Turbine Sensor Upgrade: Interfacing Old Pickup and Tachogenerator Signals to a Siemens S7 PLC

Overview

Retrofitting an aging turbomachinery train with a modern Siemens SIMATIC S7 controller is one of the most common lifecycle extensions performed on industrial turbines and centrifugal compressors. According to ABB's gas turbine control upgrade paper, the dominant drivers for such a retrofit are unwanted trips caused by component aging, obsolescence of legacy governors (Woodward 505, GE Mark II/IV/V), unavailable spare parts, and the need to comply with current safety standards such as IEC 61508 / IEC 61511 SIL ratings. The mechanical side of the train — the rotor, bearings, casings, magnetic pickups, and DC tachogenerators — usually remains serviceable for decades, but their low-level analog outputs (often less than 10 V peak) were never designed to feed a 24 V discrete input or a 4–20 mA analog front end of a modern PLC.

This application note focuses on the two most common sensor classes encountered during an upgrade:

• Magnetic pickup (speed probe): sinusoidal output, typically 1 V RMS to 30 V RMS depending on rotor speed and air gap.
• DC tachogenerator: bipolar DC voltage proportional to shaft speed, often 0–30 V, 0–60 V, or 0–110 V full scale.

The objective is to bring these signals inside the input range of a Siemens S7-300 or S7-400 analog or digital input module, with adequate noise immunity, galvanic isolation, and surge protection, while preserving measurement bandwidth and accuracy.

Legacy Turbine Sensor Signal Characteristics

Magnetic Pickup (Variable Reluctance Probe)

A passive magnetic pickup consists of a permanent magnet, a pole piece, and a many-turn coil wound around the pole piece. As gear teeth or a slotted wheel pass the tip, the reluctance of the magnetic circuit changes and an EMF is induced in the coil. Open-circuit output voltage follows:

where N is the number of teeth per second passing the probe, A is the effective core area, and dB/dt is the rate of change of flux linkage. Because both N and the slope of the flux density depend on shaft speed, the output amplitude is strongly speed-dependent. A pickup rated "1 V at 100 RPM with a 60-tooth gear" will produce roughly 6 V at 600 RPM, and 30 V at 3000 RPM. The signal is also a function of the air gap: halving the gap approximately doubles the amplitude.

Typical machine-side measurements encountered in field retrofits:

DC Tachogenerator

A DC tachogenerator is a permanent-magnet DC machine used as a generator. Its output voltage is linear with shaft speed and bipolar (positive for one rotation direction, negative for the other). Common ratings on older turbine trains are 0–30 V, 0–60 V, 0–100 V, or 0–200 V DC at rated speed, with a typical linearity of 0.5 % to 1 % and a ripple of 1 % to 3 % RMS at rated speed. Output impedance is low (typically 50–500 Ω), so the source can drive long cables with minimal attenuation but is sensitive to ground-loop induced offset errors.

Siemens S7 Analog and Digital Input Module Selection

Before designing the signal conditioning, the engineer must identify the destination module and verify which input ranges it supports. The two most commonly used module families for retrofit work are listed below. Confirm the exact catalog number against the Siemens Industry Online Support portal, as several revisions exist (e.g. -0AB0 vs -0AB1) with different feature sets.

SM 331 Analog Input Modules (S7-300)

SM 431 Analog Input Modules (S7-400)

SM 421 Digital Input Modules (S7-400) for High-Voltage Tachogenerator

Signal Conditioning Strategies

Signal conditioning serves four roles on a turbine retrofit:

1. Amplify low-amplitude signals (e.g. 1 V pickup at cranking) to a usable level.
2. Attenuate and scale high-amplitude signals (e.g. 30 V tachogenerator) to the module's input range.
3. Provide galvanic isolation to break ground loops and protect the PLC from surges.
4. Filter electrical noise induced by the VFD, exciter, or adjacent HV cabling.

Passive Voltage Divider for the Tachogenerator

For an inexpensive, isolated solution, a precision resistor divider is the first option, but it must be loaded by a high-impedance input. The general transfer function is:

For a 0–30 V tachogenerator scaled to the 0–10 V input of an SM 331 / SM 431, the ratio is 1:3:

A practical choice is R1 = 20 kΩ (0.1 %, 25 ppm/°C) and R2 = 10 kΩ (0.1 %, 25 ppm/°C). At 30 V input the divider draws 1 mA, negligible for any DC tachogenerator. However, the input impedance of the analog input module (typically 100 kΩ to 1 MΩ) is in parallel with R2, which introduces a scaling error:

With R2 = 10 kΩ and Z_in = 100 kΩ, the parallel combination is 9.09 kΩ, producing a −9.1 % error — unacceptable. The solution is to use R2 = 1 kΩ and R1 = 2 kΩ together with a unity-gain op-amp buffer (or to specify a 1 MΩ input module such as the 6ES7 431-7QH00-0AB0).

Active Op-Amp Attenuator / Buffer

A more accurate topology is a non-inverting op-amp with a feedback divider, which provides gain accuracy set by precision resistors and presents a high input impedance to the tachogenerator:

For 30 V → 10 V, use R_g = 20 kΩ, R_f = 10 kΩ. Power the op-amp from a ±15 V (or +24 V single supply with rail-to-rail output) and add a 10 kΩ / 100 nF RC low-pass filter at the output to attenuate the tachogenerator ripple. Cut-off frequency:

For a 3600 RPM (60 Hz) shaft with 4-pole tachogenerator, the fundamental ripple is 120 Hz — comfortably attenuated. For higher-frequency content choose a lower cut-off (e.g. R = 4.7 kΩ, C = 1 µF → f_c ≈ 34 Hz).

Frequency-to-Voltage Conversion (Recommended for Pickup Sensors)

A magnetic pickup is intrinsically a frequency signal. The cleanest interface is to convert frequency to voltage or to 4–20 mA, using a dedicated F/V converter or signal conditioner such as:

• Acromag 800T Series frequency isolator / F-to-I transmitter.
• Philips / Pepperl+Fuchs KFU8-FSSP-1.D frequency-to-current converter.
• Weidmüller ACT20C-FREQ or ACT20P-FREQ.
• Phoenix Contact MINI MCR-2-F-UI frequency transducer (2902045).

A typical configuration scales 0–10 kHz to 4–20 mA, with 24 V DC loop power and 1.5 kV isolation. This yields a 4–20 mA signal scaled to RPM, terminated on a 250 Ω precision resistor to give 1–5 V, fed to the 0–10 V input of the SM 331.

Pickup Sensor (1 V) Interface Design

Two interface paths exist depending on whether the application needs an analog speed feedback or a digital speed-prove / overspeed trip.

Path A: Analog Speed Feedback

1. Use a dedicated F-to-I signal conditioner (Phoenix Contact 2902045, Acromag 800T-F-IC, Weidmüller ACT20C-FREQ) rated for magnetic-pickup inputs (typically 10 mV RMS sensitivity).
2. Configure the input range to match the lowest expected pickup amplitude (e.g. 50 mV to 30 V RMS) and the output to 4–20 mA corresponding to 0 RPM to maximum RPM.
3. Loop-power the conditioner from the 24 V DC PLC power supply and terminate the current loop on a 250 Ω, 0.1 % resistor to obtain 1–5 V at the SM 331 / SM 431 input.
4. In the STEP 7 hardware configuration (HW Config) of TIA Portal or STEP 7 Classic, set the analog input channel to 0–10 V and apply a linear scale in the user program: RPM = (AIW − 5530) / 27648 × (RPM_max − RPM_min) + RPM_min with the 1 V / 5 V calibration points set against zero and full scale.

Path B: Digital Speed-Prove (Two-Out-Of-Three Voting)

Turbine overspeed protection systems (API 670, IEC 61508 SIL 2/3) typically use three independent speed probes with a 2-out-of-3 voting logic. The signal conditioning block is the same as above, but the conditioner output is wired to a 24 V digital input of the SM 321 (or SM 421 for S7-400) through a threshold comparator. A zero-crossing detector on the conditioned sine wave (or a Schmitt trigger on the square-wave output of the F-to-I converter) provides a clean 24 V square wave to the PLC's high-speed counter (e.g. SM 338, 6ES7 338-4BC01-0AB0, or the built-in counters of the CPU 317 / CPU 416).

For SIL-rated overspeed, use a separate, hardwired safety relay (Pilz PNOZ, Sick Flexi Soft, or Siemens SIRIUS 3SK1) as the final trip element. The PLC is for indication and control only.

Tachogenerator (30 V) Interface Design

1. Confirm the tachogenerator's rated output voltage, maximum RPM, and output impedance. Typical turbine-train ratings: 0–30 V at 3600 RPM, 0–60 V at 6000 RPM.
2. Select a dedicated tachogenerator isolator/transmitter. Acromag 831T-1000 and Weidmüller ACT20P-UI-2RCO accept 0–50 V input and provide 0–10 V, 0–20 mA, or 4–20 mA output with 1.5 kV isolation.
3. Wire the tachogenerator output to the conditioner, observing polarity. Reverse polarity produces negative speed, which a properly designed bipolar conditioner can accept.
4. Scale the output to the 0–10 V input of the SM 331 or SM 431.
5. In STEP 7 / TIA Portal, calibrate the channel against a reference tachometer. Many older tachogenerators exhibit a 1–3 % non-linearity, so consider a 5-point calibration table in the PLC.

Wiring, Shielding, and Grounding

No amount of signal conditioning will overcome a poorly grounded installation. On turbine retrofits the single largest source of measurement error and channel damage is ground loops between the machine bedplate, the cable tray, and the PLC cabinet.

• Use twisted, shielded pair cable (BELDEN 8761, LAPP ÖLFLEX 110 CY) for the pickup and tachogenerator runs.
• Ground the shield at one end only — typically at the PLC cabinet entry — to avoid ground loops. Use a dedicated EMC cable gland bonded to the cabinet backplane.
• Maintain at least 200 mm separation from VFD power cables, DC exciter cables, and any cable carrying >50 A.
• Cross power cables at right angles when separation is impossible.
• Install surge protection devices (e.g. Dehn DEHNguard, Phoenix Contact PT-IQ) on any field cable entering the cabinet from the turbine skid.
• Bond the turbine skid, the cable tray, and the PLC cabinet backplane to a single equipotential ground ring. Reference: IEC 61000-5-2 and IEEE 1100 (the Emerald Book).
• For F-to-I signal conditioners, mount them on DIN rail as close as possible to the PLC analog input terminal block to keep the analog section short.

PLC Configuration and Scaling

Modern Siemens controllers (S7-300, S7-400, S7-1500) configure analog inputs in two steps: hardware configuration in TIA Portal (or STEP 7 Classic) and software scaling in the user program.

Hardware Configuration Example (TIA Portal V18, CPU 315-2 PN/DP)

1. Open the device view and add an SM 331 (6ES7 331-7KF02-0AB0) in slot 4.
2. Open the module properties and select channel 0.
3. Set "Measurement type" = Voltage, "Range" = 0 to 10 V.
4. Disable diagnostics for over-range if the application tolerates it; otherwise leave enabled to detect broken wiring.
5. Download the hardware configuration to the CPU.

Software Scaling (SCL example for 0–3600 RPM from 1–5 V → 0–27648 counts)

FUNCTION_BLOCK FB_ScaleTach
VAR_INPUT
  AIW : INT;       // 0..27648
END_VAR
VAR_OUTPUT
  RPM : REAL;      // 0..3600
END_VAR
VAR
  V_raw : REAL;
END_VAR
BEGIN
  V_raw := INT_TO_REAL(AIW) / 27648.0 * 10.0;
  IF V_raw < 1.0 THEN
    RPM := 0.0;
  ELSIF V_raw > 5.0 THEN
    RPM := 3600.0;
  ELSE
    RPM := (V_raw - 1.0) * (3600.0 / 4.0);
  END_IF;
END_FUNCTION_BLOCK

Ladder Logic Example (STEP 7 Classic, FC scale for 0–5000 RPM)

NETWORK 1  // Scale AIW (speed) to RPM
  L     AIW    288   // analog input word, 0..27648
  ITD              // convert to double integer
  DTR              // convert to real
  L     2.7648e+004
  /R               // normalize to 0.0..1.0
  L     5.0e+003   // 5000 RPM full scale
  *R               // scale to 0..5000
  T     MD100      // RPM output as REAL

Commissioning and Verification

1. With the turbine stationary, measure the pickup output with a true-RMS multimeter or oscilloscope. Verify that the signal is < 50 mV at turning gear speed and well above the noise floor of the chosen signal conditioner (typically 10–20 mV).
2. Run the turbine to rated speed and verify that the conditioned signal (4–20 mA loop or 0–10 V) reaches the expected end-of-scale value.
3. Use the PLC's online watch table (TIA Portal) or VAT table (STEP 7) to confirm the scaled RPM value matches the reference tachometer within ±0.5 %.
4. Perform an overspeed trip test: command the speed reference above the trip setpoint and confirm the safety relay actuates at the configured threshold (typically 110 % of rated speed for mechanical turbines, per API 670).
5. Conduct an EMC / surge immunity check by applying a 1 kV / 1.25 µs transient to the field wiring per IEC 61000-4-4 and IEC 61000-4-5. Verify the PLC channel does not latch up.
6. Document the calibration sheet with the date, the field engineer, the reference instrument, and the as-installed scaling constants. File the document in the plant's SIS / control-system lifecycle record.

Troubleshooting Matrix

Spare Parts and Lifecycle Considerations

Modernization projects often stall because the team procures the wrong generation of module. As of 2024, the active S7-300 spare-parts horizon extends to 2028 for selected catalog numbers, while S7-400 spares remain available but are no longer recommended for new installations. For new turbine retrofits, Siemens recommends migrating directly to the SIMATIC S7-1500 family (CPU 1515-2 PN, SM 531 AI 8xU/I/RTD/TC, 6ES7 531-7KF00-0AB0) and a SINAMICS drive front-end, with the S7-300 / S7-400 used only when the rest of the plant is still on those platforms.

For the controller itself, the alternative platforms proven in the turbomachinery space include:

• Woodward control system upgrades (MicroNet, 505, easYgen) for steam, gas, and hydro turbines.
• GE Vernova Mark VIe control system upgrade for heavy-duty and aeroderivative gas turbines.
• ABB Ability 800xA with the Turbine Controller library, often paired with ABB Symphony Plus for combined-cycle plants.

Each of these platforms ships with native signal-conditioning modules sized for magnetic pickups and DC tachogenerators, which can reduce the field wiring scope considerably when the entire governor is being replaced.

Field-Proven Caveats

• On machines with high ambient temperatures (>55 °C near the turbine casing), mount the signal conditioner inside the PLC cabinet, not in the field junction box. Commercial-grade conditioners are typically rated 0–60 °C; industrial-grade 0–70 °C; some Phoenix Contact MINI Analog variants are rated −40 to +85 °C.
• Many older tachogenerators are ungrounded (floating) outputs. Verify the configuration of the chosen signal conditioner — some require a grounded input reference, others accept floating sources up to ±60 V common mode.
• If the turbine is used as a generator in parallel with the grid, the tachogenerator output may contain significant common-mode noise at the generator's fundamental frequency. Select a conditioner with at least 1.5 kV isolation and a common-mode rejection ratio (CMRR) above 80 dB at 50/60 Hz.
• Always de-rate the analog input module to 60 % of full scale for continuous inputs. This prevents ADC saturation during transients and improves linearity.
• Capture the as-installed wiring diagrams, the scale constants, the module serial numbers, the firmware versions of the CPU and the SM 331/SM 431, and the calibration certificate. This information is required for the plant's Functional Safety file and for any future migration to a different controller platform.

FAQ