WO2000023859A1

WO2000023859A1 - Power positioner with digital communication

Info

Publication number: WO2000023859A1
Application number: PCT/US1999/023786
Authority: WO
Inventors: Alvin R. Steffen; Douglas E. Simmers
Original assignee: Rosemount Analytical Inc.
Priority date: 1998-10-17
Filing date: 1999-10-13
Publication date: 2000-04-27
Also published as: AU1445000A

Abstract

A pneumatic power positioner (24) is provided with a loop communication module (42) which is couplable to a process communication channel (32) and adapted for digital communication. The power positioner (24) is also couplable to an air supply (26) such that the power positioner (24) can effect actuation based upon digital signals received from the process communication channel (32).

Description

POWER POSITIONER WITH DIGITAL COMMUNICATION

BACKGROUND OF THE INVENTION The process control industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gases in chemical, pulp, petroleum, pharmaceutical, food and other food processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process variable transmitter provides an output related to a sensed process variable over a process communication channel to a controller such that the process can be monitored and controlled. The controller can then issue a command across the process communication channel to a process control device, such as a pneumatic power positioner to cause the device to effect a physical change upon the process. For example, the physical change can entail actuating a valve or damper to a desired position to modify or control an industrial process such as combustion.

Pneumatic power positioners are used by a variety of industries to provide quick and accurate positioning when required. For example, pneumatic power positioners have been used in a variety of applications ranging from steel mills to refineries. Pneumatic power positioners generally include a pneumatic cylinder which can range in size from approximately 2 1/2 inches in diameter to about 12 inches in diameter. Stroke lengths of such cylinders in the power positioners range from about 5 inches to about 48 inches. Pneumatic power positioners can provide linear or rotary actuation. A rotary pneumatic power positioner can generate torques up to and including 4,600 ft. - lbs. (6238 Nm) , while linear pneumatic power positioners can generate a thrust of 1950 lbs. (885 Kg) or more. Such positioners can be adapted to mount on walls, swivels or floors. Power positioners have been limited with respect to communication over the process communication channel. Specifically, known power positioners can receive inputs in the form of 3-15 or 0-30 psig and 4-20 mA as well as a voltage input, for example ranging between 1-5 volts. As technology progresses and process control becomes more complex, there is an ever increasing need to provide more information about the process, as well as the devices coupled to the process communication channel. Further, as control of the process becomes more distributed there is a need to provide more flexible communication.

SUMMARY OF THE INVENTION A pneumatic power positioner is provided with a loop communication module which is couplable to a process communication channel and adapted for digital communication. The power positioner is also couplable to an air supply such that the power positioner can effect actuation based upon digital signals received from the process communication channel . BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagrammatic view of process control system 10 illustrating the environment of power positioners in accordance with the embodiments of invention. Fig. 2 is a system block diagram of a power positioner 40 in accordance with an embodiment of the invention.

Fig. 3 is a perspective of a blower in which embodiments of the invention are useful. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the invention will be described with reference to embodiments of pneumatic power positioners, 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. For example, the invention will be described with respect to pneumatic rotary power positioners controlling air flow in blowers, however other types of pneumatic power positioners such as linear power positioners actuating any variety of industrial machinery are contemplated. Further, although the invention will be described with respect to communicating enhanced control function data and diagnostic data, a power positioner in accordance with an embodiment of the invention can also digitally transmit information related to position and/or speed of the actuating member. Further, a positioner in accordance with an embodiment of the invention can be responsive to various process conditions to enhance process safety or reduce process cost. Further still, although embodiments of the invention will be described with respect to various modules, such description is provided for clarity since various modules can be incorporated on circuitry such as an application specific integrated circuit, or embodied in software executed by a microprocessor.

Fig. 1 is a diagrammatic view of process control system 10 illustrating the environment of embodiments of the invention. Blower 12 generates an air flow in duct 14 in the direction of arrows 16. Dampers 20 are interposed between blower 12 and duct 16 to selectively control air flow through duct 16. Dampers 20 are operably coupled to linkage 22 which couples to power positioner 24. Power positioner 24 is also coupled to an air supply 26 through air conduit 28. Further, power positioner 24 is coupled to controller 30 through process communication channel 32. Power positioner 24 includes actuating arm 34 which pivots about pin 36 to actuate linkage 22. In operation, power positioner 24 receives an input signal from controller 30 through process communication channel 32. The input signal can be, for example, a 4-20 mA signal; a 1-5 VDC signal; a 1-9 VDC signal; or a 0-10 VDC signal. Alternately, power positioners such as power positioner 24 can receive an input signal in the form of air pressure such as a 0-30 psi signal or a 3-15 psi signal. An air cylinder within power positioner 24 receives a quantity of air from air conduit 28 which is responsive to the input signal. The air cylinder is coupled to actuator arm 34 such that actuation of the air cylinder causes displacement of actuator arm 34. In this manner, power positioner 24 relates an input signal to specific output displacement. For example, an input signal of 4 mA may correspond to one angular displacement of actuator arm 34, while an input signal of 20 mA may correspond to another angular displacement of actuator arm 34. Thus, process control systems are able to actuate and control large machinery such as blower 12 based on control signals such as in a 4-20 mA loop.

As will be described in greater detail, one aspect of embodiments of the invention includes providing a power positioner such as power positioner 24 with enhanced processing electronics and communication electronics such that power positioner 24 may communicate digitally upon process communication channel 32. Examples of such digital communication include employing communication protocols such as HART^®, Devicenet, Foundation™ Fieldbus (Fieldbus) , Modbus, Profibus, and Ethernet. Further, such digital communication includes transmitting and receiving information through wires; fiberoptic media; and via wireless methods such as infrared and radio-frequency. Thus, a process communication channel can include electronic media, fiberoptic media, or wireless media. FIG. 2 is a system block diagram of a power positioner 40 in accordance with an embodiment of the invention. Power positioner 40 includes digital communication module 42, first proportional-integral- derivative (PID) module 44, second PID module 46, diagnostics module 48, historian module 50, positioner transducer 52 and positioner element 54. Power positioner 40 is any device which provides actuation with an air pressure acting upon an air cylinder based at least in part on digital signals received by the power positioner. As such, those skilled in the art will appreciate that a power positioner in accordance with embodiments of the invention can be used for a variety of applications including modulation of dampers (such as dampers 20 shown in FIG. 1) , fan vanes, oscillating burners, diverters, and other suitable applications. Further, a power positioner in accordance with an embodiments of the invention may be used for speed control as well -as position control.

Loop communicator 42 is adapted to couple to a process communication channel. Communicator 42 is any configuration of hardware, software, or both which is adapted for digital communication with external devices either through media connected to terminals 56, or via wireless technology. Communicator 42 can include a microprocessor for enhanced control and communication functions. FIG. 2 shows two terminals 56 which are adapted to couple to a two-wire process communication channel in accordance with an aspect of the present invention. However, as disclosed above, various other communication methods and protocols are useful for the invention. Thus, if digital communication module 42 is to communicate on an ethernet, a suitable number of terminals 56 would be provided to couple communicator 42 to the ethernet. Further, if digital communicator 42 is to communicate wirelessly, then communicator 42 itself would be adapted for such wireless communication and terminals 56 need not be used. For example, if communicator 42 is to communicate in accordance with the HART^® protocol which can utilize a digital signal superimposed upon an analog signal, then communicator 42 is adapted for such communication, through hardware, software, or a combination of the two.

First PID module 44 is coupled to communicator 42 and is adapted to receive setpoint information 58 from communicator 42 or local operator input 66. First PID module 44 also receives process information 60 from either communicator 42 or local input 62. The setpoint information 58 can include temperature, flow, pressure, level, or other appropriate analytical setpoint information. Similarly, process information 60 can include temperature, flow, pressure, level, or other suitable analytical process information. First PID module 44 receives the setpoint information 58, and process information 60, and performs a control algorithm such as a proportional-integral-derivative control algorithm to arrive at a position setpoint output which is related to the setpoint information and process information. First PID module 44 provides the position setpoint output to second PID module 46. Second PID module 46 is coupled to first PID module 44 and positioner transducer 52. As stated above, second PID module 46 also receives a position setpoint output from first PID module 44. Second PID controller 46 also receives a position feedback output from positioner transducer 52 which feedback is related to a position of positioner element 54. Second PID module 46 receives the position setpoint output, position feedback output, and performs a proportional- integral-derivative control algorithm based upon the position setpoint output and the position feedback output to arrive at a positioner output which is provided to positioner element 54.

Although modules 44 and 46 have been described as implementing proportional-derivative-intergral control functions, other control algorithms are contemplated .

Positioner element 54 is coupled to second PID module 46, and air supply 64. Positioner element 54 may be any actuator which in response to a position output signal, utilizes energy from a pneumatic source to effect a physical movement. Thus, positioner element 54 includes rotary positioner elements as well as linear positioners. Positioner element 54 receives the positioner output from second PID module 46 and provides actuation based upon the positioner output.

Positioner transducer 52 is operably coupled to positioner element 54 such that positioner transducer 52 provides a signal which is indicative of actuation provided by positioner element 54. Positioner transducer 52 is any device which provides a signal indicative of positioner actuation. Thus, positioner transducer 52 can include limit switches, potentiometers, optical encoders, and other suitable devices .

Diagnostics module 48 is operably coupled to communicator 42, historian module 50, positioner transducer 52, and air supply 64. Diagnostics module 48 responsively provides diagnostic information to communicator 42 for the transmission over the process communication loop. Further, the diagnostic information provided by diagnostics module 48 may also be provided to historian module 50 which may store and accumulate a variety of diagnostic information over time.

Illustrative examples of diagnostic information provided by diagnostics module 48 to communicator 42 include stiction information, backlash information, positional error information, stroking speed information, and air pressure information.

Stiction information is related to the amount of air pressure that must be exerted within positioner 54 before actuation occurs as indicated by the position feedback signal from positioner transducer 52. Thus, this information relates to the degree to which components within positioner element 54 adhere to one another prior to positioning.

Backlash information relates to the degree to which actuator components are subject to a jarring reaction or strike back caused by a poor fit. Such information may indicate wear of the positioning components within positioner element 54.

Positional error information can be determined by applying a pre-selected position output (such as 30 degrees) to positioner element 54 and a determining the resulting position from positioner transducer 52 (such as 35 degrees) . Such information may indicate an obstruction or other failure. Stroking speed information relates to the rate at which positioner element 54 provides actuation. By comparing stroking speed with line pressure, diagnostics module 48 may provide information related to dynamic friction within positioner 54.

Air pressure information relates to the air pressure supplied by air supply 64. By testing line pressure from air supply 64, positioner 40 can detect overpressure, underpressure leaks, or other undesirable conditions.

By providing such diagnostic information to communicator 42, a variety of functions are available for a power positioner in accordance with embodiments of the invention. For example, one aspect of the invention includes the ability to query power positioner 40 through a process communication channel coupled to terminals 56 to determine the readiness of power positioner 40 for actuation. Further, by providing diagnostic information related to device wear, such as stiction or backlash, power positioner 40 can provide, through a process communication channel, information related to a positioner lifetime estimation. Thus, power positioner 40 may be able to signal, through a process communication channel, when the power positioner is in need of repair or replacement.

Communicator 42 can also be adapted, -to provide enhanced control functions for power positioner 40. Illustratively, such enhanced control functions include safety overrides, trim control with analytic inputs, local flow control with local input signals, selectable failure mode control, and remote configuration.

An illustrative example of a trim control includes coupling power positioner 40 to another process device, such as a down-stream oxygen sensor through a process communication channel and controlling positioner element 54 based upon digital signals received through the process communication channel from the downstream oxygen sensor. Thus, in accordance with an aspect of the invention, power positioning may be effected without the use of a separate controller on the process communication channel.

An illustrative example of local flow control with local input signals includes coupling a flow meter to local input 62 to provide fluid flow information, related to a fluid flow controlled by power positioner 40, to first PID module 44 such that the combination of the flow meter and power positioner 40 may provide closed-loop flow control.

An illustrative example of selectable failure mode control includes commanding power positioner 40, through a process communication channel, to enter a preselected position (such as fully open or fully closed) upon fault recognition. One example of fault recognition includes recognizing that air pressure from air supply 64 is below a selected threshold.

An illustrative example of local operator interface includes manually entering setpoint information into local operator input 66 to provide setpoint information to power positioner 40 on-site.

An illustrative example of remote system configuration includes configuring parameters of power positioner 40, remotely, through the process communication channel. Such configuration can include sending new or revised PID control parameters to power positioner 40 to modify characteristics of the control algorithms employed by first PID module 44 or second PID module 46. Further, such remote configuration can be done using functional block graphical methods to simplify configuration.

FIG. 3 is a perspective view of another blower in which embodiments of the invention are useful. FIG. 3 depicts a single width single inlet centrifugal fan with backward blades an inlet vane control . In the fan shown in FIG. 3, flow is controlled by adjusting vane pitch.

Claims

WHAT IS CLAIMED IS;

1. A power positioner with digital communication couplable to a process communication channel and an air supply, the positioner comprising: a positioner element couplable to the air supply and adapted to provide actuation with energy received from the air supply based upon an actuation signal; and a loop communicator couplable to the process communication channel and operably coupled to the positioner element, the loop communicator adapted to provide the actuation signal to the actuation element based upon digital information received from the process communication channel .

2. The actuator of claim 1, wherein the actuation is rotary.

3. The actuator of claim 1, wherein the process communication channel is a two-wire process communication channel.

4. The actuator of claim 1, wherein the digital information is superimposed upon an analog signal .

5. The actuator of claim 4, wherein the digital information is in accordance with the HART^® protocol.

6. The actuator of claim 1, wherein the digital information is in accordance with the Fieldbus protocol.

7. The actuator of claim 1, wherein the process communication channel includes a plurality of wires.

8. The positioner of claim 1, and further comprising: a first controller coupled to the loop communicator and operably coupled to the positioner element, the first controller