Patent Publication Number: US-2013241530-A1

Title: Measuring Device for Determining and/or Monitoring at Least One Process Variable

Description:
The present invention relates to a measuring device for determining and/or monitoring at least one process variable with a sensor unit having at least one sensor sensitive for the process variable. The process variable is, for example, fill level, density, viscosity, temperature, pressure, or pH-value of a medium. 
     A multitude of varied measuring devices are known for determining and/or monitoring process variables in process- and automation technology. An example of a fill-level measuring device is that manufactured and sold by the assignee under the mark, “Liquiphant”. The “Liquiphant” device uses a vibronic measuring principle and its sensor unit has an oscillatory fork, which can be excited to execute mechanical oscillations. For use in explosion-endangered regions, such measuring devices often have a galvanic isolation, by which, for example, the medium-contacting sensor part located in the explosion-endangered region is isolated from energy supply, user interface, etc. Across the galvanic isolation, there occurs, most often, not only energy supply of the sensor unit, but also communication with the sensor unit. As a rule, the measuring device includes, for this, two electronic units, between which a galvanically isolated interface is present. This is implemented, for example, by at least one capacitor or transformer. 
     A form of energy- and data transmission via an interface equipped with a transformer is described in DE 10 2006 051 900 A1. In such case, data transmission from the first electronics unit to the second electronics unit occurs via modulation of the frequency and/or amplitude of the energy transmission, while data from the second to the first electronics unit are transmitted by means of modulation of the electrical current requirement of the second electronics unit. Both electronic units contain a processor. Activation of the transformer occurs via analog components, which set the parameters of the signal transmission, such as, for example, pulse- or pause duration. The fixed parameters bring the disadvantage that the efficiency of energy transmission is maximum only under certain conditions. Modification of the hardware circuit, which would enable influencing the efficiency by changing the parameters, would be associated with impractical circuit complexity. 
     A further disadvantage comes about in the case of start-up of the measuring device. During start-up, certain specifications must be adhered to. Thus, 4-20 mA devices, for example, must not assume values, which lie within this region associated with measured values, in order to prevent the triggering of corresponding reactions of following devices. Since analog components make use of no control function and, consequently, can not detect an exceeding of predetermined limits, it is usual to perform the start-up of the device with an electrical current lying clearly below or above the measured value range, e.g. &lt;3.6 mA. Start-up requires then correspondingly more time. A measuring device should, however, be operational as rapidly as possible. 
     An object of the invention is to provide a measuring device of the above described type, whose energy- and/or data transmission is matched to existing requirements. Furthermore, a method for operating such a measuring device is to be provided, which enable a matching of the energy- and/or data transmission to the existing requirements. 
     The object as regards the measuring device is achieved by features including that the first microcontroller produces at least one signal, which can vary in at least one characteristic, and supplies the signal to the transmission unit, that the signal controls energy- and data transmission, that the first microcontroller and/or the second microcontroller determines at least one current operating parameter, and that the first microcontroller sets at least one characteristic of the signal as a function of the operating parameter. 
     The first and/or the second microcontroller determines preferably the current operating state or the current efficiency of the energy transmission as operating parameter. Different operating states are, for example, start-up, measurement operation, and standby or energy saving mode. The settable characteristics of the signal are, for example, frequency, pulse duration, pause duration or the pulse duration to pause duration ratio. 
     For a long time now, processors possibly present in the electronics unit have delivered only a binary control signal for control of downstream components, which then produce the actual signals for energy- and/or data transmission. According to the invention, the primary side microcontroller now directly produces the control signal for energy- and/or data transmission. In this way, a high flexibility in the form of the signal is enabled. By way of the variable frequency or the variable pulse to pause ratio, the signal is adaptable as regards present, existing requirements, such as e.g. the energy requirement. The first microcontroller adapts the signal dynamically to the present requirements, respectively enables, furthermore, an adapting of the signal to changing environmental conditions, such as, for example, temperature. The characteristics of the signal are not fixedly preset, but, instead, are regularly controlled. 
     In a first embodiment of the solution of the invention, the first microcontroller adapts at least one characteristic of the signal in such a manner to a current operating state of the measuring device and/or sets the characteristic in such a manner that the efficiency of the energy transmission is optimized. Especially, in the case of continuous monitoring of efficiency, options include a control of the efficiency by optimally adapting the characteristics and/or the making of a statement concerning the state of the measuring device with an eye toward predictive maintenance. Instead of efficiency, for example, the output voltage can be optimized. 
     In an embodiment, the characteristic of the signal set by the first microcontroller is the frequency of the signal and/or the pulse duration to pause duration ratio. For example, at least one of the microcontrollers determines the present efficiency as operating parameter and the first microcontroller adapts the pulse duration to pause duration ratio of the signal for optimizing the efficiency. 
     In the case of a further development of the invention, the transmission unit includes at least one transformer. In such case, it can be a forward converter or a flyback converter. 
     In an embodiment of the measuring device of the invention, the transmission unit is a push pull converter, preferably a push pull flux converter. Two control signals are required for push pull activation, so that the first microcontroller correspondingly produces a first and a second signal, wherein the second signal is essentially an inverted first signal. 
     An advantageous embodiment of the measuring device provides that the first microcontroller registers a present input power of the transmission unit, that the second microcontroller registers a present output power of the transmission unit, and that the ‘first microcontroller and/or the second microcontroller ascertains from the registered input power and the registered output power the present efficiency of the energy transmission. Preferably, the first microcontroller ascertains the efficiency. Based on the efficiency, the optimal working point of the energy- and data transmission is ascertainable. By the variability of the signal for controlling the energy- and data transmission, the characteristics of the signal are controllable in such a manner that the transmission, at any time, occurs at the optimal working point. This makes the measuring device very energy efficient. 
     In an especially advantageous form of embodiment, the measuring device evaluates the efficiency for diagnostic purposes, especially as regards predictive maintenance. By continuously monitoring efficiency, a prognosis can be made as to when a declining efficiency will be under a value required for undisturbed operation of the measuring device and, thus, maintenance or a replacement of the measuring device will be necessary. A worsening of the efficiency can be caused by aging of the components of the electronic units, as well as by environmental conditions, such as e.g. high moisture content or high temperatures. In an embodiment, the measuring device further includes a temperature sensor for monitoring temperature. The information concerning temperature can likewise be used for diagnosis. 
     In a therewith connected embodiment, the first microcontroller and/or the second microcontroller determines from the ascertained efficiency whether a coil break is present in the transformer and produces in the case of a coil break a corresponding defect report. The measuring device is able, by evaluating the ascertained efficiency, to monitor itself for coil breaks. 
     An embodiment of the invention provides that the first microcontroller has different operating modes, which are characterized by certain values of at least one characteristic of the signal, that an operating mode is associated with each operating state of the measuring device, and that the microcontroller sets the operating mode as a function of the current operating state. For an operating mode, at least one set of starting values for the characteristics is defined. As time goes on, the first microcontroller adapts the characteristics, as required, to the present requirements. If, for example, the energy requirement of the sensor unit is smaller than expected, so that a smaller energy transmission is sufficient, the first microcontroller lessens the pulse duration, or the pulse duration to pause duration ratio, of the signal, in order to control the energy transmission. If the need arises, the first microcontroller brings the ratio back up. 
     In a further development, the first microcontroller registers an identification, or ID, of the sensor unit, and sets, as a function of the sensor unit present, the at least one characteristic of the signal in such a manner that the energy transmission is matched to the energy requirement of the respective sensor unit. Additionally, the characteristics are dynamically controllable. 
     The object of the invention is achieved furthermore by a method for operating a measuring device having the above features. The solution includes features that at least one signal having at least one variable characteristic is produced and fed to the transmission unit, that energy- and data transmission are controlled by the signal, that at least one current operating parameter is determined, and that at least one characteristic of the signal is set as a function of the operating parameter. 
     The terminology, characteristics, means variables characteristic for the signal, such as frequency and pulse duration to pause duration ratio. In a preferred embodiment of the transmission unit, such comprises a push pull converter. Preferably, for control, consequently, a first signal and a second signal are produced, wherein the second signal is essentially an inverted first signal. In an embodiment of the method, the at least one signal is matched to the operating conditions, i.e., for example, the sensor-side energy consumption is ascertained and the energy transmission correspondingly controlled. Thus, the efficiency of the energy transmission is optimized. In a variant, the first electronics unit recognizes the sensor type that is present, and the characteristics are set corresponding to the typical requirements of such sensor type, such as e.g. energy requirement. As time goes on, the characteristics are dynamically adapted to current requirements. The sensor recognition occurs, in such case, typically via communication with the microcontroller of the secondary side or via a correspondingly set port. In an embodiment, the operating state is ascertained and the signal correspondingly produced. For example, a start routine is defined, with which the system can be safely transferred from the turned-off state or from the resting state into the active, measuring state. 
    
    
     
       The invention will now be explained in greater detail based on the appended drawing for measuring device and method together. The figures of the drawing show as follows: 
         FIG. 1  a schematic representation of the components of a measuring device of the invention; 
         FIG. 2  a block diagram of the electronic components. 
     
    
    
       FIG. 1  shows the essential components of a measuring device of the invention. Sensor  3  and the second electronics unit  6  form the sensor unit. Sensor  3  is, for example, an oscillatable unit, especially an oscillatory fork, which is excited by a drive unit to execute mechanical oscillations. This is connected with the first electronics unit  5  via a galvanically isolated interface in the form of transmission unit  4 . The first electronics unit  5  supplies energy to the sensor unit, so that the side of the galvanic isolation associated with the sensor  3  can be referred to as the secondary side and the side associated with the first electronics unit  5  as the primary side. Besides the energy source, there can be located on the primary side other components such as e.g. display elements and interfaces to interaction elements or servicing elements. 
       FIG. 2  discloses a schematic block diagram of the first electronics unit  5 , the second electronics unit  6  and transmission unit  4 . The first electronics unit  5  includes a first microcontroller  1 , which has a digital interface  11 , a timer  12  and an analog/digital converter  13 . The first microcontroller  1  obtains the energy required by it via a voltage regulator  55  from the voltage source Vcc, which provides the supply voltage for transmission unit  4 . Transmission unit  4  is preferably a push pull converter, especially a push pull flux converter, having a transformer with center tapping. On the primary side, the transformer middle tap is connected with the supply voltage Vcc. The respective other ends are connectable with ground via a controllable switch  71 ,  72 . 
     Operation of the push pull converter occurs digitally by means of the first microcontroller  1 . The digital interface  11  controls the input of the timer  12 . For example, the digital interface  11  is a UART. In an alternative embodiment, the timer  12  is operated internally, i.e. by a signal within the first microcontroller  1 . Timer  12  has two outputs, on which a first signal S 1  and a second signal S 2  are output. During applied low-level, timer  12  produces the first signal S 1  with a frequency f 1 ; during high-level a frequency f 2 . In other words, the timer  12  produces a frequency modulated signal S 1 , wherein the frequency f 1  corresponds to a logical zero and the frequency f 2  a logical one. The first signal S 1  controls the first switch  71 . The second signal S 2  controls the second switch  72  and corresponds essentially to an inverted first signal S 1 . The edges of the pulses of the second signal S 2  are, however, opposite those of the first signal S 1  in such a manner, offset by a pause time tp, that at no time are the two signals S 1 , S 2  simultaneously at high-level, since this would cause a short circuit. Switches  71 ,  72  can be implemented, for example, in the form of transistors. 
     The different frequencies f 1  and f 2  serve for communication from the primary side to the secondary side. For transmission of a logical 0, the timer  12  produces signals with the frequency f 1 ; for transmission of a logical 1, the frequency f 2 . At the same time, also energy transmission is controlled with the signals S 1  and S 2 . The longer the pulse duration of the signals S 1 , S 2 , the greater is the amount of energy fed, wherein the transmission of this energy has limits set by saturation of the transformer core. Outside of the communication phases, for example, the signal S 1 , S 2  with logical 1 has the frequency f 2 . The pulse duration of the signal S 1 , S 2  with the frequency f 2  determines, consequently, the transferred energy. According to the invention, the energy transmission is matched to the current energy requirement. For this, the second microcontroller  2  determines on the secondary side the current energy requirement and informs the first microcontroller  1  thereof, whereupon this sets the pulse duration to pause duration ratio correspondingly. The frequencies f 1 , f 2  are factors of the system clock of the first microcontroller  1  and can be adjusted as much as desired. Since the second signal S 2  essentially equals the inverted first signal S 1 , this means that, in the case of a change of the characteristics, the characteristics of the first signal S 1  and the characteristics of the second signal S 2  are equally changed. 
     Same as in the case of the first electronics unit  5 , the second electronics unit  6  includes a second microcontroller  2 , which has a timer  22 , a digital interface  21  and an analog/digital converter  23 . The energy supply of the second microcontroller  2  comes from the voltage controller  65  of the transmission unit  4 . 
     Voltage controller  65  controls the transferred voltage to that required for the input voltage of the second microcontroller  2 . The ground potential of the second microcontroller  2  is labeled here with Vss. 
     For decoding the transmitted information, the secondary side second electronics unit  6  includes a demodulator  63 . This converts the frequency modulated signal back into a binary signal. For example, the demodulator  63  is a lowpass or bandpass filter, whose limit frequency, respectively limit frequencies, is, respectively are, set corresponding to the frequencies f 1  and f 2 . 
     The second electronics unit  6  includes a current converter  61  and a level converter  62 . The current converter  61  contains a resistor for changing the applied voltage into a corresponding electrical current and leads this to the analog/digital converter  23 . The level converter  62  receives the transferred voltage and forwards it to the analog/digital converter  23 , wherein the voltage is adjusted to the digitizable region of the analog/digital converter  23 , respectively the supply voltage of the second microcontroller  2 . From the present values of electrical current and voltage, the second microcontroller  2  determines the present power. 
     For communication with the primary side, the secondary side uses a pulse-like electrical current modulation. For this, the electrical current modulator  64  connected with the signal output of the digital interface  21  increases the electrical current requirement of the secondary side during the presence a logical one. In an advantageous embodiment, the electrical current requirement is increased not uniformly for the entire pulse duration of the signal output by the digital interface  21 , but, instead, there are produced during the pulse duration a number of short electrical current pulses, so that the additional electrical current consumption is kept small. 
     In order to decode the electrical current modulation and, thus, the information transmitted from the secondary side, the first electronics unit  5  includes a current converter  51  on the primary side. This is connected after the two switches  71 ,  72  and connected with the ground of the first microcontroller  1 . Current converter  51  contains at least one resistor of known value, via which the electrical current is converted into a corresponding voltage. For ascertaining the electrical current consumption, the current converter  51  supplies this voltage to the analog/digital converter  13 . Additionally, the current converter  51  contains a demodulator, which prepares the communication signal for the digital interface  11 , to which it then supplies the prepared signal. 
     The ascertained electrical current value serves, besides for ascertaining the information content transmitted by the electrical current modulation, for determining the present primary side power. For this, additionally knowledge of the currently applied voltage is necessary. The information concerning the currently applied voltage is provided to the first microcontroller  1  from the level converter  52 . This adapts the voltage to the digitizable region of the analog/digital converter  13  and forwards such thereto. In the case of known division ratio, the microcontroller  1 , thus, knows the actual voltage, so that it can then ascertain the power. 
     Preferably, the second microcontroller  2  transmits to the first microcontroller  1  the power measured on the secondary side. The first microcontroller  1  determines then the present efficiency of the transmission. If the efficiency is not optimal, the first microcontroller  1  controls, for example, the characteristics of the signals S 1 , S 2  in such a manner that the efficiency increases. If the efficiency, in spite of readjustment of the characteristics, increases not as expected, probably a coil break is present and the microcontroller  1  produces a defect report. In an embodiment, the microcontroller  1  first operates the two coils separately and determines the efficiency for each case. In another embodiment, the first microcontroller  1  increases the supplied energy, when the secondary side reports a decrease of power or transmitted energy. If the power, respectively transmitted energy, does not increase, the first microcontroller  1  produces an error report. The error report is effected, for example, by turning-on an LED on the device or issued in the form of a request for maintenance via a bus system. 
     Preferably, different operating modes are implemented in the first microcontroller  1 , which are distinguished by a set of certain characteristics. Associated with each operating state is then an operating mode. The operating state is automatically recognized and the corresponding operating mode set, i.e. the signals S 1 , S 2  are produced with the corresponding frequencies f 1 , f 2  and corresponding pulse duration to pause duration ratio. 
     An operating state is, for example, the bringing of the measuring device up in the case of start-up or from standby. Preferably, a startup routine with dynamic electrical current control is specified. The electrical current consumption of 4-20 mA devices must lie during start-up outside of the 4-20 mA range. According to the invention, there is implemented with the current converter  51  a control function in the first electronics unit  5 , with which the current electrical current consumption is registerable. Preferably, consequently, an electrical current lying only scarcely below the impermissible region is used and, in given cases, controlled down, in case the electrical current consumption rises. This accelerates the startup procedure and increases, thus, the availability of the measuring device. 
     A further operating state is that of measurement operation. Predominantly, during measurement operation, energy is transmitted from the primary side to the secondary side, wherein the signal corresponding to logical one lies on the transmission unit. The corresponding frequency and the pulse duration to pause duration ratio are matched to the energy requirement of the sensor unit, respectively the secondary side. Preferably, the primary side possesses a function for automatic sensor type recognition and selects the characteristics corresponding to the sensor-specific energy consumption. Advantageously, the first microcontroller  1  has for each sensor unit connectable with the primary side an operating mode associated with measurement operation of this sensor unit, wherein one operating mode can also be associated with a plurality of sensor units. Of course, there occurs also here, in case required, a dynamic adapting of the characteristics during measurement operation, e.g. in the case of an increased electrical current requirement or for optimizing efficiency. 
     List of Reference Characters 
       1  first microcontroller 
       11  digital interface 
       12  timer 
       13  analog/digital converter 
       2  second microcontroller 
       21  digital interface 
       22  timer 
       23  analog/digital converter 
       3  sensor 
       4  transmission unit 
       5  first electronics unit 
       51  current converter 
       52  level converter 
       55  voltage controller 
       6  second electronics unit 
       61  current converter 
       62  level converter 
       63  demodulator 
       64  electrical current modulator 
       65  voltage controller 
       71  switch 
       72  switch 
     S 1  first) signal 
     S 2  second signal 
     Vss reference potential of the second microcontroller 
     Vcc direct voltage source