Weighing Scale

Weighing Scale

The system consists mainly of a 24-bit ADC for load cell sensing, a DSP for signal processing and a 8-bit microcontroller for interface.

ADS1255 implementation

A 24-bit converter was selected since the aim of the board is to serve as an interface to any of the load cells used across the different weighing points, without the need of an amplification stage. Thus making the calibration easier, in comparison with the current weighing system that requires manual adjustment of the amplifier gain.

The menu interface is user-friendly. Calibration is achieved just by pressing a few buttons. First, zero offset correction is corrected by selecting "Calibration -> Zero". Then a full scale load is placed on the weighing platform, the option "Calibration -> Pattern" is selected, the user inputs the weight expressed in grams and then confirms. The system calculates the necessary parameters after which the system is ready to be used.

Main board

Further configurations are available depending upon the needs of the particular case. The number of decimal places, 3 levels of filtering and tare value can be selected from Calibration menu. Serial output at 4 different speeds can be enabled as well as a TCP server can be set up to communicate from "Serial" and "Ethernet" menus respectively.

Main board with Ethernet module and IOs

The system also offers several IOs to better control the process. An AC output indicates when weight value is stable. Two 24v optocoupled outputs and two inputs allow the user to trigger events like weight printing, relay/PLC input activation. A trigger input for a mechanical switch is available when manual printing is necessary. Finally an optocoupled 220v TRIAC driven output turns on when the weight is stable.

The system as a whole has been tested under tough conditions like high humidity, large temperature variations, strong vibrations and EMI; showing outstanding performance and stability over time. Load cells of 20 Kg, 200 Kg and 1 ton capacity have been successfully interfaced.

The design implements the 24-bit ADC ADS1255, featuring series and parallel filtering on the PDN as well as on the acquisition path. A dsPIC33 interfaces the ADC by configuring it and implementing discrete filtering functions. It also handles the TCP socket. A PIC18F46K80 manages the LCD screen, the keyboard and the IOs.

Weighing head front view

Main screen

Main menu

Calibration menu