Sensor interface and commodity monitoring apparatus

Abstract

A sensor interface 10 comprising a processing unit 12, at least one input 14 and two or more outputs 42 for each input, each input comprising: an input connector having excitation 18, 20, sensing 22,

Description

This invention relates to a sensor interface and a commodity monitoring apparatus, and in particular, but without limitation, to a strain gauge interface for monitoring the content of silos and 5 bulk containers. It has been common practice for many years to use sensors in industrial plant and machinery to monitor processes such that controllers and actuators can be used, in real-time, to adjust that process. Examples of sensors that are commonly used include flow sensors, strain gauges, valve position sensors and so on. 10 Each type of sensor generally either has a proprietary or a standardised interface, which can either be a logical standard in terms of the pinout configuration of the sensor, or a physical standard, such as a particular type of DIN interface physically connecting the sensor to other systems and / or other sensors. Even with a degree of sensor standardisation, there are nevertheless significant differences oftentimes between the onward software implementation of the sensor outputs, which 15 can lead to interoperability issues. CO The particular situation with which the present invention is concerned is in relation to ^M monitoring stores of commodities. CM Bulk commodities are frequently stored in silos or bulk containers (IBCs) and it is commonplace to use a strain gauge to estimate or measure the quantity of commodity contained within the CM 20 container. For example, a silo may weigh 3 tonnes when full, or 1 tonne when empty and by affixing a strain gauge to one or more legs of the silo, it is possible to measure the strain in the legs, which is a function of the weight of the silo itself. Therefore, by using a strain gauge it is possible to monitor the content of the silo and thus use that information for inventory control, re-ordering, usage monitoring and so on. 25 Most strain gauges employ a Wheatstone bridge-type circuit that provides a DC output voltage, which is proportional to the strain in the strain gauge. Most strain gauges are resistive sensors such that increasing the length of the strain gauge causes its resistance to increase. Measuring the resistance directly can be difficult - especially where the strains involved are very small, in the microstrain or millistrain range. An excitation voltage is therefore typically applied to a strain gauge, 30 which means that the resistance can be calcualted using Ohms Law or some equivalent formula. To compensate for various factors, such as thermal expansion, the excitation voltage is measured at the bridge terminals. This arrangement usually results in a strain gauge having a 6-pin connector, which has 2 pins for applying the excitation voltage, 2 pins for measuring the output voltage (and hence the strain measured) and a further 2 pins for applying a bias or reference voltage and / or monitoring the excitation voltage at the gauge to compensate for resistance in the cable from the excitation source. The output of the strain gauge is usually connected, using a length of multi-core cable to a strain gauge reader, which has a corresponding connector on it into which the cable can be plugged. 5 The strain gauge reader therefore outputs the excitation voltage and the biasing voltage (where required), reads the reference voltage, and measures the voltage across the sensing pins and then converts that voltage into some numerical value, which is typically displayed on a screen. Strain gauges are virtually ubiquitous nowadays and their design has remained fundamentally unchanged for many years. The reasons for this are backward compatibility as well as reliability and standardisation and so 10 the de facto configuration of a strain gauge is essentially as described above. Nowadays, however, it is becoming increasingly necessary or desirable to have multiple strain gauge readers connected to a single strain gauge. In an agricultural situation, for example, where poultry feed is stored in a silo for use on a poultry farm, it is necessary for on-site personnel to be able to monitor the content of the feed silos to ensure that the supply does not run out and also to monitor 15 how much feed is actually being consumed on a day-by-day or an hour-by-hour basis. The strain gauge CO readers are typically located in a central control room, which has CCTV feeds, plant controls as well as ^M all of the monitoring sensors for the facility. This means that one or several site operators can run and CM manage the entire facility from a single location - at least as far as the top-level control is concerned. There is currently no reliable way to duplicate or replicate the sensor readings in the control CM 20 room to a remote facility. This may be useful or desirable, for example, where the facility is part of a group of facilities and an overall central management system is required, or more frequently, where the facility is linked to a supplier who can replenish supplies as they become depleted. The current state of the art involves manually replicating the data from the sensors into a separate system, for example, by manually typing the sensor readings into a web-based interface, and 25 it is those replicated values that are 'shared' with other third parties. This has inherent problems in terms of it being time-consuming and error-prone and so the possibility of connecting multiple strain gauge readers to a single strain gauge has been contemplated. It will be appreciated from the foregoing description that most strain gauges are essentially analog devices connected over lengthy cable runs between the strain gauge reader and the strain 30 gauge itself. The cables are subject to external interference as well as variations arising due to thermal expansion / contraction, movement, as well as signal losses along the length of the cable itself. With this in mind, it is actually quite surprising that the reliability of strain gauges and strain gauge readers is as good as it is, but by the same token, it will be appreciated that if the signal were to be split at the reader end of the cable run, that the signal quality would be degraded to such an extent that no useful readings could be obtained. One solution to this would be to provide multiple strain gauges each feeding separate readers. Another solution might be to split the signal at the sensor end of the cable run and then run two cables to separate readers. Both solutions increase the amount of infrastructure required and are not easily retrofittable. A need therefore exists for a solution to this problem, which 5 enables multiple reading devices to be connected to single sensors. Aspects of the invention are set forth in the appended independent claim or claims. Preferred and / or optional features are set forth in the appended dependent claims. A first aspect of the invention provides a sensor interface comprising a processing unit, at least one input and two or more outputs for each input, each input comprising: an input connector having 10 excitation, sensing and reference contacts for connection to the corresponding contacts of a strain gauge; an analog-to-digital converter for converting a magnitude of a voltage input at the sensing contacts into a digital value; and a data interface for passing the digital value to the processing unit; the processing unit being configured, in use, for each received digital value: to convert the digital value into a PWM signal; and to output a copy of the PWM signal to two or more outputs of the processing 15 unit; and each output comprising: an output connector having excitation and sensing contacts for CO connection to the corresponding contacts of a strain gauge reader; an analog isolator, which converts ^M each copy of the respective PWM signal into an analog voltage at the sensing contacts; and a CM connection to a power supply, which provides an excitation voltage at the excitation contacts of the output connector, whereby: the strain gauge input at the or each input connector is substantially CM 20 replicated at the respective plurality of output connectors. The invention therefore provides a sensor interface with a processing unit interposed between, for each 'channel' a single input and two or more outputs. The processing unit is essentially used to split the incoming signal into several output signals, but by using the processing unit instead of simply splicing the input to several outputs, the signal quality is preserved. 25 The sensor interface has an input connector, which is connected to the output of a strain gauge, in use. The input connector has excitation, sensing and reference contacts, which connect to the corresponding contacts of the strain gauge. The processor is configured to output a voltage appropriate to the strain gauge and referenced to the excitation voltage provided by the externally connected equipment. The sensor interface 30 connects to the sensing contacts of the input connector, and this means that the processing unit is able to measure the output of the strain gauge as an analog voltage. An analog-to-digital converter is provided, which converts the magnitude of the voltage input at the sensing contacts into a digital value. This means that the analog-to-digital conversion is only done once and then after the strain gauge's analog signal has been digitised, it is possible to reliably replicate it within the processing unit. A data interface, such as an SPI or I2C data interface, is used to pass the digital value from the ADC to the processing unit. The processing unit then converts that into a PWM signal, which could be 5 stored temporarily in an internal register of the processing unit itself. The processing unit is programmed or otherwise configured to replicate the PWM signal on two or more of its output pins and this means that the incoming analog signal is essentially duplicated as a set of PWM signals at output pins of the processing unit. An output of the sensor interface has an output connector, which is similar to, or the same as, 10 the connector of the strain gauge itself. This means that the sensor interface can be placed in-line or in-series between a strain gauge and an existing strain gauge reader, and this increases the retrofitability of the invention to an existing installation. However, rather than simply passing the PWM signal back out, it is first passed through an analogue low-pass filter and isolator, which converts the copies of the PWM signal into respective analog voltages at two or more sets of sensing contacts. By 15 using an analog isolator, this isolates the signal voltage from any supply or excitation voltage, and it CO means that the signal that is passed on is insensitive to extraneous signals, noise and other errors. In order to be able to fit 'invisibly' between an existing strain gauge and an existing strain gauge CM reader, the sensor interface uses the external excitation voltage at its output connector, such that the strain gauge reader 'thinks' that it is connected to the strain gauge itself, rather than to the sensor CM 20 interface. The excitation contacts can be configured to output a voltage to the strain gauge reader or to receive an excitation voltage from the strain gauge reader. That voltage can be passed through the sensor interface, via a power supply, which is independent of the biasing voltage and the strain gauge output voltage. The advantage of this is that it enables the sensor interface to replicate a strain gauge reader's 25 excitation voltage output where this is necessary; or in certain embodiments, to supply an independent excitation voltage to the strain gauge without the strain gauge reader' knowing it'. This means that different types of strain gauges, and in particular those that may not be directly compatible with a strain gauge reader, can nevertheless be connected thereto via the sensor interface of the invention. 30 In either case, the strain gauge input at the or each input connector is substantially replicated at the respective plurality of output connectors. This enables several strain gauge readers or similar devices to be connected to a single strain gauge without any of the aforesaid drawbacks. Suitably, the processing unit comprises a programmable microcontroller. A programmable microcontroller has the advantage of enabling the operation and configuration of the sensor interface to be customised for each installation or application. This means that certain features can easily be added or omitted simply by changing the programming of the microcontroller. Suitably, the copy of the PWM value is substantially identical to the PWM signal derived from the received digital value. By making the output PWM signal identical to the input signal, the strain 5 gauge signal is effectively replicated identically. This can avoid interoperability issues where a particular strain gauge reader has in-built corrections or other software that requires the strain gauge output to be a 'raw' output. In other embodiments, however, the processing unit comprises signal processing software, which processes the received digital value before converting it to the PWM signal derived from the 10 received digital value. The signal processing may involve applying any one or more of a high-pass filter, a low-pass filter, an offset voltage, a smoothing algorithm, a sampling algorithm, and an averaging procedure to the received digital value. By using a signal processor, it is possible to reduce or remove noise from the data signal and / or to 'boost' the signal if necessary; or to convert the signal into some other required format. 15 Suitably, the processing unit comprises signal processing software which processes the CO received digital value before converting it to the PWM signal derived from the received digital value. ^M Suitably, the excitation voltage at the output connector is substantially the same as the CM excitation voltage at the input connector. The processing unit may thus measure the excitation voltage at the input connector and control the power supply to output a substantially identical excitation CM 20 voltage at the output connector or vice-versa. Suitably, the processing unit controls the power supply to output a user-defined excitation voltage at the output connector. Suitably, the analog isolator comprises an optocoupler. The use of an optocoupler galvanically isolates the output signal from the input signal and can also be used to galvanically isolate the 25 excitation signals from the sensor signals. A low-pass or a high-pass filter may suitably be interposed between the analog isolator and the output connector. The use of a low-pass or a high-pass filter can be useful to remove noise from a data signal as will be understood by the person skilled in the art, e.g., a low-pass filter following the optical isolator can convert the PWM signal into an analogue value. 30 Suitably, the at least one input connector and its corresponding output connectors are of the same physical type, albeit likely of opposite genders. A second aspect of the invention provides a commodity monitoring apparatus comprising: a commodity storage unit; a strain gauge affixed to the storage unit for estimating a weight of a commodity contained within the commodity storage unit, the strain gauge having an output connected to an input of the sensor interface of any preceding claim; and two or more strain gauge readers connected to respective outputs of the sensor interface as described herein. Suitably, a first one of the strain gauge readers is a local strain gauge reader, and wherein a second one of the strain gauge readers is a remote strain gauge reader. 5 Suitably, the remote strain gauge reader is operatively connected to the sensor interface via a wired or wireless connection. Suitably, the wired or wireless connection comprises: a transmitter module connected to one of the respective outputs of the sensor interface, the transmitter module comprising an ADC for converting the excitation voltage into a digital signal, a transmitter for transmitting the digital signal to 10 a receiver module; the receiver module comprising receiver for receiving the digital signal, a DAC for converting the digital signal into an analog voltage corresponding to the measured voltage applied at the input of the transmitter module, and an output connector having excitation and sensing contacts for connection to the corresponding contacts of the remote strain gauge reader. Suitably, the wired or wireless transmitter and receiver comprise a modem for transmitting 15 and receiving, respectively, the digital signal over a landline or mobile telephone connection. CO Suitably, the wired or wireless transmitter and receiver comprise a network interface for transmitting and receiving, respectively, the digital signal over a Wi-Fi, ethernet or mobile data CM connection. Suitably, the commodity storage unit comprises a bulk storage container or silo, wherein the a CM 20 first one of the strain gauge readers is located locally on-site, and wherein the second one of the strain gauge readers is located at a central monitoring station. An embodiment of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic circuit diagram for a sensor interface in accordance with an 25 embodiment of the invention; and Figure 2 is a schematic diagram of a commodity monitoring apparatus in accordance with an embodiment of the invention. Referring to Figure 1 of the drawings, a sensor interface 10 in accordance with the invention comprises a microcontroller 12 interposed between an input 14 and an output 16. The input 14 has 30 six contacts, namely the excitation contacts 18, 20, the signal contacts 22, 24 and reference contacts 26, 28. The contacts of a load cell or strain gauge 30 are connected to those contacts 18-28 respectively. An analog-to-digital converter 32 converts the signal voltage measured across contacts 22, 24 into a digital signal, which is passed via a data interface 34, in this case, an SPI interface, to the microcontroller 12. The microcontroller 12 can, in certain embodiments, control the ADC 32 via the SPI interface 34 to apply an excitation voltage across contacts 18, 20 as well as to bias or offset the excitation voltage using reference pins 26, 28. It will be noted that the strain gauge itself is formed as a resistor 36 forming one leg of a 5 Wheatstone bridge 38, which means that small changes in resistance caused by changes in applied load / strain produce a useful output voltage at pins 22 and 24. The microcontroller therefore measures (and optionally controls) the strain gauge 30 output and receives it as a digital signal via its SPI interface 34. The microcontroller 12 then replicates the digital signal and outputs it via two or more PWM pins 40,42. Because the microcontroller 12 is simply 10 duplicating the digital signal, it is actually isolated from the actual signal 22, 24 and splitting it does not, therefore, change or alter the PWM signals 40 / 42 in any way compared to the SPI input 34. This means that the microcontroller can duplicate, triplicate or otherwise multiply the input signal via the SPI interface 34 any number of times as needs / requirements dictate. On the output side 16 of the sensor interface 10, there is provided an analog isolator 42, which 15 comprises an optocoupler 44 and a low-pass filter 46 for each channel 40, 42. Each set of analog CO isolators and low-pass filters produces its own strain gauge outputs 48, 48' which are essentially the same as those at pins 22, 24 on the input side 14 of the sensor interface 10. CM As previously described, the microcontroller 12 can perform various other functions including signal processing and the skilled reader will readily understand how this can be implemented in CM 20 software / coding, or via a dedicated signal processing module or IC (not shown). Turning now to Figure 2 of the drawings, the sensor interface 10 has been incorporated into a commodity monitoring apparatus 100, which has a number of commodity storage units 102,104,106, which are, in this case, silos. The legs of the silos 102,104,106 are each fitted with a strain gauge 30 the output of which can indicate the respective fill level 108 of the silos 102, 104, 106. The strain 25 gauges 30, 30' 30" are connected to inputs 14,14', 14" of a sensor interface as previously described with reference to Figure 1. The sensor interface has multiple outputs 16, 16', 16" corresponding to each of the inputs 14, 14', 14". For each input 14 there is an output 16 connected to a remote load cell reader 120, 122, 124, which indicates numerically the fill levels 108 of the respective silos, 102, 104, 106. 30 A second output for each channel 14, 14', 14" is connected to a further strain gauge reader 126,128,130, which essentially duplicates the displays of the local readers 120, 122,124. Strain gauge readers 120, 122 are duplicated via a wireless connection, which comprises a transmitter module 132 and a receiver module 134, which receives a wireless signal 136 from the transmitter 132. The output CM CM 16,16' of the sensor interface 10 is digitised inside the transmitter module 132 before it is transmitted 136, received 134 and decoded and then displayed on the remote readers 126,128. The third silo 106 content 108 is indicated on a further remote reader 130, which receives a signal from the output side 16" of the sensor interface 10 and this is sent via a modem 138, via a data 5 cable 140 or a cloud-based network 142, to a receiving modem 144 forming part of a receiver module 144, which has an output 146 that replicates display 124 on receiver module 130. It will be appreciated from the foregoing that the invention provides a solution to the problem set out in the preamble of this disclosure and that various modifications or changes could be made without departing from the scope of the invention. 10 The invention is not restricted to the details of the foregoing embodiment, which is merely exemplary of the invention.

Claims

1. A sensor interface comprising a processing unit, at least one input and two or more outputs for each input,each input comprising: an input connector having excitation, sensing and reference contacts for connection to the corresponding contacts of a strain gauge; an analog-to-digital converter for converting a magnitude of a voltage input at the sensing contacts into a digital value; and a data interface for passing the digital value to the processing unit;the processing unit being configured, in use, for each received digital value: to convert the digital value into a PWM signal; and to output a copy of the PWM signal to two or more outputs of the processing unit; andeach output comprising: an output connector having excitation and sensing contacts for connection to the corresponding contacts of a strain gauge reader; an analog isolator, which converts each copy of the respective PWM signal into an analog voltage at the sensing contacts; and a connection to a power supply, which provides an excitation voltage at the excitation contacts of the output connector, whereby:the strain gauge input at the or each input connector is substantially replicated at the respective plurality of output connectors.

2. The sensor interface of claim 1, wherein the processing unit comprises a programmable microcontroller.

3. The sensor interface of claim 1 or claim 2, wherein the copy of the PWM is substantially identical to the PWM signal derived from the received digital value.

4. The sensor interface of claim 1 or claim 2, wherein the processing unit runs signal processing software which processes the received digital value before converting it to the PWM signal derived from the received digital value.

5. The sensor interface of claim 4, wherein the signal processing comprises any one or more of: applying a high-pass filter to, applying a low-pass filter to, offsetting the voltage of, smoothing, sampling, and averaging the received digital value.

6. The sensor interface of any preceding claim, wherein the excitation voltage at the outputconnector is substantially the same as the excitation voltage at the input connector.

7. The sensor interface of claim 6, wherein the processing unit measures the excitation voltage at the input connector and controls the power supply to output a substantially identical excitation voltage at the output connector, or vice-versa.

8. The sensor interface of any of claims 1 to 5, wherein the processing unit controls the power supply to output a user-defined excitation voltage at the output connector.

9. The sensor interface of any preceding claim, wherein the analog isolator comprises an optocoupler.

10. The sensor interface of any preceding claim, further comprising a low-pass or a high-pass filter interposed between the analog isolator and the output connector.CO11. The sensor interface of any preceding claim, wherein the at least one input connector and CM its corresponding output connectors are of the same physical type, albeit of opposite gender.1—CM 12. A commodity monitoring apparatus comprising: a commodity storage unit; a strain gauge affixed to the storage unit for estimating a weight of a commodity contained within the commodity storage unit, the strain gauge having an output connected to an input of the sensor interface of any preceding claim; and two or more strain gauge readers connected to respective outputs of the sensor interface of any preceding claim.

13. The commodity monitoring apparatus of claim 12, wherein a first one of the strain gauge readers is a local strain gauge reader, and wherein a second one of the strain gauge readers is a remote strain gauge reader.

14. The commodity monitoring apparatus of claim 13, wherein the remote strain gauge reader is operatively connected to the sensor interface via a wired or wireless connection.

15. The commodity monitoring apparatus of claim 14, wherein the wired or wireless connection comprises:a transmitter module connected to one of the respective outputs of the sensor interface, the transmitter module comprising an ADC for converting the excitation voltage into a digital signal, a transmitter for transmitting the digital signal to a receiver module;the receiver module comprising receiver for receiving the digital signal, a DAC for converting the digital signal into an analog voltage corresponding to the sensed voltage applied at the input of the transmitter module, and an output connector having excitation and sensing contacts for connection to the corresponding contacts of the remote strain gauge reader.

16. The commodity monitoring apparatus of claim 14 or claim 15, wherein the wired or wireless transmitter and receiver comprise a modem for transmitting and receiving, respectively, the digital signal over a landline or mobile telephone connection.

17. The commodity monitoring apparatus of claim 14 or claim 15, wherein the wired or wirelesstransmitter and receiver comprise a network interface for transmitting and receiving, respectively, the digital signal over a Wi-Fi, ethernet or mobile data connection.

18. The commodity monitoring apparatus of any of claims 12 to 17, wherein the commodity storage unit comprises a bulk storage container or silo, wherein a first one of the strain gauge readers is located locally on-site, and wherein the second one of the strain gauge readers is located at a central monitoring station.

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