Diagnostic device, system and method

1. A method for detecting play between a drive element of a valve drive and a valve member of a process valve driven by the drive element, the method comprising the steps of:

detecting that the deviation between a current position of the drive element and a target position of the drive element prescribed by a first position signal is greater than a predetermined threshold value; and

detecting that the target position of the drive element prescribed by the first position signal is in a different movement direction relative to a movement direction of a last executed movement of the drive element; and

performing a play detection procedure in response to the detected deviation between the current position of the drive element and the target position prescribed by the first position signal being greater than the predetermined threshold value, and the detected target position of the drive element being in a different movement direction relative to the movement direction of the last executed movement of the drive element, the play detection procedure comprising:

recording a time profile of at least one of a position of the drive element, a velocity of the drive element, or a pressure provided to the valve drive;

detecting a signal characteristic within the recorded time profile, the signal characteristic comprising at least one of a reduction in the velocity of the drive element or an increase in the pressure provided to the valve drive; and

detecting the play between the drive element and the valve member on the basis of the detected signal characteristic; and

providing play information according to the detected play, the play information comprising a play value indicating an extent of the play between the drive element of the valve drive and the valve member of the process valve driven by the drive element.

2. The method according to claim 1 , further comprising determining, in accordance with at least one of a time and a signal value of the signal characteristic, the play value indicating the extent of the play.

3. The method according to claim 1 , further comprising performing the play detection procedure in response to the first position signal.

4. The method according to claim 3 , further comprising taking into account, during the play detection procedure, the extent of a position change caused by at least one of the first position signal and the movement direction of the position change caused by the first position signal.

5. The method according to claim 3 , further comprising taking into account, during the play detection procedure, an earlier position of the drive element, which was caused by a second position signal provided before the first position signal.

6. The method according to claim 1 , wherein the play detection procedure is performed in a normal operation of the positioning device during a normal movement of the drive element, which normal movement serves to influence a process fluid flow in the normal operation.

7. The method according to claim 1 , wherein the play is detected without directly detecting the position of the valve member.

8. The method according to claim 1 , wherein the detected signal characteristic comprises a temporary drop of the velocity of the drive element to zero.

9. The method according to claim 1 , wherein the detected signal characteristic comprises a temporary drop of the velocity of the drive element from a first velocity level, at which the velocity remains temporarily, to a velocity lowest point.

10. The method according to claim 9 , wherein the detected signal characteristic further comprises a subsequent increase from the velocity lowest point to a second velocity level.

11. The method according to claim 9 , wherein the predetermined threshold value is a predetermined minimum difference between the current target position and the current position of the drive element, which minimum difference ensures that the drive element will move long enough reach the first velocity level.

12. The method according to claim 1 , wherein the increase in the pressure provided to the valve drive comprises a temporary increase of a slope of the pressure.

13. The method of claim 12 , wherein the detected signal characteristic comprises a subsequent reduction in the slope of the pressure provided to the valve drive.

14. The method of claim 1 , further comprising detecting at which position of the drive element the signal characteristic occurs and determining the play value according to this position.

15. The method of claim 1 , further comprising checking whether an opening degree of one or more valves of the fluid device actuating the drive element is, for at least a predetermined time duration, above the threshold value, and executing the play detection procedure in response to the opening degree being above the threshold value for the predetermined time duration.