Today, let’s talk about the automation control of sand and gravel production lines. I’ve been working on this for over a decade, and I’ve disassembled and assembled at least eighty production lines. Although the sand and gravel line looks simple, there are many nuances to fully automate it! Using the Siemens S7-1500 PLC (Programmable Logic Controller) for control is truly effective. I remember last year at a mine in Xuzhou, we achieved full automation from feeding to screening using the S7-1500, which directly improved efficiency by 40% compared to manual operations! Now, let’s get straight to the essentials.
△ Wiring Knowledge Points for PLC and Field Devices
To be honest, no matter how advanced the program is, if the wiring is incorrect, it’s pointless. The DI (Digital Input, which is a switch signal) module of the S7-1500 typically operates at 24V DC, while the DO (Digital Output) can be either 24V DC or relay output. Common devices on the sand and gravel line, such as vibrating feeders and belt conveyors, usually use a 24V control signal to activate a relay, which then controls the high voltage.
|--| |--|/|-------------( )--|
| | | | | |
| Switch Stop Contactor |
This program is essentially doing the most basic start-stop control, where pressing the switch keeps it on, and pressing stop disconnects it.
Warning ⚠️: Be very careful not to mix AC (Alternating Current) and DC (Direct Current) during wiring! One of my apprentices burned out an AI (Analog Input) module because of this mistake, and it was quite painful!
Field Tip: Take a photo before wiring! I habitually take a picture of the original wiring, so if I make a mistake, I can refer back to it. Also, make sure the terminal connections are tight; the sand and gravel line operates in a highly vibrating environment, and loose connections can cause inexplicable faults.
△ Logical Control of Sand and Gravel Production Line
The most critical aspect of the sand and gravel line is the interlocking control and protection sequence. When starting, you must go from back to front (take note!), and when stopping, from front to back. For example, the screening machine must start first, followed by the conveyor belt, and finally the feeder. Otherwise, the materials will get jammed!
The Siemens S7-1500 is programmed using TIA Portal (Totally Integrated Automation Portal), creating an FB (Function Block) to handle the interlocking logic:
|--| |--|/|------------|/|---( )--|
| | | | | | | |
| Start Stop Downstream Device Device 1|
|--| |--|/|------| |----( )--|
| | | | | | | |
| Start Stop Device 1 Device 2 |
This program is essentially performing device interlocking control, ensuring that upstream devices can only start when downstream devices are running, thus maintaining the safety sequence of the production line.
Field Tip: Create an HMI (Human-Machine Interface) monitoring screen in TIA Portal to display the operating status of each device; this makes issues immediately apparent. I like to use a traffic light system for clarity.
If the screening machine isn’t vibrating, it’s likely due to the motor’s thermal protection tripping or a break in the control circuit. First, check if the MCP (Motor Control Protector) has tripped, and then measure whether the 24V control circuit is normal.
△ Practical Project: Automatic Control of Feeder Flow Rate
Let’s practice with a simple example: automatically adjusting the vibration intensity of the feeder based on feedback from the belt scale to maintain a stable material flow rate.
Hardware Preparation:
- Siemens S7-1500 CPU
- One AI module (to connect to the belt scale’s 4-20mA signal)
- One AO module (to output 4-20mA to the inverter)
- Power supply, belt scale, inverter, feeder
The program is structured as follows:
// SCL language is more suitable
"Belt Scale Flow" := NORM_X("AI_Input", 27648.0, 0.0, 100.0, 0.0); // Normalization
"Deviation" := "Set Flow" - "Belt Scale Flow";
"Output Adjustment" := "Current Output" + "Deviation" * 0.5; // Simplified version of the proportional part in PID
"AO_Output" := SCALE_X("Output Adjustment", 100.0, 0.0, 27648.0, 0.0); // Proportional conversion
This program is essentially performing closed-loop control, collecting the flow rate from the belt scale, comparing it with the set value, and adjusting the vibration intensity of the feeder to achieve automatic control of the material flow rate.
Warning ⚠️: Make sure to set the inverter parameters correctly! Especially the acceleration and deceleration times; the sand and gravel feeder cannot be too fast, as it may overload and cause blockages. I recommend setting the acceleration time to at least 15 seconds.
Field Tip: In case of sudden stops, make sure to empty the material in the hopper before restarting. Mitsubishi PLCs are not as intuitive as Siemens; Siemens allows for online program modifications without affecting other logic operations, which is a real convenience during debugging!
Let me tell you, back when I was at a stone factory in Anhui, this flow control system increased production by 30%, and the boss gave me a raise! The key is its stability, unlike manual control which can fluctuate wildly.
Small Exercise: Try adding a delayed start, so the feeder starts 10 seconds after the belt starts, to prevent the belt from running empty or overloading.
As you can see, while the control of the sand and gravel line may seem complex, it really boils down to these few techniques.