Open CNC: how the Tout-en-Un VPLC532e vision loop drives ribbon strip motion

• 1
  What an open CNC architecture actually changes
• 2
  Where the milliseconds go in the vision and motion loop
• 3
  Why the strip gauge and the fixture decide the camera choice
• 4
  How the Tout-en-Un VPLC532e sits inside the control layer
• 5
  When an open loop is the wrong choice
• 6
  Step by step: setting the trigger and reject window
• 7
  Open CNC cell vs closed vision sensor on a strip line

A closed CNC keeps motion, PLC logic and I/O inside one vendor's box. You tune it with the parameters the builder gives you, and that is usually the end of the story. An open CNC moves the trajectory planner and the machine logic onto a PC or an industrial controller, so the vision system and the axis can read the same clock and write to the same program.

That single change matters for ribbon strip work. The strip is thin, long and flexible, so the inspection head has to fire at a fixed distance from the cut, not at a fixed time after the start signal. When the controller owns both the encoder position and the camera trigger, the trigger stays locked to the strip position even when the line speed drifts.

The cost is real. An open system puts integration work on your team. You choose the fieldbus, you map the I/O, you decide what happens when the vision process misses a frame. None of that arrives in a box.

• 1
  Same clockMotion and vision share one time base, so trigger position does not drift with line speed.
• 2
  Same programReject logic and axis moves live in one project instead of two talking over a handshake.
• 3
  Your integrationFieldbus choice, I/O mapping and fault handling become your engineering tasks.

Every vision-guided move spends time in four places: exposure, transfer, decision and actuation. Exposure is set by light and strip reflectivity. Transfer is the frame moving from sensor to controller. Decision is the inspection algorithm. Actuation is the servo or pneumatic axis reacting to the result.

On a strip line running at 200 mm/s, one millisecond equals 0.2 mm of travel. If your reject gate sits 300 mm downstream of the camera, you have 1.5 seconds of budget. That sounds generous until you subtract a 12 ms frame transfer, a 20 ms algorithm pass and a 30 ms valve response. The margin shrinks fast on faster lines.

Software trigger beats hardware trigger when the encoder resolution is fine and the controller jitter is under 100 μs. Above that, use a hardware trigger from the encoder card and let the vision system only pass results back. Engines that ignore jitter end up with reject windows that wander by a few tenths of a millimeter across a shift.

Log the timing on every part, not once at commissioning. Thermal drift in the camera and in the servo amplifier will move those numbers.

Ribbon strip for automotive use is commonly 0.1 mm to 1.2 mm thick and 10 mm to 60 mm wide. At that gauge, the strip will vibrate in free span. A camera with a 5 MP sensor and a 35 mm lens can resolve 20 μm per pixel at a 300 mm working distance, but only if the strip stays inside a 1 mm depth of field. A loose span will not hold that.

So the fixture comes first. Support the strip on both sides of the inspection window, keep the free span under 150 mm, and add a light tension. Then pick the lens. If the feature you must measure is a 0.3 mm burr, you need about 6 pixels across it, which means roughly 50 μm per pixel or finer at the strip surface.

Backlighting works well for edge position and width. Coaxial or dome light works better for surface defects on rolled or plated strip because the surface scatter is directional. Reusing one light for both jobs rarely holds tolerance across a full run.

The same logic applies to the machined fixture blocks that hold the strip. Flatness and parallelism on those blocks feed straight into measurement repeatability. We hold ±0.005 mm on such fixture plates and inspect 100% before shipment.

The VPLC532e is an all-in-one box: vision processing, PLC logic and motion output in one unit. In an open CNC setup it usually acts as the cell controller. The CNC handles the cut path, the VPLC532e handles inspection and the reject decision, and the two exchange a small set of signals over the fieldbus.

Keep that signal set small. Position, part ID, inspection result and a heartbeat are enough for most strip cells. Every extra tag is another place for a mapping error to hide during a shift change.

When the vision box also commands the reject axis, the safety chain has to cover it. A reject gate that moves is a motion hazard, so it belongs in the same safety assessment as the CNC axes. Do not leave it on a soft signal.

For low-volume runs, the same box can drive a second station with a different recipe. Load the recipe by part number and the camera settings, light levels and reject offsets change together.

• 1
  Small tag listPosition, part ID, result, heartbeat. Extra tags hide mapping faults.
• 2
  Safety scopeA powered reject gate belongs in the CNC safety assessment, not on a soft bit.

Open architecture is not free. It costs engineering hours at integration and again at every major change. If your strip line runs one part number at one speed for years, a closed vision sensor tied to a fixed trigger will do the job with less risk and less code to maintain.

It is also the wrong call when your team has no one who can read a fieldbus trace or a controller log. When the line stops at 2 a.m., a closed box has a vendor hotline. An open cell has your own engineers, or nobody.

Skip the open route when the measurement you need is purely geometric and static. A laser profile sensor on a fixed gauge station gives width and thickness without a full vision stack. Adding a camera there just adds a failure mode.

Choose the open route when the part mix changes often, when the inspection recipe must follow the CNC program, or when you need to correlate a defect with the exact axis position that produced it.