MXPA96004042A - Positioner of e efficient - Google Patents

Abstract

The present invention relates to a valve positioner receiving the power of a process control circuit comprising: an interface circuit coupled with a circuit for receiving a desired valve position, an actuator coupled with a valve for positioning the valve in response to a control output, a microprocessor to calculate a digital control signal, and an analog circuit that provides a control output to the actuator in response to the desired valve position as a function of a control equation implemented in the analog circuit, where the control equation is a function of the digit control signal

Description

"EFFICIENT VALVE POSITIONER" BACKGROUND OF THE INVENTION The present invention relates to a valve positioner for controlling a valve that effects a process variable. More specifically, the present invention relates to a valve positioner having a microprocessor. Various types of positioners are used in the process control industry. Some positioners are mechanically coupled with an actuator while some incorporate the positioner into the actuator. The actuator provides a means for physically positioning the valve and can be electric, hydraulic or pneumatic. The electric actuators have a current signal that drives a motor that places the valve. The hydraulic actuators have means filled with oil to place the valve. Of course, the most common in the process control industry is a pneumatic actuator that has a piston or a combination of a spring and a diaphragm that positions the valve. Depending on the application and the level of control integration the positioners receive various input types from a controller that are representative of the desired valve position. A type is a current input having a magnitude of 4 to 20 mA or 10 to 50 mA, a second is a digital signal superimposed on the current signal and a third is a fully digital input such as Fieldbus or Modbus (R ). Alternatively, the positioner can receive a pneumatic input of .211 to 1,054 kilograms per square centimeter or of .422 to 2.11 kilograms per square centimeter, representative of the desired position of the valve. Depending on the level of integration and application as well, the positioners have different types of outputs. Some positioners provide an output current to an electric drive motor while still others have a hydraulic output in rapid response. The most common type of positioner output is a pneumatic output of 0 to 14.06 kilograms per square centimeter. The positioner, as the word is used in this application, includes all instruments mounted in the field, including the different inputs and outputs, and their respective means to position the valves, if applicable. In the most common case of a spring and diaphragm actuator, the diaphragm is deflected with the pressure supplied by the positioner, thereby exerting a force or torque on a rod of the control valve of the rotating member, respectively, to to change the position of the valve. Many positioners have a mechanical or electronic position sensor to provide a position signal that is fed back to the control section based on the microprocessor of the positioner. Regardless of the specific means for supplying force to place a valve, the positioners that have microprocessors are already known. The existing microprocessor-based positioners provide good dynamic circuit response, but have limited bandwidth so that their use is limited to slow control circuits such as the one that controls the level in a tank or the temperature in a reactor. With the introduction of microprocessors in the valve positioners, the microprocessor is required to carry out considerably complex functions, resulting in higher total power requirements for the positioner. In control systems that use, for example, the 4 to 20 mA, 10 to 50 mA, Fieldbus and Modbus (R) protocols, the operating power of the positioner is limited, since the positioner operates exclusively on the power supplied through the control circuit. This limitation has placed an upper limit on the amount of the valve and the clock frequency (dynamic response) of the microprocessor in the valve positioner. As technology has advanced, it has become possible to obtain more computing power from a microprocessor without increasing the electrical power requirements. However, the desire to increase the control and operation of the valve positioners has exceeded the capacity of the microprocessor to supply the requirements under the aforementioned electrical power restrictions. A low power positioner with wide bandwidths under computer control is what is needed.

COMPENDIUM OF THE INVENTION A valve positioning assembly includes a valve positioner that provides a control pressure to a pneumatic actuator operably coupled to a valve. The valve positioner has an interface coupled with a process control circuit to receive a desired valve position and an analog PID controller that generates a control output to control the value, based on the desired valve position and a position of valve detected. A microprocessor calculates the PID constants, each of which controls a variable impedance of the analog PID controller. The digital control signal is stored in a hook and is periodically updated by the microprocessor. In one embodiment, the first and second power supply devices provide the power to the positioner as received through the control circuit. The supply devices are connected in series.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a functional diagram of a control circuit including a valve positioner according to the invention. Figure 2 is a functional diagram of the valve positioner of Figure 1. Figure 3 is a functional diagram of the control circuit of Figure 2. Figure 4 is a simplified electrical schematic diagram of a PID controller of Figure 3 Figure 5 is a simplified electrical schematic diagram of the power supply circuit used to energize the valve positioner of Figure 2.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES Figure 1 shows a process control circuit generally in which typically includes a transmitter 12 of two wires that detect the process variable representative of the process 14, a controller 16 that can be in a room 18 of control and receiving the process variable detected from the transmitter 12, and a valve positioning set 20 that receives a desired valve set point from the controller 16 and controls a physical variable in the process 14. A transmitter 12 provides the variable of process detected through a first two-wire circuit formed by the wires 22a and 22b, and the controller 16 communicates with the positioning assembly 20 through the circuits 24a and 24b. A valve positioning circuit 26 in the set 20 of the positioner receives a current of 4 to 20 mA representative of the desired valve position and provides a pneumatic output 28 of .211 to 1,054 kilograms per square centimeter to the pneumatic actuator. The actuator 30 controls the valve 32 which controls a flow in the process 14. The actuator 30 provides its pressure and position supply 34 to the positioner 26. In Figure 2, the positioner 26 includes a communications interface 40 for receiving a signal of current of 4 to 20 mA representative of the desired position of the valve 32 from the controller 16 through the wires or a digital set point 24a / 24b. Interface 40 also provides power supply outputs VS1 and VS2 generated from circuit 24a / 24b. The circuit 24a / 24b can also be a three or four wire circuit as used in the process control industry. The pneumatic outlet 28 of the positioner 26 is connected to the valve 32 through the actuator 30 so that the applied pressure biases the diaphragm 42 which exerts a force on the spring 44 and moves the rod 46 inside the valve 32. The plug 48 it coincides with a rod 46 to slideably obstruct a passage in the valve 32 and to control the flow of fluid in the line 50. Other types of valves such as the rotary valves can also be used. The microprocessor 52, which is synchronized by the oscillator 56, includes a program memory 54 in the pickup and storage and is preferably manufactured from a low power technology such as CMOS. The microprocessor 52 is connected with a decoder and the decoding circuit 58 and an analog-to-digital (A / D) converter 60. The analog-to-digital converter 60 is coupled to the MUX multiplexer 62.

The summing node circuit 64 is coupled to the interface / power supply 40 and receives a signal representative of the desired valve position of the interface 40. The summing node 64 receives three signals from the latching and decoding circuit 58: control of zero reach and entry elimination. The summing node 64 provides a position request signal to the controller 66. The control circuit 66 receives four digital control constants from a latching and decoding circuit 58 representative of the proportional gain signal (P), the integral (I) ), and the derivative (D), together with a position request adjustment signal. The PID control equation that implements the analog circuit 66 is provided by: Equation 1: r ddee ((tt); Control output (Command) = P-e (t) + I \ e (t) dt + D * + C0 dt where e (t) is the difference between the position of the desired valve and the position of the current valve, t is the time, and CQ is the desired control output (command). The control circuit 66 also receives the feedback 34 from the actuator position and pressure feedback circuits 68 and 76. The control circuit 66 provides a control signal to the drive circuit 70 which also receives a pneumatic biased control input from the circuit 58. The I / P transducer 78 (which includes a boost stage) regulates the pressurized air from the source 80 pneumatic as a function of the driving voltage signal. The position sensor 82 engages with the valve stem 46 and the temperature sensor 84 is positioned proximate the valve stem 46. The signals from the sensors 82 and 84 are provided to the multiplexer 62 through the position interface 86 and the temperature interface 88, respectively. The circuit 68 sends a position signal compensated in temperature to the control circuit 66. The pressure interface 76 of the actuator receives a feedback signal 34 from the pressure sensor 90, which measures the pressure in the pressure line 28 and provides an input to the multiplexer 62 and feedback to the control circuit 66. The microprocessor 52 periodically samples the inputs to the MUX 61 through the A / D converter 60. There is a continuing need to improve the accuracy with which the control valves are placed. Existing valve positioning devices with a microprocessor allow for the placement of an improved control valve. In addition, the microprocessor can be used to monitor the operation and diagnosis of the control valve. The benefits of an electronic microprocessor-based positioner are extensive and provide many advantages to control the operation of a valve. In the present invention, energy reduction is achieved by incorporating an analog circuit controlled by the microprocessor 52. The analog circuit performs certain of the intense calculation functions (notably the PID control equation) carried out otherwise by the microprocessor 52, thereby allowing the microprocessor 52 to perform other functions without sacrificing the bandwidth of the control system. Therefore, the invention provides PID control of low energy at high speed and broad bandwidth. The microprocessor 52 periodically updates the constants of the PID equation. In one mode, there are four cases that affect the update rate: calibration, start, constant state operation, and response to transient currents. During start-up, an initial set of constants is selected based, for example, on the previous starts or predetermined ones. During constant state operation, constants are updated infrequently but with a high level of accuracy and control. When responding to sudden changes, the constants are updated frequently. Typically, when constants need frequent updating, its accuracy is less important and therefore providing faster processing. For example, updates can occur every 0.1 to 0.25 seconds. As the values more closely resemble their final value, updating may be less frequent allowing the microprocessor 52 a greater time interval for greater accuracy. The analog circuit includes the sum / set and zero adjustment node circuit 64, the control block 66, the position feedback circuit 68, the driving circuit 70 and the diagnostic circuit 72. The control equation implemented in the analog circuit is determined by the microprocessor 52 which provides digital control signals to the circuit. The circuit 66 outputs a control output in accordance with Equation 1. Figure 3 is a simplified schematic diagram showing a portion of the control circuit 66. The circuit 66 includes a summing node 100, the PID circuit 102, and the variable gain circuit 104. The summing node 100 receives a representation of a desired valve position from the circuit 64, an adjustment input from the latching and decoding circuit 58 which allows the microprocessor 52 to adjust the request, and a position feedback signal from the circuit 68. The node 100 adds these signals and provides them to the PID circuit 102. The PID circuit 102 receives the digital control signals P, I and D engaged from the circuit 58 and provides a control signal output to the variable gain circuit 104. The gain of the circuit 104 is controlled by a pressure feedback output from the pressure feedback circuit 76 and provides a control signal output to the driver 70. Figure 4 is a simplified schematic diagram of the PID circuit 102 in accordance with the present invention. The PID circuit 102 includes the operation amplifier 110, the analog switch 112, the analog switch 114 and the analog switch 116. The switches 112 to 116, respectively, control the constants of P, I and D of the circuit 102. The reversing input of the OPAMP 110 is connected to the resistor 118 which is connected to the node 100 through the resistor 120. The input of non-inverting the OPAMP 110 is coupled to the ground of the signal and the output is coupled to the impeller 70. The analog switch 112 is connected in series with the capacitor 122 through the resistor 120. Analog switches 112 to 116 include the resistors R_ to Rg and the transistors Q ^ a Qg of field effect. The switch 116 controls P, the switch 114 and the capacitor 123 control I, and the switch 112 and the capacitor 122 controls D. The gate circuits of the FETs, Q] _ to Qg are coupled with the circuits 58a, 58b and 58c of the hooking and decoding circuit 58 shown in Figure 2. The decoding lock circuits 58a, 58b and 58c receive the inputs P, I and D of the microprocessor 52. For example, the decoder 58c receives a digital value from the constant (D) derived from the microprocessor 52. The decoding and latching circuit 58c decodes this value towards a combination of analog voltage signals that is provided to the circuits being shielded from the transistors Qi to Qg, selectively controlling in this way whether each of the transistors Qi to Qg is connected or disconnected. Circuit 58c of the decoder and latch engages this data after the decoding step so that the microprocessor 52 does not continuously supply the digital control signals to the decoder 58c. Based on the combination of the connected transistors Q ^ to Qg, the resistance that is provided by the analog switch 112 is controlled to determine the derived constant of the PID circuit 102. The decoders 58a and 58b work in a similar manner whereby the proportional (P) and integral (I) constants are sent by the microprocessor 52 and selectively modify the resistors 114 and 116, respectively. The valve positioner 20 operates using a hybrid of the digital and analog circuit and thus achieves advantages of both types of circuit. The analog circuit performs the majority of the control that high speed and wide bandwidth requires so that the microprocessor 52 operates at reduced power. However, the microprocessor 52 controls the operation of the analog circuit through the latching and decoding circuit 58 such that the control of the valve 32 is not sacrificed. Referring to FIG. 2, the microprocessor 52 receives a position request. desired from the two-wire control circuit 24a / 24b and the feedback from the analog-to-digital converter 60. Based on these inputs, the microprocessor 52 configures the analog circuit by supplying the digital control constants to the latch and decode circuit 56. Figure 4 is an example of the PID circuit 102 operating under the control of the microprocessor 52. The microprocessor 52 also controls the sum / zero and range node circuit 64, the rate of the drive driver 70, and the request setting from the position to the circuit 66 of Figure 3. Although the specific example shows the adjustment of a resistance value through the switches 112 to 116, other analog parameters can be adjusted by the microprocessor 52 such as current, inductance, capacitance , etc., using a similar circuit in accordance with the invention. The normal circuits in most journals of engineering applications can be controlled using the present techniques. For example, a voltage controlled gain circuit is controlled, providing a voltage output from the A / D converter 60 to the input voltage of the circuit. Figure 5 is a simplified diagram of the valve positioner 26 showing a power supply connection in accordance with the present invention. The positioner 26 is shown receiving all electrical power through the circuit 24a / 24b and does not require an additional power source. Figure 5 shows the interface 40 which includes the input / output circuit 140, a power supply 142 of 4 volts and a power supply 144 of 9.5 volts connected in series with the two-wire circuit 24a / 24b. The four volt power supply 142 provides the voltage VS1 for the operation of the microprocessor 52 and another digital circuit. The supply 144 provides VS2, a regulated power supply of 9.5 volts for the analog circuit. The I / O circuit 140 receives the desired position signal through the circuit 24a / 24b in accordance with the techniques known in the process control industry. In the embodiment shown in Figure 5, the decoding circuit 58 and the sensor interface circuit are energized by the power supply 144 of 9.5 volts. The circuit of Figure 5 shows "stacked" energy supplies using almost the maximum of 15 volts allowed between wires 24a and 24b. If the stacked power supply shown in Figure 5 is employed, a level shift circuit (not shown) is necessary to shift the levels of the various signals. Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes in shape and detail can be made without departing from the spirit and scope of the invention. For example, actuators other than actuators based on pneumatic operation may be employed and the decoding circuits 58 may comprise a ROM.

Claims (12)

CLAIMS:

1. A valve positioner receives the energy of a process control circuit comprising: an interface circuit coupled with a circuit for receiving a desired valve position; an actuator coupled with a valve for positioning the valve in response to a control output; a microprocessor for calculating a digital control signal; and an analog circuit that provides a control output to the actuator in response to the desired valve position as a function of a control equation implemented in the analog circuit, wherein the control equation is a function of the digital control signal.

2. The valve positioner according to claim 1, wherein the control equation is a PID equation of the form: r of (t) Command Output = Pe (t) + I [e (t) dt + D- + C0 dt where e (t) is the difference between the desired valve position and the current valve position, t is the time, and Co is the desired control output (command).

The valve positioner according to claim 1, wherein the analog circuit includes a switch that responds to the digital control signal in order to selectively couple the analog circuit with a preselected electrical component.

The valve positioner according to claim 1, including a latch coupled with the microprocessor to engage the digital control signal, wherein the microprocessor periodically sends an updated digital control signal and activates the latch to engage the signal of updated digital control.

The valve positioner according to claim 1, which includes a sensor that detects a functioning parameter of the operation of the valve and provides feedback to the analog circuit.

The valve positioner according to claim 1, which includes a power supply circuit comprising: a first power supply coupled to the control circuit providing a first regulated voltage output to the first power circuit in the positioner valve; and a second power supply coupled with the control circuit and the first power supply provides a second regulated voltage output to the second power circuit in the valve positioner.

7. A valve positioner for use in a process control circuit comprising: an interface adapted to couple with the circuit to receive a desired valve position and electrical power to operate the valve positioner from the circuit; an actuator adapted to place a valve in response to a control output, a microprocessor circuit for calculating the values of at least one constant in a PID control equation; and an analog PID control circuit that provides the control output to the actuator based on the PID control equation configured by the PID constant, which is provided by the microprocessor.

8. The valve positioner according to claim 7, wherein the PID control equation is of the form: / of (t) Command output = P-e (t) + I (e (t) dt + D- + CQ) dt where e (t) is the difference between the desired valve position and the current valve position, t is time, and Co is a desired control output (command).

The valve positioner according to claim 7, which includes a switch that responds to the PID constant that is provided by the microprocessor in order to selectively couple the analog PID control circuit with a preselected electrical component.

The valve positioner according to claim 7, including a coupling coupled with a microprocessor for engaging the PID constant, wherein the microprocessor periodically sends an updated PID constant that is retained in the latch.

11. The valve positioner according to claim 7, which includes a sensor that detects a functioning parameter of the operation of the valve and that provides feedback to the analog PID control circuit. The valve positioner according to claim 7, which includes a power supply circuit comprising: a first power supply coupled to the control circuit providing a first regulated voltage output to the first power circuit in the positioner of valve; a second power supply coupled with the control circuit and the first power supply providing a second regulated voltage output to the second power circuit in the valve positioner.