MLD (Manual Loader) Logic Block Documentation

Overview

The MLD (Manual Loader) function block is used to manage the flow of analog signals in Auto/Manual selection scenarios. It allows users to manually override an input signal during commissioning, testing, or manual operations.

In Manual Mode, the operator can input a desired value directly using MAN_IN, within defined limits (IN_LO and IN_HI). In Auto Mode, the INPUT signal passes directly to the output, ensuring seamless automation.

This block is particularly useful for systems requiring manual override of analog signals such as setpoints, actuator positions, or control outputs.

Functional Description

• Manual Mode (am_sel = FALSE):
• The operator can manually feed a value through MAN_IN, limited by IN_LO and IN_HI.

• This mode is useful during testing or commissioning phases.

• Auto Mode (am_sel = TRUE):

• The external INPUT value is directly routed to the OP (output), bypassing the manual setting.

This block simplifies the switch between auto and manual signal handling in analog control systems.

Showing the MOTOR block in the above picture.

The above picture shows how the inputs and outputs are connected in the MOTOR block.

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Input and Output Parameters

Signal	Type	Description
INPUT	REAL	The analog input signal received from external logic or controller
OP	WORD	Output value to be sent to the next process (analog output channel)
MAN_IN	REAL	Manual input value provided by the operator
IN_LO	REAL	Lower limit for manual input
IN_HI	REAL	Upper limit for manual input
am_sel	BOOL	Auto/manual mode selection TRUE = Auto, FALSE = Manual

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Operational Logic

• When am_sel = FALSE:
• OP := Clamp(MAN_IN, IN_LO, IN_HI)

• The MAN_IN value is constrained within the limits and passed to the output.

• When am_sel = TRUE:

• OP := INPUT
• The block routes the external signal directly to the output.

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Use Case Scenario

• Auto Mode Example:
• Process Setpoint → INPUT

• OP sends the Setpoint directly to the controller.

• Manual Mode Example:

• Operator sets MAN_IN = 55.5
• IN_LO = 40.0, IN_HI = 60.0
• OP becomes 55.5 if within range, else clipped to limits.

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Best Practices

• Always configure IN_LO and IN_HI according to system safety constraints.
• Display am_sel mode clearly on the HMI for operator awareness.
• Log the transitions between Auto and Manual for traceability.
• Use MLD where testing or manual override is frequently required without affecting control loops.

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Summary

The MLD block ensures flexibility and control in systems where analog signal override is necessary. It streamlines Auto/Manual signal switching and is highly effective in both process commissioning and normal operations.

Ideal for analog signal override scenarios in critical control systems such as VFD speed control, actuator position control, etc.