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Oven Heating Control System Failure

Oven Heating Control System Failure – Siemens S7-300 STEP 7

Callout to site – Failure of an oven heating control system – Automotive application. Initially, a routine visit to commission a Calorifier in late December 2015 led us to a component forming press we had visited many times before. However, this breakdown had a perfect storm aspect to it.

The problem arose in the preheat oven section used for heating a flat 9ft by 9ft component. The heating pattern was controlled by a recipe-controlling infrared elements system, pulsing (using Pulse Width Modulation) at various rates, modulated by an infrared temperature sensor. This system soaked the component in heat via Solid State Relays, applying power to 25 different heat zones, each with a 6-stage heating sequence. Operators could introduce adjustments based on their experience, depending on the component’s quality.

An obsolete specialized controller had failed, and there was no technical support, application, or programming software available. The Italian machine manufacturer’s personnel would not attend site so close to the holiday period.

My customer needed the machine back in full operation at the start of January, and in the meantime, they were dipping into their stock to keep the end user supplied. However, the machine manufacturer’s retrofit could not be installed and commissioned as they required a 10-day outage in May.

We had to come up with a solution. Utilizing a dedicated Siemens S7-315, we ensured fast scan time using interrupts, as a fancy software design was not feasible given the time constraints. The reaction time of the new system was the scan time (3mS) multiplied by 2, plus the propagation delay of the A to D conversation. Solid-state digital outputs and switching time of the SSRs would give us a respectable reaction time to modulate the AC waveform.

Designing, wiring (Frank from maintenance handled the wiring – nice job, Frank), programming, and commissioning within 18 hours was incredibly tight, but that was what we achieved. By 7 am, we had a system making components, and a representative from Jaguar Land Rover came over and shook my hand – a nice gesture.

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