Hoist: brake sequencing, overload and anti-sway

Objectives

1. Program the brake-release and brake-apply handshake of a hoist drive (Chapter 4).
2. Implement a fail-safe upper limit switch and the graded overload response of a crane.
3. Compute the anti-sway timing $T/2 = \pi\sqrt{L/g}$ in the PLC and generate the two-step speed command of the zero-vibration shaper.

Background

Chapter 4 insists that the holding brake is a holding device: it is released only once the drive has proved it can carry the load, and applied only once the motion has stopped. The chapter also describes a graded overload response (warning, hoisting inhibited while lowering stays possible, then trip) and the two-pulse input shaping that cancels load sway, with $T = 2\pi\sqrt{L/g}$ (worked example: $L = 9$ m gives $T/2 \approx 3.01$ s).

The stepper plays the hoist in Parts A and B and the trolley in Part C. The load and the rope length, which the trainer cannot provide, are read from the CPU's two analogue adjustment potentiometers under the front cover: SMB28 and SMB29, each 0 to 255.

Bench set-up

With the trainer OFF: Q0.0 → STEP MOTOR DIR, Q0.1 → STEP MOTOR CK, ENCODER A → I0.0, ENCODER B → I0.1; micro-switch COM → 24 V+ and NC → I1.0; buzzer BZ+ → RQ0.7, BZ− → 24 V−, common RC6 → 24 V+. The DC motor is not wired: the micro-switch is used on its own, and you press its lever by hand to simulate the hook block reaching the upper limit.

Preparation

1. Draw the GRAFCET of the hoist motion with the steps braked, magnetising, brake lifting, moving, stopping and brake applying. The delays are 0.3 s to prove the drive, 0.2 s for the brake to lift, and 0.3 s after the brake is applied before the drive is disabled.
2. Write the scaling expressions from SMB28 to load in percent and from SMB29 to rope length in metres.
3. For $L$ = 4 m, 9 m and 12 m, compute $T/2$. For a trolley command of 100 Hz (full speed) preceded and followed by a 50 Hz segment lasting $T/2$, compute the number of pulses $n_1$ of each half-speed segment and check that $2n_1 \lt 480$.

MOVB   1, VB500
MOVW   +20000, VW501
MOVW   +0, VW503
MOVD   +96, VD505

Part A: brake handshake and limits

1. Program the GRAFCET of your preparation. HOIST or LOWER held starts the sequence; releasing it decelerates the motor (a decreasing-frequency PTO segment), and the brake is applied only when the pulse train has finished (PTO idle bit SM76.7).
2. The upper limit stops hoisting but must still allow lowering.
3. ESTOP_NC opening removes DRIVE_ENABLE and applies the brake at once, whatever the step.

Part B: graded overload

1. Above 90 % of rated load, light WARN_90. Above 100 %, block hoisting but allow lowering (HOIST_INHIBIT). Above 110 %, stop, apply the brake, sound the buzzer and latch TRIP until RESET with the load back below 100 %.
2. Read the load cell only when the hoist is stationary, and hold the last value while moving (a real load cell reads high during acceleration).

Part C: anti-sway command

1. On TRAVEL, move the trolley (the stepper) by 480 pulses. Without shaping, use a single segment at 100 Hz.
2. With SHAPER_ON, the program computes the rope length from SMB29, then $T/2 = \pi\sqrt{L/g}$ in real arithmetic, then the pulse count $n_1$ of a 50 Hz segment lasting $T/2$. Build a three-segment profile: $n_1$ pulses at 50 Hz, $480 - 2n_1$ at 100 Hz, and $n_1$ at 50 Hz. The stop must be shaped as well as the start.
3. Measure the duration of the first segment with a timer and compare it with your preparation.

Rehearse on the virtual bench

Test your program before the session: paste or open your exported .awl, choose the wiring of this TP, and run the procedure and the fault-injection tests on the simulated trainer.

Fault-injection tests

Questions

1. Why must the brake never be released before the drive is proved, and never be used to stop a moving hoist in normal operation?
2. Explain why the upper limit switch is wired on its NC contact. What would a wiring fault do with the NO contact?
3. Why is lowering allowed above 100 % of rated load when hoisting is not?
4. Your shaper is tuned from the rope length set on SMB29. Using the anti-sway simulation of Chapter 4, explain what happens if the operator changes the hoist height after the travel has started.