Table of Contents
- What is a Weigh Feeder and How does it work?
- What is a Weigh Feeder?
- How does a Weigh Feeder work?
- What are the Instruments used in Weigh Feeder System?
- Load Cell in Weigh Feeder
- Tacho – Generator
- AC Drive
- Application of Weigh Feeder
- Frequently Asked Questions
What is a Weigh Feeder and How does it work?
What is a Weigh Feeder?
- Weigh feeder is a closed-loop control system.
- Weigh feeder monitors & regulates material flow using an electronic controller along with various instruments.
- A weigh feeder is a special type of flow meter equipped for granular or powdered solids.
- In each process industry, bulk material movement with high accuracy & with high-speed weighment is a key role.
- The commonly used weigh feeder design consists of a conveyor belt with a roller coupled with load cells.
- The easiest way to transport material in large quantities is to convey these materials over a moving belt
- The fixed length of the conveyor belt section is continuously weighed by the load cell to produce the exact weight of a material per linear distance on the belt.
- It is necessary to weigh bulky items with high accuracy to control process requirements.
- Weigh feeder receives an external command from process computers such as PLC, DCS, SCADA, & HMI.
- A tachometer is a type of speed sensor to measure the speed of the belt.
- The product of two variables such as the speed of the conveyor belt and the exact weight of material gives the mass flow rate of solid material through the weigh feeder.
- The mass flow rate of solid material is given by the following relation.
W = F*v/d
- W = Mass flow rate.
- F = Force of gravity acting on the weighted belt section.
- v = Belt speed.
- d = Length of the weighted belt.
- A smaller-sized weigh feeder with a length of about two feet is shown in the figure above
- This weigh feeder is used to feed powdered soda ash into the water at a municipal filtration plant unit to neutralize the pH of water.
- A set of rollers that supports the belt weight along with the powdered soda ash.
- This load cell fixed gives an accurate measurement of material per foot present in the conveyer belt length.
- The speed sensor called a tachometer will measure the net speed of the conveyer belt feet per minute (feet/min).
- These AC voltage and frequency signals are the desired output signal that meets the ratio of volts per hertz.
- Generally, during normal operation, these weigh feeders express belt loading fluctuations calculated in terms of mass flow rate.
- In some cases, these load fluctuations don’t coincide on the display screen.
- The load cell measures the actual weight of a predefined area on the belt.
- The encoder measures the speed of the belt.
- AC Drive controls the speed of the motor linked to the gearbox to drive the conveyor belt.
How does a Weigh Feeder work?
The working of weigh feeder system is shown in the below diagram
The working procedure of Weigh Feeder system is shown in below steps.
- Ensure all field instruments and devices are properly calibrated for accurate functioning to obtain a ready signal, otherwise check for any error present.
- A feeder operator issues a ready command to start the system as soon as the system gets ready to function without any error.
- PLC or DCS issues a Feeder ON command to the controller.
- After receiving the Feeder ON command from PLC or DCS a controller issues a BELT ON command signal for the AC drive to initiate the belt.
- The rotation of a conveyor belt begins after the AC drive receives the start command.
- AC drive doesn’t issue any RUN F/B to PLC or DCS if any kind of error is detected.
- The PLC /DCS reset the Feeder ON command to the controller to generate an error.
- After the error gets cleared or if no fault is present then AC drive issues RUN F/B to PLC or DCS.
- At this time an electronic controller called ON/OFF Controller receives necessary F/B signals from both Load Cell, & Tacho-generator.
- The load cell generates Load in Kg/m, & Tacho gives speed in m/s.
- Both F/B signals are multiplied & transmitted to PLC/DCS so that the Flow rate is the product of two which is t/h.
- An electronic (ON/OFF) controller receives a flow rate Set Point from PLC and current flow (Process Variable).
- The controller continuously compares this current flow with a given Set Point to generate and issue set speed command in proportion to AC drive PID settings.
- The speed gets increased if Set Point is greater than the current flow, and the speed gets reduced if the current flow goes higher than Set Point.
What are the Instruments used in Weigh Feeder System?
Closed Loop Controller
- The closed-loop controller is a microcontroller-based instrument.
- A closed-loop controller can be programmed based on user demand.
- Basically, the closed-loop controller receives electronic signals from both the load cell, & Encoder.
- Both these received signals are transfigured according to field instrument rating.
- These signals are now multiplexed together to gain an appropriate current flow rate (Process Variable).
- This closed-loop controller receives a set value through a 4 to 20mA electrical signal to produce a flow rate equal to an electrical signal of 4 to 20mA.
- This flow rate controls the set flow given to the controller through an external controller or process computer.
- There is a provision to produce an alarm or error signal field environment outside the given parameter.
- The controller releases an electrical signal to regulate the motor speed through the AC drive proportional to the set flow rate
Load Cell in Weigh Feeder
- A load cell is a force transducer.
- Load cell transfigures load or force into a proportional electrical signal.
- A strain gauge is the main element of the load cell.
- The strain gauge is bonded to a thin dielectric layer, & is made up of ultra-thin heat-treated metallic foil.
- Resistance in a strain gauge is varied in proportion to the applied force on it.
- In a Wheatstone bridge, the configuration consists of four strain gauges bonded in it.
- Load cell input supply is in Volts called excitation voltage with sense wires.
- Load cell output signal is in milli-Volts
Tacho – Generator
- The Tacho generator measures the speed of the conveyer belt driven by the return belt.
- Basically, this tacho generator includes of proximity type speed sensor activated by a disc installed inside the tacho generator.
- The input of the proximity-type speed sensor is in Volts, & produces an output signal in frequency keeping amplitude constant.
- AC Drive controls the speed of the motor.
- Generally, AC Drive is called Variable Frequency Drive (VFD).
- The operation of a Variable Frequency Drive is not so complicated.
- Initially, Rectifiers transfigure both AC input voltage & frequency Signals to the DC voltage source.
- Finally, an inverter converts transfigured DC voltage to the original AC voltage.
Application of Weigh Feeder
- Weigh feeder is used in various industries like
- Steel Plant,
- Cement Plant,
- Fertilizer and
- Chemical industries.
Frequently Asked Questions
What is the working principle of weigh feeder?
- Weigh feeder works on the principle of “Flow rate held constant by the varying speed in inverse proportion to the load on the belt”.
- Generally, Weigh Feeder is a small belt conveyer with a belt weight and speed control system.
What is the accuracy of weigh feeder?
Industry weighs feeder has a standard accuracy of about ±½% of net weight or ±1.0% of the set rate.
How does loss in weight feeder work?
- Loss-in-weight feeders are gravimetric feeders that directly measures materials’ weight to obtain a calculated feed rate.
- The feed rate is a measure of weight per time (e.g. kg/h).
- This Set-Point is programmed manually in the feeder control.
How does the belt feeder work?
- A belt feeder includes hopper fixed directly to belt conveyor.
- The belt conveyor pulls material from the hopper.
- As the conveyer belt moves in the forward direction, A material height is controlled by an adjustable vertical strike-off plate.
What are the different types of weigh feeders?
- Controlled Belt Speed Weigh Feeders
- Metering Hopper Weigh Feeders