Scenario
A production line experienced intermittent “no product detected” alarms, miscounts, inconsistent reject triggering, and fluctuating speed readings – leading to unplanned downtime.
The line relied on multiple detection points and speed feedback to:
- confirm product presence,
- count units,
- trigger a reject station,
- and maintain stable throughput.
- Symptoms observed on the line
- intermittent “no product detected” alarms, even when product was present
- batches stopping early (miscounts)
- reject station triggering late or inconsistently
- speed readings fluctuating, causing downstream timing faults
- troubleshooting taking too long because faults were intermittent
The result was unplanned downtime – and worse, the team couldn’t confidently predict when it would happen.
Why conveyor downtime happens (the “real root causes”)
When conveyors fail from a controls perspective, the cause is often not a major mechanical breakdown. More commonly, it’s one of these:
1) Unstable detection (sensor mismatch or installation issues)
- wrong sensor type for target
- detection too close to max range
- bracket vibration and sensor drift
- optical sensors affected by dust or inconsistent product reflectivity
2) Weak signal integrity (wiring, connectors, noise)
- damaged cable sheaths at high-flex points
- connector pin corrosion or loose locking rings
- electrical noise from motors/VFDs causing phantom triggers or missed pulses
3) Unreliable speed feedback (wrong sensor or wrong input method)
- speed feedback calculated indirectly instead of measured reliably
- pulses too fast for a standard PLC input
- encoder/speed sensor signals not shielded correctly
- a “rate” problem treated as a “count” problem
The diagnostic approach (what the team did first)
Before replacing anything, the maintenance and controls team ran a structured check:
Step A: Confirm whether the fault was detection or logic
- Does the sensor LED change state when the product passes?
- Does the PLC input change state reliably?
- Are there double triggers or flicker?
- Step B: Identify where “invisible failures” happen
They found that some sensors were switching locally, but the PLC input state was inconsistent. That pointed to:
- cable damage at vibration points,
- and noise pickup near VFD cabling.
- Step C: Validate speed feedback accuracy
Speed was being derived from a single pulse source that was not stable at higher load. The PLC then overcompensated timing, causing the reject station to misbehave.
The solution (what was changed)
The fix was a combination of better detection, better signal integrity, and better speed monitoring.
1) Improve detection reliability at critical points
- The line used detection at:
- infeed presence confirmation
- count point
- reject confirmation
- end-of-line “clear” signal
What changed:
- Sensor placement was adjusted to ensure a stable target and a sufficient margin to the sensing range.
- Where product reflectivity caused optical inconsistency, detection was improved by selecting a better-suited sensing approach (based on environment and target type).
- Mounting was reinforced to reduce vibration drift.
Recommended Products:
- Proximity sensors: https://switches.co.za/product-category/proximity/
- Optical/photoelectric sensors: https://switches.co.za/product-category/optical/
- Limit switches (where physical position confirmation is better): https://switches.co.za/product-category/limit/
Why this matters:
A sensor that triggers 98% of the time sounds “okay” until you realise that at high throughput, 2% failure equals constant downtime and quality issues.
2) Upgrade signal integrity (connectors and cable routing)
Two sensor cables were routed alongside a VFD power cable run. Over time, vibration and cable strain caused intermittent continuity and noise pickup.
What changed:
- Cable routing was adjusted to separate signal and power.
- Higher-quality connection points were used and strain relief was improved.
- Connector standardisation reduced the risk of incorrect pinouts during repairs.
Outcome:
Faults that previously appeared “random” became stable and predictable because the signal path was robust.
3) Add reliable speed feedback (and make it visible)
The biggest breakthrough came from treating the speed feedback as a measurement problem, not a “guess and compensate” logic problem.
What changed:
- A dedicated speed signal source was used to provide stable pulse feedback.
- Pulse handling and filtering were checked to avoid missed counts.
- A local display was added so maintenance could validate speed immediately during troubleshooting.
Recommended Products:
- Speed sensors: https://switches.co.za/product-category/speed/
- Rotary encoders: https://switches.co.za/product-category/rotary-encoders/
- JR-D (6-Digits) for local rate/RPM/counter readout: https://switches.co.za/product/jr-d-6-digits/
Why the local display mattered:
Instead of “is the PLC calculation correct?”, technicians could see live speed and identify slip or abnormal variance immediately.
Results:
In this case, the outcomes were:
- Operational outcomes observed
- fewer “false detection” stops
- more consistent batching and counting
- reject station triggering became predictable
- faster troubleshooting because speed became visible
- reduced callouts caused by loose connectors/cable issues
Fewer false stops, consistent batching and reject timing, faster troubleshooting, reduced callouts due to connector/cable faults.
Download the PLC Commissioning Checklist.
Small improvements – better sensor selection, better connectors, and reliable speed monitoring – can eliminate “mystery downtime” and stabilise throughput.
