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

2. 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. 8/13/2015 2 ECE265

3. 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. 8/13/2015ECE265 3

4. 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. 8/13/2015ECE265 4

5. The Load Cell setup  The basic setup to measure weight using a strain guage. 8/13/2015ECE265 5

6. Wheatstone bridge circuit  To get a voltage proportional to weight 8/13/2015ECE265 6

7. 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 8/13/2015ECE265 7

8. Interface to the microcontroller  Practical load cell circuit –  Ready for input to conditioning 8/13/2015ECE265 8

9. Signal conditioning  Signal needs to be conditioned (scaled) for input to the 68HC11.  IA = Instrumentation Amplifier 8/13/2015ECE265 9

10. Fig continued.  The full setup for interfacing 8/13/2015ECE265 10

11. If the signal is noisy 8/13/2015ECE265 11

12. Software design  An overall flowchart /pseudocode  Start at this high level and then break down each function 8/13/2015ECE265 12

13. The enter subroutine  Again have modules to break down 8/13/2015ECE265 13

14. The measure subroutine  Note level of action in each box.  What type of code is this? 8/13/2015ECE265 14

15. The little things  A subroutine to round the cents in the price.  A more complex routine 8/13/2015ECE265 15

16. Lecture summary 8/13/2015ECE265 16  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!!!

17. Assignment 8/13/2015ECE265 17  None