Fan Speed Control with the EMC2102 Device

5.2.2Basic RPM Based Fan Control

The EMC2102 defaults will enable closed-loop operation. Prior to other options, examine the various parameters on ChipMan window page 3: Fan Settings. The registers of most interest are the FAN Minimum Drive (55h), and Valid Tach Count (56h). The closed-loop controller will not drive below the minimum drive value, and will not respond to a TACH Target RPM speed less than the speed defined by the Valid Tach Count register.

Note: The TACH reading is a reciprocal function of the fan speed, i.e. a higher TACH reading value means a slower fan rpm speed. Refer to 6 "Appendix" for more details.

Same as Experiment 1, select Fan Drive Setting (51h), TACH Target (57h), and TACH Reading (58h) for plotting. Start the plot windows in order to see the response, and command a series of Tach Targets, starting at 4500 RPM, and going up in 500 RPM steps. The plots in Figure 5.9, "Plots for Proper Control Settings" will be representative of the response generated.

TACH Target (57h)

Fan Driver Setting (51h)

TACH Reading (58h)

Figure 5.9 Plots for Proper Control Settings

Using this basic setup, it is possible to explore how well the fan is controlled using default values.

5.2.3Effects of Loading

The EMC2102 has the ability to overcome changes in fan current requirements for a given RPM setting due to aging, a blocked vent, dust, etc. This is because the controller does not rely on absolute drive settings, but rather on driving the PID loop error to "0" (i.e., driving the TACH reading to equal to the TACH target). To illustrate this feature, simply command an RPM setting (6000 RPM for this experiment), and then use a piece of paper to cover the window of the fan. This will reduce the air flow through the fan and decrease the wind resistance. With the same Fan Drive Setting value (~240d in register 51h, set by devices’ RPM controller), less wind resistance will make the fan running faster. Once the PID controller inside the EMC2102 detects the difference between the TACH reading and the TACH target (6000 RPM), it will adjust the output values (down to ~210d in this experiment) to bring the fan speed back to the target, as shown in Figure 5.10, "Effects of Loading".

Revision 0.2 (09-17-07)

USER MANUAL

SMSC EMC2102

14