Electro-pneumatic converter linearization

Abstract: A temperature compensation method for a microprocessor based electro-pneumatic converter device (10) including an initial calibration sequence, a compensation sequence, and an operation sequence. Linearization of an electro-pneumatic positioner including the steps of adjusting the positioner feedback linkage (30) until a predetermined reference position is attained, reading a position feedback value and determining the required feedback linearization value to achieve a linear relationship between the output/input.

Description: This invention relates to electro-pneumatic converter devices and in particular to microprocessor based current to pressure transducer/positioner devices. Background Of The Invention Electro-pneumatic converters, such as current to pressure transducers are in common use as field instruments mounted in pipeline systems for controlling the process fluid. Accordingly, these devices are installed in potentially hazardous explosive environments as well as a variety of temperature conditions. Such devices generally receive a variable electrical input signal (i.e. a variable current input signal of between 4-20 mA or a variable voltage input signal of between 1-5 V) and eventually provide a variable pressure output to an actuator for a fluid control valve or other similar control device. In addition, such electro-pneumatic devices require calibration which entails making manual mechanical adjustments to linkages or potentiometers or some combination thereof, typically on the "bench", before installation of the units, to attain a degree of static accuracy. Normally, for instance, there is a linearly moving valve actuator arm and a non-linearly moving position sensing device (a rotating potentiometer, for instance) which are joined by a positioner feedback linkage. When mounting these units in the field, any slight misalignment between the benchset and the field mounting will reduce the linearity and therefore the accuracy of the device in operation. Furthermore, to initially calibrate or to recalibrate such electro-pneumatic devices, it is required that covers and protective elements of the device be removed to allow access to the adjusting components. This can be inconvenient, particularly in the case of explosion proof installations that require a major effort to obtain permission from the plant supervisor so that the explosion proof seals and joints can be violated while making the necessary adjustments. Another potentially damaging aspect is that every time the device needs recalibrating, the internal components will be exposed for a period of time to the plant environment in which the device is located. This can lead to degradation of the components and eventual reduction in the reliability of the device. Accordingly, it is desired to provide an improved microprocessor based electro-pneumatic device which can be adapted for calibration purposes. It is further desired to enable remote calibration of the microprocessor based electro-pneumatic device so that no mechanical adjustments or intrusions to the inside of the instrument need to be made for recalibration. It is also desired to provide an easier more flexible method of initially calibrating the device or to accommodate a wider range of process control applications to which the device might be utilized. It is another desire to provide an electro-pneumatic device which may be calibrated with greater accuracy and wherein no inaccuracy is added to the linearization process such as presently occurs in prior art linearization procedures. EP-A-0 294 918 discloses an electro-hydraulic servo system which has automatic calibration capabilities, includes a microprocessor which assesses calibration parameters stored in a non-volatile memory unit. EP-A-0 075 507 discloses an electro-hydraulic servo system with a linearized transfer function. US-A-3 753 350 discloses a reversible hydraulic actuator with selectable fail/safe operation, in particular a hydraulic actuator where the movement of piston is coupled through chain, pulley and shaft to mechanically adjust feedback potentiometer. Summary Of The Invention Thus, in accordance with the invention, there is provided a post mounting linearization method according to claim 1. In accordance with the invention, a post mounting linearization method for an electro-pneumatic converter device such as a positioner ("post mounting" referring to the mechanical assembly or mounting of the positioner to the valve actuator). In accordance with this aspect of the present invention, a post mounting linearization method is provided to linearize the output of the feedback sensor normally associated with a rotating potentiometer in non-linear movements with respect to movements of a linearly moving valve actuator stem, where the feedback sensor potentiometer and the valve stem are interconnected by a positioner feedback linkage. In particular, the linearization method of the present invention includes mounting one end of the positioner feedback linkage to the non-linearly moving feedback position sensor and the other end of the positioner feedback linkage to the linearly moving fluid valve stem. The position of the feedback linkage is then adjusted to a predetermined reference position. A position feedback value is then read from the feedback position sensor and a position feedback linearization reference value is determined sufficient to align a compensation algorithm to the feedback linkage non-linearity and to thereby linearize the configuration, with the reference value being stored in memory. The compensation algorithm required is determined by the specific feedback linkage geometry and is easily derived by one skilled in the art. Adjusting of the position of the positioner feedback linkage to a predetermined reference position can be provided by incrementing the positioner output pressure utilizing the electrical signal input until the predetermined reference position is reached; or adjusting an independent pressure source to the actuator until the positioner feedback linkage is at the predetermined reference position; or mechanically adjusting the positioner feedback linkage to the predetermined reference position. Alternatively, the linearization can be provided by initially storing a pre-established position feedback linearization value in memory; mounting the positioner to the actuator; varying the positioner electrical signal input and independently measuring the actuator position to determine linearity; and adjusting the position feedback linearization data as required and storing this value in memory. Brief Description Of The Drawings The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the several figures and in which: Figure 1 is a front elevational view illustrating a current to pressure transducer/positioner; Figure 2 is a cross-sectional view taken along section lines 2-2 of the current to pressure transducer/positioner shown in Figure 1, with certain components removed for clarity; Figure 3a is a schematic block diagram of a current to pressure positioner in accordance with the principles of the present invention; Figure 3b is a schematic block diagram of a current to pressure transducer in accordance with the principles of the present invention; Figure 4 is an information flow diagram of the steps involved in temperature compensation of the current to pressure transducer/positioner for illustrative purposes only; Figure 5 is an information flow diagram showing the steps involved in alternative methods for providing post mounting linearization in accordance with another aspect of the present invention; and Figure 6 is an information flow diagram showing the steps for calibrating and automatically recalibrating a current to pressure transducer/positioner for illustrative purposes only. Detailed Description The present invention is illustrated and will be described in connection with a preferred embodiment comprising a current to pressure transducer/positioner. It is to be understood that the teachings herein can as well be applied to other electro-pneumatic converter devices to solve problems similar to those which are solved by the present invention. Referring now to Figures 1 and 2, there is illustrated a current to pressure transducer/positioner having a housing 12 with one portion forming an enclosure defining a hollow interior 14. The housing 12 includes a field terminal box portion 16 including a field terminal strip 18 for suitable connection to an electrical signal cable for receiving a current control signal from a distributing control system, so as to for instance monitor a process. End cap 20 is removable from the housing so that the appropriate cable wiring connections can be made to terminal 18. Housing 12 also includes an inlet 22 for receiving a supply pressure from a pneumatic supply source, and an outlet port 24 through which the output pressure can be suitably coupled to a positioner or directly to a valve actuator. Typically, in response to a variable 4-20 mA current control signal, current to pressure transducer/positioner 10 provides a variable pressure output at outlet 24. Device 10 contains the electrical components and the pneumatic components required for operation of the device as a current to pressure transducer/ positioner. Typically, this will consist of a current to pressure converter device such as a I/P nozzle block 46 having a flapper for converting the variable current control signal input into a variable nozzle pressure signal; a pressure relay 48 receiving the variable nozzle pressure signal and providing a variable pressure output on outlet 24; a pressure gauge 50 monitoring the supply pressure coupled to inlet port 22; a second pressure gauge 52 monitoring the pressure output on outlet 24; and electronic equipment such as a pressure sensor and a printed circuit board with circuitry to process the electrical signals as required. A feedback position sensor potentiometer 26 has a shaft 28 coupled to positioner feedback linkage 30 which in turn is coupled to a fluid valve stem 32 for sensing the position of the valve stem being driven by a valve actuator 34. Reference may be made to Figures 3a and 3b, wherein there are illustrated block diagrams showing current to pressure positioner/transducer respectively in the overall configuration with a microprocessor 36, an analog to digital converter 38, and a digital to analog converter 40 to provide a pressure output signal: on output port 24 of the I/P converter or position output of valve stem 32 of the I/P positioner. The device includes input sensors for monitoring input values of temperature, current, pressure or position as shown in Figures 3a and 3b. Referring to Figure 4, there is illustrated a temperature compensation method for all input functions of the transducer or positioner incorporated for illustrative purposes only. The method includes measuring of the internal temperatures as well as the various input values at various points over a specified operating range and storing compensation values so that in operation the device can correct for all temperature effects. In particular, Figure 4 shows a calibration sequence, a compensation sequence, and an operation sequence. In the calibration sequence, a first temperature is applied to the A/D converter 38 and predetermined known input values of current and pressure also are applied to the A/D converter. Next, the actual temperature is sensed and read and in the next step a correction value is determined for the various inputs and stored. In the preferred technique for determining the correction values, for each input one ...N (i.e. current, pressure, etc.), a comparison is made between the input value at the present temperature versus the input value at a reference temperature; a correction value or difference is calculated; and this correction value is stored. This determination of the correction value sub-sequence is continued by applying the next or second temperature as illustrated in the calibration sequence of Figure 4 until all the temperatures have been completed. Accordingly, there has been thereby compiled a table of correction values with known calibration temperature relationships for the particular device. Subsequently, after the electro-pneumatic converter device has been installed in the field, a compensation sequence of steps shown in Figure 4 can be performed to compensate and thereby calibrate the device for the present temperature. As illustrated in Figure 4, in the compensation sequence, the present temperature is sensed and read. Next, the correction value for the various inputs at the present temperature is determined from the table, and the active correction value for the various inputs is stored for later use. One technique for determining the correction value for the particular input at the present temperature is to read the correction values stored in the memory for two temperatures closest to the present temperature and interpolating an active correction value at the present temperature. In the operation sequence shown in Figure 4, the stored active correction values determined during the compensation sequence are read from memory for the required input and applied so that the transducer/positioner device can be operated using the corrected inputs. Reference may be made to Figure 5 which along with Figure 3 shows a method for obtaining linearization of the output of the feedback sensor for corresponding movements of the valve stem. For instance, with reference to Figure 3a, it can be seen that the pressure output on output port 24 may actuate a valve actuator 34 which as schematically illustrated drives a valve stem 32 in linear movement. The movement of valve stem 32 is coupled by position feedback linkage 30 coupled to shaft 28 of a feedback potentiometer 26 for indicating position values into the A/D converter 38. As shown in Figure 3a, because of the non-linearity produced by the feedback linkage, some form of linearization must be provided. The present invention not only readily provides such linearization, but can do so with a minimum of effort so as to reduce the overall set up time compared to prior art devices. With reference to Figure 5, four methods are illustrated in Figures 5(a) through 5(d). In the preferred method of Figure 5(a) there is indicated that the first step is mounting the positioner to the actuator which includes interconnecting valve actuator 34 and valve stem 32 with feedback linkage 30 and the feedback potentiometer 26. Next, the pressure output on line 24 is incremented by changing the input current signal until the positioner feedback linkage 30 is at a predetermined reference position which is at 90° to the valve stem. The reference axis line corresponding to the predetermined reference position is marked as line 42 in Figure 3a. The position feedback value is sensed by the position sensor and inserted so that the correct linearization value can be determined and stored in memory. The position feedback linearization value provides alignment of the linearization algorithm to the feedback linkage non-linearity so that the processed output of the positioner feedback linkage will provide an accurate linear reading of the actual valve position. Figures 5(b) and 5(c) show alternative methods for adjusting of the positioner feedback linkage until the linkage is at the predetermined reference position as shown in Figure 3. Thus, rather than incrementing the positioner output pressure as set forth in the method steps of Figure 5(a), in Figure 5(b) the adjustment is made using an independent pressure source. In Figure 5(c) the adjustment of the positioner feedback linkage is made by a mechanical adjustment. In the alternative method of linearization shown in Figure 5(d), initially, a pre-established position feedback linearization value is stored in memory, and next the positioner is mounted to the actuator with the feedback linkage. Then, the positioner input is varied and the actuator position is independently measured to determine linearity. From the measured non-linearity, one can determine a corrected compensation - position feedback linearization value and an adjustment is then made in the pre-established value and stored in memory. It is to be understood, that while the linearization method of Figure 5(a) has been indicated as the preferred embodiment, the alternative linearization methods shown in Figures 5(b) - (d) can be chosen for particular usage and with certain advantages as desired. Referring to Figure 6, there is illustrated a calibration procedure and a configuration (independent of calibration) procedure for electro-pneumatic positioners or transducers for illustrative purposes only. Figure 6 illustrates an initial calibration sequence when utilizing the microprocessor based current to pressure transducer/positioner 10 shown in Figures 3a and 3b. Initially, a reference input current, etc. is applied and the sensor inputs are read and stored in memory. Next, the drive values are determined for the unit which are required to achieve predetermined feedback values and from these drive values, positioning or pressure algorithm parameters are derived and stored in memory. Once the unit has been calibrated, the configuration values can be adjusted to the particular gain, and range values desired for the operation. Accordingly, the desired gain, action, input and feedback range values are communicated and stored. Next, during operation, the independent use of configuration and calibration data is obtained in accordance with another aspect of the present invention. As shown in Figure 6, initially the calibration values and the configuration values are read from memory. Next, the servo algorithm parameters are derived using the calibration and configuration values and the servo algorithm parameters are stored. Execution of the servo algorithm can then be provided using the parameters from the prior step. For illustrative purposes, it will be appreciated that the input signal range or pneumatic output pressure or position range can be changed without recalibration of the control device. In addition, the gain of the servo algorithm used to control pneumatic output pressure or position can be changed without recalibration of the control device. As can be seen from Figure 6, this is provided by the table of stored values containing known calibration relationships between input and output as initially determined in the calibration sequence of Figure 6. Configuration values can be manipulated by the user in the configuration sequence without changing the known calibration relationships, such as changing the range of input - output relationship without the need for recalibration by deriving the required servo algorithm parameters in the operation sequence shown in Figure 6. The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

Claims: A post mounting linearization method for linearizing the mechanical linkage in an electro-pneumatic postioner device having a microprocessor receiving an electrical input signal and a current to pressure converter coupled to the microprocessor for providing an output pressure signal, said electro-pneumatic converter device output coupled to a linearly moving fluid valve stem through an intermediate positioner feedback linkage coupled at one end to the linearly moving valve actuator stem and at the other end to a non-linearly moving feedback position sensor whose sensing position corresponds to the valve actuator stem position, said method comprising the steps of: mounting one end of said positioner feedback linkage (30) to said non-linearly moving feedback position sensor (26) and the other end of said positioner feedback linkage (30) to said linearly moving fluid valve stem (32); adjusting the position of said positioner feedback linkage (30) to a predetermined reference position; reading the position feedback value from said feedback positon sensor (26); determining a position feedback linearization value; and storing said position feedback linearization value. A post mounting linearization method for an electro-positioning device according to Claim 1, wherein the step of adjusting includes incrementing said positioner device until said positioner feedback linkage is at said predetermined reference position. A post mounting linearization method for an electro-positioning device according to Claim 1, wherein the step of adjusting includes mechanically adjusting the positioner feedback linkage to said predetermined reference position.

Inventors: GASSMANN GEORGE W, GRUMSTRUP BRUCE F, SEBERGER STEPHEN G, GASSMANN, GEORGE W., GRUMSTRUP, BRUCE F., SEBERGER, STEPHEN G.
Applicants: FISHER CONTROLS INT, FISHER CONTROLS INTERNATIONAL, INC.
Application year: 1993
Main classification: G05D16
IPC codes: G05D16, F15B19, G05B19