Calculating Light Curtain Distance for Siemens S7 Press Retrofit

Overview

A 10-year-old hydraulic press rated at 1250 kN is being migrated from electromechanical relay logic to a Siemens SIMATIC S7 PLC. The original safety chain combines a Sick LCUX safety light curtain controller, a Pilz PNOZ X2.1 e-stop/perimeter relay, and a Pilz PNK overrun (top-dead-centre) relay. The retrofit must satisfy two engineering questions that drive the design:

1. What is the minimum safe distance between the light curtain and the moving ram, and how is it verified against the actual stopping performance of the press?
2. How is the dual-contact (N/O + N/C) start button, originally interlocked by two interposing relays, replaced with PLC logic that still preserves single-stroke initiation without compromising operator safety?

This article works through both problems using the press data supplied by the original engineer, the ANSI/RIA 15.06 / EN ISO 13855 distance formula, and concrete Siemens S7-1200/1500F sample code. It also addresses a frequently misquoted standard (EN 962) and corrects it to EN 693 (hydraulic presses) or EN 692 (mechanical presses) as appropriate.

Press Specification (As-Found)

The machine is a hydraulic press, so the controlling European standard is EN 693:2001+A1:2009. If the machine is later reclassified as a mechanical press with flywheel and clutch, EN 692:2005+A1:2009 applies instead. The difference matters because EN 693 carries stricter requirements for hydraulic energy isolation and valve monitoring than EN 692 does for clutch/brake monitoring.

Standard Discrepancy: EN 962 Is Not a Press Standard

Light Curtain Safety Distance Formula

The minimum safety distance D_S from the light curtain sensing field to the nearest pinch point is given by:

D_S = K * (T_S + T_C + T_R) + D_PF

Where:
  D_S   = minimum safety distance (mm or inches, match units to K)
  K     = operator approach speed
           EN ISO 13855 (standing reach over field):  K = 1600 mm/s
           ANSI/RIA R15.06 (upper limb / hand):       K = 1600 mm/s (63 in/s)
  T_S   = stopping time of the hazardous motion (s), worst case measured
  T_C   = worst-case stopping time of the safety control system (s)
  T_R   = response time of the safeguarding device including interface (s)
  D_PF  = depth penetration factor (mm or inches), depends on optical resolution

Depth penetration factor values per EN ISO 13855 Table 1 (rectangular approach):

Worked Distance Calculation for the 1250 kN Press

Step 1 — Total measured stopping time. The dominant hazard is the rapid-advance stroke at 180 mm/s. Stop the press at maximum working pressure (260 bar) with the heaviest die fitted and measure with a calibrated measurement device (e.g. Pilz PMC, Sick nanoScan3 with timing output, or a calibrated linear encoder plus oscilloscope on the safety contactor). For a 1250 kN hydraulic press at 180 mm/s the expected value is roughly:

• Hydraulic valve shutoff propagation: 20–40 ms (proportional valve plus pilot stage)
• Ram deceleration under load (260 bar relief on accumulator): 80–180 ms
• Brake/auxiliary clamp: 0–50 ms (if fitted)
• Safety contactor dropout (3RT20 series or Sirius 3RT): 15–30 ms

Assume worst-case measured value T_S = 250 ms until you measure your own machine.

Step 2 — Safety control system response time. The PLC F-runtime itself contributes a small, deterministic additional delay:

• Siemens S7-1500F F-cycle: typically 2–10 ms depending on PROFIsafe configuration
• S7-1200F F-cycle: typically 5–20 ms

Take T_C = 20 ms as a conservative design value pending the actual configured PROFIsafe monitoring time.

Step 3 — Light curtain response time. The Sick LCUX controller plus the C4000/C4000 Fusion safety light curtain has a typical response time of 12 to 25 ms depending on resolution and protective field height. The Sick datasheet for the installed model must be consulted; assume T_R = 25 ms.

Step 4 — D_PF selection. If the LCUX is paired with a 30 mm resolution curtain (typical for hand/limb detection at press workstations), D_PF = 90 mm.

Step 5 — Compute.

T_sum = T_S + T_C + T_R
      = 0.250 + 0.020 + 0.025
      = 0.295 s

D_S = 1600 mm/s * 0.295 s + 90 mm
    = 472 mm + 90 mm
    = 562 mm

=> Minimum distance from light curtain sensing field to pinch point = 562 mm

Round up to the next achievable mounting position (e.g. 600 mm) and physically fix the curtain bracket so that it cannot be repositioned closer without tooling. The original engineer’s observation that the curtain “looks too close” should be quantified: physically measure the current installed distance and compare against 562 mm. If it is below 562 mm, reposition or add a fixed perimeter guard to compensate.

Stop Time Measurement Procedure

The calculated distance is only valid if T_S is measured, not assumed. The procedure below is consistent with EN ISO 13855 §6 and is required by notified bodies during CE assessment of refurbished presses.

1. Fit the heaviest production die, set the working pressure to 260 bar, set the rapid advance to 180 mm/s.
2. Install a calibrated position transducer (e.g. Sick W10 laser, Balluff BTL linear encoder, or a string pot) on the ram, signal captured on a Yokogawa DL850 or Hioki MR6000 scope at ≥ 1 kHz sample rate.
3. Trip the press using the light curtain (simulate a breach by inserting a test rod) and capture position vs. time.
4. Repeat ten cycles at full pressure and full speed. Take the maximum measured value as T_S.
5. Verify that the value is stable across at least one full die change and at ambient minimum/maximum temperature if the press is in an unheated workshop.
6. Document the measured T_S, the calibration certificate of the measurement device, and the test conditions in the technical file.

If the measured T_S is more than 10 % above the design value (250 ms), recalculate D_S and either reposition the curtain, add a hard guard, or fit a faster-acting valve/clamp to bring T_S down.

Target Hardware Architecture

The PLC choice governs how the safety chain is implemented. For a retrofit of a 10-year-old hydraulic press, three architectures are viable:

For most retrofit jobs with an existing Pilz PNOZ chain, Option A is the most economical: keep the PNOZ X2.1 and PNK as the safety stop elements, replace the relay-logic motion sequencer with the S7-1200F, and bring the PNOZ outputs into a fail-safe input card that the PLC monitors in addition to the hard-wired stop path. This preserves the proven e-stop and overrun functions while adding the diagnostic and edge-detection benefits of a PLC.

Light Curtain Wiring to S7-1200F

The Sick LCUX controller provides two OSSD outputs. Wire them into a fail-safe input module so the PLC can both monitor and act on the field:

Terminal layout (SM 1226 F-DI, channel group 0):

  +24V  --[F-DI 0]-- OSSD1 (LCUX pin 1)
  +24V  --[F-DI 1]-- OSSD2 (LCUX pin 2)
  0V    --[F-DI 2]-- EDM feedback (LCUX pin 5, normally closed aux contact)

  LCUX pin 4 (0V) --- 0V of F-DI group

  EDM must pulse low when the outputs drop out, otherwise the
  F-I/O will raise a discrepancy fault after the configured
  discrepancy time (default 100 ms).

In TIA Portal, configure the F-DI channel group with sensor evaluation 1oo2 (two-channel equivalent), discrepancy time = 100 ms, and short-circuit monitoring enabled. Assign the channel to the LC_Field_Breached safety tag, used both in the safety program (immediate stop) and in the standard program (HMI alarm).

Start Button Logic — Two Implementation Options

The original circuit used a mechanical N/O + N/C pair and two interposing relays, so that a stuck contact or welded relay would not be able to start the press silently. Two PLC replacements are possible:

Option 1 — Single contact + rising edge + time supervision

This matches what the original engineer suggested. It detects a stuck closed contact but does not detect a stuck open contact (e.g. broken wire or failed contact).

// SCL for S7-1500, global DB "PressCtrl"
// Tags:
//   StartButton   : BOOL  (F-input mapped to standard BOOL)
//   StartPulse    : BOOL  (rising edge pulse)
//   StartTimer    : TON   (max 500 ms press-on window)
//   StartOK       : BOOL  (validated single-stroke request)
//   LastStartTime : DTL   (last accepted start, for anti-repeat)

#StartTimer(IN := #StartButton,
            PT := T#500ms);

IF #StartTimer.Q THEN
    // Button held beyond 500 ms is a fault
    "HMI_Alarm".StartButtonStuck := TRUE;
    "Safety_Stop".Latch := TRUE;
END_IF;

#StartPulse := #StartButton AND NOT "EdgeMem".StartMem;
"EdgeMem".StartMem := #StartButton;

// Anti-repeat: must release button between strokes
IF #StartPulse
   AND "HMI_Alarm".StartButtonStuck = FALSE
   AND NOT "Safety_Stop".Latch
   AND "Light_Curtain".FieldClear
   AND "Cycle_Complete".Flag
THEN
    #StartOK := TRUE;
    "Cycle_Complete".Flag := FALSE;
END_IF;

This pattern is acceptable for a non-safety part of the cycle (e.g. clamp/unclamp or feeder advance), but it does not meet EN 693 §5.5 single-stroke initiation requirements on its own because a single N/O contact provides no fault coverage for a stuck-open failure (the press would simply fail to start, which is acceptable, but a stuck-open detection only is not enough for category 3 or higher per EN ISO 13849-1).

Option 2 — Dual-channel discrepancy monitoring (preferred)

This preserves the original mechanical interlock philosophy: both contacts must transition within a defined window. If the N/C contact fails to open when the N/O closes (or vice versa), the press is inhibited.

// Two channels of the dual-contact start button, mapped to
// standard digital inputs (not F-inputs; the safety stop path
// is handled by the PNOZ X2.1 and PNK relays).

//   Start_NO : BOOL  -- the normally-open contact
//   Start_NC : BOOL  -- the normally-closed contact
//   Disc_OK  : BOOL  -- output, used to qualify StartOK

// Discrepancy window = 500 ms. Within that window both
// transitions must occur.

IF "Start_NO" AND NOT "EdgeMem".NO_Closed THEN
    "EdgeMem".NO_Closed := TRUE;
    "EdgeMem".NO_Time  := "SystemClock".Now;
END_IF;

IF NOT "Start_NC" AND "EdgeMem".NC_WasClosed THEN
    "EdgeMem".NC_Closed := TRUE;
    "EdgeMem".NC_Time   := "SystemClock".Now;
END_IF;

IF "EdgeMem".NO_Closed AND "EdgeMem".NC_Closed THEN
    "Disc_OK" := TRUE;
ELSIF (T_Diff := "SystemClock".Now - "EdgeMem".NO_Time) > T#500ms THEN
    "Disc_OK" := FALSE;
    "HMI_Alarm".StartContactMismatch := TRUE;
END_IF;

// Reset on button release
IF NOT "Start_NO" AND NOT "Start_NC" THEN
    "EdgeMem".NO_Closed := FALSE;
    "EdgeMem".NC_Closed := FALSE;
    "Disc_OK" := FALSE;
END_IF;

Combine this with the same time-window, anti-repeat, and light-curtain-clear checks from Option 1. The result is functionally equivalent to the original relay interlock and survives a single contact failure without causing an unsafe start.

Recommended Wiring for Dual Contact Monitoring

Use a Cat 3 or Cat 4 rated industrial pushbutton (e.g. Eaton M22-PV/K20 with N/O + N/C blocks) so the mechanical linkage between the contacts is positive. A standard low-cost pushbutton with separate N/O and N/C blocks is not acceptable for press single-stroke initiation because the two contacts are not mechanically linked.

Single-Stroke Initiation Logic (EN 693 §5.5)

EN 693 requires that a single-stroke press cycle can be initiated only when the following conditions are all true at the moment of initiation:

1. The operator’s hand is clear of the danger zone (light curtain clear).
2. The initiation device is actuated and then released before the next initiation (anti-repeat).
3. The ram is at top dead centre (PNK overrun relay confirms).
4. All safety devices (e-stop, perimeter, overrun) are reset.
5. The cycle is not already in progress.

The PLC must latch these conditions at the rising edge of the start pulse and drop the latch on light-curtain breach, e-stop, or end-of-stroke. The anti-repeat flag is critical: once a stroke has been initiated, the operator must release the button fully before the next stroke can be armed. A typical ladder rung for the arming bit is shown below.

[ StartNO ]--[ NOT StartNC ]--[ LightCurtainClear ]--[ CycleNotRunning ]
   |                                    |
[ Edge_StartOK ]                       [ PNK_TDC_Confirmed ]
   |                                    |
[ NOT EStopActive ]                  [ NOT CycleCompleteLatch ]
   |
( ) StrokeArmed              ; arms the down-valve via F-DO

If any of these conditions drops out, the arming bit must clear within one PLC scan. Use the F-DO channel of the S7-1500F/SM 1226F for the down-valve enable output; do not route the safety stop through a standard DO.

Performance Level (PL) Selection per EN ISO 13849-1

The original e-stop chain is built around a Pilz PNOZ X2.1, which is rated to PL e / Cat 4 in typical configurations. The PNK overrun relay is PL d / Cat 3. To preserve the original performance level during the PLC retrofit:

• Light curtain OSSD wiring must remain in a Cat 4 / PL e configuration (two-channel with cross-circuit monitoring).
• The down-valve enable must be a fail-safe output that drops out within the calculated stop time.
• The start button dual-contact logic must not be the only safety boundary — the light curtain and overrun relay must still independently drop the down valve.

Run a SISTEMA calculation on the whole safety function and file it in the technical file. EN ISO 13849-1:2023 is the current revision; previous files on the press may cite EN 954-1 categories, which are obsolete and must be re-evaluated to PL.

Verification Checklist Before Re-Energising

Document each row with the test date, instrument used, measured value, and the engineer’s signature. This document is the validation evidence required for CE re-marking under the Machinery Regulation (EU) 2023/1230, which replaced the Machinery Directive 2006/42/EC in January 2027.

Press Controller Alternatives

If the press will be CE re-marked by a notified body, consider a dedicated press safety controller rather than a general-purpose S7F. Pilz markets the PSS 4000 system with a validated press application block that implements EN 693 single-stroke, inch, and continuous modes without bespoke programming. Siemens offers the SIMATIC S7-1500F with the “Press Control” library in TIA Portal. Both reduce the validation effort dramatically compared with writing the press state machine from scratch.

Field-Proven Caveats

• The PNOZ X2.1 is an older relay with screw terminals. Its response time must be re-measured if it is more than 10 years old; relay contact wear and dust ingress on a shop press commonly degrades dropout time by 5–15 ms.
• The Sick LCUX controller has configurable EDM and reset behaviour. Verify that the EDM loop is wired; if left open, the F-DI on the S7 will latch a discrepancy fault and the press will not start, which is sometimes diagnosed incorrectly as a PLC fault.
• Hydraulic oil viscosity at cold-start can add 50–100 ms to stop time compared with operating temperature. If the press is in an unheated workshop, measure T_S at the minimum ambient temperature the press will see, not at workshop comfort temperature.
• The previous robot loader left electrical hardware in the cabinet (cable runs, unused I/O blocks, undocumented terminals). A clean rewire is faster and safer than trying to reuse unknown wiring. Tag and remove anything not in the new drawing set.
• For a 1250 kN press, the down-valve is typically a proportional or pilot-operated directional valve with a manual override. The PLC must not interfere with the manual override path; the override is reserved for setup and maintenance under hold-to-run control.

References and Tools

Standards to obtain the latest revisions of:

• EN ISO 13855:2024 — Positioning of safeguards with respect to the approach speeds of parts of the human body
• EN ISO 13849-1:2023 — Safety-related parts of control systems
• EN 693:2001+A1:2009 — Hydraulic presses safety
• EN ISO 13850:2015 — Emergency-stop devices

Manufacturer documentation:

• Pilz PNOZ X product family (incl. PNOZ X2.1 and PNK)
• Pilz PSS 4000 control system
• Sick LCUX safety light curtain controller
• Siemens SIMATIC S7-1500 system manual
• Siemens SIMATIC S7-1200 system manual

FAQ