On-line computer to reduce response time in a viscometer system having analog feedback

Abstract

A bob-in-cup viscometer system is described that incorporates a guard ring to minimize surface effects at the fluid–air interface. Effectiveness of the guard ring requires that the torque-sensing member not rotate, and analog feedback from an angle sensor is used to supply electrical countertorque to maintain zero angle. The usual technique for determining the applied torque is to measure the electrical potential producing the countertorque. However, consistent with feedback systems in general, rapid changes in the torque being measured cause excursions from zero angle, producing overshoot in the countertorque. This effectively limits the system response time to signals (torque) changing more slowly than the overshoot time. The present system is unusual in utilizing an on-line computer to calculate the net torque directly from Newton’s second law through digital inputs of actual angular positions in time, electrical countertorque, and the moment of inertia. So long as the analog feedback system maintains angular excursions small enough for guard ring effectiveness, and within the limits of digital angle sampling rates for determination of acceleration, the net torque can be determined exactly in principle, even during overshoot in angle and countertorque. This technique should be applicable to other well-defined systems utilizing analog feedback where effective response time is limited by overshoot.