Question

Explain with sketch torque measurement using (i) Stroboscope method, (ii) Capacitive method.

Answer 1

(i) Stroboscope method:--

Explanation : The arrangement of two flanges A and B placed at a distance on the shaft is as shown. Flange A carries a scale while flange a pointer. When a torque is applied on the shaft it causes angular displacement of pointer relative to the scale due to the angular twist. The deflection of the shaft may be read off directly when the shaft is stationery. However this is not possible if the shaft is rotating. In such cases, stroboscope is used. The flashing light of the stroboscope is applied onto the scale and the flashing frequency is adjusted till a stationery image is obtained. The scale reading can now be taken for measuring the angular twist. This method is simple and inexpensive. However, the accuracy of measurement is poor.

(ii) Capacitive Method : The principles of capacitive measurement and ratio metric calculation provide the advantage of robustness towards environmental influences In order to measure the torsion angle resulting from the torque transmitted by a rotating shaft, we have modified the capacitive angle/angular speed sensor by mounting two asymmetric rotors on two concentric shafts between the sensor stators. Both grounded rotors realise a single effective rotor with a variable geometry, depending on the relative angle between the rotating shafts. These modifications extend the abilities of the sensor to measure the absolute angle (360°-range) and the angular velocity of the effective rotor with the ability to measure the difference–angle between the two rotors. Figure 1 shows a typical mechanical construction for converting the torsion on as haft in to an angle between The two rotors. The torsion shaft carrying one rotor is mounted concentrically in a hollow and stiff shaft Carrying the second rotor. From the measured torsion angle and the length and the G-modulus of the shaft the transmitted torque is calculated using Hooke’s Law

Figure 2 shows the electrode structure of the capacitive sensor. One stator plate is used as transmitter with 16 transmitting segments with centre angles of 22.5°, the other stator contains the receiving ring electrode. Two rotors with a symmetrically arranged blades with a centre angle of 60° (Figure 2) are mounted mirror-symmetrically on two concentric shafts as shown in Figure 1. The shafts electrically connect the conductive rotors to ground potential. The zero position of the relative angle is defined for overlapping rotors building two blades with centre angles of 75° and 105°. From this asymmetrical geometry results the ability to determine the direction of the torsion or the sign of the torque, respectively, and the maximum range of the difference angle of ±15°. As the applied torque changes the angle between the rotors, the electrical effective size of the rotor blades is changed. These changes influence the capacitive coupling between the transmitting segments and the receiving electrode.

In one measurement cycle a pulse sequence is applied to each segment. Depending on the rotor position and the effective size of the rotor the received signals change for each segment. By applying a radiometric algorithm to the received signals, the signed relative angle between the rotors is calculated.