Industrial process indicating/recording apparatus using stepping motor

Abstract

A compact ribbon-indicator and chart-recorder instrument for use in process control systems adapted to indicate and simultaneously record the values of up to three independent process variables. The motive power for each pen/indicator drive system is supplied from a corresponding relatively high-speed stepping motor. Each such motor is servo-controlled by a feedback circuit including a potentiometer coupled to the pen drive cable for developing a d-c feedback signal to be compared with the d-c measurement signal. The system is arranged to actuate the motor immediately upon development of a deviation signal outside of a fixed dead-band, but the extent of initial motor movement is limited to a pre-set small amount (preferably one step). If the deviation signal still is outside of the dead-band after a predetermined period of time, a clock pulse oscillator is turned on to actuate the motor at a relatively rapid rate until the deviation signal returns to the dead-band. The instrument also includes special limit circuitry for preventing motor activity when the pen has been driven to either end of its full-scale stroke. An optional addition is an alarm system which makes it possible to adjust the alarm set-point while observing its value on the indicator scale at the front panel, thereby avoiding need for a precision calibrated set-point potentiometer.