Data Sheet

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Dead Band Plus Hysteresis Estimation with

ValveLinkt Diagnostics

Background

Dead band and hysteresis are important nonlinearities that can adversely affect process control. Figure 1 shows input

and output graphs for hysteresis, dead band, and dead band plus hysteresis. Hysteresis is a path dependent

characteristic that can be attributed to materials not being able to return to their original shape and size after being

stretched or deformed. Dead band represents the amount of signal change required to reverse direction and is

characterized by a zero slope region where the input signal reverses. Dead band can usually be attributed to valve

friction (packing, actuator seal rings, and valve seal rings) and pneumatic dead zones in the instrumentation.

Figure 1. Hysteresis, Dead Band, and Dead Band plus Hysteresis Curves

HYSTERESIS

ERROR

100%

OUTPUT

100% INPUT

INPUT

OUTPUT

DEAD BAND

HYSTERESIS

ERROR WITH

DEAD BAND

ERROR

100%

INPUT

100%

OUTPUT

DEAD BAND

For control valves, dead band is the dominant nonlinearity whereas hysteresis is a secondary effect. Hysteresis can be

estimated by subtracting dead band from the dead band plus hysteresis data. However, since hysteresis is generally

negligable and difficult to discern from experimental error, dead band and hysteresis are not separated and test results

are reported as the sum of these nonlinearities.

Allowable dead band plus hysteresis is process dependent but is typically specified to be ≤ 1% for the entire valve

assembly, which comprises the valve, actuator, and positioner.

Dead Band plus Hysteresis Test Definition

Dead band plus hysteresis can be estimated at the factory using a Series 3 Factory ValveLink test. Although dead band

plus hysteresis is a factory test, results can be imported and viewed in all versions of ValveLink software. Dead band

plus hysteresis tests can be run at nominal travels of 5%, 25%, 50%, 75%, and 95%. If a test point is not specified, dead

band plus hysteresis will be evaluated at a nominal travel of 50%.

ValveLink Software

D103549X012

Product Bulletin

62.1:ValveLink Software(S3)

October 2012

Summary of content (4 pages)

PAGE 1
Product Bulletin ValveLink Software 62.1:ValveLink Software(S3) October 2012 D103549X012 Dead Band Plus Hysteresis Estimation with ValveLinkt Diagnostics Background Dead band and hysteresis are important nonlinearities that can adversely affect process control. Figure 1 shows input and output graphs for hysteresis, dead band, and dead band plus hysteresis.
PAGE 2
Product Bulletin ValveLink Software 62.1:ValveLink Software(S3) October 2012 D103549X012 Test Procedures Dead band plus hysteresis is a static measurement that can be estimated using a quasi steady‐state test signal. All tests are performed using standard digital valve controller diagnostic procedures with the cutoffs and characterization disabled and with the travel integrator disabled.
PAGE 3
Product Bulletin ValveLink Software 62.1:ValveLink Software(S3) September 2012 D103549X012 The ideal response of a control valve is a straight line with unity slope. Dead band plus hysteresis can be estimated by drawing lines parallel to the ideal response that band the data. The width between the bands provides the best estimate of dead band plus hysteresis. Figure 3. Typical Travel Set Point and Travel Time Series Plots DATA TARGET Note: The first cycle is a break-in cycle.
PAGE 4
Product Bulletin 62.1:ValveLink Software(S3) October 2012 ValveLink Software D103549X012 Data Export Dead band plus hysteresis data from factory ValveLink can be exported as a standard step study test. When imported into ValveLink 11.5 or higher, time series and dead band plus hysteresis X-Y plots can be displayed. For earlier versions of ValveLink, only time series data will be displayed. References [1] ANSI/ISA75.05.012000 (R2005), “Control Valve Terminology.” [2] ANSI/ISA75.13.