Force-measuring device with current control circuit

Abstract

To provide a force measuring device of sufficient accuracy combined with an appropriate and simple configuration.SOLUTION: A force-measuring device based on the principles of electromagnetic force compensation, comprising an electronic circuit driven by a direct current, the electronic circuit comprising an inductor, in particular a coil, a control unit for controlling a compensation force by controlling a current through the inductor as a function of a force to be measured, and means for providing a measurement output indicative of the force to be measured, wherein the control of the current comprises a dynamic switching of two switch states associated with two conducting directions of the inductor (1L, 2L).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a force measuring device, in particular a balance, which operates on the principle of electromagnetic force compensation (also called electromagnetic force restoration) and which has an electronic circuit for controlling the current. In particular, the present invention relates to a force measuring device based on the principle of electromagnetic force compensation, which comprises a DC-powered electronic circuit with an inductor, in particular a coil, a control device for controlling the current through the inductor to control the compensation force, said control being responsive to the force to be measured, and means for providing a measurement output indicative of the force to be measured. [Background technology]

[0002] Various force measuring devices, including, for example, various electronic scales, operate by generating and modulating a magnetic compensation force, typically via induction.

[0003] In many cases, this force acts to repel a fixed magnet, displacing a part within the device and ultimately adjusting the compensation force until an equilibrium position is reached from which the force to be measured (weight in the case of a scale) can be estimated. In this way, equilibrium is reached very quickly (almost immediately) through magnetic compensation, so that substantial physical movement of the part does not occur; typically, feedback control is used so that any tendency to move is immediately recognized by a position sensor, and the maximum deviation from the equilibrium position is very small. A typical example of a scale with such rapid feedback control is a weighing cell, as known to those skilled in the art.

[0004] This adjustment can be continuous or discrete over a number of control cycles, depending on the technology. In many balances, the induction force is controlled by varying the current strength (magnitude) through the induction device. Such systems are disclosed, for example, in DE 3324402 A1 and US 4212361 A. In these systems, temperature fluctuations due to the changing current are controlled by a compensation network to achieve accurate measurements.

[0005] To improve accuracy, it is also known to provide a suitable reference unit to provide a precise reference, including a more complex arrangement with multiple coils, as disclosed in EP1898193B1. Summary of the Invention

[0006] The object of the present invention is to provide a force measuring device of sufficient accuracy combined with an appropriate simple construction.

[0007] This object is achieved by providing a force measuring device according to claim 1. Advantageous embodiments of the invention are further specified in the dependent claims.

[0008] The present invention therefore provides a force measuring device having a current control circuit as described in the introduction, the essential feature being that it controls the current, including dynamically switching between two switching states associated with the two conduction directions of the inductor, preferably even operating with a fixed current.

[0009] Only a single inductor, preferably a coil, is required, and both conduction directions of that coil are used during a switching cycle, which is preferably of a predetermined length of time Tc, although variable cycle time intervals are generally also implementable.

[0010] According to one embodiment of the present invention, a conduction direction is associated with two switch states, with only one switch state being active at a time, each switching on to cause current to flow in the respective direction.

[0011] The switching between the two states is dynamic. In a preferred embodiment, one state, "State 1," is on at the beginning of the cycle, and at some point in the cycle, Ts, "State 2" is switched on and "State 1" is simultaneously switched off until the end of the cycle. The switching point in each cycle is determined in a manner that depends on the force being measured.

[0012] The compensation force is adjusted via the control unit by varying the time T1 that the system is in State 1 and the time T2 that it is in State 2 during the cycle. In the preferred embodiment, this is achieved by varying the switching time TS as a percentage of the cycle duration TC=T1+T2.

[0013] The force measuring device can be brought to a zero state, which corresponds to the situation where no weight is placed on the weighing system, i.e., there is no actual force to measure, and the system is in a zero-state equilibrium position due to its physical components. The implementation of such a zero state can be achieved by a switching time TS.0. Then, for example, by using TS.0 / TC as a control parameter for feedback control, the zero state has the above-mentioned control parameter value TS / TC. Here, typically, when the sensor detects a deviation from equilibrium caused by the force to be measured / the weight placed on the scale, TS is increased, i.e., the control parameter qT, defined as TS / TC, is increased. For example, this is done in a feedback loop including the sensor and a control unit controlling qT.

[0014] As a result of the dynamic changes within the cycle, i.e., an increase in TS and an increase in qT, the coil experiences a change in magnetic field, which creates an effective opposing force depending on the ratio of switching states. This opposing force can be expressed as qT. On average, the magnetic field increases, and therefore the magnetic force towards equilibrium also increases.

[0015] Thus, when implemented in a weighing system (e.g. a balance), the invention provides a weighing system with a measurement mechanism based on the electromagnetic force compensation principle, comprising a magnet, a coil connected to an electric circuit driven by direct current, and a control (unit) for controlling the electric circuit depending on information about the relative position of the coil and the magnet, wherein the control essentially has a control mode in which the electric circuit is alternately switched between a first phase and a second phase, wherein in the first phase the electric circuit is in a first switch state and a current flows through the coil in one direction, and wherein in the second phase the electric circuit is in a second switch state and a current flows through the coil in the opposite direction, and wherein a degree of asymmetry between the first and second phases is set depending on said relative position information, wherein this degree of asymmetry in particular comprises or consists of the duration of the first and second phases.

[0016] Therefore, T1 / Tc or qT is suitable for expressing the degree of asymmetry. As mentioned above, a parameter suitable for expressing this asymmetry, for example qT, can be used as a control parameter for the feedback control of the control unit, and the measurement result will depend on the feedback result at that equilibrium. In particular, the measurement result may depend on, and in particular be proportional to, qT - qT,0, where qT - qT,0 is Ts,0 / Tc.

[0017] It is understood that if the force being measured is continuous in time, any feedback loop time will be shorter than the time scale associated with changes in the force being measured, and the measurement output signal will be obtained based on a local equilibrium state in time.

[0018] In a preferred embodiment of the present invention, this force is adjusted until it compensates for the force being measured, at which point the forces are in equilibrium and no movement occurs to components within the device.

[0019] This is particularly true in static weighing, however, the invention is not limited to static weighing with regard to the measurement process, but is also applicable to weighing systems such as so-called check-weighers, in which measurements are taken while the object to be measured is being transported.

[0020] In the case of balance / weighing systems, this adjustment is achieved using position sensors that detect the displacement from a reference position corresponding to the equilibrium of forces, for example the position of the end region of the last lever in the lever arrangement of the load / weighing cell (e.g. monoblock cells or cells with components made of die-cast aluminum and rolled bending bearings).

[0021] As mentioned above, there is an equilibrium position at the zero state, and the equilibrium position in the measurement process is controlled to the equilibrium position at the zero state. However, the equilibrium state may be reset as needed. In particular, the present invention encompasses systems having a reference weight whose force transmission path leads to the same electromagnetic force compensation arrangement, for example, a coil / compensation arrangement at the end of the last lever of the load / weighing cell lever system.

[0022] The sensor converts this mechanical deflection into an output signal that is fed to a control unit, preferably via a feedback loop, for adjusting the compensation force. Details of electromagnetic force compensation are generally known to those skilled in the art and will not be described further herein.

[0023] According to the invention, the device includes means for providing a measurement output indicative of the controlled current, from which the compensation force, and hence the force to be measured, is deduced, in particular when local or complete balance is achieved.

[0024] Preferably, the measured output in a cycle depends on the ratio of the switch on time T1 to the total cycle time Tc.

[0025] More preferably, the current supply is fixed / constant and does not require increasing current magnitude as an input for determining the measurement result. The current magnitude is reflected in the measurement result via an average value over the cycle time TC, the average value occurring at the time difference between the first and second phases (the time the system is in the first and second states).

[0026] In a preferred embodiment of the present invention, a bridge circuit, in particular an H-bridge circuit, is used to obtain the two switch states.

[0027] An H-bridge circuit consists of four switches, with a connection between two points in the circuit that separates the switches into two pairs.

[0028] In a preferred embodiment, the inductor is placed at the connection separating the switches, ie, in the center of the H-bridge.

[0029] In a preferred embodiment of the present invention, a capacitor is connected in parallel with the inductor to smooth out current changes during switching. An additional benefit of the capacitor is that it prevents any sounds, particularly audible sounds, that may otherwise be generated during operation of the device. The capacitor also serves to limit the voltage across the inductor.

[0030] In a preferred embodiment of the present invention, the device operates at a constant current, and in particular uses a fixed current source / sink, preferably including circuitry with a cascode MOSFET to suppress unwanted voltage peaks that can adversely affect a fixed current supply. By supplying a constant current, temperature fluctuations due to current changes through the shunt resistor of the current source are suppressed, reducing the need for thermal considerations as in prior art systems.

[0031] In a preferred embodiment of the present invention, the switches are configured as MOSFETs, particularly n-channel and p-channel MOSFETs in enhancement mode.

[0032] This preferred embodiment may include a switching auxiliary circuit for preventing the MOSFET from switching into a conductive state when the voltage at the inductor terminal exceeds the applied voltage. In particular, this switching auxiliary circuit may be configured with two pairs of MOSFETs and resistors.

[0033] In another embodiment of the present invention, the switch may be configured as an analog / integrated switch.

[0034] In a preferred embodiment, the frequency corresponding to the cycle time Tc is on the order of kHz. In particular, 1 / Tc is preferably at least 0.5 kHz, more preferably at least 1 kHz, even more preferably at least 2 kHz, and even more preferably at least 3 kHz. On the other hand, the frequency is preferably not more than 20 kHz, more preferably not more than 16 kHz, even more preferably not more than 12 kHz, and especially not more than 9 Hz.

[0035] The magnitude of the DC current is in the milliampere range, which is a preferred size for applications, particularly for implementing weighing cells with a resolution of at least 500,000 points, more preferably 1 million points or more, and even more preferably 2 million points or more. For example, 1 million points for a weighing range of 1 kg means a resolution of 1 μg (microgram). The current intensity is preferably at least 0.5 mA or more, more preferably 2 mA or more, and even more preferably 4 mA or more. Furthermore, the current intensity is preferably 30 mA or less, more preferably 24 mA or less, and even more preferably 16 mA or less. Regarding the switches in the circuit, when performing dynamic / alternating switching, it is desirable that the switching time for switching from the first switch state to the second switch state is shorter than the cycle time TC, preferably at least 4 times shorter, more preferably at least 10 times shorter, even more preferably at least 40 times shorter, particularly 100 times shorter, or even 200 times shorter. Higher ratios are also possible, but are limited by the physical characteristics of the implemented switches.

[0036] In a particularly preferred embodiment, a capacitor is connected in parallel with the coil as described above, particularly including a selected ratio between the switching time and the cycle time, thereby enabling the present invention to provide a force measuring device, in particular a balance / weighing system, that can maintain a constant current intensity even at low current levels with a single coil, which offers advantages in terms of energy efficiency compared to implementations of electromagnetic force compensation using two or more coils.

[0037] Although the above description has focused on a weighing system, the uses and advantages of the present invention are not limited to this particular application and can be used to measure different forces in different environments. The cycle time can also incorporate additional states, such as a quiescent state, but it is important that there are two switching states. The invention is explained in more detail below with reference to the drawings. [Brief explanation of the drawings]

[0038] [Figure 1] This is a circuit diagram showing a basic H-bridge circuit (1H) centered around an inductor (1L). [Figure 2] FIG. 10 shows a circuit diagram for controlling the current through the inductor (2L) according to a preferred embodiment of the present invention. [Figure 3] FIG. 1 shows a schematic diagram of several switching cycles. [Figure 4] FIG. 1 is a simplified explanatory diagram of a weighing system equipped with an induction unit (I) including a current control circuit. DETAILED DESCRIPTION OF THE INVENTION

[0039] As shown in Figure 1, the H-bridge circuit (1H) is composed of four switches (S1-S4). Furthermore, a fixed voltage source (1V) is connected between the upper switches (S1, S2) of the H-bridge circuit (1H), and a fixed current source / sink (1I) is connected between the lower switches (S3, S4) of the H-bridge circuit (1H). In addition, both the voltage source (1V) and the current source (1I) are connected to ground (1G).

[0040] The two switch states described above correspond to the diagonal switch pairs in the H-bridge circuit (1H), i.e., one of the upper switches (S1, S2) and the opposite lower switch (S4, S3) are simultaneously closed, and the other pair is open. In the illustrated example, one of the two switch states is shown, where the upper right switch (S2) and the lower left switch (S3) are closed, and the upper left switch (S1) and the lower right switch (S4) are open.

[0041] Therefore, by adjusting the operating times of each diagonal switch configuration, the system effectively operates as averaged over the cycle time, resulting in an average current as an effective quasi-steady magnetic field in the electromagnetic force compensation.

[0042] As shown in FIG. 2, an H-bridge circuit (2H) centered around an inductor (2L) is composed of four MOSFET switches (M5L, M5R, M3L, M3R). In one embodiment, the upper switches (M5L, M5R) are enhancement-mode p-channel MOSFETs, and the lower switches (M3L, M3R) are enhancement-mode n-channel MOSFETs. A fixed current source / sink (2I) is connected between the lower switches (M3L, M3R) of the H-bridge circuit (2H), with the other end connected to ground (2G). A capacitor (C) is connected in parallel with the inductor (2L). The capacitance of the capacitor (C) is at least 0.2 μF or more, preferably greater than 0.5 μF, and more preferably greater than 0.8 μF, and / or less than 6 μF, more preferably less than 4 μF, and even more preferably less than 2 μF.

[0043] The switching sub-circuits are each composed of a pair of MOSFETs and resistors (M4L, M4R, RL, RR), connected in parallel with each other. The resistors of each pair are connected in series between the upper switch (M5L, M5R) and the inductor (2L), while the MOSFETs of each pair are connected to opposite wires of the H-bridge circuit (2H) between the inductor (2L) and its corresponding lower switch (M3R, M3L).

[0044] The circuit further includes a voltage source (2V) connected to a second ground (GV) on one hand and having multiple connections to the H-bridge circuit (2H) on the other hand. One connection is connected to the H-bridge circuit (2H) between the upper switches (M5L, M5R). There are four more connections: two between the upper-left switch (M5L) and the lower-left switch (M3L), and two between the upper-right switch (M5R) and the lower-right switch (M3R). Each of these connections has an electronic component between the voltage source (2V) and the H-bridge circuit (2H).

[0045] The first connection between the left switches (M5L, M3L) is closer to the lower switch (M3L) than the second connection between the left switches (M5L, M3L) and includes a diode (D1) with its cathode connected to the 2V voltage source. The second connection between the left switches includes a resistor (R1). The first connection between the right switches (M5R, M3R) is closer to the lower switch (M3R) than the second connection between the right switches (M5R, M3R) and includes a diode (D2) with its cathode connected to the 2V voltage source. The second connection between the right switches includes a resistor (R2). Two capacitors (C1, C2) are also provided between each lower switch (M3L, M3R) and its first connection to the 2V voltage source.

[0046] In Figure 3, the horizontal axis represents time, with the scale indicating cycles. The vertical axis represents each switching state, whether it is on or off at that time. In each cycle, the first state is on for a certain period of time while the second state is off, then the second state is on and the first state is off for the remainder of the cycle.

[0047] In the first two cycles, the first state is on for 30% of each cycle. The horizontal dashed lines in the diagram indicate that there are multiple cycles between the first two cycles and the last cycle shown—cycle n—that are omitted from the diagram. In cycle n, the first state is on for 50% of the cycle.

[0048] These simplified examples may for example show the zero state (no metering) setting described above, or the final setting after feedback control during measurement.

[0049] A simplified diagram of an example weighing system is shown in Figure 4. A weight (W) is placed on a weighing pan (T) and monitored by a sensor (S). Information about the pan's position is sent to a control unit (CT), which controls an induction unit (I) based on this information. The induction unit includes a current control circuit and generates a compensation force. This force interacts with a magnet (M), thereby adjusting the position of the weighing pan (T). This control is preferably performed in a feedback loop. The measurement output of the measuring means (MS) is obtained from the control unit (CT) and / or the induction unit (I).

[0050] As described above, the current supply is a DC power supply controlled to provide a constant magnitude current. Even in this constant magnitude configuration, the magnitude of the current can be varied by switching the operating mode of the system / device. This allows for a change in the metering range, and in one embodiment, the system has at least two operating modes, each with a different current magnitude.

[0051] In the following, a preferred embodiment of the present invention will be described in more detail, taking as an example a weighing system.

[0052] Such a system can be described schematically based on the simplified diagram shown in Fig. 4, which includes a weighing pan (T) on which the mass to be measured is placed, which is connected (e.g. via a Roveral mechanism, in particular a lever system connected thereto) to a position sensor (S). The position sensor detects the displacement from a reference position corresponding to a reference switching time Ts,0. This displacement information is sent to a control unit (CT), which adjusts the switching time Ts for the next switching cycle or for the next switching cycles.

[0053] In this embodiment, the switching time is adjusted by feedback control until the compensation force and the weight reach equilibrium and the weighing pan returns to the reference position.

[0054] The measuring means then outputs information about the switch state within the cycle, as shown in FIG. 3, on the basis of which the compensation force is derived and the weight is estimated.

[0055] To technically implement this conceptually simple design, the preferred embodiment shown in Figure 2 uses MOSFET switches (M3L, M3R, M5L, M5R) with switch transition times preferably less than 60 ns, more preferably less than 45 ns, and even more preferably less than 25 ns, with the lower limit determined by available technology. On the other hand, if integrated / analog switches are used, switch transition times can be less than 5 ns, or even less than 2 ns, or even less than 1 ns.

[0056] In the preferred embodiment shown in Figure 2, the device operates at a constant current, preferably about 10 mA, with an applied voltage of about 6 V and a cycle time of approximately 6 kHz. Generally, when operating with MOSFETs, the applied voltage is sufficient to operate the elements in the circuit, and is preferably less than 200 V, less than 50 V, less than 20 V, less than 10 V, and / or preferably greater than or equal to 4 V. On the other hand, for integrated / analog switches, the applied voltage may be less than 4 V, less than 2 V, or even less than 0.5 V, but is preferably greater than or equal to 0.5 V.

[0057] The present invention may be embodied and implemented in other specific forms, such as operating in more than two switch states, which will be apparent to those skilled in the art and do not depart from the essential characteristics of the invention. Therefore, the embodiments disclosed herein are to be considered in all respects as illustrative and not restrictive. The present invention is limited only by the scope of the appended claims, and not by the details of the specification and embodiments.

Claims

1. A force measuring device based on the principle of electromagnetic force compensation, an electronic circuit driven by DC, the electronic circuit including an inductor (1L, 2L), in particular a coil; a control unit for controlling the compensation force by controlling the current flowing through the inductor in response to the force to be measured; means for providing a measurement output indicative of said measured force; Equipped with The control of the current includes dynamic switching between two switch states associated with two conduction directions of the inductors (1L, 2L). Force measuring device.

2. 2. The force measuring device according to claim 1, wherein the electronic circuit is a bridge circuit.

3. 3. The force measuring device according to claim 2, wherein the bridge circuit is an H-bridge circuit (1H) having four switches (S1 to S4).

4. 4. A force measuring device according to claim 3, wherein pairs of switches at opposite corners (S1 and S4, S2 and S3) in the H-bridge circuit (1H) are coupled together, and each of the two switch states corresponds to one of the coupled pairs being closed and the other being open.

5. 5. A force measuring device according to any one of claims 1 to 4, wherein a capacitor (C) is wired in parallel to the inductor (2L) to smooth out current changes during switching.

6. 6. A force measuring device according to any one of claims 1 to 5, which is driven with a fixed current, in particular with a fixed current source / sink (1I).

7. 7. A force measuring device according to claim 1, wherein the switches (M3L, M3R, M5L, M5R) are configured as MOSFETs.

8. 8. A force measuring device according to claim 7, when dependent on claim 2, further comprising a switching auxiliary circuit for preventing one or more MOSFET switches (M3L, M3R, M5L, M5R) from becoming conductive due to the end voltage of the inductor exceeding the applied voltage (2 V), in particular preventing said becoming conductive during switching.

9. 9. A force measuring device according to claim 8, which relies on claim 3, wherein the switching auxiliary circuit is constituted by two sets of MOSFETs and resistors (M4L, M4R, RL, RR), the two sets being wired in parallel with each other, each of the two sets of resistors being connected in series between a corresponding upper switch (M5L, M5R) and the inductor (2L), and each of the MOSFETs of each set being connected to wiring on the opposite side of the H-bridge circuit (2H) between the inductor (2L) and the corresponding lower switch (M3R, M3L).

10. Force measuring device according to any one of claims 1 to 6, wherein the switches (S1-S4) are analog / integrated switches.

11. 11. A force measuring device according to any one of claims 1 to 10, wherein the measurement output in a cycle depends on the ratio of the time that each switch state is on during that cycle.

12. A weighing system comprising a force measuring device according to any one of claims 1 to 11.

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