Rate-of-Change (ROC)
Introduction
Beyond the nominal torques being transmitted which can be measured accurately with MagCanica's torque sensor system, torque variations are ubiquitous in the rotating parts of machines and can be measured with MagCanica's rate-of-change of torque (ROC) sensor system. Sensing and measurement of the variational torque components can often provide more detailed information concerning machine function than might be apparent from a measurement of torque alone, which is often dominated by its larger, more steady state components. This benefit is analogous to that obtained by using an accelerometer in an airbag sensor system to detect an impending crash, as opposed to a vehicle speed sensor which would not be able to detect a collision rapidly enough in the first place, and sometimes not at all due to the insufficient sensitivity to rapid velocity changes that an accelerometer is able to detect but a vehicle speed sensor is not.
The Need for ROC Measurement
Torque variation can be fully characterized by its time rate-of-change (ROC). Continuous measurement of ROC of torque can provide sufficient information to identify specific torque producing events, recognize "signature" abnormalities, and signal the need for controlling action. While ROC information can, in principle, be obtained by differentiating a torque dependent signal, such an approach would fundamentally limit signal content and quality. MagCanica's ROC sensor system overcomes these issues to reliably detect several forms of time-varying torque that can be of interest such as:
  • Torque variations reflective of the actual function of the machine (e.g., impulse wrenches, rock crushers, etc.)
  • Torque variations originating from kinematic features (e.g., oscillating or reciprocating parts)
  • Major torque variations originating from inconstant rates of energy input or usage (e.g., as in piston engines, air compressors, etc.)
  • Torque variations that vary with rotational position such as those arising in electric motors (e.g. cogging in machines having permanent magnet rotors or ripple in brushless motors)
Some sources of variational torque, such as those which arise on, or from the rotation of, ship propellers, helicopter rotors or wind turbine rotors, are more subtle in that they arise simply from the variation with rotational position of the blade's proximity to the hull or fuselage or frame, respectively. Moreover, all such active variational torques can stimulate torque oscillations at frequencies dependent on the dynamic relationship between inertia and elasticity within individual or interconnected rotating parts.
MagCanica's ROC Sensor System - Architecture and Benefits
MagCanica's ROC system has been successfully implemented on such varied applications as automotive engines and transmissions, electric motors, and machine tools. The system operates according to the measurement chain shown in the diagram below:
ROC SIGNAL PROCESSOR AND INTERPRETER
Signal Flow Diagram for MagCanica's rate-of-change of torque (ROC) sensor system
MagCanica's ROC system provides a number of complementary benefits beyond those provided by the torque sensor system:
  • The frequency response of an ROC system is much greater than that of any existing torque sensor system
  • The detectable amplitude of a given variational torque is higher for higher frequencies in an ROC signal than that which can be practically obtained through a differentiated torque sensor signal
  • Small amplitude, rapid changes in torque representing potentially very important diagnostic or control data, are either not easily observable by measuring torque alone or not at all, whereas they are detectable by measuring ROC directly
  • The ROC system has fewer restrictions on types of acceptable shaft steels
  • The ROC system can withstand very high temperatures, even in excess of 300C