Siemens LOGO! AM2 AQ 0-10V Output Configuration with PT100 Input

1. Overview

The Siemens LOGO! 8 logic module series can be expanded with dedicated analog signal modules to drive proportional damper actuators, valves, and variable-speed drives. The combination of an AM2 AQ (analog output, 0-10 V / 4-20 mA) and an AM2 RTD (PT100/PT1000 input) provides a complete closed-loop signal chain from temperature sensor to actuator position command. This reference explains the hardware wiring, the LOGO! Soft Comfort V8 program structure, the unit-to-voltage scaling rules, and the practical limits of the AM2 AQ when engineers ask whether the module can output a low-resistance 0-130 Ω signal.

2. AM2 AQ Module Specifications

The Siemens LOGO! 8 System Manual lists the AM2 AQ as a two-channel analog output module. The key electrical and functional specifications are summarized below.

The two channels are wired on the front of the module. Each channel supports either voltage or current output, but not both simultaneously. Configuration is performed in LOGO! Soft Comfort V8 by placing an Analog Output block and selecting the desired source memory (AQ1/AQ2) and range type.

3. AM2 RTD Module for PT100/PT1000 Inputs

For closed-loop temperature control, the AM2 RTD module (part number 6ED1055-1MD00-0BA1) connects two PT100 or PT1000 sensors in 2- or 3-wire mode. The module scales the resistance to a program value of -50 to +200 °C mapped onto 0 to 1000 internal units, so an external scaling amplifier is unnecessary.

If the application uses a 0-10 V or 4-20 mA temperature transmitter instead of an RTD, the standard AM2 (6ED1055-1MA00-0BA1) module is sufficient and supports both voltage and current inputs on two channels.

4. Scaling: 0 to 1000 Program Units

The internal representation of every analog value inside the LOGO! is a 16-bit signed word. The convention used by the AM2 AQ for output is unambiguous and must be applied in all FBD/LAD networks that drive AQ1 or AQ2.

The mapping is linear across the full range. To convert a desired voltage V to the program value required at the input of the AM2 AQ function block, apply the formula:

ProgramValue = V / 10.0 × 1000 = V × 100

For example, a 6.5 V damper position command is 650 program units. For current output in 4-20 mA mode, the conversion is:

ProgramValue = (mA − 4) / 16 × 1000

Using the Analog Amplifier block (B001) in LOGO! Soft Comfort, the engineer scales the engineering-unit value (0-10 V) to the 0-1000 integer expected by the output block. The amplifier gain is then 100 with zero offset for a 0-10 V output.

5. PT100 to 0-10 V Closed-Loop Program Structure

A typical demand for the AM2 AQ is to drive a Belimo or similar proportional damper actuator based on a PT100 supply-air sensor. The minimal program chain is described below. All blocks are standard in the LOGO! Soft Comfort V8 function library; no user-defined functions are required.

5.1 Block Sequence

1. AI1 (AM2 RTD channel 1) delivers the raw PT100 value scaled to 0-1000 (corresponds to -50 to +200 °C).
2. Analog Amplifier (B001) re-scales the process variable from 0-1000 to engineering units. With sensor_min = 0 °C and sensor_max = 100 °C, gain = 100/1000 = 0.1; offset = 0.
3. Up/Down Counter (B003) or setpoint from TDE arrows provides the setpoint, also in engineering units.
4. PI Controller (B014) executes the closed-loop regulation. SP = setpoint, PV = measured value. Output is in program units 0-1000.
5. Analog Output block maps the PI controller output directly to AQ1 of the AM2 AQ (range: 0-10 V).

5.2 FBD Reference Implementation

     AI1 (AM2 RTD)              Up/Down Counter
         |                              |
         v                              v
   [B001 Analog Amplifier]      [B001 Analog Amplifier]
   Gain=0.1, Offset=0            Gain=0.1, Offset=0
         |                              |
         v                              v
   [B008 Analog Comparator] (limits PV within sensor range)
         |                              |
         +------------+ +---------------+
                      | |
                      v v
              [B014 PI Controller]
                SP   PV
              KC = 1.0
              TI = 30 s
              DIR = +
                      |
                      v
              [Analog Output -> AQ1]
                      |
                      v
             AM2 AQ (channel 1, 0-10 V)

Operator setpoint adjustment is implemented through the LOGO! TDE (text display, 6ED1055-4MH08-0BA1) using the four cursor keys (Esc, OK, Left, Right) bound to the Up/Down Counter's Cnt and Dir inputs. The setpoint value is shown in °C and is also written to the program variable feeding the PI controller.

6. Why 0-130 Ω Output Is Not Possible on AM2 AQ

A common application question is whether the AM2 AQ can be used as a programmable resistance source to simulate a PT100 sensor for bench testing of third-party controllers. The answer is no, for three hardware reasons documented in the LOGO! system manual.

1. Output topology: The AM2 AQ is a low-impedance voltage or current source, not a programmable resistor. The output stage uses a buffered DAC and an output amplifier; the terminal voltage is fixed by the DAC, not by a sense resistor and an internal shunt.
2. Minimum load resistance: In voltage mode the output amplifier requires a minimum load of approximately 1 kΩ to maintain linearity and avoid saturation. A 130 Ω load would either shut the output stage down or drive it into current limit, depending on firmware revision.
3. No PT100 ratiometric output: PT100 simulation requires an excitation current source plus a sense input, neither of which exists on the AM2 AQ terminals.

For bench simulation of a PT100 signal the proper tools are:

• A dedicated programmable resistance decade (e.g., Burster 1424, IET Labs PRS-300).
• A PT100 simulator with 4 mA excitation, e.g., the WIKA CEP100.
• A PLC with an integrated resistance output card (e.g., Siemens SM 431 6ES7431-1KF20-0AB0 with 0-600 Ω range) used in calibration/test mode.

7. Minimum Load Resistor Calculation

For voltage-mode operation the AM2 AQ specifies a minimum load R_min to keep the output amplifier inside its linear region. The Siemens datasheet gives R_min = 1 kΩ absolute minimum and recommends 5 kΩ for the published accuracy of ±1.5 % of full scale. The relationship is:

Rload = Vout / Iout_max

where I_{out_max} is the short-circuit current of the output stage, typically 25-30 mA. The worst-case output current for a 10 V command is therefore:

I = 10 V / 1 kΩ = 10 mA

Most industrial damper actuators (Belimo LM.., NM.., SM.. series) present a control input impedance of 100 kΩ or higher, so the 1 kΩ limit is satisfied by an enormous margin. The issue appears only with very low-impedance loads such as older pneumatic I/P converters (typically 130-500 Ω) or direct solenoid coils.

8. Wiring Topology

The following diagram shows the physical layout of a LOGO! 8 base with the AM2 RTD and AM2 AQ on the right side, the PT100 sensor on the left, and the proportional damper actuator on the far right. The TDE display is connected to the base module's second interface.

8.1 PT100 Wiring Notes

• Use 3-wire connection for run lengths above 5 m to cancel lead resistance.
• Keep the PT100 cable in a dedicated conduit, at least 200 mm from VFD output cables.
• Connect the shield at the AM2 RTD end only; leave the sensor end floating.

8.2 AM2 AQ Wiring Notes

• Output terminals are V+ and I+ combined on the front of the module. The M terminal is the 0 V reference.
• In voltage mode, connect V+ to the actuator positive input and M to the actuator negative input.
• In current mode, connect I+ to the actuator positive input and M to the actuator negative input.

9. Program Construction in LOGO! Soft Comfort V8

9.1 Library Block Selection

9.2 PI Controller Tuning

Start with conservative gains and tune using the Ziegler-Nichols open-loop step method:

1. Disable the I-term: set TI = 99 s, KC = 0.5.
2. Apply a setpoint step of 10 % of span and observe the open-loop response on the LOGO! TDE or LOGO! Web Editor.
3. Measure the dead time L (s) and the time constant T (s) from the response curve.
4. Compute Ziegler-Nichols PI gains: KC = 0.9 × T / L; TI = 3.33 × L.
5. Enter these values into the PI Controller block, then verify setpoint tracking and disturbance rejection.

10. Verification and Commissioning

1. Power-up check: With the LOGO! in RUN, the AM2 AQ status LED must be solid green; a red LED indicates a bus fault or missing base module handshake.
2. Open-circuit voltage test: Disconnect the actuator wires and measure the AQ1 terminal with a calibrated multimeter. Force AQ1 = 0 in online test mode; the meter must read 0.00 V ± 30 mV. Force AQ1 = 1000; the meter must read 10.00 V ± 100 mV.
3. Scaling test: Force AQ1 = 250, 500, 750 in sequence; the voltages should read 2.50 V, 5.00 V, 7.50 V ± 1 % of full scale.
4. Sensor substitution test: Replace the PT100 with a precision decade box set to 100.00 Ω (0 °C). The LOGO! TDE must display 0 °C ± 0.5 °C. Verify the corresponding AQ1 output reaches the setpoint value within the PI controller's settling time.
5. Load test: Reconnect the actuator and verify that AQ1 voltage remains within ±1 % of the commanded value at 50 %, 75 %, and 100 % command. If voltage drops, the load impedance is too low; switch to current mode or use a signal conditioner.
6. TDE setpoint adjustment: Use the TDE arrow keys to raise and lower the setpoint across the full range. The AQ1 output must track without dead band or overshoot greater than the PI controller design value.

11. Troubleshooting Matrix

12. Frequently Asked Questions