Siemens LOGO! Conveyor Chain Pulse Monitor: Jam Detection Setup

Overview

The classic small-continuous-conveyor fault mode is a clutch-protected drive that is electrically commanded to run while the chain has seized against the sprocket. A single proximity sensor aimed at the sprocket teeth is the cheapest way to detect this, but a plain off-delay on that one bit fails under exactly the failure mode you need to catch: a tooth that parks under the sensor keeps the input true, so the timer never expires and the PLC happily reports "moving" while the chain is welded in place. A Siemens LOGO! 8 logic module with a frequency / pulse threshold trigger (sometimes called the frequency monitor or threshold trigger block) instead counts edges over a sliding measurement window and treats a stalled stream as a fault, regardless of the static state of the discrete input.

This reference documents the working pattern: pick a sprocket-tooth count and a normal running speed, derive a minimum pulses-per-second value, configure a Frequency Threshold Trigger with an On-Delay upstream to mask the start-up ramp, latch the fault output into a coil that drops the motor contactor, and provide a manual reset path. The same pattern applies to belt-slip detection on a tachometer roller, auger rotation monitoring, and slow-speed detection on mixer shafts.

Prerequisites

System Architecture

The conveyor is treated as three logical objects: the drive command (a Start pushbutton plus maintained run permissive), the motion evidence (pulse train from the sprocket sensor), and the fault latch (a self-holding coil that drops the contactor on either Start release, E-stop, or pulse-train loss). The motion evidence is the new element; the drive command and fault latch are conventional LOGO! patterns.

Figure 1 — Logical flow: drive command and motion evidence feed the decision block; the decision block drives the contactor output.

Sensor Selection and Mounting

An inductive proximity sensor aimed at the tooth root of the driven sprocket is the most common, lowest-cost choice. Mount it within its rated switching distance (Sn) of the tooth tip; a typical M12 sensor has Sn = 4 mm with a 1 mm hysteresis band. For a worn chain with possible tooth deformation, mount on a smooth roller that the chain rides over and let every roller tooth pulse the sensor — that gives a higher effective pulses-per-revolution and more headroom against the threshold.

Wiring the Sensor into LOGO!

Wire the brown lead to +24 V on the LOGO! sensor-supply terminal, blue to 0 V, and black (PNP output) to a fast digital input. On a LOGO! 8 base with relay outputs (e.g., 6ED1052-1MD08-0BA1), inputs I1–I4 are the high-speed capable inputs used by the counter, frequency trigger, and high-speed counter blocks. Connect the E-Stop NC contact in series with the Start pushbutton into a different input (I1), and the sprocket sensor into I3. The motor contactor coil returns to a relay output (Q1) or transistor output rated for the coil inrush.

Shielded cable is recommended for runs longer than 5 m; ground the shield at the cabinet end only to avoid ground loops. If the conveyor runs through a VFD-controlled section, route the sensor cable at least 200 mm from VFD motor cables and cross them at 90 degrees when unavoidable.

Computing the Threshold Frequency

The frequency threshold trigger fires when the measured frequency falls below a configurable threshold for a configurable on-delay time. The threshold must be set below the normal running frequency and well above zero so that a single missed pulse does not trip, but a stalled chain trips within a small number of pulse periods.

Use the formula:

where N_motor is motor RPM at running speed, Z_sprocket is the number of teeth being sensed, and i_gear is the overall gear ratio between motor and sprocket shaft. For a 1450 RPM motor, a 14-tooth sprocket, and a 1:5 reducer: f_run = 1450 × 14 / (5 × 60) ≈ 67.7 Hz. A reasonable threshold is 70% of that, or about 47 Hz, with a 2-second on-delay inside the trigger block. At 47 Hz the inter-pulse gap is 21 ms; a 2-second window therefore expects ~94 missed pulses before the fault fires, which absorbs chain stretch and transient tooth bounce without allowing a true stall to look healthy.

For the source application — "one pulse every five seconds when speed is correct" — that is f_run = 0.2 Hz. In that regime, the trigger block's threshold trigger with hysteresis parameter set to 0.1 Hz and an on-delay of 10 s is more appropriate, because the discrete on/off nature of the measurement dominates at such low rates. The user explicitly noted the 5-second figure is adjustable, so either increase the sensed target (sense every tooth on a multi-tooth sprocket so f_run climbs above 1 Hz) or shift to the threshold-trigger block with appropriate parameter scaling.

Programming the LOGO! Program

Build the program in LOGO!Soft Comfort as a Function Block Diagram (FBD). The minimum viable logic consists of four blocks chained together:

1. On-Delay (B001): Input is the drive command. Output goes true T_start seconds after drive command rises. Used to bypass the frequency trigger during start-up.
2. Frequency Threshold Trigger (B002): Sensor input I3 feeds the trigger. Threshold f_th, on-delay T_on, gate input = B001 output. While the gate input is false, the trigger output is forced false (no fault). While the gate is true, the trigger output goes true when f < f_th for longer than T_on.
3. RS Latch / Self-holding (B003): Set input = Start PB (or drive command rising edge). Reset input = the OR of E-Stop and the trigger output. Output Q is the motor run command.
4. Output (B004): Q1 drives the contactor coil or VFD enable from the latch output.

Wire the blocks with the FBD editor as follows:

I1 (Start/E-Stop) ──►[B003 RS]──►[B004]──► Q1 Motor
I2 (E-Stop)      ──┘                  ▲
I3 (Sensor)   ──►[B002 F.Thresh]──┐   │
                  gate: B001_out ──┘   │
B001: On-Delay      trigger ──────────┘
    input: I1 (drive command)
    T_start = 3.0 s
B002: Frequency Threshold Trigger
    input: I3
    f_th = 40 Hz
    T_on = 2.0 s
    g (window) = 0.5 s
    gate = B001 output

Timing Behavior

Figure 2 — Timing: the gate (B001) holds the fault logic suspended for T_start after the drive command rises, after which the frequency trigger is allowed to drop Q1 if pulses vanish for T_on.

Latching, Reset, and Indication

The fault must latch — a momentary pulse-loss event should not allow the conveyor to restart on its own, since the operator needs to walk the line and clear the jam. The RS latch block (B003) handles this: set on Start command rising edge, reset on either E-Stop NC opening or on the rising edge of the trigger output. To force a manual reset, route the Start pushbutton to a second contact that resets the latch, or add a dedicated Reset pushbutton into I4 wired to the reset input of B003.

For HMI / panel indication, expose three flags on unused outputs:

If a text panel (LOGO! TDE or external HMI) is present, transmit the measured frequency (B002 measured-value output) and the threshold as network inputs so the operator can view real-time pulse rate and tune T_start without re-programming.

Alternative Approaches

Two alternative block selections achieve the same goal with different tradeoffs:

• Threshold trigger with hysteresis (B007 family): simpler — output is true while input is below threshold, false while above. Combine with an on-delay to create a "below threshold for T seconds" detector. Suitable when f_run is in the 0.1–10 Hz range where the frequency trigger's resolution is overkill.
• Up/Down counter with comparator: count edges on I3 over a fixed measurement window using a pulse generator to gate the counter. Compare count to expected minimum. More flexible (you can compute expected pulses exactly for a known window) but more parameters to maintain.
• High-speed counter block: only required if f_run exceeds the 5 kHz input limit of the standard counter, or if you need absolute position rather than just speed.

Verification

1. Static test (stopped conveyor): Energize the LOGO!. Press Start. The drive command should be asserted, B001 should begin its T_start countdown, B001 output should be false until T_start elapses, then the gate opens and the trigger output should immediately rise (since f = 0). Q1 should drop the contactor. This proves the latch and trigger wiring.
2. Spin-up test (running conveyor): Press Start while the conveyor is free. After T_start, the gate opens; the trigger should evaluate f ≥ f_th and stay false; Q1 should remain energized. The "Bypass active" indicator should extinguish after T_start.
3. Simulated jam: With the conveyor running, manually stall the sprocket (use a gloved hand or wedge) without opening the E-Stop. Pulses stop. After T_on seconds, Q1 drops. Indicator Q3 illuminates. The latch holds Q1 off until reset.
4. Single-tooth parking test: The case that defeats a plain off-delay. Stop the sprocket with a tooth directly under the sensor head. Sensor input stays true. Frequency trigger still trips within T_on because there are no edges, only a static level. This is the diagnostic value of the design.
5. Slow-leak test: Reduce motor speed via the VFD to the minimum expected (e.g., 30% of nominal). f falls below f_th; Q1 should drop. Raise the VFD; manual reset required because the latch holds.

Commissioning Parameter Sheet

Photograph the LOGO!Soft Comfort program and the parameter sheet on commissioning. The measurement-window g parameter is the one most often mis-tuned in the field: too short and single skipped teeth trip; too long and the trigger becomes sluggish. Start at 0.5 s for f_run > 10 Hz and increase linearly with 1/f_run for slower belts.

Troubleshooting Matrix

Edge Cases and Field Notes

• VFD braking to stop: when an operator commands stop, f drops through f_th and the trigger would briefly try to trip. The latch accepts a reset on the falling edge of the drive command, so this is normally harmless. If nuisance tripping occurs during normal stops, suppress B002 evaluation when the drive command itself is false by ANDing the gate with the drive command.
• Reverse rotation: the frequency trigger counts edges without direction. A jammed chain that is mechanically back-driven by an adjacent conveyor (rare but possible on shared-shaft machinery) will look healthy. If reverse rotation is plausible, replace the frequency trigger with an up/down counter plus direction input from a second sensor.
• Sensor supply collapse: a LOGO! 24 V DC base module can source roughly 200 mA total across all sensor supplies. If multiple sensors and a TDE are powered, brown-outs during motor inrush are possible. Verify supply voltage at the sensor under motor-start conditions.
• Chain stretch at end of life: as the chain stretches, fewer pulses per unit time appear because the sprocket diameter effectively decreases relative to the pitch. A threshold set with no headroom will false-trip on a worn chain. Aim for f_th ≤ 0.5 × f_run and schedule chain replacement before f_run falls to f_th.

Standards and Reference Reading

The pulse-based motion proof pattern aligns with category 1 / category 2 safe stop functions described in ISO 13849-1 when combined with a monitored contactor. A standalone frequency trigger is a process interlock, not a safety function — a sensor that breaks off and reads zero will stop the line just like a real jam, so the system is fail-safe in the de-energize direction. If personnel protection is the goal, add a category 1 safety stop circuit in series with the LOGO! output and certify the safety function separately.

For programming reference, consult the LOGO! 8 system manual on the Siemens Industry Online Support portal: support.industry.siemens.com. Block parameter definitions for the Frequency Threshold Trigger and Threshold Trigger blocks are in the LOGO! 8.3 / 8.4 manual set under "Special functions" → "Counter and frequency" → "Frequency threshold trigger".

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