Weight Measurement System - Microprocessor Based Systems - Lecture Slides, Slides of Microprocessor and Interfacing

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Lecture 16

Complete System Example

A Weight Measurement System

Lecture Overview

• System application example
  • A point of sale weight measurement system
  • Involves
    • Load cells and signal conditioning
    • Application program
• REF: Chapter 14
• Many chapters plus the 68HC11 reference manual.

Some background

• Load cells are transducers that covert applied force, such as weight, into a differential electrical output voltage signal.
• Load cells – foil type strain gauges
  • Use a Wheatstone bridge circuit to output a signal that can be input to the microcontroller A/D.
  • Foil type strain gauges measure the deflection of the sensing beam to which they are attached. The beam deflects when a load is applied and thus the ability to measure weight is achieved.

The Load Cell setup

• The basic setup to measure weight using a strain guage.

Load Cell Specification

• Parameters to specify
  • Excitation voltage – typically a maximum
  • Terminal resistance input – used to determine power consumption – manufacturers specify minimum resistance.
  • Rated capacity – in pounds of kilograms
  • Rated output – in mV/V – specifies the output signal per pound given an input voltage of V

Interface to the microcontroller

• Practical load cell circuit –
• Ready for input to conditioning

Fig continued.

• The full setup for interfacing

If the signal is noisy

The enter subroutine

• Again have modules to break down

The measure subroutine

• Note level of action in each box.
• What type of code is this?

Lecture summary

• In this lecture we have looked at a system implementation where a microcontroller would be used.
• System in an integration of both hardware and software design and implementation.
• The software for the system is 8 ½ pages in chapter.
• Note the documentation within the software code!!!

Assignment

 None