Industrial Weighing System: Load Cell and PLC Integration

Load Cell Fundamentals for Industrial Weighing

Load cells are the force-sensing elements at the heart of every electronic weighing system. In India, industrial weighing is critical for compliance with the Legal Metrology Act 2009 and the Standards of Weights and Measures (Packaged Commodities) Rules 2011.

Common load cell types used in Indian industry:

• Single-point load cells: Used in bench scales, packaging machines, and check weighers. Capacity: 5kg to 300kg. Cost in India: ₹400-3,000
• Shear beam load cells: Used in platform scales, hopper scales, and tank weighing. Capacity: 100kg to 50 tonnes. Cost: ₹1,500-15,000
• Compression load cells: Used in truck scales, silo weighing, and heavy industrial applications. Capacity: 10 tonnes to 500 tonnes. Cost: ₹5,000-50,000
• S-type (tension/compression) load cells: Used in crane scales, batching systems, and material testing. Capacity: 5kg to 20 tonnes. Cost: ₹2,000-20,000

Indian-made load cells from manufacturers like HBM India, Mettler Toledo India, Essae, and local brands offer good quality at competitive prices.

HX711 Load Cell Amplifier: Arduino Integration

The HX711 is the go-to load cell amplifier for Arduino-based weighing projects. At ₹50-100 in India, it provides 24-bit ADC resolution — sufficient for 0.01% accuracy applications.

// Arduino HX711 Weighing System
#include "HX711.h"
#include <LiquidCrystal_I2C.h>

#define LOADCELL_DOUT_PIN 3
#define LOADCELL_SCK_PIN 2
#define TARE_BUTTON 4

HX711 scale;
LiquidCrystal_I2C lcd(0x27, 16, 2);

float calibrationFactor = -7050; // Adjust during calibration
float weight = 0;

void setup() {
  Serial.begin(9600);
  lcd.init();
  lcd.backlight();
  pinMode(TARE_BUTTON, INPUT_PULLUP);

  scale.begin(LOADCELL_DOUT_PIN, LOADCELL_SCK_PIN);
  scale.set_scale(calibrationFactor);
  scale.tare(); // Zero the scale

  lcd.print("Scale Ready");
  delay(1000);
}

void loop() {
  if (digitalRead(TARE_BUTTON) == LOW) {
    scale.tare();
    lcd.clear();
    lcd.print("Tared!");
    delay(500);
  }

  weight = scale.get_units(10); // Average 10 readings

  lcd.setCursor(0, 0);
  lcd.print("Weight: ");
  if (weight < 1000) {
    lcd.print(weight, 1);
    lcd.print(" g   ");
  } else {
    lcd.print(weight / 1000.0, 3);
    lcd.print(" kg  ");
  }

  // Check weight tolerance (Legal Metrology)
  float target = 500.0; // 500g target
  float tolerance = target * 0.02; // 2% tolerance
  lcd.setCursor(0, 1);
  if (abs(weight - target) <= tolerance) {
    lcd.print("PASS - In Range ");
  } else if (weight < target - tolerance) {
    lcd.print("FAIL - Under    ");
  } else {
    lcd.print("PASS - Over     ");
  }

  delay(200);
}

Industrial Check Weigher Design

A check weigher automatically verifies that each product on a conveyor meets the target weight. This is mandatory for packaged goods in India under the Legal Metrology Act.

Architecture of an industrial check weigher:

[Product on conveyor]
       |
[In-feed section] → stabilise product position
       |
[Weighing section] → load cell platform, isolated from conveyor vibration
       |
[Reject section] → pneumatic pusher for underweight products
       |
[Out-feed section] → continue to packaging

Key design considerations for Indian conditions:

• Vibration isolation: Indian factory floors often lack vibration dampening. Use rubber isolation mounts under the load cell platform and keep the weighing section mechanically separate from the conveyor frame.
• Speed matching: The weighing section must run at the same speed as the conveyor. Speed mismatch causes product sliding, affecting accuracy.
• Settling time: The product must stabilise on the load cell before the reading is taken. For a 30 products/minute line, you have approximately 1 second per product — design the weighing section length accordingly.
• Environmental: Temperature and humidity affect load cell output. Use temperature-compensated load cells (standard in quality Indian/imported cells).

PLC Integration for Production Weighing

For production environments, connecting the weighing system to a PLC enables automated batching, sorting, and data logging.

Connection Methods

• Analog output (4-20mA): Weighing indicator sends a proportional current signal to PLC analog input. Simple but limited resolution.
• Modbus RTU: Digital communication via RS-485. Weight value as a holding register. Best balance of cost and capability.
• Analog input module: Connect HX711 output directly to a high-resolution PLC analog input. Requires shielded cabling.

Ladder Logic for Batching

// Rung 1: Read weight from Modbus register
|--[MOV: MODBUS_REG_40001 → CURRENT_WEIGHT]--|

// Rung 2: Start filling when weight below target
|--[LT: CURRENT_WEIGHT = TARGET - DRIBBLE_RANGE]----( SLOW_FILL )--|

// Rung 4: Stop fill at target
|--[GE: CURRENT_WEIGHT >= TARGET_WEIGHT]----( FILL_COMPLETE )--|
|--[FILL_COMPLETE]----( FILL_VALVE OFF )--|

// Rung 5: Log and advance
|--[FILL_COMPLETE]--[TON: 1s]----( ADVANCE_CONVEYOR )--|
|--[FILL_COMPLETE]--[MOV: CURRENT_WEIGHT → LOG_REGISTER]--|

This batching logic fills a container to a target weight with fast fill followed by slow dribble feed for accuracy. The 1-second delay after completion allows the weight to stabilise before logging.

Sensors for Weight Monitoring and Environmental Compensation

Accurate weighing in Indian conditions requires monitoring ambient conditions. Temperature and humidity affect both load cell output and product weight (especially for hygroscopic products like flour, sugar, and pharmaceutical powders):

Install a BME280 or DHT22 near the weighing station and log the environmental data alongside weight readings. This helps diagnose accuracy drift and is required for quality audits in pharmaceutical and food industries.

Calibration and Legal Metrology Compliance in India

Indian legal requirements for industrial weighing:

• Legal Metrology Act 2009: All weighing instruments used for trade must be verified and stamped by the Department of Legal Metrology
• OIML R76: International standard for non-automatic weighing instruments, adopted by India
• Accuracy classes: Class III (commercial) — 3000 divisions. Class IIII (industrial) — 1000 divisions
• Verification interval: Annual re-verification is mandatory

For DIY/Arduino-based systems used for internal process control (not trade), legal verification is not required. However, regular calibration using known reference weights is essential for quality control.

Calibration procedure:

1. Power on and warm up for 30 minutes
2. Zero/tare with no load
3. Place a known reference weight (80% of capacity) on the platform
4. Adjust calibration factor until display matches reference weight
5. Verify with 3-4 different reference weights across the range
6. Document calibration date, reference weights used, and results

Frequently Asked Questions