Conveyor systems are the lifeblood of many industrial operations, from manufacturing and logistics to food processing and mining. Their primary function is to transport materials from one point to another, often in a complex, multi-stage process. While the mechanical components of a conveyor are straightforward, the control logic that orchestrates their movement is critical. Without a well-designed control system, a simple task like starting and stopping can lead to catastrophic consequences, including material pile-up, equipment damage, and serious safety hazards. This article delves into the fundamental principles of controlling conveyor ON and OFF sequences, highlighting the crucial role of Programmable Logic Controllers (PLCs) in ensuring a safe, reliable, and efficient operation.
The Basic Conveyor Control Circuit
At its most basic level, controlling a single conveyor seems simple. You need a way to turn it on and a way to turn it off. This is typically achieved with two pushbuttons: a normally open (NO) start button and a normally closed (NC) stop button. The core of this logic, in PLC programming, is a simple latching circuit, also known as a seal-in circuit.
The logic works as follows: when the start button is pressed, the PLC's input is energized. A rung of ladder logic in the PLC program then turns on a corresponding output, which in turn energizes the conveyor motor starter. What makes this a latching circuit is that once the motor is running, a contact from the PLC output is used in parallel with the start button. This "seals in" the circuit, allowing the operator to release the start button without the conveyor shutting down. The conveyor will continue to run until the stop button is pressed, which de-energizes the stop input and breaks the latch, or until an emergency stop is activated. This simple but robust logic is the foundation for all more complex conveyor systems.
The Challenge of Multiple Conveyors
The real complexity arises when you have multiple conveyors in a line, especially if they are feeding material to one another. Consider a system with three conveyors: Conveyor 1 feeds to Conveyor 2, which then feeds to Conveyor 3. In this scenario, simply turning on all conveyors at once would be disastrous. If Conveyor 1 starts before Conveyor 2, material will be dumped onto a stationary belt, causing an immediate pile-up and potential equipment damage. Similarly, stopping the entire system at once without a proper sequence can also lead to a mess.
To prevent these issues, a PLC must be programmed to handle both a sequential start-up and a sequential shut-down.
The Sequential ON Logic: The Importance of Upstream Control
The golden rule for starting a conveyor system is to always start from the last conveyor in the sequence and work your way backward. This ensures that when material is dropped from an upstream conveyor, there is a downstream conveyor already running to accept it.
Here’s how the PLC logic for a three-conveyor system would work:
An operator presses the master start button.
The PLC program checks to ensure all safety conditions are met.
The PLC immediately energizes the output for Conveyor 3 (the last conveyor in the line).
Once the PLC confirms that Conveyor 3 is running (this can be done via a dedicated feedback sensor or a timer), it then initiates a time delay. This timer, typically a TON (Timer ON-Delay), ensures that Conveyor 3 has reached its operational speed before the next conveyor starts.
After the timer for Conveyor 3 has elapsed, its timer's output bit becomes true. This bit is used as a condition to enable the start of Conveyor 2.
The same process repeats: Conveyor 2 starts, a timer begins, and once it is complete, its output bit is used to enable the start of Conveyor 1.
This cascading sequence ensures a smooth, non-destructive start-up. Timers are the most common method for implementing these delays, providing a simple and reliable way to sequence the start of each conveyor. More advanced systems might use proximity sensors or speed sensors on each belt to confirm it is running before allowing the next one to start, adding a layer of security.
The Sequential OFF Logic: Clearing the Line
The shutdown process is just as critical as the start-up, and the logic is reversed. To prevent material from piling up on a belt that has just stopped, you must stop the conveyors in the order of material flow—from the first conveyor to the last.
Here's the sequential shut-down process:
The operator presses the master stop button, or a limit switch is activated, indicating the end of a production run.
The PLC program immediately de-energizes the output for Conveyor 1 (the first conveyor in the line). Material flow from the source is now stopped.
A timer begins for Conveyor 2. This delay allows any remaining material on Conveyor 1 to be fully transferred onto Conveyor 2.
After the timer for Conveyor 2 has elapsed, the PLC de-energizes the output for Conveyor 2.
A final timer begins for Conveyor 3, allowing any remaining material on Conveyor 2 to be transferred off.
Once this final timer has elapsed, the PLC de-energizes the output for Conveyor 3, and the entire system is stopped.
This timed shutdown ensures that all material has been cleared from the system, leaving the conveyors empty and ready for the next cycle.
The Unconditional Stop: Prioritizing Safety
While sequential logic is vital for normal operations, it must be overridden in a critical safety event. A system should always have an unconditional stop feature, often tied to a hardwired emergency stop (E-stop) button, safety light curtains, or a motor overload relay.
In a PLC program, the E-stop button is connected to a safety input. This input is then programmed to bypass all sequential logic and immediately de-energize every single conveyor output in the system. The same principle applies to motor overload relays, which detect mechanical stress or jams. When a motor trips, a fault input to the PLC must be used to trigger an immediate, complete shutdown of all conveyors in the system. This prevents a single fault from cascading into a larger, more dangerous problem.