NARROWER THE BETTER

Since its inception, the air bearing shape meter width measurement has been reduced through the reduction of rotor width. Rotor widths have decreased from 4 inches to 3 inches and down to 2 inches. Shape meters are often supplied with a mix of rotor widths with the narrower rotors located at the edge of the strip. Further reductions in rotor width seem unlikely. As the rotor width decreases, measurement linearity suffers. Figures 1 and 2 show the variation of measurement accuracy as a function of load position. This characteristic is particularly relevant when obtaining the measurements at the edge of the strip. Although the calibration is good when carried out with a load carefully positioned at the center of each rotor, it can be seen that the accuracy is reduced dramatically as the load is moved from the center of the rotor. In actual operation, the condition of central loading only occurs when the rotor is fully covered with strip having uniform tension. Near the strip edge this condition is hardly ever met, and of course, for partially covered rotors, it is never met.

The HiRes shape meter reduces the measurement width by incorporating two sensor tapings into each rotor width. Dual tapings yield additional information about the pressure field between the rotor and the arbor. This information is used to produce two signals proportional to the average load on each half of the rotor. The result is that 3-inch rotors can produce 1.5" measurement resolution and 2" rotors can produce 1-inch measurement resolution.