Siemens PLC Control Technical Solution for Juice Extraction Process

The automated juice production line can significantly improve production efficiency and product quality, with the key being precise control of various process parameters and state transitions.

1. Hardware Configuration

PLC and Expansion Module Selection Description

The core of the juice process control system uses the Siemens S7-1200 series CPU 1214C DC/DC/DC, which has 14 digital input points, 10 digital output points, and 2 analog input points. Considering the need to control multiple motors, valves, and detect various sensor signals during the juice extraction process, the following expansion modules are configured:

• SM 1231 8AI module: Used for collecting analog signals such as temperature, pressure, and liquid level
• SM 1222 16DO module: Expands digital output points to control more actuators
• CM 1241 RS485 communication module: Connects to inverters and other intelligent devices

I/O Point Allocation Table

| Address | Function Description | Device Type |

|——|———|———|

| I0.0 | Feed Sensor | Photoelectric Switch |

| I0.1 | Raw Material Position | Proximity Switch |

| I0.2 | Juicer Operation Feedback | Auxiliary Contact |

| I0.3 | Filtration System Feedback | Differential Pressure Switch |

| I1.0 | Filling Position Sensor | Photoelectric Switch |

| I1.1 | Emergency Stop Button | Safety Switch |

| IW64 | Temperature Sensor | PT100 |

| IW66 | Pressure Sensor | 4-20mA |

| Q0.0 | Feed Motor Control | Contactor |

| Q0.1 | Juicer Start | Contactor |

| Q0.2 | Juicing Speed Control | Inverter |

| Q0.3 | Filtration System Control | Solenoid Valve |

| Q0.4 | Filling Pump Control | Contactor |

| QW80 | Juicing Pressure Control | Analog Output |

System Wiring Key Points

1. All sensors are connected using shielded cables to reduce interference
2. Power lines and signal lines are wired separately to avoid electromagnetic interference
3. Inverter control lines use dedicated shielded cables with good grounding
4. Analog signal lines are routed separately, away from high-power devices
5. The emergency stop circuit uses independent hard wiring to ensure safety and reliability

2. Control Program Design

Variable Definition Specification

Variable naming follows the “function_device_property” format for better understanding and maintenance:

// Global variable example

"Juice_Feed_Start" : Bool    // Feed start signal

"Juice_Press_Speed" : Real   // Juicer speed setting

"Juice_Filter_Pressure" : Real // Filtration pressure value

"Juice_Temp_Actual" : Real   // Current temperature value

"Juice_Alarm_Filter" : Bool  // Filter fault alarm

Program Architecture Design

The program adopts a hierarchical structure, clearly dividing functional blocks:

• OB1: Main program loop, calling various function blocks
• OB100: Startup initialization program
• FB10: Feed control function block
• FB20: Juicing control function block
• FB30: Filtration control function block
• FB40: Filling control function block
• FB50: Temperature control function block
• FB60: Alarm handling function block
• DB10-DB60: Data blocks corresponding to each function block

Function Block Design Example

Taking the juicing control FB20 as an example:

FUNCTION_BLOCK "FB_Juice_Pressing"

{ S7_Optimized_Access := 'TRUE' }

VERSION : 0.1

   VAR_INPUT 

      Start : Bool;           // Start signal

      Stop : Bool;            // Stop signal

      FeedReady : Bool;       // Feed ready signal

      SetPressure : Real;     // Set pressure value

      SetSpeed : Real;        // Set speed value

   END_VAR


   VAR_OUTPUT 

      Running : Bool;         // Running status

      Completed : Bool;       // Completion signal

      ActualPressure : Real;  // Actual pressure

      PressureError : Bool;   // Pressure abnormality

   END_VAR


   VAR 

      State : Int;            // Internal state

      Timer1 : TON;           // Pressing time timer

      PressTime : Time := T#30S; // Pressing time setting

   END_VAR


BEGIN

   // State control logic

   CASE State OF

      0:  // Standby state

         IF Start AND FeedReady THEN

            State := 1;

            Running := TRUE;

            Completed := FALSE;

            // Start juicer

            "Q0.1" := TRUE;

         END_IF;
         

      1:  // Start acceleration phase

         // Set initial speed and gradually rise to set value

         "QW80" := 0;  // Initial pressure is 0

         State := 2;
         

      2:  // Normal juicing phase

         // PID adjustment based on set pressure

         "QW80" := REAL_TO_INT(SetPressure * 27.648);
         

         // Check if actual pressure is within a reasonable range

         IF ABS(ActualPressure - SetPressure) > 10.0 THEN

            PressureError := TRUE;

         ELSE

            PressureError := FALSE;

         END_IF;
         

         // Start timer

         Timer1(IN := TRUE, PT := PressTime);

         IF Timer1.Q THEN

            State := 3;

         END_IF;
         

      3:  // Juicing completion phase

         "QW80" := 0;  // Release pressure

         "Q0.1" := FALSE;  // Stop juicer

         Completed := TRUE;

         Running := FALSE;

         State := 0;
         

   END_CASE;
   

   // Emergency stop handling

   IF Stop THEN

      "QW80" := 0;

      "Q0.1" := FALSE;

      Running := FALSE;

      State := 0;

      Timer1(IN := FALSE);

   END_IF;
END_FUNCTION_BLOCK

State Control Design

The juicing process uses a state machine to manage the process flow, with the main states including:

1. Initialization State (S0): System startup self-check
2. Standby State (S1): Waiting for start command
3. Feed State (S2): Raw material delivery in place
4. Juicing State (S3): Executing juicing process
5. Filtration State (S4): Juice filtration processing
6. Filling State (S5): Finished product filling
7. Fault State (S99): System exception handling

State transitions are driven by trigger conditions and completion signals to ensure orderly process flow.

3. System Debugging Methods

Step-by-Step Debugging Method

The debugging of the juice control system adopts a “unit-joint-system” three-level debugging strategy:

1. Unit Debugging

• Check each I/O point one by one to confirm correct wiring
• Calibrate sensor signals, especially temperature and pressure sensors
• Test actuators individually to verify control response
• Test function blocks individually to check logical correctness

2. Joint Debugging

• Feed-juicing linkage test
• Juicing-filtration linkage test
• Filtration-filling linkage test
• Parameter linkage impact verification

3. System Debugging

• Full process automatic operation test
• Abnormal condition simulation test
• Production rhythm optimization adjustment
• Long-term stability test

Parameter Tuning Steps

Key parameter tuning methods for the juice system:

1. Juicing Pressure Parameter

• Start from the minimum pressure and gradually increase until the best juice yield is obtained
• Record the best pressure parameters for different raw materials
• Establish pressure-juice yield curve

2. Juicing Time Parameter

• Start from the shortest time and gradually extend until the juice volume stabilizes
• Avoid excessive time leading to product oxidation
• Determine the best time parameter

3. Filtration Parameter

• Adjust filtration speed to ensure clarity meets standards
• Monitor filtration differential pressure to prevent blockage
• Optimize backwash cycle

Abnormal Simulation Testing

System robustness testing includes:

• Emergency stop test: Verify that all actuators can stop safely and immediately
• Sensor fault simulation: Disconnection, short circuit, signal drift, etc.
• Power fluctuation test: System response during voltage fluctuations
• Communication interruption test: System behavior during network anomalies
• Operational error test: System protection mechanisms under erroneous operations

4. Fault Diagnosis and Troubleshooting

Common Fault Analysis

Common faults and solutions for the juice system:

1. Feed Not Smooth

• Symptoms: Feed sensor signal unstable or no raw material in place signal
• Causes: Sensor position deviation, insufficient sensing distance, raw material blockage
• Solutions: Adjust sensor position, clear feed channel, check motor drive circuit

2. Juicing Pressure Abnormal

• Symptoms: Pressure cannot reach set value or fluctuates too much
• Causes: Hydraulic system leakage, pressure sensor failure, inappropriate PID parameters
• Solutions: Check hydraulic system sealing, calibrate pressure sensor, optimize PID parameters

3. Filtration Efficiency Decrease

• Symptoms: Increased filtration differential pressure, decreased flow rate
• Causes: Filter blockage, pump efficiency decrease
• Solutions: Execute backwash program, replace filter, check pump operating status

4. Temperature Control Abnormal

• Symptoms: Temperature exceeds set range
• Causes: Temperature sensor failure, control loop abnormality
• Solutions: Verify temperature sensor, check heating/cooling control loop

Troubleshooting Methodology

The system fault handling adopts a “four-step method”:

1. Observe Phenomenon: Record fault performance, check HMI alarm information
2. Locate Link: Determine which process link the fault occurred in
3. Analyze Cause: Analyze possible fault causes through monitoring data
4. Verify Handling: Implement repair measures and verify effectiveness

5. Operation Interface Design

Interface Layout Description

The HMI interface design follows the principles of simplicity and practicality, mainly including:

1. Main Screen: Displays the status of the entire juice production line, including:

• Dynamic process flow diagram
• Running status indicators for each unit
• Real-time values of key parameters
• Production statistics information

2. Parameter Setting Screen:

• Process parameter configuration form
• Permission control buttons
• Parameter import/export functions
• Preset scheme selection

3. Alarm Screen:

• Real-time alarm list
• Historical alarm records
• Alarm confirmation button
• Fault handling guide link

4. Trend Curve Screen:

• Historical trends of key parameters
• Multi-parameter comparison function
• Time period selection
• Data export function

Parameter Setting Description

System parameters are divided into three levels:

• Operator Level: Daily production parameters, such as juicing time, filling volume
• Engineer Level: Process adjustment parameters, such as pressure range, PID parameters
• Administrator Level: System configuration parameters, such as communication settings, data backup

Parameter modification adopts a double confirmation mechanism to avoid misoperation leading to production anomalies. Key parameter settings have upper and lower limit alarms to prevent out-of-range settings.

Siemens PLC control of the juice extraction process requires both process understanding and technical implementation. This article serves as a framework, and actual applications need to be deeply customized based on specific equipment. We welcome discussions on optimization solutions!