Lab 18: U-Make Force Probe
- LCHS Physical Science
 LCHS Main SiteSD271 Main siteUof I New Century Classroom  
Objective: The learner will...
...utilize previous programming skills and electromagnetic knowledge to design, build and calibrate a working digital probe from common materials
 


INTRO:
A 50 KOhm linear slide resistor seems to be the best suited and most readily available value for the force probe. The 3 shown below were made with simple materials, including: slider resistor (Radio Shack), block of wood, hot glue, elastic band or spring, and wire/paperclip. After calibration, the rubberband force probe on the left had an accuracy of 1/2 of 1% throughout its entire range of 0 to 500 grams!!!

The band/spring must be slightly loose when there is no weight, hence the need to be able to adjust band/spring placement. The weight limit is determined by the strength/number of bands/spring. Once the weight has moved the switch to the very bottom and you need more capacity, more bands or a stronger spring must be added and the probe must be recalibrated. Fortunately, this particular probe is easy to calibrate due to it's high linearity. For the first calibration point just pull on the hook and let it return the top and read this value as zero (Newtons/pounds). The 2nd (and last) point is gotten by hanging the maximum capacity weight and using its value.

PROCEDURE:
Plug the constructed force probe into the supplied gameport adaptor, type in the Quick Basic Program below and RUN.

SCREEN 9 : CLS
START:
Y=STICK(0)
LOCATE 12,10 : PRINT Y;" "
A$=UCASE$(INKEY$)
IF A$="Q" THEN
CLS : GOTO START
END IF

To calibrate, carefully hold the force probe and pull the wire hook down, allowing the rubber band or spring to rebound back. Record this computer display value as X1 (Y1 = 0 grams). Now hang a 100 gram mass on the hook and let it rebound. Record this computer display value as X2 (Y2 = 100 grams).
DATA: X1 = X2 =

Press "Q" to stop the program and now replace the line
Y=STICK(0) with Y=(ABS(X2 - STICK(0))*100/(X2 - X1))+100
replacing the values X1 and X2 with your data.
This new line will calibrate the probe!

NOW, hang an unknown mass (watch, calculator, glasses) on the scale (that does not excede its capacity), RUN the program again and record the output.
Experimental Mass = grams
LASTLY, place the unknown mass on an accurate gram scale, record the ACTUAL mass and compute the error.
Actual = grams
Error = 100% x (Experimental - Actual) / Actual = %

Finally, update your website with the details of this lab experience including a short descriptive summary, all data, calculations and error conclusions! The instructor will grade this lab web entry according to this rubric.