Control Loop | AseptSoft Documentation

A Control Loop defines a PID (or other) control strategy for continuous process control. Control loops manage setpoints, tuning parameters, modes, failure behavior, and phase interaction — providing a complete specification for how a process variable is controlled.

💡 In pharmaceutical terms: Control loops are the backbone of automated process control. They continuously regulate critical parameters like temperature during SIP sterilization, flow rate during CIP circulation, or pressure during product transfer. A well-defined control loop ensures your process stays within validated ranges.

📋 Properties

Identity

Property	Type	Default	Description
Name	Text	—	Unique identifier (e.g., "TIC-101", "FIC-201")
Description	Text	—	Detailed description of the loop's purpose
Loop Number	Text	—	Loop number/tag for identification
Criticality	Selection	Non-Critical	See table below
Pattern	Selection	Basic PID	See table below

🎯 Criticality Levels

Criticality	What It Means	Verification Required
Quality-Critical (Critical Process Parameter)	This loop controls a parameter that directly impacts product quality — highest verification level	Full qualification (IQ/OQ/PQ), periodic re-verification
Safety-Critical	This loop controls a parameter that impacts personnel or equipment safety	Safety integrity verification, redundancy assessment
Non-Critical	This loop controls a parameter that does not directly impact quality or safety	Standard commissioning

📊 Process Variable (PV)

Property	Type	Description
PV Instrument Tag	Text	Tag name of the measuring instrument
PV Units	Text	Measurement units (e.g., "°C", "bar")
PV Range Low	Decimal	Lower range of the PV
PV Range High	Decimal	Upper range of the PV
PV Filter Expression	Text	Optional signal filter expression
Bad PV Behavior	Text	What to do when PV signal is bad

🎚️ Setpoint Sources

Each loop can have multiple setpoint source configurations, one per operating mode:

Property	Type	Default	Description
Mode	Selection	—	Which mode this source applies to (Auto, Manual, Cascade, Remote SP)
Source Type	Selection	Operator	Where the setpoint comes from (see below)
Default SP	Decimal	—	Default setpoint value
SP Range Low	Decimal	—	Minimum setpoint limit
SP Range High	Decimal	—	Maximum setpoint limit
Ramp Rate	Decimal	—	Setpoint ramp rate
Notes	Text	—	Additional notes

Setpoint Source Types:

Source Type	What It Means
Operator	Setpoint entered by the operator via HMI
Recipe	Setpoint comes from the recipe/phase parameter
Engineering	Fixed engineering value (not changeable during operation)
Remote Loop	Setpoint received from an external system
Master Loop	Setpoint driven by the output of a cascade master loop

🔧 Controller

Property	Type	Default	Description
Algorithm	Selection	PID	P (proportional only), PI (proportional + integral), PID (full PID), or On/Off (two-position)
Action	Selection	Reverse	See table below
Action Rationale	Text	—	Explanation for the control action choice
Derivative on PV	Yes/No	Yes	Whether derivative acts on PV (not error)
Scan Time (ms)	Decimal	250	Controller scan time in milliseconds

Control Actions:

Action	What It Means	Typical Use
Direct acting (output increases with error)	When the process variable rises above setpoint, the output increases	Cooling applications — more cooling when temperature rises
Reverse acting (output decreases with error)	When the process variable drops below setpoint, the output increases	Heating applications — more steam when temperature drops

📤 Output

Property	Type	Default	Description
Output Range Low	Decimal	0	Lower output range
Output Range High	Decimal	100	Upper output range
Output Limit Low	Decimal	0	Lower output limit
Output Limit High	Decimal	100	Upper output limit
Output Rate Limit	Decimal	—	Maximum output change rate

🏭 Final Control Elements

Each loop can control one or more Final Control Elements (valves/instruments):

Property	Type	Default	Description
Valve Name	Text	—	Name of the controlled valve/instrument
Split Range Low	Decimal	0	Lower split range boundary
Split Range High	Decimal	100	Upper split range boundary
Overlap Deadband	Text	—	Overlap/deadband for split range

🔄 Mode Handling

Property	Type	Default	Description
Allowed Modes	List	Auto, Manual	Available modes: Auto, Manual, Cascade, Remote SP
Default Mode	Selection	Auto	Initial mode when loop starts
Bumpless Transfer Enabled	Yes/No	Yes	Whether bumpless transfer is active
Bumpless Transfer Method	Text	—	Method description

⚠️ Failure Behavior

Property	Type	Default	Description
On Bad PV	Selection	Hold last output	What to do when the process variable signal is bad
On Communication Loss	Selection	Hold last output	What to do when communication with the instrument is lost
On Device Fault	Selection	Force manual mode	What to do when the final control element has a fault

Failure Response Options:

Response	What It Means
Hold last output	Keep the output at its last known good value
Drive to safe position	Move the output to a predefined safe value
Force manual mode	Switch the loop to manual mode for operator control
Trigger interlock	Fire an associated Interlock for safety response

🔗 Phase Interaction

Property	Type	Description
Phase Interaction Notes	Text	General notes on how the loop interacts with phases
On Phase Start	Text	What the loop does when a phase starts
During Phase Run	Text	Behavior during phase execution
On Phase Hold/Abort	Text	Behavior on phase hold or abort

📡 Demand and Ownership

Property	Type	Default	Description
Default Demand Source	Selection	Manual/Operator	Where the demand comes from: Manual/Operator (HMI), Sequence/Phase (ISA-88 phase), or Control Loop (automatic)
Ownership Arbitration Notes	Text	—	Notes on ownership arbitration

🖥️ HMI

Property	Type	Description
HMI Faceplate Requirements	Text	Faceplate display requirements
Operator Permissions	Text	Permission requirements for operator actions

🧪 Testing

Property	Type	Description
Loop Alarm Notes	Text	Alarm configuration notes for this loop
Acceptance Criteria	Text	Acceptance criteria for loop commissioning
Test Note	Text	Testing notes

📐 Loop Patterns

Pattern	What It Means	Example
Basic PID	Standard feedback control — one sensor, one controller, one output	Temperature control with one thermocouple and one steam valve
Cascade	Master loop output drives the setpoint of a slave loop — tighter control	Temperature cascade: master on product temp, slave on jacket temp
Ratio	Setpoint is computed as a ratio of another measurement	Chemical dosing proportional to flow rate
Split Range	One controller output drives two or more final elements across different output ranges	Heating/cooling with a single temperature controller driving both steam and coolant valves
Override	Constraint/selector control — high/low select between multiple controllers	Primary flow control with high-pressure override protection
On/Off	Simple two-position bang-bang control — fully on or fully off	Level switch controlling an on/off pump

Cascade-Specific Properties

Property	Type	Description
Master Loop Name	Text	Reference to the master loop
Slave Loop Name	Text	Reference to the slave loop
Slave SP Limit Low / High	Decimal	Setpoint limits for the slave loop
Slave Saturation Behavior	Text	What happens when the slave saturates

Ratio-Specific Properties

Property	Type	Description
Ratio Base Tag	Text	The base measurement tag
Ratio Value	Decimal	The ratio multiplier
Ratio Range Low / High	Decimal	Allowable ratio range
Ratio Base Bad Behavior	Text	Behavior when the base measurement is bad

Override-Specific Properties

Property	Type	Description
Primary Controller Description	Text	Description of the primary controller
Constraint Controller Description	Text	Description of the constraint controller
Selector Type	Text	"High Select" or "Low Select"

📖 How To: Define a Temperature Control Loop for SIP

Open Module Data — Navigate to the Data panel in the Module Ribbon and open the Module Data window.
Go to the Control Loops tab — Select the Control Loops section.
Create a new Control Loop — Name it (e.g., "TIC-201") and provide a description ("SIP steam temperature control").
Set the pattern — Choose "Basic PID" for a standard temperature loop.
Set the criticality — For SIP sterilization, choose "Quality-Critical (Critical Process Parameter)" since temperature directly impacts sterility assurance.
Configure the Process Variable — Set the PV instrument tag (e.g., TT-201), units (°C), and range (0–150 °C).
Configure the Setpoint — Set the default setpoint (e.g., 121 °C for SIP), range, and source type ("Recipe" for batch-driven or "Operator" for manual entry).
Set the Controller — Choose PID algorithm, "Reverse acting" (output increases when temperature drops), and a scan time of 250 ms.
Define the Final Control Element — Set the steam valve (e.g., TV-201) as the final element.
Configure Failure Behavior — Set "On Bad PV" to "Drive to safe position" (close steam valve) for safety.
Add Phase Interaction — Define what the loop does on phase start (set SP from recipe, switch to Auto), during run (maintain SP), and on hold/abort (hold last output or drive safe).

🏭 Example: SIP Temperature Control Loop

Property	Value
Name	TIC-201
Description	SIP steam temperature control — maintains sterilization temperature
Loop Number	201
Pattern	Basic PID
Criticality	Quality-Critical (Critical Process Parameter)
PV Instrument	TT-201 (Temperature Transmitter)
PV Range	0–150 °C
Algorithm	PID
Action	Reverse acting (increase steam output when temperature drops below SP)
Scan Time	250 ms
Final Element	TV-201 (Steam Control Valve)
Default Mode	Auto
On Bad PV	Drive to safe position (close steam valve)
On Communication Loss	Hold last output
Phase Start	Set SP from recipe parameter (121 °C), switch to Auto
Phase Hold	Hold last output, maintain temperature
Phase Abort	Drive to safe position (close steam valve)

🏭 Pharma context: During SIP sterilization, the temperature must be maintained at ≥121 °C for the validated hold time (typically 20–30 minutes at the coldest point). The control loop must respond quickly to temperature deviations and drive the steam valve to maintain the setpoint. If the temperature transmitter fails, the loop must safely close the steam valve to prevent overheating.

🏭 Example: CIP Flow Rate Control Loop

Property	Value
Name	FIC-101
Description	CIP circulation flow rate control
Pattern	Basic PID
Criticality	Non-Critical
PV Instrument	FT-101 (Flow Transmitter)
PV Range	0–10000 L/h
Final Element	FV-101 (Flow Control Valve) or VSD on CIP Pump
Default SP	5000 L/h
Action	Reverse acting
On Bad PV	Hold last output

📋 Module Data — All module data types
🚨 Alarm — Alarm definitions that may relate to control loops
🛡️ Interlock — Safety interlocks that interact with control loops
📏 Parameter — Parameters used as setpoint sources
Engineering Item — Instruments and valves referenced by control loops
🧠 Algorithm Design — Condition logic that interacts with control loops