Lab 18: Two-Hand Anti-Tie-Down Control

Application Scenario

A pneumatic punch press in a metal shop brings its ram down with enough force to cut steel. The most common press injury is a hand in the point of operation when the ram comes down. A classic safeguard is two-hand control: the operator must press two buttons, spaced far apart, with both hands to make the ram move. As long as both hands are on the buttons, neither hand can be under the ram.

Operators under pressure sometimes try to defeat two-hand control by tying down one button (with tape, a wedge, or a zip tie) so they can run the press with one hand. A proper two-hand control circuit therefore has three extra features:

These requirements come from machine-safety standards for two-hand controls (for example, ISO 13851 and ANSI B11.19). In this lab:

Learning Outcomes

After this lab you will be able to:

• Build a two-hand control circuit with concurrency, maintained actuation, and anti-repeat.
• Use relays to "copy" a push button when its single contact is needed in more than one place.
• Use an interval timer as a short window that closes on its own.
• Explain how each part of the circuit defeats a specific way of cheating the safeguard.

Background

Why button relays? Each push button on the trainer has only one contact. The circuit needs each button in two places: in the "both pressed" (AND) logic and in the "either pressed" (OR) logic that starts the window. So each button drives its own relay: PB1 → CR1 and PB3 → CR2. The relays then supply as many contacts as the logic needs.

The concurrency window. The window timer TR3 is an interval timer. Its coil is energized when either button relay is ON (CR1B and CR2B in parallel). Its N.O. contact TR3A closes immediately and opens again after 0.5 s. The press can start only while TR3A is closed. Because TR3's coil stays energized as long as any button is held, the window cannot reopen until both buttons are released — this gives anti-tie-down and anti-repeat with the same timer.

The press output. TR4, set to interval mode, acts as the press output relay: its contacts close at once when its coil is energized. Its coil is fed through CR1A and CR2A (both hands) and the window TR3A. A seal-in contact TR4B bypasses the window only — not the hand contacts — so the stroke continues after the window closes but stops the instant either hand is released. As a bonus, TR4's interval time (5 s) limits the stroke: if the buttons are held longer, the output drops, like the stroke-time monitor on a real press.

Timing Diagram

• Good cycle: PB3 is pressed 0.3 s after PB1, inside the window. The press starts, and continues after the window closes. Releasing PB3 stops it immediately.
• Tie-down attempt: PB1 is held down continuously. The window opens and closes 0.5 s later. PB3, pressed afterward, is too late: no stroke. As long as PB1 stays down, the window can never reopen.

Functional Requirements

1. G1 (PRESS) turns ON only if PB1 and PB3 are pressed within 0.5 s of each other.
2. G1 stays ON while both are held, and turns OFF immediately when either is released.
3. If one button is held more than 0.5 s before the other is pressed, G1 does not turn ON.
4. After a stroke, the next stroke requires both buttons to be released first.
5. G1 turns OFF after at most 5 s, even if both buttons are still held.
6. R1 lights for 0.5 s each time the window opens.

Ladder Diagram — Step-by-Step Drawing Guide

This design uses 8 grid lines.

Step 1 — Rungs 1–2: button relays

1. Grid line 1: L1 → PB1 (N.O.) → N2 → CR1 coil (2–7).
2. Grid line 2: L1 → PB3 (N.O.) → N3 → CR2 coil (14–13).

Step 2 — Rungs 3–4: window timer (either button)

1. Grid line 3: L1 → CR1B N.O. (8–6) → N4 → TR3 coil (2–7).
2. Grid line 4: CR2B N.O. (10–6) from L1 to N4, in parallel with CR1B.

Step 3 — Rungs 5–6: press output (both buttons)

1. Grid line 5: L1 → CR1A N.O. (1–3) → N5 → CR2A N.O. (9–5) → N6 → TR3A N.O. (1–3, window) → N7 → TR4 coil (2–10).
2. Grid line 6: TR4B N.O. (11–9) seal-in from N6 to N7, bypassing only TR3A.

Step 4 — Rungs 7–8: lamps

1. Grid line 7: from N7, G1 to L2 (in parallel with the TR4 coil).
2. Grid line 8: L1 → TR3B N.O. (8–6) → N9 → R1 to L2.

Step 5 — Trace your drawing with a pencil

1. PB1 is pressed, then PB3 0.2 s later. Trace the path to the TR4 coil. Then trace it again at t = 2 s (window closed).
2. While the press is running, PB1 is released. Which contact opens? Is there any other path to the TR4 coil?
3. PB1 has been taped down. PB3 is pressed 3 s later. Which contact blocks the stroke?
4. After a good stroke, PB3 is released and pressed again while PB1 is still held. Why is there no second stroke?
5. What would happen if the seal-in TR4B were drawn from L1 to N7?

Step 6 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

List every terminal on each node (with device names), chain them, and fill in the wiring table. The N7 row is done for you.

Get instructor approval before wiring.

Key Wiring Points

1. AND for the stroke, OR for the window. CR1A and CR2A in series feed the output; CR1B and CR2B in parallel feed the window timer.
2. The window timer resets only when both buttons are released, because its coil is fed by either button. That single fact provides anti-tie-down and anti-repeat.
3. The seal-in bypasses the window only, never the hand contacts.
4. TR4 in interval mode is an output relay with a time limit.

Wiring Procedure

1. Power OFF. Set TR3 to interval 0.5 s; set TR4 to interval mode, 5 s.
2. Wire the circuit from your approved wiring table, one node at a time.
3. Meter check, power still OFF:

   Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.

   • L1 to CR1:2 should read high, and near 0 Ω while PB1 is pressed.
   • L1 to CR2:14 should read high, and near 0 Ω while PB3 is pressed.
   • CR1:6 to CR2:6 and CR1:6 to TR3:2 should read near 0 Ω (N4).
   • CR2:5 to TR3:1 and CR2:5 to TR4:11 should read near 0 Ω (N6).
   • TR3:3 to TR4:2 should read near 0 Ω (N7).
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below. Work with your lab partner: one of you presses the buttons, the other watches the lamps.

Test & Acceptance

Troubleshooting

Review Questions

1. For each of the three rules (concurrency, maintained actuation, anti-repeat), name the device or contact in your circuit that enforces it.
2. Why does the window timer's coil use OR (parallel) contacts, while the output uses AND (series) contacts?
3. Why must the two buttons on a real press be spaced far apart and shielded (shrouded)? What does the circuit alone not prevent?
4. A two-hand control protects only the operator who is pressing the buttons. Name a situation in which another person could still be injured, and a safeguard that would help.
5. Real two-hand control buttons each have an N.O. and an N.C. contact, and certified two-hand control relays monitor both. What failure could an N.C. contact help to detect?

Applying This Circuit Pattern

The window pattern: an interval timer started by the first of several events defines a short time in which the other events must happen. Combined with an AND output and a seal-in that bypasses only the window, it enforces "together, and on purpose."

Design question for the future: In Lab 17 you built an E-stop with a switched bus L3. How would you add the E-stop to this press so that it removes the output even while both buttons are held?

Extension

Change TR3 to 2 s and repeat Test 4 with a 1-second delay between buttons. Then set it back to 0.5 s. In a few sentences, explain why the standard window is as short as 0.5 s.