Electronic weighing system

[darrins]

I'm working on an appliance that dispenses a powder from a hopper using a feed screw. I want to control the dispensed quantity by measuring the weight loss of the hopper. This change in weight will be on the order of 7 grams to 40 grams.

I figure that this could be done by either:

a) using a spring and measuring the deflection by a LVDT

or

b) using a strain gage load cell

I'd prefer to use the load cell, but they are quite expensive. I have some literature on how to build a load cell, but I need to find out where I can buy inexpensive metal foil strain gages. Any suggestions?

Thanks.

Darrin

[hotwaterwizard]

Why not use a Timer to stop the Screw from turning after a certain time..
Use an adjustable Delayed off timer and just keep weighing the powder until you get the desired weight for the time you set. Then it will dispense the powder according to the limit you set.

[darrins]

Why not use a Timer to stop the Screw from turning after a certain time..
Use an adjustable Delayed off timer and just keep weighing the powder until you get the desired weight for the time you set. Then it will dispense the powder according to the limit you set.

I considered using a timer. This is definitely a more economical and simpler approach. My appliance will dispense different kinds of powders with different characteristics, e.g. density, so I'll have to experiment with this to see if it is accurate enough.

I appreciate the feedback.

Darrin

[ante]

Darrin,

The screw suggested by John can be highly accurate if you use a stepper motor to pull it. Then you can decide a number of steps for a dose and there will be no influences from the motors speed that can be the case with a timer. Just a thought!

Ante ::)