The use of electrical division in servo error detection

Servo systems in which reference and feedback signals are subtracted suffer from two weaknesses; their sensitivity to common-mode noise and the partial dependence of their accuracy upon signal magnitude. It is shown that if the subtraction process is replaced by division, improved performance can result. The required division circuit is synthesized from a simple resistive attenuator after noting that a voltage e
1
may be divided by a second voltage e
2
if the attenuator's shunt resistance is caused to vary properly with e
2
. The functional relationship between these quantities is expressed mathematically and the parametric resistor's
\upsilon-i
characteristic is analyzed. Results indicate that a semiconductor diode can be used to match the required function over a wide voltage range. Design problems are discussed, and experimental data from measurements on a prototype circuit are presented and compared with theoretical performance.