ECE265
ECE 265 – LECTURE 16
Complete System Example
A Weight Measurement System
4/13/2015
1
Lecture Overview
2
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System application example
 A point
of sale weight measurement system
 Involves
 Load
cells and signal conditioning
 Application program
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REF: Chapter 14
Many chapters plus the 68HC11 reference
manual.
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Statement of the Problem
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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.
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Some background
4
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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.
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
The Load Cell setup
5
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The basic setup to measure weight using a strain
guage.
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Wheatstone bridge circuit
6
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To get a voltage proportional to weight
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Load Cell Specification
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Parameters to specify
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
 Excitation
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Interface to the microcontroller
8
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Practical load cell
circuit –
Ready for input to
conditioning
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Signal conditioning
9
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Signal needs to be conditioned (scaled) for input to
the 68HC11.
IA = Instrumentation Amplifier
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Fig continued.
10
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The full
setup for
interfacing
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
If the signal is noisy
11
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Software design
12
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An overall
flowchart
/pseudocode

Start at this high
level and then
break down each
function
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
The enter subroutine
13
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Again have
modules to break
down
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
The measure subroutine
14
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Note level of action
in each box.
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What type of code is
this?
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
The little things
15
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A subroutine to
round the cents in
the price.

A more complex
routine
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Lecture summary
16
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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!!!
 Joanne E. DeGroat, OSU
ECE265
4/13/2015
Assignment
17
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None
 Joanne E. DeGroat, OSU
ECE265
4/13/2015