CN112384767A

CN112384767A - Electronic circuit of a weighing system with one or more load cells

Info

Publication number: CN112384767A
Application number: CN201980044485.5A
Authority: CN
Inventors: J·C·巴斯霍尔姆
Original assignee: Linak AS
Current assignee: Linak AS
Priority date: 2018-07-09
Filing date: 2019-07-09
Publication date: 2021-02-19
Anticipated expiration: 2039-07-09
Also published as: CN112384767B; WO2020011319A1

Abstract

An electronic circuit comprising a printed circuit board (2a) provides a supply voltage to a load sensor and receives a differential measurement signal from the load sensor. A connector (3) having terminals (6, 7) for receiving differential measurement signals from the load sensor is arranged on the printed circuit board (2a), and circuit traces (12, 13) connect these terminals to corresponding pins on an analog-to-digital converter (18) for converting the differential measurement signals into digital signals. The voltage divider is connected to the supply voltage to provide an intermediate voltage, and an additional circuit trace (15) connected to the intermediate voltage is arranged on the printed circuit board between the connector terminal for receiving the differential measurement signal and an adjacent connector terminal. In this way, the risk of leakage currents is reduced and a more robust weight measurement is achieved which is less sensitive to humidity and temperature variations.

Description

Electronic circuit of a weighing system with one or more load cells

Technical Field

The invention relates to an electronic circuit for providing a supply voltage to a load cell and receiving a differential measurement signal from the load cell, wherein the electronic circuit comprises a connector for connecting the electronic circuit to the load cell. The invention also relates to a weighing system comprising one or more load cells and an electronic circuit, and to a piece of furniture comprising the weighing system.

Background

Weighing systems that include one or more load cells may be used in many applications. Typically, such load sensors are based on four strain gauges or other strain detectors arranged in a Wheatstone bridge configuration, but load sensors comprising only one strain gauge (quarter-bridge) or two strain gauges (half-bridge) may also be used. In that case, a standard resistor having the same level of resistance as the strain gauge replaces the rest of the Wheatstone bridge. In the case where a fixed voltage is applied to the Wheatstone bridge, a differential voltage proportional to the weight applied to the load cell will develop across the Wheatstone bridge.

The Wheatstone bridge of the load sensor is typically connected to the electronic circuit by cables and connectors, where the differential voltage may be detected by, for example, an analog-to-digital converter. On the electronic circuit, which is typically a printed circuit board, circuit traces connect the differential voltage signals from the terminals of the connector to the analog-to-digital converter. Typically, the same connector is also used to provide a fixed voltage to the load sensor, and therefore the circuit traces and terminals for the differential voltage would have to be arranged relatively close to the circuit traces and terminals for the fixed voltage.

Load sensors are often used in environments where humidity and temperature vary widely, which can result in leakage currents on the printed circuit board if the surface of the circuit board is not completely clean. Since the voltage level of differential voltages is typically very low, often in the range of μ V or even nV, leakage currents between circuit traces and terminals for differential voltages and circuit traces and terminals for fixed voltages can easily interfere with the measurement of differential voltages. In other words, the measurement of the weight applied to the load sensor is sensitive to such leakage currents and therefore also to variations in humidity and temperature in the environment of the load sensor.

Disclosure of Invention

It is therefore an object of embodiments of the present invention to provide an electronic circuit for a weighing system with one or more load cells, which weighing system can perform a more robust weight measurement that is less sensitive to leakage currents and thus also to humidity and temperature variations in the environment of the load cells.

According to an embodiment of the invention, the object is achieved with an electronic circuit for providing a supply voltage to a load sensor and receiving a differential measurement signal from the load sensor, the electronic circuit comprising: a printed circuit board; a voltage source for providing the supply voltage; a connector for connecting an electronic circuit to a load sensor, the connector being arranged on the printed circuit board and comprising terminals arranged for providing a supply voltage to the load sensor and terminals arranged for receiving a differential measurement signal from the load sensor; an analog-to-digital converter for converting the differential measurement signal into a digital signal; and circuit traces disposed on the printed circuit board and connecting terminals for the differential measurement signals to corresponding pins on the analog-to-digital converter. The object is achieved when the electronic circuit further comprises the following components: a voltage divider connected to the supply voltage to provide an intermediate voltage; and an additional circuit trace connected to the intermediate voltage and disposed on the printed circuit board between the connector terminal for receiving the differential measurement signal from the load sensor and an adjacent connector terminal.

When the additional circuit traces are arranged between the connector terminals for the differential measurement signal (i.e. the terminals sensitive to leakage currents) and the adjacent connector terminals and are connected to an intermediate voltage (e.g. a voltage near the common mode level of the differential measurement signal), the risk of disturbing the measured leakage currents is greatly reduced or even eliminated. In this way, a more robust weight measurement is achieved, which is less sensitive to leakage currents and thus also to changes in humidity and temperature in the environment of the load sensor.

In an embodiment, the printed circuit board is a two-layer board and the additional circuit traces are disposed on both sides of the board and connected with the at least one plated through hole. Alternatively, the printed circuit board is a multi-layer board and the additional circuit traces are disposed on multiple layers of the board and connected with the at least one plated through hole. In this way, leakage currents in both or all layers of the printed circuit board may be reduced or prevented.

Additional circuit traces may also be disposed on the printed circuit board between the circuit traces connecting the terminals for the differential measurement signal to corresponding pins on the analog-to-digital converter and adjacent circuit traces to also protect sensitive differential measurement signals from leakage currents along the circuit traces carrying the signals to the analog-to-digital converter. In an embodiment, the additional circuit traces are further arranged to completely surround said circuit traces connecting the terminals for the differential measurement signals to the corresponding pins on the analog-to-digital converter.

In an embodiment, the connector further comprises a terminal arranged for receiving a voltage reference signal from the load sensor, and the additional circuit trace is arranged on the printed circuit board between the connector terminal for receiving the differential measurement signal from the load sensor and the connector terminal for receiving the voltage reference signal from the load sensor. The use of a voltage reference signal prevents measurement errors due to, for example, a voltage drop in the terminals supplying the supply voltage to the load sensor, since the voltage actually applied to the load sensor is connected to the analog-to-digital converter. Additional circuit traces between the voltage reference signal and the differential measurement signal prevent leakage currents between these signals.

A filter comprising two resistors and a decoupling capacitor may be arranged in each of the circuit traces connecting the terminals for the differential measurement signal to the corresponding pins on the analog-to-digital converter, the decoupling capacitor being connected to the additional circuit trace, and a further decoupling capacitor being arranged between the additional circuit trace and ground. The filter prevents radio frequency interference caused by radio frequency noise radiated by other devices in the vicinity of the weighing system, and the connection of the decoupling capacitor to the additional circuit trace reduces the risk of leakage currents in the decoupling capacitor that can interfere with the measurement.

In an embodiment, an electronic circuit is configured to provide a supply voltage to a plurality of load sensors and to receive differential measurement signals from the plurality of load sensors, the electronic circuit comprising a connector for each of the plurality of load sensors. This is advantageous in weighing systems using multiple load cells.

The weighing system may comprise a load cell and an electronic circuit as described above, wherein the load cell is connected to the electronic circuit via said connector. In this way, the weighing system benefits from the described advantages of the electronic circuit. The weighing system may further comprise a plurality of load cells and an electronic circuit as described above, wherein each load cell is connected to the electronic circuit via one of said connectors.

A piece of furniture may include a weighing system as described above. In this way, the piece of furniture also benefits from the advantages described. The piece of furniture may be a hospital bed. The hospital bed may further comprise an actuator system comprising at least one linear actuator comprising a reversible DC electric motor; a spindle driven by the reversible DC motor; a spindle nut mounted on the spindle and fixed against rotation, the spindle nut being arranged to move between two end positions; and a load sensor for registering a force, the linear actuator being exposed to the load sensor; a controller; and at least one driver circuit configured to drive the at least one linear actuator under control of the controller, wherein the weighing system is integrated in the actuator system.

Drawings

Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which,

fig. 1 shows an example of a weighing system, which comprises a load cell connected to an electronic circuit,

figure 2 shows an example of how the electronic circuit of figure 1 may be implemented as a printed circuit board,

fig. 3 shows a weighing system, which includes a load cell connected to an electronic circuit with additional circuit traces connected to a voltage divider,

figure 4 shows an example of how the additional circuit traces of figure 3 may be arranged on a printed circuit board,

fig. 5 shows an example of a two-layer board, where additional circuit traces are arranged on both sides of the board and connected to plated through holes,

fig. 6 shows the printed circuit board of fig. 5, wherein the additional circuit trace is extended, such that the circuit trace is surrounded by the additional circuit trace over most of its shunt line on the analog-to-digital converter,

fig. 7 shows the printed circuit board of fig. 5, wherein the additional circuit traces are arranged as closed circuit traces that completely surround the sensitive circuit traces,

fig. 8 shows the electronic circuit of fig. 3, wherein a filter of RC elements is arranged in each sensitive circuit trace,

fig. 9 shows the electronic circuit of fig. 8, wherein a filter of RC elements is also arranged in the circuit trace for the reference voltage,

fig. 10 shows the electronic circuit of fig. 8, with the decoupling capacitor of the RC filter in the sensitive circuit trace connected to the additional circuit trace,

fig. 11 shows the electronic circuit of fig. 9, with the decoupling capacitor of the RC filter in the sensitive circuit trace connected to the additional circuit trace,

figure 12 shows an example of how the electronic circuit of figure 11 may be implemented as a printed circuit board,

fig. 13 shows a weighing system, including a plurality of load cells connected to an electronic circuit with additional circuit traces connected to a voltage divider,

figure 14 shows an example of how the electronic circuit of figure 13 may be implemented as a printed circuit board,

fig. 15 shows a schematic side view of a hospital bed, wherein the weighing system of fig. 13 can be used,

fig. 16 shows a schematic top view of the upper frame of the hospital bed of fig. 15, wherein four load sensors are arranged in the corners of the frame,

fig. 17 shows a schematic side view of a hospital bed with an actuator system, wherein a weighing system can be used,

fig. 18 schematically shows an example of a linear actuator, and,

figure 19 shows an example of an actuator system for the hospital bed of figure 17.

Detailed Description

Fig. 1 shows an example of a weighing system 10 comprising a load cell 1 connected to an electronic circuit 2 via a connector 3. The load cell 1 is a transducer adapted to generate an electrical signal whose magnitude is proportional to the force measured by the load cell. The load cell 1 is shown as a strain gauge load cell, but other types of load cells may be used.

The strain gauge load cell 1 comprises four strain gauges S arranged in a Wheatstone bridge configuration₁、S₂、S₃And S₄. The strain gauge is in principle a planar resistor which deforms when the material of the load cell deforms due to the weight applied to the load cell. This deformation of the strain gauge changes its resistance by an amount proportional to the applied strain. Two of the strain gauges (e.g. S)₁And S₄) Designed to stretch and thereby increase its resistance when weight is applied to the load cell, and two other strain gauges (i.e., S)₂And S₃) Designed to compress when weight is applied to the load cell, thereby reducing its resistance. Typically, the resistance of each strain gauge of the bridge is about 350 Ω.

Thus, when a stabilized voltage (e.g., 5V or 10V) is applied from the electronic circuit 2 to the load sensor 1 via the

terminals

4 and 9 of the connector 3, a change in the resistance of the strain gauge due to the applied weight will cause a differential voltage across the Wheatstone bridge that is proportional to the applied weight. This differential voltage signal is taken as signal V via

terminals

6 and 7 of connector 3_in+And V_in-An analog-to-digital converter 18 connected to the electronic circuit 2. Differential voltage of V_in+-V_in-. As will be described further below, since the differential voltage signal is typically a very small signal, the analog-to-digital converter 18 should be highA resolution analog-to-digital converter, such as a 24-bit analog-to-digital converter. Alternatively, the differential voltage signal may be amplified in a differential amplifier before being applied to the analog-to-digital converter.

In fig. 1, the voltage actually applied to the load sensor 1 is also taken as the reference voltage signal V_ref+And V_ref-To an analog-to-digital converter 18 on the electronic circuit 2 via

terminals

5 and 8 of the connector 3. Reference voltage signal V_ref+And V_ref-The purpose of (a) is to avoid measurement errors due to, for example, voltage drops in the

terminals

4 and 9. However, it is noted that, although as mentioned above, the reference voltage signal V is used_ref+And V_ref-The measurement error due to the voltage drop in the

terminals

4 and 9 is reduced, but it is of course also possible to couple the supply voltage V₊And ground is connected directly to the analog-to-digital converter 18.

It is noted that the load sensor may also comprise only one strain gauge (quarter-bridge) or two strain gauges (half-bridge). In that case, the remaining strain gauges of the Wheatstone bridge are replaced by standard resistors having the same resistance (i.e., around 350 Ω).

On an electronic circuit 2, which may be realized on a printed circuit board 2a, connections in the form of circuit traces 11, 12, 13 and 14 couple a signal V_ref+、V_in+、V_in-And V_ref-The

slave terminals

5, 6, 7 and 8 are connected to an analog-to-digital converter 18. The output of the analog-to-digital converter 18, which is a digital signal indicative of the weight applied to the load sensor 1, is connected to a control circuit 19, in which control circuit 19 it can be used, for example, to control the display of the indicated weight or to make decisions or generate other control signals depending on the weight. It is noted that although the control circuit 19 is shown here as being part of the electronic circuit 2, it may also be replaced by a control circuit in an external system, such as an actuator system, in which the output of the analog-to-digital converter 18 may be used directly.

Fig. 2 shows an example of how the electronic circuit 2 with the connector 3 and the analog-to-digital converter 18 can be implemented as a printed circuit board 2 a. For the sake of simplicity, only the

terminals

5, 6 are shown7 and 8 are connected to circuit traces 11, 12, 13 and 14 of an analog-to-digital converter 18. By way of example, analog-to-digital converter 18 may be implemented with an integrated circuit ADS1231 from Texas Instruments, Inc. (Texas Instruments) in a 16-pin package, with signal V_ref+、V_in+、V_in-And V_ref-Connected to

pins

10, 7, 8 and 9, respectively.

When a voltage of, for example, 5V is applied from the electronic circuit 2 to the load sensor 1 via the

terminals

4 and 9 of the connector 3, two voltages V_in+And V_in-Will be close to 2.5V with a small differential voltage between them proportional to the weight applied to the load cell 1. A full-scale differential voltage (i.e., a differential voltage-V corresponding to a maximum weight (e.g., 500kg) for the load sensor 1)_in+-V_in-) Typically will be ± 0.3% of the voltage applied to the load sensor 1. Thus, in the case of applying a voltage of 5V to the load sensor, the full-scale differential voltage will be + -15 mV. This means that if the weight applied to the load sensor 1 is increased or decreased by, for example, 500g, the differential voltage will be increased or decreased by 15 μ V. In some cases, the requirement is that the system be able to detect a weight change of 50g corresponding to 1.5 μ V. To ensure this, the measurement accuracy should be four times higher, which means that the system should be able to detect 12.5g or 375nV in practice.

V_in+And V_in-These low voltage levels of the differential voltage therebetween may make the system very sensitive to leakage currents on the printed circuit board 2 a. Thus, as can be seen from fig. 2, the distance between the circuit traces connected to the

terminals

5, 6, 7 and 8 of the connector 3 is relatively small and cannot be changed, since it is predefined by the dimensions of the connector 3. The voltage level at terminal 5 is 5V, the voltage level at terminal 8 is 0V, and the voltage levels at

terminals

6 and 7 are approximately 2.5V. Thus, as an example, in the case of a voltage difference of 2.5V between the

terminals

5 and 6, a leakage current of as little as 2nA between these two terminals is connected in parallel to the strain gauge S₂Corresponds to a resistance of 1.25G Ω. This will cause the voltage V to be_in+Increased by 350nV and voltage V_in-Remain unchanged. Therefore, this small leakage current will almost haveThe same effect as a weight change of 12.5g that the system on the load cell 1 should be able to detect. As is the leakage current between

terminals

7 and 8. Leakage currents in this range are not uncommon due to dirt and other impurities on the surface of the printed circuit board 2a, especially when the printed circuit board is located in an area with high humidity.

As described below, such sensitivity of the

terminals

6 and 7 to impurities on the surface of the printed circuit board 2a can be mitigated. As shown in fig. 3 and 4, on the printed circuit board 2a of the circuit 2 for the input signal V_in+And V_in-And for the voltage V_ref+And V_ref-With additional circuit traces 15 disposed between

adjacent terminals

5 and 8. If the voltage V is not used_ref+And V_ref-Then the adjacent terminal is typically used instead for V₊And ground, V₊And ground is a voltage at the same level. This additional circuit trace 15 is connected to a voltage divider comprising a voltage divider connected to V respectively₊Two identical resistors R to ground₁And R₂. As an example, resistor R₁And R₂May have a value of 1k omega. This means that the additional circuit trace 15 will be at an intermediate voltage level of 2.5V, which is related to the voltage V_in+And V_in-Are substantially the same.

Therefore, even in the presence of impurities on the surface of the printed circuit board 2a in the area between the

terminals

6 and 7 and the additional circuit traces 15, there will be substantially no voltage difference, which allows only very small leakage currents. In addition, the impedance of such impurities is typically non-linear and requires a certain threshold voltage to conduct leakage current. Thus, in practice, there will be no leakage current to or from the

sensitive terminals

6 and 7.

Note that there will be a voltage difference of 2.5V between the additional circuit trace 15 and each of the

terminals

5 and 8, so here leakage currents will occur, but they will not affect the

sensitive terminals

6 and 7.

Of course, the intermediate voltage may be provided in other ways than by a resistor divider. An example may be a 2.5V zener diode in series with a resistor.

Note also that the intermediate voltage does not have to be exactly 2.5V. As mentioned above, the intermediate voltage and the voltage V are such that a certain threshold voltage is required to conduct the leakage current_in+And V_in-A voltage difference within this threshold (e.g., a few hundred mV) between the voltage levels of (a) would be acceptable. Even if there is a large voltage difference, the possible leakage current (although not eliminated) will be less than if the adjacent voltage were the supply voltage or ground.

In addition, note that in the case where the strain gauges of the load sensor 1 do not have the same resistance, that is, for example, if the strain gauge S₁And S₂Is higher than the strain gauge S₃And S₄Resistance of, then V_in+And V_in-Will be different from 2.5V and therefore the intermediate voltage should also be adjusted accordingly.

In some embodiments, the printed circuit board 2a of the circuit 2 may be a single layer board with circuit traces on only one side of the board, and in that case the circuit traces may be arranged as shown in fig. 4. However, in many cases, due to the complexity of the circuitry, a two-layer board or even a multi-layer board will be preferred, and in that case there will typically be pads or circuit traces on both sides of the board (or even in all layers) for mounting the terminals of the connector 3. For each terminal, these pads will be connected with plated through holes that serve as electrical tunnels through the insulating substrate of the board. Such holes are also called through holes. Thus, for example, the risk of leakage currents between the

sensitive terminals

6 and 7 and the

terminals

5 or 8 will also be present on both sides of the board. It would therefore be advantageous to arrange at least one plated-through hole in the additional circuit trace 15, so that the additional circuit trace 15 may be present on both sides of the board 2 a. This is illustrated in fig. 5 with plated through

holes

16 and 17, where additional circuit traces 15 between the holes are arranged on both sides of the board.

Dependent on the layout of the printed circuit board 2a, for sensitive input signals V between the

terminals

6 and 7 and the analog-to-digital converter 18_in+And V_in-Circuit traces 12 and 1 of3 may also have to be arranged elsewhere so close to other circuit tracks that leakage currents may occur in these places. This may be prevented by extending the additional circuit trace 15, as shown in figure 6, so that the sensitive circuit traces 12 and 13 are surrounded by the additional circuit trace 15 over most of their branch lines from the

terminals

6 and 7 to the analogue to digital converter 18. If there is sufficient space between the pins of the analog-to-digital converter 18, the additional circuit trace 15 may even be arranged as a closed circuit trace completely surrounding the sensitive circuit traces 12 and 13, as shown in fig. 7.

Due to the input signal V_in+And V_in-And thus these signals may also be sensitive to radio frequency interference caused by radio frequency noise radiated by other devices in the vicinity of the weighing system. The susceptibility of the system to such radio frequency noise (susceptability) can be reduced in a number of well known ways, one of which is illustrated in fig. 8, in which a filter with an RC element is arranged in each of the connections or circuit traces 12 and 13 between the

terminals

6 and 7 of the connector 3 and the analog-to-digital converter 18. As shown in fig. 8, a resistor R is arranged in the connection 12 between the terminal 6 and the analog-to-digital converter 18₃And R₄And a decoupling capacitor C₁And a resistor R is arranged in the connection 13 between the terminal 7 and the analog-to-digital converter 18₅And R₆And a decoupling capacitor C₂The filter of (2). Decoupling capacitor C₁And C₂Connected between the midpoint of the respective resistor and ground for the purpose of decoupling the radio frequency signal to ground.

Although between

terminals

5 and 8 of connector 3 and analog-to-digital converter 18 for reference voltage signal V_ref+And V_ref-Are less susceptible to radio frequency signals due to their voltage levels, but it is often advantageous to arrange similar filters in these connections. This is illustrated in fig. 9 as including a resistor R₇、R₈、R₉And R₁₀And a decoupling capacitor C₃And C₄The filter of (2).

However, in FIGS. 8 and 9, noteIt is intended that the decoupling capacitor C₁And C₂Leakage currents may also occur and such leakage currents may affect the measurement accuracy of the weighing system, similar to the leakage currents between the terminals of the connector 3, as described above. However, this can be achieved by decoupling the capacitor C₁And C₂Connected to the additional circuit trace 15 instead of connecting them to ground and between the additional circuit trace 15 and ground (i.e. with the resistor R)₂Parallel) arrangement of a further decoupling capacitor C₅To alleviate this problem. In this way, the capacitor C₁And C₂The DC voltage across will be very low and thus no leakage currents will occur, as described above, and at the same time the radio frequency signal will still pass through the capacitor C₁、C₂And C₅Decoupled to ground.

This solution is illustrated in fig. 10 and 11, and fig. 12 shows an example of how the electronic circuit 2 of fig. 11 can be implemented on a printed circuit board 2 a. Again, for simplicity reasons, only the circuit traces relevant to the present invention are shown. It is further noted that here the additional circuit trace 15 may extend as shown in fig. 6 and 7, such that the sensitive circuit trace is over most of its branch from the

terminals

6 and 7 to the analog-to-digital converter 18 or is completely surrounded by the additional circuit trace 15.

Note that the decoupling capacitor C₃And C₄Instead of connecting them to ground, additional circuit traces 15 may be connected.

In some cases, the weighing system may use multiple load cells, and thus the electronic circuitry may also be configured to process signals from the multiple load cells. As an example, fig. 13 shows a weighing system 20 comprising four

load cells

21, 22, 23 and 24 connected to an electronic circuit 25 via

connectors

26, 27, 28 and 29. Also here, the load sensor is shown as a strain gauge load sensor comprising four strain gauges S arranged in a Wheatstone bridge configuration₁、S₂、S₃And S₄However, other load cell types may be used.

Thus, when connected via

connectors

26, 27, 28 and 29When the electronic circuit 25 applies a stabilized voltage of, for example, 5V or 10V to the

load sensors

21, 22, 23 and 24, the change in resistance of the strain gauges of one of the load sensors due to the applied weight will cause a differential voltage proportional to the applied weight to appear across the Wheatstone bridge of that load sensor. These differential voltage signals being signals V_in+And V_in-With corresponding reference signal V_ref+And V_ref-Together connected to analog-to-

digital converters

31, 32, 33 and 34 on the electronic circuit 25 via

connectors

26, 27, 28 and 29.

The outputs of the analog-to-

digital converters

31, 32, 33 and 34, which are digital signals indicative of the weight applied to the respective load sensors, are connected to control

circuits

35, 36, 37 and 38, where they can be used, for example, for controlling the display of the indicated weight or for making decisions or generating other control signals depending on the weight. Alternatively, the outputs of the analog-to-

digital converters

31, 32, 33 and 34 may be connected to a common control circuit arranged on the electronic circuit 25, or to a control circuit in an external system (such as an actuator system), wherein the outputs of the analog-to-

digital converters

31, 32, 33 and 34 may be used directly.

For simplicity, the circuit of fig. 13 is shown corresponding to the circuit of fig. 3, i.e., without the filters shown in fig. 8, 9, 10 or 11. However, these filters can of course also be used in the circuit for the connection to a plurality of load sensors.

Fig. 14 shows an example of how circuit traces may be arranged on a printed circuit board 25a of the circuit 25. Only a portion of the printed circuit board is shown. In this case the printed circuit board is shown as a two-layer board, with circuit traces between the terminals of the

connectors

26, 27, 28 and 29 and the corresponding analog-to-

digital converters

31, 32, 33 and 34 arranged on the top side of the board. The additional circuit trace 15 includes a plurality of plated through

holes

41, 42, 43, 44 and 45, so the additional circuit trace 15 may be present on both sides of the board below the connector, for example between plated through

holes

41 and 42 and between plated through

holes

43 and 44. However, between the connectors, for example between plated through

holes

42 and 43 and between plated through

holes

44 and 45, the additional circuit traces 15 are only present on the underside of the board, which is shown with dotted lines, and therefore the additional circuit traces 15 can be arranged between the associated terminals of all the connectors without interfering with the circuit traces between the connector terminals and the corresponding analog-to-digital converters. Also, the embodiments shown in fig. 6 and 7 may of course be used in combination with the circuit 25 of fig. 13.

As an example of a piece of furniture in which the weighing system 20 of fig. 13 may be used, fig. 15 shows a hospital bed 50 comprising a lower frame 51 and an upper frame 52. The lower frame 51 is equipped with a drive wheel 53 and two lifting

columns

54 and 55, which allow height adjustment of the patient bed and connect the lower frame 51 and the upper frame 52. As also shown in fig. 16, four

load sensors

21, 22, 23, and 24 are arranged on the upper frame 52, i.e., in the corners of the upper frame 52, and a support frame 56 is supported by the four

load sensors

21, 22, 23, and 24. In use, the support frame 56 typically supports a mattress, not shown. The control box 57 comprises, for example, a power supply and control circuits for the different functions of the patient bed. In this case, the control box 57 further includes the electric circuit 25, and the four

load sensors

21, 22, 23, and 24 are thus connected to the electric circuit 25 in the control box 57.

The weighing system 20 with the

load sensors

21, 22, 23 and 24 can be used to monitor the weight of the patient and/or the position of the patient in the patient bed 50. In addition, it may be configured to detect and give an indication whether the patient is in the process of getting out of bed or has got out of bed. Since hospital beds are often used in environments where large variations in temperature and humidity may occur, the weighing system 20 ensures a more robust monitoring or detection that is less sensitive to such variations.

As another example of a piece of furniture in which the weighing system 2 or weighing system 20 may be used, fig. 17 shows a patient bed 60 comprising a lower frame 61 and an upper frame 62. The lower frame 61 is equipped with a drive wheel 63. An adjustable support surface 64 for a mattress, not shown, is mounted to the upper frame 62. The support surface includes a backrest portion 63, a hinged leg rest portion 64 and a fixed intermediate portion 65 therebetween. The backrest 63 and the legrest portion 64 may be adjusted with

linear actuators

66, 67 so that the support surface may assume different contours. At each end, the upper frame 62 is connected to the lower frame 61 with

links

68, 69. The upper frame 62 can be raised and lowered by means of a pair of

linear actuators

70, 71 connected to

links

68, 69, allowing height adjustment of the bed.

The

linear actuators

66, 67, 70 and 71 are connected to a control box 72, which control box 72 comprises at least a power supply, a controller and a driver circuit for the linear actuators. In this case, the control box 72 also comprises one of the

electronic circuits

2 or 25 described above, for connection to one or more load sensors. As will be described below, the load sensors may be arranged in one or more linear actuators. A connection box 73 is connected to the control box 72 for connection to e.g. a hand-held remote control 74 or a control panel 75 integrated in the head or the pedals. The system comprising the

linear actuators

66, 67, 70 and 71, the control box 72 and the

control units

74 and 75 is also referred to as an actuator system.

As mentioned, load sensors may be arranged in one or more of the

linear actuators

66, 67, 70 and 71. Fig. 18 schematically shows an example of the linear actuator 66 having a load sensor. The linear actuator 66 includes a reversible electric motor 82, a transmission or reduction gear 83, typically having several stages, a spindle 84 having threads 85, a spindle nut 86 engaged with the threads 85, and a tubular activation element 87. At the end of the activation element 87, a mounting bracket 88 is arranged for mounting the linear actuator 66 to, for example, a carrier element. Accordingly, the rear mounting bracket 89 may be disposed at the other end of the linear actuator. The spindle nut 86 is fixed against rotation. In some linear actuators, the spindle nut is directly connected to, for example, a carrier element without the use of an activation element. When the spindle 84 is rotated by the motor 82, the spindle nut 86 moves along the spindle 84, thereby converting the rotation into a linear movement of the spindle nut 86 and/or the activation element 87 between the two end positions. Note that for some motor types, the reversible electric motor 82 may directly drive the main shaft 84, and thus the transmission 83 may be avoided. The reversible electric motor 82 is typically a reversible DC electric motor, although other types of electric motors may be used.

The linear actuator 66 also includes a load sensor 90 for registering the force to which the linear actuator is exposed and the relative change to this force. In fig. 18, the load cell 90 is positioned in connection with the activation element 87, but it may also be positioned in connection with the rear of the main shaft 84 or a rear mounting bracket 89. As described below, differential voltage signals from the load sensors indicative of the force applied to the linear actuator 66 are connected to the electronic circuit 2 in the control box 72.

Fig. 19 shows a block diagram illustrating the actuator system 91 of the bed shown in fig. 17. The

linear actuators

66, 67, 70, and 71 are connected to a control box 72 via cables, and the control box 72 includes a power supply 92, a controller 93, and a driver circuit (i.e.,

driver circuits

94, 95, 96, and 97) for each linear actuator. Each driver circuit, and thus the electric motor of

actuators

66, 67, 70 and 71, is controlled individually by control signals from controller 93. Typically, the controller 93 includes a microcomputer. The power supply 92 is typically connected to the main AC supply grid by a power cable, but batteries may also be used, either alone or in combination with a power supply connected to the utility grid.

As shown in fig. 17, the control box 72 is also connected to a junction box 73 for connection to, for example, a hand-held remote controller 74 or a control panel 75 integrated in the head or pedals, allowing the operation of the linear actuators to be controlled by a person in the vicinity of the bed 60. As shown in fig. 19, the connection to the handheld remote controller 74 or the control panel 75 may be a wired connection, but a wireless communication system (such as a radio link or an infrared link) may also be used.

In this case, the linear actuator 66 comprises a load cell for registering the force to which it is exposed, and the control box 72 therefore comprises a circuit 2 connected to the load cell as described above. The output signal from the electronic circuit 2 is connected to a controller 93. This output signal may be either an output from the controller 19 or directly from the analog-to-digital converter 18. Note that of course, one or more of the three other

linear actuators

67, 70 and 71 may also include a load sensor, and in that case, the electronic circuit 25 may be used instead of the electronic circuit 2.

For the bed 50 in fig. 13, the weighing system may here also be used to monitor the weight of the patient and/or the position of the patient in the patient bed 60, and it may be configured to detect and give an indication whether the patient is in the process of leaving the bed or has left the bed.

These changes will be registered in the actuator 66 by the load sensor when the patient is sitting on the bed and thus potentially may be in the process of exiting the bed or has exited the bed. Information relating to these changes is sent as a differential voltage signal to the electronic circuit 2 in the control box 72. Based on these changes, the controller 93 in the control box may thus determine whether an alert should be generated or associated with the bed 60 or to a caregiver. If the alarm is generated in relation to the bed, this may occur via an audible, visual or tactile alarm connected to or integrated in the actuator system. The actuator system may be connected to the control box 72 and/or the

control unit

74 or 75, for example. In the event that an alarm should be sent to a caregiver or another person who can attend to the patient, this may be done by connecting the actuator system to a paging system or alarm system used in a given hospital or nursing home. Such connection to the actuator system may be achieved in a variety of ways depending on the paging system or alarm system in a given hospital or nursing home. Furthermore, the connection may be by cable (cabled) and/or wireless. Where the connection is by cable, this may be done, for example, via a cable running from the control box 72 to a wall plug 98 near the bed. If the connection is wireless, control box 72 may generate a signal that is transmitted by transceiver 100 via transceiver 101 to a paging system or alarm system used in a given hospital or nursing home.

Control box 72 may thus convert information from the load sensors in linear actuator 66 into signals suitable for the communication protocol used by the paging system or alarm system. The wall plug 98 and/or the transceiver 100 may also be incorporated in the control box 72 or the junction box 73.

The above-described weighing

system

10 or 20 is therefore integrated here in the actuator system 91 of the bed 60.

In other words, an electronic circuit 2 is disclosed; 25 for applying a force to the load cell 1; 21. 22, 23, 24 provide a supply voltage V₊And from the load cell 1; 21. 22, 23, 24 receive a differential measurement signal V_in+、V_in-An electronic circuit 2; 25 comprises: a printed circuit board 2 a; 25 a; a voltage source for providing the supply voltage V₊(ii) a A connector 3; 26. 27, 28, 29 for connecting the electronic circuit 2; 25 is connected to the load cell 1; 21. 22, 23, 24, said connector being arranged on said printed circuit board 2 a; 25a and comprises a voltage supply arranged to supply a supply voltage V to the load cell₊And arranged for receiving a differential measurement signal V from a load cell_in+、V_in-The

terminals

6, 7; an analog-to-digital converter 18; 31. 32, 33, 34 for converting the differential measurement signal V_in+、V_in-Converting into digital signals; and circuit traces 12, 13 arranged on the printed circuit board 2 a; 25a and will be used for the differential measurement signal V_in+、V_in-Are connected to an analog-to-digital converter 18; 31. 32, 33, 34. The electronic circuit further comprises: voltage divider R₁、R₂Is connected to said supply voltage V₊For providing an intermediate voltage; and an additional circuit trace 15 connected to said intermediate voltage and arranged on the printed circuit board 2 a; 25a between said

connector terminals

6, 7 for receiving differential measurement signals from the load cell and adjacent connector terminals.

When the additional circuit traces are arranged between the connector terminals for the differential measurement signal (i.e. the terminals sensitive to leakage currents) and the adjacent connector terminals and are connected to an intermediate voltage (e.g. a voltage near the common mode level of the differential measurement signal), the risk of disturbing the measured leakage currents is greatly reduced or even eliminated. In this way, a more robust weight measurement is achieved, which is less sensitive to leakage currents and thus to changes in humidity and temperature in the environment of the load sensor.

In an embodiment, the printed circuit board 2 a; 25a is a two-layer board and the additional circuit traces 15 are arranged on both sides of the board and electrically connected to at least one plated through

hole

16, 17; 41. 42, 43, 44, 45. Alternatively, the printed circuit board 2 a; 25a is a multi-layer board and the additional circuit traces 15 are arranged on multiple layers of the board and electrically connected to at least one plated through

hole

16, 17; 41. 42, 43, 44, 45. In this way, leakage currents in both or all layers of the printed circuit board may be reduced or prevented.

Additional circuit traces 15 may also be disposed on the printed circuit board 2 a; 25a to be used for the differential measurement signal V_in+、V_in-Is connected to an analog-to-digital converter 18; 31. 32, 33, 34, thereby also protecting sensitive differential measurement signals from leakage currents along the circuit traces carrying the signals to the analog-to-digital converter. In an embodiment, the additional circuit trace 15 is further arranged to completely enclose the signal to be used for the differential measurement signal V_in+、V_in-Is connected to an analog-to-digital converter 18; 31. 32, 33, 34, respectively, of the circuit traces 12, 13 of the corresponding pins.

In the embodiment, the connector 3; 26. 27, 28, 29 further comprises a voltage reference signal V arranged to receive a voltage reference signal from the load cell_ref+、V_ref-And additional circuit traces 15 are arranged on the printed circuit board for receiving the differential measurement signal V from the load cell_in+、V_in-And a

connector terminal

6, 7 for receiving a voltage reference signal V from a load cell_ref+、V_ref-Between the

connector terminals

5, 8. The use of a voltage reference signal prevents measurement errors due to, for example, a voltage drop in the terminals supplying the supply voltage to the load sensor, since the voltage actually applied to the load sensor is connected to the analog-to-digital converter. Additional circuit traces between the voltage reference signal and the differential measurement signal prevent leakage currents between these signals.

Comprising two resistors R₃、R₄；R₅、R₆And a decoupling capacitor C₁；C₂Can be arrangedWill be used for the differential measurement signal V_in+、V_in-Are connected to an analog-to-digital converter 18; 31. 32, 33, 34, the decoupling capacitor C in each of the circuit traces 12, 13 of the corresponding pin₁；C₂Connected to said additional circuit trace 15 and a further decoupling capacitor C₅Is arranged between the additional circuit trace 15 and ground. The filter prevents radio frequency interference caused by radio frequency noise radiated by other devices in the vicinity of the weighing system, and the connection of the decoupling capacitor to the additional circuit trace reduces the risk of leakage currents in the decoupling capacitor that may interfere with the measurement.

In an embodiment, the electronic circuit 25 is configured to provide a supply voltage to the plurality of

load sensors

21, 22, 23, 24 and to receive differential measurement signals V from the plurality of

load sensors

21, 22, 23, 24_in+、V_in-The electronic circuit comprises a

connector

26, 27, 28, 29 for each of the plurality of load sensors. This is advantageous in weighing systems using multiple load cells.

The weighing system 10 may comprise a load cell 1 and an electronic circuit 2 as described above, wherein the load cell 1 is connected to the electronic circuit 2 via said connector 3. In this way, the weighing system benefits from the described advantages of the electronic circuit. The weighing system 20 may further comprise a plurality of

load cells

21, 22, 23, 24 and an electronic circuit 25 as described above, wherein each

load cell

21, 22, 23, 24 is connected to the electronic circuit 25 via one of said

connectors

26, 27, 28, 29.

A piece of furniture 50; 60 may include a weighing system 10 as described above; 20. in this way, the piece of furniture also benefits from the advantages described. The piece of furniture 50; 60 may be a hospital bed. The hospital bed 60 may further comprise an actuator system 91, the actuator system 91 comprising at least one linear actuator 66, the linear actuator 66 comprising: a reversible DC electric motor 82; a main shaft 84 driven by the reversible DC motor 82; a spindle nut 86 mounted on the spindle 84 and fixed against rotation, the spindle nut 86 being arranged to move between two end positions; and a load sensor 90 for registering a force, the linear actuator 66 being exposed to the load sensor 90; a controller 93; and at least one driver circuit 94 configured to drive the at least one linear actuator 66 under the control of the controller 93, wherein the weighing system 10; 20 are integrated in the actuator system 91.

While various embodiments of the invention have been described and illustrated, the invention is not limited thereto but may also be embodied in other ways within the scope of the subject matter defined in the following claims.

Claims

1. An electronic circuit (2; 25) for supplying a supply voltage (V) to a load cell (1; 21, 22, 23, 24)₊) And from the load cell (1; 21, 22, 23, 24) receive a differential measurement signal (V)_in+，V_in-) An electronic circuit (2; 25) the method comprises the following steps:

a printed circuit board (2 a; 25 a);

a voltage source for supplying the supply voltage (V)₊)；

-a connector (3; 26, 27, 28, 29) for connecting an electronic circuit (2; 25) to a load cell (1; 21, 22, 23, 24), the connector being arranged on the printed circuit board (2 a; 25a) and comprising:

is arranged for providing a supply voltage (V) to the load sensor₊) The terminals (4, 9); and

is arranged for receiving a differential measurement signal (V) from the load sensor_in+，V_in-) The terminals (6, 7);

an analog-to-digital converter (18; 31, 32, 33, 34) for converting the differential measurement signal (V)_in+，V_in-) Converting into digital signals; and

-circuit tracks (12, 13) arranged on the printed circuit board (2 a; 25a) and intended for differential measurement signals (V)_in+，V_in-) Is connected to an analog-to-digital converter (18; 31, 32, 33, 34),

the electronic circuit is characterized in that the electronic circuit further comprises:

a voltage divider (R)₁，R₂) Connected to said supply voltage (V)₊) For providing an intermediate voltage; and

an additional circuit trace (15) connected to said intermediate voltage and arranged on the printed circuit board (2 a; 25a) between said connector terminal (6, 7) for receiving a differential measurement signal from the load sensor and an adjacent connector terminal.

2. An electronic circuit according to claim 1, characterised in that the printed circuit board (2 a; 25a) is a two-layer board and that additional circuit tracks (15) are arranged on both sides of the board and are connected to at least one plated-through hole (16, 17; 41, 42, 43, 44, 45).

3. An electronic circuit according to claim 1, characterised in that the printed circuit board (2 a; 25a) is a multilayer board and that additional circuit tracks (15) are arranged on the multilayer of the board and connected to at least one plated-through hole (16, 17; 41, 42, 43, 44, 45).

4. An electronic circuit according to any one of claims 1 to 3, characterised in that an additional circuit trace (15) is also arranged on the printed circuit board (2 a; 25a) to be used for the differential measurement signal (V)_in+，V_in-) Is connected to an analog-to-digital converter (18; 31, 32, 33, 34) of a corresponding pin and adjacent circuit traces (12, 13).

5. An electronic circuit according to claim 4, characterized in that the additional circuit trace (15) is further arranged to completely enclose the signal (V) to be used for differential measurement_in+，V_in-) Is connected to an analog-to-digital converter (18; 31, 32, 33, 34) of the corresponding pin.

6. An electronic circuit according to any of claims 1-5, characterized in that the connector (3; 26, 27, 28, 29) further comprises a voltage reference signal (V) arranged to be received from a load sensor_ref+，V_ref-) And additional circuit tracks (15) are arranged on the printed circuit board (2 a; 25a) for receiving a differential measurement signal (V) from a load cell_in+，V_in-) And a connector terminal (6, 7) for receiving a voltage reference signal (V) from a load sensor_ref+，V_ref-) Between the connector terminals (5, 8).

7. An electronic circuit according to any one of claims 1 to 6, comprising two resistors (R)₃，R₄；R₅，R₆) And a decoupling capacitor (C)₁；C₂) Is arranged to be used for differential measurement signals (V)_in+，V_in-) Is connected to an analog-to-digital converter (18; 31, 32, 33, 34) of the corresponding pin, the decoupling capacitor (C) in each of the circuit traces (12, 13) of the corresponding pin₁；C₂) Connected to the additional circuit trace (15) and a further decoupling capacitor (C)₅) Is arranged between the additional circuit trace (15) and ground.

8. A weighing system (10; 20) comprising an electronic circuit (2) according to any one of claims 1-7 and a load cell (1), wherein the load cell (1) is connected to the electronic circuit (2) via the connector (3).

9. Piece of furniture (50; 60) comprising a weighing system (10; 20) according to claim 8.

10. The piece of furniture of claim 9, wherein the piece of furniture (50; 60) is a hospital bed further comprising an actuator system (91), the actuator system (91) comprising:

at least one linear actuator (66) comprising:

-a reversible DC electric motor (82);

-a main shaft (84) driven by the reversible DC motor (82);

-a spindle nut (86) mounted on the spindle (84) and fixed against rotation, the spindle nut (86) being arranged to move between two end positions; and

-a load sensor (90) for recording a force, the linear actuator (66) being exposed to the load sensor (90);

a controller (93); and

at least one driver circuit (94) configured to drive the at least one linear actuator (66) under control of a controller (93),

wherein the weighing system (10; 20) is integrated in the actuator system (91).