Pulse Relay Control of 3-Point Valve with LOGO! 8 (0BA8)

Application Overview

A weather-compensated hydronic heating system typically uses a three-way mixing valve to blend flow from the boiler with return water, holding the supply temperature at a value derived from the outdoor temperature and the indoor setpoint. When the existing installation has a Belimo three-point (open/close) actuator wired to two mechanical pulse relays, the controller must reproduce that behaviour with solid-state logic: a short pulse on relay Q1 drives the actuator toward "more flow" and a short pulse on Q2 drives it toward "less flow". The actuator's integral capacitors and self-holding gearing keep the last commanded position once the pulse ends.

This reference documents a working implementation on a Siemens LOGO! 8 base module type 6ED1052-1CC08-0BA1 (LOGO! 24CEo, hardware revision 0BA8) extended with an AM2 RTD (6ED1055-1MD00-0BA8), an AM2 AQ (6ED1055-1MM00-0BA8), and a DM8 12/24R (6ED1055-1MB00-0BA8). All temperatures, setpoints, alarm limits, and PI gains are exposed on the LOGO! TDE text display (6ED1055-4MH08-0BA1) and are editable without a PC.

Three actuators are involved in the complete system:

• Primary mixing valve (3-point, pulse relay control)
• Outside-air damper (analog 0-10 V from AM2 AQ channel 1)
• Extract-air damper (analog 0-10 V from AM2 AQ channel 2)

The article focuses on the digital pulse-relay branch and on the AM2 RTD analog acquisition, because those are the parts that give beginner installations the most trouble.

Hardware Reference: LOGO! 8 Base Module 0BA8

The 0BA8 generation (also marketed as LOGO! 8.1 / 8.2 / 8.3 depending on firmware) introduced a 10x faster scan engine, a real-time clock, an Ethernet port, and micro-SD project storage. The 24CEo variant has the following key ratings that matter for this application:

The four on-board relay outputs of the 24CEo are typically allocated to one of the pulse-relay directions and the boiler enable. The remaining pulse direction lives on the DM8 12/24R expansion module, which provides 4 relay outputs rated 5 A resistive. For full module specifications see the Siemens LOGO! product page and the LOGO! 8 system manual on the Siemens Industry Online Support portal.

Expansion Module Detail

The 0BA8 firmware (≥ 8.3) is required to configure the AM2 RTD measurement mode through the slot Properties menu; older 0BA6 / 0BA7 modules are not pin-compatible and the same project file will not bind the AM2 RTD correctly. Always confirm the exact order number on the side label of the module before wiring.

Belimo 3-Point Actuator Behaviour

Belimo small rotary and linear actuators that carry the "3-point" or "open/close" suffix (LR..-3, NR..-3, AR..-3, TR..-3, LM..-3) drive an integrated synchronous motor through a gear train. Three conductors leave the actuator:

• Lead 1 (typically black/blue) - 24 V AC/DC common
• Lead 2 (typically red) - "Open" / increase
• Lead 3 (typically white/grey) - "Close" / decrease

When 24 V is applied across leads 1 and 2 the actuator runs toward the "open" position; when applied across leads 1 and 3 it runs toward "closed". The actuator runs as long as voltage is present and stops as soon as the voltage is removed. There is no minimum pulse length in the strict sense, but the internal starting capacitor needs at least 100 ms to latch the start winding.

Typical running times for residential Belimo 3-point actuators:

Because the actuator self-holds once motion stops, the controller can send a short pulse and then release the relay. Pulse length is the only mechanism for fine-positioning: a 200 ms pulse on a 90 s actuator moves the valve about 0.22 % of its travel. Use longer integrated pulses (PI controller output) for normal operation and short fixed pulses (1 s) for manual jog from the TDE.

Pulse Relay Wiring, Interlock and Contact Protection

The classic 3-point relay stage is two SPDT contacts in an H-bridge-like configuration with mechanical or electrical interlocking. With LOGO! relays a software interlock is mandatory because the contacts are physically separate and both directions share the same common terminal inside the module.

For a Belimo 24 V AC actuator the same circuit is used but the 24 V AC supply is taken from a control transformer. The contact rating of the LOGO! relay (3 A inductive at 230 V AC, 2 A inductive at 24 V DC) easily covers a single small Belimo. For larger LM.. or AF.. actuators (5 to 7 W running), put an interposing relay (Finder 55.34, 10 A) between the LOGO! output and the actuator.

Software Interlock (mandatory)

In LOGO! Soft Comfort, place two AND blocks that receive the open/close request and the negated state of the other direction:

Open_pulse_enable  = Open_request  AND NOT Close_request
Close_pulse_enable = Close_request AND NOT Open_request

If the controller ever sets both requests high (for example after a download during runtime), the AND gates block both outputs. Add a 1 s on-delay and 1 s off-delay to the enable signal to keep the pulse length deterministic and prevent relay chatter. A state-machine implementation is cleaner and is shown below.

Contact Protection

A Belimo motor winding looks like an inductor with a ferrite-cored motor. When the contact opens, the collapsing field generates a back-EMF that can reach 5-10x the supply voltage and pits the silver-alloy contact. Even though Belimo actuators contain an RC network on the input terminals, contact life is roughly tripled with an external suppressor wired as close as possible to the actuator:

Snubbers also reduce the radiated EMI that can disturb the PT1000 lead on the AM2 RTD if both cables run in the same duct. Keep a minimum 200 mm separation between 24 V switching cables and the screened sensor cable.

AM2 RTD Wiring and Analog Signal Conditioning

The AM2 RTD accepts two PT100 or PT1000 sensors on the front plug. Three terminals per channel: I1a, I1b, I1c (and I2a, I2b, I2c). For 2-wire connection, the sensor is wired between Ixa and Ixc with Ixb left open. For 3-wire connection (recommended for any lead over 3 m), the two same-colour leads go to Ixa and Ixb and the third lead to Ixc.

PT1000 is the right choice for residential heating: the lead resistance of a 20 m run of 0.75 mm² copper is 0.95 Ω, producing a worst-case 2-wire error of 0.25 °C. PT100 on the same cable produces a 2.5 °C error, which is enough to swing the heating curve badly.

The screen is bonded to ground at the LOGO! end only. Do not ground both ends; that creates a ground loop that injects 50 Hz into the measurement. See the AM2 RTD wiring diagram in the LOGO! 8 system manual for the recommended shield termination.

Configuration in LOGO! Soft Comfort

1. Open the project, right-click the AM2 RTD slot, choose Properties.
2. For channel 1 (outdoor sensor): type = PT1000, wire = 3-wire, smoothing = 4 samples.
3. For channel 2 (flow sensor on the heating pipe): type = PT1000, wire = 3-wire, smoothing = 8 samples.
4. Click PC > LOGO! to send the project and write the slot configuration to the module.

If the AM2 RTD reports a constant -50 °C, +200 °C, or 0 °C, the sensor type has not been written to the module. Power-cycle the LOGO! and re-send the program.

Analog Amplifier and Display Scaling

The AM2 RTD output is a normalised integer in the LOGO! range 0 to 1000 (representing -50 °C to +200 °C with a gain of 4 LSB/°C). The "Analog Amplifier" block in LOGO! Soft Comfort performs the scaling:

The displayed value on the TDE is the real engineering unit (for example "21.4 °C"). Without the amplifier the TDE would show the raw LOGO! integer, which is meaningless to the user.

Weather-Compensated Setpoint Calculation

The supply temperature setpoint is calculated from the outdoor temperature and the indoor setpoint using a linear heating curve:

T_flow_set = T_room_set + slope * (T_room_set - T_outdoor)

where:

• T_flow_set is the supply temperature in °C
• T_room_set is the desired room temperature (default 21 °C)
• T_outdoor is the measured outdoor temperature in °C (signed, so -15 °C is -15)
• slope is a curve gradient (typical 0.8 to 2.5 for floor / radiator mix)

Example: T_outdoor = -10 °C, T_room_set = 21 °C, slope = 1.6 gives T_flow_set = 21 + 1.6 * (21 - (-10)) = 21 + 1.6 * 31 = 70.6 °C. The boiler is asked to deliver 70 °C water to the radiators when it is freezing outside. Typical slope values:

Implement the math using two "Mathematical Instruction" blocks: one to compute the difference (T_room_set - T_outdoor), one to multiply by slope and add T_room_set. The LOGO! has a built-in PI controller block; pass the flow setpoint to its SP and the measured flow temperature to its PV. The CV output of the PI is then converted to a pulse width using a pulse-width modulator (clock generator + counter) before being routed to Q1 / Q2.

PI Controller Tuning

Start with conservative gains and trim during commissioning:

Use LOGO! Soft Comfort's online monitor to watch the PV and CV while injecting a manual setpoint step change. The CV is a signed percentage: positive drives Q1 (more flow), negative drives Q2 (less flow). A CV deadband of ±3 % prevents continuous dithering of the relay contacts.

For a 3-point valve, Kp = 2.0 and Tn = 120 s gives a settling time around 4-5 minutes for a 10 °C step on a 90 s actuator. If the actuator never reaches the setpoint, raise Kp in steps of 0.5 and re-test.

Pulse Generator: Converting CV to Relay Pulses

The CV of a LOGO! PI controller is a value from -1000 to +1000, not a time. Convert it to a pulse duration with a pulse generator built from a clock and an up/down counter:

T_pulse_ms = 100 + 1000 * ABS(CV) / 1000

where 100 ms is the guaranteed motor-start pulse and the second term scales linearly to a maximum of 1100 ms at full output. On a 90 s actuator that produces a step of about 1.2 % of travel per cycle.

Use a 5 s cycle clock. At the start of every 5 s window, the open or close output fires for T_pulse_ms then releases. The PI loop runs every cycle so the valve slowly tracks the setpoint. A typical block diagram in LOGO! Soft Comfort is:

[PI Controller] --CV--> [Comparator sign] --> [AND with open request] --> [Pulse generator Q1]
                       \---> [Comparator sign negated] --> [AND with close request] --> [Pulse generator Q2]

The two AND gates enforce the software interlock described earlier. Add an off-delay of 4.9 s to Q1 and Q2 so the next pulse cannot start until the cycle window closes.

TDE Text Display Parameters

The LOGO! TDE (6ED1055-4MH08-0BA1) shows the following editable values via the message text function. Each row uses the bar graph or numeric display element:

Editable parameters map to LOGO! variable memory (VM) addresses, which the message text element reads and writes through Tools > Parameter VM Mapping. The retention flag must be enabled (Rem = yes) for setpoints and curve values to survive a power cycle. Without Rem = yes the controller always reverts to the project default on re-start, which is the most common cause of "my setpoint keeps disappearing" reports.

Commissioning and Diagnostic Procedure

1. Disconnect the Belimo actuator leads from the LOGO! outputs and verify the actuator runs correctly by applying 24 V directly to leads 1-2 and 1-3 in turn. Listen for the gear noise; the actuator must reach the end stop in 90 s ±5 s.
2. Power the LOGO! with the AM2 RTD, AM2 AQ and DM8 modules. Send an empty program and verify all three modules appear on the diagnostic page (Tools > Diagnostics) with green status.
3. Wire the PT1000 outdoor sensor to AM2 RTD channel 1. Confirm the temperature reading on the TDE matches a reference thermometer within ±0.5 °C after the smoothing filter has settled (8 samples = 2 s).
4. Wire the flow sensor to AM2 RTD channel 2. Compare to the boiler's built-in gauge or a clamp-on Pt1000 reader.
5. Reconnect the Belimo leads. From the TDE, set Manual = Open, verify the actuator runs toward the open end stop, then set Manual = Close to confirm the second direction. Return to Auto.
6. Trigger a setpoint step change of +10 °C on T_flow_set. Watch the CV bar in the online monitor and confirm only one direction pulses. Verify the interlock by forcing both Open_request and Close_request = 1 in monitor mode; the controller must keep both Q1 and Q2 OFF.
7. Tune Kp and Tn for non-oscillatory settling within three minutes. Record the values on the TDE.
8. Leave the system in Auto and log the CV and PV traces for 24 h. A correctly tuned loop has CV changing sign at most 4-6 times per hour and PV within ±1 °C of setpoint.

Troubleshooting Matrix

FAQ