Synthesis of feedback control unit for beam microbalances

Abstract

A feedback system was designed for a Cahn RG microbalance with rational use of control system theory to obtain critical damping and error integration. The former efficiently cuts transient effects and settling time, while the latter makes the steady-state error zero so that effects of nonlinearity vanish. The required transfer functions were realised with operational amplifiers and suitable impedances. The modified balance responds quickly (step response error less than 1% in 5 s), the noise level is reduced to 0.3 mu g, and in the steady state the null position is load-independent. Because coil current, rather than voltage, was chosen as the feedback quantity, the temperature coefficient of the compensating electromagnetic torque was reduced to less than 2*10-4 K-1, making temperature compensation superfluous. Detailed prescriptions are given so as to permit application of the procedure to other electromagnetic measuring devices, including balances, with the improvement in performance this may give.