4.2 Process Controls and Quality Gates

A digital system that only "records" production is a passive historian. To prevent defects, the MES must act as an active gatekeeper, physically blocking the process when conditions are violated. Move beyond "inspection" (finding defects) to "control" (preventing defects) by interlocking the physical machine with the digital logic.

The Interlocking Strategy (The "Gate")

The machine must never be the decision maker. It is the muscle; the MES is the brain. Establish a strict "Request/Grant" handshake for every cycle.

The Handshake Protocol

1. Request: Machine reads barcode and asks MES: "Can I process Unit X?"
2. Validate: MES checks:
   • Is the Previous Step = Pass?
   • Is the Route Correct?
   • Is the Recipe Match = True?
3. Grant: MES sends logic signal: "Enable Conveyor/Start Cycle."
4. Deny: MES sends logic signal: "Lock Machine + Display Error."

The "Zombie Unit" Risk

• If a unit fails a test but is physically pushed to the next station → The Next Station must Reject it.
• Reasoning: Operators will try to bypass logic to meet quotas. The machine must refuse to work on "Dead" (Failed) units.

Parametric vs. Binary Data

"Pass/Fail" is insufficient for engineering analysis. Storing only binary results hides process drift until it is too late.

Data Storage Logic

• Binary (Boolean): Useful only for simple routing decisions.
• Parametric (Float): Mandatory for Critical to Quality (CTQ) characteristics.
  • Example: Do not store "Torque: OK". Store "Torque: 5.2 Nm".

Limit Management

• If limits are stored in the machine PLC → You have a maintenance nightmare.
• Then master the limits in the MES/PLM. The machine should fetch Min/Max/Target values at the start of the cycle.

Pro-Tip: When defining limits, define "Warning Limits" (Yellow Zone) inside the "Spec Limits" (Red Zone). Trigger a maintenance alert if 3 consecutive units hit the Yellow Zone, even if they are technically "Passing."

Statistical Process Control (SPC) Integration

Detect drift before it becomes a defect. Real-time SPC is not about drawing charts for management; it is about stopping the line automatically.

Automated Rule Sets

Do not rely on humans to interpret control charts.

• Rule 1 (Out of Spec): 1 point > Upper Spec Limit (USL). Action: Stop Line.
• Rule 2 (Trend): 7 points consecutively increasing. Action: Email Process Engineer.
• Rule 3 (Shift): 7 points on one side of the Mean. Action: Alert Supervisor to check calibration.

Sampling Logic

• If Process Cpk > 1.67 (Robust) → Sample 1 in 10.
• If Process Cpk < 1.33 (Unstable) → Force 100% Test.

Poka-Yoke (Mistake Proofing)

Digital Poka-Yoke prevents the operator from making an error, rather than warning them after the fact.

Common Digital Controls

• Parts Match: Operator scans the "Reel" before loading the feeder.
  • Logic: If Reel P/N ≠ BOM P/N → Feeder Lock.
• Tool Interlock: DC Torque driver is disabled until the correct barcode is scanned.
  • Logic: Tool enables for exactly 4 screws. If operator tries 5th → Tool ignores trigger.
• Revision Match: Operator scans the Work Instruction revision.
  • Logic: If WI Rev < Active Route Rev → Screen Lock.

Final Checklist

Category	Metric / Control	Threshold / Rule
Interlock	Handshake	100% of Machines request "Start Permission" from MES
Data	Granularity	Critical Tests store Value + Unit (e.g., 5.0 V)
Limits	Storage	Mastered in MES, pushed to Machine
SPC	Automation	Line Stop triggered automatically on Rule 1 Violation
Poka-Yoke	Tooling	Connected Tools (Torque/Press) interlocked to Job
Safety	Override	"Force Pass" requires Engineering Manager Password
Calibration	Enforcement	Tool disabled if Calibration Date < Today