ECE 265 – LECTURE 16 Complete System Example A Weight Measurement System 8/13/2015 1 ECE265.

ECE 265 – LECTURE 16

Complete System Example

A Weight Measurement System

04/19/23

1ECE265

Joanne E. DeGroat, OSU

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.

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Statement of the Problem

A chain of hardware stores wants to automate checkout of small items to make the checkout process more efficient.

For nail, screws, washers, and other similar small items.

Each carries a different price and each items has an item weight.

When a bulk purchase of one of these items is made, the order is placed on the scale of the system and the clerk enters the product id.

The microcontroller weighs the order, the number of items and total price calculated.

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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.

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The Load Cell setup

The basic setup to measure weight using a strain guage.

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Wheatstone bridge circuit

To get a voltage proportional to weight

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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

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Interface to the microcontroller

Practical load cell circuit –

Ready for input to conditioning

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Signal conditioning

Signal needs to be conditioned (scaled) for input to the 68HC11.

IA = Instrumentation Amplifier

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Fig continued.

The full setup for interfacing

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If the signal is noisy

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Software design

An overall flowchart /pseudocode

Start at this high level and then break down each function

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The enter subroutine

Again have modules to break down

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The measure subroutine

Note level of action in each box.

What type of code is this?

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The little things

A subroutine to round the cents in the price.

A more complex routine

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Lecture summary

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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

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None

ECE 265 – LECTURE 16 Complete System Example A Weight Measurement System 8/13/2015 1 ECE265.

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Transcript of ECE 265 – LECTURE 16 Complete System Example A Weight Measurement System 8/13/2015 1 ECE265.