Configuring Fine and Coarse Dosing with SIWAREX FTA in PCS 7

The SIWAREX FTA weighing module is a Siemens solution for high-speed, high-accuracy dosing applications integrated natively into the SIMATIC S7-300 / ET 200S ecosystem. Two-stage dosing — using a coarse flow to deliver the bulk of the material and a fine flow to dribble the final weight with high accuracy — is the canonical operating mode for filling, batching, and bagging processes. This reference covers the complete configuration path from Siwatool parameter assignment through PCS 7 driver block wiring, including the special cases of coarse-only and fine-only dosing and the resolution of technology error message 043 "Inhibition time violation Fine".

1. SIWAREX FTA Dosing Overview

The FTA (Flexible Technology, Automation) variant of the SIWAREX family differs from the FTC (Continuous) and FTM (Multi-channel) variants in that it is optimized for discrete, high-speed weighing operations rather than continuous flow measurement. Dosing is one of its primary use cases, alongside filling, level measurement, and checkweighing.

Two-stage dosing is implemented inside the SIWAREX FTA firmware. The module monitors the live weight from a connected load cell, compares it against operator-supplied setpoints, and independently drives two digital outputs that correspond to the coarse and fine flow valves. The transition between stages is handled entirely inside the FTA — the PCS 7 controller only commands "start dose" and reads the "dose complete" result.

For the full Siemens application reference on compound dosing with SIWAREX FTA inside PCS 7 (covering tolerance evaluation, taring, and good/bad dose classification), see the official Siemens support entry: Compound Dosing with SIWAREX FTA in the PCS 7 Environment. The companion PDF pcs7_siwarex_fta_V91_en.pdf details the tolerance signal evaluation and step-by-step dosing flow.

2. Hardware Architecture and Module Variants

The SIWAREX FTA is a SIMATIC function module that plugs into the S7-300 rack or an ET 200S distributed I/O station. Load cells connect to the front terminals; the host PLC exchanges data over the S7 backplane bus. The module exposes digital I/O for valve control and status LEDs for local diagnostics.

3. Dosing Theory: Coarse and Fine Flow Stages

Two-stage dosing is required whenever a single-stage approach cannot simultaneously meet the targets of throughput and accuracy. Opening a single valve fully throughout the dose fills the container quickly, but the overshoot caused by the in-flight material (the material still in the air or in the feed screw between the valve closing and the material landing in the container) will exceed the target weight by an unacceptable margin. A second, smaller valve is therefore used for the final dribble portion:

1. Start of dose. Both coarse and fine valves open. The bulk of the material is delivered through the coarse flow at high rate.
2. Coarse → Fine transition. When the live weight reaches the configured Fine weight setpoint (the switch-off threshold for the coarse flow), the coarse valve closes. Only the fine valve continues to deliver material.
3. Fine → End of dose. When the live weight reaches the configured Setpoint weight, the fine valve closes. The dose is complete; the in-flight material will continue to add a small predictable amount, which is absorbed by the tolerance window.
4. Tolerance evaluation. After a short settling time, the measured final weight is compared against the configured upper and lower tolerance limits. The dose is classified as good (within tolerance) or bad (out of tolerance).

The arrangement of the two valves (solenoid-actuated butterfly valves, slide gates, or screw feeders) is application-specific, but the SIWAREX FTA treats both as binary outputs: ON while their stage is active, OFF otherwise.

4. Data Record (DR) Parameters for Dosing

All dosing setpoints and timing values live in data records (DRs) on the SIWAREX FTA. Siwatool exposes these records as named fields and writes them to the module over the service port. The dosing-specific DRs that are central to the fine/coarse question are summarised below; the parameter names and DR numbers below are the ones referenced in the operational correspondence for the FTA and are visible directly in the Siwatool dosing view.

The exact setpoint position and the dwell-time windows depend on the physical mechanics of the feeder (free-fall distance, screw pitch, valve closing time). Always derive Fine weight empirically: start with a value that is approximately 90–95 % of Setpoint, run several doses, and adjust so the in-flight overshoot lands the final weight consistently within the tolerance band.

5. Siwatool Configuration Environment

Siwatool FTA is the offline and online configuration tool supplied by Siemens for the SIWAREX FTA. It is the only tool that can reach the data records at the level of granularity used here. The configuration PC connects to the module over the front-panel service port (USB on current hardware revisions; RS-232 on earlier modules). Siwatool also supports loading a saved parameter set from disk, which is the recommended way of reproducing a project configuration from one module to another.

Typical Siwatool workflow for a dosing application:

1. Connect Siwatool to the module, perform Read module to load the current DR contents.
2. Open the Dosing view (the layout shows the Setpoint, Fine weight, Tolerance, and Times tabs).
3. Enter the setpoint values in DR21, DR22, DR23, DR24 as per the application.
4. Save the parameter set to a project file.
5. Use Write to module to download the configuration; the module stores the DRs in non-volatile memory.
6. Run a manual dose from Siwatool's Service view to verify the coarse/fine sequencing before connecting to the PCS 7 driver.

6. PCS 7 Integration: Driver Blocks and Signal Mapping

Under PCS 7 the SIWAREX FTA is exposed as a typed channel block (driver block) inside a CFC chart. The driver block exposes the coarse and fine command bits, the Dosing in progress status, the Dose complete status, the Good / Bad dose result, and the live weight to the PCS 7 OS picture and to the SFC / SFB automation logic. The CFC block name and signal layout depend on the PCS 7 version and the SIWAREX driver library that is installed in the master project.

The two push buttons in the original problem — Fine dosing and Coarse dosing — do not map directly to a PCS 7 input that the FTA evaluates. The buttons are operator selections that the application logic must translate into one of three behaviours, as described in the next section.

7. Step-by-Step: Two-Stage Dosing Configuration

This procedure produces the canonical two-stage behaviour, with the coarse flow delivering the bulk of the material and the fine flow dribbling the remainder.

1. Set DR21 — Setpoint weight. Enter the target dose weight (e.g. 50.000 kg). This is the live weight at which the fine valve closes.
2. Set DR22 — Fine weight. Enter a value strictly less than the setpoint (e.g. 49.500 kg). When the live weight reaches this value, the coarse valve closes; the fine valve continues.
3. Set DR23 — Inhibition time Fine. Enter a small positive value (e.g. 100 ms) to debounce the fine signal. Do not enter 0 unless you have a reason (see the error 043 section).
4. Set DR24 — Tolerance +/−. Enter the upper and lower tolerance limits that define a "good" dose.
5. Wire the operator buttons in PCS 7. The Coarse and Fine push buttons should select the dose recipe (e.g. DR21, DR22, DR24 values), not the valve directly. A single Start command triggers the dose; the FTA then drives both valves internally.
6. Add operator authorisation in the OS picture. Place the Start, Stop, Good/Bad indicators, and live weight display on the OS faceplate. Confirm the coarse/fine output indicators are visible so the operator can see the staging in real time.
7. Save the Siwatool project to the project archive so the parameter set is reproducible.

8. Single-Stage (Coarse-Only or Fine-Only) Configuration

The application as originally described requires the silo to be dosed on fine only or on coarse only, depending on which push button is pressed. The SIWAREX FTA supports this through the special case of DR22 Fine weight = 0.0 plus external selection of which output drives the valve.

8.1 Coarse-and-fine simultaneous (single output)

Set DR22 Fine weight = 0.0. The FTA then energises the coarse and fine outputs at the same time, and de-energises them at the same time when the setpoint is reached. The dosing is effectively a single-stage dose driven by both outputs in parallel. To use only one physical valve, wire that valve to a single output of the FTA (typically the coarse output, since it is the default in most driver blocks) and leave the fine output unused, or vice versa.

8.2 Coarse-only dosing

Use only the coarse signal from the SIWAREX FTA to drive the valve. The fine output remains unwired or is ignored in the application logic. The dosing will be fast but relatively coarse in accuracy — the overshoot is bounded only by the in-flight material between the coarse valve closing and the material landing in the silo.

8.3 Fine-only dosing

Use only the fine signal from the SIWAREX FTA to drive the valve. The dose is slow but accurate; the coarse valve is not used. This is the configuration to select when the operator presses the Fine dosing push button — it trades throughput for accuracy, which is the typical trade-off for a hand-finish or top-up operation.

8.4 Push-button selection in PCS 7

The two operator push buttons in the original question are best implemented as mode selectors that load a different recipe into DR21/DR22/DR24 before each dose, not as physical valve-selection switches. The PCS 7 CFC then writes the recipe to the module through the driver block and starts the dose. This keeps all dosing decisions in the module and reduces the latency penalty of routing the live weight through the PLC for valve control.

9. Technology Error 043: Inhibition Time Violation

Technology error 043 "Inhibition time violation Fine" is raised by the SIWAREX FTA firmware when the fine signal is requested to switch states more frequently than the configured Inhibition time Fine parameter in DR23 allows. The inhibition time is a debounce / latch that protects the fine valve and the dosing algorithm from a noisy or rapidly toggling weight signal. It is also raised if the fine signal is configured in a way that is inconsistent with the rest of the dosing parameters (e.g. DR22 Fine weight = 0.0 but DR23 Inhibition time Fine > 0, which is internally contradictory).

9.1 Resolution when fine is intentionally unused

When the dosing is configured for coarse-only or for simultaneous coarse-and-fine via DR22 Fine weight = 0.0, the fine output is not expected to be active in the normal sequence. The cleanest fix is to disable the inhibition time check on the fine path:

1. Open Siwatool and connect to the module.
2. Read the current DRs.
3. Navigate to DR23 — Inhibition time Fine (msec).
4. Set the value to 0. This disables the inhibition check on the fine signal.
5. Write the updated DRs back to the module.
6. Power-cycle the S7 station (or perform the equivalent reset on the module) to clear the latched technology error.
7. Run a manual dose from Siwatool's Service view to confirm that error 043 is no longer raised.

9.2 Resolution when fine is in use

If the fine path is genuinely in use, error 043 indicates a real problem. Investigate in this order:

1. Mechanical check. The fine valve may be chattering due to a sticky solenoid, a low air-supply pressure, or contamination in the seat.
2. Weight signal integrity. Excessive mechanical vibration on the scale can produce a rapidly changing weight near the fine threshold. Add mechanical damping, lower the measurement bandwidth, or widen the fine-threshold hysteresis.
3. Setpoint placement. The fine weight may be set so close to the setpoint that the in-flight material is repeatedly re-triggering the fine output. Move DR22 further from DR21 (e.g. from 49.8 kg to 49.4 kg for a 50.0 kg dose) so the fine stage has more time to complete.
4. Inhibition time tuning. Increase DR23 Inhibition time Fine so the firmware tolerates a brief toggling burst without raising the error.

10. Verification and Commissioning

After configuration, the following sequence verifies the dosing system end-to-end. Run this with the silo empty or with a known starting weight, and a controlled material supply.

1. Tare the scale from Siwatool (or send a tare command through the PCS 7 driver block). Confirm the live weight reads 0.000 kg ± 1 display count.
2. Start a manual dose from Siwatool in the Service view. Observe the coarse and fine output indicators. Both should energise at the start; the coarse should de-energise when the live weight reaches DR22; the fine should de-energise when the live weight reaches DR21.
3. Record the final weight and compare against the tolerance window. A correctly tuned system will land within tolerance on the first 3 of 5 consecutive doses.
4. From PCS 7, start a dose through the OS faceplate. Confirm the Dose complete, Good dose, and Bad dose indicators behave as expected.
5. Cycle through both push-button selections (fine only, coarse only). Confirm the dosing behaviour changes as designed, and confirm no technology error 043 is raised during a 30-minute run.
6. Trigger a power cycle on the S7 station. Confirm the dosing parameters are retained (DRs are non-volatile) and that the module re-establishes communication with the PCS 7 driver block without operator intervention.

11. Frequently Asked Questions