Click the blue text to follow!
Yesterday, a friend working on a packaging line complained that their control system frequently malfunctioned—sometimes the conveyor belt would suddenly stop, and sometimes the packaging would not execute in order. I asked him, “Have you used state machine programming?” He looked puzzled. Many beginners overlook this powerful tool, so today I will unveil its mysteries.
I remember when I first started, the control programs I wrote were like a bowl of mixed stew—if statements nested seven or eight layers deep, variables flying everywhere, and modifying them later was a nightmare. It wasn’t until I learned about state machine programming that my code became clear and organized, making debugging and maintenance much easier.
1
What is a State Machine?
A state machine is essentially a model of a system transitioning between different states. Imagine an automatic washing machine: it has states like soaking, main wash, rinsing, and spinning, each performing different tasks and transitioning under specific conditions—this is a typical state machine!
In industrial control systems, the core of a state machine includes three elements: states, events, and transitions. For example, a packaging machine may have states like “idle”, “feeding”, “packaging”, and “discharging”, while button presses and sensor triggers are events that initiate transitions between states.
2
Methods for Implementing State Machines
On microcontrollers, state machines are typically implemented using a switch-case structure. Here’s a simplified control code:
switch(currentState) {case IDLE: / Idle state logic /case FEEDING: / Feeding state logic /}
In PLCs, we can implement state machines using state registers in ladder diagrams. For example, we can use D0 to store the current state and then write corresponding logic blocks for different state values. Each state segment is responsible for its own operations and the judgment of the next state.
I once guided a beverage bottling line project where the previous program used a large amount of chained logic, making it cumbersome to maintain. After switching to a state machine, the program was divided into clear states of “position detection → filling → capping → labeling”, executing only the logic of the current state, making error handling simple and intuitive.
3
Practical Tips for State Machines
Clearly define states—each state should have clear entry and exit conditions. For example, the entry for the “filling state” is “bottle in position and system normal”, and the exit is “liquid level meets standards or timeout”. Vague state definitions are breeding grounds for errors!
Creating a state transition table is crucial! I usually start by drawing all states and transition conditions, forming a “roadmap”, before I begin coding. This helps avoid missing states or transition conditions.
For complex systems, consider using hierarchical state machines—where the main state machine manages large operational modes (like “automatic”, “manual”, “maintenance”), and each mode has sub-state machines handling specific processes. This structure keeps the program clear and flexible.
4
Common Issues and Solutions
A common issue for beginners is “state lock”—the system enters a certain state and cannot proceed. The usual cause is forgetting exception handling. The solution is to set a timeout mechanism for each state and implement a global reset function.
Another trap is “race conditions”—when multiple transition conditions are satisfied simultaneously, the system behavior becomes uncertain. You should clearly define priorities or use mutex conditions to avoid this situation.
I once encountered a strange case where a stirring system kept restarting; after checking, I found it was due to sensor jitter causing frequent state switching. The solution was to add a simple “debounce” delay to ensure the signal was stable before switching states.
5
Practical Case: Simple Sorting System
Imagine a simple object sorting system with three stations: detection, classification, and conveying. Using a state machine, we can define:
WAIT_ITEM (waiting for item) → DETECTING (detecting) → SORTING (sorting) → CONVEYING (conveying) → back to WAIT_ITEM
Each state only concerns its own task. For example, the DETECTING state is only responsible for reading sensor data and determining the type of item, and once completed, it stores the result in a shared variable and switches to the SORTING state. This modular design makes the program easy to understand and maintain.
State machines not only clarify control logic but also inherently support emergency stop functions—if an emergency stop signal is detected in any state, it immediately switches to the EMERGENCY state. In real industrial equipment, safety is always the top priority!
Next time you design a control system, don’t forget to consider the state machine approach. It will make your program more structured, reliable, and easier to explain to team members how the system works. May your automation journey become broader!
