Hello Lawrence & PIC.ers,

Y'might want to try the basic principle used in many laboratory balances.

The platen has a very small range of movement, with NO springs.
Powered-down the platen rests at the bottom of its range, powered-up it is
raised by a solenoid till it interrupts a slotted type
opto-coupler. There is no hysteresis with this arrangement, the opto can
look through an angled appendage attached to the platen.
The solenoid current is controlled by your PIC, in closed loop, so that the
opto output current remains constant at some convenient set-point value,
i.e. the platen remains at a fixed position.
When the platen is loaded externally (someone
dumps material on it, or in your case Mrs. Jones pie)
the closed-loop increases the solenoid current and brings the
opto output back to set-point again.
The compensating solenoid current is directly proportional to the
load on the platen.

This arrgt. is real simple and capable of tremendous accuracy, even if
know you don't need to weigh pies to +/- 0.001 gm.


        best regards,   John

e-mail from the desk of John Sanderson, JS Controls.
Snailmail:          PO Box 1887, Boksburg 1460, Rep. of South Africa.
Tel/fax:            Johannesburg  893 4154
Cellphone no:   082 741 6275
email:                jsand@pixie.co.za
Manufacturer & purveyor of laboratory force testing apparatus, and related
products and services.

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads