DE102018121561A1 - High-resolution induction / frequency measurement with a slow microcontroller - Google Patents

Abstract

The invention relates to an inductive sensor (15) comprising an LC oscillator (18), which comprises an LC resonant circuit (17) with a capacitor (19) and coil (16), which can generate a high-frequency alternating magnetic field, and an evaluation unit which is set up to detect a change in the signal of the LC oscillator (18) by means of a metallic object introduced into the alternating field, the capacitance of the capacitor (19) being adjustable and the LC oscillator (18) being part of a phase locked loop or Frequency control loop of the evaluation unit, which comprises a reference oscillator (22), which provides a reference signal, and a detector (21), which is designed to compare the output signal of the LC oscillator (18) with the reference signal, the output signal the control loop for controlling the resonance frequency of the LC resonant circuit (17) is fed back to the capacitor (19) for setting the capacitance of the capacitor (19), so d ass the resonance frequency of the LC resonant circuit (17) corresponds to the frequency of the reference oscillator.

Description

The present invention relates to an inductive sensor with the features of the preamble of claim 1, an electromechanical power steering system for a motor vehicle having the inductive sensor and a method for high-resolution measurement of an angle of rotation with an inductive sensor with the features of the preamble of claim 9.

Steering angle sensors measure the turning angle of the steering wheel of a motor vehicle using turning angle sensors. Accordingly, the term “steering angle” in the present description should be understood to mean the angle of rotation of the steering shaft and the steering wheel coupled to it, but not the turning angle of the steered wheels of the vehicle.

Angle sensors are also used for torque sensors. Here, two shaft parts that can be rotated to a limited extent are elastically coupled to one another via a torsion spring. If a shaft part is rotated against the other shaft part by a torque applied by the driver of the vehicle, the relative angle of rotation is essentially proportional to the torque introduced. For an accurate determination of the torque, it is important to be able to measure the angle of rotation precisely.

Inductive rotation angle sensors are known which have a high accuracy. Essentially, an inductive sensor consists of three main elements. An oscillator, which is a self-oscillating system, the frequency measuring unit, which both compares and evaluates the data, and the output unit, which converts the values into an electrical signal. In operation, inductive sensors generate a magnetic, high-frequency alternating field with the help of a magnetic coil. The induced eddy currents cause a change in the impedance of the magnetic coil. This impedance counteracts the magnetic field and is electronically converted into a switching signal. If a metallic object moves in the resulting magnetic field, an eddy current is induced in the object, which counteracts the magnetic field and thus withdraws energy. The resulting change in frequency is measured with a reference oscillator.

Microcontrollers are generally used in inductive sensors. Microcontrollers are small computers on a single integrated circuit. They contain one or more processor cores as well as non-volatile program memory, SRAM data memory and programmable peripheral devices such as timers, ADCs and DACs. Inductive rotation angle sensors require a high resolution of the frequency or period measurement for their use in steering shafts. This is done with a microcontroller that has a timer with a high clock frequency. Such timers are usually only used in fast microcontrollers that are expensive.

The disclosure EP 1 217 334 A2 discloses an inductive rotation angle sensor with an oscillator circuit which drives an oscillating circuit with a pulsed signal at a predetermined frequency. The resonant circuit comprises a resistor, the excitation coil of the sensor and a capacitor. The impedance fluctuations of the excitation coil lead to a phase shift of the voltage signal on both sides of the capacitor, which is measured by a detection unit in order to then determine a steering angle from it.

It is an object of the present invention to provide an inductive sensor which has a high resolution and is nevertheless inexpensive.

This object is achieved by an inductive sensor with the features of claim 1 and by a method for high-resolution measurement of an angle of rotation with an inductive sensor with the features of claim 9.

Accordingly, an inductive sensor has an LC oscillator, which comprises an LC resonant circuit with a capacitor and coil, which can generate a high-frequency alternating magnetic field, and an evaluation unit, which is set up to change the signal of the LC oscillator by means of a to detect a metallic object introduced into the alternating field. The capacitance of the capacitor is adjustable and the LC oscillator is part of a phase locked loop (PLL) or frequency locked loop (FLL) of the evaluation unit, which comprises a reference oscillator that provides a reference signal and a detector that is designed to output the signal compare the LC oscillator with the reference signal, the output signal of the control loop for controlling the resonance frequency of the LC resonant circuit being fed back to the capacitor for adjusting the capacitance of the capacitor, so that the resonant frequency of the LC resonant circuit corresponds to the frequency of the reference oscillator . The resonance frequency of the LC oscillator can thus be kept constant and at a predetermined value. A fast microcontroller can thus be dispensed with, which makes the sensor particularly cost-effective.

The capacitance of the capacitor is preferably adjustable by means of a varactor diode. The In this case, the returned signal is used to control the varactor diode.

Depending on which control loop is used, the detector comprises at least one detector from the following list: phase-frequency detector, phase detector, frequency detector.

A charge pump is preferably provided, which is set up to convert the output signals of the detector into an equivalent current and to pass them on to a loop filter. It is advantageous if the loop filter is set up to convert the output current of the detector into a voltage and at the same time to filter out high-frequency interference components. In a preferred embodiment, the PLL / FLL is implemented as part of a slow microcontroller by means of programmable logic modules.

The inductive sensor is preferably an inductive rotation angle sensor for measuring a rotational position of a steering shaft.

Furthermore, an electromechanical power steering system is provided for a motor vehicle, comprising a steering shaft which is rotatably mounted about a steering shaft axis of rotation and can assume various rotational positions, an electric motor to support a steering movement and an inductive rotation angle sensor described above.

• • Einstellen einer Resonanzfrequenz des LC-Schwingkreises mittels der einstellbaren Kapazität des Kondensators;
• • Bereitstellen eines Referenzsignals durch den Referenz-Oszillator;
• • Vergleichen des Ausgangssignals des LC-Oszillators mit dem Referenzsignal in einem Detektor,
• • Einstellen der Resonanzfrequenz des LC-Schwingkreises auf die Referenzfrequenz des Referenz-Oszillators mittels Rückführung des Ausgangssignals der Regelschleife zur Varaktordiode oder zum variablen Kondensator.

In addition, a method for high-resolution measurement of an angle of rotation of a steering shaft with an inductive sensor has an LC oscillator, which comprises an LC resonant circuit with a capacitor and a coil, which can generate a high-frequency alternating magnetic field, and an evaluation unit which is set up to to detect a change in the signal of the LC oscillator by means of a metallic object introduced into the alternating field, the LC oscillator being part of a phase locked loop or frequency locked loop of the evaluation unit, which comprises a reference oscillator, and that the method comprises the following steps:

• • Setting a resonance frequency of the LC resonant circuit by means of the adjustable capacitance of the capacitor;
• Provision of a reference signal by the reference oscillator;
• Comparing the output signal of the LC oscillator with the reference signal in a detector,
• • Setting the resonance frequency of the LC resonant circuit to the reference frequency of the reference oscillator by means of feedback of the output signal of the control loop to the varactor diode or to the variable capacitor.

The capacitance of the capacitor is preferably adjustable by means of a varactor diode. In this case, the returned signal is used to control the varactor diode.

Depending on which control loop is used, the detector comprises at least one detector from the following list: phase-frequency detector, phase detector, frequency detector.

• • Umwandeln des Ausgangssignals des Detektors in einen äquivalenten Strom mittels einer Ladungspumpe,
• • Umwandeln und Filtern des Signals der Ladungspumpe mittels eines Schleifenfilters,
• • Rückführung des Ausgangssignals des Schleifenfilters an den Kondensator zum Einstellen der Resonanzfrequenz des LC-Schwingkreises.

The method can also include the following steps:

• Converting the output signal of the detector into an equivalent current by means of a charge pump,
• Converting and filtering the signal of the charge pump by means of a loop filter,
• • Feedback of the output signal of the loop filter to the capacitor for setting the resonance frequency of the LC resonant circuit.

Furthermore, a method for determining a rotational angle of the rotational position of a rotatably mounted steering shaft of a motor vehicle is provided, the rotational position of the rotation of the steering shaft being measured with the inductive rotation angle sensor which has the fixedly arranged coil which is part of the LC resonant circuit , wherein the coil scans at least one electrically conductive track rotating with the steering shaft and extending around the steering shaft, in which a change in a resonance frequency of the LC resonant circuit is detected by means of the previously described method.

• 1: eine schematische Darstellung einer elektromechanischen Hilfskraftlenkung mit einem induktiven Drehwinkelsensor, sowie
• 2: ein Blockschaltbild eines induktiven Drehwinkelsensors.

A preferred embodiment of the invention is explained in more detail below with reference to the drawings. Identical or functionally identical components are provided with the same reference symbols in all figures. Show it:

• 1 : a schematic representation of an electromechanical power steering system with an inductive rotation angle sensor, and
• 2 : a block diagram of an inductive rotation angle sensor.

In the 1 is an electromechanical motor vehicle power steering 1 with a steering wheel 2 that with a steering shaft 3 is rotatably coupled, shown schematically. Via the steering wheel 2 the driver brings a corresponding torque into the steering shaft as a steering command 3 on. The torque is then over the steering shaft 3 on a steering pinion 4 transfer. The pinion 4 combs with a tooth segment in a known manner 50 a rack 5 , The steering pinion 4 forms together with the rack 5 a steering gear. The rack 5 is slidably mounted in a third steering housing in the direction of its longitudinal axis. The rack is at its free end 5 with tie rods 6 connected via ball joints, not shown. The tie rods 6 themselves are in a known manner via steering knuckles with one steered wheel each 7 connected to the motor vehicle. One turn of the steering wheel 2 leads over the connection of the steering shaft 3 and the pinion 4 to a longitudinal displacement of the rack 5 and thus swiveling the steered wheels 7 , The steered wheels 7 learned about a lane 70 a reaction that counteracts the steering movement. For swiveling the wheels 7 consequently a force is required that has a corresponding torque on the steering wheel 2 makes necessary. An electric motor 8th with a rotor position sensor (RPS) of a servo unit 9 is provided to assist the driver in this steering movement. The servo unit 9 can be used as an auxiliary support device 10 . 11 . 12 either with a steering shaft 3 , the steering pinion 4 or the rack 5 be coupled. The respective assistant support 10 . 11 . 12 carries an auxiliary torque in the steering shaft 3 , the steering pinion 4 and / or in the rack 5 a, which supports the driver in the steering work. The three different ones 1 assistants shown 10 . 11 . 12 show alternative positions for their arrangement. Usually only one of the positions shown is assisted.

It is a torque sensor unit 13 , which is part of an integral unit that is a steering angle sensor unit 14 includes, provided. The torque sensor unit 13 detects the rotation of an upper steering shaft 30th against a lower steering shaft 31 as a measure of that on the upper steering shaft 30th torque applied manually by the driver. The steering angle sensor unit 14 however measures the current steering angle of the lower steering shaft 31 ,

The upper steering shaft 30th and the lower steering shaft 31 are coupled to each other in a torsionally flexible manner via a torsion bar. The twist between the upper steering shaft 30th and the lower steering shaft 31 can be determined using a rotation angle sensor. This angle of rotation sensor is also referred to as a torque sensor. Depending on the torque sensor unit 13 measured torque, the servo unit provides auxiliary support for the driver.

2 shows a block diagram of the inductive sensor, preferably an inductive angle sensor 15 a steering angle sensor unit 14 for measuring the steering angle. A coil 16 of the inductive sensor is part of an LC resonant circuit 17 of an LC oscillator 18th , The LC resonant circuit 17 is the frequency selective component. The capacitance of the capacitor 19 the LC resonant circuit can be set using a varactor diode (also called varicap or capacitance diode). The capacitance of such a diode has a voltage dependency and can be changed in a certain range by applying a control voltage. Due to the change in inductance, there is also a change in the resonance frequency ω ₀ LC resonant circuit 17 , At a desired low resonance frequency ω ₀ , especially in a range from 1 to 10 MHz, the capacitance of the varactor diode must be increased.

The oscillator 18th is part of a phase locked loop 20th (PLL, phase locked loop) or frequency locked loop (FLL, frequency locked loop), which is generally used to generate a stable, adjustable frequency. With a phase locked loop 20th a phase comparison is carried out in which a phase error remains as a control residue; the relative accuracy of the output frequency corresponds to that of the reference frequency if frequency dividers are used in the control system. In contrast, in a frequency comparison in a frequency control loop, a frequency error always remains as the control residue. For this reason, the frequency control loop is mostly only used in automatic tuning circuits (AFC), where extreme frequency accuracy is not important.

Depending on whether a phase-locked loop or frequency-locked loop is used, the control loop comprises a phase-frequency detector (PFD), phase detector (PD) and / or frequency detector (FD), in which 2 as a detector 21 shown, and also an external reference oscillator 22 as well as a loop filter 23 ,

The following is an exemplary PLL 20th described. The external reference oscillator 22 Generates with high quality, characterized by high stability and accuracy, an input signal fref serving as a reference signal for the PLL 20th , A PFD 21 compares the output signal of the LC oscillator 18th with respect to its phase with the reference signal fref of the reference oscillator 22 and generates a signal at its output that is proportional to its phase difference.

If the phase of the LC oscillator signal leads the phase of the reference signal fref, the PFD generates 21 a negative signal at its output. If, on the other hand, there is the opposite case in which the phase of the LC oscillator signal lags behind the phase of the reference signal fref, the PFD generates 21 a positive signal at its output. The Output signals of the phase-frequency detector 21 are converted into an equivalent current by means of a charge pump (not shown) and to the loop filter 23 passed on.

The loop filter 23 serves the output current of the PFD 21 convert into a voltage and at the same time filter out high-frequency interference. The output voltage is as smooth as possible. The output voltage is sent to the adjustable capacitor 19 of the voltage controlled LC oscillator to control the varactor diode to correct any deviation from the intended output frequency. The output signal serves as the measurement signal of the sensor. The frequency of the measurement signal of the inductive sensor can thus be set to a certain extent by means of the varactor diode. This can be used to avoid EMC problems, for example, and to be insensitive to nearby interference fields. Since the frequency is kept constant, several sensor elements with different frequencies can be used. Interference between the sensor elements can thus be avoided. The LC oscillator can be implemented as part of the microcontroller using programmable logic modules. The loop filter can be implemented in the form of software.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• EP 1217334 A2 [0006]

Claims (13)

Inductive sensor (15) comprising an LC oscillator (18), which comprises an LC resonant circuit (17) with a capacitor (19) and a coil (16), which can generate an alternating magnetic field, and an evaluation unit that does so is set up to detect a change in the signal of the LC oscillator (18) by a metallic object introduced into the alternating field, characterized in that the capacitance of the capacitor (19) is adjustable and the LC oscillator (18) is part of a phase locked loop or Frequency control loop of the evaluation unit, which comprises a reference oscillator (22) which provides a reference signal, and a detector (21) which is designed to compare the output signal of the LC oscillator (18) with the reference signal, the output signal the control loop for controlling the resonance frequency of the LC resonant circuit (17) is fed back to the capacitor (19) for setting the capacitance of the capacitor (19), so that the Resonance frequency of the LC resonant circuit (17) corresponds to the frequency of the reference oscillator. Inductive sensor after Claim 1 , characterized in that the capacitance of the capacitor (19) is adjustable by means of a varactor diode. Inductive sensor after Claim 1 or 2 , characterized in that the detector (21) comprises at least one detector from the following list: phase-frequency detector, phase detector, frequency detector. Inductive sensor according to one of the preceding claims, characterized in that a charge pump is provided which is set up to convert the output signals of the detector (21) into an equivalent current and to pass them on to a loop filter (23). Inductive sensor after Claim 4 , characterized in that the loop filter (23) is set up to convert the output current of the detector (21) into a voltage and at the same time filter out high-frequency interference components. Inductive sensor according to one of the preceding claims, characterized in that the LC oscillator (18) and the PLL or FLL logic is implemented as part of a microcontroller by means of programmable logic modules. Inductive sensor according to one of the preceding claims, characterized in that the inductive sensor (15) is an inductive rotation angle sensor for measuring a rotational position of a steering shaft. Electromechanical power steering system for a motor vehicle, comprising a steering shaft (3, 31) which is rotatably mounted about a steering shaft rotation axis and can assume various rotational positions, an electric motor (8) for supporting a steering movement and an inductive rotation angle sensor (15) according to Claim 7 , Method for high-resolution measurement of an angle of rotation of a steering shaft with an inductive sensor (15) comprising an LC oscillator (18), an LC resonant circuit (17) with a capacitor (19) and a coil (16), which have a high-frequency alternating magnetic field can generate, and an evaluation unit which is set up to detect a change in the signal of the LC oscillator (18) by a metallic object introduced into the alternating field, characterized in that the LC oscillator (18) is part of a phase locked loop or frequency control loop of the evaluation unit, which comprises a reference oscillator (22), and that the following method steps are provided: • Setting a resonance frequency of the LC resonant circuit (17) by means of the adjustable capacitance of the capacitor (19); • providing a reference signal by the reference oscillator (22); • Compare the output signal of the LC oscillator (18) with the reference signal in a detector (21), • Adjust the resonance frequency of the LC resonant circuit (17) to the reference frequency of the reference oscillator (22) by means of feedback of the output signal of the control loop to one Varactor diode or to a variable capacitor (19). Procedure according to Claim 9 , characterized in that the capacitance of the capacitor (19) is adjustable by means of the varactor diode. Procedure according to Claim 9 or 10 , characterized in that the detector (21) comprises at least one detector from the following list: phase-frequency detector (PFD), phase detector (PD), frequency detector (FD). Method according to one of the preceding Claims 9 to 11 , characterized in that the method comprises the following steps: • converting the output signal of the detector (21) into an equivalent current by means of a charge pump, • converting and filtering the signal of the charge pump by means of a loop filter (23), • feedback of the output signal of the loop filter ( 23) to the capacitor (19) for setting the resonance frequency of the LC resonant circuit (17). Method for determining an angle of rotation of the rotational position of a rotatably mounted steering shaft (3, 31) of a motor vehicle, wherein in the method the rotational position of the rotation of the steering shaft (3, 31) is measured with an inductive angle of rotation sensor (15) which detects the fixed coil ( 16), which is part of the LC resonant circuit (17), the coil (16) sensing at least one electrically conductive track rotating with the steering shaft (3.31) and extending around the steering shaft (3.31), in which a change in a resonance frequency of the LC resonant circuit (17) by means of the method according to one of the Claims 9 to 12 is detected.

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