High_Low Alarm

Owned by Gary Cane
May 09, 2022


Operator Menu Location: Signal Comparators

Operator Bitmap:
 

Functional Description:
The High/Low Alarm operator compares an analog input signal to high and low alarm levels. The operator's digital output is set LOW if the input signal is between the alarm levels, and is set HIGH otherwise. The output will change only if the alarm condition is maintained for a user-specified length of time-the hysteresis time. That is, the output y is defined by

where x is the input, is the high alarm level, is the low alarm level, H is the hysteresis time, and t is the loop sampling time.
For example, suppose that the initial output is y(0) = 0, the high alarm level is , the low alarm level is , the hysteresis time is H = 2 seconds, and the input signal has the following profile:

Under these conditions, the output of the High/Low Alarm operator will be

Initially, the output is y(0)=LOW (as specified by the initial condition property). The output remains LOW while x(t)=50 since an alarm condition is not met.

Although an alarm condition exists while x(t)=90, the output remains LOW because the condition is not sustained for two seconds (the hysteresis time). The output switches to HIGH at t=9 seconds because an alarm condition is maintained for at least two seconds. It remains HIGH even while x(t)=75 (within alarm levels) because the condition is not sustained for two seconds. At t=20 seconds, the input signal satisfies the no-alarm condition, but the output does not switch to LOW until t=22 seconds. (when the input signal has remained within the alarm levels for at least two seconds.) Likewise, the output will go HIGH (and remain HIGH thereafter) at t=27 seconds (because the low alarm condition is sustained).

User-Defined Properties:
Object Name. A string label that identifies the operator

Initial State. The output state (HIGH or LOW) at startup (at time t=0)
Low Alarm. The low alarm level,
High Alarm. The high alarm level,

Hysteresis. The length of time a signal must satisfy the alarm condition, for the output to switch state. The hysteresis time is denoted by H in the equation above, and is expressed in seconds.

Comments:
• Design Pad will issue an error on processing if the input pin is not connected
•The high alarm level must exceed the low alarm level,
•The hysteresis time must be greater than or equal to zero, H >=0

See Also:
=, >=, >, <=, <, and != Comparators; Thresholding