Diagnosing Spurious Sensor Glitches in SIMIT Simulation with the CONTEC Conveyor Library

In SIMIT Simulation (V9.x and V10.x) using the CONTEC conveyor technology library (Fördertechnik-Bibliothek), spurious sensor glitching or 'wiping pulses' (where a proximity sensor turns to 1, drops to 0 upon material clearing, and then briefly pulses back to 1 for a single simulation cycle) are caused by spatial boundary discretization errors, geometric sensor placement at the exact conveyor transition boundary, or floating-point rounding during discrete coordinate handoffs. To resolve this, position the sensor at least one simulation step distance away from the conveyor boundary (Pos < Length - v * CycleTime), verify that adjacent conveyor elements are snapped together with identical geometry, and configure an appropriate hysteresis or debounce filter on the sensor output block.

Mechanism of SIMIT CONTEC Discrete Coordinate Modeling

The SIMIT simulation platform enables virtual commissioning of SIMATIC PCS 7 systems by simulating field device behavior, conveyor material handling, and process dynamics. The SIMIT CONTEC library provides specialized kinematic components—such as roller conveyors, belt conveyors, turntables, and position sensors—to model material transport systems.

In SIMIT CONTEC, material items (Fördergut) are treated as geometric entities characterized by length, width, and an absolute or relative coordinate along a 1D/2D transport track. In each discrete simulation cycle $\Delta t$ (typically 50 ms or 100 ms), SIMIT calculates the displacement of each material item using the transport velocity $v$:

$$\Delta s = v \cdot \Delta t$$
$$x(t + \Delta t) = x(t) + \Delta s$$

Conveyor sensors (such as optical photo-eyes or proximity switches) detect material when the item's geometric bounding envelope intersects the sensor's configured coordinate window:

$$Pos_{sensor} - \frac{L_{sensor}}{2} \le x_{item}(t) \le Pos_{sensor} + L_{item} + \frac{L_{sensor}}{2}$$

When an item traverses the conveyor track, the sensor output transitions from 0 to 1 as the leading edge enters the detection zone, remains 1 while the body crosses, and drops back to 0 when the trailing edge exits.

Root Causes of Spurious Sensor Bouncing (Wischer)

In virtual commissioning projects, engineers frequently observe a sensor artifact referred to in German engineering literature as a 'Wischer' (wiping pulse). The sensor activates, deactivates as the material moves away, and then abruptly pulses 1 for a single simulation cycle before returning to 0.

This behavior arises from three technical factors within the discrete simulation engine:

Boundary Coordinate Boundary Collision: If a sensor is placed at or immediately adjacent to the physical end of a conveyor element (e.g., sensor position set to $Pos = Length$), the material handoff between Conveyor Element $A$ and downstream Conveyor Element $B$ involves a multi-step coordinate transformation. If Element $A$ has not purged the item from its internal list during the handoff cycle while Element $B$ begins processing its leading edge, coordinate wrap-around causes Element $A$'s sensor to evaluate the item's coordinate as intersecting the sensor for one cycle.
Discrete Time Step Overshoot: At higher conveyor speeds (e.g., $v = 1.5\text{ m/s}$) and larger simulation cycle times (e.g., $\Delta t = 100\text{ ms}$), the displacement per step is $\Delta s = 0.15\text{ m}$. If the sensor detection window is narrow (e.g., $0.05\text{ m}$), spatial undersampling causes discrete position jumps. In certain geometries, rounding truncation in SIMIT's internal floating-point coordinate registry re-evaluates the boundary condition on the step immediately following item exit.
Misaligned Snap Connectors: If adjacent conveyor blocks in SIMIT are visually aligned but their logical CONTEC connection ports (In and Out material flow connectors) are not properly snapped, items do not transition smoothly. The item briefly stalls or oscillates across the boundary interface before being transferred to the downstream segment.

Engineering Solutions and Configuration Rules

To eliminate ghost pulses and stabilize sensor signals in SIMIT CONTEC models, implement the following engineering corrections:

Rule 1: Set Sensor Position Inside Safe Mechanical Margins

Never place a sensor coordinate at the extreme edge of a conveyor component ($Pos = 0.0$ or $Pos = Length$). Maintain a clearance margin equal to at least twice the step displacement:

$$Pos_{min} \ge 2 \cdot (v_{max} \cdot \Delta t)$$
$$Pos_{max} \le Length - 2 \cdot (v_{max} \cdot \Delta t)$$

For example, on a 2.0-meter conveyor running at $1.0\text{ m/s}$ with a $50\text{ ms}$ cycle time ($\Delta s = 0.05\text{ m}$), place the exit sensor at $Pos \le 1.90\text{ m}$ rather than $2.0\text{ m}$.

Rule 2: Ensure Logical Snap Connection Between Elements

In the SIMIT graphical canvas, select the conveyor segment.
Check the connection nodes. Ensure the green connection handle of Conveyor 1 snaps directly into the input handle of Conveyor 2.
Verify in the component properties that both conveyors share matching track widths and material transfer parameters.

Rule 3: Implement Signal Debounce / Smoothing in SIMIT or PLC

If mechanical positioning requires placing the sensor close to the boundary, insert a signal conditioning block between the CONTEC sensor output and the PLC coupling interface:
1. In SIMIT, route the sensor binary output through a Pulse / Delay component (e.g., an off-delay timer set to $2 \times \Delta t$).
2. Alternatively, in the PCS 7 CFC program, use the Pcs7DiIn driver block with its input filter parameter FltrOn = 1 and FltrTi set to $100\text{ ms}$. This suppresses any single-cycle discrete glitch before it reaches the control logic.

SIMIT Conveyor Simulation Parameter Matrix

The following table summarizes the parameter relationships governing CONTEC sensor behavior:

Parameter NameSIMIT Block LocationRecommended ValueOperational FunctionPos (Position)Sensor Component > Parameters$0.1 \cdot L < Pos < 0.9 \cdot L$Longitudinal coordinate of the sensor along the conveyor trackWidth / LengthSensor Component > GeometrySlightly larger than item detection tabGeometric detection volume along transport axisVelocity ($v$)Conveyor Component > Parameters$0.2\text{ m/s} - 1.5\text{ m/s}$Linear transport speed of the conveyor belt/rollersSimulation Cycle ($\Delta t$)SIMIT Project Settings > Time Slice$20\text{ ms} - 50\text{ ms}$ (Slice 1 or 2)Fundamental integration step size for kinematic calculationsFltrTi (Filter Time)CFC > Pcs7DiIn Channel Driver$50\text{ ms} - 100\text{ ms}$Software debounce filtering out simulation boundary artifacts

Verification

To verify that the sensor ghost pulse is resolved:
1. In SIMIT, open the Chart / Trend view and add the raw CONTEC sensor output signal and the PLC input tag.
2. Start the simulation and release test material items through Conveyor S4 across the sensor.
3. Observe the trend recorder: The sensor signal must cleanly transition from 0 to 1 as material enters, maintain 1 during presence, and drop to 0 cleanly upon exit without any single-cycle spikes.
4. Verify in the PCS 7 CFC online view that the connected Pcs7DiIn block or interlock logic does not trigger false edge-detection interrupts (R_TRIG).

Limitations and Operational Precautions

Observe the following technical precautions when simulating conveyor dynamics in SIMIT:
- Time Slice Allocation: Assign CONTEC kinematic components to SIMIT Time Slice 1 or 2 ($20\text{ ms}$ to $50\text{ ms}$). Running conveyor physics in slow time slices ($500\text{ ms}$) causes severe spatial discretization errors.
- Virtual vs. Real Sensors: In physical installations, optical sensors may experience optical bouncing due to reflective packaging or mechanical vibration. Do not mask physical process sensor flaws by over-filtering without validating field switch behavior.
- PLCSIM Coupling Latency: When coupling SIMIT to PLCSIM Advanced or standard S7-PLCSIM, ensure the communication cycle matches the simulation time slice to prevent I/O buffering jitter.

Engineer FAQ

What is a 'Wischer' in German automation terminology?

A 'Wischer' refers to a brief wiping or bouncing pulse—an instantaneous false pulse (typically lasting a single PLC or simulation cycle) generated by sensor contact bounce or boundary calculation errors.

Why does decreasing the SIMIT simulation time slice eliminate the pulse?

Decreasing the cycle time (e.g., from 100 ms to 20 ms) reduces the distance traveled per step (Delta s). Smaller steps prevent the item coordinate from overshooting the sensor detection window and boundary calculation zones.

Can I use standard SIMIT Basic blocks instead of the CONTEC library?

Yes. SIMIT Basic library provides elementary components (such as integrators, limiters, and flip-flops) to build custom material tracking, but CONTEC provides pre-built 2D/3D physics, collision detection, and routing.