What are dead band and hysteresis in control valves and how do they affect control?

Dead band is the range of input signal change that produces no output movement - caused by mechanical clearances and friction. Hysteresis is the difference in output position for same input signal depending on direction of travel - caused by friction and backlash. Combined effect: output takes different paths for increasing vs decreasing signals. Impact on control: causes limit cycling, slow response, and poor control quality. Acceptable limits: typically <1% for good control, <0.5% for precise control. Reduce through: low-friction packing, stiffer actuators, and digital positioners with tight control. Measure during positioner calibration.