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Control of pH Based on a Wiener Model
What does this application do?
Used to control a pH loop whose reagent→pH relationship is an S-shaped (nonlinear) titration curve. The AOI applies the inverse of the nonlinearity to the Setpoint and the Process Variable, allowing the loop to be closed with a linear PIDE that performs stably across the entire pH range (acid, neutral and basic).
General Features
This development includes the following:
- pH_WienerLin Add-On Instruction (Wiener linearization) in Studio 5000
- Preconfigured PIDE instruction for control in the linear domain
- Call routine and associated tags
Advantages:
- Provides an initial reference for the control of highly nonlinear processes (pH, neutralization)
- Enables stable performance without re-tuning the controller when changing pH zone
Limitations/Disadvantages
- Limited to a SISO loop (one controlled variable, one manipulated variable); for multivariable control with constraints, PlantPAx MPC is recommended
- Requires prior identification of the titration curve for PWL mode
Downloads
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Need Help?
If you need help with an application or have feedback from the Innovation Center, please contact us.
Is this application relevant to me?
If you need to control a pH or neutralization loop where a conventional PID oscillates, overshoots or overdoses reagent.
Application áreas - Can be applied in water treatment, effluent neutralization, chemical industry, food and beverage.
How can I make it work?
Requirements: products, tools, prior knowledge.
Hardware
- Any Logix controller (the program is tested on a 1756-L81E ControlLogix 5580)
- pH transmitter with temperature compensation
- Final element: control valve or reagent dosing pump
Software
- Studio 5000 (the program is developed in version 37)
- MS Excel
Prior Knowledge
Basic knowledge of programming and configuration in:
- Studio 5000 (Add-On Instructions, PIDE instruction, Structured Text)
- Process regulatory control
- pH control fundamentals / Wiener models
Implementation Guide
- Step 1
- Step 2
- Step 3
- Step 4
- Step 5
- Step 6
- Step 7
- Step 8
- Step 9
- Step 10
- Step 11
Step 1
Extract and open Extract the files of the delivered program (".ACD" project).
control-of-ph-based-on-a-wiener-model_16x9-phControl_step1_imagen1.jpg
Open the Studio 5000 program by double-clicking the .ACD file. For testing this application, a ControlLogix 5580 controller (1756-L81E) was used.
control-of-ph-based-on-a-wiener-model_16x9-phControl_step1_imagen1.jpg
Step 2
Understand the program structure. Review the main components of the program in the "Controller Organizer":
- pH_WienerLin: Linearizes pH by applying the inverse of the titration curve
- pH_PIDE (PID_ENHANCED): Regulatory control in the linear domain
- Program P_pH_Control: Contains the call routine
- Task T_Process: Periodic task that executes the control
control-of-ph-based-on-a-wiener-model_16x9-phControl_step2_imagen1.jpg
Step 3
Understand the pH_WienerLin AOI
- Open the AOI in the Controller Organizer and review the Logic tab.
- The AOI transforms pH into the linear domain x through the inverse of the nonlinearity: x=10−pH−10(pH−pKw)x = 10^{-pH} - 10^{(pH - pK_w)}x=10−pH−10(pH−pKw)
- It delivers two outputs: Out_SPlin (linearized SP) and Out_PVlin (linearized PV), which feed the PIDE. The Cfg_Mode parameter selects the method: 0 = analytical inverse; 1 = PWL table (actual titration curve).
control-of-ph-based-on-a-wiener-model_16x9-phControl_step3_imagen1.jpg
Step 4
Understand the call routine. In the P_pH_Control routine, verify the execution sequence:
control-of-ph-based-on-a-wiener-model_16x9-phControl_step4_imagen1.jpg
Step 5
Configure the field tags (I/O mapping). Link the program tags to your input/output modules:
| Tag | Type | Link to |
|---|---|---|
| pH_CVout | REAL | Analog output to valve/pump |
| pH_PVFault | BOOL | Channel diagnostics / transmitter fault |
| pH_SP | REAL | Setpoint (initial value 7.0) |
| pH_PV | REAL | Analog input from the pH transmitter |
control-of-ph-based-on-a-wiener-model_16x9-phControl_step5_imagen1.jpg
Step 6
Configure the PIDE instructions.
Open the properties of pH_PIDE and adjust:
| Parameters |
Recommended value |
Issue – Action Taken |
|---|---|---|
| CVEUMax / CVEUMin |
100 / 0 % |
Valve/Pump Output Range |
| PVEUMax / PVEUMin |
+0.1 / −0.1 |
Linear Domain Scaling (Adjust to Match Your Curve) |
| PGain / IGain |
1.2 / 0.05 (seed value) |
Retune After Model Identification |
| DGain |
0 | Common for pH Loops Due to Measurement Noise. |
control-of-ph-based-on-a-wiener-model_16x9-phControl_step6_imagen1.jpg
Step 7
Populate the titration curve (pH_XBrk / pH_YBrk):
- In the tag monitor, complete the 16 values of each array (indices 0–15).
- pH_YBrk: ascending pH (1.0 → 14.0).
- pH_XBrk: linear domain x values (calculated or from actual titration).
- Verify that pH_YBrk is strictly ascending and without consecutive repeated values
control-of-ph-based-on-a-wiener-model_16x9-phControl_step7_imagen1.jpg
Step 8
Download the program to the controller:
- Configure the communication path (RSLinx / FactoryTalk Linx).
- Verify the project with Verify (Ctrl+K) — it must compile without errors.
- Download the project and set the controller to Remote Run mode
control-of-ph-based-on-a-wiener-model_16x9-phControl_step8_imagen1.jpg
Step 9
Test in Manual mode
- Set the PIDE to Manual from the pH_PIDE tag.
- Verify that when varying pH_PV (or forcing values in simulation), Out_PVlin changes sign when crossing pH 7 (positive in the acid zone, ~0 in neutral, negative in basic).
- Confirm Sts_Linealizado = 1 and that pH_CVout responds manually.
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Step 10
Tune and test in Automatic
- With the PIDE in Manual, use the built-in open-loop autotuner (applies a step in CV and estimates the process model).
- Apply the suggested gains (start with a slow/medium response, suitable for pH).
- Switch to Auto and validate with small SP steps
Step 11
Validation across the pH range. Run SP steps in the three zones and confirm homogeneous performance:
| Zona | test SP | What to observe |
|---|---|---|
| Acid |
3 → 4 | Stable response, no oscillation |
| Neutral |
6 → 8 | Critical zone — no overshoot despite the high gain |
| Basic |
10 → 11 | Consistency with the other zones |
- Operation verification (checklist)
- Project verified without errors (Ctrl+K)
- pH_PIDE with reverse action confirmed
- Field tags mapped to I/O
- pH_XBrk / pH_YBrk arrays correctly populated
- Manual test: linearization and output correct
- Tuning applied and Auto test satisfactory
- Range validation (acid/neutral/basic) homogeneous
control-of-ph-based-on-a-wiener-model_16x9-phControl_step11_imagen1.jpg
Control of pH Based on a Wiener Model
Version 1.0 - August 2026
