PLC-Based Controller for a Precision Laser Driller

Simulating a digital replacement for a twenty-year-old analog control system, validated in ladder logic before touching the real machine.

Central Manufacturing Technology Institute, Bangalore · with Amrita Vishwa Vidyapeetham

Ladder Logic PLC Simulation Safety Interlocks Legacy System Migration

10Software: Zeliosoft2 v4.5 12PLC: 6 DI / 4 DO 14Status: Validated

Fig. 2 — Prior Art Limitations Sheet 02 of 06

01 — The Problem

Twenty years of analog control, running out of runway

The Raytheon precision laser driller/welder at CMTI Bangalore was built around analog controllers roughly two decades earlier. Sourcing components for maintenance and feature upgrades had become difficult — and the case for a digital controller kept getting stronger.

16Components scarce — analog parts from that generation were no longer readily available.

18Not expansible — adding capability meant redesigning around a fixed analog architecture.

20Bulky and expensive — with a complex design that made troubleshooting slow.

22Hard to diagnose — faults were difficult to trace through the analog signal chain.

A PLC/DSP-based digital controller offered a modular, user-friendly alternative — with no signal loss from A/D–D/A conversion, better noise immunity, and lower long-term cost.

Fig. 3 — Process Flow Diagram Sheet 03 of 06

02 — System Overview

From three-phase supply to a single laser pulse

10THREE-PHASE
SUPPLY

→

12HV
CONTACTOR

→

14RESERVOIR /
CAPACITOR BANK

→

16PFN
(PULSE-FORMING)

→

18FLASHLAMP

→

20LASER
ROD

The reservoir voltage is manually set and fed back to confirm it has built up to the target level before firing is armed. The Pulse Forming Network — seven identical inductor–capacitor loops — shapes that stored energy into a clean rectangular pulse for the flashlamp.

3 sReservoir charge time before FIRE is armed

7Identical inductor–capacitor loops in the PFN

7 msMaximum pulse width — 1 ms per loop, full bank

Fig. 4 — I/O Reference Table Sheet 04 of 06

03 — Simulation Approach

Every switch, sensor, and interlock, given an I/O tag

A PLC variant with six discrete inputs and four output relays was selected and programmed in ladder logic using Zeliosoft2 v4.5, mapping the analog system's switches, interlocks, and drive signals onto simulated I/O.

Table 1. Simulation I/O Point List
Inputs		Outputs
I1	HV switch	Q1	Charging SCR
I2	Trigger (FIRE) switch	Q2	Discharging SCR
I3–I9	Interlocks — overcurrent, overpower, overfrequency, cooling water low-flow / over-temp, water leak, misfire	Q3	PFN
IA	Emergency STOP	Q4	Laser rod
		Q5	Interlock activation indicator
		Q6	Emergency STOP indicator

Fig. 5 — Control Logic Diagram Sheet 05 of 06

04 — Validated Behavior

Timing the pulse to the microsecond

24I1 HV 26TON 3s 28I2 FIRE 30Q1

Simplified rung — HV, then a 3-second timer, then FIRE, before the charging SCR can energize.

An asynchronous timer staggered the charging and discharging SCRs by 500 microseconds in each direction, so the two could never conduct at once — and any interlock trip cut power ahead of the HV contactor before it reached the rest of the system.

OVERCURRENT → TRIP MISFIRE → TRIP E-STOP → ISOLATE NOMINAL → RUN

500 µsCharge/discharge SCR stagger

7Distinct interlocks reproduced in the ladder program

6 DI / 4 DOPLC variant used for the simulation

Fig. 6 — Summary of Results Sheet 06 of 06

05 — Conclusion

Digital control, validated before the retrofit

The ladder-logic simulation reproduced every function and safety interlock of the original analog system — confirming that a modern digital controller could take over the Raytheon laser driller/welder without losing any of its safety behavior.

Ladder Logic PLC Simulation Safety Interlocks Legacy System Migration

Feasibility
Confirmed