Inductive proximity sensor and method for teaching target classes

Abstract

Inductive proximity sensor for determining the distance (z) of an electrically conductive target (0), wherein the inductive proximity sensor an oscillating circuit (1) having an inductance (2) and a capacitance (3), a measuring circuit (4) suitable for determining the frequency (f) of the resonant circuit (1), and a control unit (5) with a digital memory (6), wherein the inductance (2) is designed as a printed circuit board coil with a temperature sensor (7), characterized in that that the inductive proximity sensor is designed to classify the target (0) with regard to its shape in a learning process comprising at least two measurements, to generate a form factor (F) in order to determine the distance (z) of the target (0) as an absolute distance signal as a millimeter value from the form factor (F) and the oscillating circuit frequency (f).

Description

[0001] The invention relates to a contactless inductive proximity sensor with a measuring circuit for determining the distance of an electrically conductive measuring object according to claim 1. The independent claim 5 relates to a method according to the invention.

[0002] Inductive proximity sensors are used in contactless electronic switching devices, primarily in automation technology, but also as measuring devices.

[0003] Such inductive proximity sensors are widely used and are also manufactured and distributed by the applicant.

[0004] They have at least one sensor coil and can be operated with either pulsed current or continuous, usually sinusoidal alternating current. In the latter case, the sensor coil is usually part of an LC oscillator and thus determines the frequency.

[0005] Inductive sensors should respond equally to switching flags (objects or targets) with high conductivity and low permeability, such as aluminum and non-ferrous metals, as well as to targets with lower conductivity and high permeability, such as steel or stainless steel. This behavior is referred to as "K=1."

[0006] However, the implementation is anything but simple, since different effects are supposed to lead to the same measurement result.

[0007] The switching distances are specified for an ambient temperature of 23°C. They are designed to vary by less than 10% within a temperature range of -25°C to +85°C, or sometimes even from -40°C to 100°C.

[0008] Since the interaction between the transmitting coil and the target decreases very rapidly with increasing distance, temperature compensation is of immense importance.

[0009] For example, DE 36 06 878 C2 proposes placing a temperature sensor corresponding to the effective value of the loss resistance of the resonant circuit coil in the immediate vicinity of the resonant circuit coil and connecting it to an input and the output of the oscillator amplifier. While this is extremely effective, it is not sufficient to achieve the specified temperature stability even at long switching distances.

[0010] DE 39 31 892 A1 assumes operating a sensor resonant circuit well outside its Q-factor. However, this is counterproductive for the switching distance, so that the designers eventually returned to operating at the Q-factor by implementing additional temperature compensation measures. It is proposed to measure the coil temperature with an additional bifilar coil winding acting as a temperature sensor, and to influence the oscillator amplitude and / or the switching threshold of the sensor using the temperature measurement.

[0011] EP 0 049 304 B1 discloses a method for compensating temperature-related measurement errors by measuring the DC resistance of the sensor coil, in which a correction signal representing the temperature is generated based on the voltage drop across the measuring element (the sensor coil). DE 41 41 065 A1 teaches something similar for an inductive pulse sensor.

[0012] Taking technical progress into account, DE 195 27 174 C2 proposes repeatedly recording the temperature dependence of the components and their tolerances in a teach-in process using an externally connectable programming device in the fully assembled state, storing the measured values, and making the switching threshold variable depending on the temperature. A semiconductor diode arranged near the resonant circuit is proposed as the temperature sensor.

[0013] Because a parallel resonant circuit has the properties of an ohmic resistor in the case of resonance, the temperature response of the copper resistance of the coil should not have a major influence on its resonance frequency despite its high value of approximately 4000 ppm / °C.

[0014] Such considerations ultimately led to the decision to measure not the coil quality factor and / or the losses of the sensor coil based on an amplitude, but rather a frequency, or more precisely, the change in frequency caused by a nearby target, and to convert it into a distance signal. This measurement principle is also extremely advantageous because the aforementioned condition K=1 is inherently fulfilled at measurement frequencies above approximately 2 MHz, as the Weiss domains in the ferromagnetic materials can no longer follow the pattern. However, a ferrite coil core is not required.

[0015] Texas Instruments offers “Inductance-to-Digital-converter” (LDC), which instead of measuring the amplitude (resonant circuit quality) measures the ratio between the sensor frequency and a self-generated reference frequency (ratio of f sensor / f ref ) in digitized form with 28-bit resolution. Further information can be found in the "Technical Data Sheet TEXAS INSTRUMENTS: LDC1612, LDC1614 Multi-Channel 28-Bit Inductance to Digital Converter (LDC) for Inductive Sensing. December 2014-Revised March 2018" and in the "Application Report" - "Dallas (TX), 2018 (SNOSCY9A). 67 pages. - Company publication TEXAS INSTRUMENTS: Application report - LDC1000 Temperature Compensation. September 2013. Dallas (TX), 2013 (SNAA212). 6 pages. - Company publication".

[0016] The object of the invention is to at least partially overcome the aforementioned disadvantages of the prior art and to provide an inductive proximity sensor which, despite incomplete temperature compensation, provides a highly precise distance signal (in mm), independent of the geometric shape and the electrical conductivity of a (measurement) object referred to as target.

[0017] This object is achieved according to claim 1. The subclaims relate to advantageous developments of the invention. The subordinate claim 5 relates to a method according to the invention.

[0018] The essential idea of ​​the invention is, on the one hand, to dispense with complete temperature compensation and replace it with measuring the coil temperature with subsequent correction of the measurement results. The user has the option of fine correction in which each target is classified (with regard to its shape) by approximating its characteristic curve using at least two measuring points and storing the data in a memory. According to the invention, each measured value is first corrected based on the coil temperature and only then adapted to a characteristic curve stored in the control unit of the proximity sensor. This fine correction enables a distance measurement independent of the (target) material and its shape (form factor), so that the distance to the target can be specified in linear units (mm).

[0019] The proximity sensor is therefore designed to permanently measure the coil temperature and to classify the target in a learning process comprising at least two measurements and finally to determine the distance of the target from the temperature T measured with a temperature sensor and the oscillating circuit frequency f, or its change.

[0020] The measured values ​​are first thermally corrected by multiplying them by temperature coefficients ϑ (T) stored as a table or polynomial. The proximity sensor is then calibrated at the factory using a standard target, e.g., a 60 mm x 60 mm steel plate, so that it can display a distance in mm without the need for further (subsequent) calibration.

[0021] For this purpose, a frequency-distance characteristic curve (fz characteristic curve) (e.g. consisting of 32 measuring points) can be recorded and saved.

[0022] A sensor coil according to the invention is advantageously designed as a single- or multi-layer printed circuit board coil that supports at least one temperature sensor, whereby a printed circuit board can also be understood as a ceramic body or a flexible carrier. The temperature sensor advantageously comprises a semiconductor diode.

[0023] The invention is explained in more detail with reference to the drawing: Fig. 1 shows an inductive sensor with the integrated circuit LDC1612, Fig. 2 shows the typical course of a measurement curve f(x) and its linearization.

[0024] The Fig. 1 shows an inductive sensor with the integrated circuit LDC1612 in a representation limited to the most essential components with a target 0, an oscillating circuit 1 consisting of an inductance 2 and a capacitor 3, a measuring circuit for inductance determination 4, the aforementioned LDC, a control unit 5, preferably a microcontroller µC, without limiting the invention thereto, and a memory module 6, which can also belong to control unit 5.

[0025] The inductor 2 is a flat coil mounted on a circuit board. The circuit board also carries a temperature sensor 7 coupled to the coil area, which in the simplest case is a semiconductor diode.

[0026] The preferred (measurement) frequency is in the range greater than or equal to 2 MHz, and is only limited by the LDC, so that frequencies from 0.1 - 10 MHz are possible, without limiting the invention to a specific frequency.

[0027] For fine adjustment of the inductive proximity sensor according to the invention, an expected target is placed at a first distance x1, e.g., at 10% of the measuring range (monitoring area), during a learning process, and a first measurement is performed. Subsequently, at least one second measurement is performed, for example, at the end of the measuring range (at 100%).

[0028] Since each measurement is connected with a temperature measurement according to the invention, at least two (frequency) measured values ​​f1(x1, T1) and f2(x2, T2) are available, where the quotients f sensor 1 / f ref or f sensor 1 / f ref are to be understood.

[0029] Due to the short time interval, the temperature T usually remains unchanged, so one measurement is often sufficient, but a second one should not be ruled out. These measured values ​​are first thermally corrected by multiplying them by temperature coefficients ϑ (T) stored as a table or polynomial. This results in thermally corrected measured values ​​f1. korr and f2 korr which correspond to the measured values ​​f1 (23°C) and f2 (23°C) expected at 23°C.

[0030] The Fig. 2 shows the typical course of a measurement curve and its linearization, whereby it can remain undecided whether these are the original measured values ​​or the thermally corrected ones, because ultimately f (x, 23°C) = f korr (x, 23°C).

[0031] From these measured values ​​(in the simplest case) a form factor F = Δf korr / Δx is generated in the form of a straight line. It should be understood that approximations using polynomials of higher degree (with multiple measurement points), tables, or any other suitable mathematical method can also be used.

[0032] During measurement, all measured values ​​f (x, T) are first calculated with ϑ, whereby temperature-corrected measured values ​​f korr (x, T0) with T0 = 23°C, which are then calculated with the form factor F, resulting in a distance value z (f, x, T): z(f, x)=x2+(f1−f2) / (x1−x2)*(f−f2 )

[0033] For those in Fig. 2 shown measurement (at 23°C) one obtains with f1= 5, f2=1, x2=5 x1=1 a straight line slope of (f1-f2) / (x1-x2) = (5-1) / (1-5) = -4

[0034] According to equation [1], for f = 3.5, a distance value z = 2.5 mm is obtained.

[0035] As mentioned above, the sensor is factory calibrated with a standard target and is therefore ready for use. However, the user has the option of recalibrating if using a target that deviates significantly from the standard target. This may be necessary when using a different material, such as aluminum, or if there is a significant deviation from the specified geometric shape (reduction in size). This allows different object classes to be differentiated and taken into account by storing object-class-specific calibration curves.

[0036] In an advantageous development of the invention, unknown targets can be assigned (classified) to an existing class by test measurements, which saves the user a new (more precise) calibration with several measuring points. List of reference symbols 0 Target, (measurement) object 1 resonant circuit 2 Inductance, resonant circuit inductance, e.g. a two-layer circuit board coil 3 Capacitance (resonant circuit capacitance) 4 Measuring circuit for inductance determination, inductance-to-digital converter 5 Control unit µC (microcontroller, signal processor (DSP), ASIC, or similar) 6 Digital memory module, preferably an SDRAM 7 Temperature sensor ϑ(T) temperature coefficient, table or polynomial, stored in a memory F Form factor, geometry factor of the target 0 x Actual distance of the target (measurement object) Δx Difference between two distances x f(x) Frequency as a function of distance x, in particular normalized as f sensor / f ref (x) Δf Difference between two (frequency) measured values ​​fsensor2 / fref (x2) - fsensor1 / fref (x1) T Temperature z Displayed double corrected distance value [mm]

Claims

[1] Inductive proximity sensor for determining the distance (z) of an electrically conductive target (0), wherein the inductive proximity sensor an oscillating circuit (1) having an inductance (2) and a capacitance (3), a measuring circuit (4) suitable for determining the frequency (f) of the resonant circuit (1), and a control unit (5) with a digital memory (6), wherein the inductance (2) is designed as a printed circuit board coil with a temperature sensor (7), characterized by , that the inductive proximity sensor is designed to classify the target (0) with regard to its shape in a learning process comprising at least two measurements, to generate a form factor (F) in order to determine the distance (z) of the target (0) as an absolute distance signal as a millimeter value from the form factor (F) and the oscillating circuit frequency (f). [2] Inductive proximity sensor according to claim 1, characterized bythat object class-specific calibration curves can be stored in the digital memory (6). [3] Inductive proximity sensor according to one of the preceding claims, characterized by that unknown targets can be assigned to an existing class through test measurements. [4] Inductive proximity sensor according to one of the preceding claims, which is designed to: - to measure temperature and frequency in the learning process at at least two known distances (x1, x2) of the target, whereby measured values ​​f1 and f2 are produced. - to correct these two using stored temperature coefficients ϑ(T), - to determine the form factor F = Δf / Δx from the two corrected measured values, - in the following measuring operation, all measured values ​​f(x, T) are to be calculated with ϑ(T), whereby temperature-corrected measured values ​​f korr (x, T0) with T0 = 23°C, - to calculate these temperature-corrected measured values ​​with the form factor F, - to display or output a twice corrected distance value z (f, T). [5] Method for operating an inductive proximity sensor according to claim 4, characterized by , that - in the learning process, temperature and frequency are measured at at least two known distances (x1, x2) of the target, whereby the measured values ​​f1 and f2 are generated, - these are first thermally corrected by multiplying them using stored temperature coefficients ϑ(T) and then generating the form factor F=Δf / Δx, - in the following measuring operation, all measured values ​​f(x, T) are first calculated with ϑ(T), - where temperature-corrected measured values ​​f korr (x,T0) with T0 = 23°C, - which are then offset against the form factor F, and - a twice corrected distance value z (f, T) is displayed or output.

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