Independent intelligent limit switch

A digital valve positioning system is disclosed. The system comprises a valve, a valve controller having a controller microprocessor and a controller memory and a valve actuator having an output coupled to the valve to control the position of the valve over a range of motion in response to an output from the valve controller. The system further comprises a first position sensor for determining the position of the valve actuator and an intelligent limit switch for generating a limit switch state signal indicative of a state of the valve being above or below a threshold set point. The limit switch comprises a second position sensor generating an output signal indicative of the position of the valve, a limit switch memory, a limit switch microprocessor communicatively coupled to the limit switch memory, an isolated communication link between the controller microprocessor and the limit switch microprocessor for transferring the set point data from the controller to the limit switch. The limit switch microprocessor includes a first limit switch microprocessor output, the limit switch microprocessor being responsive to the second position sensor output signal and the set point data stored in the limit switch memory to generate the state signal at the first limit switch microprocessor output.

BACKGROUND OF THE INVENTION

Conventional limit switch implementations are generally categorized as: 1) a dependent-embedded system or 2) an independent physical system. The dependent-embedded design utilizes position feedback data from the feedback element within the instrument to provide a virtual limit switch. The main disadvantage of this embedded implementation is that the “switch” is not isolated or independent from the operation of the instrument and is wholly dependent on the instrument's operation. These types of limit switches cannot be used for an interlock application.

The independent physical limit switch solves the isolation issue associated with the embedded design, but since it is not integrated within the microprocessor-based instrument, the instrument's calibration and limit switch trip point set are not coupled. The complete and “un-intelligent” aspect of the independent, physical switches requires resetting the trip point each time the instrument is recalibrated. Additionally, the setup of the limit switches is typically blind and requires taking the valve out of operation to specifically stroke the valve to establish the trip points. Some manufacturers utilize both implementations, but still do not solve the aforementioned disadvantages.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

This invention incorporates a low-cost dedicated microprocessor to physical limit switches to provide an independent, intelligent limit switch assembly. The limit switch embedded controller can communicate to the instrument microprocessor to establish trip points, but still provide isolated operation appropriate for interlock operation.

A digital valve positioning system, generally designated10, is illustrated inFIGS. 1 and 2. The system10comprises a valve12and a valve controller14. The valve controller14has a controller microprocessor16and a controller memory18. As is well known, the controller microprocessor16controls the position of the valve12by issuing commands to a current to pressure (I/P) converter19via a digital to analog (D/A) converter19a.

The system10further includes a valve actuator20having an output20acoupled to the valve12to control the position of the valve12over a range of motion in response to an output from the valve controller14. A first position sensor24, such as a conventional Hall effect sensor, determines the position of the valve12.

As is well known, the controller memory stores calibration data defining the range of motion of the valve12.

In accordance with the invention, the system10further includes an intelligent limit switch30for generating a limit switch state signal indicative of a state of the valve12being above or below a threshold set point.

The limit switch30utilizes an operators interface32communicatively coupled to the controller microprocessor16for entering limit switch set point data defining the location of the threshold set point into the controller memory18. The operators interface32includes a display33, such as an LCD display34and an LCD control/driver34, which can be used to indicate the state of the limit switch30. The operators interface32also includes a pushbutton interface35. In the present embodiment, the limit switch30utilizes the same operators interface30as is used to perform conventional communication with the controller microprocessor16, such as to enter the calibration data.

The limit switch30further includes a second position sensor36, also such as a Hall effect sensor, and a limit switch memory38. The second position sensor36generates an output signal indicative of the position of the valve12. The limit switch memory38is communicatively coupled to a limit switch microprocessor40.

A communication link44including a first optical isolation unit45provides isolated communication between the controller microprocessor16and the limit switch microprocessor40for transferring the limit switch set point data from the controller memory18to the limit switch memory38.

The limit switch microprocessor40includes a first limit switch microprocessor output40a. The limit switch microprocessor40is responsive to the second position sensor output signal and the limit switch set point data stored in the limit switch memory38to generate the state signal at the first limit switch microprocessor output40a.

A first limit switch output46is communicatively coupled to the first limit switch microprocessor output40afor generating a first limit switch output signal indicating the state of the limit switch. The first output signal is preferably a current control signal, such as a 1–4 mA signal.

The limit switch30also provides a second output, permitting the limit switch30to function as two limit switches. Specifically, the operators interface32is communicatively coupled to the controller microprocessor16for entering, into the controller memory18, second limit switch data defining the location of a second threshold set point.

The isolated communication link44transfers the second limit switch set point data from the controller memory18to the limit switch memory38. The limit switch microprocessor40includes a second limit switch microprocessor output40b. The limit switch microprocessor40is responsive to the second position sensor output signal and the second limit switch set point data stored in the limit switch memory38to generate a second state signal, indicative of a second state of the valve12being above or below the second threshold set point, at the second limit switch microprocessor output.

The limit switch30includes a second output49coupled to the limit switch microprocessor second output for generating a second output signal indicating the secondary state of the limit switch. The second output signal is a current control signal, such as a 1–4 mA signal.

A second optical isolation unit50optically isolates the second limit switch from the first limit switch.

The limit switch data defines the location of the threshold set point as a percentage of the range of motion. Thus if the valve fails, or otherwise must be replaced, it need only be recalibrated. The limit switch set point data does not need to be changed and the limit switch calibration data is automatically adjusted through the communication link44as part of the valve calibration process.

The limit switch30is powered by the current control signal, which is independent of the controller power to the valve control. Thus failure of the controller power will not affect the operation of the limit switch30.

The limit switch30operates independent of the valve controller14. Thus failure of the valve controller14will not affect the operation of the limit switch30.

The system10includes a connector54for communicatively coupling the controller microprocessor16to a network55via a conventional communications interface56. This permits analog and/or digital communication with other devices on the network55, such as a process controller60.

The present invention has been described with respect to a certain embodiment, which is not meant to limit the invention. Those skilled in the art will understand that variations from the embodiment described herein may be made without departing from the invention as set forth in the appended claims.