SP_RampDN Logic Block Documentation

Overview

The fn_SP_RampDN function block is designed to smoothly decrease a set point (SP) from its current value to a target value over a predefined duration. This is especially useful in process control systems where sudden changes to the set point could cause instability or stress to the system.

By employing the fn_SP_RampDN block, the reduction is controlled, systematic, and safe, thereby protecting equipment and improving the overall reliability of operations.

In the above picture we are showing the fn_SP_RampDN block

Functional Description

• This block gradually ramps down the set point based on the time period defined by TM_PT.
• It ensures a linear and predictable transition from the initial set point to the required set point.
• When the target is achieved, the output Finish is set to TRUE.

Here is an illustration showing how the block behaves with real-time data and calculated outputs.

Signal Connection Example

Below is a typical example showing how the pins are connected to the fn_SP_RampDN block within a control logic diagram.

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Pins Information

Signal Name	Type	Description
TM_PT	WORD	The period required for the set point to reach the target
Last_SP	REAL	The last known set point value before ramping begins
Init_SP	REAL	Current set point from which the ramping down will start
Reqd_SP	REAL	Desired final set point after ramping
Reqd_SP_Fctr	REAL	Factor applied to scale the required set point (e.g., for percentage reduction)
Calc_SP	REAL	Intermediate calculated set point based on elapsed ramping time
Final_SP	REAL	Final set point once ramping is complete
Finish	BOOL	Status output: TRUE when ramping is complete, otherwise FALSE

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Operational Behavior

1. Initialization:
   On activation, the block reads the Init_SP as the starting point and calculates the target value using Reqd_SP × Reqd_SP_Fctr.

2. Ramping:
   The block gradually reduces the set point from Init_SP to the scaled Reqd_SP over the period defined in TM_PT.

3. Completion:
   Once the calculated time has elapsed and the Final_SP is reached, the Finish output is set to TRUE.

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Example Use Case

For instance, in a temperature control system, you might want to reduce the heater’s set point gradually over 5 minutes to prevent sudden cooling, which could impact product quality. Setting TM_PT = 300 (5 minutes in sec) with appropriate Init_SP and Reqd_SP values would accomplish this smoothly.

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Best Practices

• Always provide accurate TM_PT based on system dynamics to avoid overshooting or undershooting.
• Use Reqd_SP_Fctr = 1.0 if no scaling of Reqd_SP is needed.
• Ensure that Last_SP and Init_SP are consistently updated per cycle for accurate ramping.
• Monitor Finish output to trigger subsequent control steps only after ramp-down is complete.
• This block is ideal for PID transitions, flow rate reductions, energy-saving shutdown procedures, etc.

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The fn_SP_RampDN block offers a structured and safe method to implement set point ramp-downs, minimizing system instability and ensuring a smooth transition to the desired target values over a time-controlled ramp period.