S7-1200 Laser Tank Level Sensor: 0-10V Selection and TIA Setup

This application note covers the selection, wiring, and TIA Portal integration of a top-mounted laser distance sensor for two vertical oil storage tanks (8 m and 7 m nominal height) feeding a Siemens SIMATIC S7-1200 PLC. The fluids are coconut oil and palm kernel oil, both low-viscosity edible oils with refractive indices near 1.45. The original requirement was a 0-10 V analog output proportional to oil level; this document also covers the 4-20 mA alternative that is strongly preferred for industrial sites, and shows how to use a single 500 Ω precision resistor to derive 0-10 V from a 4-20 mA loop if the PLC input card must remain a voltage channel.

1. Application Overview

Top-mounted laser time-of-flight sensors measure the empty distance between the sensor and the liquid surface. The PLC subtracts this measured distance from the calibrated tank height to obtain the oil level, then converts level to volume using the tank's geometry. For cylindrical tanks, the volume is:

V = π × (D / 2)² × h_level

where D is the inner tank diameter in metres and h_level is the calculated oil height. For non-cylindrical tanks, a strapping table (height vs. volume) is required; this can be stored in a DB and indexed by scaled level in TIA Portal.

2. Fluid Properties Relevant to Level Measurement

Both coconut oil and palm kernel oil are clear-to-pale-yellow edible oils. Before selecting an optical (laser) sensor, verify that the surface is compatible with the chosen measurement principle.

3. Measurement Technology Comparison

Several non-contact and contact technologies are available for tank level. Match the technology to the application constraints.

For two open or vented tanks of 7-8 m height with clear refined oils, a laser time-of-flight sensor is the best price/performance compromise. If the tank is closed with agitation and significant foam, move up to FMCW radar (VEGA VEGAPULS 64, Siemens SITRANS LR560, or Endress+Hauser Micropilot FMR6x).

4. Recommended Laser Distance Sensors

The following devices cover the 7-8 m range and ship with both 0-10 V and 4-20 mA analog outputs (or selectable). They are widely available and supported with datasheets, manuals, and 3D models.

5. Signal Type: Why 4-20 mA Beats 0-10 V in the Field

The original request specified 0-10 VDC. In an industrial environment with cable runs typically 20-100 m, voltage signals are highly susceptible to:

• Capacitive pickup from VFD cables and motor leads
• Ground-loop offsets (mV-level errors that look like real level changes)
• Voltage drop on long cables, biasing the zero point

A 4-20 mA current loop is immune to all three. The PLC input sees a current, not a voltage, so loop resistance does not degrade accuracy as long as the loop supply voltage is sufficient. If the field engineer has already committed to 0-10 V (for example, the chosen sensor only ships with a 0-10 V output, or the PLC is configured for voltage input), follow the recommendations in section 5.1.

5.1 Converting 4-20 mA to 0-10 V at the PLC

For sensors that have both outputs, prefer the 4-20 mA channel. Place a precision 500 Ω, 0.1 % resistor across the analog-input terminals of the S7-1200 SM 1231 (configured for ±10 V) to develop 2 VDC at 4 mA and 10 VDC at 20 mA. The transfer is:

V_AI = I_loop × 500 Ω

V_AI (V) = 2.0 + 0.5 × (Level % × 10)

Use a 500 Ω resistor with ≤ 50 ppm/°C drift (e.g., a Vishay Z201 or equivalent) to keep the loop accurate over industrial temperature swings. The PLC sees 2.0-10.0 V; in TIA Portal, treat the input as 0-10 V mapping 0-100 % of span with a 20 % offset in software, or use the ±10 V range and offset/subtract in the scaling block.

6. S7-1200 Analog Input Hardware

The S7-1200 family supports analog input through:

• SM 1231 signal modules on the right side of the CPU
• SB 1231 signal boards on the front of the CPU (1 channel, ±10 V or 0-20 mA)

For two sensors, a single 4-channel SM 1231 (6ES7231-4HD32-0XB0) is the most economical choice. For higher accuracy, use the 16-bit SM 1231 (6ES7231-5ND32-0XB0) which gives 15-bit signed resolution and better than 0.1 % accuracy over the industrial temperature range. The default Siemens S7-1200 System Manual covers all module wiring and parameter pages.

7. Wiring and Shielding

1. Use twisted-pair shielded cable, e.g., Belden 3082A (1 pair 18 AWG) or Lapp ETHERLINE® Sensor IP67 for short runs. For long runs (>30 m) the 4-20 mA loop is preferred because voltage drop on the 18 AWG pair is ~6.4 mΩ/m, which would shift the 0-10 V signal by 1 % for a 30 m run at 5 mA sensor load.
2. Connect the shield at one end only — at the PLC side — to a clean earth ground bar (not the DIN-rail mounting screw, which can carry cabinet return currents).
3. Keep analog signal cables at least 200 mm away from VFD output cables and three-phase motor leads, crossing at 90° if a crossover is unavoidable.
4. Provide a 24 VDC, 0.5 A minimum supply to each sensor from the same 24 VDC bus that feeds the PLC inputs, so ground references are common.
5. Wire M12 connectors with a torque-limiting tool to 0.6 Nm. Field-wireable M12 connectors from Phoenix Contact (SACC-M12MS-4QO) or Binder (713 series) are recommended for sensor cables.

8. TIA Portal Configuration

After wiring, configure the SM 1231 in the TIA Portal device view. Open Devices & networks → CPU → signal board / SM 1231 → Properties → Analog inputs → Channel 0 and set:

Repeat for channel 1. Save and download to the CPU. The analog-input process image is now at %IW64 (channel 0) and %IW66 (channel 1) for a SM 1231 in slot 0; the exact addresses depend on slot order, which the TIA Portal device view confirms in the I/O mapping table.

9. Scaling the Analog Input

The S7-1200 returns a 16-bit INT (or REAL after the SCALE / NORM_X instructions). Use the following reference values:

Add a data block DB_Sensors with the following structure:

DATA_BLOCK "DB_Sensors"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
NON_RETAIN
  STRUCT
    iRaw_Tank1 : INT;        // %IW64  raw AI value, ±10 V range
    iRaw_Tank2 : INT;        // %IW66  raw AI value, ±10 V range
    rVoltage_Tank1 : REAL;   // scaled voltage 0.0-10.0 V
    rVoltage_Tank2 : REAL;   // scaled voltage 0.0-10.0 V
    rDist_Tank1_m : REAL;    // measured distance from sensor to oil surface, m
    rDist_Tank2_m : REAL;
    rLevel_Tank1_m : REAL;   // calculated oil level, m
    rLevel_Tank2_m : REAL;
    rVolume_Tank1_m3 : REAL; // calculated oil volume, m³
    rVolume_Tank2_m3 : REAL;
  END_STRUCT;
END_DATA_BLOCK

Build a function block FB_LevelScale that performs normalization, conversion to engineering units, subtraction from tank height, and volume calculation. The following SCL (Structured Control Language) source works for both sensors, with a true/false flag selecting the 0-10 V or 4-20 mA interpretation.

FUNCTION_BLOCK "FB_LevelScale"
VAR_INPUT
    iRaw          : INT;     // raw value from %IW64 or %IW66
    bIs4to20mA    : BOOL;    // TRUE = 4-20 mA across 500 Ω (2-10 V at AI)
    rTankHeight   : REAL;    // nominal tank height in metres, e.g. 8.0
    rEmptyVoltage : REAL;    // voltage at empty tank (sensor sees full distance), V
    rFullVoltage  : REAL;    // voltage at full tank (sensor sees minimum distance), V
    rEmptyDistance: REAL;    // distance from sensor to oil at empty, m
    rFullDistance : REAL;    // distance from sensor to oil at full, m
    rDiameter     : REAL;    // tank inner diameter, m
    bRisingVoltage: BOOL;    // TRUE if voltage rises with level
END_VAR
VAR_OUTPUT
    rDistance_m   : REAL;
    rLevel_m      : REAL;
    rVolume_m3    : REAL;
    bError        : BOOL;
END_VAR
VAR
    rVolt         : REAL;
    rFraction     : REAL;
END_VAR
BEGIN
    bError := FALSE;

    // 1. Normalize raw value to engineering voltage
    IF bIs4to20mA THEN
        // 0-10 V range; usable 2-10 V, raw 5529-27648
        rVolt := NORM_X(MIN := 0,    VALUE := INT_TO_REAL(iRaw), MAX := 27648) * 10.0;
        IF rVolt < 1.5 OR rVolt > 10.5 THEN
            bError := TRUE;        // wire break or sensor fault
        END_IF;
    ELSE
        // 0-10 V unipolar, raw 0-27648
        rVolt := NORM_X(MIN := 0,    VALUE := INT_TO_REAL(iRaw), MAX := 27648) * 10.0;
        IF rVolt < -0.2 OR rVolt > 10.5 THEN
            bError := TRUE;
        END_IF;
    END_IF;

    // 2. Convert voltage to distance (sensor-specific, linear over 10-90 % of range)
    rFraction := (rVolt - rEmptyVoltage) / (rFullVoltage - rEmptyVoltage);
    // Clamp to 0-1
    rFraction := LIMIT(0.0, rFraction, 1.0);
    rDistance_m := rEmptyDistance - rFraction * (rEmptyDistance - rFullDistance);

    // 3. Convert distance to level (m of oil in tank)
    rLevel_m := rTankHeight - rDistance_m;
    IF bRisingVoltage THEN
        // Sensor output rises with level (use voltage to drive level directly)
        rLevel_m := rFraction * rTankHeight;
    END_IF;

    // 4. Volume for a cylindrical tank (m³)
    rVolume_m3 := 3.14159265 * (rDiameter / 2.0) * (rDiameter / 2.0) * rLevel_m;
END_FUNCTION_BLOCK

9.1 Calling the FB in OB1 (cyclic main)

// Tank 1: 8.0 m, Ø2.5 m, 0-10 V laser with V = 0 V at empty (8.0 m distance), V = 10 V at full (0.5 m distance)
"i_DB_Tank1"(iRaw := %IW64,
             bIs4to20mA := FALSE,
             rTankHeight := 8.0,
             rEmptyVoltage := 0.0,
             rFullVoltage  := 10.0,
             rEmptyDistance:= 8.0,
             rFullDistance := 0.5,
             rDiameter     := 2.5,
             bRisingVoltage:= FALSE,
             rDistance_m   => "DB_Sensors".rDist_Tank1_m,
             rLevel_m      => "DB_Sensors".rLevel_Tank1_m,
             rVolume_m3    => "DB_Sensors".rVolume_Tank1_m3,
             bError        => "DB_Sensors".bErr_Tank1);

// Tank 2: 7.0 m, 4-20 mA laser with 500 Ω resistor, 2 V at empty, 10 V at full
"i_DB_Tank2"(iRaw := %IW66,
             bIs4to20mA := TRUE,
             rTankHeight := 7.0,
             rEmptyVoltage := 2.0,
             rFullVoltage  := 10.0,
             rEmptyDistance:= 7.0,
             rFullDistance := 0.5,
             rDiameter     := 2.0,
             bRisingVoltage:= FALSE,
             rDistance_m   => "DB_Sensors".rDist_Tank2_m,
             rLevel_m      => "DB_Sensors".rLevel_Tank2_m,
             rVolume_m3    => "DB_Sensors".rVolume_Tank2_m3,
             bError        => "DB_Sensors".bErr_Tank2);

10. Filtering, Averaging, and HMI Display

Raw laser distance values often exhibit 1-3 mm of cycle-to-cycle noise. Add a first-order lag filter on the level and on the volume to stabilize HMI display:

// First-order IIR filter: y[n] = α·x + (1-α)·y[n-1]
rLevel_Tank1_filtered := rLevel_Tank1_filtered * 0.9 + "DB_Sensors".rLevel_Tank1_m * 0.1;

A coefficient of 0.1 (one-second time constant at 100 ms cycle) removes the visible noise without slowing the level response during fill/draw. For a step change in flow, the 90 % settling time is ~2.3 s, which is acceptable for inventory tracking.

On the HMI (Siemens KTP700 or WinCC Unified), display the volume in m³ and in litres (× 1000), and add a bar graph tied to the scaled raw value 0-100 %. Include an alarm for bErr_Tank1 or bErr_Tank2 = TRUE that latches until acknowledged by the operator.

11. Installation Considerations for Oil Tanks

1. Mount the laser at least 150 mm above the maximum fill level. The sensor's minimum measurable distance is typically 0.2 m; the configured full-tank distance of 0.5 m gives a safety margin of 300 mm.
2. Add a bracket or swing-arm assembly that allows the sensor to be aimed at the centre of the tank, not the wall. A wall-aimed beam picks up wall reflection on shiny oil.
3. Vent the tank headspace. If the tank is sealed with positive pressure, laser windows can fog and bias the measurement. Add a Gore-Tex vent (e.g., Donaldson P171505) to equalise pressure without admitting moisture.
4. Tilt the sensor 5-10° off vertical if the oil surface becomes glossy. This turns a specular return into a slightly diffuse one for triangulation sensors.
5. Disable the sensor's laser during fill operations if the fill stream crosses the beam. Many SICK and IFM devices support a teach-in to ignore an exclusion zone.
6. Use UV-resistant cable jackets (e.g., TPE or PUR) if the conduit runs outdoors; PVC jacket oils will embrittle in 2-3 years in tropical installations.

12. Troubleshooting Matrix

13. Commissioning Procedure

1. Power up the S7-1200 with the sensor cables disconnected. Download the project and confirm the CPU goes to RUN with no I/O faults.
2. Connect the sensor for tank 1 only. In TIA Portal online, monitor %IW64. The value should sit between -27648 and 27648 with the sensor aimed at the empty tank.
3. Run the sensor's teach-in sequence per the manufacturer manual: press the teach button for 3 s at the empty condition, then for 3 s at the calibrated full condition (water or a known-height substitute during commissioning).
4. Verify the AI value spans the expected range. For an 8 m tank with 0-10 V sensor, the empty reading should be ~0 V (raw 0) and the full reading should be ~10 V (raw 27648). With 4-20 mA across 500 Ω, expect 2-10 V (raw 5529-27648).
5. Watch the filter response in the online monitor for 5 minutes; confirm no bError is latched.
6. Repeat for tank 2. Document the final empty/full raw values in the sensor's commissioning sheet.
7. Perform a step test: pump in 200 L, watch the volume tick up, and verify against a sight glass or hand-dipped measurement.

14. When to Look Beyond Laser

If during commissioning the laser return from the oil is unstable, the next step is a 80 GHz FMCW radar. Devices such as the VEGA VEGAPULS 64, Endress+Hauser Micropilot FMR62, or Siemens SITRANS LR560 all deliver ±1 mm accuracy, are unaffected by foam, vapour, or surface texture, and natively output 4-20 mA + HART. The price premium (typically 3-5× a laser) is recovered in reduced recalibration and fewer false alarms. Hydrostatic pressure and ultrasonic remain valid as low-cost backup methods but should not be specified as the primary on a refined oil service where accuracy and surface stability matter.

15. Summary

For two vertical oil tanks of 7-8 m with refined coconut and palm kernel oil, a laser time-of-flight sensor such as the SICK DT50, IFM O1D106, Banner LT7, or Pepperl+Fuchs VDM28 mounted 0.5 m above the maximum fill point gives a clean 0-10 V or 4-20 mA output. Wire the 4-20 mA channel into the S7-1200 SM 1231 (6ES7231-4HD32-0XB0 or the 16-bit 6ES7231-5ND32-0XB0) configured for ±10 V through a 500 Ω precision resistor. Configure the module in TIA Portal, call a scaling FB (NORM_X → linearise → volume formula) once per tank in OB1, and present the engineered level and volume on the HMI. Add first-order filtering to suppress surface noise, and provide an operator-facing error flag for wire break or out-of-range conditions.