Designing a Scale for Bakery Ingredient Measurement

[morgan82]

I need to design a scale. It is a simple 2-D problem. All you need to know is that it's a plate on a vertical spring. Each end of the plate slides a little ways into an indent. It is a scale for weighing ingredients in a bakery. A weight of 22lbs (+or- .25 lbs) is acceptable. If there isn't enough ingredients, the plate hits the top and electrically signals that it isn't enough. If there is too much ingredients, the plate hits the bottom and signals that it is too much.

How would I go about this? I need to figure out a good spring length and stiffness. So do I say the L (unstretched length) is when the plate touches the top (when F=0)? What about the bottom? I am going to try to say that the unstretched length of the spring is 4 inches. How do I figure out the ideal stiffness and Lo? I know how to calculate it, I just don't understand how to make the +,- work.

 

[dlgoff]

Once you've determined which spring (stiffness), you could use micro-switches to determine the top and bottom locations. Maybe to sound an alarm or trigger some other action.