Force measuring device with current control circuit

Abstract

The invention relates to a force measuring device based on the principle of electromagnetic force compensation, comprising: an electronic circuit driven by a direct current, comprising an inductor, in particular a coil; a control unit that controls a current flowing through the inductor and thereby controls a compensation force, the control being responsive to a force to be measured; and means for providing a measurement output indicative of the force to be measured. According to the invention, current control involves dynamic switching between two switching states associated with two conduction directions of the inductor.

Description

Technical Field

[0001] This invention relates to a force measuring device, and more particularly to a balance operating based on the principle of electromagnetic force compensation (also known as electromagnetic force restoration) and having electronic circuitry for controlling current. Specifically, it relates to a force measuring device based on the principle of electromagnetic force compensation, comprising: an electronic circuit driven by direct current, the circuit including an inductor (particularly a coil); a control unit controlling the current flowing through the inductor and thereby controlling the compensation force, the control being responsive to the force to be measured; and a device for providing a measurement output indicating the force to be measured. Background Technology

[0002] Various force measuring devices (such as a series of electronic balances) typically operate by inductively generating and adjusting magnetic compensation forces.

[0003] In many cases, the force acts repulsively on the stationary magnet, causing movement of components within the device. This eventually leads to the adjustment of the resulting compensating force until an equilibrium position is reached, from which the force to be measured (weight in the case of a balance) can be deduced. The process of achieving equilibrium through magnetic compensation can be so rapid (near instantaneous) that the components do not actually exhibit noticeable physical movement; the tendency to move can be immediately identified by a positioning sensor, and typically through feedback control, the maximum displacement from the equilibrium position is very small. Weighing sensors known to those skilled in the art are typical examples of balances with this rapid feedback control.

[0004] Depending on the technology, adjustment can be performed continuously or discretely over multiple control cycles. In many balances, the inductive force is controlled by changing the intensity (magnitude) of the current flowing through the sensing device. Such systems are disclosed, for example, in DE3324402A1 and US4212361A1. In these systems, the temperature change caused by the current variation is controlled by a calibration network to obtain accurate measurement results.

[0005] To improve accuracy, as disclosed in EP1898193B1, it is known to provide a precise reference by providing a suitable reference cell, which involves a more complex arrangement using multiple coils. Summary of the Invention

[0006] The purpose of this invention is to provide a force measuring device that combines satisfactory accuracy with a simple layout.

[0007] This objective is achieved by providing the force measuring device as described in claim 1. Advantageous embodiments of the invention are further defined in the dependent claims.

[0008] Therefore, the present invention provides a force measuring device with current control circuit as described above, the basic feature of which is that the current control involves dynamic switching between two switching states associated with two conduction directions of the inductor, preferably, it can operate even under a fixed current.

[0009] Only a single inductor (preferably a coil) is required, and during the switching cycle (preferably a predetermined duration T) C The coil can be used in both conduction directions within a time period (although a variable periodic time interval can also be achieved).

[0010] According to the present invention, the conduction direction is associated with two switching states, and when the corresponding state is turned on, the trigger current of the switching state flows in the corresponding direction, while only one switching state is active at any given time.

[0011] The switching between the two states is dynamically triggered. In a preferred embodiment, one state (state 1) is activated at the beginning of the cycle, and the other state (state 2) is activated at a certain time T within the cycle. S The circuit is switched on, while state 1 is disabled, and the circuit remains on until the end of the cycle. The switching time for each cycle is determined based on the force being measured.

[0012] The compensation force is adjusted by the control unit changing the time T1 when the system is in state 1 and the time T2 when it is in state 2 within a cycle. In a preferred embodiment, this is achieved by changing the switching time T... S The period duration T C (T C It is achieved by changing the percentage of (T1 + T2).

[0013] The force measuring device can be in 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 be measured, but the system itself is already in equilibrium through its physical components in the zero state. This zero state can be achieved through a switching time T. S，0 Give it. Then, for example, use T. S，0 / T C As a control parameter for feedback control, the zero state has the value T of this control parameter. S / T C Currently, displacement from equilibrium caused by the force to be measured / the weight placed on the balance is typically sensed by a sensor, T. S (i.e., defined as T) S / T C control parameter q T ) will increase, for example, when involving sensors and control q T Within the feedback loop of the control unit.

[0014] Due to the dynamic changes in the period (i.e., T) Sor q T The increase in the coil causes a change in the magnetic field, which in turn generates an effective reaction force that depends on the change in the switching state ratio, and can be transmitted through q. T This can be represented by the symbol . On average, the magnetic field increases, therefore the magnetic force towards equilibrium increases.

[0015] Therefore, when used in a weighing system (e.g., a balance), the present invention provides a weighing system having a measuring mechanism based on electromagnetic force compensation and including a magnet, a coil connected to a circuit driven by direct current, and a control unit that controls the circuit in response to relative position information of the coil and the magnet. Its essential feature is that the control unit has a control mode in which the circuit alternately switches between a first stage and a second stage, in which the circuit is in a first switching state and current flows through the coil in one direction; and in the second stage, the circuit is in a second switching state and current flows through the coil in the opposite direction; and the degree of asymmetry between the first stage and the second stage is set according to the relative position information, the degree of asymmetry specifically comprising (or consisting of) the duration of the first stage and the second stage.

[0016] Therefore, T1 / T C The ratio or q T This is suitable for describing the degree of asymmetry. As mentioned earlier, a suitable parameter (e.g., q) can be used to represent this asymmetry. T The measurement result depends on the feedback result at equilibrium, and is used as the control parameter for feedback control of the control unit. Specifically, the measurement result may depend on (particularly) q. T -q T，0 Proportional, where q T，0 For T S，0 / T C .

[0017] When the force to be measured is continuous over time, it is understandable that the time of any feedback loop is lower than the relevant time scale of the change in the force to be measured. In this case, the instantaneous local equilibrium condition is used as the basis for measuring the output signal.

[0018] In a preferred embodiment of the invention, the force is adjusted until it compensates for the force to be measured, at which point the forces are balanced and the components in the device do not move. This is particularly suitable for static weighing. Of course, the invention is not limited to static weighing during the weighing process, but also applicable to weighing systems where the items to be weighed are transported and measurements are taken during transport, such as so-called checkweighers.

[0019] In the case of a balance / weighing system, this adjustment is achieved by a position sensor that detects displacement from a default position (which is also in stress balance), such as the position of the last lever end region in a load / weighing sensor lever assembly (e.g., an integral sensor or a sensor consisting of die-cast aluminum and rolled bending bearing components).

[0020] As previously stated, a zero state exists as an equilibrium position, and the equilibrium position of the corresponding measurement process to be controlled must reach the zero state equilibrium position. However, the equilibrium state can be reset as needed. In particular, the present invention also includes a system with a reference weight whose force transmission path terminates at the same electromagnetic force compensation device, such as a coil / correction device at the final lever end of a load / weighing sensor lever system.

[0021] The sensor then converts this mechanical deviation into an output, which is fed to the control unit to adjust the compensation force, preferably via a feedback loop. Details of electromagnetic force compensation are well known to those skilled in the art and will not be described further here.

[0022] According to the invention, the device includes a means for providing a measurement output indicating a controlled current from which a compensating force can be inferred, and further inferred, the force to be measured, particularly when local equilibrium or balance is achieved.

[0023] Preferably, the period measurement output depends on the switch on-time T1 and the total period time T. c The ratio.

[0024] More preferably, since the current source is fixed / constant, there is no need to use the increased current magnitude as an input to determine the measurement result, and the current magnitude still affects the periodic time T caused by the time difference between the first and second stages (the time the system is in the first and second states). C The average value can influence the measurement results.

[0025] In a preferred embodiment of the invention, a bridge circuit (especially an H-bridge circuit) is used to obtain two switching states.

[0026] An H-bridge circuit consists of four switches, with additional connections between two points in the circuit that divide the switches into two pairs.

[0027] In a preferred embodiment, the inductor is located at the connection of the disconnector switch, also known as the middle of the H-bridge.

[0028] In a preferred embodiment of the invention, the capacitor is connected in parallel with the inductor to smooth current changes during switching. Another advantage of the capacitor is that it prevents noise (especially audible noise) that would otherwise be produced during device operation. The capacitor also limits the voltage across the inductor.

[0029] In a preferred embodiment, the device operates with a constant current, particularly using a fixed current source / well, preferably including circuitry with a common-source MOSFET to suppress unwanted voltage spikes that negatively impact the fixed current supply. The fixed current supply suppresses temperature variations caused by current changes in the current source shunt resistor and reduces the need for countermeasures compared to prior art systems.

[0030] In a preferred embodiment of the invention, the switch is configured as a MOSFET, particularly an enhancement-mode n-channel and p-channel MOSFET.

[0031] This preferred embodiment may further include a switch support circuit to prevent the MOSFET from switching to the on state when the voltage across the inductor exceeds the applied voltage. Specifically, the switch support circuit may consist of two pairs of MOSFETs and resistors.

[0032] In another embodiment of the invention, the switch may be configured as an analog / integrated switch.

[0033] In a preferred embodiment, with period time T C The relevant frequencies are on the order of kHz. Specifically, 1 / T is preferred. C The frequency is at least 0.5 kHz, more preferably at least 1 kHz, even more preferably at least 2 kHz, and even at least 3 kHz. On the other hand, the frequency is preferably no more than 20 kHz, particularly no more than 16 kHz, more preferably no more than 12 kHz, and particularly no more than 9 kHz.

[0034] The magnitude of the DC current is in the milliampere range, which is particularly preferred for applications, especially for load cells that achieve a resolution of at least 500,000 points per range interval (particularly one million points or higher, preferably two million points or higher). For example, for a 1 kg interval, one million points means a resolution of 1 μg. The current intensity is preferably at least 0.5 mA or higher, preferably 2 mA or higher, particularly 4 mA or higher. Furthermore, the current intensity is preferably no more than 30 mA, particularly no more than 24 mA, and particularly no more than 16 mA. Regarding the switches in the circuit implementing dynamic / alternating switching, the switching time from the first switching state to the second switching state is preferably longer than the cycle time T. C At least 4 times smaller, more preferably at least 10 times smaller, more preferably at least 40 times smaller, and especially 100 times smaller or even 200 times smaller. Although higher ratios are envisioned, there are natural limits due to the physical characteristics of the switches being implemented.

[0035] A particularly preferred embodiment includes the capacitor connected in parallel with the coil, and specifically selects the ratio of the switching time to the cycle time. Therefore, the present invention provides a force measuring device (particularly a balance / weighing system) that maintains a constant current magnitude even at low current levels by using only one coil, which also offers energy-saving advantages compared to any technique that uses two or more coils in electromagnetic force compensation implementation.

[0036] Although the foregoing description focuses on the weighing system, the uses and advantages of this invention are not limited to this specific application and can also be used to measure different forces in different scenarios. Additional states (e.g., rest cycle states) can be integrated into the cycle time; importantly, there are two switching states. Attached Figure Description

[0037] The invention will now be explained in more detail with reference to the accompanying drawings, wherein:

[0038] Figure 1 A circuit diagram of the basic H-bridge circuit 1H surrounding the inductor 1L is shown.

[0039] Figure 2 A circuit diagram for controlling the current flowing through inductor 2L is shown according to a preferred embodiment of the present invention.

[0040] Figure 3 Multiple switching cycles are illustrated schematically.

[0041] Figure 4 A simplified illustration of a weighing system with sensing unit I, including a current control circuit, is shown. Detailed Implementation

[0042] like Figure 1 As shown, the H-bridge circuit (1H) consists of four switches (S1-S4). In addition, a fixed voltage source (1V) is connected between the upper switches (S1, S2) and a fixed current source / sink (1I) is connected between the lower switches (S3, S4). Both the voltage source (1V) and the current source (1I) are connected to ground (1G).

[0043] The two switching states previously described correspond to each pair of diagonal switches in the H-bridge circuit (1H), where one of the upper switches (S1, S2) and the lower switch (S4, S3) on the opposite side are closed simultaneously, while the other pair of switches is open. In the figure, one of the two switching states is shown with the upper right switch (S2) and the lower left switch (S3) closed, while the upper left switch (S1) and the lower right switch (S4) are open.

[0044] Therefore, by changing the activation time of the corresponding diagonal switch configuration, the system effectively presents an average situation within the period, thereby generating an average current as an effective quasi-static magnetic field in electromagnetic force compensation.

[0045] like Figure 2 As shown, the H-bridge circuit (2H) surrounding the inductor (2L) consists of four MOSFET switches (M5L, M5R, M3L, M3R). In an exemplary embodiment, the upper switches (M5L, M5R) are enhancement-mode p-channel MOSFETs, while the lower switches (M3L, M3R) are enhancement-mode n-channel MOSFETs. A fixed current source / well (2I) is connected between the lower switches (M3L, M3R) to the H-bridge circuit (2H) and to ground (2G) on the other side. A capacitor (C) is wired in parallel with the inductor (2L). Its preferred capacitance is at least 0.2 μF, preferably greater than 0.5 μF, more preferably greater than or equal to 0.8 μF and / or less than 6 μF, preferably less than 4 μF, more preferably less than 2 μF.

[0046] The switch support circuit consists of pairs of MOSFETs and resistors (M4L, M4R, RL, RR) connected in parallel. The corresponding resistor in each pair is connected in series between each upper switch (M5L, M5R) and the inductor (2L). The corresponding MOSFET in each pair is connected to the wires of the H-bridge circuit (2H) between the inductor (2L) and the lower switch (M3R, M3L) on the opposite side.

[0047] In addition, the circuit includes a voltage source (2V) connected to a second ground (GV) on one side and multiple connection points on the other side connecting to the H-bridge circuit (2H). One connection point connects to the H-bridge circuit (2H) between the upper switches (M5L, M5R). There are four additional connection points: 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 between the voltage source (2V) and the H-bridge circuit (2H) includes an electronic component.

[0048] For the first connection point between the left switches (M5L, M3L), which is closer to the lower switch (M3L) than the second connection point between the left switches (M5L, M3L), the component is a diode (D1) with its cathode located on the voltage source (2V) side. For the second connection point between the left switches, the component is a resistor (R1). For the first connection point between the right switches (M5R, M3R), which is closer to the lower switch (M3R) than the second connection point between the right switches (M5R, M3R), the component is a diode (D2) with its cathode located on the voltage source (2V) side. For the second connection point between the right switches, the component is a resistor (R2). Finally, two additional capacitors (C1, C2) are provided between the lower switch (M3L, M3R) on each side and the corresponding first connection point of the voltage source (2V) on that side.

[0049] exist Figure 3 In the diagram, the horizontal axis represents time, the period is indicated by scale lines, and the vertical axis shows whether each switching state is active within that time period. Within each cycle, the first state is active for a short period, during which the second state is deactivated. Then, the second state is active again, and the first state is deactivated, until the cycle ends.

[0050] In the first two cycles, the first state is on for 30% of the corresponding cycle time. The horizontal points represent several cycles not shown in the diagram between the first two cycles and the last cycle shown (cycle n). In cycle n, the first state is on for 50% of the cycle time.

[0051] These simplified examples could be, for instance, the zero-state (unweighed) setup described above and the final setup after feedback control during measurement.

[0052] Figure 4 A simplified schematic diagram of a possible weighing system is shown. A weight (W) is placed on a weighing pan (T), which is monitored by a sensor (S). Information about the pan's position is fed to a control unit (CT), which controls a sensing unit (I) based on this information. The sensing unit includes a current control circuit and generates a compensating force. This force interacts with a magnet (M), thereby adjusting the position of the weighing pan (T). This control is preferably performed within a feedback loop. The measurement output at the measuring device (MS) is obtained from the control unit (CT) and / or the sensing unit (I).

[0053] The current supply described so far is a DC supply controlled to provide a constant magnitude of current. Even in this constant magnitude configuration, the magnitude level can be varied in different operating modes of the system / device. Thus, the weighing range can be changed, and in one embodiment, the system has at least two operating modes in which the current magnitude differs between the different modes.

[0054] Preferred embodiments of the invention operating in a weighing system are described in more detail below.

[0055] Indicatively, such a system can be based on Figure 4 The simplified diagram shown includes a weighing tray (T) on which the mass to be weighed is placed, the tray being coupled (particularly relating to the Robervail mechanism and the lever system coupled thereto) to a position sensor (S), which detects deviations corresponding to a default switching time T. S，0 The displacement is the default position. This displacement information is then transmitted to the control unit (CT), which in turn adjusts the switching time T for the next switching cycle or several switching cycles. S .

[0056] In this embodiment, the switching time is adjusted by feedback control until the compensating force and weight are balanced and the tray returns to the default position.

[0057] Then, the measuring device is used to output information about the switching state during the cycle (e.g. Figure 3 As shown in the figure, the compensating force can be deduced from it, and the weight can be deduced from this force.

[0058] As a practical technical implementation of this conceptually simple design Figure 2 The preferred embodiment shown uses MOSFET switches (M3L, M3R, M5L, M5R) with a switching transition time of less than 60 nanoseconds, preferably less than 45 nanoseconds, and most preferably less than 25 nanoseconds, with the lower limit set by available technology. Otherwise, if an integrated / analog switch is used, the switching transition time can be less than 5 nanoseconds, or even less than 2 nanoseconds or 1 nanosecond.

[0059] In such Figure 2 In the preferred embodiment shown, the device operates with a constant current (preferably about 10mA), an applied voltage (about 6V), and a cycle time (about 6kHz). Typically, the applied voltage when using a MOSFET is sufficient to operate the components in the circuit. Preferably, it can be below 200V, below 50V, or even below 20V or below 10V, and / or preferably above 4V. For integrated / analog switches, it can be below 4V, below 2V, or even below 0.5V; preferably, it should remain above 0.5V.

[0060] This invention may be embodied in other specific forms, such as using more than two switching states, as will be apparent to those skilled in the art without departing from its essential characteristics. Therefore, the embodiments disclosed herein are to be considered illustrative rather than restrictive in all respects. The invention is defined by the scope of the appended claims, and not by the details in the description of the specification and embodiments.

Claims

1. A force measuring device based on the principle of electromagnetic force compensation, comprising: An electronic circuit driven by direct current, which includes inductors (1L, 2L), and in particular coils; A control unit that controls the current flowing through the inductor and thereby also controls the compensation force, the control being responsive to the force to be measured; as well as A device for providing a measurement output indicating the force to be measured. The characteristic feature is that the current control involves dynamic switching between two switching states associated with two conduction directions of the inductor (1L, 2L).

2. The apparatus according to claim 1, wherein, The circuit is a bridge circuit.

3. The apparatus according to claim 2, wherein, The bridge circuit is an H-bridge circuit (1H) with four switches (S1-S4).

4. The apparatus according to claim 3, wherein, The diagonal switch pairs (S1 and S4, S2 and S3) of the H-bridge circuit (1H) are paired such that two switch states correspond to one pair of the paired switch pairs being closed and the other pair being open.

5. The apparatus according to any one of the preceding claims, wherein, The capacitor (C) is connected in parallel with the inductor (2L) to smooth current changes during switching.

6. The apparatus according to any one of the preceding claims, wherein, The device is driven by a fixed current, particularly by a fixed current source / sink (1I).

7. The apparatus according to any one of the preceding claims, wherein, The switches (M3L, M3R, M5L, M5R) are configured as MOSFETs.

8. The apparatus according to claims 2 and 7, wherein, The device has an additional switch support circuit that prevents one or more of the MOSFET switches (M3L, M3R, M5L, M5R) from being in the on state due to the voltage across the inductor exceeding the applied voltage (2V), especially during switching.

9. The apparatus according to claims 3 and 8, wherein, The switch support circuit consists of two pairs of MOSFETs and resistors (M4L, M4R, RL, RR) connected in parallel. The corresponding resistor in each pair is connected in series between each upper switch (M5L, M5R) and the inductor (2L). The corresponding MOSFET in each pair is connected to the H-bridge circuit (2H) between the inductor (2L) and the lower switch (M3R, M3L) on the opposite side.

10. The apparatus according to any one of claims 1 to 6, wherein, The switches (S1-S4) are analog / integrated switches.

11. The apparatus according to any one of the preceding claims, wherein, The measurement output for one cycle depends on the ratio of the on-time of each switching state within that cycle.

12. A weighing system having a force measuring device according to any one of the preceding claims.

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