Siemens LOGO! 8 Pool Heating: Hysteresis Control & Web HMI

This reference walks an automation engineer familiar with SIMATIC S7 / TIA Portal through building a complete pool heating controller on a Siemens LOGO! 8.3 base module. It covers hardware selection, LOGO! Soft Comfort project creation, analog scaling, hysteresis control, Text Display (TDE) parameter editing, and the dual web server configuration. Field-verified values, scaling math, and commissioning checks are included.

1. Project Overview and Control Philosophy

The target installation is a small outdoor pool / spa, heated year-round including winter conditions. Ground (unheated) water temperature is approximately 20 °C. The control loop is a classic two-position (on/off) thermostat with selectable hysteresis:

• Process variable (PV): Water temperature, 0–50 °C from a 0–10 V transmitter (e.g. PT1000 + head transmitter).
• Setpoint (SP): Operator-adjustable on the TDE 6ED1055-4MH08-0BA1 or via the LOGO! web server.
• Hysteresis band: 2–3 °C below SP turns the heater ON; 2–3 °C above SP turns it OFF.
• Output: One digital output driving a contactor / SSR for the heating cartridge.

Two-position control is the correct strategy here: a pool has high thermal mass, slow dynamics, and the heater is a single electrical load — a continuous PID would be wasteful and over-engineered. The block to use is the analog comparator with hysteresis (analog threshold switch with adjustable upper and lower thresholds) or the on/off hysteresis block.

2. Hardware Selection

2.1 Base Module

For one analog input (0–10 V) and one digital output, a LOGO! 8 base module is sufficient — no AM2 / AM2 RTD / DM8 expansion is required. The recommended variant is:

Choose the 12/24 RCE when you have a 0–10 V temperature transmitter powered from 24 V DC and need dry-contact relay outputs to switch a 230 V AC contactor coil. The relay contacts handle the inrush of AC-1 / AC-3 contactor coils comfortably within the 5 A rating.

2.2 Text Display (TDE)

The TDE is optional if you use the integrated web server, but recommended for outdoor pool installations where a physical keypad survives touchscreen-unfriendly conditions (wet fingers, gloves).

2.3 Software Tools

Download LOGO! Soft Comfort V8.3 SP1 or later from the Siemens Industry Online Support. The installer includes the documentation and example projects.

3. Software Installation and First Project

3.1 Install LOGO! Soft Comfort

1. Run the installer LOGO!SoftComfort_V8.3_Setup.exe as administrator.
2. Select destination (default C:\Program Files\Siemens\LOGO!SoftComfort V8.3).
3. Install the LOGO! Access Tool if remote mobile HMI is required.
4. Launch LSC and accept the licensing prompt (no hardware dongle — V8 license is a soft key bound to the LOGO! over Ethernet on first upload).

3.2 Project Skeleton

1. File → New. In the project dialog select device LOGO! 8.3 12/24 RCE (6ED1052-1MD08-0BA1). This determines the available I/O and instruction set.
2. Set the Ethernet IP address in Tools → Ethernet Connections (default: 192.168.0.10, mask 255.255.255.0).
3. Set time/date in Tools → Set Clock. The LOGO! has a capacitor-backed real-time clock (typical 80 h backup).
4. Save the project as pool_heating.lsc.

There is no HW Config equivalent. The block diagram implicitly consumes I/O: placing an Analog Input block onto terminal AI1 binds physical input I1 to the block. Compiler diagnostics will flag mismatches between the selected base module and blocks you place.

4. Analog Input Scaling (0–10 V → 0.0–50.0 °C)

A LOGO! analog input converts 0–10 V to the integer range 0–1000. Display with one decimal place is achieved by the Gain / Offset math of the analog amplifier block (B001 family).

4.1 Block Math

The amplifier formula is:

Y = (X - Offset_in) × Gain + Offset_out

Simplified form (zero input offset):

Y = X × Gain + Offset

For 0–50.0 °C display, the engineering range the downstream blocks must work with is 0–500 (one decimal). For full-scale:

• X_max = 1000 (10 V at input)
• Y_max = 500 (50.0 °C)
• Gain = Y_max / X_max = 500 / 1000 = 0.50
• Offset = 0

Configure the amplifier block parameters:

4.2 Example: 10 V Input

With sensor delivering 10 V (full scale), X = 1000. Calculation through the block:

Y = 1000 × 0.50 + 0 = 500

Rendered on the TDE / web display as 50.0 °C (one decimal place = divide by 10 at HMI layer).

4.3 Example: 5 V Input (25 °C)

Y = 500 × 0.50 + 0 = 250 → 25.0 °C on the display.

4.4 Bipolar Sensor Scaling (−30 to +60 °C)

For an outdoor air sensor with range −30 to +60 °C and 0–10 V output, the engineering range with one decimal is −300 to +600. Solve for linear mapping:

• Y(X) = m·X + b
• At X = 0: Y = −300 → b = −300
• At X = 1000: Y = 600 → m·1000 + (−300) = 600 → m = 0.9

5. Hysteresis (On/Off) Control Programming

5.1 The Two Comparator Thresholds

Implement a Schmitt-trigger style thermostat using a single analog threshold switch with hysteresis. The block (B007 in older LSC, Analog Threshold Switch in V8.x with Hysteresis option) has two thresholds:

• ON threshold (Th_low): Switches output ON when PV falls below this value.
• OFF threshold (Th_high): Switches output OFF when PV rises above this value.

For a 2 °C hysteresis band centered on SP:

Th_low = SP − 2 °C = SP − 20 (engineering units)

Th_high = SP + 2 °C = SP + 20

5.2 Reference Circuit (FBD)

1. Place an Analog Input block on AI1 → 0–10 V.
2. Insert an Analog Amplifier block (B001) with Gain = 0.50, Offset = 0, 1 decimal place. Output PV = water temperature × 10.
3. Place a Setpoint block (B002) that holds the operator-adjustable SP. Tag it as editable from the message text and TDE.
4. Place an Analog Threshold Switch with Hysteresis block (B003):
   • Input Ax = PV (B001 output)
   • Th_low = B002 SP − 20
   • Th_high = B002 SP + 20
   • Gain = 1 (no scaling inside this block)
5. Route B003 output Q to digital output Q1 (heater contactor).
6. Optional: AND B003 Q with a "Heating Enable" flag (from a digital input or scheduled timer) to allow summer shut-off.

5.3 Sample Equivalent Statement List (LSC View → STL)

While LOGO! 8 does not offer text-based programming, the equivalent netlist is generated by the compiler. The relevant boolean equivalent:

// PV is in 0..500 (×0.1 °C)
// SP is in 0..500 (×0.1 °C), editable
// HYST = 20  (i.e. 2.0 °C)

IF PV < (SP - HYST) THEN
    Q1 := TRUE;          // heater ON
ELSIF PV > (SP + HYST) THEN
    Q1 := FALSE;         // heater OFF
END_IF;

6. Text Display (TDE) Setup

6.1 Physical Connection

The TDE 6ED1055-4MH08-0BA1 ships with a 3 m RJ45-to-RJ45 cable. Plug one end into the LOGO! base module's second Ethernet port, the other into the TDE. Power is supplied via PoE-style phantom (no separate supply needed). Maximum cable length is the standard Ethernet limit (≤ 100 m with Cat 5e / Cat 6).

6.2 Message Text with Editable Setpoint

To allow the operator to change the setpoint from the TDE:

1. In the LSC diagram, double-click the Setpoint block (B002). Tick Parameter set / Remanence as required and check Editable from message text / TDE.
2. Place a Message Text block on the diagram.
3. Set Display target:
   • LOGO! display only
   • TDE only
   • Both LOGO! and TDE
   • + Web server (simple web server can render the message text)
4. In the message text configuration, set the line content to SP: followed by the B002 tag, and tick Editable for the SP field.

6.3 Editing a Setpoint from the TDE

1. Navigate to the message text containing the setpoint.
2. Hold ESC for > 2 s. A black highlight bar appears over the first editable field.
3. Use ▲ / ▼ to move the bar between fields.
4. Press OK on the chosen field — it turns into a blinking box.
5. Use ◀ / ▶ to move between digit positions, ▲ / ▼ to change the digit value.
6. Press OK to commit, ESC to exit edit mode.

6.4 Manual Push-Buttons F1–F4

The TDE has four function keys F1–F4 that map to LOGO! digital inputs (M17–M20 in the FBD flag area). Wire them as digital flags into AND / OR logic to:

• F1 — Manual filter pump ON (latched).
• F2 — Manual filter pump OFF.
• F3 — Force heating program enable.
• F4 — Acknowledge alarms.

Create a latching RS flip-flop for each manual load so that the operator can start and stop the filter pump from the keypad independent of the automatic program.

7. Web Server Configuration

LOGO! 8.2 and 8.3 ship with two web servers running in parallel. The MAC address label and QR code on the front of the base module provide the default IP (192.168.0.10).

7.1 Simple Web Server

URL: http://<LOGO_IP> (default port 80). No SD card required. The simple web server mirrors the on-device display and TDE:

• Virtual LOGO! display and TDE representation.
• Programmed message texts render here too (if "Display target" includes the web server).
• Buttons are clickable from a browser / LOGO! Access Tool app.
• Setpoint edit and acknowledgment work the same as on the TDE.

7.2 Extended Web Server

URL: http://<LOGO_IP>/<user_dir> (default port 80, separate path). The extended web server serves user-defined HTML pages authored in LOGO! Web Editor (LWE). Features:

• Custom graphics, your pool layout, gauges, trend plots (JavaScript-driven from JSON status).
• Read/write of all LOGO! process variables, markers, and block parameters.
• Programmed message texts are not rendered here — only your custom pages.

7.3 SD Card Requirements (Extended Web Server)

7.4 Transferring an LWE Project to the LOGO!

1. Author the web pages in LWE on a PC. Use placeholders linked to LOGO! variable tags.
2. Build the LWE project (LWE exports a ZIP archive and an upload index file).
3. Insert the SD card into the PC and copy the LWE export files into the directory structure expected by the extended web server (default user folder under /<user>).
4. Insert the SD card into the LOGO! base module. The LOGO! will detect logo_user.aff / logo_user.lwf and mount it.
5. Browse to http://<LOGO_IP>/userdir/index.htm to view the custom pages.

8. Commissioning Procedure

1. Pre-power checks: Verify the 24 V DC supply polarity, 0–10 V sensor wiring (signal +, ground), and that the heater contactor coil voltage matches the relay contact rating (5 A / 250 V AC).
2. Power up LOGO!. Observe the RUN/STOP LED — should be solid green within 5 s.
3. Connect LSC: Tools → Ethernet Connections → Connect. Online → observe should show all block I/O states live.
4. Sensor calibration: Immerse the PT1000 (with transmitter) in an ice/water slurry (0 °C) — adjust transmitter zero until the LOGO! reads 0.0 °C. Place in boiling water (100 °C, altitude-adjusted) — adjust transmitter span. Most 0–10 V transmitters have multi-turn pots on the front.
5. Force the heater output: In LSC online view, force Q1 to TRUE; verify the contactor pulls in and the heater draws rated current.
6. Functional test of hysteresis: Lower SP below current PV; heater should turn OFF. Raise SP above PV + hysteresis; heater should turn ON after the band is crossed.
7. Test the TDE setpoint edit per the procedure in §6.3.
8. Test web access: From a phone on the same SSID, open http://192.168.0.10. Verify the virtual TDE renders and setpoint changes propagate to the program.
9. Test extended web server (if used): Browse the custom LWE pages; toggle a flag from the web and confirm the LOGO! responds.
10. Run for 24 hours: Log PV and Q1 history if you wired the LOGO! data log to a micro SD card. Verify cycle rate is reasonable (a few cycles per hour, not short-cycling).

9. Field-Proven Caveats

• SD card holder alignment. The micro SD socket is spring-latched. If the card will not seat fully, the card is misaligned, not stuck — pull it out, inspect the pins for bent tines, and re-insert squarely. Forcing it bends the pins permanently; the holder is a replacement part (not a service item, replace base module).
• Sensor cable length. A 0–10 V signal is sensitive to induced noise on long cable runs. Use shielded cable, ground the shield at the LOGO! end only, and keep the analog cable physically separated from contactor coil wiring.
• PT1000 vs 4–20 mA. PT1000 + head transmitter is the most common residential pool solution. If the pool is > 30 m from the LOGO!, switch to 4–20 mA with a 250 Ω shunt — the LOGO! AI cannot read 4–20 mA directly; use an external 250 Ω → 1–5 V conversion resistor and re-scale.
• Heater inrush. A 3 kW heating cartridge switched by a 25 A contactor has a coil inrush of ~ 10× holding current for 50 ms. The LOGO! relay is rated 5 A steady; derate to 3 A for > 100 k cycles. The contactor coil is well within the relay's surge capability, but a dedicated interposing relay is wise if a latching contactor or three-phase contactor is used.
• Winter operation. At ~ 0 °C ambient, condensation inside the cabinet is a real risk. Use a 20 W cabinet heater and an IP65 enclosure.
• Time-of-use tariffs. If the pool is heated on a time-of-use tariff, add a weekly timer block and gate the heater enable with the cheap-rate window.

10. Troubleshooting Matrix

11. Acceptance Checklist

• [ ] AI1 reads 0.0 °C in ice water and 50.0 °C in boiling water (±0.5 °C).
• [ ] Heater turns ON when SP is raised 5 °C above PV, OFF when PV exceeds SP + 2 °C.
• [ ] Hysteresis band is symmetric (2 °C below, 2 °C above) and the cycle rate is < 6 starts/hour.
• [ ] TDE setpoint is editable per the §6.3 procedure; new value persists across power cycle.
• [ ] Simple web server is reachable from a phone on the LAN; setpoint edit propagates to the program within 1 s.
• [ ] Extended web server (if used) loads the LWE custom page; toggle a flag and verify LOGO! response.
• [ ] All message texts configured for both LOGO! and TDE display targets.
• [ ] SD card (if used) is FAT32, ≤ 32 GB, and seated correctly without play.
• [ ] Manual F-key push-buttons toggle the filter pump and heater enable as expected.
• [ ] 24-hour soak test recorded with cycle count < 30 (1 start every 1.5–2 h typical for 3000 L pool).