Apparatus for optically determining concentration of a fluid substance

Abstract

An apparatus for optically determining the concentration of a fluid substance is proposed, comprising a measurement amplifier for amplifying a measured photocurrent, wherein the measurement amplifier comprises at least one linear transimpedance amplifier for detecting and amplifying the photocurrent.

Description

Technical Field

[0001] This invention relates to an apparatus and method according to the claims in the parallel patents. Background Technology

[0002] In process industries, particularly biotechnology, optical measurements, such as infrared (IR) absorption or turbidity measurements, are used to ensure process quality and to control or regulate processes. These measurements are typically based on the attenuation of a light beam as it interacts with the fluid (absorption, scattering), and in some cases, on the detected increase in intensity (scattering, fluorescence, changes in polarization state, changes in propagation speed).

[0003] Publication DE 10 2013 111 696 A1 discloses a measuring device for measuring the flow velocity of fluid in a pipe. This measuring device particularly includes a measuring amplifier that detects and amplifies a measuring signal, such as a measuring voltage. However, these prior art measuring devices have many disadvantages, as described below.

[0004] The parameters being monitored can be, in particular, cell density in a bioreactor, color changes for identification, and / or separation of product or product phase in a pipeline, or ultraviolet absorption (single-wavelength or dual-wavelength) for controlling protein separation processes in biotechnology. Ideally, the measuring device should be universally applicable to these measurements.

[0005] The evaluation / visualization of measurement data and process control are increasingly being achieved through the use of, for example, programmable logic controllers combined with graphical displays for showing user interfaces. In corresponding, especially bidirectional, connections from the measuring device to the controller, the local operating interface on the measuring transmitter can often be omitted.

[0006] Measurement sensors, measurement amplifiers, and measurement transmitters with different implementations are known, such as the Optek immersion probes ASD12 or ASD25. In these examples, a NIR LED emits a light beam that passes through a window into a measurement gap filled with a fluid medium (pipe, biotechnology fermenter). The light passes through the medium and attenuates through absorption and / or scattering. The beam leaves the measurement volume and illuminates a silicon photodiode, where a photocurrent proportional to the light intensity is generated. The properties of the medium can be inferred from the attenuation of the light. For example, for cell or bacterial fermenters, there is a correlation between cell density and photocurrent attenuation.

[0007] For these applications, the typical scenario is that the intensity to be measured spans multiple orders of magnitude, and the incident light intensity is limited or should be limited. Therefore, it is advantageous to be able to detect the smallest possible photocurrent with high accuracy, linearity, and repeatability. For some sensors, the incident light intensity can be modulated over time, and in particular, it can be turned off.

[0008] If the measurement transmitter is to be universal, it is advantageous that it can also be used with sensors whose intensity radiated into the interaction region is approximately constant over time and which, in particular, cannot be specifically time-modulated. To achieve universal availability of the measurement amplifier, the response time should be as short as possible, with approximately 100 ms to 200 ms considered sufficient to identify phase boundaries of fluids or control valve units, for example, in chromatographic material separation, and this has been proven in practice.

[0009] The aforementioned measuring devices are intended for process control and monitoring. Therefore, they should provide at least quasi-continuous measurement signals in time, even if the measured variable varies over a wide range of parameters as described above. The device should be compact, robust, and cost-effectively manufactured even in medium-volume production. Furthermore, a wide operating temperature range is desired without compromising measurement accuracy.

[0010] For photometric measurements, such as those in spectrophotometers, where small photocurrents need to be detected, a system is typically employed where the measurement light is time-modulated with a high dynamic range (electrical or mechanical chopping or chopping) to separate slowly changing interfering parameters (e.g., using a lock-in amplifier for evaluation). In some such devices, a dual-beam scheme is also used, where the source intensity sometimes interacts with the measurement volume and is sometimes provided to the photodetector via a path unaffected by the measurement volume for normalization purposes.

[0011] In these solutions, the modulation frequency of the measured radiation (and the resulting photocurrent) must be high enough to follow changes in the measured parameters quickly enough, even considering the necessary signal processing. This principle requires additional overhead to modulate the light source, transmit and / or detect the frequency and phase of the modulation, and filter the signal (e.g., in the sense of phase-sensitive rectification).

[0012] However, omitting such measures increases the requirements for the magnitude and stability of interference parameters that would otherwise be uncorrectable. Important aspects of suitable circuitry for detecting photocurrent are discussed in US20220214212A1 (PTL 1). However, in the applications targeted by its mission objectives, continuous measurement is only necessary to a limited extent, and measurements are interrupted for the purpose of configuring and determining interference parameters.

[0013] In the solutions described in this literature, for example, slowly varying DC components should be corrected using reference measurements. However, this presupposes that the optical measurement radiation can be interrupted at appropriate times to detect the effects of interfering parameters and perform corrections during the measurement. The teachings of this literature apply only partially to use cases where the measurement radiation cannot be selectively interrupted or should not be interrupted, or where the measurement signal remains available even when the measurement parameters change drastically.

[0014] Measuring devices with periodically or intermittently interrupted radiation measurements are effective for correcting slowly changing interference parameters, but they do not offer an inherent solution for providing sufficient dynamic range. In this regard, photocurrent amplifiers are advantageous, as they convert the primary photocurrent into a nonlinear, typically logarithmic, characteristic curve (e.g., into voltage), which can then be measured, for example, by means of a common linear ADC (analog-to-digital converter). Such solutions are also derived in PTL1.

[0015] In absorption measurements, the photocurrent is typically correlated approximately with the concentration of the absorbing substance using the Lambert-Beer law. This results in a continuous linear range with respect to the substance concentration, which fully utilizes the resolution of the analog-to-digital converter. However, accurate logarithmic amplifiers are technically challenging, sometimes requiring temperature control, producing unpredictable rise times, and / or having input stages that interfere with the linearity of the connected photocurrent source (photodiode) due to input (leakage) current and input offset voltage. Existing technology is the linear transimpedance amplifier (TIA) for detecting photocurrent, where the characteristic curve is defined by a negative feedback resistor (transimpedance) that is constant for the range, and the photocurrent is proportionally converted to a voltage.

[0016] However, achieving a wide range actually presupposes range switching by influencing the transimpedance, as described in PTL1. However, discontinuous range switching interferes with signal filtering and makes it difficult to track dynamic signals fast and accurately enough. This is why PTL1 uses a scheme with a non-linear (e.g., logarithmic) characteristic curve.

[0017] Furthermore, power needs to be supplied to the switching elements. Semiconductor switches, as proposed as a possible solution in PTL1, are low-wear and fast; however, they inject difficult-to-compensate interfering charges that are indistinguishable from the photocurrent and disadvantageously limit the achievable range to lower current directions. The photocurrent, converted to voltage, can be converted to a digital value using known analog-to-digital converters. This appears advantageous even when using analog interfaces (e.g., 4-20 mA current interfaces) to transmit measurements to an upper-level controller.

[0018] According to existing technology, 24-bit Σ-Δ converters are available as integrated components with relatively low power consumption and good cost-effectiveness. They also offer highly efficient internal filters relative to interference at common power supply frequencies (50 or 60 Hz). However, such low-cost converters with good filtering and low power consumption reduce data rates and increase latency to such an extent that achieving signal rise times of 100 ms–200 ms becomes difficult. Consequently, range switching is also delayed in time, leading to problems when tracking time-varying signals for visualization, control, or conditioning.

[0019] For example, delayed switching may cause the measurement signal to exceed the drive limit in the measurement chain, thus temporarily providing only strongly and nonlinearly distorted measurements, which may interfere with the filtered values ​​in the long term. Summary of the Invention

[0020] Therefore, the objective of this invention is to overcome the shortcomings of the prior art, and in particular to provide an improved apparatus or an improved method for optically determining the concentration of fluid substances.

[0021] This task is solved by the subject matter described in the independent claims. Advantageous extensions of the invention are described in the dependent claims. The scope of the invention also includes all combinations of at least two features described in the specification, claims, and / or drawings. Values ​​within the specified range, especially those falling within the limits, are also disclosed as limiting values ​​and can be claimed in any combination.

[0022] The present invention relates to an apparatus for optically determining the concentration of a fluid substance, comprising: a measuring amplifier for amplifying the measured signal, particularly photocurrent, wherein the measuring amplifier includes at least one linear transimpedance amplifier for detecting and amplifying the signal, particularly photocurrent.

[0023] In this disclosure, the characteristic "matter concentration" includes not only matter concentration in the strict sense, but also matter concentration in a particular electronic and / or spatial configuration, as well as the relative ratio of such concentrations or the amount of spillover from such configuration.

[0024] The present invention further relates to a method for optically determining the concentration of a fluid substance, wherein a measuring amplifier amplifies the measured signal, particularly photocurrent, wherein the measuring amplifier includes at least one linear transimpedance amplifier for detecting and amplifying the signal, particularly the photocurrent.

[0025] According to the invention, it is particularly advantageous to provide continuous measurement data with no jumps and low time delays. Furthermore, it is particularly advantageous to eliminate the need for a logarithmic amplifier.

[0026] All features described below with respect to the device according to the invention also apply to the method according to the invention, and vice versa.

[0027] In particular, it also provides range switching and effective filtering.

[0028] In a preferred embodiment, the device further includes: The light source of the emission source spectrum A wavelength selection device positioned in front of the measuring volume. At least the measurement space that defines the measurement volume in the optical path, and A detector used to measure the wavelength-dependent absorption of the measurement spectrum passing through the measurement volume.

[0029] In another preferred embodiment, the device further includes: The light source of the emission source spectrum A wavelength selection device positioned in front of the measuring volume. At least the measurement space that defines the measurement volume in the optical path, and A detector that measures the light scattered in a measurement volume.

[0030] The term "light" or "optical radiation" is not intended to be limited to the visible light range, but rather refers to one or more sub-ranges of the spectrum of radiation from the ultraviolet to the infrared. In addition to photodiodes (semiconductor diodes, such as those based on silicon or germanium) commonly used in measurement techniques, any detector that predictably converts light into photocurrent can be used to convert light intensity into photocurrent. This invention allows for the handling of both positive and negative currents at the active measurement input, thereby enabling the connection of both the cathode and anode in the photodiode to the active measurement input.

[0031] In a preferred embodiment, the measuring amplifier is configured to continuously detect the photocurrent and provide it to a higher-level controller. This particularly advantageously enables continuous, uninterrupted, and seamless monitoring, which allows for the control and / or adjustment of process-related parameters, real-time control of the separation process, and seamless process archiving for quality assurance purposes.

[0032] In another preferred embodiment, it is specified that the delay of the measuring amplifier is <500 ms. This low delay is particularly advantageous in enabling better and more stable process parameter tuning, and is advantageous in controlling the separation process, for example by switching fluid valves when phase boundaries are identified.

[0033] In another preferred embodiment, it is specified that the range of the measurement amplifier is <4 orders of magnitude. By limiting the dynamics of the range between switching points, it is particularly advantageous to leave sufficient margin to ensure adequate resolution at the lower limit of the range and sufficient, still measurable signal overflow at the upper limit of the range, so that even with range switching, the dynamically changing signal can be tracked at the switching point without significant information loss.

[0034] In another preferred embodiment, the measuring amplifier is specified to include a range switch. This is particularly advantageous in enabling a particularly wide total range of dynamic measurements without the need for a logarithmic amplifier.

[0035] It is also preferred that the range switching includes a reed relay. This provides a particularly advantageous way to achieve a range switching suitable for detecting unmodulated photocurrents, as it consistently ensures high switching dynamics (high isolation from interfering currents and low on-resistance).

[0036] In another preferred embodiment, the measuring amplifier is specified to include input protection via a diode connected to the input potential of the measuring amplifier. Amplifier inputs with low leakage current (typically based on insulated field-effect transistors) are sensitive to electrostatic discharge. This advantageously enables reliable operation, where the potential difference is kept as low as possible by the protection diode to minimize leakage current.

[0037] The input stage of a transimpedance amplifier, with its low interference parameters, contains sensitive components that can be damaged by even minor loads (such as electrostatic discharge / ESD). Therefore, reliable operation depends on appropriate protection circuitry. Protecting the amplifier input with semiconductor diodes, by directly shunting voltage peaks to the amplifier's positive or negative supply voltage, can result in unacceptable, temperature-dependent interference currents through the diodes due to the high voltage applied to them.

[0038] One solution involves connecting two reverse-biased diodes in parallel to a voltage very close to their respective input voltages. Alternatively, a solution can be used that combines a protection diode, a voltage follower, and a shunt diode in a single package or integrated circuit.

[0039] In another preferred embodiment, a metal housing is provided that has a low-impedance connection to the reference input of the measurement amplifier. This advantageously achieves low sensitivity relative to electric field interference that should be expected in the technical environment.

[0040] In another preferred embodiment, it is specified that the photocurrent input terminal of the measurement amplifier is electrically isolated from the power supply unit and / or communication unit. Electrical isolation advantageously minimizes interference currents in sensitive parts of the device or measurement amplifier, and therefore also minimizes interference voltages caused by these currents across the inductors and conductive path resistances. This particularly reduces interference effects on the reference potential (e.g., the analog-to-digital converter relative to the non-inverting input of the transimpedance amplifier).

[0041] In another preferred embodiment, the measurement amplifier is specified to have >100 dB suppression of 50 / 60 Hz interference. This high suppression of 50 / 60 Hz interference is particularly advantageous in ensuring that the measurement remains unaffected even when measuring small photocurrents.

[0042] In another preferred embodiment, the device or method is configured to detect unmodulated radiation. Its advantage lies particularly in its ability to evaluate signals from measurement sensors that do not provide light source modulation or use light sources that are difficult to modulate. Attached Figure Description

[0043] Other advantages, features, and details of the invention will become apparent from the following description of preferred embodiments and from the accompanying drawings, which are illustrated schematically, respectively: Figure 1 shows the schematic circuit diagram of the measurement amplifier of the device according to the present invention. Figure 2 shows a measurement amplifier circuit board for a measurement amplifier used in a device according to the present invention, wherein the left side is a top view, the middle side view, and the right side is a bottom view. Figure 3 shows a graph in which the relative coefficient parameter b depends on the coefficient index a. Figure 4 shows the passband curve (Durchlasskurve). Figure 5 shows the housing of the measurement amplifier of the device according to the invention, and Figure 6 illustrates an exemplary embodiment of the device according to the present invention. Detailed Implementation

[0044] In the accompanying drawings, the advantages and features of the invention according to embodiments of the invention are indicated by reference numerals, wherein components or features that have the same function or effect are indicated by the same reference numerals.

[0045] According to the present invention, photocurrent detection is performed using a linear transimpedance amplifier (TIA) as shown in FIG1. ​​The photocurrent is fed to the inverting input terminal 101 of operational amplifier 102, and there is a negative feedback from the output terminal to the inverting input terminal 101, consisting of at least resistor 103, as transimpedance. Operational amplifier 102 and resistor 103 constitute a linear transimpedance amplifier.

[0046] To achieve multiple measurement ranges, one or more resistors 104 and 105 can be connected in parallel with resistor 103 for switching. Here, the characteristic curve (measuring the amplifier output voltage as a function of the input current) is designed to be approximately linear, thus eliminating the need for nonlinear, especially exponential or logarithmic, negative feedback.

[0047] In order to measure the smallest possible photocurrent, it is necessary to minimize and / or compensate for the leakage current.

[0048] Reed relays 106 and 107 can be advantageously used as switching elements. They achieve dynamics that cannot be achieved using semiconductor switches (high isolation resistance in the non-operated state and extremely low residual resistance in the on state). Furthermore, the isolation between the control circuit and the switched circuit is superior to that achievable using semiconductor switches.

[0049] The theoretically achievable finite number of switching cycles does not constitute a lifespan limitation in most practical applications when the time behavior of the measurement parameters is taken into account, especially when the range switching design has sufficient hysteresis (e.g., at least 10% of the more sensitive range, preferably 30%, and most preferably more than 40%).

[0050] Switching elements require space, are expensive, and are potential sources of interference current. Their number should therefore be minimized, particularly within the influence range of the inverting input of the TIA. To further reduce interference current, the actuator coil is preferably connected at connection 108 to a potential very close to the potential of the non-inverting connection 109 of the transimpedance amplifier, and especially to its low impedance. In the uncontrolled state, the entire actuator coil is at this potential, which is very close to the potential of the inverting input in the steady state of the transimpedance amplifier. Therefore, there is no significant voltage driving unwanted leakage current between the actuator coil and the photocurrent input.

[0051] A less sensitive range can be achieved by connecting a resistor in parallel with a negative feedback resistor for the most sensitive range. Switching elements in the path of the highest negative feedback resistor should be omitted. Therefore, all switching elements are inactive within the most sensitive range, and the effects of switching element operation can be minimized. When a relay is used as the switch, all actuator coils are therefore at a completely non-critical voltage within the most sensitive range. In a preferred embodiment, the transimpedance amplifier is equipped with three ranges (MB4, MB3, MB2).

[0052] Operational amplifier 102 uses an integrated module with low input offset voltage, low input current, and FET input transistors. The desired performance level is achieved without a discrete structure. The effective transimpedance within the corresponding range is approximately 100 megohms 103 (most sensitive range, MB4), and can be paralleled with a 560 kilohm 104 (MB3) and an additional 3000 ohms 105 (MB2). The TIA has a drive range of approximately ±5 V. Switching to another range occurs, if present, at values ​​above 4.75 V or below 0.0125 V.

[0053] This ensures a sufficient distance between the switching level and either the upper drive limit (signal limitation) or the lower drive limit (insufficient resolution) to track changing signals. It also introduces significant switching hysteresis, preventing frequent range switching under real-world process conditions. To limit the bandwidth of the transimpedance amplifier and reduce unwanted oscillation tendency, it is appropriate to connect capacitors 110, 111, and 112 in parallel with the negative feedback resistor. These capacitors 110, 111, and 112 must be adapted to the characteristics of the amplifier used, particularly the input capacitor at the inverting input terminal 101.

[0054] If a high input capacitance is expected, a larger negative feedback capacitor must be selected. If the cable length used to connect the photodiode to the transimpedance amplifier is variable, a large negative feedback capacitor must be selected for stability reasons. Using a short cable with low capacitance can lead to significant variations in signal bandwidth.

[0055] A drastic change in signal bandwidth can be offset by intentionally increasing the effective input capacitance through parallel capacitor connections. While this reduces the maximum achievable bandwidth, it results in a significant reduction in the bandwidth dependence on cable length (or other sources of input capacitance), which simplifies further signal processing, such as frequency filtering.

[0056] In a preferred embodiment: 33 pF for MB4, additionally 220 nF for MB3, and additionally 1 nF for MB2. These capacitors allow operation with an input capacitance of 50 nF, which reduces the further impact of external cable capacitance, which is at most 30 m at 200 pF / m.

[0057] The input stage of a transimpedance amplifier with low interference parameters contains sensitive components that can be damaged even by minor loads (such as electrostatic discharge / ESD). Therefore, reliable operation depends on appropriate protection circuitry. Protecting the amplifier input with semiconductor diodes, by directly shunting voltage peaks to the amplifier's positive or negative supply voltage, can result in unacceptable, temperature-dependent interference currents flowing through these diodes due to the high voltage applied to them.

[0058] An alternative solution is to connect two reverse-biased diodes in parallel to a voltage very close to their respective input voltages. Alternatively, a solution can be used that combines protection diode 114, voltage follower 115, and shunt diodes 116 and 117 in a single package or integrated circuit.

[0059] However, the voltage follower introduces an additional voltage offset across the protection diode due to its input offset voltage (and thus generates additional interference current). This can be avoided by connecting the protection diode directly to the non-inverting input 109 with a low impedance via resistor 118 or a wire bridge. In this case, the voltage follower is both decoupled and protected against overload via a resistor (which may already be integrated). Voltage follower 115 and resistor 119 can also be omitted if the integration scheme allows.

[0060] In the region of the transimpedance amplifier and its input leads 109, 113, and 101, the measurement amplifier is sensitive to the effects of electric fields and interference currents. This is particularly true for the circuit section connected with a low impedance to the inverting input of the transimpedance amplifier (the active measurement input—connected in the preferred embodiment to the cathode of photodiode 120, which in the preferred embodiment is not part of the measurement amplifier but part of an external measurement sensor—to the reed relay, the feedback element, especially the feedback element of the most sensitive range).

[0061] The electric field component can be minimized by electrical shielding 121 with low impedance connected to the non-inverting input 109. In a preferred embodiment, a shielded enclosure made of a metal plate is used to protect sensitive circuit components (particularly the transimpedance amplifier, which includes feedback components, switching elements, and ESD protection circuitry). Some mechanical characteristics of the preferred embodiment can be read from Figure 2. The shielded enclosure with the transimpedance amplifier 201 is soldered to the circuit board during circuit board assembly, where a hermetically sealed connection with the circuit board is emphasized. Furthermore, the circuit board itself is designed to be hermetically sealed within the shielded enclosure area (e.g., no open vias).

[0062] The interior of the shielding housing is accessible via a pluggable cover 202 made of a metal plate. This allows the shielding housing to be used as a demolding material for the insulating potting compound. This potting compound protects the sensitive areas of the transimpedance amplifier from contamination and moisture intrusion. The circuit board is slightly larger than the shielding housing and has serrated contact surfaces at its edges for mechanical and electrical contact. It forms a module together with the shielding housing and potting compound, which is then soldered onto the main circuit board 203.

[0063] Sensitive areas, both inside and outside the potting compound, are carefully protected, particularly from circuit sections that may present different potentials. Conductive structures 121 are used here as shielding (e.g., wires), which are either directly connected to the non-inverting input 109 of the TIA or to its reference voltage. They almost completely, or at least largely, surround the wires of the sensitive circuit components. To ensure effectiveness, these metal structures have exposed surfaces (uncoated).

[0064] To minimize leakage current within the circuit board, vias are introduced into the shielded conductors. Their primary function is to absorb leakage current; that is, they are not necessary for guiding operating current. Therefore, even under conditions of increased temperature and humidity, leakage current can be controlled to levels that, in the preferred embodiment, it is not necessary to use circuit board substrates other than glass fiber reinforced epoxy resin, and in particular, fluoroplastics are not required as substrates.

[0065] Therefore, in a preferred embodiment, the uncompensated interference current of the measuring transmitter can be reduced to less than 500 fA over a wide temperature and humidity range. Due to the small photocurrent being measured, dielectric absorption effects can cause significant interference, particularly in the photocurrent circuitry (capacitance between the anode and cathode leads of the photodiode, or between the inverting and non-inverting inputs of the TIA, and capacitance in the feedback path of the TIA), especially within the sensitive measurement range. In a preferred embodiment, capacitors with plastic dielectrics are used in these locations, particularly those with dielectrics composed of PP, PET, or PPS, with SMD capacitors preferably being used. To optimize noise behavior, signal amplification has already been performed in the transimpedance amplifier, eliminating the need for further voltage amplification before or during the analog-to-digital converter (ADC) 123.

[0066] The input offset voltage of the transimpedance amplifier manifests as an offset voltage at its output. Where the drive range of the transimpedance amplifier allows, it is meaningful to select a voltage range higher than that allowed by subsequent circuitry (e.g., the input dynamic range of the ADC) and reduce it before its input by a voltage divider 122, as the interfering input offset voltage is reduced by the voltage division ratio. In a preferred embodiment, the output voltage of the TIA is nominally halved. This voltage divider can then be easily extended into an RC circuit by a capacitor (not shown in Figure 1) connected in parallel with the differential measurement input of the ADC, which limits the signal to a reasonable bandwidth before analog-to-digital conversion. This reduces the risk of the analog-to-digital converter entering overload range due to interference. It also reduces interference components due to undersampling. In a preferred embodiment, the time constant is approximately 3.3 ms.

[0067] Depending on the polarity of the photocurrent (e.g., whether the anode or cathode of the photodiode is connected to the inverting input), the TIA produces a negative or positive output voltage. If, for example, for shielding reasons (e.g., the photodiode's case is connected to the cathode), the anode is connected to the inverting input of the transimpedance amplifier, a disadvantage arises with respect to the ADC, because the voltage to be detected is negative relative to the reference potential. Mirroring the measurement signal to the reference voltage or offsetting it by a constant introduces additional interference parameters (e.g., the operational amplifier's input offset voltage).

[0068] Therefore, in the preferred embodiment, an ADC capable of detecting both positive and negative input voltages (nominally between -5V and +5V relative to reference point 109) is used, although the available range actually utilizes only positive voltages. This reduces the number of components. The resulting reduction in usable analog-to-digital converter resolution from 24 bits to 23 bits is at least partially compensated for by the reduced effect of additional interference parameters. Interference parameters (e.g., leakage current and input offset of the transimpedance amplifier, but also electromagnetic interference whose average value decays over time) therefore do not result in small signals falling below the detection range of the analog-to-digital converter and thus no longer being reliably detected. Therefore, the inclusion of a compensation current in PTL1 to ensure a fixed polarity of the input signal can be omitted.

[0069] Furthermore, this also leads to a fundamentally symmetrical design of the power supply voltages for both the TIA and ADC relative to the reference signal. This simplifies the filtering of the power supply voltage 124, which can be shared by both the TIA and ADC, and reduces the residual influence of the power supply voltage on the measurement results. As schematically shown in Figure 1, the analog-to-digital converter 123 can be connected to the microcontroller 126, for example, via a serial SPI bus.

[0070] In a preferred embodiment of the invention, signal sampling is performed at a modulator frequency of 256 kHz for the Σ-Δ analog-to-digital converter. From a plurality of filter functions implemented in the converter, a combination is selected that processes data with low latency (the transient response of the filter chain occurs within a single filter / decimation cycle; in continuous operation, the signal delay corresponds to the time interval between two data points at the output) and a relatively high output data rate of approximately 400 data points per second, wherein the data width is 24 bits.

[0071] Eliminating further decimation and / or filtering in the analog-to-digital converter, particularly the use of sinc³ filters, ensures a continuous data stream with relatively low latency. Sufficient computing power, programmable, and data memory can be provided at low cost by using common microcontrollers based on existing technology (such as ARM-Cortex-M4 derivatives) to compute even relatively high-order digital filters.

[0072] In a preferred embodiment, a 60th-order FIR filter (61 symmetric real positive coefficients for standard form 1, 16-bit coefficient resolution) is used. The relative distribution of the coefficients can be obtained from Figure 3; the normalized deviation from 1 can be considered at other points in the calculation.

[0073] This enables very high suppression over the range of interference caused by the power grid (50 Hz and / or 60 Hz) and its resulting harmonics (e.g., integer multiples of the fundamental frequency generated by rectifying the grid voltage, where the interference amplitude decreases sharply with increasing frequency). Here, the delay is kept within the limits acceptable for the aforementioned applications: it is recognized that the effect of the input signal on the filter output due to the principle can be delayed by up to 61 / (400 / s), or approximately 150 ms. At least 50% of the transition is reflected at the output after 75 ms. Figure 4 shows the calculated effect of the filter.

[0074] Using the selected filter, combined with the transfer function of the analog signal chain and the transfer function of the analog-to-digital converter in the selected operating mode, the fundamental frequency of interference at 50 Hz can theoretically be suppressed by more than 160 dB (20 log(input / output)); interference of full input amplitude is attenuated to such an extent that the nominal amplitude at the filter output will be less than 1 bit (at 24-bit resolution). This high level of suppression is available up to 350 Hz and above. This means that very high suppression of up to 7 times the fundamental frequency (50 Hz) or up to 6 times the fundamental frequency (60 Hz) can be achieved.

[0075] The transimpedance amplifier is switched to achieve a wide dynamic range. Therefore, the scaling of the ADC output signal changes at the switching limits. In a preferred embodiment, necessary normalization is taken into account at the output of the digital filter. At the switching moment, all existing ADC values ​​in the filter are converted to the new scaling factor to prevent unwanted transient responses in the filter.

[0076] After switching between the two ranges, the transimpedance amplifier exhibits transient behavior. In a preferred embodiment, this behavior is slowest when switching to the most sensitive range. Here, it is helpful to choose a relatively high bandwidth that is still within the stable operating range of the TIA in order to achieve rapid stabilization. In a preferred embodiment, fewer than 10 output values ​​are affected by the switching.

[0077] Interference can be minimized by replacing the affected analog-to-digital converter (ADC) value with an evaluated value after switching. In a preferred embodiment, the last output value of the digital filter (considering proper scaling) is used as the replacement value. Depending on the expected time-varying behavior of the input parameters, this value represents a better replacement estimate than, for example, the last available ADC value, because short-term interference effects have been filtered out. Under practical conditions, the selected device can measure currents up to 50 pA with a noise component of 0.5 pA. At the most sensitive range and an ambient temperature of 65°C, the uncorrected interference parameter is less than 500 fA with respect to the input. Currents up to 1.5 mA and above can be measured.

[0078] The measurement ranges are individually characterized at the factory based on the slope (effective transimpedance) and axial intercept (TIA input offset and leakage current), and the measurements are calibrated and normalized accordingly before filtering. No recharacterization is required during the equipment's lifespan. Residual jumps occur at the switching points between ranges, which are negligible compared to the filtered noise components.

[0079] The measuring transmitter typically also provides power to the light source in the form of electricity. In a preferred embodiment, a nearly constant voltage is provided, which is switchable, particularly by a microcontroller. The current consumed by the light source is monitored, specifically measured in a time-resolved manner. In the event of a functional failure (e.g., unacceptable high current consumption, source disconnection), the power supply voltage can be shut off by the microcontroller. Furthermore, it is possible to detect the time-modulated current consumption by the light source controller and thus transmit information (e.g., regarding the operating status of the light source, operating duration, or environmental conditions) to the measuring amplifier. In the event of a failure, the power supply voltage can be periodically switched on to check if the fault condition persists and to restore normal operation if it has been eliminated.

[0080] To communicate with the upper-level control unit, the measurement transmitter has an interface; in a preferred embodiment, it has an electrical differential interface, particularly a 2-wire RS485 interface with an additional reference potential lead, through which data is transmitted bidirectionally in a manner compatible with conventional MODBUS installations (MODBUS RTU, data rates of 9600, 19200, and 38400 bits / second). The measurement transmitter can be identified (device type, individual identifier / serial number) and configured (bus address, transmission parameters) via bidirectional digital communication. The user can also set a starting value, outputting measured values ​​calculated relative to that starting value. These measured values ​​can be provided either calculated with or without the starting value, including a logarithmic ratio value relative to the starting value.

[0081] The calculated measurements are updated with a cycle time of approximately 100 ms. Response time is typically less than 150 ms, and less than 200 ms at switching points. In a preferred embodiment, the local user interface of the measurement amplifier is limited to a few LEDs used to signal power supply voltage and operational readiness, receive digital data, and indicate identified fault conditions. These LEDs are visible through light-transmitting openings in the front panel 502. A display for showing variable symbols, text, or numbers is omitted, and the entire configuration and functionality are accessible via the digital interface. Furthermore, a reset button is provided for restarting and restoring factory settings.

[0082] In a preferred embodiment, the measurement amplifier is used to operate a sensor such as the Optek ASD 12, where the LED light source and photocurrent sensor are spatially adjacent to each other within a metal housing and are subject to similar interference (EMV) effects. Furthermore, the LED power supply and photocurrent are routed through a common cable. Therefore, electrical isolation between the voltage supply to the light source and the photocurrent measurement circuitry is eliminated. However, electrical isolation is provided between the photocurrent measurement input and the power supply voltage, between the photocurrent input and the digital communication interface (RS485), and between the digital external communication interface and the power supply, wherein at least a functional isolation of a nominal voltage of 250 V AC is provided.

[0083] To improve immunity to interference, particularly surge voltage, the coupling between potential islands is designed with low capacitance, especially parasitic capacitance <5 nF. The measurement amplifier has multiple connection options for the PE (protective earth) conductor to achieve connection to local equipotential. This connection can be used to locally connect the possible shielding layer of the RS485 cable to the protective earth and to achieve a high-impedance connection from the photocurrent circuit to ground potential. Ground potential maintains the positive effect of potential isolation but allows the discharge of static charges generated by friction, which could introduce interfering displacement currents or sporadic discharges with a wide interference spectrum in the measurement circuit.

[0084] To facilitate installation of the measuring transmitter, in a preferred embodiment, it is equipped with a narrow plastic housing (Figure 5), which allows mounting on a standard top-cap rail via spring clips 501. It has a main circuit board 203 (Figure 2) positioned perpendicular to the mounting direction of the top-cap rail and parallel to the main extension direction of the housing. Other circuit boards 204 are mounted in parallel on the main circuit board, carrying a transimpedance amplifier and its shielded housing, switching elements, and their feedback elements. For access to external circuit components, the housing has four terminal blocks 205, each with three or four screw terminals.

[0085] The terminal blocks are perpendicular to the main circuit board in their longitudinal direction (the direction in which the terminal blocks are arranged), and the terminal blocks contain contact elements that are directly soldered to the circuit board. The terminal blocks for the active measurement inputs (anode branch in a preferred embodiment) of the photocurrent circuit are placed between the passive measurement inputs (cathode branch in a preferred embodiment) and shielded connection terminals arranged at nearly the same potential. This arrangement of the terminal blocks and signals allows for a large air gap, or creepage distance, around the active measurement inputs. Where these gaps / distances are small, at least the potential difference between the conductive components and the active measurement inputs is ensured to be small. The active measurement inputs are thus well shielded against interference currents. A nominal 24 V ± 20% DC voltage and approximately 5 watts of power are supplied.

[0086] In the first preferred embodiment described above, the measuring transmitter is used to power a single-channel absorption system, such as an Optek absorption probe ASD12 or ASD25.

[0087] In a second exemplary embodiment, the measuring transmitter is used to operate a dual-channel absorption sensor. The second embodiment is very similar to the first embodiment. However, in this case, the connected light source provides radiation at two different wavelengths, particularly in the ultraviolet range, wherein the radiation for both wavelengths is substantially constant in the on-state. The measuring sensor provides photocurrent signals for both wavelengths via photodiodes.

[0088] The measuring transmitter is correspondingly equipped with two photocurrent inputs, each fitted with a TIA and an ADC, which transmit digital signals to a common microcontroller. Because the photodiodes are spatially adjacent to each other in this arrangement and are therefore subject to the same interference, there is no need to electrically isolate the photocurrent circuits from each other; the two TIAs can operate at a common reference potential and can use a shared power supply voltage for both TIAs and both ADCs.

[0089] The two TIAs are each housed in their respective shielded enclosures. The shielding electrodes and shielding connections can be shared by the two photocurrent circuits. The data transmitted to the higher-level controller on the digital bus is expanded compared to the first preferred embodiment; starting values ​​can be detected jointly or individually for both photocurrent inputs via the bus according to commands, and both raw data and data calculated based on the starting values ​​can be provided for both inputs, either in current proportional units or absorption units (logarithmic scale). The difference between the measured and absorbed values, proportional to the quotient of the two currents, is also provided for both photocurrent inputs. The width of the enclosure used is twice that of the first preferred embodiment; in other respects, the statements made for the first embodiment also reasonably apply to the second embodiment.

[0090] Further configurations or embodiments can be derived from the foregoing solutions. In some cases, it is advantageous to integrate the measurement transmitter with the measurement sensor (or a portion thereof) into a common housing (the transmitter). In such cases, particularly for biotechnology applications, it is often meaningful to design the housing to be hermetically sealed, easy to clean, and steam-sterilizable.

[0091] In addition to one or two photocurrent inputs, more photocurrent inputs can be implemented, which can be either electrically isolated from each other or arranged in one or more non-isolated groups, depending on the measurement sensor. For example, this arrangement also allows for the detection of incident intensity for, for example, normalization purposes, when the intensity emitted by the radiation source varies sharply over time.

[0092] To switch the range of the TIA, in addition to the preferred reed relay, other forms of electromagnetic relays, including micromechanical relays, can be used to reduce requirements. If the requirements for switching dynamics or interference current are low, for example, because the light source is time-modulated and can be essentially turned off, then using a semiconductor switch may be the preferred solution. This allows for the assessment and correction of slowly changing interference parameters when the light source is off. Identification regarding range switching and transient behavior is then applied accordingly to this operating condition.

[0093] Containing the transimpedance amplifier (TIA) in a separate enclosure represents a flexible, modular solution. However, if the measurement inputs are not electrically isolated from each other, it may also be worthwhile to house multiple TIAs in a shielded enclosure. The number of ranges and range limits can vary. They can also differ in this respect when multiple measurement inputs are configured. Regarding the power supply to the light source, it can be omitted or configured for multiple power supplies and monitoring of multiple light sources. The use of the aforementioned digital filters represents a good solution, especially under adverse EMV conditions or process conditions that change rapidly over time. Lower-order filters with less attenuation at critical interference frequencies can also be used if requirements are lower. When requirements are very low, ADC operating modes that produce higher delays or lower data rates can also be selected.

[0094] In some applications, it may be meaningful to provide discrete digital inputs and / or outputs in addition to the digital interface, for fast or local applications, to indicate the starting value or the arrival of a limit value. These outputs can then be designed to be fully or partially electrically isolated from other connections. The measurement transmitter can also generate further measurement functions from the detected photocurrent signal; for example, conversion into measurement parameters such as transmittance, turbidity expressed in turbidity units (FTU, NTU, PPM), chromaticity units such as Hazen or ICUMSA.

[0095] In situations with high microcontroller loads (e.g., due to the use of complex digital filters to implement multiple photocurrent inputs), it may be meaningful to construct multiple subsystems of the aforementioned invention, each equipped with its own microcontroller. Logically, data is aggregated and provided to the higher-level controller via digital connections of known technologies (serial transmission via UART, SPI, IIC, CAN bus, or similar parallel digital bus connections). Spring terminals or sockets for accommodating spring or screw terminals, or plug connectors, can also replace screw terminals. Communication with the higher-level controller can also be performed via alternative digital communication systems.

[0096] Suitable physical transmission methods include wired transmission (RS485, Ethernet-type physical layer, APL, T1-L), but may also include optical transmission via fiber optics or wireless connections (Wi-Fi, mobile networks). Protocols that can be used include, for example, TCP / IP, Profinet, EtherNet / IP, OPC UA, and MQTT.

[0097] If data is available to the microcontroller at a relatively high data rate and low latency, as described above, this provides expanded possibilities for predictive measurement evaluation and range switching. Additional filters and algorithms can be connected in parallel with the actual filters used for measurement generation, where these additional filters and algorithms evaluate the signal curve, for example, with lower latency in relation to the first-order or even higher-order time derivatives. Therefore, better decisions can be made regarding range switching, and precisely matched alternative values ​​can be estimated for measurements affected during transient phases.

[0098] Given the available computing power, using a high-order FIR filter is an easily manageable solution. It can be replaced by a functionally similar IIR filter. In the context of this invention, the microcontroller can also be replaced by a microprocessor with external memory or peripheral components, or by a controller implementation within an FPGA.

[0099] Figure 6 schematically illustrates a device 600 according to the invention. Device 600 includes a measurement sensor 601 with a measurement gap 602 and a media seal (O-ring) 603. The measurement sensor 601 is connected via a cable 604 to a connection terminal 605 for power supply and a connection terminal 606 for the photocurrent input terminals of measurement amplifiers 607 (viewed from below) and 608 (viewed from above). Measurement amplifiers 607 and 608 have a connection terminal 609 for communication with a controller and a connection terminal 610 for power supply and PE.

[0100] The basic operating principle of this device is known to those skilled in the art. For details regarding the specific operation of measurement amplifiers 607 and 608, please refer to the above description.

[0101] The above embodiments are merely illustrative of the present invention and do not limit the basic inventive concept in any way.

[0102] List of reference numerals 101 Inverting Input Terminal 103 resistor 102 operational amplifier 104, 105 resistors 106, 107 Reed Relay 108 connector 109 In-phase connection terminal 110, 111, 112 capacitors 113 capacitor 114 protection diode 115 voltage follower 116, 117 shunt diodes 118, 119 resistors 120 photodiode 121 Shielding Structure 122 voltage divider 123 ADC 123 124 filter power supply 125 digital bus 126 microcontroller 201 with TIA shielding enclosure 202 cover 203 main circuit board 204 module circuit board 205 terminal block a coefficient index b Relative coefficient parameter 501 retainer, clip 502 Front Panel 600 equipment 601 Measurement Value Sensor 602 Measurement Gap 603 Media Seal (O-ring) 604 Cable with plug connector 605 is used for power supply connection terminals. 606 Connection terminal for photocurrent input 607 Measurement Amplifier (viewed from below at an angle) 608 Measurement Amplifier (viewed from an oblique top) 609 is a connection terminal for communication with the controller. 610 is used for the connection between the power supply and the PE.

Claims

1. An apparatus (600) for optically determining the concentration of a fluid substance, comprising a measuring amplifier (607, 608) for amplifying the measured photocurrent, characterized in that, The measurement amplifiers (607, 608) include at least one linear transimpedance amplifier (102, 103) for detecting and amplifying the photocurrent.

2. The device according to claim 1, wherein the measurement amplifier (607, 608) is configured to continuously detect the photocurrent and provide it to a higher-level controller.

3. The device according to at least one of the preceding claims, wherein the delay of the measurement amplifier (607, 608) is < 500 ms.

4. The device according to at least one of the preceding claims, wherein the range of the measuring amplifier (607, 608) is less than 4 orders of magnitude.

5. The device according to at least one of the preceding claims, wherein the measurement amplifier (607, 608) includes a range switch.

6. The device according to claim 5, wherein the range switching comprises a reed relay (106, 107).

7. The device according to any one of the preceding claims, wherein the measurement amplifier (607, 608) includes input protection via a diode connected to the input potential of the measurement amplifier (607, 608).

8. The device according to at least one of the preceding claims, the device comprising a metal housing having a low-impedance connection to a reference input of the measurement amplifier (607, 608).

9. The device according to at least one of the preceding claims, the device comprising electrical isolation between the photocurrent input terminal of the measurement amplifier (607, 608) and the power supply unit and / or communication unit.

10. The device according to at least one of the preceding claims, wherein the measurement amplifier (607, 608) has a suppression of >100 dB for interference at 50 / 60 Hz.

11. The device according to at least one of the preceding claims, wherein the device is configured to detect unmodulated radiation.

12. A method for optically determining the concentration of a fluid substance, particularly using the apparatus according to at least one of the preceding claims, wherein a measuring amplifier (607, 608) amplifies the measured photocurrent, characterized in that... The measurement amplifiers (607, 608) include at least one linear transimpedance amplifier for detecting and amplifying the photocurrent.

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