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Dec
11

Getting a Kick Out of Stick-Slip Compensation

Last week, we discussed the deadly and sticky situations of some control valves. I once thought a variable speed drive (VSD) was the solution until Bob Heider pointed out that some VSD packages may not be as sensitive as a well designed throttling control valve. A VSD resolution may be 0.4% whereas a sliding stem control valve with low friction packing and digital positioner can be 0.1%. Jae Park looked at the specs for a particular model VSD and found out the following:

I Speed control

a. Speed regulation without the feedback (without the encoder): 0.1% of base speed across 120:1 speed range

b. Speed regulation with the feedback (with the encoder): 0.001% of base speed across 120:1 speed range

II Command signal accuracy

a. Typically 9 to 12 bits for analog in VFD from different manufactures. If one bit is a sign bit this corresponds to a resolution of 0.05% to 0.4%.

b. If using digital input, accuracy is 0.01% of set output frequency.

Obviously, here the limitation is the analog command signal and a low resolution A/D.

If you refer your project manager to this website and you are still not set free to buy a fine final element and are stuck with a sloppy control valve, where the backlash plus sticktion can range from 0.5% to 10%, you may need to resort to desperate measures. This resorting can be retirement to Stan’s country (Naples) or adding a resolution and deadband compensator from a library of composite templates to the controller output.

A deadband (backlash) compensator can be as simple as an addition or subtraction of a half deadband offset to the controller output when it reverses direction. The following screen prints show the configuration of the composite block.

Deadband Compensator

The compensation of resolution (stick-slip) is a bit dicey. One implementation uses the relay auto tuner method of a single step (kick) of the output in the direction to return the process variable (PV) to its set point (SP) when the PV gets out of the noise band. However, in this case the kick is equal to the stick and is not necessarily large enough to cause the PV to cross back over the SP. This method requires that the control action and valve action be correctly provided as inputs. The block also shows an optional dither of the loop to help reduce the sticktion since some valves have trouble breaking free when stuck in one position for a long time (often called freezing in position even when it caused by hot temperatures). The worst case is if the valve is normally closed and designed for tight shutoff. The whole motionless gig is kind of like me in a lazy boy chair getting commands from my spouse. I can be as sensitive as the finest example of my gender but as I get older my joints get stiffer the longer I sit.

Since constant dither can wear out valve or body parts, dither amplitude and frequency is important from both a maintenance and variability view point. The dither is more suitable for composition, gas pressure, or temperature loops on a large well mixed volume, because these have a slower natural period (less frequent dither) and larger process time constant (more effective filtering). The following file shows the reduction in oscillation amplitude in the primary temperature loop of a cascade temperature control system by the addition of a resolution compensator on the secondary (coolant) temperature controller’s output. Also shown are screen prints of the configuration of the composite block.

Resolution Compensator

These compensators need to be tested and adjusted carefully because of many practical issues. The deadband and stick-slip are never constant. For example, the value depends upon the throttle position, magnitude and direction of the change in the controller output, and the time in service of the valve. The slip can also be greater than stick, particularly if the actuator is undersized or the valve is coming off the seat. Fine valves do not age like fine wines. Crud can build up on the stems and trim (another reason why dither may help). A kick or offset that is too large will create additional slip and do more harm than good so underestimates of the deadband and resolution limit are wise. Some software packages can identify the deadband and resolution automatically online as documented in the paper “Valve Diagnostics in an Adaptive Control Loop”.

The following file, which is Appendix A of the paper, describes the use of a composite block that has a concise code for the simulation of valve dead band and resolution. The block also models the rate limited second order second order response of the actuator. The user can set the pre-stroke dead time, the slewing rate for increasing and decreasing directions, and second order lags. For large valves, the actuator dynamics are significant because it takes time to move enough air in and out of the actuator to move its shaft. These dynamics are particularly important for compressor anti-surge control valves. Until recently, when a supplier provided the dynamic response of a control valve, it was for an actuator not connected to a control valve even though not too many of these were sold this way. The dynamic response did not include the effects of backlash and sticktion.

Valve Diagnostics Appendix A