Process transmitter isolation unit compensation

A process transmitter includes an isolation unit, a process sensor, a compensation circuit, and an output circuit. The isolation unit is configured to engage a process and includes a medium. The process sensor is configured to produce a process signal that is a function of a parameter of the process that is communicated through the medium. The compensation circuit is configured to compensate the process signal for a response time of the isolation unit, and output a compensated process signal. The output circuit is configured to produce a transmitter output as a function of the compensated process signal.

BACKGROUND OF THE INVENTION

Embodiments of the present disclosure relate to the measurement of parameters of a process, such as an industrial process, using a process sensor. More specifically, embodiments of the present disclosure relate to compensating process measurements performed by the process sensor for a time constant of an isolation unit through which the parameter is communicated to the process sensor.

Industrial processes are used in the manufacturing and transport of many types of materials. In such systems, it is often required to measure different parameters within the process. One such parameter is a pressure, such as a differential pressure, for example. The differential pressure is the pressure difference between one point in the process and another point in the process. Such differential pressure measurements may be useful in determining the flow rate of a process fluid in a pipe of the process, or to measure a height of a process fluid in a container, or to provide another process parameter measurement.

In industrial processes, process sensors, such as pressure sensors, are typically contained in, or coupled to, a process transmitter. The process transmitter is usually located at a remote location, and transmits the process measurement related information to a centralized location, such as a control room. The transmission is frequently over a process control loop. For example, a two-wire process control loop is often used in which two wires are used to carry both information, as well as power to the transmitter. Wireless process control loops may also be used.

Process transmitters, such as pressure transmitters, typically include an isolation unit that separates the process sensor from the process being measured. This protects the process sensor from process conditions that may damage the sensor, and/or adversely affect the measurement of the process parameter, for example.

Such isolation units introduce delays to the measurement of the process parameter due to the medium used, the structure of the isolation unit, and possibly other factors. Such a delay has a direct impact on the response time of the process parameter measurement by the process sensor. Furthermore, the delay varies depending on the operating conditions of the process transmitter. As a result, control system designers must design or tune the system for the worst-case response of the process transmitter. This may lead to an inefficient process, which can erode profitability of the process under control.

SUMMARY OF THE INVENTION

Embodiments of the present disclosure relate to process transmitters and methods for compensating process signals for time constants of an isolation unit through which process parameters are communicated. Some embodiments of the process transmitter include an isolation unit, a process sensor, a compensation circuit, and an output circuit. The isolation unit is configured to engage a process and includes a medium. The process sensor is configured to produce a process signal that is a function of a parameter of the process that is communicated through the medium. The compensation circuit is configured to compensate the process signal for a response time of the isolation unit, and output a compensated process signal. The output circuit is configured to produce a transmitter output as a function of the compensated process signal.

Some aspects of the method include a method for producing a process transmitter output. In some embodiments, a process signal is produced that is a function of a parameter of a process that is communicated through a medium of an isolation unit of the process transmitter using a process sensor. The process signal is compensated for a response time of the isolation unit and generating a compensated process signal using a compensation circuit. The process transmitter output is produced as a function of the compensated process signal using an output circuit.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present disclosure are described more fully hereinafter with reference to the accompanying drawings. Elements that are identified using the same or similar reference characters refer to the same or similar elements. The various embodiments of the present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art.

Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it is understood by those of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, circuits, systems, networks, processes, frames, supports, connectors, motors, processors, and other components may not be shown, or shown in block diagram form in order to not obscure the embodiments in unnecessary detail.

It will be understood that when an element is referred to as being “connected,” “coupled,” or “attached” to another element, it can be directly connected, coupled or attached to the other element, or it can be indirectly connected, coupled, or attached to the other element where intervening or intermediate elements may be present. In contrast, if an element is referred to as being “directly connected,” “directly coupled” or “directly attached” to another element, there are no intervening elements present. Drawings illustrating direct connections, couplings or attachments between elements also include embodiments, in which the elements are indirectly connected, coupled or attached to each other.

Embodiments of the present disclosure may also be described using flowchart illustrations and block diagrams. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed, but could have additional steps not included in a figure or described herein.

Embodiments of the present disclosure are directed to compensating process transmitter parameter measurements to improve the response time of the process transmitter. This is generally accomplished by compensating a process signal produced by a process sensor that is a function of a parameter of the process being measured for a response time of an isolation unit that separates the process sensor from the process. The improved response time of the process transmitter can improve the efficiency of the process, and allows the process transmitter to be used in processes where high speed process parameter measurements are required.

FIG. 1is a simplified block diagram of a process transmitter100, which is formed in accordance with one or more embodiments of the present disclosure, interacting with a process102. In some embodiments, the process102includes an industrial process that involves a material, such as a fluid, moving through pipes and tanks to transform less valuable materials into more valuable and useful products, such as petroleum, chemicals, paper, food, etc. For example, an oil refinery performs industrial processes that can process crude oil into gasoline, fuel oil, and other petrochemicals. Industrial process control systems use process devices, such as process transmitters, as measurement instruments for sensing and measuring process parameters, such as pressure, flow, temperature, level, and other parameters, in combination with, for example, control devices, such as valves, pumps and motors, to control the flow of materials during their processing.

In some embodiments, the process transmitter100includes an isolation unit104, a process sensor106, a compensation circuit108, and an output circuit110. The isolation unit104is configured to engage the process102and isolate the process sensor106from the process102. The isolation unit104operates to communicate a parameter of the process102to the process sensor106for measurement through a suitable medium111, such as a fluid, or other suitable medium. The process sensor106is configured to produce a process signal112that is a function of the measured parameter of the process102that is communicated through the medium111of the isolation unit104.

The isolation unit104does not communicate the process parameter instantaneously to the process sensor106. Rather, a delay in the communication and measurement of the process parameter is induced by the isolation unit104. This delay relates to a response time of the isolation unit104. In some embodiments, the response time of the isolation unit104is dependent on one or more variables, such as a temperature of the medium111, a pressure, structures of the isolation unit104(e.g., diaphragms), materials forming the isolation unit, and/or other variables.

Effects of the delay in communicating and measuring the process variable using the sensor106include a limit on the measurement bandwidth of the process parameter being monitored. Specifically, the delay acts as a low-pass filter whose cutoff frequency drops in response to an increase in the delay or a decrease in the response time. As a result, changes in the process variable occurring above the cutoff frequency are rendered undetectable by the process sensor106. Embodiments of the present disclosure operate to reduce or eliminate the delay by reducing the response time, thereby decreasing the cutoff frequency and the loss of potentially valuable information.

The compensation circuit108is configured to compensate the process signal112for the response time of the isolation unit104, and output a compensated process signal114, in which the delay corresponding to the response time is reduced. The output circuit110is configured to produce a transmitter output116as a function of the compensated process signal114. In some embodiments, the compensation circuit108is configured to compensate the process signal112for the response time of the isolation unit104that is dependent on one or more variables, such as those mentioned above.

The compensation circuit108may comprise analog circuitry and/or digital circuitry. In some embodiments, the compensation circuit108represents one or more processors that are configured to execute instructions, which may be stored locally in memory of the compensation circuit108, or in memory that is remote to the transmitter100, to perform one or more functions described herein.

In some embodiments, the response time of the isolation unit104is dependent on a temperature of the medium111through which the process parameter is communicated to the process sensor106. In some embodiments, the process transmitter100includes a temperature sensor117having a temperature signal118that is indicative of the temperature of the medium111of the isolation unit104, and is used by the compensation circuit108to perform the compensation of the process signal112for the response time of the isolation unit104.

In some embodiments, the response time of the isolation unit104is dependent on an operating pressure that the isolation unit104is subjected to during use. In some embodiments, the compensation circuit108uses the operating pressure to compensate the process signal112for the response time of the isolation unit104. In some embodiments, the operating pressure is input to the compensation circuit108as a fixed variable, such as when the operating conditions of the isolation unit104are known. In some embodiments, a pressure sensor, such as a line pressure sensor, or a differential pressure sensor, may be used to determine the operating pressure of the isolation unit104, and provide the compensation circuit108with a pressure signal that is indicative of the operating pressure. In some embodiments, the process sensor106, or another sensor, operates as the pressure sensor that provides the compensation circuit108with the operating pressure of the isolation unit104.

In some embodiments, the transmitter100is an analog device, in which the signals112,114, and116are analog signals. In some embodiments, the compensation circuit108includes a microprocessor that is configured to process the process signal112in the digital domain. In some embodiments, the process transmitter100includes an analog-to-digital converter (ADC)119that digitizes the analog process signal112into a digital process signal112′ for processing by the compensation circuit108. In some embodiments, the compensated process signal114is in a digital signal, and the process transmitter100includes a digital-to-analog converter (DAC)120that converts the compensated process signal114to an analog compensated process signal114′ that is received by the output circuit110.

In some embodiments, the output circuit110is configured to transmit the transmitter output116to a suitable controller that uses the transmitter output116to control aspects of the process102. In some embodiments, the output circuit110transmits the transmitter output116to a controller that is remote from the process transmitter100, such as in a remote control room.

In some embodiments, the output circuit110is connected to a controller122over a two-wire loop121, as illustrated in the process measurement system124shown inFIG. 2. In some embodiments, the two-wire loop121is configured to transmit all power to the process transmitter100. In some embodiments, the output circuit110communicates the transmitter output116over the two-wire loop to the controller122by modulating a current flow that varies between 4-20 milliamps. Alternatively, the output circuit110may be configured to transmit the transmitter output116to the controller122wirelessly in a point-to-point configuration, a mesh network, or other suitable configuration with the process transmitter100having its own power source.

In some embodiments, the process transmitter100is in the form of a pressure transmitter, an exemplary embodiment of which is illustrated inFIGS. 2 and 3.FIG. 3is a simplified side view of the exemplary pressure transmitter100with a cross-sectional view of an exemplary isolation unit104, in accordance with embodiments of the present disclosure. In some embodiments, the process102being measured is in the form of a pressure of a fluid or other material that is contained within a pipe130, to which the process transmitter100is attached, as shown inFIG. 2.

In some embodiments, the transmitter100includes a housing131that may contain, for example, the compensation circuit108and the output circuit110, as shown inFIG. 3. In some embodiments, the isolation unit104includes a housing132that can be attached to the housing131. In some embodiments, the housing132includes one or more ports134(FIG. 3) that are connected to the process102through a suitable connection, such as through impulse lines136(FIG. 2), for example. A pressure of the process102is received at each of the ports134.

In some embodiments, the isolation unit104includes one or more fill tubes or isolation tubes138in the housing132. In some embodiments, the housing132includes a flexible diaphragm140for each of the fill tubes138that seals an end142of the fill tube138and isolates the sense element146from the process102. In some embodiments, each of the fill tubes138is filled with a fluid, such as a hydraulic fill fluid.

In the exemplary pressure transmitter100, the process sensor106is a pressure sensor that includes a sense element146and electronics148. The sense element146is positioned at an end150of the one or more fill tubes138. Each diaphragm140deflects in response to a pressure of the process102received through the corresponding port134, which applies the pressure to the fill fluid in the fill tube and communicates the pressure to the sense element146. The sense element146senses the pressure and produces the process signal112as a function of the pressure using the electronics148.

The sense element146may be any suitable sense element for detecting a line or differential pressure. The process or pressure signal112may be generated by the electronics based on changes in an electrical capacitance, changes in electrical resistance, changes in a resonant frequency, or using another suitable technique.

When the pressure transmitter100is in the form of a differential pressure transmitter, as shown inFIG. 3, the pressure sense element146is in the form of a differential pressure sense element, which may include a pair of pressure sense elements, and receives two pressures from the process102through the separate fill tubes138, as shown inFIG. 3. The electronics148of the pressure sensor106generates the process signal112in response to a difference between the two pressures, in accordance with conventional techniques.

As discussed above, the process signal112is communicated to the compensation circuit108, which compensates the process signal112for a response time of the isolation unit104, and produces the compensated process signal114. The following example applies to the pressure transmitter100utilizing the fluid medium111in the fill tubes138to communicate the process pressure to the process sensor106.

It can be shown that for an ideal system, the effect of the fluid medium111in the tube138of the isolation unit104can be modelled as a first-order linear time-invariant system in the time domain as:

where Pin(t) is the pressure of the process102(pressure input signal) and Ps(t) is the pressure at the sensor106. τ can be modelled as:

where R represents the sum of impedances to hydraulic flow in the isolation unit104, such as in the tubes138, and S represents the sum of the stiffnesses of the isolation diaphragm140and the sense element146, shown inFIG. 3.

In the frequency domain Equation 1 becomes:

where s is the complex variable jω. The term

11+s⁢⁢τ
is thus the transfer function of the fluid medium111on the pressure input signal.

Accordingly, the original signal Pin(s) can by recovered by applying the inverse of the hydraulic transfer function, 1+sτ, to Ps(s):

where PHC(s) represents the compensated pressure signal114. This result indicates that with knowledge of the time constant of the fluid medium111, τ, the first order effect of the hydraulics can be fully compensated.

As shown in Equation 2, there are two primary factors that influence the variable τ: S, the sum of the stiffnesses of the isolating diaphragm140and the sense element146, and R, the sum of impedances to hydraulic flow. The stiffnesses of the diaphragm140and the sense element146are largely a function of the magnitude of the absolute value of the pressure to be measured and are fixed by design. Larger pressures utilize larger stiffness values for the diaphragm140and the sense element146, and therefore exhibit smaller time constants.FIG. 4is a chart that illustrates the effect of a stiffness change on the step response of two different isolated pressure sensors106. Both sensors106assume an identical value for R.

Stiffnesses are characteristic of the design of the isolation unit104and the pressure sensor106, and generally do not change significantly under varying environmental conditions. Therefore, in some embodiments, the compensation circuit108uses a constant value for S in Equation 2 for the system124, which may be identified through design or testing.

The second factor, R, in Equation 3 is the sum of hydraulic impedances. This term, too, is largely a function of design: it is derived from factors such as fluid path cross-sectional areas, roughness of surfaces, etc. The different forms of hydraulic impedance take on the generic form
R=Σi=1nkiv=vΣi=1nkiEq. 5)

where kirepresents a constant associated with the ithimpedance factor, n is the number of resistance terms and V represents the kinematic viscosity of the fluid medium111. From Equation 5 we see that the sum of the hydraulic impedances is in fact linear with the kinematic viscosity of the fluid. Thus, while the kiterms are mostly constant over environmental conditions, the viscosity of the fluid medium111plays a significant role in determining R, as it can vary widely over temperature.

FIG. 5is a chart that illustrates the role that temperature plays in affecting the step response of the same pressure sensor106used to generate the response of the smaller pressure sensor in the chart ofFIG. 4.

This behavior can be traced to the aforementioned sensitivity of viscosity to temperature. Viscosity for most fluids is highly non-linear with temperature exhibiting a logarithmic relationship. Fluid viscosities in a cold environment can be 5 to 20 times larger (or more) than at room temperature (fluid type dependent).FIG. 6is a chart that illustrates the change in viscosity of an exemplary fluid medium111in the form of a 5 cSt oil over temperature. Accordingly, embodiments of the pressure transmitter100include the temperature sensor117that provides the temperature of the medium111to the compensation circuit108, which uses the temperature of the medium111to accurately compensate for hydraulic effects of the medium111based on the temperature. The temperature signal118may also be used to compensate for other non-ideal sensor related temperature effects.

Assuming that the temperature effects are negligible (relative to the fluid viscosity) on the stiffnesses and kicomponents, Equation 2 can be re-written as:

The hydraulic compensation transfer function, 1+sτ, from Equation 4 can then be re-written as:
TFHC(s)=1+sτ=1+sK·v(T)  Eq. 7)

Thus, as illustrated in the block diagram ofFIG. 7, the communication of the process pressure parameter at the process102that is detected by the pressure sensor106(i.e., process signal112) is affected by the time constant of the isolation unit104, which is set by the temperature and viscosity of the fluid medium111, and possibly the stiffnesses of the diaphragm140and the sense element146, for example. The compensation circuit108operates to compensate the process signal112to generate the compensated process signal114that reduces the effects of the time constant of the isolation unit104, such as the effects that are based on the temperature of the medium111, by using the temperature signal118.

Application of the transfer function in Equation 7 can be applied anywhere after the sensor pressure signal112, Ps(s), is obtained. When the process transmitter100operates in the analog domain, the compensation circuit108may include an appropriate analog high-pass filter to provide the desired compensation to the signal112and produce the compensated process signal114. In some embodiments, a temperature of the fluid medium111is obtained using the temperature sensor117, and is used to establish the viscosity v(T) of the fluid medium111before being fed to the analog filter in order to provide reasonable levels of compensation.

Preferably, the compensation circuit108operates in the digital domain and converts the pressure signal112from the sensor106using the ADC119to digital form before performing the necessary compensations to produce the compensated signal114. Additionally, the temperature signal118and other compensating terms, are preferably converted to digital form for processing by the compensation circuit108. An example of this method is provided below.

Once the isolation unit104and the sensor106subsystem is designed, its configuration is static. Thus, the stiffnesses, hydraulic pathways etc. that comprise the subsystem are not subject to any fundamental physical change. In this example, while there may be some physical effects due to pressure, these effects will be considered to be insignificant in relation to the effects of temperature on the viscosity of the fluid medium111. Thus, the term K in Equation 7 becomes a parameter defined by the particular sensor subsystem design. The transfer function can then be defined as:
fHC(T)=K·v(T)=τ  Eq. 8)

to represent the time constant, τ, of Equation 7.

The compensation circuit108receives the temperature signal118that indicates the temperature of the fluid medium111, and uses the temperature to determine fHC(T) by fitting the viscosity of the fluid medium111to the temperature, such as by performing a fit for the curve inFIG. 7, for example. This can be done via any standard curve fitting process (look-up table, polynomial fit, etc.). This fit can incorporate the constant K so that the fit result would actually be the desired τ for the hydraulic compensation transfer function. Depending on the accuracy needs, in some embodiments, a “family” curve fit could be employed for this function where the curve fit coefficients would be representative of a particular family of sensor subsystem. Where ultimate compensation accuracy is desired, a characterization of τ vs. T could be done for each individual sensor subsystem.

While the high-pass filter of Equation 7 compensates for the hydraulic damping, in some embodiments, this high-pass gain is limited to less than the Nyquist rate of the digital sampling performed by the ADC119to avoid aliasing high frequency noise into the measurement. This requires the application of a low pass filter with its cutoff frequency at less than half the sampling frequency of the ADC119:

where fsis the sampling rate of the process signal112by the ADC119. Combining this low-pass filter with the hydraulic compensation transfer function gives us our final applied compensation transfer function:

We have substituted in the variable g in Equation 10 with the restriction that it be less than or equal to π to satisfy the Nyquist criterion.

The transfer function of Equation 7 is then converted to a difference equation using well developed Z-transform methods suitable for implementation in the microprocessor of the compensation circuit108. The result is the infinite impulse response filter:

where PHCnrepresents the nth sample of compensated process signal114and Psnrepresents the nth sample of the process signal112at the sensor106. In Equation 11, if one selects g=2 the last term is eliminated while meeting our Nyquist requirement; this action turns the filter into a guaranteed stable finite impulse response filter:

Some embodiments are directed to a method for producing a process transmitter output116. In some embodiments of the method, a process signal112is produced by a process sensor106in response to a process parameter (e.g., a pressure, a temperature, a humidity, etc.) communicated through a medium111of an isolation unit104. The process signal112is compensated for a response time of the isolation unit104, and a compensated process signal114is generated using a compensation circuit108. The process transmitter output116is produced as a function of the compensated process signal114using an output circuit110. In some embodiments, the compensation circuit108compensates the process signal112for the response time of the isolation unit104using one or more of the equations described above, such as Equation 12.

In some embodiments of the method, a temperature signal118is produced as a function of a temperature of the medium111using a temperature sensor117. The process signal112is compensated for the response time of the isolation unit104using the temperature signal118.

In some embodiments of the method, the isolation unit104includes a housing132, a fill tube138, and a diaphragm sealing an end142of the fill tube138, as shown inFIG. 3. In some embodiments, the medium111comprises a fluid in the fill tube138. In some embodiments, the process sensor106includes a pressure sensor or sense element146at an end150of the fill tube. In some embodiments, the process signal112is produced as a function of a pressure communicated to the pressure sensor106or sense element146through the fluid medium111in the fill tube138.

The following are exemplary compensations performed in accordance with embodiments of the present disclosure. In a first example, a step response of a pressure sensor106is hydraulically isolated from the process102by an isolation unit104having a time constant of 50 ms. Also, the process or pressure signal112produced by the sensor106is digitally sampled by the ADC119(FIG. 1) at 100 Hz.FIG. 8is a chart illustrating the pressure at the sensor106(i.e., pressure indicated by the process signal112) and the compensated pressure determined by the compensation circuit108(i.e., the pressure indicated by the compensated process signal114) through the application of Equation 12 to the process signal112.

FIG. 8also includes an inset chart that illustrates the first few data points of the sampled process signal112and the compensated process signal114. As shown, the first few data points of the compensated signal114may not precisely synchronize with the input pressure at the process102due to the sampling frequency. In this example, the delay is approximately 7 ms and is associated with a deadtime due to sampling rate. However, the second sample of the compensated process signal114is substantially synchronized with the input pressure at the process102. By increasing the sampling frequency, it is possible to reduce the deadtime associated with sampling and further improve synchronization with the input pressure at the process102.

In a second example, the system of the first example is used, but the hydraulic time constant of the isolation unit104is increased by an order of magnitude due to a temperature change to 500 ms. The results of the compensation are illustrated inFIG. 9with the inset chart illustrating the first few sampled data points of the compensated signal114. A comparison betweenFIGS. 8 and 9clearly illustrates that the compensated signal114substantially mirrors the input pressure at the process102, and substantially compensates (e.g., within 1, 3 or 10 data points) for the hydraulic time constants of the isolation unit104.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. While specific examples of isolation unit time constant compensation that relate to the communication of a process parameter in the form of a pressure through a medium of the isolation unit, it is understood that embodiments of the present disclosure relate to isolation unit time constant compensation for other types of process parameters that are communicated to process measurement sensors through a medium of an isolation unit. Additionally, while exemplary embodiments of the process transmitter utilize a fluid medium of the isolation unit to communicate process parameters, it is understood that other types of mediums that are capable of communicating process parameters may also be used.