Weighing scale is an engineering art that requires precision. The introduction of digital system to weighing scale design further pushes the limit demanded by customer. It brings the technology to a different level with wider application such as linkage with Point Of Sales systems (POS), Body Mass Index measurement (BMI), etc.

Digitization reduces system dependency on mechanical and analogue accuracy. It produces more stable yet accurate readings and allows ease on calibration. Digitized system is also more reliable in the long term, with lesser influence from wear and tear.





Product Overview



The weighing scale solution developed by the System Solution Team (SST) of WPG South Asia employs the Analog Devices Inc (ADI) μconvertor ADuC847, an 80C51 core system with 24-bit Sigma-Delta (Σ-Δ) Analog to Digital Conversion (ADC) interface with a precision load cell to provide a display resolution of 30,000 counts.

The design is simple yet with sufficient precision for mid end application. The system is fine tuned with linearity correction through software calibration. The solution reduces the design time for customers. It is cost saving on both manpower and bill of material.





Application

Advance Health scale

Platform scales

Bench scales

Truck scales

Retail scales ( price computing )

Counting scales





Fig 1. Types of weighing application and their resolution requirement.





The WPG's ADI weighing scale design is a Validated Reference Design. Further customization is also possible and should there be a need for this, customers can approach WPG SA System Solution Team (SST).



Weighing machine specification

300,000 counts of internal resolution

3,000 to 30,000 counts of Display resolution

Zero Tracking (Auto Zero) of up to +/- 0.1 Digit Display Count

Virtual Zero range :+/- 3,500 internal count (include Dead-load & Offset)

22-bits of effective resolution (RMS)

19-bit p-p noise-free code resolution







Fig 2. Scaling from 300K Raw ADC to 10K Display resolution (Linear zone)





Features of Weighing Solution



Hardware

STN LCD panel for reading display.

Programmable buttons for customization.

Indication LEDs programmable to support customer's development.

UART interface for external communication (to printer, to POS).

In system programming (ISP) through UART interface for quick firmware updates.

Multiple input channels allowing advance development using dual, four or more load cells application.

DC powered (Utilizing 7.5V to 9V)





Fig 3. Block diagram of the ADI weighing scale design





Software

Motion detection with indicator (optional)

Zero tracking (auto zero) reducing offset voltage drift to < 50nV / oC

Tare function

Over/Under load indication

Zero indicator when no load

Software (Auto) calibration

Linearity correction

Digital filter to set cutoff, output rate and settling time

Gross/Net weight indicator (optional)



Optional

High speed check weighing function with Hi-Lo, Accept Indicators

Set point with relay contact/TTL output (2 set points)

IR remote control

Computer control protocol for automated weighing application

Multiple display, eg. Price computing





The selection of ADI, ADuC847 as the core chip for the solution simplifies the hardware design and reduces component count ( Microcontroller + 24 Bit Σ-Δ ADC in one Package) . The complexity of design is migrated to the firmware development which allows more flexibility changes and improvements.







ADuC847

High performance single Cycle 8052 Core to 12.58 MIPs

Buffered Programmable Gain, Self Calibrating high resolution sigma-delta 24-Bit ADC

Offset drift 10 nV/°C, gain drift 0.5 ppm/°C chop enabled

10-Channel Single-Ended / 5-Channel Differential Input Options

Two Independent, Differential Reference Input Options

62 K-Byte In-Circuit Re-Programmable Flash Program Memory

4 K-Byte Read/Write Accessible Non-Volatile Flash Data Memory