DE2918961C2 - Evaluation circuit for an inductive short-circuit ring displacement sensor - Google Patents

Description

State of the art

The invention is based on an evaluation circuit for an inductive short-circuit ring displacement sensor according to the preamble of the claim.

Such an evaluation circuit, which processes the output signal of a short-circuit ring displacement sensor with an inductance that can be changed depending on the displacement, is known from DE-OS 24 16 237. The single coil of the short-circuit ring displacement sensor is alternately connected to a first and second operating voltage by an operational amplifier connected as a voltage comparator. This creates an alternating increasing and decreasing current in the coil. The current flow is converted into a voltage in a working resistor that is used to control the comparator. This creates a triangle-like voltage, the period of which is a measure of the position of the short-circuit ring displacement sensor. Interferences on the inductance of the coil, which lead to a change in inductance regardless of a change in displacement, cannot be detected with such a circuit and therefore cannot be eliminated.

An electronic length measuring device with an electronic converter is known from DE-AS 19 03 051. The converter has two coils that are in a circuit that includes a pulse generator that delivers electrical pulses at intervals to temporarily release the flow of current through the coils, as well as a device for measuring an electrical state variable that depends on the pulses and the coil inductance. The output signals of the two coils must be processed in separate circuits.

The invention is based on the object of increasing the accuracy of the evaluation of the variable inductance by comparing it with the inductance of a second coil.

According to the invention, the measures specified in the characterizing part of patent claim 1 are provided for the solution.

Advantages of the invention

The evaluation circuit according to the invention for an inductive short-circuit ring position sensor has the advantage that the accuracy of the position measurement is increased by forming the difference between the further processed output signals of two coils compared to a design with one coil. By forming the difference, interference is eliminated, since it affects both coils in the same direction. An option to adjust the inductance of at least one coil simplifies adaptation to the required measuring range.

The circuit is characterized by an extremely simple and therefore reliable structure. With the help of two switching elements, which are alternately controlled by a bistable multivibrator, the two measuring coils are alternately connected to the supply voltage via a load resistor. The increase in current in the two measuring coils leads to an increase in voltage at the load resistor, which is fed to an input of the operational amplifier connected as a comparator. The current reduction in the coils is carried out by freewheeling diodes. The comparator generates a pulse train at its output, the period duration of which is proportional to the measured path over two switching processes. The pulse pause after the switching pulse is assigned to the output signal of one coil and the pulse pause after the next switching pulse is assigned to the output signal of the other coil. An increase in inductance in both measuring coils, for example due to environmental influences, has the effect that one pulse pause is extended and the following pulse pause is shortened. The period duration over two switching processes is therefore independent of interference that affects both coils.

The invention is described and explained in more detail below with reference to several embodiments shown in the drawing.

Fig. 1 shows an inductive displacement sensor which converts the adjustment path of a membrane 1 into a change in the inductance of a coil 2 which is arranged on the center leg 3 of an iron core 4. The membrane 1 is connected in a pressure-tight manner to a housing 5 , the interior of which is connected via a pipe socket 6 to the intake pipe of an internal combustion engine (not shown) and is intended to convert the intake air pressure prevailing there, which depends on the driving speed, the throttle valve position and the load state of the internal combustion engine, into an analog electrical quantity with which an operating quantity of the internal combustion engine, for example the ignition adjustment angle or the amount of fuel injected, can be adapted to the prevailing operating conditions.

A helical spring 7 is arranged in the housing 5 , which is supported on the one hand against the membrane 1 and on the other hand against the housing 5 and is arranged coaxially with a stop 8 , which can be adjusted in the axial direction by means of a screw thread 9 and limits the adjustment path s of the membrane center.

In the center of the membrane 1 there is a tubular support 10 made of insulating material for a short-circuit ring 11 attached to its front side, which surrounds the center leg 3 and increases the inductance of the coil 2 the more the pressure in the intake pipe and accordingly the pressure prevailing in the housing 5 drops compared to the external atmospheric pressure and the more the center of the membrane 1 moves in the direction of the path arrow s .

In order to be able to convert the changes in the inductance L 1 of the coil 2 resulting from the changes in the path s into a square wave 12 , an evaluation circuit 13 is provided, described in more detail below, with which a signal U _A analogous to the frequency, the duty cycle or the period of the square wave voltage 12 can be obtained.

According to the invention, the evaluation circuit can be constructed according to the circuit diagram shown in Fig. 2. The main components of this evaluation circuit are an operational amplifier O used as a voltage comparator, a bistable flip-flop FF with two opposing outputs Q and Q, and two switching transistors T 1 and T 2 which serve to alternately switch the coil 2 of the inductive displacement sensor according to Fig. 1 with its variable inductance L 1 and then a second coil 15 , whose inductance L 2 can be set to a fixed value, into the control circuit of the voltage comparator O. For this purpose, the two coils 2 and 15 are fed to a common connection point 16 and connected to a resistor R leading to the common operating current line 17 connected to the positive pole of a vehicle battery (not shown), and are also connected via a line 18 to the inverting input 20 of the voltage comparator O. This is connected via a decoupling resistor 21 , just like the plus input 22 of the comparator via a decoupling resistor 23, to the tap of a voltage divider formed from two resistors 24 and 25. A resistor 26 also leads from the plus input of the comparator O to ground.

From the collectors of the two transistors T 1 and T 2 , which are connected to the coils 2 and 15 respectively, a diode D 1 or D 2 leads to the cathode of a Zener diode Z , the anode of which is connected to the two coils and the resistor R.

The transistors T 1 and T 2 act as switches and control the coils 2 and 15 alternately. The voltage taken off across the resistor R is proportional to the current flowing through the coil being controlled, the temporal progression of which is indicated in Fig. 3 by the line 27. If U _B denotes the voltage on the positive line 17 , a control voltage U _S results at the negative input 20 of the comparator O according to the equation °=c:30&udf54;&udf53;vu10&udf54;&udf53;vz2&udf54;&udf53;vu10&udf54;where t ₁/ t ₂ is the duty cycle of the square wave.

If the threshold voltage set at the comparator O is undershot, the comparator switches its output to a positive supply voltage. The subsequent flip-flop changes its output state and thus controls the other coil via the corresponding transistor T 1 or T 2. While the current in the non-controlled coil is extinguished via the associated diode D 1 , D 2 and the Zener diode Z , the activated coil integrates the current in proportion to its inductance until the threshold voltage of the comparator is reached again.

If one uses the square wave voltage shown in the top line of Fig. 3 at the Q output of the flip-flop FF , which is represented by the line 28 in Fig. 3, the following results by transforming the above equation: °=c:30&udf54;&udf53;vu10&udf54;&udf53;vz2&udf54;&udf53;vu10&udf54;

This means that the pulse duration t 1 is proportional to the inductance L 1 and the pause duration t 2 is proportional to the inductance L 2 of the comparison coil 15. The output signal indicated at U _A in Fig. 2 can be further processed both digitally and analogously by averaging. The switching pulses 29 produced at the output of the comparator O can be used for digital evaluation; these are indicated in the lowest line in Fig. 3.

Fig. 4 shows a second inductive short-circuit ring sensor which is designed as a differential angle sensor and allows the conversion of the angle of rotation α of a shaft 30 , for example the throttle valve shaft of an internal combustion engine, into an analog electrical quantity.

In detail, the rotary encoder according to Fig. 4 has two concentric, semi-cylindrical ring segments 31 and 32 , which are connected to one another by radial magnetic webs 33 and 34. Close to these webs there is one of two coils 35 and 36 , respectively, the inductances of which are designated L 1 and L 2 , respectively. On the shaft 30 there is a cantilever 37 made of metal, which ends in a short-circuit ring 38 , which encloses the outer ring segment 31 and, when the shaft 30 rotates, changes the inductances L 1 and L 2 of the two coils 35 and 36 in opposite directions, namely the inductance of the coil 35 is reduced, while at the same time the inductance L 2 of the coil 36 is increased and vice versa.

The circuit shown in Fig. 2 can be used to evaluate the two inductances L 1 and L 2 which can be changed depending on the swivel angle α. Fig. 5 shows another evaluation circuit which also oscillates automatically and produces an output voltage U _A at its output A which is analogous to the clock ratio, frequency and/or period.

In detail, the evaluation circuit according to Fig. 5, in which the components corresponding to the circuit according to Fig. 3 are provided with the same reference numerals, contains the operational amplifier O used as a voltage comparator, two switching amplifiers S 1 and S 2 for the two inductances L 1 and L 2 as well as a clock amplifier V connected between the output of the operational amplifier O and the input of the flip-flop FF and two isolation amplifiers P 1 and P 2 connected to the outputs Q and ≙.

The evaluation circuit according to Fig. 5 works in principle in the same way as that according to Fig. 3, so that a detailed functional description is unnecessary.

Claims (1)

Evaluation circuit for an inductive short-circuit ring displacement sensor, which carries a coil on an iron core, the inductance of which can be changed by a short-circuit ring which encloses a leg of the iron core and can be displaced along this leg by a distance, the respective size of which is converted into an analogue electrical signal, in particular a voltage, with the aid of an operational amplifier connected as a voltage comparator, which generates a square wave whose period depends on the respective inductance value of the coil, characterized in that
a) a second coil ( 15 , L 2 ) is provided on the iron core ( 31 ), b) the two coils ( 2 , L 1 , 15 , L 2 ) can be connected to a supply voltage (U _Batt ) by switching means (T 1 , T 2 ), c) the voltages applied to the two coils ( 2 , L 1, 15 , L 2 ) are fed to the operational amplifier (O) connected as a voltage comparator and d) the output of the comparator (O) is followed by a bistable multivibrator (FF) , to whose two complementary outputs (Q, ≙) one of the two switching means (T 1 , T 2 ) is connected.