Eurotherm 6100, Руководство пользователя

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100/180MM DISTRIBUTED GRAPHIC RECORDER: USER GUIDE
HA029324
Issue 11 Oct 14 Page 71
4.3.3 CHANNEL/ALARM CONFIGURATION (Cont.)
PARAMETERS
Figures 4.3.3d, e and f illustrate the following terms for the different alarm types.
Threshold For Absolute alarms, this defi nes the value (in engineering units) at which an alarm is
triggered. The alarm also returns to its non-active state at this value (unless a hyster-
esis value has been set). If a dwell value is set, the alarm does not become active until
this dwell time has elapsed.
Hysteresis Defi nes a ‘deadband’ (in engineering units) to eliminate spurious triggering if the sig-
nal value is hovering around the trigger point. The deadband lies:
Below Absolute High thresholds
Above Absolute Low thresholds
Outside the deviation band for Deviation-in alarms
Inside the deviation band for Deviation-out alarms.
Dwell Allows a dwell period to be entered in seconds. The alarm does not take effect until
this period has expired. If an alarm clears before the dwell period has expired, the
alarm is ignored.
Reference For Deviation alarms, this is the central value of the deviation band.
Deviation For Deviation alarms, this value defi nes the width of the deviation band, each side of
the reference value. I.E. the total width of the deviation band is 2 x Deviation value.
Amount For Rate-of-change alarms, this value defi nes the minimum amount by which the sig-
nal value would have to change, within the ‘Change Time’ period (below), in order for
the alarm to become active.
Change Time For Rate-of-change alarms, this selects the time period (Per second, Per minute, Per
hour) within which the change in signal value must exceed the Amount value (entered
in the preceding fi eld) in order for the alarm to become active. See Rate-of-Change
example below for more details.
Average time For rate-of-change alarms, this allows an average period to be entered for signal
smoothing.
HYSTERESIS EXAMPLE
Threshold = 100 units; Hysteresis = 5 units
With the above settings, an absolute high alarm would become active if its input were to rise above 100
and would remain active until its value fell to below 95 units. An absolute low alarm would become active
if its input fell below 100 units, and would remain active until its input rose above 105 units. Deviation
alarms behave in a similar manner.
RATE-OF CHANGE ALARM EXAMPLE
Rate-of-change alarms allow the user to enter an ‘amount’ (say 3 degrees) and a time period (say 1
minute), and if the process value changes by the specifi ed amount or more, within the specifi ed time
period (more than 3 degrees in a minute in this example), then the alarm becomes active.
The recorder uses its iteration rate of 125 msec (1/8th second) as the time base for it calculations. For our
example, 3 degrees per minute equates to 3/60 degrees per second or 3/(60 x 8) = 0.00625 degrees per
iteration. If a change greater than this is detected, then the alarm becomes active.
In order to reduce sensitivity, an averaging period can be confi gured. This means that an average of all
the 1/8th second samples is taken over the specifi ed period, and the alarm becomes active only if the
average value exceeds the specifi ed rate of change.
Confi guring a dwell time can also reduce ‘jitter’ because, if during the specifi ed dwell time, any one sam-
ple is non active, then the elapsed dwell time is reset to zero. The alarm becomes active only after the
dwell time has elapsed i.e. only if the rate of change has been exceeded for every software cycle through-
out the specifi ed dwell time.