Loss-in-Weight Feeder Product

Overview

A loss-in-weight feeder is an advanced gravimetric dosing device used in industrial processes where precise material flow rate is critical. Unlike a volumetric feeder (which assumes consistent density), a loss-in-weight feeder directly measures the mass of material discharged and adjusts the discharge rate in real-time to maintain a target setpoint.

The device consists of a Hopper Assembly suspended on three Load Cell Suspension, a Discharge Screw that discharges material, and a Weight-Based Controller that implements a proportional-integral-derivative (PID) feedback loop. As material exits the hopper, its weight decreases; the controller measures this weight loss rate and adjusts the Discharge Screw speed (via a Variable-Frequency Drive) to achieve the desired discharge rate (e.g., 500 kg/h).

This approach offers superior accuracy compared to open-loop speed control, because it accounts for variations in material density, compaction, particle size, and moisture content—all of which affect volumetric flow but not gravimetric mass flow.

How It Works

Material is loaded into the Hopper Assembly. Three Load Cell sensors suspend the hopper and measure its instantaneous weight. A Weight-Based Controller reads these load cell signals continuously (typically every 100–500 milliseconds) and calculates the instantaneous weight loss rate: ΔWeight / ΔTime = discharge rate in kg/second (or tons/hour).

The operator (or automated process) sets a target discharge rate (e.g., 500 kg/h) via a local keypad or networked interface. The PID control loop in the Control Processor compares the measured discharge rate to the setpoint:

Error = Setpoint − Measured Rate

The controller outputs a 0–10 V command to the Variable-Frequency Drive, which drives the Motor and gearbox. If the measured rate is too low, the VFD increases the Discharge Screw speed (in RPM); if too high, it slows down. This closed-loop adjustment continues until the measured rate stabilizes at the setpoint.

When the hopper weight drops below a pre-configured refill threshold (e.g., 10% of capacity), the Weight-Based Controller opens the Refill Gate Valve (a motorized solenoid Gate Valve Body) to allow fresh material to fall from an upstream hopper into the active feeder. Once the weight exceeds the refill threshold, the valve closes automatically, and the feeder resumes normal operation. This cycle repeats throughout the shift.

Control Algorithm & PID Tuning

The PID control law is:

Control Output = Kp × Error + Ki × ∫Error dt + Kd × dError/dt

Where:

• Kp (Proportional): Responds immediately to the current error; higher Kp increases responsiveness but can cause oscillation.
• Ki (Integral): Eliminates steady-state error by accumulating historical error; slower response but ensures zero offset.
• Kd (Derivative): Predicts future error by measuring the rate of change; improves stability and reduces overshoot.

For a loss-in-weight feeder, typical tuning:

• Kp: 0.5–2.0 (tuned by operator to target 10–30% overshoot on step response)
• Ki: 0.01–0.1 (eliminates offset over 30–60 seconds)
• Kd: 0.1–0.5 (damping to prevent ringing)

Poor tuning leads to instability: if Kp is too high, the screw speed oscillates wildly between fast and slow, and the discharge rate chatters around the setpoint with ±5–10% error. If Kd is too high, the feeder becomes sluggish and takes a long time to reach setpoint.

Operator training and documentation (like the Tuning Documentation) typically include step-by-step tuning procedures and rules of thumb for different material types.

Mechanical Design

The Hopper Assembly is a conical or wedge-shaped stainless steel vessel (20–200 liters) with a 60° half-angle for gravity-assisted free flow. The hopper is suspended by three Load Cell Suspension mounted on a Hopper Mounting Plate plate secured to a fixed structural frame. Three [[loss-in-weight-feeder-hopper-brace|stabilizing braces]] prevent the hopper from swinging laterally as the Discharge Screw rotates, which would introduce dynamic loads into the load cells and corrupt the measurement.

The Discharge Screw is a variable-speed auger (25–50 mm diameter, 1–2 m long) rotating at 10–500 RPM. The screw shaft has helical flights welded or molded onto it; as it rotates, the flights push material from the hopper inlet toward the discharge outlet. A Screw Tube Casing (stainless steel casing) surrounds the screw to prevent material spillage and protect the rotating element.

The Screw Shaft is hardened alloy steel designed to resist bending under the load of material in the screw. Screw Bearings at both ends (sealed deep-groove ball type) support the shaft and handle thrust loads; they are typically rated for 5–10 metric tons axial force.

Drive & Control Electronics

The Motor is typically a 0.5–2 kW three-phase or single-phase AC induction motor. A Variable-Frequency Drive (Variable-Frequency Drive) adjusts the motor speed by varying the supply frequency (0–50 Hz). The VFD receives a 0–10 V analog command signal from the Weight-Based Controller; higher voltage = higher frequency = faster motor speed.

The Speed Reduction Gearbox (20:1 to 30:1 ratio, helical gears) reduces the motor speed while increasing torque. The output shaft drives the Discharge Screw via a Motor Coupling (flexible jaw or disc type) that accommodates minor misalignment.

The Control Panel is a NEMA 4X stainless steel panel housing the Control Processor (typically an ARM Cortex-M4 PLC or x86 embedded PC running real-time Linux), a Signal Conditioning Board (multi-channel A/D converter and relay outputs), and terminal blocks for field wiring.

The Refill Gate Valve is a motorized solenoid Gate Valve Body (slide gate or butterfly valve) with a Position Sensor (inductive proximity switch) detecting open/closed state. The valve is controlled by the Control Processor: when hopper weight < low threshold, the solenoid opens the refill gate; when weight > high threshold, it closes.

Calibration & Accuracy

Loss-in-weight feeders are calibrated in two steps:

1. Load Cell Zero/Span: Using Test Weights (ASTM Class 1, typically 10, 25, 50 kg), a technician places weights in the hopper and records the displayed weight. The Control Processor stores zero and span trim coefficients.

2. Material Calibration: The feeder is run with the actual material to be dispensed (not test weights, which may have different flow characteristics). The operator runs the feeder at a known slow speed (e.g., 20 RPM), collects material for exactly 5 minutes, and weighs the collected material. This establishes the relationship between screw RPM and actual discharge rate, accounting for material density, particle size, and friction. The result is stored in the controller's material database.

Once calibrated, typical accuracy is ±1–2% of the setpoint in steady state (after PID loop has settled). Transient response (time to reach setpoint after a step change) is typically 30–60 seconds, depending on tuning and material compaction.

Advantages & Limitations

Advantages:

• Superior accuracy vs. volumetric feeders, especially for materials with variable density or moisture.
• Real-time feedback allows autonomous operation without drift.
• Handles material variation gracefully; PID loop self-corrects.
• Precise dosing improves product quality and reduces material waste.

Limitations:

• Higher cost than open-loop feeders ($10,000–50,000 depending on capacity and automation).
• Requires skilled technician tuning; poor PID tuning leads to instability.
• Load cell hysteresis and creep (though small) can accumulate over hours, requiring periodic re-zeroing.
• Not suitable for very sticky or cohesive materials (e.g., wet clay) that may jam in the screw or load cells.

Applications

• Polymer Extrusion: Precise ratio control of multiple resin feeds in a co-extrusion line.
• Concrete Batching: Weighing cement, sand, and aggregates for consistent concrete mix.
• Tablet/Capsule Manufacture: Dosing active pharmaceutical ingredients and excipients.
• Animal Feed Blending: Mixing grains, vitamins, minerals in precise ratios.
• Powder Coating: Metering colored pigment into clear resin base.
• Food Production: Controlling ingredient addition (flour, sugar, salt) in bakery or snack manufacturing.
• Chemical Processing: Dispensing catalysts, additives, or precursors in precise stoichiometric ratios.

Related Reading: Hopper Assembly, Discharge Screw, Weight-Based Controller, Variable-Frequency Drive.