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A Safety Instrumented System for use with a process control system receives pressure of process fluid in the process piping. A valve positioner positions a valve which controls flow of process fluid through the process piping. The valve positioner is caused to perform a partial stroke of the valve o

A Safety Instrumented System for use with a process control system receives pressure of process fluid in the process piping. A valve positioner positions a valve which controls flow of process fluid through the process piping. The valve positioner is caused to perform a partial stroke of the valve or otherwise introduce a perturbation into the process. A resulting change in sensed pressure due to the perturbation is used to diagnose operation of the process.

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What is claimed is: 1. A Safety Instrumented System for use in diagnosing operation of a basic process control system (BPCS) connected by a process control loop, comprising: a pressure sensor coupled to process piping and the process control loop configured to measure a pressure of process fluid fl

What is claimed is: 1. A Safety Instrumented System for use in diagnosing operation of a basic process control system (BPCS) connected by a process control loop, comprising: a pressure sensor coupled to process piping and the process control loop configured to measure a pressure of process fluid flows in the process piping; a valve positioner coupled to the process control loop position configured to control in response to a control signal from the process control loop a valve which controls flow of process fluid through process piping to there by control a process; and diagnostic circuitry applying a perturbation signal having a frequency relative to the control signal to cause the valve positioner to perform a partial stroke of the valve, the diagnostic circuitry configured to compare a change in pressure sensed by the pressure sensor due to the partial stroke with a stored pressure signature and responsively diagnose operation of the process control system based upon the comparison. 2. The apparatus of claim 1 including a pressure transmitter which couples to the pressure sensor. 3. The apparatus of claim 2 wherein the diagnostic circuitry is implemented in the pressure transmitter. 4. The apparatus of claim 1 wherein the comparison comprises an amplitude comparison. 5. The apparatus of claim 1 wherein the comparison comprises a phase comparison. 6. The apparatus of claim 1 wherein the diagnostic circuitry is configured to learn the stored pressure signature in a learning mode. 7. The apparatus of claim 1 including a logic solver and wherein the diagnostic circuitry is implemented in the logic solver. 8. The apparatus of claim 1 wherein the diagnostic circuitry causes the valve positioner to perform a plurality of partial strokes of the valve. 9. The apparatus of claim 8 wherein the plurality of partial strokes provide a substantially continuous signal. 10. The apparatus of claim 9 wherein the signal comprises noise. 11. The apparatus of claim 8 wherein the plurality of partial strokes reduce stiction of the valve. 12. The apparatus of claim 1 wherein the comparison comprises a frequency response comparison. 13. The apparatus of claim 1 including a memory configured to store information related to the performance of a diagnostic test by the diagnostic circuitry. 14. The apparatus of claim 1 including a memory configured to store information related to a probability of failure on demand (PFD) and the diagnostics circuitry is configured to provide an output related to the probability of failure on demand and a interval between proof tests. 15. The apparatus of claim 14 wherein a probability of failure on demand value is reset to an initial value upon the occurrence of a diagnostic test performed by the diagnostics circuitry. 16. The apparatus of claim 1 including a proof test output which indicates that proof testing is required. 17. The apparatus of claim 16 wherein the diagnostics circuitry responsively performs a diagnostic test in response to the proof test output. 18. The apparatus of claim 1 wherein the partial stroke is of a limited duration. 19. The apparatus of claim 1 wherein a control point or process variable output is provided to a control system prior to the partial stroke. 20. A diagnostic system in a Safety Instrument System (SIS) for diagnosing operation of a process control system having a process control loop, comprising: a pressure sensor coupled to process piping and the process control loop configured to measure a pressure of process fluid in the process piping; a valve positioner coupled to the process control loop responsive to a control signal from the process control loop configured to control position of a valve which controls flow of process fluid through the process piping; diagnostic circuitry configured to cause small variations at a frequency in the control signal during normal operation of the process control system, the diagnostic circuitry further configured to compare changes in pressure sensed by the pressure sensor due to the variations in the control signal with a stored pressure signature and responsively diagnose operation of the process control system. 21. The apparatus of claim 20 including a pressure transmitter which couples to the pressure sensor. 22. The apparatus of claim 20 wherein the variations in the control signal are of a limited duration. 23. The apparatus of claim 20 wherein the diagnostic circuitry is implemented in the pressure transmitter. 24. The apparatus of claim 20 wherein the comparison comprises an amplitude comparison. 25. The apparatus of claim 20 wherein the comparison comprises a phase comparison. 26. The apparatus of claim 20 wherein the diagnostic circuitry is configured to learn the stored pressure signature in a learning mode. 27. The apparatus of claim 20 including a logic solver and wherein the diagnostic circuitry is implemented in the logic solver. 28. The apparatus of claim 20 wherein the diagnostic circuitry causes the valve positioner to perform a plurality of partial strokes of the valve. 29. The apparatus of claim 28 wherein the plurality of partial strokes provide a substantially continuous signal. 30. The apparatus of claim 29 wherein the signal comprises noise. 31. The apparatus of claim 20 wherein the comparison comprises a frequency response comparison. 32. The apparatus of claim 28 wherein the plurality of partial strokes reduce stiction of the valve. 33. The apparatus of claim 20 including a memory configured to store information related to the performance of a diagnostic test by the diagnostic circuitry. 34. The apparatus of claim 20 including a memory configured to store information related to a probability of failure on demand (PFD) and the diagnostics circuitry is configured to provide an output related to the probability of failure on demand and a interval between proof tests. 35. The apparatus of claim 34 wherein a probability of failure on demand value is reset to an initial value upon the occurrence of a diagnostic test performed by the diagnostics circuitry. 36. The apparatus of claim 20 including a proof test output which indicates that proof testing is required. 37. The apparatus of claim 36 wherein the diagnostics circuitry responsively performs a diagnostic test in response to the proof test output. 38. A method in a Safety Instrumented System for use in diagnosing operation of a process control system housing a process control loop, comprising: sensing a pressure of a process fluid carried in process piping; controlling operation of the process by applying a control signal from the process control loop to a valve coupled to the process piping; causing the valve coupled to the process piping to perform a partial stroke by applying a perturbation signal at a frequency to the control signal; monitoring pressure changes of the process fluid following the partial stroke; and diagnosing operation of the process control system based upon the monitored pressure changes of the process fluid. 39. The method of claim 38 including storing information related to performing the step of diagnosing operation. 40. The method of claim 38 including providing a proof test output related to probability of failure on demand (PFD). 41. The method of claim 38 wherein the diagnosing comprises an amplitude comparison. 42. The method of claim 38 wherein the diagnosing comprises a phase comparison. 43. The method of claim 38 including learning a pressure signature in a learning mode. 44. The method of claim 38 including causing the valve to perform a plurality of partial strokes of the valve. 45. The method of claim 38 wherein the diagnosing comprises comparing a frequency response comparison. 46. A method in a Safety Instruments System (SIS) of diagnosing operation of a process control system having a process control loop, comprising: introducing small variations at a frequency into a controlled position of a valve coupled to the process control loop and to the process piping; monitoring pressure of the process fluid following the introduction of small variations into the controlled positioned of the valve; diagnosing operation of the process control system based upon the monitored pressure of the process fluid. 47. The method of claim 46 including storing information related to performing the step of diagnosing operation. 48. The method of claim 46 including providing an output related to probability of failure on demand (PFD). 49. The method of claim 46 wherein the diagnosing comprises an amplitude comparison. 50. The method of claim 46 wherein the diagnosing comprises a phase comparison. 51. The method of claim 46 including learning a pressure signature in a learning mode. 52. The method of claim 46 wherein the diagnosing comprises a frequency response comparison. 53. The apparatus of claim 1 wherein the process control loop comprises a two wire process control loop. 54. The apparatus of claim 20 wherein the process control loop comprises a two wire process control loop. 55. The method of claim 38 wherein the process control loop comprises a two wire process control loop. 56. The method of claim 46 wherein the process control loop comprises a two wire process control loop.