Abstract: A variable gain amplifier with digital output is described. The amplifier is useful for but not limited to input range selection in chart recorders or other measuring instruments. To minimize the number of expensive presision resistors used in the gain selection network each selected range is approximated by switching. A software routine then adjusts the gain after digitizing to the correct value for the selected range. Both range switching and gain correction are controlled by a general purpose microprocessor which also is available for other duties in the instrument.

Abstract: A direct current servo motor operates through a toothed belt linkage to drive the pen carriage of a chart recorder. The position of the drive is monitored by a sectored encoder disc on the motor shaft. A pair of optical sensors is located relative to the encoder disc to provide alternately phased transition signals as corresponding edges of the encoder disc sectors pass the sensors when the motor rotates. A microprocessor interrupted by a transition signal goes to a sub-routine that determines which way the encoder disc is rotating and stores a count signal of the corresponding sign in a register. Further the microprocessor compares an input signal with the stored count signal and directs the motor to rotate in the direction that will reduce the difference or error signal to zero thus causing the pen to follow the magnitude of the input signal.

Abstract: An improvement to a chart recorder where the chart paper is driven by a motor responsive to pulses counted by a microprocessor makes it possible for the operator to return the pen at any time to the starting point of the last chart scan by merely pressing a key. Using a programmed counter the microprocessor keeps track constantly of the number of counts since the start of the scan. On key command the microprocessor directs the motor to return through this number of counts plus 50; then to go forward 50 counts thus taking up any backlash in the chart drive so as to return exactly to the initial point of the scan. The subroutine also raises the pen during return and lowers it at the starting point.