Abstract
An analog output stage of a field device employed in process automation is provided. The analog output stage regulates an analog output, for example, a loop current flowing in a two wire current loop, based on an input, for example, a process variable such as temperature, pressure, etc., detected by the field device. The analog output stage includes a regulator module and a switching module. The switching module, via a switching pulse width modulated signal, alternately applies to the regulator module, a first analog value associated with the input detected by the field device and a predefined analog output, and a second analog value associated with the analog output. The regulator module includes an integrator and a differential amplifier. The regulator module generates a differential analog output based on the first analog value and the second analog value and regulates the analog output based on the differential analog output.
Claims
1. An analog output stage of a field device employed in process automation, for regulating an analog output based on an input detected by the field device, the analog output stage comprising: a regulator module comprising an operational amplifier and an integrator having a resistor and capacitor, wherein the regulator module is configured to control the analog output; and a switching module communicatively coupled to the regulator module, the switching module configured to alternately apply to the regulator module, a first analog value associated with the input detected by the field device and a second analog value associated with the analog output, wherein the integrator of the regulator module is configured to smoothen an output from the switching module, which is applied to an input of the operational amplifier of the regulator module.
2. The analog output stage of claim 1, wherein the input detected by the field device comprises a process variable.
3. The analog output stage of claim 1, wherein the switching module is driven by a switching pulse width modulated signal generated by a processor of the field device.
4. The analog output stage of claim 1, wherein the first analog value is set by a reference pulse width modulated signal generated by a processor of the field device, and wherein the reference pulse width modulated signal is generated based on the input detected by the field device and a predefined analog output that corresponds to the input detected by the field device.
5. The analog output stage of claim 4, wherein the first analog value comprises a coarse component set by a first reference pulse width modulated signal generated by the processor of the field device and a fine component set by a second reference pulse width modulated signal generated by the processor, the fine component being a function of the coarse component, and wherein the switching module is configured to alternately apply to the regulator module, the coarse component, the fine component, and the second analog value.
6. The analog output stage of claim 5, wherein one or more of the first analog value and the second analog value are processed by one or more processing modules, before being applied to the regulator module, the processing modules comprising a filter and an amplifier.
7. The analog output stage of claim 4, wherein one or more of the first analog value and the second analog value are processed by one or more processing modules, before being applied to the regulator module, the processing modules comprising a filter and an amplifier.
8. The analog output stage of claim 1, wherein the first analog value comprises a coarse component set by a first reference pulse width modulated signal generated by a processor of the field device and a fine component set by a second reference pulse width modulated signal generated by the processor, the fine component being a function of the coarse component, and wherein the switching module is configured to alternately apply to the regulator module, the coarse component, the fine component, and the second analog value.
9. The analog output stage of claim 8, wherein one or more of the first analog value and the second analog value are processed by one or more processing modules, before being applied to the regulator module, the processing modules comprising a filter and an amplifier.
10. The analog output stage of claim 1, wherein one or more of the first analog value and the second analog value are processed by one or more processing modules, before being applied to the regulator module, the processing modules comprising a filter and an amplifier.
11. A method of regulating an analog output of a field device employed in process automation based on an input detected by the field device, the method comprising: providing the field device comprising an analog output stage communicatively coupled to a processor of the field device, the analog output stage comprising a regulator module and a switching module; obtaining, by the processor, the input detected by the field device, wherein the input comprises a process variable; generating, by the processor, a reference pulse width modulated signal configured to output a first analog value based on the input detected by the field device and a predefined analog output that corresponds to the input detected by the field device; detecting, by the processor, the analog output; alternately providing to the regulator module the first analog value and a second analog value by the switching module, wherein the switching module is driven by a switching pulse width modulated signal generated by the processor, and wherein the second analog value is associated with the detected analog output; integrating, by the regulator module, one or more of the first analog value and the second analog value from the switching module, wherein an integrator of the regulator module is configured to smoothen an output from the switching module, which is applied to an input of an operational amplifier of the regulator module; generating, by the regulator module, a differential analog output based on the first analog value and the second analog value; and regulating, by the regulator module, the analog output of the field device based on the differential analog output.
12. The method of claim 11, further comprising: processing one or more of the first analog value and the second analog value, by one or more processing modules of the analog output stage, before being applied to the regulator module, wherein the processing modules comprise a filter and an amplifier.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The present disclosure is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:
(2) FIG. 1 illustrates a conventional equivalent circuit of a two-wire current loop in a process automation system.
(3) FIG. 2 illustrates a conventional current output stage of the field device illustrated in FIG. 1.
(4) FIG. 3 illustrates a circuit diagram of a current output stage of a field device for process automation according to one aspect of an analog output stage.
(5) FIG. 4 illustrates a circuit diagram of the current output stage of the field device shown in FIG. 3, according to another aspect of the analog output stage.
(6) FIG. 5 illustrates a graphical representation of an output of the regulator module based on a switching cycle of the switching module of the current output stage illustrated in FIG. 4.
(7) FIG. 6 illustrates a circuit diagram of a voltage output stage of a field device, according to another aspect of the analog output stage.
(8) FIG. 7 illustrates a process flowchart of an exemplary method of regulating an analog output of a field device employed in process automation based on an input detected by the field device, in accordance with the aspects of the analog output stage illustrated in the FIGS. 3-5.
(9) The foregoing summary, as well as the following detailed description, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the disclosure, exemplary constructions are shown in the drawings. However, the disclosure is not limited to the specific methods and structures disclosed herein. The description of a method act or a structure referenced by a numeral in a drawing is applicable to the description of that method act or structure shown by that same numeral in any subsequent drawing herein.
DETAILED DESCRIPTION
(10) FIG. 3 illustrates a circuit diagram of a current output stage 300 of a field device 101 for process automation according to one aspect of an analog output stage. The current output stage 300, at its output, regulates an analog output, that is, a loop current Io, flowing in a current loop 100 shown in FIG. 1. The loop current Io is a function of a process variable such as an analog real world signal including pressure, temperature, etc., that the field device 101 senses and measures via a sensor, transducer, etc. The loop current Io lies in a range of about 4 mA to about 20 mA. However, for designing of various components of the current output stage 300, a range of 0 mA to 25 mA is considered. For example, a loop current Io below 4 mA, that is, between 0 mA and 3.6 mA, or above 20 mA, that is, between 22.8 mA and 25 mA, may indicate malfunctioning of the field device 101, e.g. malfunctioning of one or more components in the current output stage 300. The current output stage 300 includes a regulator module 302 communicatively coupled to a switching module 301. The regulator module 302 is communicatively coupled to the terminals 101a and 101b of the field device 101, with which the field device 101 communicates with the current loop 100. The terminal 101b is a negative terminal and the terminal 101a is a positive terminal. The regulator module 302 is connected to the terminals 101a and 101b via a current regulation circuitry 204 that converts a differential voltage output of the regulator module 302 into a corresponding loop current Io. The regulator module 302 includes an operational amplifier (OPAMP) 302a which is configured as a differential amplifier 302a regulating input voltages applied to its input terminals. That is, the OPAMP 302a acts as a nullor that drives its differential output voltage to zero. The regulator module 302 also an integrator including a resistor R and a feedback capacitor C. The resistor R and the feedback capacitor C integrate, that is, smoothen, the output of the switching module 301 which is applied to the input of the OPAMP 302a. The temperature coefficient and long-term stability of the components R and C have a near zero effect on the stability of the current output stage 300, thereby, eliminating need of employing costly components for improving an overall stability. Moreover, the capacitor C may be selected from capacitors available at a lower price and higher long-term stability and temperature characteristics.
(11) The switching module 301 is, for example, a single pole double throw switch alternately applying to the regulator module 302, a first analog value, that is, a set point voltage Vsp associated with the input detected by the field device 101, and a second analog value, that is, a feedback voltage Vfb associated with the loop current Io. The voltage Vfb is measured based on the loop current Io flowing over a sense resistor Rs. A switching pulse width modulated signal drives the switching module 301. A processor 303 of the field device 101 generates the switching pulse width modulated signal. In order to design components of the current output stage 300, for example, a duty cycle of the switching pulse width modulated signal, the loop current Io to be generated by the current output stage 300 is considered to be maximum, that is, 25 mA. Assuming, the sense resistor Rs is of 30 Ohms, the voltage Vfb, at a switching position 2 of the switching module 301, is equal to product of the loop current Io and the sense resistor Rs=Io*Rs=25 mA*30 Ohms=0.75V. As disclosed in the detailed description of FIG. 2, the switching module 301 replaces the resistor divider module 202 and therefore, the duty cycle of the switching pulse width modulated signal is computed based on a ratio of the resistors R1 and R2 of the resistor divider 202. That is, a ratio of the set point voltage Vsp applied at a switching position 1 of the switching module 301 and the feedback voltage Vfb is equal to the ratio of the resistors R1 and R2. By using this computation, Vsp can be calculated as 2.048V for Vfb of 0.75V at Io=25 mA, for example, based on commonly available resistor values of R2=100 kilo Ohm and R1=36.5 kilo Ohm. The set point voltage Vsp is applied at terminal 203 of the current output stage 300. Thus, the loop current Io ranging from 0 mA to 25 mA is mapped to a set point voltage Vsp of 0V to 2.048V. This Vsp is set by a reference pulse width modulated signal generated by the processor 303 based on the input detected by the field device 101 and a predefined loop current Io to be generated in accordance with the input detected by the field device 101, using the mapping range disclosed above. For example, consider a process variable input having a sensing range of 0 degrees Celsius to 100 degrees Celsius. Assuming that at a particular instant the process variable input is about 64 degrees Celsius which maps to a predefined loop current Io being as 16 mA. Therefore, Vsp would be set at a value of 1.311V based on the 0V-2.048V range of Vsp mapped to 0 mA to 25 mA operating range of Io. The duty cycle of the switching pulse width modulated signal is calculated as ratio of Vfb to Vsp=36%. Therefore, for maximum output current Io=25 mA, the processor 303 generates a switching pulse width modulated signal such that the feedback voltage Vfb is applied to the regulator module 302 for 64% of a time interval t and the set point voltage Vsp is applied for 36% of the time interval t. Thus, lesser the loop current Io, lesser is the set point voltage Vsp, and higher is the duty cycle percentage, such that Vsp will be applied for a longer duration compared to Vfb.
(12) FIG. 4 illustrates a circuit diagram of the current output stage 300 of the field device 101 shown in FIG. 3, according to another aspect of the analog output stage. As shown in FIG. 4, the first analog value, that is, Vsp is split into a coarse component 203a and a fine component 203b. A first reference pulse width modulated signal generated by the processor 303 sets the coarse component 203a and a second reference pulse width modulated signal generated by the processor 303 sets the fine component 203b. The fine component 203b being a function of the coarse component 203a. The coarse component 203a and the fine component 203b are passed through low pass filters 401a and 401b respectively. The low pass filters 401a and 401b average or smooth the coarse component 203a and the fine component 203b. A reaction time of the current output stage 300 required to generate and regulate the loop current Io, may be dependent on a clocking frequency of the processor 303, a frequency of the reference pulse width modulated signal used to generate Vsp, and a resolution of the reference pulse width modulated signal. The clocking frequency of the processor 303 affects power consumption of the current output stage 300 and therefore, cannot be increased. Similarly, the frequency of the reference pulse width modulated signal cannot be arbitrarily decreased because this affects the analog signal, that is, Vsp, being generated. Therefore, by dividing the Vsp into a coarse component 203a and a fine component 203b and by filtering each of these components 203a and 203b, a resolution of the reference pulse width modulated signal is increased without compromising on the reaction time of the current output stage 300 required to regulate the output current Io.
(13) The switching module 301 alternately applies to the regulator module 302, the coarse component 203a, the fine component 203b, and the feedback voltage Vfb. For example, for a loop current of Io=25 mA, the switching pulse width modulated signal driving the switching module 301 is set at a duty cycle such that Vsp is applied to the regulator module 302 for 64% of a time interval t and the coarse component 203a and the fine component 203b together are applied for 36% of the time interval t. Out of the 36% time interval, the coarse component 203a is applied for a longer duration compared to the fine component 203b. For example, a ratio of 2:1 is selected for the coarse component 203a to the fine component 203b. In this example, the coarse component 203a represents upper bits of the reference pulse width modulated signal and the fine component 203b represents the lower bits of the reference pulse width modulated signal.
(14) FIG. 5 illustrates a graphical representation 500 of an output of the regulator module 302 based on a switching cycle of the switching module 301 of the current output stage 300 illustrated in FIG. 4. The switching module 301 switches from a position 3 to a position 2 and then to a position 1, that is, applies the feedback voltage Vfb followed by the fine component 203b and then the coarse component 203a. The set point voltage Vsp charges the feedback capacitor C of the regulator module 302, as shown in FIG. 5 from position 2 onwards, and the feedback voltage Vfb discharges the feedback capacitor C, as shown in FIG. 5 during position 3. Thus, an average output at the regulator module 302, represents a sum of all the input signals applied thereto.
(15) FIG. 6 illustrates a circuit diagram of a voltage output stage 600 of a field device 101, according to another aspect of the analog output stage. In this aspect, the analog output stage is configured as a voltage output stage 600 including the regulator module 302 and the switching module 301 especially as disclosed in the detailed description above. The switching module 301 alternately applies to the regulator module 302 the set point voltage Vsp and the feedback voltage Vfb. Vsp is a set point voltage predefined based on an input detected by the field device 101. The regulator module 302 integrates the voltages Vsp and Vfb. The regulator module 302 generates a differential output voltage Vo which may be further fed to a voltage transmission component in a process automation system.
(16) FIG. 7 illustrates a process flowchart 700 of an exemplary method of regulating an analog output of a field device 101 shown in FIG. 1, employed in process automation based on an input detected by the field device 101, in accordance with the aspects of the analog output stage such as the current output stage 300 illustrated in the FIGS. 3-4 and/or the voltage output stage 600 illustrated in FIG. 6. At act 701, the method provides the field device 101 having an analog output stage 300, 600 communicatively coupled to a processor 303 of the field device 101. The analog output stage 300, 600 includes a regulator module 302 and a switching module 301 especially as disclosed above in the detailed description of FIGS. 3, 4, 6. At act 702, the processor 303 obtains the input detected by the field device 101, such as, a process variable. At act 703, the processor 303 generates a reference pulse width modulated signal that outputs a first analog value, that is, Vsp, based on the input detected by the field device 101 and a predefined analog output that corresponds to the input detected by the field device 101. At act 704, the processor 303 detects the analog output, that is, the existing analog output, such as, the loop current Io shown in FIGS. 3-4 or a feedback voltage Vfb shown in FIG. 6. At act 705, the switching module 301 alternately provides to the regulator module 302, the first analog value Vsp, and a second analog value Vfb associated with the detected analog output. At act 706, the regulator module 302 generates a differential analog output based on the first analog value Vsp and the second analog value Vfb. At act 707, the regulator module 302 regulates the analog output of the field device 101 based on the differential analog output, for example using the current regulation circuitry 204 of a current output stage 300 in a current loop transmission or by summing the differential analog output with the existing analog output voltage in a voltage transmission component of a process automation system.
(17) It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present disclosure. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.
(18) While the present disclosure has been described above by reference to various embodiments, it may be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.
