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May
04

What Have I Learned? – Cost and Source of Oscillations (Part 3)

If you want to know how to minimize oscillations from final elements and don’t have time to read the supporting information you can use the following rules of thumb and move on to more important tasks like reading email. The final elements considered here are throttling control valves and variable speed drives (VSD) on pumps or fans.

• Use a sliding stem throttling valve with a properly tuned digital positioner (position feedback) or a VSD with a properly tuned speed controller (tachometer feedback) to minimize the amplitude of the limit cycle from a final element

• Make sure the DCS and final element I/O cards have at least 12 bits

• Enable “Dynamic Reset Limit” in PID block and use position or speed feedback as PV for BKCAL_OUT of AO block to prevent a burst of unstable oscillations when PID reset action is faster than valve or VSD response

• Set IDEADAND in the PID block equal to the limit cycle amplitude from the final element to kill the limit cycle during quiet periods of operation (e.g. periods when there are no disturbances or set point changes) for a self-regulating loop

Resolution is the minimum change in the element’s output. Changes in the output smaller than the resolution cannot be made. For a control valve, the resolution limit is the result of friction in the packing, seat, and seal. For a VSD, the resolution limit is the result of an artificially imposed deadband, which is really a dead zone or from a speed sensing element resolution limit. Resolution can also result from a quantize limit from the number of bits in a microprocessor or I/O card. The number of bits in A/D and D/A cards for most DCS has increased from 12 bits to 16 bits. In both cases, the resolution limit from these I/O cards is negligible. However the standard input card of some VSD manufacturers is only 8 bit causing a significant resolution limit. The resolution in the stroke of a control valve or in the speed a variable speed drive will cause a limit cycle in any loop with integral (reset) action.

The term deadband is often used in automation systems to specify a dead zone (a bandwidth around a reference value where there is no response). Examples are deadband (dead zone) specifications in VSD configuration for noise rejection and in a PID configuration for integral action suspension.

For final elements, deadband has a significantly different definition. Here deadband is the change in signal required upon a reversal of direction to get a change in the element’s output. Once the output reverse direction, deadband places no limit on how small a change can be made in the same direction. In reality, valve deadband is usually accompanied by a resolution limit. In the stroke of a control valve, deadband is the result of backlash from gaps or play in linkages and shaft or stem connections. Deadband normally doesn’t exist in a VSD. Deadband will cause a limit cycle if there are two integrators in series in the control system. Multiple integrators in series can occur from a PID with integral action on a process with an integrating response such as level. Alternately, the limit cycle can occur if there is a cascade control loop where there is integral action in more than one controller. If both the temperature and flow PID blocks have integral (reset) action in a temperature to flow cascade control system, then deadband can cause a limit cycle. Most people forget that a positioner or digital valve controller creates a cascade loop where the positioner controller is the secondary loop. Positioners until recently were proportional only controllers.

The amplitude of the limit cycle is the smallest change in flow associated with the smallest possible change in valve position or speed multiplied by the process gain (change in process variable in engineering units divided by the change in flow). To get the smallest possible change in flow of a control valve, multiply the valve’s resolution limit in % of stroke by the installed characteristic curve for the valve at its operating point. Note that valve stick-slip and the resolution gets worse near the seating or sealing surface. The manufacturer’s quoted numbers are at a 50% throttle position. To get the smallest possible change in flow of a VSD multiply the resolution limit of the input card resolution of the tachometer sensing element, or noise deadband, whichever is largest, and convert to flow based on the interpolated shift in the installed characteristic curves with speed for the pump or fan. Be careful, many VSD have an adjustable deadband (dead zone) to prevent the VSD from responding to noise. This adjustment is often set with no regard to the effect on loop performance.

Resolution limits and deadband add dead time to the control loop for slow disturbances because it takes time for the PID output work through the zone of no final element response. The dead time is the resolution limit or deadband divided by the rate of change of the controller output. This additional deadtime increases the peak and integrated error for the upset. Note that step changes in the controller output larger than the resolution limit or deadband will not reveal the deadtime.

Control valves have an inherent velocity limit from the limitations imposed by actuator fill and exhaust rates. VSD have an application set velocity limit from the motor load limitations imposed by the impeller inertia. Make sure the valve actuator and VSD motor have enough muscle for the valve sticktion and pump inertia, respectively or you can get into poor valve position or speed control and hence even bigger loop problems.

Use the “dynamic reset limit” option of a PID block in a DCS, such as DeltaV, where the PID uses a positive feedback network for its integral action. The BKCAL_OUT for the AO block which in connected to the BKCAL_IN of the PID block should be actual valve position or VSD speed. Select the PV (position or speed) option in the AO block for the BKCAL_OUT. This feedback of actual position or speed to the PID enables the PID algorithm to curtail its integral contribution to the PID output so that the PID output from reset action does not change faster than the valve or drive can respond. If this protection is not in place, everything may look OK until the loop gets a disturbance large enough PID to cause the PID output to change faster than the final element. The mysterious bursts of instability for big load upsets often go unresolved.

Set the IDEADBAND option in the PID block to a value about equal to the limit cycle amplitude. IDEADBAND will suspend the integral action when the PID error is less the IDEADBAND. This suspension will stop limit cycles from a resolution limit or deadband for a self-regulating process at a steady state. It will not stop the limit cycle on a process with an integrating response because the process has no steady state and will continue to ramp until the process variable exceeds the IDEADBAND.

For more info on final element response, check out the “Deal or No Deal” Control Talk column in Control magazine, the article “What is your Valve Trying to Tell You” in Control Design magazine, and “Improve Control Loop Performance” in Chemical Processing magazine.