CAPACITIVE READING DEVICE FOR A POINTING INSTRUMENT

Description

[0001] The invention relates to a capacitive reading device for a pointer instrument.

[0002] Pointer instruments already installed in the field, such as pressure gauges or thermometers, are usually read on-site by a person. There is a need to read these pointer instruments remotely, without requiring a person to go on-site. Furthermore, the reading of the measured value indicated by the pointer position should be automatic and provided as a digital measured value.

[0003] Often, there is no electrical power supply or data line on-site. A technical device that automatically reads the pointer position should therefore consume as little energy as possible and be capable of transmitting the measured value wirelessly. Furthermore, this technical device must be inexpensive to make such a retrofit worthwhile for the operator.

[0004] Optical reading devices, such as those known from WO 2007 / 147609 A2, US 6,157,311 A or US 4,275,393 A, are sensitive to interference from extraneous light and may therefore require active illumination.

[0005] Capacitive reading devices known from EP 3 150 972 B1 and DE 38 91 367 C2 comprise a plurality of electrodes arranged in a circle or arc on a support opposite the rotating pointer of the pointer instrument. The measurement resolution is directly dependent on the number of electrodes and is limited by the fact that only a small number of electrodes, e.g., 8 or 16, can be connected to the measurement electronics via conventional multiplexers. Large multiplexers or solutions with cascaded multiplexers are complex and expensive.

[0006] US Patent No. 4,999,734 A discloses a device for reading a measuring instrument with a pointer having a plurality of electrodes. When a metal portion on the pointer passes over an electrode, a capacitance change is induced, which can be detected, so that the pointer position can be determined.

[0007] US Pat. No. 4,743,838 A discloses a multiplexer belonging to a capacitive display device. The capacitive display device includes electrodes that are divided into a sine wave pattern by an insulating section. Receiving electrodes are mounted opposite the electrodes and are movable relative to the electrodes. The multiplexer is configured to detect the displacement resulting from the capacitance changes.

[0008] Patent application US 2018 / 328762 A1 discloses a capacitive rotary encoder comprising a capacitive sensor array. The capacitive sensor array comprises a plurality of individual sensors arranged in several concentric rings on a PCB disk. Several individual sensors, i.e., individual capacitances, are radially aligned and configured to form a data word with multiple bits in the radial direction.

[0009] The invention is based on the object of detecting a continuous analogue pointer position using simple means and good measurement resolution.

[0010] According to the invention, the object is achieved by the capacitive reading device defined in claim 1, of which advantageous developments are specified in the subclaims.

[0011] The subject of the invention is therefore a capacitive reading device for a pointer instrument with a support provided for arrangement opposite a rotating pointer of the pointer instrument, on which a plurality of n radial electrodes are arranged in a circular arc alternating with radial counter electrodes, wherein the counter electrodes are electrically connected to one another and connected to a first measuring terminal of a capacitance-to-digital converter, wherein the n electrodes are connected to a second measuring terminal of the capacitance-to-digital converter via an nxk connection matrix and a downstream 1-out-of-k multiplexer, and wherein the nxk connection matrix connects each of the n electrodes to one of the k channels of the 1-out-of-k multiplexer. To determine a position of the pointer, the connection pattern varies along the circular arc. The connection pattern can vary to such an extent that, viewed along the circular arc, a non-repeating sequence results for the channels, at least in sections, for the connection pattern. By detecting a capacitive influence on a plurality of channels, the position of the pointer can be determined using the non-repeating pattern.

[0012] For example, the connection pattern along the circular arc can vary with n variations of b out of k, where n > k ≥ b > 1.

[0013] Furthermore, the n variations of v can be formed from k channels, taking into account a specific order of the channels. Taking the order into account provides a way to uniquely determine the position of the pointer.

[0014] Furthermore, the claimed capacitive reading device can be configured to determine the strength of a capacitive influence on a channel. As a result, it is possible to determine, among other things, which channel is most capacitively influenced by the pointer. A channel with the most capacitive influence indicates that it is closest to the pointer. A channel with a lesser capacitive influence can indicate that it is adjacent to the channel with the most capacitive influence. A channel with an even lesser capacitive influence can be located two positions next to the most capacitively influenced channel. This applies analogously to even more distant channels. In conjunction with the connection pattern, the position of the pointer can be unambiguously determined.

[0015] The radial electrodes do not necessarily have to be arranged or formed in a strictly straight radial direction; they can also be slightly radially inclined and / or slightly curved.

[0016] The angular range of the circular arc in which the electrodes are arranged around the pointer's rotational axis should at least correspond to the deflection range of the pointer of the pointer instrument. To enable the optical reading device to be used largely independently of the design of the pointer instrument, the electrodes can be arranged in a closed circle.

[0017] Using the nxk connection matrix, it is possible to uniquely represent each pointer position on a significantly smaller number of k channels of the 1-out-of-k multiplexer, even with a comparatively high number of n electrodes. Assuming a "measurement window" of k adjacent electrodes within which the pointer capacitively influences b electrodes (b ≤ k), then, in the worst case scenario where the pointer influences these b electrodes equally, i.e., indistinguishably, there are N min = k! / [(kb)!·b!] combinations of b out of k measured values. In the best case scenario, the pointer influences the b electrodes to the right and left of it equally, but otherwise differently with increasing distance from it. Then there are N max = k! / [2·(kb)!] variations ("combination taking the order into account") of b out of k measured values. This makes it possible to output each pointer position uniquely to k channels of the 1-out-of-k multiplexer with n electrodes (N max > n > N min ).

[0018] The invention will be explained below using exemplary embodiments and with reference to the figures of the drawing; in detail: Fig. 1 shows an embodiment of a capacitive reading device on a pointer instrument, Fig. 2 shows a block diagram of the capacitive reading device, Fig. 3 shows a simple example of the connection of the electrodes to the 1-out-of-k multiplexer via an nxk connection matrix and Fig. 4 shows an example of the determination of the connection pattern of the nxk connection matrix.

[0019] Identical reference symbols have the same meaning in the various figures. The illustrations are purely schematic and do not represent proportions.

[0020] Fig. 1 shows an exemplary embodiment of a capacitive reading device 1 with a flat pick-up sensor 2, which is glued to the viewing window 3 of a pointer instrument 4, in this case a pressure gauge. The pick-up sensor 2 can also be attached to the pointer instrument 4 in another way, for example, by clamping it on. The pointer instrument 4 is a conventional round instrument with a pointer 5, which rotates behind the viewing window 3 around an axis 6 above the scale or dial 7 of the pointer instrument 4.

[0021] The pick-up sensor 2 is arranged and configured such that, on the one hand, it is opposite the pointer 5 in such a way that the latter completely detects the deflection range of the pointer 5, and, on the other hand, it still allows a view of the scale inscription and, at least partially, the pointer 5. In particular, the pick-up sensor 2 can be designed to be largely transparent.

[0022] The pick-up sensor 2 is designed as a carrier 8 for a plurality of n electrodes 9, which are arranged in a circular arc around the axis 6 of the pointer 5 and are aligned essentially radially. The circular arc with the electrodes 9 does not necessarily have to be closed if the deflection range of the pointer 5 is less than 360°, e.g., 270°.

[0023] Fig. 2 shows a block diagram of the capacitive reading device 1. The radial electrodes 9 are arranged alternately with radial counter electrodes 10, the arrangement in Fig. 2 For the sake of simplicity, it is not shown circularly, but rather as a straight line. The counter electrodes 10 are electrically connected to one another and connected to a first measuring terminal 11 of a capacitance-to-digital converter 12. The n electrodes 9 are connected to a second measuring terminal 15 of the capacitance-to-digital converter 12 via an nxk connection matrix 13 and a downstream 1-out-of-k multiplexer 14.

[0024] High-resolution capacitance-to-digital converters (CDCs) are available from Analog Devices, Inc., for example, under the designations AD7745 / AD7746. This CDC measures a capacitance range from 0 to 8 pF with 24-bit resolution. The capacitance is measured between the first measurement terminal 11 and the second measurement terminal 15 and output as a 24-bit value 16. The second measurement terminal 15 controls the charge and discharge cycle of the capacitance being measured. It is a low-impedance voltage source with a square-wave signal.

[0025] As shown below with reference to Fig. 3 As explained, the nxk connection matrix 13 connects each of the n electrodes 9 to one of the k channels of the 1-out-of-k multiplexer 14.

[0026] The 1-out-of-k multiplexer 14 connects one of the k channels to the second measurement terminal 15 of the capacitance-to-digital converter 12 in response to a control signal 17, while simultaneously connecting the remaining k-1 channels to ground, neutralizing them for capacitance measurement and additionally acting as a shield. A typical example of such a 1-out-of-k multiplexer 14 is the 74xx238 integrated circuit from the 74xx logic family. This 3:8 multiplexer / decoder has 8 channels, each of which can be selected depending on 3 control-side input bits of the 3-bit control signal 17 in this case.

[0027] The control signal 17 is generated by an evaluation device 18 in order to periodically switch each of the channels 1 to k through to the second measuring connection 15 of the capacitance-to-digital converter 12 and to determine the respective current position of the pointer 5 on the basis of the successively measured 24-bit capacitance values ​​16 and to output it as a measured value 19.

[0028] Back to Fig. 1 The pick-up sensor 2 is connected via a line 20 to an electronics unit 21, which, for example, contains the evaluation device 18 or parts thereof and has a battery or solar cell for the power supply and a radio interface 22, such as Bluetooth Low Energy (BLE), for transmitting the determined pointer position to a higher-level location. The capacitance-to-digital converter 12, the nxk connection matrix 13, and the 1-out-of-k multiplexer 14 can be integrated into the pick-up sensor 2. As an alternative to the example according to Fig. 1 the electronics 21 can also be integrated into the pick-up sensor 2.

[0029] Fig. 3 serves to explain the nxk connection matrix 13 using a simple example. The number k, here e.g. k = 4, denotes the width of a measuring window 23 of k adjacent electrodes 9 in order to detect the position of the pointer 5. In the example shown, it is assumed that the pointer 5 capacitively influences three (b = 3) adjacent electrodes 9, i.e. the capacitances between these electrodes 9 and the counter electrodes 10 located between them. For this purpose, the pointer does not have to be made of metal; it is sufficient that its dielectric constant is different from that of air. In the example, it is further assumed that the pointer 5 influences the b electrodes 9 to the right and left of it equally, but influences them differently with increasing distance from it. In line 24, the capacitive influences on the electrodes 9 by the pointer 5 in its current position are illustrated by different gray levels.At the pointer position shown, of the n electrodes 9, the electrode with the consecutive number 4 is most strongly influenced, the electrodes to the right and left of it with the numbers 3 and 5 are less influenced, and all other electrodes are unaffected. The k capacitance values ​​measured in the measurement window 23 are made available to the k channels of the 1-out-of-k multiplexer 14 via the nxk connection matrix 13.

[0030] Fig. 4 The upper line 25 illustrates the four electrodes located in the measuring window 23 at the current position of the pointer 5. The numbers 1, 2, 3, 4 = k indicate the positions of the electrodes within the measuring window 23.

[0031] Block 26 shows all four (4 = k! / [(kb)!·b!]) possible combinations without repetition of the b = 3 electrodes influenced by the pointer 5 from the k = 4 electrode positions in the measuring window 23.

[0032] The six blocks 27 and 28 together show all twenty-four (24 = k! / (kb)!) possible variations of the b = 3 electrodes influenced by the pointer 5 from the k = 4 electrode positions in the measurement window 23. Since, as mentioned above, the b electrodes are influenced differently with increasing distance from the pointer 5 but are capacitively influenced equally to the right and left of it, repetitions occur, with the variations in the gray-shaded blocks 28 representing repetitions of the variations in blocks 27. For example, the influence pattern of the electrodes at positions 1, 2, 3 is identical to the influence pattern of the electrodes at positions 3, 2, 1.

[0033] There are therefore twelve (12 = k! / [2-(kb)!] = N) variations without repetition of the b = 3 electrodes influenced by the pointer 5 from the k = 4 electrode positions in the measuring window 23. It is therefore possible to place the measuring window N times next to each other, each offset by the amount of one electrode position, whereby different variations of the electrodes influenced by the pointer 5 are recorded in the different measuring windows 23. This makes it possible for the example shown to uniquely record any pointer position with the help of n = N-1+b, i.e. fourteen adjacent electrodes, and to output it to the k = 4 channels of the 1-out-of-k multiplexer 14 without any loss of information.

[0034] In the example in Fig. 3 are those from blocks 27, 28 of the Fig. 4 removable variations arranged as follows: 2 1 4 1 4 3 4 3 1 etc., For the reasons stated above, variations with the same influence pattern may only be used once. For example, in the example above, after using the variation "4 3 1," the variation "1 3 4" can no longer be used.

[0035] The result is the Fig. 3 shown arrangement of fourteen (n = 14) adjacent electrodes 9, which are connected by the nxk connection matrix 13 in the following order to the four (k = 4) channels of the 1-out-of-k multiplexer 14: 2 1 4 3 1 4 2 3 1 2 4 3 2 1 .

[0036] From the simple example described, the following conditions can be derived in order to achieve a high measurement resolution in the capacitive reading device 1 according to the invention with the largest possible number of electrodes 9: The width of the measuring window 23, ie the number k of electrodes 9 encompassed by the measuring window 23, is predetermined by the 1-out-of-k multiplexer 14 used in each case.

[0037] The width of the electrodes 9 in the circumferential direction of the circular arc is optimal if it allows as many variations as possible without repetition of the b electrodes influenced by the pointer 5 from the k electrode positions in the measuring window 23. This is the case when the pointer 5 influences as many of the electrodes 9 as possible within the measuring window 23 differently. The number of possible variations is reduced the more the more electrodes the pointer influences, either equally or not at all, i.e., in both cases, indistinguishably.

[0038] Since several to many electrodes are connected simultaneously to each channel of the 1-out-of-k multiplexer 14 via the nxk connection matrix 13, measurement interference can occur if the number b of electrodes simultaneously and measurably influenced by the pointer 5 is greater than the width k of the measurement window.

[0039] In the two cases mentioned above, the above-mentioned Fig. 4 explained sequence of variations, a distance rule must be introduced as a condition, according to which there must be a minimum distance e < k between all electrodes connected to one and the same channel of the 1-out-of-k multiplexer 14, within which there are e-1 further electrodes connected to other channels. Examples:

[0040] With b = 3 and k = 4, one obtains 30 variations for creating the connection pattern of the nxk connection matrix 13. With e = 4, the number of variations is reduced to 23 and one obtains the following connection pattern (n = 23-1+b = 25): 2 1 3 4 2 1 5 3 2 4 1 3 5 4 1 2 3 4 5 2 3 1 5 2 4 .

[0041] With b = 3, k = 8 and e = 7, 117 of the original 168 possible variations can be strung together, resulting in the following connection pattern (n = 117-1+b = 119): 2 1 3 4 5 6 7 2 1 8 3 4 6 5 2 1 7 3 4 8 5 2 6 1 3 7 4 5 2 8 1 3 6 4 5 7 2 8 3 1 4 6 7 5 2 3 8 4 6 1 5 2 7 3 8 6 1 4 2 5 3 7 6 1 8 4 5 3 2 6 7 1 4 5 8 2 3 6 7 4 1 5 8 3 6 2 4 7 1 8 5 6 3 4 2 7 1 5 6 8 4 2 3 1 5 7 8 6 4 2 1 5 3 8 7 4 6 2 1 .

[0042] The sequence of variations for creating the connection pattern of the electrodes 9 with the channels of the 1-out-of-k multiplexer 14 can be done by simple automatic numerical calculation.

Claims

1. Capacitive reading apparatus (1) for a pointing instrument (4), with a carrier (2, 8) provided for arrangement in relation to a rotating pointer (5) of the pointer instrument (4), on which carrier a large number of n radial electrodes (9) are arranged in a circular arc in alternation with radial counter electrodes (10), wherein the counter electrodes (10) are electrically connected with one another and are connected to a first measurement terminal (11) of a capacitance-to-digital converter (12), wherein the n electrodes (9) are connected via an n x k connection matrix (13) and a downstream 1-out-of-k multiplexer (14) to a second measurement terminal (15) of the capacitance digital converter (12), and wherein the n x k connection matrix (13) connects each of the n electrodes (9) with one of the k channels of the 1-out-of-k multiplexer (14) in each case, wherein the connecting pattern varies along the circular arc to form an ascertainment of a position of the pointer (5).

2. Capacitive reading apparatus (1) according to claim 1, characterised in that the connecting pattern varies along the circular arc with n variations of b out of k and n > k ≥ b > 1.

3. Capacitive reading apparatus (1) according to claim 2, characterised in that the n variations of b out of k are formed taking a sequence of the channels into account.

4. Capacitive reading apparatus (1) according to one of claims 1 to 3, characterised in that the capacitive reading apparatus (1) is designed to ascertain a strength of a capacitive influencing of at least one channel.

5. Capacitive reading apparatus (1) according to one of claims 1 to 4, characterised in that the electrodes (9) are arranged in a closed circle.

6. Capacitive reading apparatus (1) according to one of claims 1 to 5, characterised in that the 1-out-of-k multiplexer (14) is designed and controlled to successively select one of the k channels and connect the remaining k-1 channels to earth.