Implementing Tare Function in Siemens LOGO! for Bag Filling

An automatic bag-filling machine must establish a true zero reference on every cycle because empty-bag weight, container seating, and material residue drift the load cell baseline. In a Siemens LOGO! 8 logic relay the tare function can be implemented either by writing the captured offset back to the Analog Amplifier block or by adding the captured tare value to the target weight. This reference details both methods, the supporting hardware selection, the LOGO! Soft Comfort V8 programming sequence, and the field commissioning checks required to achieve ±0.05 kg repeatability with a 30 kg load cell.

1. Operating Principle and Signal Flow

The 30 kg single-point load cell operates in a Wheatstone bridge configuration. At zero applied force the bridge output is nominally 0 mV/V; at full scale (30 kg) it produces the rated sensitivity, typically 2.0 mV/V. The external signal conditioner applies an excitation voltage (commonly 5 VDC or 10 VDC), amplifies the millivolt signal, and produces a 0–10 V output that is proportional to applied weight.

The Siemens LOGO! receives this 0–10 V signal on an analog input. The LOGO! internal analog representation is 0–1000 units (10-bit resolution), with a refresh interval of 250 ms on the base module analog inputs. The Analog Amplifier block maps this 0–1000 internal count to engineering units of 0.0–30.0 kg using a gain and offset pair. The Analog Threshold Trigger block compares the scaled weight to the setpoint and energizes the fill valve output when the weight is below setpoint and de-energizes it at the cutoff threshold.

The tare function captures the load cell reading at the moment the empty bag is placed but no product has been dispensed, then subtracts this baseline so the net weight shown to the control logic represents only the dispensed product. Two implementation paths are detailed in Section 5 and Section 6.

2. Prerequisites

2.1 Hardware

Refer to the Siemens LOGO! product page and the LOGO! 8 System Manual on the Siemens Industry Online Support portal for the full selection of base modules, expansion modules, and accessories.

2.2 Firmware

LOGO! 8 base modules shipped after mid-2018 carry firmware 1.08.xx or later. The Analog Math block, User-Defined Functions (UDF), and direct memory write to block parameters require LOGO! 8.1 (FS:04) or later. Update the firmware using LOGO! Soft Comfort V8.3 or V8.4 and a standard Ethernet cable to the LOGO! Ethernet port (port 1). The current firmware version is visible on the LOGO! onboard display under Menu → Diagnostics → Software, or via the integrated web server at http://<LOGO IP>/.

To update the firmware, open LOGO! Soft Comfort, connect to the LOGO!, and select Tools → Firmware Update. The firmware file (LWE file format) is bundled with the LOGO! Soft Comfort installation under C:\Program Files\Siemens\LOGOComfort_V8\Firmware\. The transfer takes 60–90 seconds and the LOGO! reboots automatically. The Ethernet configuration and program are retained across the firmware update.

2.3 Software

LOGO! Soft Comfort V8.3 (order number 6ED1058-0BA08-0YA1) or later. The V8.4 release adds support for the LOGO! 8.3 base module (FS:05) and the LOGO! CMR (cellular router) and LOGO! CSM (unmanaged switch) modules. Program in FBD (Function Block Diagram) because the Analog Amplifier, Analog Threshold Trigger, and Analog Math blocks are native FBD elements. The Ladder Diagram (LD) editor does not natively expose analog blocks; a small FBD subroutine is required even when the top-level program is in LD.

2.4 Cabling and Shielding

Use a shielded 4-conductor cable between the load cell and the signal conditioner; ground the shield at the conditioner end only. Run the 0–10 V output on a twisted, shielded pair to the LOGO! AI terminal. Avoid routing the analog cable parallel to VFD output cables; if crossing is unavoidable, cross at 90° and maintain a minimum 200 mm separation. Apply a ferrite bead (e.g., Würth Elektronik 74270097) on the analog cable within 50 mm of the LOGO! terminal to suppress common-mode noise.

3. Load Cell and Signal Conditioner Wiring

3.1 Load Cell Mechanical Mounting

Mount the load cell on a rigid, vibration-isolated platen. The platen must be free to tilt slightly to avoid side-loading the cell. A typical mounting kit uses a single M10 stud at the top of the cell and a button-load foot at the bottom. The button foot allows the cell to return to its vertical neutral position when the load is removed. Do not over-tighten the mounting hardware; the cell's specified deflection at full scale is typically 0.2–0.5 mm, and an over-tight mount preloads the cell and shifts the zero.

For a 30 kg single-point load cell, the recommended mounting surface is a steel plate of at least 10 mm thickness, machined flat to within 0.05 mm. The platen should be larger than the bag's footprint (typically 400 mm × 600 mm) but smaller than the load cell's load introduction area (typically 350 mm × 350 mm for a 30 kg cell) to ensure the applied force is centered on the cell's sensing element.

3.2 Load Cell Electrical Connection

A 4-wire load cell uses two excitation lines (EX+, EX-) and two signal lines (SIG+, SIG-). The conditioner applies a stable 5 VDC or 10 VDC across EX+ / EX- and reads the millivolt difference between SIG+ and SIG-. The output stage produces 0 V at 0 kg and 10 V at the load cell's calibrated full scale (30 kg in this application).

3.3 Signal Conditioner to LOGO! Input

3.4 Calibration of the Signal Conditioner

With no load on the load cell, adjust the conditioner zero potentiometer until the output reads 0.00 VDC ± 5 mV measured at the LOGO! terminal. Apply a known calibration mass of 20 kg (or a series of known masses spanning 0–30 kg) and adjust the span potentiometer until the output reads 6.667 VDC (20 kg / 30 kg × 10 V). Repeat zero and span twice to compensate for interaction. After final adjustment record the calibration certificate with date, masses used, output voltage at each step, and ambient temperature.

4. LOGO! Analog Input Configuration

4.1 Base Module Analog Inputs

The LOGO! 12/24 RCEo base module provides four analog inputs: AI1, AI2, AI3, and AI4. These share their physical terminals with digital inputs I7, I8, I1, and I2 respectively. AI1 and AI2 (terminals I7 and I8) are 0–10 V only. AI3 and AI4 (terminals I1 and I2) can be configured as 0–10 V (default) or 0–20 mA by inserting the appropriate jumper on the bottom of the module. The internal representation is 0–1000 counts (10-bit, 0.010 V/count).

For an AM2 analog expansion module the inputs are addressed as AI5/AI6 (first AM2), AI7/AI8 (second AM2), and so on. AM2 inputs can be 0–10 V or 0–20 mA, configured per-channel by LOGO! Soft Comfort block parameters. The AM2 module also has a hardware filter jumper with positions 0.5 s, 1 s, 2 s, 4 s, and 8 s; for a noisy bag-filling environment, set the filter to 1–2 s to smooth the analog reading without introducing excessive lag.

4.2 Analog Amplifier Block Parameters

The output of the Analog Amplifier is a 16-bit signed integer. With gain 30 and sensor 0–10 V the maximum output is 30000, representing 300.00 kg if the offset remains 0. The actual range is set by the gain to match the calibrated full scale. If the application uses a smaller load cell (e.g., 5 kg) the gain can be increased to 200 to maintain internal resolution; the LOGO! analog input does not change but the amplifier's output scaling becomes finer.

4.3 Analog Threshold Trigger Parameters

The Threshold Trigger output is a Boolean. In a bag-filling application the fill valve is normally closed; the LOGO! output energizes a relay that opens the valve while the weight is below cutoff. The block can be inverted to support normally-open valve logic.

The Variable Threshold option, added in firmware 1.08.xx, allows the OFF threshold to be driven by a network reference (analog marker word) instead of a constant. Enable this option in the block properties dialog. If the firmware is older than 1.08.xx, the threshold is constant and Method A (variable cutoff) cannot be implemented; the user must fall back to Method B (variable offset) or update the firmware.

5. Method A: Add Tare to Target Weight (Recommended)

This is the method validated in the field report. The tare value is captured at the start of each cycle and added to the target weight. The amplifier offset and gain remain at their calibrated values, which preserves the linearity of the scale and avoids touching the block parameters in real time.

5.1 Logic Sequence

1. Operator presses the tare button (I1, B001 in the user's project). The pulse edge enables a one-shot capture (B002).
2. The Analog Amplifier output (in engineering ×100 units) is latched into a marker word MW10 (B005) on the rising edge of the capture pulse. Because the amplifier output represents the gross weight (bag + any residue), MW10 holds the tare value.
3. The target weight is entered by the operator as a constant B006. For a 5 kg fill, B006 = 500.
4. A Math block (B007) computes the cutoff threshold as MW10 + B006. For 5 kg fill into a 0.25 kg bag, the cutoff threshold is 750 (representing 7.50 kg gross).
5. The Analog Threshold Trigger (B003) compares the live amplifier output to the computed cutoff threshold and de-energizes the fill valve when the weight reaches the threshold.
6. After the fill cycle, the fill-complete output Q1 remains latched until the bag is removed and the cycle start button is pressed again, which retriggers the tare capture and reinitializes the threshold.

5.2 Block Diagram (FBD)

5.3 Parameter Wiring (LOGO! Soft Comfort)

Set the Analog Threshold Trigger's OFF threshold to a network reference that points to the output of the Math block. The recommended reference name is NetRef_TareCutoff. The Math block inputs are NetRef_Tare (pointing to MW10) and NetRef_Target (pointing to B006). The target weight itself can be exposed to the LOGO! display (LOGO! TD or LOGO! TDE) or to a connected HMI via Ethernet.

5.4 Advantages of Method A

• The amplifier gain and offset remain at their calibrated values, so linearity is preserved across the full scale.
• No block parameter is rewritten at runtime, eliminating the risk of a partial write leaving the amplifier in an undefined state.
• The tare value is visible and can be trended, alarm-filtered, or used to detect a stuck bag (e.g., tare > 0.5 kg indicates bag-handler error).
• A running average of the last 10 tare values can be used to detect progressive drift (e.g., accumulating residue on the load cell platen).
• Negative tare (subtracting a fixture weight) is implemented naturally by inverting the sign in the Math block.

5.5 Limitations

• The Analog Threshold Trigger block expects a constant threshold in the parameter dialog. To use a variable threshold the block's OFF threshold must be linked to a network reference, which LOGO! 8 supports only on Analog Threshold Trigger blocks with the Variable Threshold option (added in firmware 1.08.xx). Confirm firmware by reading the LOGO! web server's Module Information page (Tools → Web Server → Module Info).
• The internal representation is integer (×100 kg). Sub-gram resolution is not available; the LSB is 10 g. If higher resolution is required, migrate to a Siemens S7-1200 with the SM 1231 analog input module, which supports 13- and 16-bit representations.
• The Analog Math block uses 32-bit floating point internally, but the input values are 16-bit integers. No precision loss occurs for the tare + target addition because both operands are within the 16-bit range.

6. Method B: Modify Amplifier Offset (Original Concept)

This was the user's original idea: at the moment the tare button is pressed, write the negative of the current amplifier reading back into the amplifier's offset parameter. The advantage is that the downstream threshold does not change; the disadvantage is that the amplifier block's offset is a non-retentive parameter that can only be rewritten by the program using an analog memory write, supported in LOGO! 8 firmware 1.08.xx and later.

6.1 Logic Sequence

1. The tare button (I1) triggers a one-shot pulse (B002).
2. On the rising edge of the pulse, the current amplifier output (B003.AQ) is read and its negative is computed using a Math block with function "A × -1" (B005).
3. A second Math block (B006) writes the result to a marker word MW20, which is then mapped back to the amplifier's offset parameter via a network reference.
4. The Threshold Trigger (B004) compares the now-zeroed amplifier output to the constant setpoint (e.g., 5.00 kg). The fill valve closes when the amplifier output reaches 5.00 kg.

6.2 Network Reference Setup

LOGO! 8 supports the assignment of network references to block parameters, including the Amplifier's offset parameter. In LOGO! Soft Comfort, right-click the Amplifier block, select Properties → Parameters, and bind the Offset field to NetRef_TareOffset. At runtime, writing a value to NetRef_TareOffset (a memory word) updates the amplifier's effective offset.

6.3 Limitations and Risks

• Each cycle overwrites the amplifier's calibration. If the write is interrupted (e.g., LOGO! enters Stop mode mid-cycle), the amplifier offset reverts to its last downloaded value, which can cause the next cycle to overfill or underfill.
• The amplifier's offset is intended for fixed calibration adjustment, not for per-cycle tare. Using it for tare adds latency: the LOGO! analog input is sampled at 250 ms; the offset rewrite takes effect on the next scan, typically 50–100 ms later.
• If the tare value exceeds the amplifier's full scale (i.e., the bag is heavier than the load cell capacity), the output saturates and the tare function fails silently. Add an alarm block that flags a tare value greater than, e.g., 20 % of full scale (6 kg on a 30 kg cell).
• The amplifier's output range is reduced by the offset. A -500 offset (representing a 5 kg tare) reduces the positive dynamic range from 30000 to 29500, which limits the maximum fillable weight to 29.5 kg on a 30 kg cell.

6.4 Recommended Use

Method B is appropriate when the threshold trigger is implemented in a downstream device (e.g., a panel HMI or a separate logic module) that expects the amplifier to deliver a tared value. In the LOGO!-only architecture this method is generally inferior to Method A.

7. Step-by-Step Programming in LOGO! Soft Comfort V8

7.1 Project Setup

1. Launch LOGO! Soft Comfort V8.3 (or V8.4) and create a new project.
2. Select the base module from the hardware catalog. If using LOGO! 12/24 RCEo (6ED1052-1MD08-0BA0), the catalog entry matches.
3. Add the AM2 expansion if the 4 base analog inputs are insufficient. Drag the AM2 from the catalog onto the expansion slot. The catalog entry is 6ED1055-1MA00-0BA2.
4. Set the LOGO! IP address under Tools → Ethernet Connections. Use a static address (e.g., 192.168.0.10) or enable DHCP.
5. Configure the LOGO! clock under Tools → Set Clock if any time-based logic (e.g., shift register, on-delay) is used.

7.2 Adding the Logic Blocks (Method A)

1. Open the FBD editor. Place a Digital Input block and assign it to I1. Label it Tare_Button.
2. Place an Edge-triggered Pulse Generator or a Wiping Relay to produce a single-cycle pulse on the rising edge. Label it Tare_OneShot.
3. Place an Analog Amplifier block. Assign its input to AI3 (the physical I1 terminal on a 12/24 RCEo, configured for 0–10 V). Set Sensor = 0–10 V, Gain = 30, Offset = 0. Label it Amp_30kg.
4. Place an Analog Latch or a Set/Reset latch configured for analog storage. Drive the Set input with Tare_OneShot and the data input with Amp_30kg.AQ. Label it Tare_Store.
5. Place an Analog Constant. Set the value to 500 (5.00 kg). Label it Target_5kg.
6. Place an Analog Math block with function "A + B". Wire A to Tare_Store.AQ and B to Target_5kg.AQ. Label it Cutoff_Sum.
7. Place an Analog Threshold Trigger with Variable Threshold = enabled (if firmware supports it). Wire the analog input to Amp_30kg.AQ and the threshold reference to Cutoff_Sum.AQ. Set ON = 0, OFF = (computed), Hysteresis = 10. Label it Fill_Cutoff.
8. Place a Digital Output block and assign it to Q1. Wire it to Fill_Cutoff.Q. Label it Fill_Valve.
9. Add a Cycle Start button on I2, an On-delay to debounce, and a Reset pulse to clear the latched tare when the bag is removed.

7.3 Downloading and Going Online

1. Connect the PC to the LOGO! Ethernet port. Use Tools → PC → LOGO! Connection.
2. Click Download (F8). Select PC → LOGO!. After the transfer completes the LOGO! enters Run mode automatically.
3. Open the Online Test (Ctrl+F5) to view the live state of every block: amplifier output, tare value, computed cutoff, threshold trigger output, and Q1.
4. If the transfer fails with error 0x8004F001, the LOGO! is in Stop mode or the program memory is full. Clear the program memory under Tools → Clear LOGO! and retry.

7.4 Saving the Program to the LOGO! SD Card

The LOGO! ships with a microSD slot that accepts cards up to 32 GB. Insert a Siemens-formatted card (6ED1057-1AA00-0BA0) to enable auto-backup, datalogging, and recipe storage. The recipe function (LOGO! 8.2 and later) is particularly useful for storing per-product target weights (5.00 kg, 10.00 kg, 25.00 kg) and recalling them by selector switch. Up to 64 recipes can be stored on a single SD card, each containing 32 values.

To export a recipe, open LOGO! Soft Comfort, select File → Export → Recipe, and save the .lma file to the SD card's \LOGO\Recipe\ directory. To load a recipe at runtime, use the Recipe block (added in LOGO! 8.2) and assign its selection input to a digital input wired to a selector switch.

7.5 LOGO! TD/TDE Text Display Configuration

The LOGO! TD (6ED1055-4MH08-0BA0) and LOGO! TDE (6ED1055-4MH08-0BA1) are 6-line text displays that connect to the LOGO! base module via the proprietary TD bus. To display the tare, target, and net weight on the TD:

1. Open the Message Text configuration in LOGO! Soft Comfort.
2. Insert a Message Text block. Configure the message trigger to be always on.
3. In the message body, add three numeric fields. The first references NetRef_Gross (amplifier output), the second references NetRef_Tare (latch output), the third references NetRef_Cutoff (math output).
4. Set the display format to "0.00" (two decimal places) so the internal ×100 values appear as 1.23 kg, 0.25 kg, 5.25 kg.
5. Enable the bar graph on the second line to show fill progress from 0 to cutoff.

7.6 LOGO! Web Server for Remote Monitoring

LOGO! 8 includes an integrated web server that exposes variable values over HTTP. To enable the web server, navigate to Tools → Web Server → Enable in LOGO! Soft Comfort and assign a user password. Once enabled, the current values of NetRef_Tare, NetRef_Cutoff, and the live amplifier output are visible at http://<LOGO IP>/vars. The web server also supports Modbus TCP polling on port 502, which allows SCADA systems to read the tare and cutoff values without custom protocol handling.

8. Logic Diagram and State Sequence

The following inline SVG depicts the signal flow from the load cell through the LOGO! to the fill valve.

The state machine for one fill cycle is shown below.

9. Verification and Commissioning Procedure

9.1 Static Verification (Bench Test)

1. With no load on the load cell, place the LOGO! in Run mode and observe the live amplifier output in Online Test. The value should be 0 ± 5 counts (0.00 ± 0.05 kg).
2. Apply a 1.000 kg calibration mass. The amplifier output should read 100 ± 5 counts (1.00 ± 0.05 kg).
3. Apply a 10.000 kg calibration mass. The amplifier output should read 1000 ± 5 counts.
4. Apply a 30.000 kg calibration mass. The amplifier output should read 3000 ± 5 counts.
5. Verify linearity by applying 5, 15, and 25 kg and checking the deviation against the calculated value.

9.2 Tare Function Verification

1. With the load cell empty, press the tare button. The Tare_Store block should hold 0 ± 5 counts.
2. Place a 0.250 kg empty bag on the platen. The amplifier output should read 25 ± 5 counts.
3. Press the tare button. The Tare_Store block should now hold 25 ± 5 counts, and the Cutoff_Sum block should hold 525 (5.00 kg + 0.25 kg = 5.25 kg).
4. Press the cycle start button. The fill valve (Q1) should energize.
5. Manually add weight to the load cell in 1 kg increments. The fill valve should de-energize when the amplifier output reaches 525 counts.
6. Remove the bag. The amplifier output should drop back to 0. The cycle ends.

9.3 Repeatability Test

1. Run 20 consecutive fills at 5.00 kg target into identical 0.25 kg bags.
2. Record the gross weight of each filled bag on a calibrated checkweigher.
3. Calculate the mean and standard deviation. The standard deviation should be below 0.05 kg (50 g) for a properly tuned system.
4. Compute the process capability index Cpk = (USL - μ) / (3σ) where USL is the upper specification limit (5.10 kg for a ±100 g tolerance). A Cpk greater than 1.33 indicates a capable process.
5. If the standard deviation exceeds 0.10 kg, inspect the load cell mounting for mechanical play, check the cable shielding, and verify that the LOGO! scan time is not impacted by an excessive number of communication blocks.

9.4 Field Tuning Knobs

9.5 Overfill Protection

A secondary safety check should be implemented regardless of the tare method. Place a second Analog Threshold Trigger that compares the gross amplifier output to a hard-coded maximum (e.g., 3500 representing 35.00 kg, which is 5 kg above the rated full scale). If the gross weight exceeds this limit for more than 500 ms, the overfill output energizes an alarm relay and locks out the fill valve. This protects the bag from rupture if the primary threshold fails.

Additionally, implement a maximum fill-time watchdog. Start a timer when the fill valve opens; if the gross weight does not reach the cutoff threshold within, e.g., 10 seconds, force the valve closed and trigger a "fill timeout" alarm. This catches material bridging, empty supply hopper, and valve failures.

10. Troubleshooting Matrix

11. Cross-Platform Scaling Reference

The 0–1000 internal representation used by the LOGO! is a common pattern across small-format logic relays and compact PLCs. Beckhoff's EL4374 EtherCAT terminal documents the equivalent scaling behavior for its analog input channel, including the dynamic-range restriction that occurs when tare is applied (see Tare - Beckhoff Information System). The Beckhoff documentation explicitly notes: "When using tare, the value output in the channel is shifted, which leads to a restriction of the dynamic range in the positive or negative direction." This is a useful sanity check for the LOGO! implementation: if the tare value exceeds 20 % of full scale (i.e., 6 kg on a 30 kg cell), consider reducing the bag-handler residue or repositioning the load cell.

Omron's general-purpose scaling guidance, while written for CP1W and NX-series analog modules, applies to the LOGO! architecture as well (see How can I scale an analog input or output? - Omron). The core principle is the same: the raw digital value (0–4000, 0–8000, or 0–1000 depending on the module) is mapped to engineering units by a linear equation Engineering = (Raw - Raw_Zero) × (Eng_FullScale - Eng_Zero) / (Raw_FullScale - Raw_Zero) + Eng_Zero. The LOGO!'s Amplifier block performs this calculation in a single block with Gain and Offset parameters.

For users migrating to a Siemens S7-1200 or S7-1500 platform, the analogous blocks are the analog input channel configuration in TIA Portal (Input scaling in the device configuration) and the NORM_X / SCALE_X instructions in the program. The tare function can be implemented as a single subtraction after scaling, using a retentive DB tag to hold the tare value across power cycles. The S7-1200 SM 1231 AI module supports 13-bit resolution (±10 V range), which provides 2.4 mV LSB and effectively 5 g resolution on a 30 kg cell, an improvement over the LOGO!'s 10 mV LSB.

12. Frequently Asked Questions