LOGO! Analogue PID Control for Constant-Current Electrocoagulatio

1. Application Overview

Electrocoagulation cells used in industrial wastewater treatment rely on a constant current density across sacrificial plate electrodes. As the pollutant load varies, the conductivity of the effluent changes, and as the plates erode the inter-electrode gap grows. Both effects cause the loop resistance to drift, so the applied DC bus voltage must be continuously adjusted to keep the cell current at its target setpoint. With a 0–10 V Hall-effect current transducer on the input side and a 0–10 V phase-angle controller on the AC-to-DC supply on the output side, a single Siemens LOGO! logic module with analogue I/O can implement the entire closed loop without an additional HMI PLC.

This reference design targets the LOGO! 8.3 platform (6ED1052-1MD08-0BA2 base module, or the LOGO! 8.4 release 1.82.01 firmware used in current shipping stock) combined with:

• LOGO! AM2 (6ED1055-1MA00-0BA2) — 2 analogue inputs, 0…10 V or 0/4…20 mA
• LOGO! AM2 AQ (6ED1055-1MM00-0BA2) — 2 analogue outputs, 0…10 V
• LOGO! TDE (6ED1055-4MH08-0BA0) — external 6-line text display with cursor keys
• Ethernet RJ45 on the base module for the built-in LOGO! Web Server

2. Signal Ranges and Scaling

Before any control block is placed, normalise the field signals to engineering units. The LOGO! analogue blocks operate natively in the integer range 0…1000, which represents the raw 0…10 V input. Two scaling factors map this to amperes and to the bus voltage setpoint.

LOGO! scaling is performed with the Analogue Amplifier (B002) and the Analogue Multiplexer blocks. A typical scaling equation from raw to engineering is:

I_cell [A] = AI1_raw × 32 / 1000
V_bus [V]  = AQ1_raw × 110 / 1000

Use LOGO!Soft Comfort V8.4 (or later) to enable the engineering-unit display so that AI1 is annotated as A and AQ1 as V on the TDE and web interface. This is configured under Tools → Options → Display → Analog Inputs/Outputs.

3. System Architecture

The signal path is fully linear and unidirectional apart from the feedback branch. The block diagram below shows the field wiring, the LOGO! I/O addresses, and the software blocks.

4. PID Controller Configuration

LOGO!Soft Comfort provides the dedicated PID Controller block (B014) in the Special Functions catalog. It accepts an analogue setpoint SP, an analogue process variable PV, and outputs a manipulated variable MQ on the analogue output rail. The parameters and the values that produce a well-damped response on a 50–200 L wastewater cell are listed below.

With these settings the closed loop settles in approximately 25 s after a step load change (for example a 30 % conductivity increase when acid dosing is started) and the steady-state error is < 0.1 A.

5. Variable Setpoint via TDE and Web Server

The current target has to be adjustable between roughly 10 A (light contamination) and 28 A (heavy contamination). Two operator paths are exposed:

1. TDE — the four cursor keys on the 6ED1055-4MH08-0BA0 increment and decrement a network analogue NI1 in 0.1 A steps. The current value of NI1 is shown on line 3 of the TDE, with units A declared in the display settings.
2. LOGO! Web Server — the web server hosted on the base module's Ethernet port (default 192.168.0.1, port 80) exposes all network variables as GET and POST resources. The setpoint can be written to /AJ/NI1 with a value in raw units (multiply amperes by 31.25 to convert). Enable Web Server Access → Variables → NI1 Read/Write in LOGO!Soft Comfort and assign a password under Tools → Options → Web Server.

For the web interface, the following HTML snippet can be embedded in any plant dashboard to push a new setpoint to the LOGO!:

POST /AJ/NI1 HTTP/1.1
Host: 192.168.0.1
Authorization: Basic <base64(user:pass)>
Content-Type: application/x-www-form-urlencoded

NI1=625

625 in raw units corresponds to 20.0 A, which is a typical mid-range target during initial optimisation.

6. Alarm Generation: Voltage Ceiling Reached

The PID block cannot by itself report that the bus voltage has reached its ceiling and the loop is open. Add an Analogue Threshold Trigger (B007) on the AQ1 raw value, with On=950 and Off=930 (hysteresis prevents chatter). The output of this block drives:

• Digital output Q1 — LED beacon on the panel door
• Network flag M3 — surfaced on the TDE line 4 and on the web server as a status flag
• Optional email notification via LOGO! CMR (6BK1700-0BA00-0AA0) cellular module using the Send e-mail function in LOGO!Soft Comfort V8.4

Symmetrically, an under-current condition (AI1 raw < 250 after a 30 s warm-up delay) is detected with a second threshold trigger and indicates a broken cable or blown fuse rather than a process alarm.

7. Ladder / FBD Program Layout

The program occupies 26 function blocks and fits in any LOGO! 8 base module. Block numbering is the convention used by LOGO!Soft Comfort V8.4.

1. B001 — Digital input I1 = Run enable (panel switch)
2. B002 — Analogue Amplifier, gain 0.032 on AI1 → AM1 (cell current in A ×10)
3. B003 — Analogue Threshold Trigger, AI1 < 250 → M1 (under-current fault)
4. B004 — On-delay 30 s on M1 → M2 (latched under-current fault)
5. B005 — Analogue Threshold Trigger, AQ1 > 950 → M3 (voltage ceiling)
6. B006 — Wiping relay (pulse) on TDE Up key → increments NI1 by 31
7. B007 — Wiping relay on TDE Down key → decrements NI1 by 31
8. B008 — Limit clamp on NI1 to [313, 875] (10 A…28 A)
9. B009 — PID Controller (parameters per Section 4). Input: SP = NI1, PV = AM1
10. B010 — Analogue Amplifier, gain 1.0 on PID output → AQ1
11. B011 — Q1 = M2 OR M3 (panel alarm)

Save the project as EC_PID.lsc and transfer it through Ethernet using the Tools → Transfer → PC ↔ LOGO! menu. The on-board SD card slot will retain the program through a power loss if Retain → On is checked.

8. Commissioning and Verification

Perform the verification in this order, recording every reading in the commissioning logbook.

1. Wiring check. With the supply isolated, measure the loop resistance from the DC bus bars to ground. Anything below 50 kΩ on a 110 V bus indicates a wiring error that will skew the current reading at switch-on.
2. Signal calibration. Force AQ1 = 0 in MANUAL mode and confirm AI1 < 0.1 A. Force AQ1 = 1000 and confirm AQ1 climbs to 110 V DC within 3 s. Record the no-load AQ1 voltage under PID ‘manual’ at AQ1=1000.
3. Step response. Switch to AUTO with NI1 = 500 (16 A). Apply a 10 % step increase in conductivity by adding a small dose of NaCl. The current should reach 16.0 ± 0.3 A within 30 s and stay inside the deadband thereafter.
4. Ceiling test. Set NI1 = 875 (28 A) on a dry cell. The bus voltage will saturate at ~110 V and Q1 will energise. Confirm the alarm clears within 5 s when NI1 is reduced to 500.
5. Network write test. From a laptop, push NI1 = 313 (10 A) and confirm the TDE line 3 shows SP 10.0A within one scan (~70 ms).
6. Power-loss ride-through. Remove supply for 10 s. On return, the LOGO! should auto-restart in AUTO with NI1 retained at the last value (verify with Retain flag on the network input).

9. Performance Summary and Limits

Beyond 28 A target the phase-angle controller begins to inject significant harmonic distortion into the supply. If higher currents are required, fit an isolation transformer ahead of the controller and consult the local utility for harmonic compliance.

10. Troubleshooting Matrix

11. Variations and Expansion

The same architecture scales to multi-cell installations by repeating the PID block and assigning each one a unique AQ channel on additional AM2 AQ modules. Up to 8 analogue outputs are supported by stacking modules on the right side of the base unit. For sites where conductivity fluctuates faster than the PID can compensate, add a feed-forward term from AI2 (flow rate) by inserting an Analogue Multiplexer between NI2 and the PID's SP input — the multiplexer blends the operator's setpoint with a flow-proportional offset.

For plants that must retain detailed historical data, export the analogue values over the LOGO! Modbus TCP server (register 0x0001 = AI1 raw, 0x0002 = AQ1 raw) into a small SCADA. The base module firmware 1.82.01 implements Modbus TCP without additional licensing.

FAQ