Fan_Velocity_Feedback_Control

Pennies were added to the fan to slow it down a bit, so that our DMM is sufficiently fast to monitor

the fan velocity.

An infrared beam and phototransistor are positioned to be interrupted by the fan blades. The fan velocity

is then monitored as the frequency of a square wave by the DMM.

Scope trace of the phototransistor signal.

The fan system is modeled in this feedback configuration. The fan is modeled to first order.

D is the DC sensitivity of the fan in Hz/volt. alpha is the fan response time. k/s is a digital

integrator implemented in LabView. k (V) is the integrator gain, set programmatically.

Elementary simplification of the feedback system yields a second order system.

The LabView main panel. The fan velocity (closed loop) is controlled by slider. The loop gain is

here set to .01 V.

Typical operation of the closed loop system showing the fan velocity (red) following the input

setting (green). System is slightly under damped.

Typical operation of the open loop system, for determination of the fan delay time constant alpha.

Lab View block diagram showing the digital integrator, and the DMM acquisition icon, and the fan

voltage control.

Direct measurement of fan sensitivity, using LabView to sweep control voltage from 4 V to 12 V.

Labview establishes fan sensitivity D as 16.5 Hz/V over the fan operating range.