Weighing System Selection Guidelines – Instrumentation

Weighing System Selection Guidelines

Types of Weighing Systems

• There are 2 major types of weighing systems which are used in all segments of industry –batch weighing systems and in-motion weighing systems; defined as follows:
• Batch weighing system is defined as preprogrammed system of weighing and discharging 2 or more materials using one or more multiple scales, and requiring a minimum of human intervention, for the production of product on an incremental basis rather than a continuous basis.
• In-motion weighing system is designed to capture the weight of a discrete object or continuous flow of dry material while it moves or is moved across the scale.

  

Weighing System Components

• Regardless of types of weighing systems, there are 3 basic system components – scales, material handling equipment, and information systems; therefore, each properly selected weighing system must coordinate the above 3 basic system components.

Scales

The scale typically consists of single or multiple weight modules with summing junction box, weight indicators to power the load cells, read the load cell signal, digitize the signal, convert and display the weight, and output a weight signal with either a serial or parallel BCD (Binary Coded Decimal) to an information system.

Material Handling Equipment

The flow of material onto and away from the scale has an important effect on weighment accuracy. Typical material handling equipment includes screw feeders, vibratory feeders, various types of valves, and metering feeders.

Information Systems

Information systems, as related to scales, are typically the result of requirements to combine the collection of weight information and the associated product identification information. These information items are put together in usable form for later processing by man or computer.  Information systems can be classified by the amount of operator interaction. There are 3 types of information systems, as follows:

• Automatic

An automatic system requires no human interaction. Typically, these systems involve not only the collection of information from scales and product sensors, but also the controlling of the product onto and off of a scale.

• Semi-automatic

Semi-automatic systems are tools to aid operators in accomplishing their  tasks. The operator can be directed step by step as to what to do. On the other hand, the system can also accomplish some of the steps in collection of the information automatically.

• Manual

In a manual information system, the results are totally dependent on the  operators’ ability to record and pass on the correct information to other devices.
Information systems components can be classified as follows:

Base Systems

A weight indicating and recording device form the base components of a weight information system.

Peripherals

Three categories are available for information system. The categories are input devices, output devices, and storage devices.

• Input Devices

Input devices can be segregated into 2 categories – automatic and manual.  Manual input devices include CRTs (Cathode Ray Tube) and other similar keyboard related terminals. Automatic input devices are more restrictive in that they are designed to read specific forms of information. These devices include barcode scanners, transponders, and voice recognition systems.  

• Output Devices

The printer is a common output device for information systems.

• Storage Devices

Information can be stored in either magnetic or nonmagnetic media.  Common ways of storing information for retrieval are using magnetic tapes. Nonmagnetic media include a RAM (Random Access Memory) and BUBBLE memory. RAM typically requires power to retain its state where bubble retains data without power.