaluation circuit 12 and the starting position of the controller 14 are fed to the 1 logic circuit 16. In this logic circuit 16, the two values are compared with each other, a f ault being detected if the actual value lies within a predeterminable range and the controller at the same -time is at one of its stops during a predeterminably longer time. This predeterminable range is in this case f ixed such that it covers all possible values which can be produced at the output 13 of the evaluation circuit 12 if the supply lead is interrupted, it thus lies approximately in the centre of the operating range of the sensor. The fault detection is possible since, on the one hand, if the controller is at one of its stops, the actual value would have to change after some time and consequently would have to leave the predetermined range and, on the other hand, a controller stop should not occur as long as the actual value lies within the predetermined range around the centre of the operating range.

In the event of a detected f ault or defect, an indicator 17 is triggered by the logic circuit, furthermore it is provided that in this case an actuation of the controller 14 is performed by the logic circuit 16 in such a way that the controller 14 shifts the movable part 11 into an uncritical position by means of the final control element 15.

The logic circuit 16 may be configured using analog circuit technology or as a microprocessor, the indicator 17 as a lamp, LED or as an acoustic device. The arrangement represented in the figure can be used for any differential inductive sensors.

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Claims (5)

1. Claims.

   Device for detecting defects in sensors, preferably differential inductive sensors, having an evaluatioii circuit which supplies a signal which serves as actual value f or a control device, the evaluation circuit supplying a constant output signal in the event of a fault occurring, characterised in that a defect is detected if the actual value lies within a predeterminable range and the controller is at the same time at one of its stops during a predeterminably longer time.
2. 2. Device according to Claim 1, characterised in that, if a fault is detected, an indication takes place andlor measures for influencing the controller are performed in such a way that no inadmissible operating states can occur.
3. 3. Device according to one of the preceding claims, characterised in that a further range is defined for the actual value, which range covers all values allowed and a fault is detected if an actual value occurs which lies outside this range.
4. 4. Device according to one of Claims 1 to 3, characterised in that the sensor is an inductive exhaust d isplacement check-back indicator.
5. 5. A device for detecting defects in sensors substantially as herein before described with reference to the accompanying drawing.