Measuring device and measuring method

Abstract

The invention relates to a measuring device (100), comprising: a plurality of N resistive sensors (10-1, ...,10-6) each having a first end and a second end, where N is an even number greater than or equal to 2, and wherein each first end of the N resistive sensors is connected to a star point (101); an amplifier (20) comprising a first input (201) connected to the star point, a second input (202) and an output (203) for outputting a measurement signal (Vout); N switches (30-1, ...,30-6), wherein each second end of the N resistive sensors is optionally connected, via one of the N switches, in a potential-free manner or to reference earth or to reference voltage (VDD); and a control unit (40), which is designed to connect the second ends of the N resistive sensors individually in a potential-free manner, to reference earth or to reference voltage (VDD).

Description

Measuring device and measuring method TECHNICAL FIELD [1] This document relates to embodiments of a measuring device for evaluating switched resistive sensors in a star connection, in particular for an electric drive device of a bicycle, as well as corresponding embodiments of a measuring method for evaluating switched resistive sensors in a star connection. Furthermore, this document relates to embodiments of an electric drive device comprising a measuring device and to embodiments of a bicycle comprising an electric drive device. BACKGROUND [2] Bicycles are increasingly being equipped with electric drive systems to make pedaling easier for riders. Such bicycles are commonly referred to as e-bikes or pedelecs, which also include derivatives such as cargo bikes, recumbent bikes, and the like. [3] The control of the electric drive device is based on various control parameters, e.g. a parameter whose value is indicative of the current force that the cyclist applies to the pedal. [4] If the control of the electric drive device is based on the current force (which the cyclist applies to the pedal), an accurate measurement of the corresponding parameter should be carried out. [5] Forces, mechanical stresses, and torques are often determined indirectly via the strain of a surface. For example, strain gauges are used, which are applied to the objects to be measured and change their resistance depending on the surface strain of the object. Since the stiffness of the objects cannot always be brought to a low level, The relative resistance changes are sometimes very small. For this reason, conventional resistance measurements are usually not suitable. Instead, resistive sensors, such as strain gauges, are connected, for example, in Wheatstone bridges. The full bridge exhibits good technical properties due to its symmetrical design. At least four strain gauges are used. [6] Due to the usually very small output voltage of the full bridge, deviations from the standard behavior (so-called drifts) can have a strong impact on the downstream amplifier circuit. This means that either very expensive so-called zero-drift amplifiers are used, or the measured value will fluctuate with age and temperature due to the amplifier properties, which is generally undesirable. In addition, the strain gauges and their application usually account for the highest cost share of strain sensors. Furthermore, it can be disadvantageous to combine several independent strains (and thus independent measured variables) into only one output signal, since the measured variables may no longer be able to be differentiated from one another, or can only be differentiated with difficulty. In addition, the number of strain gauges in the case of a full bridge cannot be freely selected; it must always be an integer multiple of 4. [7] Occasionally, strain gauges are used on a measuring shaft of an e-bike with the aim of measuring the torque applied to it. However, since the measuring shaft is subjected not only to torque but also to local pressures, for example, due to freewheels and clamping elements, measurement errors can occur. These local pressures lead, for example, to changes in the resistance of individual strain gauges and thus to a measured torque to which the shaft is not actually subjected. Furthermore, bending moments cannot be measured in this way. [8] The technical problem underlying the present invention is to propose an accurate and low-interference measurement based on resistive sensors. SUMMARY [9] The aforementioned technical problem is solved by the subject matter of the independent claims. Features of advantageous embodiments are specified in the subclaims.

[0010] According to a first aspect, a measuring device comprises: a plurality of N resistive sensors each having a first end and a second end, where N is an even number greater than or equal to 2, and where each first end of the N resistive sensors is connected to a star point; an amplifier comprising a first input connected to the star point, a second input, and an output for outputting a measurement signal; N switches, where each second end of the N resistive sensors is selectively connected to one of the N switches in a potential-free manner, to reference ground, or to reference voltage; and a control unit configured to individually switch the second ends of the N resistive sensors to a potential-free state, to reference ground, or to reference voltage.

[0011] According to a second aspect, a measuring method using a measuring device is proposed. The measuring device comprises a plurality of N resistive sensors, each with a first end and a second end, where N is an even number greater than or equal to 2, and where each first end of the N resistive sensors is connected to a star point; an amplifier comprising a first input connected to the star point, a second input, and an output for outputting a measurement signal; N switches, where each second end of the N resistive sensors is optionally connected to a floating state, to a reference ground, or to a reference voltage via one of the N switches. The measuring method comprises individually switching the second ends of the N resistive sensors to a floating state, to a reference ground, or to a reference voltage.

[0012] Further aspects relate to an electric drive device for a bicycle comprising a measuring device according to the first aspect and to a corresponding bicycle.

[0013] Some embodiments are described below. The features of these embodiments may be combined to form further embodiments, unless expressly stated otherwise.

[0014] Resistive sensors, for example, are strain gauges (DMS). Resistive sensors can also be thermistors or magnetic sensors.

[0015] The subject matter of the independent claims allows for a reduction in the number of resistive sensors required for the measurement, thus enabling comparatively low-cost measurement. Since measurement signals can be recorded associated with the respective switching state of the resistive sensors, it is possible to record independent measured variables, such as torque and bending moment, using the same measuring device. By switching the resistive sensors, low 1 / f noise can be achieved. Furthermore, the measuring device is highly susceptible to temperature- or lifetime-related drift. Furthermore, the measuring device allows for early detection of manufacturing or assembly errors, thus facilitating the provision of an intelligent measuring system overall, since the resistance change of each individual resistive sensor can be determined.

[0016] According to the invention, the N resistive sensors are connected in a star configuration. This means that all first ends of the N resistive sensors are at the same potential, the potential of the star point. This potential, or a potential derived from it, is fed to the first input of the amplifier. The other ends of the N resistive sensors are either potential-free or connected to reference ground or reference voltage via the respective switch.

[0017] The amplifier is designed, for example, as a differential amplifier. A further reference voltage is applied to its second input. The further reference voltage is, for example, constant and / or proportional to the first reference voltage. For example, the first input of the amplifier is non-inverting, and the second input is inverting.

[0018] Each of the switches allows the respective second end to be switched to either floating, reference voltage, or reference ground. For this purpose, the respective switch can be configured, for example, as a MOSFET pair.

[0019] The reference voltage, for example, is constant and amounts to 3.3 V or 5 V (although other voltage values are also possible, such as 1.8 V or 2.7 V). The reference ground potential deviates from this and is, for example, 0 V.

[0020] Because the second ends of the N resistive sensors can be individually switched either to floating state, to reference ground, or to reference voltage, the measuring device can be operated in a variety of different switching states. For example, it can be ensured that one half of the N resistive sensors is always connected to reference voltage, while the other half is connected to reference ground.

[0021] In one embodiment, the switching state of the N resistive sensors is changed, for example, by switching two of the N switches at a time, i.e., by changing the potentials of two second ends of the N resistive sensors (e.g., from reference ground to reference voltage or vice versa). This minimizes the number of switching transitions. In other embodiments, different sequence orders are selected, e.g., those in which the switching states of the individual switches change more frequently.

[0022] The switching state of the N resistive sensors changes, for example, with a frequency of at least 50 Hz or at least 100 Hz. However, the frequency can also be significantly higher, for example in the kHz range, such as 1 kHz to 50 kHz.

[0023] A measurement period, for example, comprises several sequentially following switching states, e.g., all possible switching states. The sequence of switching states can be predefined, and the control unit can be configured to continuously repeat this sequence. The duration of the complete sequence then corresponds to the measurement period, and the reciprocal of the measurement period corresponds to a measurement frequency fM.

[0024] Since each switching state has a complementary switching state, common-mode interference can be compensated so that, for example, drifts, regardless of their origin, are no longer a problem.

[0025] In one embodiment, the measuring device comprises an evaluation unit coupled to the output of the amplifier, which is designed to store measurement signals for the measurement period comprising several switching states, 1 / fM, and to output a corresponding evaluation signal for each measurement period based on the stored measurement signals. Additionally or alternatively, the measuring device can be designed to output an evaluation signal for each switching state. For example, it is possible to measure several measured variables from the recorded sequence of measurement signals. Furthermore, depending on the sequence, a redundancy arises that can be used to check the plausibility of the measured values.

[0026] For example, the evaluation unit is designed to acquire the respective measurement signal after a delay time has elapsed, e.g., after at least 1 / 10 or 14 of the switching period (i.e., the duration of a switching state) has elapsed since the switching state change. This way, measurement disturbances caused by the switching process, such as overshoots or other transients immediately after switching, can be avoided. The exact value of the delay time can depend on the measurement frequency and the dynamic range of the amplifier.

[0027] The evaluation unit is, for example, operatively coupled to the control unit. The evaluation unit can be configured to store the measurement signals output by the amplifier, assigned to the respective switching state. After completing a measurement period, a plurality of measurement signals are available for a corresponding plurality of switching states of the N resistive sensors. From these, the evaluation unit can determine the evaluation signal, e.g., according to the principle of least squares or maximum likelihood estimation.

[0028] For example, the resistance changes of all resistive sensors are available independently and can now be used to determine the potentially complex strain state. Local stresses can thus be easily distinguished from macroscopic stress states.

[0029] Each switching state can be present for the same switching duration. Furthermore, the sequence of switching states can be configured such that each resistive sensor is connected to reference voltage for half the measuring period and to reference ground for the other half.

[0030] The control unit and the evaluation unit can each be implemented as a microcontroller. In another embodiment, the control unit and the evaluation unit are implemented by a common microcontroller.

[0031] The evaluation signal is, for example, indicative of one or more of the following parameters: torque, bending moment, shear force, local deformation or stresses.

[0032] The present invention also relates to a measuring method which is carried out on the basis of a measuring device. The measuring device comprises a plurality of N resistive sensors, each with a first end and a second end, where N is an even number greater than or equal to 2, and where each first end of the N resistive sensors is connected to a star point; an amplifier comprising a first input connected to the star point, a second input and an output for outputting a measuring signal; N switches, where each second end of the N resistive sensors is optionally connected to potential-free, to reference ground or to reference voltage via one of the N switches. The measuring method comprises individually switching the second ends of the N resistive sensors to potential-free, to reference ground or to reference voltage.

[0033] Exemplary embodiments of this measuring method correspond to the embodiments of the measuring device described above. Reference is made to the above.

[0034] Furthermore, an electric drive device for a bicycle is proposed, which comprises a measuring device according to one of the above embodiments. The electric drive device is designed, for example, as an e-bike drive, e.g., as a mid-engine. The central placement between the front and rear wheels contributes to stability and safety.

[0035] The electric drive device comprises, for example, a controller for controlling the electric drive device, wherein the control unit receives the evaluation signal of the measuring device and controls the electric drive device on the basis of the output signal.

[0036] To measure torque, six resistive sensors, i.e., three pairs of resistive sensors, can be applied to the hollow shaft / output shaft of the bicycle, e.g., distributed circumferentially with a spacing of 120°. Other placement of the resistive sensors is also possible, for example, on the pedal spindle. One end of each of the six resistive sensors is designed to be switchable, as described above. while the remaining ends are connected in a star configuration and fed to the amplifier. All permitted switching states are controlled sequentially via the control unit (e.g. implemented as a microcontroller), and the resulting measured values are recorded by the evaluation unit (e.g. also implemented in the microcontroller). The individual deformation states of the resistive sensors or the relationship between the deformation states can then be determined. With six resistive sensors, for example, five signals result. From these, the following variables can be output as evaluation signals: torque, bending moment in two directions, transverse force, local deformations and unwanted tension in the shaft. The determined torque can be used to control the assistance of the bicycle (e-bike). The bending moment can be used to draw conclusions about the position of the pedals and the load state of the crank.The local deformations are crucial for fault diagnosis and predictive maintenance. This method allows faulty measuring shafts to be detected during production. If, for example, individual resistive sensors – such as strain gauges – are incorrectly bonded, they can be identified immediately. Furthermore, this method can be used to detect in the factory if the shaft is subjected to stress due to poorly positioned clamping elements.

[0037] Finally, a bicycle is proposed that includes an electric drive device as described above. Using the measuring device, a controller detects, for example, the torque currently generated by the cyclist on the pedals and, based on this, controls, for example, a correspondingly adjusted power output of the electric drive device. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The drawings show:

[0039] Fig. 1 shows a schematic and exemplary circuit diagram of a Measuring device according to one or more embodiments; and

[0040] Fig. 2 schematically and exemplarily shows a table illustrating a measurement period according to one or more embodiments. DETAILED DESCRIPTION

[0041] Fig. 1 shows a schematic and exemplary circuit diagram of a measuring device 100 according to one or more embodiments. Fig. 2 illustrates a measurement period using a table. Reference is made to both figures below.

[0042] The measuring device 100 comprises a plurality of N resistive sensors (six of which are shown, 10-1, ...., 10-6) each having a first end and a second end, where N is an even number greater than or equal to 2, and where each first end of the N resistive sensors is connected to a star point 101. In other embodiments, more or fewer than six resistive sensors are provided. All N resistive sensors preferably have the same nominal resistance value.

[0043] For example, the N resistive sensors are strain gauges.

[0044] The measuring device 100 further comprises an amplifier 20, which has a first input 201 connected to the star point 101, a second input 202, and an output 203 for outputting a measurement signal Vout. The amplifier 20 can be designed as a differential amplifier. The output 203 can be fed back to the first input 201 via a resistor 170.

[0045] A further reference voltage Vref is applied to the second input 202. The further reference voltage Vref is, for example, constant. The first input 201 of the amplifier 20 is, for example, inverting, whereas the second input 202 of the amplifier is non-inverting; at the same time, the output 203 can be fed back to the first input 201 via the resistor 170. The voltage at the star point 101, i.e., at the first input 201, is indicated by Vtap in Fig. 1.

[0046] The measuring device 100 further comprises N switches 30-1, ..., 30-6, wherein each second end of the N resistive sensors 10-1 to 10-6 is connected via one of the N switches either in a potential-free manner, to reference ground, or to reference voltage VDD. For this purpose, the measuring device 100 comprises a control unit 40, which is designed to connect the second ends of the N resistive sensors individually in a potential-free manner, to reference ground, or to Switch the reference voltage VDD ON. For this purpose, the control unit 40 outputs corresponding control signals, which are fed to the switches 30-1 to 30-6, as shown schematically in Fig. 1.

[0047] Each of the switches 30-1 to 30-6 allows the respective second end to be switched to either floating, reference voltage, or reference ground. For this purpose, the respective switch can be configured, for example, as a MOSFET pair.

[0048] The reference voltage VDD is constant and amounts to 3.3 V or 5 V (although other voltage values are also possible). The reference ground potential deviates from this and is, for example, 0 V.

[0049] Because the second ends of the six resistive sensors 10-1 to 10-6 can be individually switched either potential-free or to reference ground or to reference voltage VDD, the measuring device 100 can be operated in a variety of different switching states, which is illustrated in Fig. 2.

[0050] In one embodiment, the switching state of the six resistive sensors is changed, for example, by switching two of the six switches 30-1 to 30-6, i.e., the potentials of two second ends of the six resistive sensors change (e.g., from reference ground to reference voltage VDD or vice versa).

[0051] The switching state of the N resistive sensors changes due to the corresponding control by the control unit 40, for example, at a frequency of at least 50 Hz or at least 100 Hz. However, the frequency can also be significantly higher, for example in the kHz range, such as 1 kHz to 50 kHz. If the switching state of the six resistive sensors is changed by switching two of the six switches 30-1 to 30-6, the corresponding switching frequencies are 50 Hz / (N / 2) or switching frequency ranges are 1 kHz / (N / 2) to 50 kHz / (N / 2).

[0052] A measurement period, for example, comprises several sequentially following switching states. Fig. 2 illustrates 20 different switching states. Each state is indexed by a sequence index (left column). The column to the right of this shows a decimal number which, interpreted in binary terms, indicates the respective switching state. 7 (first row), for example, corresponds to the 00001111. The six columns to the right of this indicate which the resistive sensors 10-1 to 10-6 are switched to which potential, in this example either reference ground or reference voltage VDD.

[0053] The sequence of switching states is predefined there by the indices, and the control unit 40 continuously implements this sequence by actuating the switches 30-1 to 30-6 in such a way that the switching states 0 to 19 are repeatedly assumed. The duration of the complete sequence (states 0 to 19) corresponds to the measuring period, and the inverse of the measuring period corresponds to a measuring frequency fM.

[0054] A next switching state is achieved, for example, by switching two or more switches compared to the current switching state. The transition from the switching state corresponding to sequence index 0 to the switching state corresponding to sequence index 1 occurs, for example, by switching switches 30-3 (S3) and 30-4 (SQ) of resistive sensors 10-3 and 10-4. The transition from the switching state corresponding to sequence index 8 to the switching state corresponding to sequence index 9 occurs, for example, by switching both switches 30-3 (S3) and 30-1 (Si) of resistive sensors 10-3 and 10-1.

[0055] Each switching state can be present for the same switching duration. Furthermore, the sequence of switching states can be configured such that each resistive sensor 10-1 to 10-6 is connected to reference voltage VDD for half the measurement period and to reference ground for the other half.

[0056] An evaluation unit 50 coupled to output 203 of amplifier 20 stores measurement signals Vout for the measurement period comprising switching states 0 to 19, 1 / fM. Based on the stored measurement signals, the evaluation unit outputs a corresponding evaluation signal TORQ for each measurement period. Additionally or alternatively, measuring device 100 can be configured to output an evaluation signal TORQ for each of the switching states 0 to 19.

[0057] In the example shown in Fig. 1, the evaluation unit 50 is operatively coupled to the control unit 40. The evaluation unit 50 can be configured to store the measurement signals Vout output by the amplifier 20, assigned to the respective switching state. After completing a measurement period, a plurality of measurement signals are available for a corresponding plurality of switching states of the six resistive sensors. From these, the evaluation unit 50 determines the evaluation signal TORQ.

[0058] The control unit 40 and the evaluation unit 50 can each be implemented as a microcontroller. In another embodiment, the control unit and the evaluation unit are implemented by a common microcontroller.

[0059] The evaluation signal TORQ is, for example, indicative of one or more of the following parameters: torque, bending moment, shear force, local deformation or stresses.

[0060] As described in the introduction, the measuring device 100 is particularly suitable for use in an electric drive device (not shown in the drawing here) for a bicycle (also not shown in the drawing here). The electric drive device is designed, for example, as an e-bike drive, e.g., as a mid-engine. For example, the N resistive sensors are arranged on a shaft of the electric drive device.

[0061] The electric drive device comprises, for example, a controller for controlling the electric drive device, wherein the control unit receives the evaluation signal of the measuring device 100 and controls the electric drive device based on the output signal.

[0062] By means of the measuring device, the control system detects, for example, the torque currently generated by the cyclist on the pedal and, based on this, controls, for example, a correspondingly adjusted power output of the electric drive device.

[0063] Depending on the design of the resistive sensors, it may be advantageous that (option a) the second ends of the N resistive sensors are optionally connected to reference ground or to reference voltage via the respective switch (but not potential-free) or that (option b) the second ends of the N resistive sensors are optionally connected to reference ground or to reference voltage via the respective switch.

[0064] In principle, the "switch nodes" can also be left open, meaning the second ends are potential-free. Leakage currents can then flow from the second end of the resistive sensor to the reference ground and reference voltage, depending on the nature of the switch in question. These leakage currents can be exemplary and temperature dependent. However, if switches with low leakage currents and / or resistive sensors, such as strain gauges, with low resistance values are used, option (b) can be considered. In one embodiment, one resistive sensor is always connected to reference ground, another to reference voltage, and the remaining - e.g. four - resistive sensors are not switched, i.e. left potential-free. In this way, for example, 30 combinations are created, the output voltage of which can then be evaluated. After running through the 30 combinations, the resistance values can then be estimated. Option (b) offers the advantage that the power consumption is lower for the same resistive sensors, since not all resistive sensors are always current-carrying. Or, with the same power consumption, lower-ohmic and therefore cheaper resistive sensors can be used.

Claims

CLAIMS 1. A measuring device (100), comprising: a plurality of N resistive sensors (10-1, . . . , 10-6) each having a first end and a second end, where N is an even number greater than or equal to 2, and where each first end of the N resistive sensors is connected to a star point (101); an amplifier (20) comprising a first input (201) connected to the star point, a second input (202), and an output (203) for outputting a measurement signal (Vorn); N switches (30-1, . . . , 30-6), wherein each second end of the N resistive sensors is selectively connected to a potential-free state, to a reference ground, or to a reference voltage (VDD) via one of the N switches; and a control unit (40) configured to individually switch the second ends of the N resistive sensors to a potential-free state, to a reference ground, or to a reference voltage (VDD).

2. Measuring device (100) according to claim 1, wherein all N resistive sensors have the same nominal resistance value.

3. Measuring device (100) according to claim 1 or 2, wherein the control unit (40) is designed to switch the second ends of the N resistive sensors sequentially potential-free, to reference ground or to reference voltage (VDD), e.g. such that always half of the second ends of the N resistive sensors are on reference ground, while the other half of the second ends of the N resistive sensors are on reference voltage (VDD).

4. Measuring device (100) according to one of the preceding claims, wherein the control unit (40) is designed to change the switching state of the N resistive sensors at a frequency of at least 50 Hz.

5. Measuring device (100) according to one of the preceding claims, wherein the control unit (40) is designed to effect a change in the switching state of the N resistive sensors by changing the states of two of the N switches.

6. Measuring device (100) according to one of the preceding claims, wherein each of the N switches comprises a pair of MOSFETs.

7. Measuring device (100) according to one of the preceding claims, further comprising an evaluation unit (50) coupled to the output (203) of the amplifier (20), which is designed to store measurement signals (Vorn) for a measurement period comprising a plurality of switching states and to output a corresponding evaluation signal for each measurement period based on the stored measurement signals.

8. Measuring device (100) according to claim 7, wherein the evaluation unit (50) is designed to store the measurement signals associated with the respective switching state.

9. Measuring device (100) according to claim 7 or 8, wherein all possible switching states are passed through in one measuring period.

10. Measuring device (100) according to one of the preceding claims 7 to 9, wherein the evaluation unit (50) is designed to detect the respective measurement signal after a delay time has elapsed.

11. Measuring device (100) according to one of the preceding claims 7 to 10, wherein the evaluation signal is indicative of one or more of the following parameters: torque, bending moment, local deformation, transverse force or stresses.

12. Electric drive device for a bicycle, comprising a measuring device (100) according to one of the preceding claims.

13. A bicycle comprising an electric drive device according to claim 13.

14. A measuring method using a measuring device (100), the measuring device (100) comprising: a plurality of N resistive sensors (10-1, . . . , 10-6) each having a first end and a second end, where N is an even number greater than or equal to 2, and where each first end of the N resistive sensors is connected to a star point (101); an amplifier (20) comprising a first input (201) connected to the star point, a second input (202), and an output (203) for outputting a measurement signal (Vout); N switches (30-1, . . . , 30-6), wherein each second end of the N resistive sensors is selectively connected to potential-free, to reference ground, or to a reference voltage (VDD) via one of the N switches; wherein the measuring method comprises: individually switching the second ends of the N resistive sensors to potential-free, to reference ground, or to reference voltage (VDD).

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