Measuring Instantaneous Forces with a Photoelastic Force Balance

Abstract

We present a new floating element drag balance design that uses an optical measurement of the force using photoelastic stress analysis. The force-sensing element consists of pins embedded in photoelastic polyurethane pads, which generate internal stress when the floating element is loaded. A series of known loads and their corresponding fringe patterns allow a calibration matrix to be derived using a 5th-order polynomial model solved by a least square regression. The balance first measured the lift curve of the NACA0015 wing at 5m/s and 7m/s. These measurements were time-averaged. A comparison of the photoelastic balance and an ATI Mini 40 load cell showed differences of 6%. This optical approach enables accurate measurements with inexpensive and simple components inside the sensor. Following this, the balance's ability to measure dynamic forces was investigated by a series of step responses at increasing Frequencies and then by capturing a decaying pendulum. The results show that a photoelastic balance is a simple, effective, and versatile means of force measurement, able to measure forces in a time-averaged manner and instantaneously.