Siwarex FTA Multi-Silo Additive Dosing into a Common Weighing Hopper

This engineering reference covers the design, configuration, and commissioning of a Siemens Siwarex FTA-based additive weighing system where multiple silos feed a common weighing hopper through individually controlled magnetic vibratory feeders. The application pattern matches the standard PCS 7 "dosing from several components" recipe delivered in the Siwarex Multiscale configuration package, and is fully supported by the standard Siwatool commissioning environment and the AS/OS blocks of the PCS 7 library.

1. System Architecture and Application Pattern

The target topology is a star-shaped additive batching arrangement. Three (or N) storage silos each feed a common weighing hopper through a magnetic vibratory feeder (alternatively: screw feeder, slide gate, or rotary valve). The hopper is suspended on three (or four) compression load cells whose mV/V outputs are summed in a junction box and brought back to the Siwarex FTA as a single differential mV signal. The Siwarex FTA itself is plugged into the SIMATIC S7 backplane (ET 200M on PROFIBUS or PROFINET, or directly in an S7-300 station) and is integrated into PCS 7 through the standard Multiscale CFC plan and the standard Multiscale faceplates on the OS.

Operating sequence per batch:

1. Operator selects recipe; PCS 7 commands the FTA to start dosing component 1. FTA auto-tares the empty hopper, then opens the coarse flow.
2. Feeder 1 runs at high amplitude (coarse). When the actual weight reaches a coarse cut-off threshold inside the FTA, the FTA drops the analog output to a low-amplitude fine value.
3. Feeder 1 runs at low amplitude (fine). When the actual weight reaches the SP1 set point, the FTA closes the feeder (drives analog output to zero or to a configurable stop value).
4. The weight of component 1 is captured by PCS 7 from the FTA and written to the batch report.
5. The FTA automatically arms for the next component. PCS 7 re-routes the multiplexer so that the next analog signal lands on the second feeder. Coarse/fine dosing of component 2 begins additively on top of the existing gross weight.
6. The cycle repeats for components 3 through N.
7. After the last component finishes, the FTA exposes the per-component weights and the recipe total. PCS 7 opens the discharge valve. The hopper empties. The FTA zeroes/tares in preparation for the next batch.

This "additive, no empty between components" pattern is exactly what the Siwarex FTA's DR4 = "Filling (additive) without emptying" data record was designed for. The FTA holds the gross weight in the hopper between sub-dosings, accumulates the recipe, and exposes the per-component weight to the S7 side at the end of each sub-dosing, so the PCS 7 batch log gets a clean per-component weight plus a total weight.

2. Prerequisites and Component Selection

Confirm the following before commissioning:

• Siwarex FTA module (e.g., 7MH4900-1AA01) installed in ET 200M or S7-300, firmware compatible with the Siwarex Multiscale configuration package CD you will install in PCS 7. The matching configuration package is published under Siemens Support entry Siwarex Multiscale for PCS 7 V8/V9 - configuration package CD.
• Siwatool FTA commissioning software installed on the engineering station. Siwatool is used to load DR (Data Record) parameters into the FTA, calibrate the scale, run command interpreter (CMD) sequences, and capture trace recordings.
• Three (or more) load cells matched to the hopper's full-scale weight. Compression-type load cells (e.g., Siemens Siwarx R or any 4-wire / 6-wire mV/V cell with class C3 or better) are typical. All cells in a hopper must be of the same nominal capacity and matched within the manufacturer's tolerance to limit corner-load error.
• Junction box (Siwarex JB or equivalent) with sense-resistor trim, fitted with the Siwarex extension cable between the box and the FTA. The Siwarex cable is shielded and uses the dedicated 6-pin connector that mates with the FTA's sensor input; the alternative is a user-wired cable using the assignment table in the FTA manual.
• Magnetic vibratory feeders (or alternative feeders) with thyristor controllers that accept a 0/4-20 mA setpoint. The setpoint current is provided by the FTA's analog output and is switched between coarse and fine values by the FTA's dosing state machine.
• Multiplexer / channel selector (contactor bank, solid-state relay matrix, or a small PLC) that routes the analog signal and the digital enable to the currently active feeder. The PCS 7 application program is the right place to drive the multiplexer because the FTA does not know which physical silo corresponds to which "channel" in the recipe.
• PCS 7 V8.x or V9.x with the Siwarex Multiscale library installed in the master data library. Both AS blocks (CFC) and OS faceplates are delivered with the configuration package CD.

3. Wiring and Signal Topology

The signal flow is:

1. Load cells -> Junction box (parallel sum, sense-line compensation).
2. Junction box -> Siwarex extension cable -> FTA sensor input (6-pin connector on the FTA front).
3. Sensor excitation: 10 V DC provided by the FTA.
4. Analog output: FTA -> 0/4-20 mA current loop into the thyristor controller setpoint input of the currently selected feeder. Polarity is observed per the controller manual.
5. Digital output (FTA DO0 / DO1, configurable in DR30) can be wired to a feeder-enable interlock or to a status indicator. The multiplexer channel selection is not done from the FTA digital output; it is driven by the PCS 7 program from the active component index.
6. RS-232 service port on the FTA front is used by Siwatool for parameter download, calibration, and trace. A standard 1:1 serial cable (Siemens 6ES7 972-0CA20-0XA0 or equivalent) is used.

Grounding: ground the load-cell cable shield on the junction box end only; ground the FTA end at the DIN-rail ground bar. The thyristor controllers must be grounded independently per their manual; do not use the load-cell shield as a PE conductor.

4. Siwatool Commissioning Software Setup

Siwatool is the FTA's native parameterization and calibration tool. It is launched from the engineering PC, connects to the FTA's RS-232 service port (or via an S7 routing path through the CPU), and exposes the FTA's data records (DR) as editable spreadsheets.

1. Connect Siwatool to the FTA. Set the COM port and baud (default 9 600 8N1). Siwatool auto-detects the module type and firmware version.
2. Read the current parameter set. Save a backup file. This is your "golden image" for the scale.
3. Configure DR3 (calibration): nominal load, unit (kg / t / lb), resolution per digit, decimal point, zero range, span, and the calibration procedure (the FTA supports dead-weight calibration with the actual mV/V read out, and theoretical calibration when test weights are not available). With three compression cells under one hopper, dead-weight is the recommended method; calibrate after the hopper has been mechanically aligned and the feeders disconnected.
4. Configure DR4 (dosing mode). Select "Filling (additive) without emptying." This is the mode that holds the gross between sub-dosings and accumulates the recipe total.
5. Configure DR5 (limits): empty range, stability window, standstill time, dosing timeout. Dosing timeout protects the batch in the event of a feeder fault (e.g., feeder stalled, valve jammed closed). The timeout value should be set to approximately 150-200 % of the longest expected fine-dosing time plus a margin for product settling.
6. Configure DR6 (filter): low-pass filter for the load-cell signal. The default is appropriate for slow hopper applications. For a magnetic vibratory feeder that introduces mechanical disturbance, increase the filter cut-off frequency gradually until the displayed weight becomes stable during feeder operation.
7. Configure DR7 (status): enable the status bits you will use in PCS 7 (command enable, dosing active, fine/coarse, error). DR7 is read-only from the S7 side and is the primary source of state for the CFC plan.

5. DR4 - Additive Filling Without Emptying

DR4 selects the dosing mode. The supported values are:

Select DR4 = 0 (Filling, additive, without emptying) for the 3-silo shared-hopper application. With this mode selected:

• The FTA does not auto-tare after each component; it accumulates gross weight across all sub-dosings.
• The fine/coarse state machine runs once per sub-dosing, on top of the existing gross.
• At the end of the last sub-dosing, the FTA exposes the per-component weights and the recipe total.
• The PCS 7 program is responsible for resetting the recipe state and taring for the next batch.

6. DR23 - Analog Output Coarse / Fine Configuration

The FTA's analog output is a 0/4-20 mA current source that drives the active feeder's thyristor controller. The FTA itself switches the current between a "coarse" value and a "fine" value as the actual weight approaches the set point, so the feeder vibrates at high amplitude during bulk delivery and at low amplitude during in-flight / dribble correction.

Configure the following in DR23 (Siwatool -> DR23):

Field-procedure for selecting coarse and fine values:

1. Set fine to the lowest value at which the feeder still delivers material reliably (e.g., 20 %).
2. Set coarse to a value that delivers the target bulk rate without spillage or feeder bounce (e.g., 85 %).
3. Run a calibration batch. Monitor the in-flight weight (the material that falls into the hopper after the analog output has been switched to stop). In-flight is a function of material flowability, feeder geometry, and the fine current. If in-flight is large relative to the fine-cut-off, lower the fine value or shorten the fine-flow window by raising the coarse cut-off threshold.
4. Document both the values and the achieved in-flight weight in the calibration report. The PCS 7 recipe uses the per-component SP values and trusts the FTA's closed-loop control; the operator does not tune the analog values from the faceplate.

7. Component Identification via DB32 Bit Mapping

When the Siwarex Multiscale CFC plan is compiled into the AS, it allocates a per-component data structure inside DB32. The PCS 7 program uses one bit per component to identify the active sub-dosing; this is the index that drives the multiplexer channel selection. The bit addresses are:

Implementation in the PCS 7 CFC plan (SCL example):

IF "FTA"."Comp1_Active" THEN
    "MuxSel" := 1;   // route analog output + enable to feeder 1
ELSIF "FTA"."Comp2_Active" THEN
    "MuxSel" := 2;
ELSIF "FTA"."Comp3_Active" THEN
    "MuxSel" := 3;
ELSE
    "MuxSel" := 0;   // no feeder selected; close all
END_IF;

The multiplexer is a small interposing relay block (one contactor or SSR per feeder) wired such that the active set of bits closes the line from the FTA's analog output to the corresponding feeder's thyristor setpoint terminal and the digital enable to the corresponding feeder-enable input. The PCS 7 program does the routing; the FTA itself does not know which physical silo is "silo 1."

8. Multiplexer / Feeder Selection - the Digital Output Question

The FTA exposes a small number of digital outputs (DO0, DO1; quantity and assignment in DR30). The temptation is to use these to drive the multiplexer directly. The recommended architecture is not to do that, for two reasons:

1. The FTA's digital outputs are not aware of the recipe's component-to-physical-silo mapping. They follow the FTA's internal state, which is one thing; mapping that state to a physical feeder is a project-specific decision that belongs in the PCS 7 program.
2. If the recipe is changed (more components, different order, or a different physical silo for component 1), the FTA's output map does not need to change. Only the PCS 7 routing block does.

If, however, the multiplexer is driven by the FTA directly, the assignment is fixed: each DO is hard-wired to a feeder, and the recipe order is fixed. This is acceptable for a single-product plant with three physical silos and a fixed order, but it does not give the operator recipe flexibility from the OS.

9. PCS 7 Multiscale Integration - CFC and Faceplates

The Siwarex Multiscale configuration package CD installs:

• The AS blocks (CFC) for the FTA. The main block encapsulates the Siwarex FTA driver, the dosing state machine, and the per-component data. Drop one instance per physical scale.
• The OS faceplates for the standard views: current gross weight, net weight, total, set point, status (DR7), command buttons (start, stop, tare, zero), and the per-component sub-display.
• The Siwatool FTA engineering software (run on the engineering station).

Within the CFC plan, wire the following signals:

The CFC plan also contains the multiplexer-routing block, the discharge-valve driver, and the batch-step handshake (BatchStart, StepDone, BatchAbort). Standard PCS 7 batch blocks (SFC) drive the recipe.

10. Taring Strategy for Sequential Dosing

For DR4 = "Filling (additive) without emptying," the recommended taring discipline is:

• Tare once at batch start, when the hopper is empty and stable. This sets the net weight to zero and gives a clean reference.
• Do not tare between components. Taring between sub-dosings would zero out the accumulated gross, which is the recipe total in progress. The per-component weight is computed by PCS 7 as the difference between successive gross readings.
• Tare (or zero) after discharge, when the hopper is empty and stable, in preparation for the next batch. This can be commanded by PCS 7 automatically when the discharge valve closes and the weight has settled.

If a taring-on-the-fly is required (e.g., to recover from a partial discharge), the operator can issue a manual tare from the faceplate, but the recipe totals must be re-validated. In a regulated batch environment (pharma, food, feed), tare is typically locked to the operator authorization level.

11. Step-by-Step Commissioning Procedure

1. Mechanical check. Verify hopper alignment, all load cells in contact, no pipework stress transferring to the hopper, feeders free to vibrate without hard contact to the hopper structure.
2. Electrical check. With the FTA powered and the load cells connected, read the raw mV/V value in Siwatool (DR3 diagnostic). A typical 3-cell sum with 2 mV/V cells at full scale is on the order of 6 mV/V at nominal load. Zero with no load; verify the mV/V reads near 0.0000.
3. Calibration. Apply known test weights to each cell in turn (corner-load test), then apply a known full-scale weight to the whole hopper (dead-weight calibration). Re-zero, re-span, save the parameters to the FTA, and back up to Siwatool.
4. DR parameter set. Configure DR4 = 0 (additive without emptying), DR5 (limits), DR6 (filter), DR23 (analog output coarse/fine), DR30 (digital output assignment, if used). Save and back up.
5. PCS 7 CFC compile. Compile the Multiscale CFC plan into the AS. Verify the instance DBs are created and DB32 is allocated.
6. OS compile. Compile the OS, drop the Multiscale faceplate, and verify the live weight value matches the Siwatool display.
7. Multiplexer test. Force each component index from the CFC test mode and verify that the corresponding relay closes and the analog output is routed to the correct feeder.
8. Feeder test. With the hopper empty and tared, start a 5 kg dosing of component 1. Observe: coarse current -> fine current -> stop. Verify the actual weight reaches 5 kg +/- in-flight tolerance. Repeat for components 2 and 3.
9. Additive test. Run a full 3-component recipe (e.g., 10 kg + 20 kg + 30 kg). Verify: after component 1, gross = 10 kg. After component 2, gross = 30 kg. After component 3, gross = 60 kg. Per-component values reported by PCS 7 must match the recipe to within the dosing tolerance.
10. Discharge test. Open the discharge valve. Verify the hopper empties, the weight returns to zero, the discharge valve closes, and the FTA tares in preparation for the next batch.
11. Fault test. Force a feeder-stall fault (e.g., jam a piece of material into the vibratory tray). Verify the dosing timeout fires, the FTA raises the dosing-error status, the recipe aborts cleanly, and the faceplate alarm appears on the OS.

12. Verification and Diagnostics Checklist

13. Troubleshooting Matrix

14. Field-Proven Caveats

• Mechanical isolation. A magnetic vibratory feeder bolted rigidly to the hopper structure will inject vibration noise that the FTA's low-pass filter can mask at low cut-off but at the cost of dosing speed. The hopper and the feeder must be on separate support structures, with a flexible chute between them.
• Air pressure / dust. Pulse-clean filters on a hopper plenum can change the apparent weight by a few hundred grams per pulse. The DR6 filter must reject this; verify by trend-recording the weight during a filter pulse cycle.
• Temperature. Load cell zero drift over a 20 K temperature swing is on the order of 0.05 % of FS for a C3-class cell. The FTA's auto-zero function (when enabled in DR5) compensates slowly; for high-accuracy batching, do not run auto-zero in the middle of a recipe.
• Recipe integrity. Recipe parameters (SP, tolerance, set of components) live in PCS 7. The FTA's data records (DR3 / DR4 / DR5 / DR23) are the scale configuration, not the recipe. Do not modify DR records from the recipe; the Siwatool backup is the source of truth for the scale.