Understanding the APL DoseL Dosing Block in SIMATIC PCS 7: Coarse, Fine, and Overrun Compensation

In SIMATIC PCS 7 Advanced Process Library (APL), FB 1809 DoseL is a dedicated technological block engineered for precise measured dosing and batch dispensing of liquids or solids. It controls single-stage or two-stage dosing via coarse feed (OutCoarse) and fine feed (OutFine) outputs, supports pulse-counter or analog flow rate inputs, and automatically calculates dynamic post-dosing overrun compensation (DoseCorr) to achieve tight setpoint tolerances without overfilling.

Operational Purpose and Scope of FB 1809 DoseL

In chemical, pharmaceutical, food and beverage, and water treatment plants, dispensing exact quantities of ingredients into vessels or reactors is a critical process operation. While basic dosing can be manually programmed using timer and counter logic, industrial processes require sophisticated cut-off compensation, tolerance monitoring, dribble feeding, and interlock protection.

The SIMATIC PCS 7 Advanced Process Library provides DoseL (FB 1809) to standardize high-accuracy dosing control. DoseL integrates seamlessly with SIMATIC BATCH, APL faceplates, and WinCC OS operator controls. It manages the complete lifecycle of a dosing operation:
* Ready / Idle state
* Start dosing command (StartAut / StartMan)
* Two-stage feeding (High-speed bulk filling followed by precision slow trickle)
* Dynamic valve/feeder shutoff before setpoint to compensate for material in transit
* Post-dosing measurement and tolerance evaluation (TolPos / TolNeg)
* Dosing completed (DoseEnd) or aborted (DoseTerm)

Dosing Principles: Coarse Flow, Fine Flow, and Overrun

To achieve both high throughput and high accuracy, DoseL employs a two-stage dosing profile:
1. Coarse Dosing Stage (OutCoarse = 1, OutFine = 1):
At the start of dosing, both the main feed valve/screw and the auxiliary bypass valve open simultaneously. Material enters the vessel at maximum flow rate until the actual dosed quantity reaches the coarse cut-off threshold:
$$\text{Coarse Cutoff} = \text{Setpoint} - \text{FineQuantity} - \text{Overrun}$$
2. Fine Dosing Stage (OutCoarse = 0, OutFine = 1):
The coarse output closes, drastically reducing mass flow rate. The remaining material enters via the fine dosing valve, allowing precise measurement and minimizing fluid inertia.
3. Overrun Stage (Shutoff Compensation):
When fluid flows through a pipe, closing a valve does not instantly stop mass accumulation—the column of falling liquid between the valve seat and the vessel surface still lands on the load cell. DoseL closes the fine valve at:
$$\text{Fine Cutoff} = \text{Setpoint} - \text{Overrun}$$
The material that lands after valve closure is measured and used to adjust subsequent doses.

Flow Input Modes: Analog Flow vs. Digital Pulse Integration

DoseL supports two distinct primary measurement methods selectable via parameter DoseTyp:
* Analog Flow Measurement (DoseTyp = 1):
The block connects to an analog flow transmitter (e.g. mass flowmeter or magnetic flowmeter) via parameter PV. DoseL numerically integrates flow rate over time using the cyclic execution interval of the assigned cyclic interrupt OB (e.g. OB35 at 100 ms) to compute totalized dosed quantity DQ.
* Digital Pulse Counter Input (DoseTyp = 2):
The block connects to a high-speed digital pulse counter or positive displacement flowmeter via input PulseIn. Each incoming pulse represents a calibrated volume or mass increment defined by PulseVal (e.g. 0.1 liters per pulse).
* Weighing Scale / Load Cell Input (DoseTyp = 3):
The block can directly evaluate the gross/net weight difference from a SIWAREX weighing card or analog load cell amplifier.

APL Dosing Block Comparison and Feature Matrix

The following matrix compares technological parameters and operational features in FB 1809 DoseL:

Parameter / FeatureOperational DescriptionTypical Configuration / RangeSPTarget dosing quantity setpointEngineering units (kg, L, gal)DQActual dosed quantity accumulated during current cycleRead-only outputFineValQuantity dosed exclusively during fine flow stage5% to 20% of total setpointDoseCorrOverrun correction value subtracted from cutoff pointAuto-calculated or manualCorrEnEnable automatic dynamic overrun calculation1 = Adaptive learning enabledCorrWeightWeighting factor for dynamic overrun adaptation0.1 to 1.0 (filter coefficient)TolPos / TolNegUpper and lower allowable dosing error limits$\pm 0.5\%$ to $\pm 2.0\%$OutCoarseDigital output for coarse / fast feed actuatorConnected to Pcs7DiOu / ValveOutFineDigital output for fine / slow feed actuatorConnected to Pcs7DiOu / ValveDoseEndOutput pulse indicating successful dosing within toleranceSignal to batch or sequence logic

Configuring and Tuning Overrun Compensation (DoseCorr)

To configure adaptive overrun compensation:
1. Enable Automatic Adaptation:
Set input parameter CorrEn = 1. This instructs the internal algorithm to compare the final settled quantity against the target setpoint after each cycle.
2. Set the Learning Weight (CorrWeight):
Configure CorrWeight between 0.2 and 0.5. A weight of 0.3 means the newly calculated error contributes 30% to the updated overrun compensation value, while previous historical data contributes 70%. This prevents wild swings caused by single-cycle turbulence:
$$\text{DoseCorr}{\text{new}} = \text{DoseCorr})$$}} + \text{CorrWeight} \times (\text{DQ} - \text{SP
3. Establish Settling Time (PostDoseTime):
Set PostDoseTime (e.g. 3.0 seconds) to ensure fluid sloshing or scale vibration settles completely before DoseL samples the final DQ for tolerance and overrun evaluation.

Verification

Verify DoseL operation through functional testing in CFC or S7-PLCSIM:
1. Place DoseL into Manual mode (ManAct = 1) and enter a setpoint SP = 100.0 kg, FineVal = 10.0 kg, initial DoseCorr = 1.0 kg.
2. Issue start command (StartMan = 1). Verify both OutCoarse and OutFine energize (1).
3. Simulate incoming flow PV:
- When accumulated quantity DQ reaches $100 - 10 - 1 = 89.0\text{ kg}$, verify OutCoarse de-energizes (0) while OutFine remains active (1).
- When DQ reaches $100 - 1 = 99.0\text{ kg}$, verify OutFine de-energizes (0).
4. Allow PostDoseTime to expire. If final quantity lands at 100.5 kg, verify that DoseCorr automatically updates to $1.0 + 0.3 \times (100.5 - 100.0) = 1.15\text{ kg}$.
5. Confirm that DoseEnd pulses 1 and no tolerance alarms trigger if 100.5 kg is within TolPos.

Limitations and Operational Precautions

Flow integration over analog PV depends on cyclic interrupt regularity; never place DoseL in non-cyclic OBs (such as OB1) where scan cycle jitter degrades integration precision.
If process material viscosity or line pressure fluctuates violently, set CorrWeight to lower values (0.1) to avoid unstable overcompensation.
When interfacing with SIMATIC BATCH, setpoint and start signals are controlled via SFC types; ensure ModLiOp = 1 and AutAct = 1 for external automated control.

Engineer FAQ

What is the difference between DoseL and the older Dose block from classic PCS 7 libraries?

DoseL is part of the Advanced Process Library (APL) and includes standard APL features: operating modes (Auto/Manual/Local/Out of Service), structured signal status (ST), feature bits, and modern WinCC OS faceplates. The classic Dose block used elementary data types and lacked unified APL alarming.

Can DoseL control a variable speed dosing pump instead of digital valves?

Yes. While OutCoarse and OutFine are digital, DoseL provides analog speed outputs or can be combined with an analog output block (Pcs7AnOu) where coarse dosing commands 100% speed and fine dosing commands 15% speed.

What happens if an interlock activates during dosing?

If an interlock (Intlock, Protect) trips, both OutCoarse and OutFine shut down immediately. Dosing is paused (DoseHold) or terminated (DoseTerm) depending on block feature bit parameterization.