piclist 2000\06\08\093346a >
Thread: and [OT] Measuring weight using TSL214 with a PIC
face BY : paulbSTOPspam@midcoast.com.au

Rob Bakker wrote:

> I am using a PIC to measure the weight of a small animal feeder.
> Currently the feeder is hanging from a spring and I'm trying to
> measure the change in height as the feeder empties.  Feeder weighs no
> more than a couple of kilograms and the amount of feed delivered is in
> the order of 100 grams - so I need to detect approx 0-100 gram change.
> The feeder rises approx 1 cm from full to empty...

 It might just be interesting to consider the extremely elegant, being
both mechanically simple and robust, mechanism used as I understand in
most precision balances.  One advantage is that unlike the mechanism you
suggest, there is extremely little movement of the load.

 You use a single optical chopper.  The load moves this, but you
constrain its range with stops to perhaps a millimetre or two.  A
solenoid pulls against the load (or may indeed push).  The solenoid is
powered by a linear driver with current feedback.

 In general, the solenoid is feedback controlled to bring the chopper
exactly to its midpoint (ergo, it must have at least two threshold
levels).  At that equilibrium, the solenoid current is directly
proportional to the load.  The solenoid curent can either be measured by
an ADC, or simply determined by a DAC or PWM (In fact, if you use only
digital feedback, the latter is necessary to perform the former anyway).

 The minimum system is a sensor and a solenoid driven using PWM
straight from a digital driver.  You will need a PID algorithm to do
this, but the mechanical aspects certainly should be simple and robust.

 No, I haven't done it.
       Paul B.

<393FA0B2.24193160@midcoast.com.au> 7bit

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