Converting a 4-20 mA Ultrasonic Sonar to Millimeters on LOGO! 8

Problem Overview

A Schneider Electric Telemecanique XX-series ultrasonic distance sensor with a 4-20 mA analog output is wired into a Siemens LOGO! 8 base module or AM2 expansion module. The sensor's published physical range is 105 mm to 1000 mm (an 895 mm span). The user expects the LOGO! display or process value tag to read in millimeters, but the engineering-unit conversion is wrong by hundreds of millimeters. Field measurements show that 500 mm physical distance produces a raw LOGO! value of 645, 110 mm produces 992, and 270 mm produces 908. The mapping is clearly non-trivial because the sensor output is inverted: smaller physical distance corresponds to a higher raw value, not a lower one.

The objective of this procedure is to derive a single linear scaling equation that maps the LOGO! raw integer (0-1000) directly to a 0-1000 mm engineering range, implement it with the LOGO! 8 Gain/Offset (formerly "Analog Amplifier") function block, calibrate it with two known targets, and verify linearity at a third target. After implementation, the LOGO! display or process value tag should report physical distance in millimeters to within the sensor's published accuracy band, typically ±0.5 % of full scale plus the acoustic-resolution floor of the XX-series transducer.

Sensor Signal Chain and I/O Configuration

Industrial ultrasonic distance sensors in the Schneider Electric XX family (XX5, XX6, XX9 prefixes) are two-wire loop-powered devices in their 4-20 mA variant. They drive 24 VDC across the loop and modulate the current proportional to the measured acoustic time-of-flight. The 4 mA lower endpoint corresponds to one extreme of the configured measurement window, and 20 mA corresponds to the other. The polarity of that mapping (which extreme is 4 mA and which is 20 mA) is configured either by a teach-in pushbutton, by wiring convention, or by the order code of the specific XX model.

The signal chain into the LOGO! is therefore:

1. Sensor head emits a 200-500 kHz burst, receives the echo, and converts the time-of-flight to a loop current.
2. Two-wire loop carries 4-20 mA into the LOGO! AM2 analog expansion module (catalog designation 6ED1055-1MA00-0BA2 for the 12/24 V version, 6ED1055-1MA00-0BA8 for the 24 V version). See the LOGO! 8 System Manual, Edition 04/2023 for the module's input ranges and jumper positions.
3. The AM2's ADC resolves the current to a 10-bit signed integer that LOGO! Soft Comfort surfaces as a 0-1000 raw value when the module's jumper is in the 0-20 mA position, or as a 0-1000 raw value when the jumper is in the 4-20 mA position (the jumper selects the electrical range, not the engineering scaling).
4. LOGO! Soft Comfort ladder/FBD logic applies the Gain/Offset math block to translate the raw integer into a millimeters engineering value.

For a more general treatment of sonar signal-chain conditioning (analog front-end, gain stages, ADC selection, and noise floor) see the Analog Devices Sonar Signal Chains reference design overview. The general principle that an analog current or voltage front-end must be conditioned to match the ADC input span applies identically to industrial ultrasonic sensors as it does to hydrophone arrays, and a description of the underlying direct-conversion analog processing is given in the ScienceDirect Direct Analogs overview.

Understanding the Telemecanique XX Series Output Polarity

The user-reported raw values (645 at 500 mm, 992 at 110 mm, 908 at 270 mm) are monotonically decreasing as the physical distance increases, which is the diagnostic signature of a reverse-acting analog output. In this configuration, 20 mA corresponds to the near end of the measurement window (105 mm) and 4 mA corresponds to the far end (1000 mm). This is the most common default for diffuse-reflective ultrasonic sensors used for liquid level or bulk-material level, because the loop current then increases as the material rises toward the sensor (and operators expect the current to "go up" as the level rises).

If your specific XX model has been re-taught to invert this mapping, the slope sign in the formula below flips, and the procedure is otherwise identical. To confirm the polarity in the field, place a flat target at a known near distance (e.g., 150 mm) and a known far distance (e.g., 800 mm). If the higher raw integer corresponds to the nearer target, the configuration is reverse-acting and you should use the formula in this article as written.

The user's 645 at 500 mm matches the reverse-acting column, which confirms the polarity assumption used in the rest of this procedure.

The Scaling Mathematics: 4-20 mA → 0-1000 raw → mm

The conversion has two linear stages. The first stage is electrical: the AM2 module in 0-20 mA mode maps the loop current to a raw integer with the relation

mA = raw × 0.020

The second stage is the sensor's own transfer function. For a reverse-acting 4-20 mA ultrasonic sensor with a 105 mm to 1000 mm window, the engineering-unit mapping is

Distance (mm) = 1000 − ((mA − 4) / 16) × 895

Substituting the first stage into the second and simplifying yields a single linear equation in raw:

Distance (mm) = 1223.75 − raw × 1.11875

Validate this against the user's three field samples:

The 500 mm and 110 mm samples are within the sensor's typical ±1 % linearity band; the 270 mm sample is off by 62 mm, which is outside linearity for a properly aimed XX-series transducer and is most often caused by acoustic reflections off the mounting bracket, a non-perpendicular target, or air-temperature-induced sound-velocity error. The procedure's verification step is designed to catch this kind of systematic error using a third known target.

LOGO! 8 Implementation: Gain/Offset Block

The Gain/Offset block (BF: "Math") in LOGO! Soft Comfort implements the linear function

Q = a × (Ax − b) + c

where Ax is the raw analog input (0-1000), Q is the engineering output, and a, b, c are user-entered constants. To realize the formula Distance = 1223.75 − 1.11875 × raw, set:

Block programming steps in LOGO! Soft Comfort (V8.3 or later):

1. Open your project and locate the AM2 input tag in the I/O list, e.g. AI1 (this is the raw 0-1000 integer).
2. From the instruction palette, drag the Math (BF) function block onto the FBD sheet.
3. Double-click the block. In the properties dialog, set the operation mode to Gain/Offset (not "PID" or "Addition").
4. Assign the input: Ax = AI1 (the AM2 raw input).
5. Enter Gain: a = −1.11875. Enter the input-side offset: b = 0. Enter the output-side offset: c = 1223.75.
6. Set the output Q to a new tag, e.g. Distance_mm.
7. Enable block output to the LOGO! display (Tools → Message Text) and set the display format to integer with one decimal place suppressed: Distance_mm / 1 mm.
8. Compile and download to the LOGO! base module.

The Gain/Offset block runs every LOGO! scan cycle (typical 10-50 ms depending on program size), and the result is updated at the same rate. The AM2 conversion itself runs at the module's configured sample rate (default 50 ms; configurable to 12.5 ms, 25 ms, 50 ms, or 100 ms via the parameter dialog in LOGO! Soft Comfort).

Alternative: Two-Stage mA-to-mm Conversion

If the AM2 module is set to 4-20 mA jumper mode instead of 0-20 mA mode, the raw integer is mapped over the 4-20 mA span, so:

mA = 4 + (raw / 1000) × 16

The engineering formula then collapses to a much simpler form:

Distance (mm) = 1000 − (raw / 1000) × 895

which is directly realizable in the Gain/Offset block with:

This form is preferable if you have physical access to the AM2 jumper, because the integer-to-millimeter ratio is friendlier for HMI scaling and alarm thresholds. However, in 4-20 mA jumper mode the AM2 reports 0 for currents below 4 mA (open-loop or broken wire), which gives the same 0-1000 mm reading as the far limit. To distinguish "out of range low" from "open loop," wire a broken-loop diagnostic: if the raw integer ever reads exactly 0 while the sensor is supposed to be within range, the LOGO! logic should raise a Sensor Open Loop alarm. With 0-20 mA jumper mode, open-loop reads 0 mA, which is also raw 0; the diagnostic is identical.

Calibration Procedure

The scaling constants derived above are theoretical, based on the published sensor window and the AM2 jumper mode. Real-world installation introduces mounting-angle error, target reflectivity error, and acoustic-noise error, so two-point calibration is mandatory before the distance reading is used in any control loop.

1. Mechanical setup. Mount the XX-series sensor to a rigid bracket with its acoustic axis perpendicular to a flat reference target. The target should be at least the diameter of the transducer face (typically 25-30 mm for the XX5, 50 mm for the XX6) and made of a material with diffuse-reflective acoustic properties (bare plywood, MDF, or rough concrete). Mirror-finish metal targets cause specular reflection and will produce non-repeatable readings.
2. Establish the near target. Position a flat target at a precisely known distance within 10 % of the published near limit. For a 105-1000 mm sensor, set the target at 150 mm measured with a calibrated steel rule or laser rangefinder (±1 mm). Note the LOGO! raw value displayed in the online monitor (Tools → Online → AI1 in LOGO! Soft Comfort). Call this raw_near.
3. Establish the far target. Move the same target to a precisely known distance within 10 % of the published far limit, e.g. 900 mm. Note the LOGO! raw value. Call this raw_far.
4. Re-derive the gain and offset. The two known (raw, distance) pairs give a least-squares-trivial exact linear fit. Using the two-point form

a = (D_near − D_far) / (raw_near − raw_far)
c = D_near − a × raw_near

enter these recalculated a and c into the Gain/Offset block, recompile, and re-download. The slope a will typically land within ±5 % of the theoretical −1.11875 (0-20 mA mode) or −0.895 (4-20 mA mode), confirming that the wiring polarity and jumper mode are correct.

5. Verify the dead zone. Slowly move the target from 150 mm toward the sensor face. The XX-series has a published blind zone (typically 0-105 mm for the XX5, 0-100 mm for the XX6) inside which the sensor output is undefined. Confirm that the LOGO! distance reading saturates or pegs to the near-limit value when the target enters the blind zone, and that it tracks linearly once the target is outside it. If the LOGO! reads a non-saturating value inside the blind zone, the sensor is ringing on the transducer face and the target should be backed off to at least 1.2 × the published blind-zone distance.

Verification and Acceptance Criteria

After calibration, place the target at three independent verification points (for example 250 mm, 550 mm, and 800 mm) and record the LOGO! distance reading against the reference measurement. The acceptance criteria for industrial use are:

If the verification points fail the relevant tier, do not adjust the gain to "bandage" the error at one point. Instead, repeat the calibration with fresh near and far measurements and inspect the mechanical mounting for skew, standoff-distance error, or acoustic interference from adjacent surfaces. A 2° mounting angle can produce a 5-10 mm ranging error at the far end of a 1000 mm sensor.

Common Pitfalls and Troubleshooting Matrix

Wiring, Grounding, and EMC Notes

Two-wire 4-20 mA loops are sensitive to ground loops, especially when the sensor and the LOGO! are fed from different 24 VDC supplies. The recommended practice is to power the sensor from the same 24 VDC bus that feeds the LOGO! AM2 module, with the supply common tied at a single point. If separate supplies are unavoidable, use an isolated loop isolator (e.g., Phoenix Contact MACX MCR-EX-SL) between the sensor and the AM2 input. Shielded cable is required for runs longer than 5 m, with the shield grounded at the LOGO! end only. Routing the analog cable parallel to VFD output cables or servo motor leads will inject common-mode noise that the AM2's 10-bit resolution cannot reject, producing a 2-5 LSB jitter in the raw integer. Cross such cables at 90° and maintain a 200 mm separation.

The LOGO! base module's analog inputs on the unit itself (not the AM2 expansion) are 0-10 V only and cannot read 4-20 mA directly. Attempting to wire a current-output sensor to a voltage input without the 250 Ω shunt will produce a near-zero reading and may damage the AM2 input stage. Use the AM2 expansion (catalog 6ED1055-1MA00-0BA2 for 12/24 V units or 6ED1055-1MA00-0BA8 for 24 V units only) for all 4-20 mA inputs.

Expanding the Function: Hysteresis, Alarms, and Display

Once the Gain/Offset block produces a clean Distance_mm tag, the natural next steps are:

1. Hysteresis on alarm thresholds. Use a LOGO! Schmitt-trigger (block BH) or threshold-trigger (block B2) with a configurable hysteresis (e.g., 5 mm) to debounce a high-level alarm. The hysteresis prevents the alarm from chattering when the target surface is rippling or when the acoustic echo is at the resolution floor of the sensor.
2. Out-of-range diagnostics. Add a comparison block (block B2 or B3) that asserts when Distance_mm < 110 (target in the blind zone) or Distance_mm > 1000 (target beyond the far limit). The latter condition, combined with a raw integer still between 0 and 1000, indicates a sensor that has lost its echo and is reading its last valid value, not a true out-of-range. Flag this distinction in the HMI message text.
3. HMI display formatting. In LOGO! Soft Comfort, drag a Message Text block to the FBD sheet and select the Distance_mm tag as the bar-graph or numeric display. Use a 4-digit integer format and a 1-decimal suppressed layout; the LOGO!'s built-in display is 4 lines × 12 characters, and a clean "D = 532 mm" string fits comfortably.
4. Retentive storage. If the LOGO! powers down and restarts, the AM2 will lose its last reading for one scan cycle. Use a retention-enabled tag for Distance_mm if the downstream control logic cannot tolerate a single-cycle zero.

Cross-Platform Notes: Migrating the Formula to Other Small PLCs

The same scaling logic applies, with the constants unchanged, to other small PLCs that read 4-20 mA on a 0-1000 raw scale:

For the S7-1200 specifically, the standard idiom is to wire the AI module's INT output to NORM_X (normalizing the raw integer 0-27648 to a real 0.0-1.0 floating-point), then to SCALE_X (scaling 0.0-1.0 to the engineering range 0-1000 mm), with a sign-flip handled by reversing the MIN and MAX parameters of SCALE_X. The result is a LREAL in millimeters that can feed directly into a PID_Compact process variable.

Field-Proven Caveats from Industrial Deployments

Several issues recur across multiple XX-series deployments that are worth flagging before commissioning:

• Mounting standoff. The transducer face should protrude at least 5 mm past the mounting bracket surface to avoid bracket-induced acoustic reflection. A flush-mounted XX6 will produce a 20-50 mm bias error at distances below 300 mm.
• Beam cone interference. The XX5 has a 5° beam cone and the XX6 a 7° cone; at 1000 mm the beam footprint is 90-120 mm in diameter. Sidewalls within this footprint, even at oblique angles, can produce secondary echoes that bias the reading by 10-30 mm. Coning the inside of an enclosure with acoustic foam eliminates this.
• Target material. Soft materials (foam, fabric, granular bulk solids) absorb ultrasound and reduce the effective measurement range by 20-40 %. Hard, dense, flat surfaces (water, metal plate, concrete) give the cleanest readings.
• Power supply noise. A switching 24 VDC supply with 200 mV peak-to-peak ripple will inject 1-2 LSB of noise into the AM2 ADC. Linear regulation or a pi-filter on the 24 V rail feeding the sensor loop reduces this to a sub-LSB level.
• Sensor warm-up. XX-series sensors need 200-500 ms after power-up to stabilize the analog output. The LOGO! AM2 will read whatever the sensor presents during that window, including the initial 0 mA power-on pulse some units emit. A 1-second startup delay in the FBD logic (using a TON timer) suppresses this.

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