Monitoring Actuator Current with Siemens LOGO! Analog Inputs

1. Overview

Monitoring the load current of an output is a common requirement on small automation cells where the controller must distinguish between a healthy actuator and one that is jammed, mechanically overloaded, or driven past its continuous rating. With a Siemens LOGO! logic module, the relay (or transistor) output itself is not a measuring device, so current must be sensed externally and either fed back as a discrete threshold signal or as an analog value scaled into one of the LOGO!'s 0-10 V / 0/4-20 mA inputs.

This reference covers the three practical sensing paths for a LOGO! application: a current monitoring relay (digital threshold, no LOGO! AI required), an analog current transducer (0-10 V or 4-20 mA into a LOGO! AM2 / AM2 RTD / base-unit AI), and a shunt + instrumentation amplifier approach using devices such as the TI INA240. Each method is matched against the actuator type (DC solenoid, single-phase AC motor, three-phase drive) and the LOGO! firmware generation that has to read it.

By the end of the article you will be able to:

• Pick the correct current sensor class for a DC or AC actuator.
• Wire a transducer output to a LOGO! analog input and apply the right scaling gain/offset blocks.
• Implement an automatic overcurrent trip that drops the output Q without operator intervention.
• Verify the loop with a known test current and an expected AI value.

2. Prerequisites

2.1 LOGO! firmware compatibility

LOGO! 8.3 introduced the "analog comparator with offset" and "analog threshold trigger" blocks used for high/low alarming on AI values. Earlier 8.0/8.1 firmware supports only the standard analog comparator (one reference value). If your fleet is mixed, design the program around the older comparator block so the same LSC file loads everywhere.

3. Current Sensing Methods Compared

4. Choosing the Sensor for Your Actuator

4.1 DC solenoid or small DC motor (≤ 5 A)

The simplest field install is a closed-loop Hall transducer such as the NK Technologies DT series with a 0-10 V output. The DT series measures DC only and tolerates bipolar current, which is useful if the actuator is reversible. Wire its output across the AM2 AI terminals configured for voltage input. For OEM builds where you can place the shunt on the PCB, the TI INA240A1 (gain 20 V/V) with a 10 mΩ shunt gives 200 mV at 1 A, easily scaled to a 0-3.3 V ADC upstream.

4.2 Single-phase AC actuator (≤ 16 A)

Use a fixed-ratio current transformer (e.g. CR Magnetics CR8410-1000) feeding a burden resistor. The CT secondary is typically 5 A or 1 A; pick a burden that converts the maximum expected primary current into 0-10 V. For a 100 A primary CT and a 100 Ω burden, full scale gives 5 V — usable, but you lose half the LOGO! AI range. A 50 Ω burden at the same 100 A primary gives 10 V, which uses the full LOGO! input span.

4.3 Three-phase motor (≤ 50 A line current)

Two practical paths:

1. Use a three-phase current monitoring relay such as the ABB CM-IWN.1S or Schneider RM4-TM that sums and compares the three phases against an absolute window. The output is a single relay contact wired into a LOGO! digital input (I1-I8 or expansion DI).
2. Use three individual LEM transducers and read each on a separate AM2 input; the LOGO! program can then trip on the maximum of the three.

Path 1 is one AI-free input and is preferred when you only need a trip, not per-phase trending.

4.4 Motor drive (VFD) controlled actuator

If the actuator is driven by a SINAMICS V20, PowerFlex, or Altivar drive, the drive itself reports motor current. Wire the analog output of the drive (0-10 V proportional to motor current) directly into the LOGO! AI. Configure the drive's analog output to span 0-10 V across 0-200 % of motor FLA. This removes the external sensor entirely.

5. Hardware Wiring

5.1 LOGO! 230RC base unit (no built-in AI)

The 230 V variants have only digital inputs. Add a LOGO! AM2 (6ED1055-1MA00-0BA2) expansion module on the right-hand bus. The AM2 has two inputs (AI1, AI2) selectable as 0-10 V (default) or 0/4-20 mA via the two DIP switches behind the front cover.

AM2 DIP switch positions:
  Switch 1 ON,  Switch 2 ON  → AI1 = 0-10 V, AI2 = 0-10 V
  Switch 1 OFF, Switch 2 ON  → AI1 = 0-20 mA, AI2 = 0-10 V
  Switch 1 OFF, Switch 2 OFF → AI1 = 4-20 mA, AI2 = 4-20 mA

5.2 LOGO! 12/24RCE or 24RCE (built-in AI)

These base units have four built-in analog inputs (AI1-AI4) on terminals I7/I8 (AI1/AI2) and the dedicated AI3/AI4 terminals. They share their ground with the supply and accept 0-10 V directly. For 0/4-20 mA you must add an external 500 Ω precision resistor across the AI terminal to ground; this converts 20 mA into 10 V at the LOGO! pin.

5.3 Transducer → AM2 wiring (0-10 V)

Transducer 24 V supply ────┐
                            ├── Transducer V+/V-
LOGO! 24 V sensor supply ───┘

Transducer output (0-10 V) ── AM2 AI1 (+)
Transducer GND           ── AM2 GND (-)

5.4 Current monitoring relay wiring (digital trip)

MCB ── Relay contact common ── Relay contact NO
                                   │
                                   ├── LOGO! digital input I1
                                   │
                                   └── LOGO! I1 terminal block GND

Load conductor passes through relay CT aperture (single pass for full ratio).

6. LOGO! Programming — Analog Scaling and Trip Logic

6.1 Enabling analog inputs in LOGO!Soft Comfort

1. Open the project and double-click the AM2 block to open its properties.
2. Set the AI to "Used as analog input", enable "0-10 V" sensor type.
3. Save. The AM2 will appear on the analog bus as AI1 (or AI3 depending on expansion slot).

6.2 Scaling 0-10 V into 0.0-10.0 A

If your transducer is rated 0-50 A primary and outputs 0-10 V, the LOGO! must treat 10 V = 50 A. Add an Analog Amplifier block (gain = 5.0, offset = 0) to convert the raw 0-1000 count into 0-500 (= 0.0-50.0 A in 0.1 A units). Alternatively, use a Mathematic Instruction block with the equation:

Result = (AI1 / 100) * 50

This avoids a separate gain block if you need additional functions on the same value.

6.3 Threshold-trigger for overcurrent trip

Insert an Analog Threshold Trigger (LOGO!Soft Comfort block B025, available in firmware 8.3+). Configure:

The trigger's Q output feeds the reset input of an SR latch that drives the actuator's relay output Q1. When AI crosses 3.0 A, the latch resets and Q1 opens, removing power to the load. The 2.5 A hysteresis prevents chattering around the setpoint.

6.4 Pre-8.3 alternative: Analog Comparator with hysteresis

For older firmware, use an Analog Comparator block plus a second comparator to emulate the on/off hysteresis:

1. Comparator A: reference = 30, AI = AI1; output Qa trips when AI > 30.
2. Comparator B: reference = 25, AI = AI1; output Qb trips when AI < 25.
3. Feed Qa into the SR latch reset input, and Qb (delayed with an on-delay B007, 200 ms) into the SR latch set input. This gives manual reset behaviour with a 2.5 A hysteresis band.

6.5 Automatic restart vs manual reset

For an actuator that may unjam itself (e.g. a flap stuck on debris), wire the on-delay Qb output directly to the SR latch set input. The actuator re-energises after the AI falls below the off threshold. For an actuator where a trip indicates a real fault (e.g. motor bearing seizure), keep the latch reset by Qa only and provide a manual "Reset" pushbutton on a LOGO! digital input wired to the SR latch set input.

7. Verification and Commissioning

1. Test the transducer at zero load. With the actuator de-energised, the LOGO! should read 0 V on AI1 (raw count 0). If you read more than 0.05 V (count 5), the transducer has a residual offset; calibrate by subtracting it in a math block or using the AM2 zero-shift parameter.
2. Inject a known current. Apply 50 % of full scale (e.g. 2.5 A on a 0-5 A transducer) using a bench supply and a high-wattage resistor. The LOGO! AI should read 5.0 V ±0.1 V (count 500 ±10). Anything beyond ±2 % indicates a wiring problem or misconfigured sensor type.
3. Trigger the trip. Increase the load until AI crosses the on-threshold. The relay output Q1 must open within one LOGO! scan cycle (~20 ms typical). Verify by measuring the contact with a multimeter.
4. Verify hysteresis. Drop the load until AI falls below the off-threshold. If using manual reset, the relay must NOT re-close. If using auto restart, the relay must re-close only after the on-delay expires.
5. Test fail-safe behaviour. Disconnect the transducer output (simulate open circuit). The LOGO! AI should read 0 V or, on a 4-20 mA loop, 0 V indicating under-range. Configure the program to treat this as a fault and trip the relay.

8. Troubleshooting Matrix

9. Specifications Reference

9.1 LOGO! 8 base unit output ratings

9.2 AM2 analog input specifications

9.3 NK Technologies DT series (representative)

9.4 TI INA240 (shunt-based reference design)

10. Edge Cases and Field Tips

• Inrush on solenoid valves. A 24 VDC solenoid can pull 5-10× its holding current for the first 50-200 ms. Set the AI on-threshold above the inrush, or use the off-delay on the SR latch set input so the trip is disabled during the first 1-3 seconds after energise.
• Three-phase imbalance detection. If your three-phase drive does not offer a current-imbalance output, read three CTs into three AM2 inputs and use a math block to compute the ratio max/min. A ratio > 1.15 indicates a phase-loss condition; trip the contactor accordingly.
• 4-20 mA loop power. A 2-wire 4-20 mA transducer draws its operating current from the loop. The LOGO! AM2 does not source loop power; you must supply 24 V into the transducer and use a 250 Ω sense resistor at the LOGO! terminal to convert 20 mA into 5 V. If you need 0-10 V from a 4-20 mA loop, select a 500 Ω resistor or set the AM2 to 0-20 mA and scale accordingly (4 mA = 0, 20 mA = 1000 counts).
• Common-mode noise on CTs. For AC CTs, use a shielded twisted pair from the secondary to the burden resistor; ground the shield at the LOGO! end only.
• Thermal derating of the LOGO! relay. Above 55 °C ambient, the LOGO! 230RC relay contact rating derates to 6 A resistive. If your continuous load is above 6 A, route it through an external contactor driven by the LOGO! output and sense current on the load side of the contactor.
• Calibration without a known current source. If a bench supply is unavailable, energise the actuator under normal load and read the LOGO! AI value. Compare it with the actuator nameplate FLA. The program can use a one-point field calibration: at the next scheduled maintenance, scale the math block gain so the LOGO! reading matches a clamp-meter reading.