1. Application Overview
This configuration drives a 0-10 V modulating signal for a three-way mixing valve from a PT1000 water-temperature probe wired into a Siemens LOGO! logic module. The required transfer characteristic is a linear ramp bounded at both ends:
- Probe temperature ≤ +5 °C → output 0 V (valve fully cold-side)
- Probe temperature ≥ +15 °C → output 10 V (valve fully hot-side)
- Between +5 °C and +15 °C → linear interpolation at 1 V per °C
This is a direct open-loop conversion; no PID closed loop is required because the target hydraulic circuit is low-mass and proportional modulation of a mixing valve is sufficient. The hard clamping above 15 °C and below 5 °C prevents the valve actuator from being driven past its physical travel endstops, which would otherwise cause continuous stall current and accelerated wear.
2. Hardware Requirements
| Item | Catalog Number | Function |
|---|---|---|
| LOGO! 12/24 RCE base | 6ED1052-1MD08-0BA2 (8.4) / 6ED1052-1MD00-0BA2 (8.0) | Logic module with built-in 0-10 V output on AQ1 |
| AM2 RTD module | 6ED1055-1MD00-0BA2 | 2× PT100/PT1000/Ni1000 inputs |
| AM2 AQ module (optional) | 6ED1055-1MM00-0BA2 | 2× 0-10 V / 0/4-20 mA outputs, 20 mA drive |
| PT1000 Class A immersion probe | Siemens QAE26 or equivalent | 2-wire or 4-wire connection |
| LOGO!Soft Comfort V8.3+ | 6ED1058-0BA08-0YA1 | Programming software, project transfer to base |
| 3-way modulating valve with 0-10 V actuator | Various (e.g. Siemens VWG44.10) | Final control element |
The 24 V DC supply rail for the LOGO! base and the 24 V supply for the AM2 RTD module are typically the same regulated 24 V PSU. A 100 W bench supply (e.g. Siemens SITOP) is more than adequate; the LOGO! 12/24 RCE base draws approximately 1.5 W with two AI active, and the AM2 RTD adds roughly 0.8 W. A 24 V 2.5 A supply gives a comfortable headroom for the valve actuator's inrush.
3. PT1000 Sensor Electrical Characteristics
A PT1000 is a Resistance Temperature Detector with a nominal 1000 Ω resistance at 0 °C. Its temperature coefficient is approximately 3.85 Ω/°C, almost ten times that of the more common PT100. Between 0 °C and 100 °C the relationship is close to linear, exhibiting a slight upward bow of about 0.4 % of full scale across the full 100 K span — the curve becomes slightly less steep as temperature rises.
| Temperature | PT1000 resistance |
|---|---|
| −40 °C | 842.7 Ω |
| −10 °C | 960.9 Ω |
| 0 °C | 1000.0 Ω |
| +5 °C | 1019.3 Ω |
| +10 °C | 1038.5 Ω |
| +15 °C | 1057.8 Ω |
| +30 °C | 1116.7 Ω |
| +100 °C | 1385.1 Ω |
Over the 5-15 °C band of interest, the PT1000's intrinsic non-linearity is approximately 0.04 °C — negligible for valve-control purposes. Linear interpolation gives an error of under 0.05 °C, well inside the sensor's own Class A tolerance of ±(0.15 + 0.002·|t|) °C. For high-accuracy instrumentation, a Callendar-Van Dusen correction can be applied in software, but it is not required for this hydronic application. The TI application note "Analog linearization of resistance temperature detectors" provides a more thorough treatment of the polynomial compensation math for those who need it.
For cable runs over 5 m, use a 4-wire PT1000 and the 4-wire mode of the AM2 RTD module to cancel lead resistance (each 1 Ω of lead resistance translates to a 0.26 °C error in a 2-wire connection). For runs under 5 m, 2-wire is acceptable, and the AM2 configuration provides a lead-resistance compensation offset that you can dial in by measuring the loop resistance with a 4-wire ohmmeter and entering the value directly.
Further background on RTD selection, including the trade-off between PT100 and PT1000, self-heating, and noise immunity, is available in the Analog Devices article "How to Select and Design the Best RTD Temperature Sensing System".
4. Signal Flow and Linearization Math
The required conversion is a piecewise linear function:
V_out = 0 V for T ≤ 5 °C
V_out = (T − 5) for 5 °C < T < 15 °C
V_out = 10 V for T ≥ 15 °CThe slope of the active region is exactly 1.0 V/°C, so any linear block must implement a gain of 1.0 and an offset of −5.0 °C. The AM2 RTD module returns temperature in 0.1 °C increments, so the integer AI value is five °C = 50 raw units and fifteen °C = 150 raw units. The LOGO! AQ1 expects an integer 0-1000 representing 0-10 V, so the required transfer function in raw integer units is:
AQ_raw = clamp(0, 1000, (AI_raw − 50) × 10)This single formula captures both the linear ramp and the dual-end clamping. Implementation in LOGO!Soft Comfort uses an Analog Amplifier block (B043) with explicit Min/Max output limits, which delegates clamping to the amplifier's saturation logic. The output is then wired directly to the AQ1 function block.
5. Signal Flow Diagram
6. Hardware Wiring
- Mount the LOGO! 12/24 RCE base on a DIN rail and clip the AM2 RTD module to its right side. The expansion bus connector is pre-wired via the slide contact.
- Apply 24 V DC to the base terminals L+ (24 V) and M (0 V) on the base unit.
- Connect the PT1000 probe to the AM2 RTD module's I1+ and I1- terminals. For 4-wire probes, use I1+, I1-, and the auxiliary terminals; for 2-wire, jumper the compensation pair at the module's spring terminals.
- Connect the 3-way valve actuator's control input to the LOGO! base AQ1 terminals Ua+ and Ua-. These are 0-10 V referenced to the same M (0 V) terminal.
- Use shielded cable (e.g. LiYCY 2×0.5 mm²) for both the PT1000 loop and the AQ1 run. Ground the shield at the LOGO! end only — multiple ground points create a ground loop that injects noise into the analog signal.
- Keep the analog wiring at least 100 mm away from VFD motor cables and contactor coils. If a crossing is unavoidable, cross at 90°.
7. LOGO!Soft Comfort Project Setup
- Launch LOGO!Soft Comfort V8.3 or V9.0 and create a new project.
- From the Tools > Select Hardware menu, choose the LOGO! 12/24 RCE base matching your physical hardware.
- Drag the AM2 RTD module from the catalog into the I/O stack so it appears after the base unit. LOGO!Soft Comfort automatically allocates it the next free module address.
- Double-click the AM2 RTD module in the I/O field to open its properties. Set Channel 1 Sensor type = PT1000, Unit = °C, and Smoothing factor = 4 (gives an exponential time constant of roughly 1 s, sufficient to filter hydraulic / electrical noise without adding perceptible lag).
- Confirm the base module's AQ1 output is set to the default "0-10 V" mode. No further configuration is required at the hardware level — the AQ1 is a hardware DAC, not a soft output.
8. Block-by-Block Program Construction
Open the FBD editor and build the program in the following order.
- Analog Input connector (AI1): Place a connector at the I1 slot of the AM2 RTD module. In the program, this is now a wire carrying the temperature in 0.1 °C integer units. Drag from the connector to the input of the next block.
-
Analog Amplifier (B043): Insert the Amplifier block. Configure the parameters as follows:
• Sensor: "No sensor" (the input is already scaled to engineering units by the AM2 RTD module)
• Min input value: 50
• Max input value: 150
• Min output: 0
• Max output: 1000
• Gain: 1.00 (default)
• Offset: 0 (default) - Output saturation behavior: The Amplifier block does not clamp automatically on its own — it uses linear scaling. To enforce the 0-1000 saturation, set the output value range fields to 0 and 1000. Any input outside 50-150 is then mapped to 0 (below) or 1000 (above). For an additional safety net, set the LOGO! AQ1's value range to 0-1000 — the hardware DAC ignores any value outside this band.
- Analog Output (AQ1): Place the AQ1 function block. Connect the amplifier output to its input. The default 0-10 V output type and 0-1000 integer range require no changes.
- Optional Text Display block (B026): For local readout, drag a Message Text block onto the display, add a bar graph element bound to the amplifier output (0-1000 = 0-100 %), and a text field showing the AI1 raw value (÷10 to display °C with one decimal).
- Save and transfer: Press Ctrl+S, then PC → LOGO! (or F11) to send the program to the base. The LOGO! enters RUN mode automatically if no errors are detected.
The complete FBD requires only two or three functional blocks, plus a connector and the AQ1 output. The total program cycle time impact is under 200 µs on a LOGO! 8.4 base, well below the 10 ms minimum cycle — no performance concerns.
9. Parameter Reference Table
| Block / I/O | Parameter | Setting | Effect |
|---|---|---|---|
| AM2 RTD Ch1 | Sensor type | PT1000 | Selects 1000 Ω nominal RTD |
| AM2 RTD Ch1 | Unit | °C | Returns 0.1 °C integer value |
| AM2 RTD Ch1 | Smoothing | 4 | ~1 s exponential time constant |
| AM2 RTD Ch1 | Lead resistance | 0.0 Ω (2-wire) / measured (4-wire) | Compensates cable drop |
| Amplifier B043 | Min input | 50 | 5.0 °C lower bound |
| Amplifier B043 | Max input | 150 | 15.0 °C upper bound |
| Amplifier B043 | Min output | 0 | 0 V lower clamp |
| Amplifier B043 | Max output | 1000 | 10 V upper clamp |
| Amplifier B043 | Gain | 1.00 | Unity in/out |
| Amplifier B043 | Offset | 0 | No shift |
| AQ1 | Type | 0-10 V | Voltage output |
| AQ1 | Value range | 0-1000 | Integer input |
An alternative formulation uses Gain=10 and Offset=−500 on the amplifier, which produces the same end result and is more compact to display in the LOGO!'s on-device menu. The Min/Max range form is preferred for novice programmers because the engineering intent is visible in the parameter list.
10. Commissioning the 3-Way Valve
- Pre-power check: With the PT1000 disconnected and power applied, the AI1 input reads the underrange sentinel value of -3276.8 (or, on some firmware versions, the lowest possible 16-bit value). The amplifier maps this to 0 because the value is below the configured Min input of 50, so AQ1 = 0 V. Confirm this with a multimeter on the AQ1 terminals.
- Reference temperature check (0 °C): Place the PT1000 in an ice-water slurry (crushed ice + water, well-stirred). After the AM2 RTD smoothing filter stabilizes (~5 s), the AI1 should read 0 (or +1-2 due to Class A tolerance), and AQ1 should remain at 0 V.
- Mid-range check (10 °C): Move the probe to a temperature-calibrated bath (or a beaker of water with a calibrated reference thermometer) at 10.0 °C ± 0.1 °C. AQ1 should read 5.00 V ± 0.05 V. The ±50 mV band includes the LOGO! AQ1's 10-bit DAC quantization (≈ 10 mV per step), the amplifier's 0.5 % tolerance, and the PT1000's ± 0.15 °C Class A error.
- Upper-clamp check (20 °C): Move the probe to 20 °C water. AQ1 should read 10.00 V ± 0.05 V. The probe is well above the 15 °C upper input, so the amplifier saturates the output to its Max of 1000.
- Actuator connection: With the AQ1 verified, disconnect the multimeter and connect the 3-way valve actuator. Stroke the valve manually and observe its position indicator. Then command AQ1 to several setpoints (e.g. 2.5 V, 5.0 V, 7.5 V) using the on-device cursor and confirm the valve position tracks linearly.
- Hunting test: Drive the system through a full thermal transient (cold start to setpoint) and observe the AQ1 trace on the LOGO!'s online view. A well-tuned open-loop ramp with the 5-15 °C band shows a smooth monotone ramp from 0 V to 10 V. Any oscillation indicates hydraulic lag is competing with the ramp — increase the AM2 RTD smoothing to 8-16 to damp it.
11. Verification Procedure
| Test point | Probe temperature | Expected AQ1 voltage | Pass band |
|---|---|---|---|
| 1 | −10 °C | 0.00 V | Clamp — no error bound |
| 2 | 0 °C | 0.00 V | Clamp |
| 3 | +5.0 °C | 0.00 V | 0.00 to 0.05 V |
| 4 | +7.0 °C | 2.00 V | 1.95 to 2.05 V |
| 5 | +10.0 °C | 5.00 V | 4.95 to 5.05 V |
| 6 | +13.0 °C | 8.00 V | 7.95 to 8.05 V |
| 7 | +15.0 °C | 10.00 V | 9.95 to 10.05 V |
| 8 | +30 °C | 10.00 V | Clamp |
The ±0.05 V bound is dominated by the LOGO! AQ1's 10-bit quantization (10 mV per LSB) and the PT1000's ± 0.15 °C Class A error. If the measured voltages deviate beyond these limits, work through the troubleshooting matrix below.
12. Troubleshooting Matrix
| Symptom | Likely cause | Corrective action |
|---|---|---|
| AQ1 stuck at 0 V, AI1 shows -3276.8 | PT1000 open or shorted | Measure probe resistance at the AM2 terminals; replace if reading is infinite or < 800 Ω at room temperature |
| AQ1 stuck at 0 V, AI1 reads correctly | Offset polarity reversed | Verify amplifier Min input = 50, Max input = 150 |
| AQ1 stuck at 10 V, AI1 reads correctly | Min/Max output range inverted | Check that amplifier Min output = 0 and Max output = 1000, not swapped |
| 5 °C point shows ~0.5 V instead of 0 V | Wrong unit assumed (°C × 1 instead of × 10) | Confirm AM2 RTD is set to °C; AI1 should be 50 at 5 °C, not 5 |
| 10 °C point shows 0.5 V instead of 5.0 V | Gain error of 100× (using °C instead of 0.1 °C raw) | Change Min input to 50 and Max input to 150 |
| Output drifts with valve movement | AI smoothing too low (mechanical vibration coupled to probe) | Increase AM2 RTD smoothing factor to 8-16 |
| Output is noisy (jumps ± 0.3 V) | EMI from contactor or unshielded cable | Use shielded PT1000 cable, ground shield at LOGO! end only |
| LOGO! reports "AI out of range" | Sensor configured as PT100 instead of PT1000 | Reconfigure AM2 RTD sensor type in the hardware catalog |
| AQ1 reads negative voltage | Load-side ground loop or shorted actuator | Disconnect actuator, measure open-circuit voltage first |
| AI1 reads stable but AQ1 cycles 0 ↔ 10 V | AI value hovering at the boundary 50 or 150 | Add hysteresis via a second amplifier with offset/offset window |
13. Field-Engineering Notes
- Cable length. The AM2 RTD supports lead lengths up to 100 m when 4-wire is used. For 2-wire installations, keep the loop under 10 m to stay within the module's ± 0.5 % basic accuracy budget.
- Self-heating. The PT1000's self-heating at the AM2 RTD's excitation current is < 0.05 °C in still water. In a flowing pipe with the probe tip in the centre of the stream, the heating error is negligible. Avoid mounting the probe in a dead-leg or dead-end tee where heat does not dissipate.
- Calibration interval. For a hydronic valve-control loop, an annual check using a calibrated reference thermometer in the same bath is sufficient. The PT1000 Class A sensor drifts less than 0.05 °C/year in clean water service.
- Actuator direction. Most 3-way valves use the convention "0 V = fully cold-side, 10 V = fully hot-side". If your actuator is reversed, swap the Min/Max input range to 150 (cold) / 50 (hot), which inverts the curve and the upper/lower clamps.
- Firmware version. This configuration is verified on LOGO! 8.0 (6ED1052-1MD00-0BA2) and LOGO! 8.4 (6ED1052-1MD08-0BA2). On earlier LOGO! 6/7 hardware, the AM2 RTD module is not available and an external PT1000-to-4-20 mA transmitter is required.
- Why not PID? For a 3-way valve in a domestic hot-water circuit, the open-loop ramp is correct: the valve's hydraulic characteristic naturally integrates the flow. A PID is justified only if you need to hold the mixed water within a tight ± 0.5 °C band against load disturbances, in which case add a second PT1000 downstream of the valve and configure the LOGO!'s PI controller (B090) with a setpoint of, say, 38 °C.
14. FAQ
What is the resolution of the LOGO! AM2 RTD module for a PT1000, and how does it map to the AQ output?
The AM2 RTD returns temperature in 0.1 °C increments, so a 5.0 °C reading appears as AI1 = 50 and a 15.0 °C reading as AI1 = 150. After scaling through the amplifier, this is 0 V and 10 V respectively. The LOGO! AQ1's 10-bit DAC quantizes the voltage in 10 mV steps, which is the dominant quantization error in the chain.
Can I implement the 0-10 V ramp using the LOGO! PID block (B090) instead of the Amplifier?
It is technically possible but over-engineered. The Amplifier (B043) is a stateless linear function; the PID requires a setpoint, a feedback signal, and tuning of Kp/Tn/Tv. Use the PID only if you need closed-loop water-temperature regulation against a second PT1000 placed downstream of the valve. For the open-loop ramp described here, the Amplifier is the correct and smallest solution.
Why does the AM2 RTD show -3276.8 °C when the PT1000 is disconnected?
An open PT1000 presents a near-infinite resistance to the module, which the firmware interprets as below the lowest calibrated point of the sensor. The LOGO! display then shows the underrange sentinel value -3276.8. The amplifier maps this to 0 V because the input is below the configured Min of 50, which is the safe and desired behavior for a disconnected probe.
Do I need an AM2 AQ module if the LOGO! 12/24 RCE base already has a 0-10 V output?
No. The base unit's AQ1 is a hardware 0-10 V output suitable for a 0-10 V valve actuator drawing up to 10 mA. Add the AM2 AQ module (6ED1055-1MM00-0BA2) only if you need a second independent output, a 4-20 mA current output, or a higher drive current up to 20 mA for actuators with lower input impedance.
How accurate is the PT1000 linearization over the 5-15 °C band, and do I need a software correction table?
For a 10 °C band centered at 10 °C, the PT1000's intrinsic non-linearity is less than 0.05 °C. This is well inside the sensor's Class A tolerance of ±(0.15 + 0.002·|t|) °C, so no software Callendar-Van Dusen correction is required. The TI application note "Analog linearization of resistance temperature detectors" (SLYT442) provides the polynomial math in case higher accuracy is needed for a different application.
What happens to the AQ1 output if the AM2 RTD module loses communication with the base?
The AQ1 holds its last commanded value. In a valve-control application this is safe (the valve stays at its last position), but for fail-safe behavior, add an Analog Watchdog block that drives AQ1 to 0 V (or to a configurable safe value) if the AI fails to update for more than 5 seconds. This is good engineering practice whenever a process variable drives a final control element.