1. Introduction
The transition from systems based on wired relay logic to PLC (Programmable Logic Controller) based systems represents a necessary step in today's machine tool manufacturing industry. Relay architectures, once standard, now present critical technical limitations: poor flexibility in logic modifications, high diagnostic complexity and, above all, an increasing obsolescence of components which leads to prolonged downtime in the event of a fault. Furthermore, recent directives on energy efficiency (such as the Ecodesign regulations) and the need to guarantee high safety standards, in compliance with the EN ISO 13849 standard, make it difficult to maintain competitive systems based on dated electromechanical components. The retrofit to the PLC is not just a technological update, but a strategic investment for operational continuity.
2. Evaluation of the Legacy System
Before starting the retrofit, you need to examine the state of the existing system. Not every system requires immediate intervention, but the assessment must be based on objective parameters of reliability and risk.
| Criterion | Critical State | Status Acceptable |
|---|---|---|
| Age of the electrical panel | > 20 years | < 10 years |
| Failure Rate (MTBF) | < 2000 hours | > 10000 hours |
| Spare parts availability | Obsolete / Discontinued | Available in stock |
| Diagnostics | Absent (manual troubleshooting) | Local LED indicators |
| Safety Compliance | Not compliant (EN ISO 13849) | CE certified |
3. Modern Alternatives
Replacing relay logic with PLC offers immediate benefits in terms of precision and management. An essential modern component in this context is the HYDAC 42/98 0016 075 pressure transducer, which allows the monitoring of hydraulic systems to be integrated directly into the PLC control cycle, improving precision compared to old mechanical pressure switches.
| Feature | Relay Logic | PLC control |
|---|---|---|
| Logical Flexibility | Physical (rigid) cabling | Software (editable) |
| Diagnostics | Multimeter (slow) | HMI / PC interface (instant) |
| Sensor Accuracy | Low (mechanical) | High (electronics, e.g. HYDAC 42/98) |
| Encumbrance | High (multiple relays) | Reduced (CPU/IO module) |
4. Calculation of ROI (Return on Investment)
The cost analysis must consider not only the price of the components, but the overall operational impact. Let's consider a real case of an average machine tool:
- Retrofit Cost (Components + Work): €15,000
- Energy Savings (PLC + Inverter vs Relay): 3,000 kWh/year * €0.20/kWh = €600/year
- Downtime reduction: Reduction of 80 hours/year * €150/hour (fixed cost) = €12,000/year
- Maintenance Savings (Spare Parts/Hours): €1,500/year
- Total Annual Savings: €14,100
- Payback Period: €15,000 / €14,100 ≈ 1.06 years
The system pays for itself in just over a year, fully justifying the investment against the "old system still works" objection.
5. Implementation Roadmap
To minimize production disruption, a phased approach is adopted:
- Planning and Audit: Complete survey of the existing wiring and drafting of the updated electrical diagram (CEI EN 60204-1 standard).
- Procurement: Selection of reliable components (PLC, I/O, HYDAC sensors).
- Pre-wiring (Discontinued): Assembly of the new PLC panel in parallel, where possible.
- Installation: Quick replacement during a scheduled downtime (e.g. holidays, weekends).
- Commissioning: Signal testing, logic validation and commissioning.
6. Technical Challenges
The main critical issues lie in the signal transition. Older wiring may exhibit leakage or electromagnetic interference (EMI). It is essential to use shielded cables for analog signals (e.g. for the HYDAC sensor) and separate the power lines from the signal lines. The translation of the logic from relay to PLC must be documented and validated, not limiting itself to a mere copy, but taking advantage of the new diagnostic capabilities of the PLC.
7. Case Study
A customer in the turning industry replaced a relay system on an aging hydraulic press. Before: MTBF of 1,200 hours, high maintenance costs, numerous machine downtime. After: PLC integration with HYDAC 42/98 0016 075 transducers, MTBF extended to 8,500 hours, 18% reduction in energy consumption, centralized diagnostics that reduce troubleshooting time from hours to minutes.
8. Commissioning and Validation
Final validation is critical. Must include:
- FAT test (Factory Acceptance Test): Logic test on the bench.
- SAT test (Site Acceptance Test): Complete verification in the field, including safety systems (e.g. light curtains, emergency stop according to EN ISO 13850).
- Documentation: Complete update of the machine's technical file (Machinery Directive 2006/42/EC).
9. Conclusions
The transition from relay logic to PLC control is a necessary step to maintain efficiency and regulatory compliance in the modern industrial context. The reduction of downtime and the optimization of consumption guarantee a rapid economic return and a tangible improvement in production quality. For your retrofit needs and the supply of modern components such as HYDAC sensors, visit the UNITEC-D E-Catalog.
10. References
- CEI EN 60204-1: Machinery safety - Electrical equipment of machines.
- EN ISO 13849-1/2: Machinery safety - Safety-related parts of control systems.
- Directive 2006/42/EC (Machinery Directive).
- HYDAC component migration technical manuals.
