Siemens LOGO! 24 ON/OFF Level Control with SINAMICS G110 Drive

Overview

This reference covers a complete ON/OFF pump-down level control system built around three Siemens components: a LOGO! 24RCE (or LOGO! 24) logic module, a Prosonic FMU 860 ultrasonic level sensor with 4–20 mA output, and a SINAMICS G110 variable-frequency drive controlling the pump motor. The logic dictates that the drive runs when the water level falls to a low setpoint and stops when the level rises to a high setpoint. The implementation uses an external 500 Ω precision resistor to convert the 4–20 mA loop into a 0–10 V signal that the LOGO! analog input can read.

The two engineering decisions that drive every parameter in the program are:

1. 4 mA × 500 Ω = 2.0 V at the LOGO! input (empty / low setpoint).
2. 20 mA × 500 Ω = 10.0 V at the LOGO! input (full / high setpoint).

The LOGO! displays this voltage scaled to 0–1000 internal units, so the switch-on threshold sits near 200 (≈ 2 V) and the switch-off threshold near 1000 (≈ 10 V). A separate wire-break detector is added below 2 V because a true 4–20 mA loop produces 0 mA (and therefore 0 V) on a broken conductor — this lets the program raise a fault instead of interpreting an empty pipe as "low level".

System Architecture and Components

Prerequisites

• LOGO! Soft Comfort V8.3 (or compatible) installed on a PC and connected to the LOGO! via Ethernet or the programming cable.
• SINAMICS G110 parameter list / operator panel (BOP) for setting analog input scaling and digital input source.
• Calibration of the FMU 860: empty-tank (4 mA) and full-tank (20 mA) trim must be performed with the actual tank before the LOGO! program is finalised.
• Multimeter capable of reading 0–10 V and 0–30 mA with 0.1 % accuracy for verifying the 500 Ω burden.
• Two shielded twisted pairs: one for the FMU 860 loop, one for the G110 control wiring. Keep analog and motor cables in separate conduit runs.

Sensor Signal Conditioning: 4–20 mA to 0–10 V

The LOGO! 24 base unit's analog inputs accept 0–10 V only. The Prosonic FMU 860 delivers a 4–20 mA signal. Adding a 500 Ω burden resistor in parallel with the LOGO! input gives a 2.0–10.0 V measurement that preserves the "live zero" feature of the 4–20 mA standard:

The conversion formula and Ohm's law check:

Wiring the Burden Resistor

1. Connect the positive conductor of the FMU 860 loop to LOGO! terminal AI1 (e.g. terminal I7 on LOGO! 24RCE when configured as analog).
2. Connect the negative conductor of the FMU 860 loop to LOGO! terminal M (analog ground).
3. Solder or screw-clamp the 500 Ω precision resistor directly across AI1 and M, as close to the LOGO! terminals as physically practical. Lead length should not exceed 30 mm to avoid picking up noise.
4. Connect the loop supply (24 V DC +) to the positive input of the FMU 860; connect the loop supply (24 V DC –) to the LOGO! M terminal so the current path is: supply(+) → FMU 860 → AI1 → 500 Ω → M → supply(–).

Wiring the LOGO! 24 PLC to the Prosonic FMU 860

The Prosonic FMU 860 is a two-wire loop-powered transmitter. Wire it as a current source sinking into the LOGO! analog input. The shield of the signal cable should be bonded at the LOGO! cabinet gland only; the sensor end is left floating (or terminated through a 1 nF capacitor if local practice requires).

Wiring the LOGO! Output to the SINAMICS G110

You can drive the G110 from either an analog setpoint or a digital RUN/STOP command. The simplest and most reliable method for an ON/OFF level control is digital RUN/STOP. The analog setpoint can be hard-wired to a fixed value (e.g. 5 V = 30 Hz) or used to vary the pump speed if a soft-start is desired later.

Recommended Method — Digital Enable to the G110

1. On the LOGO!, allocate relay output Q1 (e.g. terminals of a 6ED1052-1CC08-0BA0 relay) to the "drive enable" function in the program.
2. Wire Q1 contact (or one pole of the SPDT output) to the G110 terminal 5 (digital input DIN1 by default for ON/OFF1 with P0701 = 1).
3. Wire the other side of the contact to the G110 terminal 9 (24 V reference) or terminal 2 depending on the G110 variant and the chosen control source.
4. Wire the G110 terminal 8 (digital common / 0 V) to the LOGO! M terminal so both reference potentials match.

Optional Method — Analog Setpoint

If you want a fixed 30 Hz pump speed when enabled, use the LOGO! Q1 to switch a 0–10 V signal to the G110 analog input (terminal 3 = AIN+, terminal 4 = AIN–). Wire a 10 kΩ potentiometer across the G110 10 V reference (terminal 1) and GND (terminal 2), and connect the wiper through a Q1 contact to terminal 3. When Q1 closes, the G110 sees the preset voltage; when it opens, the G110 falls back to 0 V and ramps down.

LOGO! Programming: Threshold Blocks and Hysteresis

The control law needs hysteresis so the pump does not chatter at the high setpoint. Implement it as two analog threshold comparators feeding an SR flip-flop, then route the flip-flop output to Q1.

Block Diagram (LOGO! Soft Comfort)

  AI1 ──┬──►[ Analog Threshold "Off" ]──┐ ON (level high)
        │                                │
        │     gain 1.0                  ──►[ RS Flip-Flop ]──► Q1 (Drive enable)
        │     On  : 1000 (10.0 V)        │
        │     Off :  900 (9.0 V)         │
        │                                │
        │     gain 1.0                  │
        │     On  :  200 (2.0 V)         │
        │     Off :  300 (3.0 V)         │
        └──►[ Analog Threshold "On" ]───┘ SET (level low)

  AI1 ──►[ Analog Threshold "WireBreak" ]──►[ NOT ]──► Q2 (Fault lamp / horn)
              gain 1.0
              On  :   50 (0.5 V)
              Off :  150 (1.5 V)

Threshold Parameters Explained

Why Hysteresis Matters

A single-threshold comparator with the tank at the high setpoint would oscillate: any surface ripple would cross the threshold repeatedly and wear the contactor / pulse the drive. The LOGO! "On" and "Off" parameters built into the Analog Threshold block implement two separate switch points (one for the rising edge, one for the falling edge) and provide inherent hysteresis equal to the difference between the two values.

SINAMICS G110 Configuration

With a 24 V digital input from the LOGO! driving the RUN command, the minimum set of G110 parameters to verify on first commissioning is:

Wire-Break Monitoring

The 4–20 mA loop can detect a broken conductor because the current will fall to 0 mA. A 0 V reading at the LOGO! is therefore a fault, not a "low tank" condition. The "WireBreak" threshold at 0.5 V (50 internal units) raises a fault output and the G110 is forced to coast to stop.

To implement the forced stop in the program, AND the SR flip-flop output with the negation of the wire-break output:

  (RS Q) ──┐
           ├──[ AND ]──► Q1 (Drive enable)
  (NOT WireBreak) ──┘

Recommended visual indicators and outputs:

Commissioning and Verification

1. Sensor trim. With the tank empty, calibrate the FMU 860 to 4 mA (zero trim). With the tank at the high level, calibrate to 20 mA (span trim). Allow at least 30 cm of measurement range for the ultrasonic blanking zone of the FMU 860.
2. Burden check. Disconnect the resistor and measure with a multimeter; the value should be 500 Ω ± 0.5 Ω. Reconnect.
3. LOGO! reading. Power the system. With the tank empty, observe the AI1 value in LOGO! Soft Comfort online mode. It should read between 190 and 210 (2.0 V ± 0.1 V). With the tank full, it should read between 990 and 1000 (≈ 10 V).
4. Threshold trip test. Force the FMU 860 to 4 mA using a calibrator. The "On" threshold must trip, Q1 must close, and the green run lamp must illuminate. Force the FMU 860 to 20 mA. The "Off" threshold must trip, Q1 must open, the green lamp must extinguish, and the drive must ramp down.
5. Wire-break test. Disconnect the positive loop conductor. AI1 must read below 50 (0.5 V), the wire-break output must assert, the red alarm lamp must come on, and Q1 must drop out to stop the drive.
6. G110 trip test. From the BOP or terminal strip, force the drive to ON and verify that it ramps to the fixed setpoint (e.g. 30 Hz) following the ramp time in P1120. Then drop the LOGO! Q1 output. The drive should ramp down following P1121 and stop.
7. End-to-end. Fill the tank to the high level, run the program, and verify the drive remains off. Open the discharge valve to draw down the level. The drive must start when the level crosses the low setpoint and stop when the level returns to the high setpoint, with no chattering across the mid range.

Troubleshooting Matrix

Field-Proven Caveats

• Internal scaling in LOGO!. The 0–1000 internal value is linear with the voltage on the analog input. A 2.0 V signal is therefore 200 units, not 360. The original 360 value comes from a common mistake of treating the sensor range as 0–10 V rather than 2–10 V — a direct consequence of using a 4–20 mA sensor with a burden resistor instead of a native 0–10 V sensor.
• Sensor blanking zone. The Prosonic FMU 860 cannot reliably measure closer than 25–40 cm to the transducer face (check the FMU 860 manual for the exact value). Set the high setpoint well below the physical top of the tank to avoid reading the false echo from the tank roof.
• Surge protection. If the sensor cable runs outdoors or in a lightning-prone area, install a surge arrester (e.g. Dehn BVT) on the loop conductors at the cabinet entry. The 500 Ω burden is not rated to dissipate a lightning surge.
• LOGO! 24RCE vs LOGO! 24RCEo. Both support 0–10 V analog inputs, but the RCEo (without display) has the same AI1–AI4 terminal block. The 500 Ω burden can be soldered directly across the screw terminals on either variant.
• Loop voltage compliance. The Prosonic FMU 860 requires a minimum of 12 V across its terminals to operate. With 24 V supply, 2–10 V across the burden, and ≈ 0.5 V drop in the conductor, the sensor still has ≈ 11.5 V — acceptably close to the limit. If the supply is reduced to 22 V (worst case at end of cable run), the sensor may drop out at 20 mA. Use a 24.5 V nominal supply or a 24 V supply with low impedance.
• Drive enable vs safety stop. The LOGO! relay output is not a safety-rated output. The E-stop circuit must hard-wire to the G110's STO (Safe Torque Off) or the contactor ahead of the drive so the system is fail-safe even if the LOGO! is unpowered or stuck.

FAQ