Specifications

Digital I/O Functions 6-6
Figure 6.5 On-Off Controller
ON-OFF CONTROLLER WITH HYSTERESIS
The simple single-value controller, by its very nature, suffers from erratic
output that may not be acceptable, particularly when high-power equip-
ment is being controlled. To lengthen the control cycle and to make the
control action smoother, hysteresis (dead band) is often used in on-off
controllers. With hysteresis, the process variable is controlled between the
two setpoints in order to lengthen the duty cycle of the control output. To
increase the control duty cycle, the hysteresis, or difference between the
setpoints, must be increased. Figure 6.6 shows the effect of hysteresis on
the control output.