Crane Automatic Sequence Control Startup to Hoisting
📋 Key Summary
For automated hoisting to run flawlessly from start to finish, sequence control is what keeps every step in order: each operation executes in a fixed sequence, and the next step only begins once the previous one is complete and its position signal is received. This article explains the state-machine logic behind sequence control, position signals, fault interruption, and how it takes a lifting operation from a single press of "start" to fully automated completion — no steps skipped, no steps out of order.
📌 Core Logic
Previous step complete → position signal received → next step starts. Each link is tightly coupled.
At its core, sequence control is a strict serial process: one step must finish before the next one is allowed to begin.
One-click start sounds simple, but behind it lies a chain of steps that must execute in a precise order: the hoisting mechanism moves first, then the crane bridge travels once hoisting is complete, then lowering begins, and only after the load is safely set down does load detachment occur. Skip a step or start one before the previous is done, and the entire lifting operation falls apart.
Sequence control is what governs this order: the next step is only permitted once the previous one is complete. Here's how it ensures no step is ever missed or out of sequence.
What Sequence Control Really Means: One Step at a Time
At its core, sequence control is a strict serial process: one step must finish before the next one is allowed to begin.
A lifting operation is a series of dependent steps: grabbing, hoisting, transporting, lowering, placing, and load detachment. The completion of each step is the precondition for starting the next.
Sequence control manages this flow using a state machine: each step is a state, and transitions between states occur based on defined conditions. When the current state completes and its conditions are met, the system moves to the next state; if conditions are not satisfied, it stays in the current state and waits.
This state-transition mechanism guarantees that steps are never skipped, never reordered, and never missed. ISO 24617, Intelligent Control Systems for Cranes, provides a framework for sequence control.
Three Pillars of Sequence Control: Condition Checks, Position Signals, Fault Interruption
For sequence control to execute every step without fail, it relies on three key elements.
Condition checks define the criteria for each state transition. Has hoisting reached its target position? Is the load stable? Is the path clear? Only when these conditions are met does the system advance to the next step. If not, it holds and waits.
Position signals provide feedback that a step has been completed. A signal is generated when grabbing is done, another when hoisting reaches its limit — these signals are what condition checks are based on. Without position signals, sequence control is effectively blind.
Fault interruption is the safety net when something goes wrong. If any anomaly occurs at any step — overload, collision, sensor failure — the sequence is immediately halted, an alarm is raised, and the error is prevented from propagating further. These three elements work together so that sequence control is both accurate and stable. Kelude identifies condition checks, position signals, and fault interruption as the three pillars of sequence control.
Implementing Sequence Control: State Machine Plus Position Signals
Sequence control is implemented through a state machine combined with position signals. GB/T 28264-2017, Safety Monitoring and Management System for Lifting Appliances, requires traceability of sequence operations.
Step one: break down the process and define states. Decompose the lifting operation into clear steps, assign each step a state, and define the transition order between states.
Step two: assign position signals. Each step gets a position signal — position reached, load stable, action complete — which serves as the condition for transitioning to the next step.
Step three: add fault interruption. Each state is linked to anomaly detection; overload, collision, or failure triggers an immediate halt. Kelude builds sequence control using this three-step approach — defining states, assigning signals, and adding interruption — to ensure lifting operations run without a single missed step.
Most Common Mistakes in Implementing Sequence Control
Mistake one: no position signals between steps. The next step starts without confirmation that the previous one is complete, leading to collisions and unintended movements. Position signals are the lifeline of sequence control.
Mistake two: sequence without interruption. If the process goes wrong and there's no fault protection, the error runs all the way through. Fault interruption must always be in place.
Mistake three: incomplete transition conditions. If a condition check misses a factor — for example, checking position but not load — the system may transition with hidden risks. Kelude ensures condition checks are comprehensive, verifying position, load, and safety factors alike.
Sequence Control: Key Mechanisms at a Glance
| mechanism | function | failure mode | implementation | priority |
|---|---|---|---|---|
| state machine | step transition control | step sequencing error | PLCstate machine | high |
| position signal | step completion feedback | inadvertent start collision | Sensorsignal | high |
| abnormal interruption | fault shutdown | error propagation | interlockabnormal interruption | high |
Quick Reference of Standard Clauses for Sequence Control
| Standard | clause essentials | andsequence controlrelationship with |
|---|---|---|
| ISO 24617 | intelligent control system for cranes | sequence controlframework |
| GB/T 28264 Safety Monitoring and Management System | safety monitoringtraceabilityrequirements | sequential operation traceability |
| TSG (Special Equipment Safety Technical Regulation) 51 Safety Technical Specification for Special Equipment-2023 Crane Safety Technical Supervision Regulation | safety interlockmonitoringrequirements | abnormal interruptioninterlock |
FAQ: Automatic Sequence Control for Overhead Cranes
Q: What is the difference between sequence control and manual operation?
A: Manual operation relies on the operator to execute each step, with the sequence and pace determined by human judgment. Sequence control locks the workflow into a state machine—each step completes, a position signal is received, and only then does the next step proceed automatically. The key difference is that the system guarantees the order instead of relying on the operator, eliminating skipped steps, missed operations, and sequencing errors.
Q: What standards govern sequence control?
A: The sequence control framework follows ISO 24617, operational traceability complies with GB/T 28264 Safety Monitoring and Management System, and abnormal interruption interlocks are governed by TSG 51-2023 Crane Safety Technical Supervision Regulation. These standards define the control framework, state recording, and fault-interlock requirements. At the implementation level, a state machine with position feedback forms the foundation, while abnormal interruption handling serves as the safety baseline.
Q: Where should I start troubleshooting when sequence control stalls at a step?
A: Start with the position signals—if the previous step completed but no signal was sent, the condition check fails and the sequence halts. Next, verify the condition logic: a missing or incorrectly set transition condition will also cause a stall. Finally, inspect the state machine for errors in the transition logic. The recommended troubleshooting order is: position signals → condition logic → state machine.
Q: Why is abnormal interruption handling essential in sequence control?
A: Because a process error must never be allowed to propagate unchecked. Sequence control follows a fixed workflow—if an anomaly occurs at any stage (overload, collision, sensor failure) and the process continues, the fault cascades into subsequent steps and can cause an accident. Abnormal interruption handling stops the equipment and triggers an alarm immediately upon fault detection, containing the error at the current state and preventing it from advancing. This is the safety baseline of sequence control.
Sequence control runs on the PLC and can be referenced against the control architecture described in Overhead Crane PLC Control System Three-Layer Architecture: S7-1500 + PROFINET + G120.
Sequence control takes hoisting from a single command to fully automatic completion—every step accounted for. Kelude Heavy Industry uses a state machine to manage the workflow, position signals for closed-loop feedback, and abnormal interruption handling as the safety net, ensuring steps are never skipped, missed, or executed out of order.
